Hermann F Fasel

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Journals/Publications

• Haas, A. P., Hader, C., & Fasel, H. F. (2024). Effects of Small Leading-Edge Bluntness on High-Speed Boundary-Layer Instabilities on Flat Plates. AIAA journal, 62(1), 162--174.
• Hader, C., & Fasel, H. F. (2023). Numerical Investigations of Nonlinearly Generated Disturbance Waves in High-Speed Boundary Layers. AIAA Journal, 61(11), 4797--4807.
• Hosseinverdi, S., & Fasel, H. F. (2023). High-order-accurate incompressible flow solver for high-fidelity numerical simulations and linear stability analysis. AIAA Journal, 61(4), 1772--1789.
• Woodward, M., Tian, Y., Lin, Y. T., Hader, C., Fasel, H., & Livescu, D. (2023). Mori-Zwanzig Modal Decomposition. arXiv preprint arXiv:2311.09524.
• Woodward, M., Tian, Y., Lin, Y. T., Hader, C., Fasel, H., Chertkov, M., & Livescu, D. (2023). Modal Analysis with Mori-Zwanzig Formalism: Application to Hypersonic Boundary Layer Flow. Bulletin of the American Physical Society.
• Browne, O., Haas, A., Brehm, C., & Fasel, H. F. (2022).

  A nonlinear compressible flow disturbance formulation for adaptive mesh refinement wavepacket tracking in hypersonic boundary-layer flows

  . Computers & Fluids, w240.
• Fasel, H. F., Bahrainirad, L., Gross, A., & Hasan, M. (2022).

  Investigation of collector flow for 1: 30 scale solar chimney power plant model

  . Solar Energy J., 241.
• Gross, A., Little, J., & Fasel, H. F. (2022). Numerical investigation of unswept and swept turbulent shock-wave boundary layer interactions 
  
  . Aerospace Science and Technology, 123.
• Fasel, H. F. (2021). 10) Haas, A. P., Hader, C., Fasel, H. F. “Linear Stability Investigation of Cross-Flow Instability for a Supersonic Swept Wing with a Biconvex Airfoil,” AIAA 2021-2848, 2021. AIAA.
• Fasel, H. F. (2021). 11) Hader, C., Fasel, H. F. “Flow control using steady blowing and suction strips in a Mach 6 Boundary Layer on a Flared Cone.” AIAA 2021-1206, 2021. AIAA.
• Fasel, H. F. (2021). 12) Hader, C., Deng, N., Fasel, H. F. “Direct Numerical Simulations of Hypersonic Boundary-Layer Transition for a straight cone at Mach 5.” AIAA 2021-0743, 2021. AIAA.
• Fasel, H. F. (2021). 13) Hader, C., Deng, N., Woodward, M., Fasel, H. F. “Direct Numerical Simulations of Laminar-Turbulent Transition for Transonic Boundary Layers,” AIAA 2021- 1658, 2021. AIAA.
• Fasel, H. F. (2021). 14) Hartman, A., Hader, C., Fasel, H. F. “Direct Numerical Simulations of laminar-turbulent boundary-layer transition for a blunt cone at Mach 6,” AIAA 2021-0744, 2021. AIAA.
• Fasel, H. F. (2021). 15) Leinemann, M., Hader, C., Fasel, H. F. “Direct Numerical Simulations of the Nonlinear Boundary Layer Transition Regime on a Flat Plate at Mach 6,” AIAA 2021- 1739, 2021. AIAA.
• Fasel, H. F. (2021). 16) Hosseinverdi, S. & Fasel, H. F. “High-Order accurate incompressible Navier-Stokes solver based on vorticity-velocity formulation for orthogonal curvilinear grids,” AIAA 2021-2741, 2021. AIAA.
• Fasel, H. F. (2021). 17) Bahrainirad, L., Hosseinverdi, S., & Fasel, H. F. “Wave Packet Development in Three-Dimensional Low-Speed Laminar Boundary Layers,” AIAA 2021-2897, 2021. AIAA.
• Fasel, H. F. (2021). 18) Borgmann, D., Hosseinverdi, S., Little, J. C. & Fasel, H. F. “Investigation of laminar separation bubbles using experiments, theory and DNS,” AIAA 2021-2898, 2021. AIAA.
• Fasel, H. F. (2021). 19) Guerra, A. G., Hosseinverdi, S., Singh, A., Little, J. C. & Fasel, H. F. “Unsteady evolution of a laminar separation bubble subjected to structural motion,” AIAA 2021-2949, 2021. AIAA.
• Fasel, H. F. (2021). 2) Meersman, J.A., Hader, C., Fasel, H.F. “Numerical investigation of Nonlinear Boundary-Layer Transition for Cones at Mach 6,” AIAA Journal Vol. 59 No. 6: 1940-1952, 2021.. AIAA J., 59, 1940-1952.
• Fasel, H. F. (2021). 3) Gross, A., and Fasel, H.F., “Numerical Investigation of Wing Section Undergoing Low-Frequency Aeroelastic Limit Cycle Oscillations,” AIAA Journal, Vol. 59, No. 4, pp. 1361-1373, 2021. AIAA J., 59.
• Fasel, H. F. (2021). 4) Meersman, J. A., Hader, C., & Fasel, H. F. “Direct Numerical Simulations of Nonlinear Entropy-Layer Instability Waves,” AIAA 2021-1737, 2021. AIAA.
• Fasel, H. F. (2021). 5) Hader, C., Fasel, H. F. “Direct Numerical Simulations of Hypersonic Boundary-Layer Transition for a Straight Cone at Mach 4: Oblique Breakdown,” AIAA 2021-2863, 2021. AIAA.
• Fasel, H. F. (2021). 6) Hader, C., Fasel, H. F. “Direct Numerical Simulations of Hypersonic Boundary-Layer Transition for a Straight Cone at Mach 7,” AIAA 2021-2865, 2021. AIAA.
• Fasel, H. F. (2021). 7) Hader, C., Fasel, H. F. “Three-dimensional wave packets in a Mach 10 Boundary Layer on a Sharp Cone,” AIAA 2021-2943, 2021. AIAA.
• Fasel, H. F. (2021). 8) Hartman, A., Hader, C., Fasel, H. F. “Direct Numerical Simulations of laminar-turbulent boundary-layer transition for blunt cones at Mach 6: Effect of Varying Nose Bluntness,” AIAA 2021-2880, 2021. AIAA.
• Fasel, H. F. (2021). 9) Singh, A., Threadgill, J. A., Flood, J. T., Craig, S. A., Little, J. C., Hader, C., Fasel, H. F. “Development of Plasma-based Controlled Disturbances for the Study of Boundary Layer Transition and Shock Boundary Layer Interaction,” AIAA 2021-2822, 2021. AIAA.
• Fasel, H. F. (2021). Barraza, B., Castillo Gomez, P., Tena, A., Gross, A., Leinemann, M., Tsakagiannis, V., and Fasel, H.F., “Machine-Learning Based Amplification Factor Transport Equation for Transition Modeling,” AIAA 2021-2706, 2021. AIAA.
• Fasel, H. F. (2021). Hartman, A., Hader, C., Fasel, H.F. “Nonlinear transition mechanism on a blunt cone at Mach 6: Oblique Breakdown,” Journal of Fluid Mechanics 915, 2021. Journal of Fluid Mechanics, 915.
• Fasel, H. F. (2021). Hasan M. K., Gross, A., Bahrainirad, L., & Fasel, H. F. “Numerical Investigation of Flow Inside the Collector of a Solar Chimney Power Plant,” AIAA 2021-0367, 2021. AIAA.
• Chynoweth, B. (2019). History and Progress of Boundary-Layer Transition on a Mach-6 Flared Cone. J. Spacecraft and Rockets, 56(3):1-14, 2019.. J. Spacecraft and Rockets, 56(3), 1-24.
• Haas, A. (2019). A time-spectral approximate Jacobian based linearized compressible Navier-Stokes solver for high-speed boundary-layer receptivity and stability. Haas, A.P., Browne, O.M., Fasel, H.F., and Brehm,. J. Comp. Phys., 405: 108978, 2019., 405: 108978, 2019, 405, 108978-109018.
• Hader, C. (2019). Direct numerical simulations of hypersonic boundary-layer transition for a flared cone: fundamental breakdown.. J. Fluid Mech., 869:341-384, 2019, vol 869, 341-384.
• Hosseinverdi, S. (2019). Numerical investigation of laminar-turbulent transition in laminar separation bubbles: The effect of free-stream turbulence.. J. Fluid Mech., 858:714-759, 2019, 858, 714-759.
• Gross, A. (2018). Active control of laminar separation - simulations, wind tunnel, and free-flight experiments,” Aerospace, vol. 5, No. 4, 114, 2018. Aerospace, 5(4).
• Gross, A., Agate, M., & Little, J. (2018). Numerical simulation of plunging wing section at high angles of attack.. AIAA J., 56.
• Hader, C. (2018). Towards simulating natural transition in hypersonic boundary layers: A random forcing approach. J. Fluid Mech., vol. 847, R3, 2018.. J. Fluid Mech., 847(R3).
• Hosseinverdi, S. (2018). An efficient, high-order method for solving Poisson equation for immersed boundaries: Combination of compact difference and multiscale multigrid methods.. J. Comp. Physics, 374, 912-940.
• Hosseinverdi, S. (2018). Role of Klebanoff modes in active flow control of separation: Direct numerical simulations. J. Fluid Mech., vol. 850, pp. 954-983, 2018. J. Fluid Mech, 850.
• Salemi, L. (2018). Synchronization of second-mode instability waves for high-enthalpy hypersonic boundary layers. J. Fluid Mech.. J. Fluid Mech., 838(r2).
• Fasel, H. F., & Balzer, W. (2016). Numerical Investigation of the Role of Free-stream Turbulence in Boundary-layer Separation. Journal of Fluid Mechanics, 801, Page no. 289-321.
• Fasel, H. F., & Gross, A. (2016). Hybrid Turbulence Model Simulations of Partially Stalled Airfoil Flow. AIAA Journal, 54(4), Page no. 1220-1234.
• Fasel, H. F., & Gross, A. (2016). Numerical Investigation of Shock Boundary-Layer Interactions. AIAA 2016-0347.
• Fasel, H. F., & Hader, C. (2016). Laminar-Turbulent Transition on a Flared Cone at Mach 6. AIAA 2016-3344.
• Fasel, H. F., & Hoseinverdi, S. (2016). Direct Numerical Simulations of Laminar-to-Turbulent Transition in Laminar Separation Bubbles in Three-Dimensional Boundary-Layer. AIAA-2016-3793.
• Fasel, H. F., & Laible, A. (2016). Continuously Forced Transient Growth in Oblique Breakdown for Supersonic Boundary Layers. Journal of Fluid Mechanics, 804, Page no. 323-350.
• Fasel, H. F., & Sivasubramanian, J. (2016). Direct Numerical Simulation of Laminar-Turbulent Transition in a Flared Cone Boundary Layer at Mach 6. AIAA 2016-0846.
• Fasel, H. F., & Sivasubramanian, J. (2016). Numerical Investigation of Shockwave Boundary Layer Interactions in Supersonic Flows. AIAA 2016-0613.
• Fasel, H. F., Gross, A., & Little, J. (2016). Numerical Simulation of Wing Section Near Stall. AIAA 2016-3947.
• Fasel, H. F., Mertens, C., Pineda, S., Little, J. C., & Gross, A. (2016). Effects of Structural Motion on the Aerodynamics of the X-56A Airfoil. AIAA 2016-2073.
• Tumin, A., Fasel, H. F., & Sivasubramanian, J. (2016). The Reynolds Number Effect on Receptivity to a Localized Disturbance in a Hypersonic Boundary Layer. AIAA Paper No. 2016-4246.
• Embacher, M., & Fasel, H. F. (2014). Direct Numerical Simulations of Laminar Separation Bubbles: Investigation of Absolute Instability and Active Flow Control of Transition to Turbulence. Journal of Fluid Mechanics, 747, 141-185.
• Mayer, C. J., Fasel, H. F., Choudhari, M., & Chang, C. L. (2014). Transition Onset Predictions for Oblique Breakdown in a Mach 3 Boundary Layer. AIAA Journal, 52(4), 882-885.
• Sivasubramanian, J., & Fasel, H. F. (2014). Numerical Investigation of the Development of Three-dimensional Wavepackets in a Sharp Cone Boundary Layer at Mach 6. Journal of Fluid Mechanics, 756, 600-649.
• Balzer, W., & Fasel, H. F. (2013). Direct numerical simulations of laminar separation bubbles on a curved plate: Part 1 - Simulation setup and uncontrolled flow. Proceedings of the ASME Turbo Expo, 6 B.
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  Abstract: The aerodynamic performance of lifting surfaces operating at low Reynolds number conditions is impaired by laminar separation. For a modern low-pressure turbine (LPT) stage, in particular when designed for high blade loadings, laminar separation at cruise conditions can result in significant performance degradation. Understanding of the physical mechanisms and hydrodynamic instabilities that are associated with laminar separation and the formation of laminar separation bubbles (LSBs) is key for the design and development of effective and efficient active flow control (AFC) devices. For the present work, laminar separation (part I) and its control (part II) were investigated numerically by employing highly-resolved, high-order accurate direct numerical simulations (DNS). © 2013 ASME.
• Balzer, W., & Fasel, H. F. (2013). Direct numerical simulations of laminar separation bubbles on a curved plate: Part 2 - Flow control using pulsed vortex generator jets. Proceedings of the ASME Turbo Expo, 6 B.
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  Abstract: Highly-accurate direct numerical simulations (DNS) are employed to investigate active flow control of laminar boundary layer separation by means of pulsed vortex generator jets (VGJs), i.e. by injecting fluid into the flow through a spanwise array of small holes. The uncontrolled flow configuration is represented by a laminar separation bubble developing on a curved-plate geometry modeling the convex suction-side curvature of the Pratt&Whitney "PackB" research blade. The simulation setup and uncontrolled flow results were presented in part I of the present paper. In this second part, particular focus is directed towards identifying the relevant physical mechanisms associated with VGJ control of low Reynolds number separation, as for example encountered in low-pressure turbine applications. The numerical results confirm findings of earlier flatplate simulations, which showed that the control effectiveness of pulsed VGJs can be explained by the fact that linear hydrodynamic instability mechanisms are exploited. When pulsing with frequencies to which the (uncontrolled) separated shear layer is naturally unstable, instability modes are shown to develop into large-scale, spanwise-coherent structures. These structures provide the necessary entrainment of high-momentum fluid causing a much sooner reattachment of the separated flow compared to the uncontrolled flow and consequently leading to a significant reduction in performance losses. © 2013 ASME.
• Brehm, C., & Fasel, H. F. (2013). A novel concept for the design of immersed interface methods. Journal of Computational Physics, 242, 234-267.
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  Abstract: The objective of this paper is to present a novel, robust, high-order accurate Immersed Interface Method (IIM) for advection-diffusion type equations. In contrast to other immersed methods that were designed for consistency and accuracy with a posteriori check of the numerical stability, we combine local Taylor-series expansion at irregular grid points with a local stability constraint as part of the design process. Stability investigations of the IIM are employed to demonstrate that the local stability constraint is sufficient for obtaining a globally stable method, as long as the Neumann number is less than its limiting value. One of the key aspects of this IIM is that the irregular finite-difference stencils can be isolated from the rest of the computational domain. To validate our novel immersed interface approach, two-dimensional and three-dimensional test cases for model equations are presented. In addition, this method is applied to the incompressible Navier-Stokes equations to conduct stability investigations of a boundary layer flow over a rough surface, and for investigations of pulsatile stenotic flows. Stability investigations of wall bounded flows are challenging for immersed methods, because the near wall accuracy is important for correctly capturing the characteristics of the hydrodynamic instability mechanisms, in particular regarding the wave relation between the wave velocity components close to the wall. © 2013 Elsevier Inc.
• Brehm, C., Gross, A., & Fasel, H. F. (2013). Open-loop flow-control investigation for airfoils at low reynolds numbers. AIAA Journal, 51(8), 1843-1860.
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  Abstract: Within the scope of a flight research project involving dynamically scaled models, active flow control was investigated for a modified NACA 64 3-618 airfoil. At low-Reynolds-number conditions, the aerodynamic performance of the modifiedNACA 643-618 airfoil is considerably reduced by flow separation. Computational-fluiddynamics simulations of the natural (uncontrolled) and controlled flow were carried out and provide the basis for a detailed analysis of the underlying physical mechanisms. The simulation results for the natural uncontrolled flow compare well with wind-tunnel measurements. Simulations and experiments show a large trailing-edge separation at lowangles of attackand a leading-edge separationbubble at highangles of attack.Tocontrol the laminar separationand improve the overall performance of the airfoil at the low-Reynolds-number conditions, time-periodic blowing and suction through a spanwise slot is employed. A strong dependence of the effectiveness of the active flow control on the forcing parameters, such as forcing frequency and forcing slot location, is found. In addition, linear stability analysis is employed to identify the instability mechanisms that are responsible for the effectiveness of the flow-control approach. Copyright © 2013 by the American Institute of Aeronautics and Astronautics, Inc.
• Dianics, J. V., Balthazar, M. A., Gross, A., & Fasel, H. F. (2013). Wind tunnel and free-flight testing of active flow control for modified NACA 64₃-618 airfoil. 31st AIAA Applied Aerodynamics Conference.
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  Abstract: A wing glove for active flow control free-fiight experiments using a one-fifth dynamically scaled Aeromot 200S Super Ximango motor glider was developed. The wing glove has a modified NACA 643-618 airfoil and is instrumented with off-the-shelf pressure transducers and amplitude-modulated synthetic jet actuators. Before the flight-testing was commenced the wing glove and the instrumentation were tested in two open return low-speed wind tunnels. Due to the low-aspect ratio of the glove, particular attention was paid to the two-dimensionality of the flow field over the wing glove. Several flight experiments were carried out and free-flight data with and without actuation were recorded. Unexpected challenges that were encountered during the flight-testing are discussed and an analysis of the wall pressure data in the frequency domain is provided.
• Fasel, H. F., Meng, F., Shams, E., & Gross, A. (2013). CFD analysis for solar chimney power plants. Solar Energy, 98, 12-22.
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  Abstract: Solar chimney power plants are investigated numerically using ANSYS Fluent and an in-house developed Computational Fluid Dynamics (CFD) code. Analytical scaling laws are verified by considering a large range of scales with tower heights between 1. m (sub-scale laboratory model) and 1000. m (largest envisioned plant). A model with approximately 6. m tower height is currently under construction at the University of Arizona. Detailed time-dependent high-resolution simulations of the flow in the collector and chimney of the model provide detailed insight into the fluid dynamics and heat transfer mechanisms. Both transversal and longitudinal convection rolls are identified in the collector, indicating the presence of a Rayleigh-Bénard-Poiseuille instability. Local separation is observed near the chimney inflow. The flow inside the chimney is fully turbulent. © 2013 Elsevier Ltd.
• Gross, A., & Fasel, H. (2013). Numerical investigation of separation control for wing sections. International Journal of Flow Control, 5(3-4), 121-141.
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  Abstract: For Reynolds numbers that are typical for general aviation aircraft separation is typically turbulent. In this paper the question is addressed if flow control strategies that are successful at low-Reynolds number conditions remain effective at higher Reynolds numbers. Towards this end hybrid simulations based on a one-equation renormalization group model were carried out for a modified NACA643-618 airfoil at a chord Reynolds number of one million. For ten degrees angle of attack a short laminar separation bubble develops near the leading edge and the flow separates turbulent from the suction side downstream of the leading edge bubble. Active flow control by harmonic blowing through a spanwise slot was investigated. The control was found to be only mildly effective or even counterproductive. In particular, the disturbances that were introduced by the control were only weakly amplified or even dampened. The unresolved eddy viscosity in the separated boundary layer lowers the effective Reynolds number and makes the flow less unstable or even stable with respect to two-dimensional disturbances. As a consequence of this, higher blowing ratios are required compared to flow control at low-Reynolds number conditions where the shear-layer instability is stronger.
• Gross, A., & Fasel, H. F. (2013). Numerical investigation of flows with three-dimensional separation. 51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 2013.
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  Abstract: When low aspect ratio geometries such as submarines, torpedoes, or missiles are operated at large angles of attack three-dimensional separation will occur on the leeward side. Separation increases losses and can result in undesirable unsteady forces. An improved understanding of three-dimensional separation is desirable as it may open the door to new methods for the control or prevention of separation. Numerical simulations of three-dimensional separation can provide detailed insight into instability mechanisms and the resultant flow structures. For most technical applications the Reynolds numbers are too high for direct numerical simulations and lower-fidelity approaches such as hybrid turbulence models become attractive. In this paper a new hybrid turbulence model blending strategy is proposed that adjusts the model contribution according to the local grid resolution. The strategy is validated for two-dimensional plane channel flow at Reτ= 395 and for the Stanford asymmetric diffuser which features a turbulent three-dimensional separation. The model is then employed for simulations of a hemisphere-cylinder geometry at 10 and 30 degrees angle of attack. The simulations demonstrate satisfactory model performance over a wide range of Reynolds numbers (5×103< ReD< 5×106). A nose separation bubble is captured for the lower Reynolds numbers and leeward vortices are observed for 30deg angle of attack regardless of Reynolds number. Different from, e.g., hemisphere-cylinder geometries asymmetric separation and roll-instability were reported for non-body-of-revolution geometries. The paper concludes with a brief discussion of simulations that were carried out for the Virginia Tech ellipsoid model at ReL= 20, 000. © 2013 by the authors.
• Gross, A., & Fasel, H. F. (2013). Numerical investigation of passive separation control for an airfoil at low-reynolds-number conditions. AIAA Journal, 51(7), 1553-1565.
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  Abstract: Two different passive flow control strategies were investigated for a modified NACA 643-618 airfoil at a chord-based Reynolds number of Re = 64,200 and an angle of attack of α = 8.64 deg. For these conditions, the laminar boundary layer separates from the suction side, resulting in a loss of lift and a drag increase. Distributed roughness elements with roughness Reynolds numbers of Rek = 136 and 446 that were mounted near the leading edge and scalloped leading edges with serration amplitudes of 5 and 0.5% of the chord were considered. The large roughness elements and the scalloped leading edge reduce the flow separation and enhance performance. The flow physics are, however, different. For Rek = 446, the roughness elements result in high-frequency shedding. The shedding results in an accelerated transition of the separated boundary layer. For the scalloped leading edge with 5% serration amplitude, laminar separation bubbles are situated in the leading-edge troughs. The turbulent wedges that originate from these bubbles coalesce near midchord. For a serration amplitude of 0.5%, the separation line is deformed in the spanwise direction in a manner that is reminiscent of stall cells. Copyright © 2013 by Christopher Porter, R. Mark Rennie, Eric J. Jumper.
• Gross, A., & Fasel, H. F. (2013). Numerical investigation of separation control for wing section at Re=300,000. 43rd Fluid Dynamics Conference.
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  Abstract: Active flow control by harmonic blowing through a slot is investigated for a modified NACA643-618 airfoil at a chord Reynolds number of Re=300,000 and for 15 degrees angle of attack. A direct simulation of the uncontrolled flow serves as a reference for hybrid simulations using a one-equation renormalization group turbulence model. Because of their lower computational expense, two-dimensional hybrid calculations are employed for active flow control parameter studies. Downstream harmonic blowing through a spanwise slot is investigated. For a blowing ratio of 0.1 the control is ineffective. For blowing ratios and dimensionless forcing frequencies between one and two the actuation results in vortex shedding of the separated boundary layer and a moderate rise in lift.
• Gross, A., Jagadeesh, C., & Fasel, H. F. (2013). Numerical and experimental investigation of unsteady three-dimensional separation on axisymmetric bodies. International Journal of Heat and Fluid Flow, 44, 53-70.
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  Abstract: The understanding of unsteady three-dimensional separation is lagging behind the understanding of two-dimensional separation due to its greater complexity and the limited amount of data for universal or canonical flow problems. Three-dimensional separation occurs, for example, when low aspect ratio devices such as submarines and torpedoes are operated at large angles of attack, α. Two low aspect ratio geometries, the DARPA Suboff bare hull geometry, which is a prototypical submarine shape, and a hemisphere-cylinder geometry were investigated. Numerical simulations and water tunnel experiments for the Suboff geometry for a Reynolds number based on diameter of Re= 10, 000 show little flow separation at α = 30°. A hemisphere-cylinder geometry was derived by replacing the Suboff forebody with a hemisphere. Simulations and water tunnel experiments were carried out for Re= 2000 and 5000. For α = 10° a large shedding separation bubble is observed on the leeward side. For α = 30° two counter-rotating leeward vortices appear and shedding is reduced. Proper orthogonal decomposition and Fourier analysis in time are employed for investigating the unsteady fluid dynamics. Conclusions are drawn with respect to the mean flow topology and possibly relevant hydrodynamic instabilities. © 2013 Elsevier Inc.
• Haas, A., & Fasel, H. F. (2013). Numerical investigation of the spatial development of a wave packet in laminar boundary layers with zero and adverse pressure gradient. 43rd Fluid Dynamics Conference.
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  Abstract: The effect of an adverse pressure gradient (without separation) on the downstream development of a wave packet in a flat plate boundary layer is investigated using threedimensional Direct Numerical Simulations (DNS) based on the incompressible Navier-Stokes equations. The wave packet was generated by a short duration pulse at the surface of the flat plate. Both the linear and nonlinear stages of the evolution of the wave packet were considered. For investigating the linear stages, the initial disturbance amplitudes were kept very small, whereas for investigating the nonlinear stages, larger amplitudes were used. The DNS results for the linear case are in very good agreement with Linear Stability Theory (LST) calculations. The pressure gradient was found to significantly increase the growth of instability waves compared to the case without pressure gradient. For the high-amplitude case, the strong disturbance growth and the ensuing nonlinear interactions quickly lead to the formation of a turbulent spot.
• Hader, C., Brehm, C., & Fasel, H. F. (2013). Numerical investigation of porous walls for a Mach 6.0 boundary layer using an immersed interface method. 51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 2013.
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  Abstract: Temporal direct numerical simulations were carried out for a Mach 6.0 boundary layer in order to investigate the effects of porous walls on stability and transition. To model the porous wall, a novel immersed interface method was implemented into the compressible Navier-Stokes Solver developed in our laboratory. Grid convergence studies were carried out to ensure that the resolution for the immersed interface simulations was sufficient. The simulation results demonstrate that the immersed interface method is well suited for investigating the effects of porous walls. Furthermore, a comparison of the results obtained with our immersed interface method with those reported in the literature for the linear stability regime, shows very good agreement. Additionally, the different terms in the kinetic disturbance energy equation were analyzed for the smooth wall and porous wall cases in order to gain physical insight into the stabilization mechanisms of porous walls. The results suggest that the pressure diffusion and viscous dissipation are the two most relevant mechanisms responsible for stabilization. In addition to investigating the effect of the porous walls on the linear stability regime, we also studied their effects with the nonlinear behavior and nonlinear breakdown mechanisms. Our simulation results for a fundamental breakdown scenario indicate that porous walls may also be effective in mitigating secondary instability mechanisms, and thus final breakdown to turbulence. However to confirm this, other breakdown mechanisms need to be investigated as well, such as for example, the subharmonic and oblique breakdown. © 2013 by the authors.
• Hader, C., Brehm, C., & Fasel, H. F. (2013). Numerical investigation of transition delay in a Mach 6 boundary layer using porous walls. 43rd Fluid Dynamics Conference.
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  Abstract: The main emphasis of this work is to investigate the effect of the porous wall on the nonlinear stability regime and to determine if the nonlinear stages of transition are affected by porous walls. Temporal direct numerical simulations were carried out for a Mach 6.0 boundary layer on a smooth and porous wall geometry. An Immersed Interface Method was implemented in a compressible Navier-Stokes code to physically resolve the porosity. The resonance onset behavior for fundamental and subharmonic breakdown was compared. Fundamental resonance was found to be stronger than the subharmonic resonance. For highly resolved breakdown simulations a strategy was adapted where the resolution was successively increased as the flow evolved from a laminar to turbulent state. In addition, the effect of numerical filtering on the transition simulations was investigated.
• Hosseinverdi, S., & Fasel, H. (2013). Direct Numerical Simulations of transition to turbulence in two-dimensional laminar separation bubbles. 51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 2013.
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  Abstract: Transition to turbulence in two-dimensional laminar separation bubbles on a flat plate is investigated by Direct Numerical Simulations (DNS). In laminar separation bubbles transition and separation are present simultaneously and interact in a physically complex manner. A set of numerical simulations has been carried out to investigate the transition process, and in particular to shed light on the development of large coherent structures, which arise during the transition. For the separation bubbles investigated, transition to turbulence appeared to be self-sustained, i.e., in contrast to zero pressure gradient boundary layers, no external forcing was required to initiate or sustain the transition process. The DNS results reveal that the streamwise vortices ('braids'), which evolve between two neighboring spanwise coherent vortical structures, may be with a consequence of a similar instability mechanisms as observed in bluff-body wakes (mode B instability). The simulation results also suggest that the development of three-dimensional disturbances is due to an absolute secondary instability. High-amplitude 2D waves were introduced through a spanwise slot to control the separation. It was shown that strong 2D forcing significantly alters the flow filed. With large amplitude two-dimensional forcing the temporal growth of 3D disturbances inside the separated flow region can be suppressed and as a consequence, the secondary absolute instability can be prevented. Consequently, the flow remains laminar in almost the entire computational domain. © 2013 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Hosseinverdi, S., & Fasel, H. F. (2013). Effect of free-stream turbulence on control of laminar separation bubbles using pulsed vortex generator jets: Direct numerical simulations. 51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 2013.
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  Abstract: Direct numerical simulations were employed to investigate the effects of free-stream turbulence on the control of laminar boundary-layer separation using pulsed vortex generator jets. Earlier research has shown that laminar separation can efficiently and effectively be controlled when an inviscid shear-layer instability is exploited. This paper addresses the question if such a control remains effective under free-flight conditions, which are characterized by free-stream turbulence. In the direct numerical simulations isotropic free-stream turbulence was introduced at the inflow boundary. For low free-stream turbulence levels and a blowing ratio of 0.6 the pulsed jets showed the same effectiveness as observed in earlier research with zero free-stream turbulence. When the free-stream turbulence intensity was increased to 3% the effectiveness of the pulsed jets slowly diminished. A proper orthogonal decomposition of the time-dependent data revealed that the dominant flow structures are two-dimensional for low to moderate free-stream turbulence levels. For 3% turbulence intensity the dominant structures were streamwise- "streaky" structures. Additional simulations with higher blowing ratios were carried out for 3% free-stream turbulence. When the blowing ratio was increased to 1.5 the separation length was reduced. For larger blowing ratios the separation length remained constant. For a blowing ratio of 2.5 the most energetic flow structures were predominantly two-dimensional. © 2013 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Hosseinverdi, S., & Fasel, H. F. (2013). Numerical investigation of the effect of free-stream turbulence on separation control by pulsed vortex generator jets. 43rd Fluid Dynamics Conference.
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  Abstract: Our previous research has shown that an effective and efficient laminar separation control is achieved when an inviscid shear-layer instability is exploited. How such active flow control strategies are affected by free-stream turbulence remains an open question. To address this question, highly resolved direct numerical simulations were carried out where isotropic turbulence was introduced at the inflow boundary. Without control, the length of the separated region was reduced when the free-stream turbulence intensity was increased. For a free-stream turbulence intensity of 3%, detailed investigations of active flow control using pulsed vortex generator jets (PVGJs) were carried out. The spanwise jet spacing, the blowing ratio, and the actuation frequency were varied. The present results indicate a clear optimum for all three parameters. A detailed analysis of the simulation data based on Fourier transforms, proper orthogonal decomposition, and instantaneous flow visualizations was carried out to provide detailed insight into the time-dependent fluid dynamics. Particular emphasis was placed on the downstream evolution of the disturbance Fourier mode amplitudes which provide indications for possible flow instabilities.
• Jagadeesh, C. S., Balthazar, M., Gross, A., & Fasel, H. (2013). Experimental investigation of the structure and dynamics of laminar separation bubbles at the onset of bursting. 31st AIAA Applied Aerodynamics Conference.
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  Abstract: A two-dimensional laminar separation bubble on a flat plate is studied experimentally using Particle Image Velocimetry (PIV) and flow visualization. The separation bubble was generated on a flat plate by an imposed adverse pressure gradient. The adverse pressure gradient was generated by using an inverted wing with a NACA 643-618 airfoil mounted above the flat plate. A parametric study of the effect of the upstream flow velocity and the induced pressure gradient on the mean flow topology and the unsteady behaviour of the separation bubble was carried out in the low-speed water tunnel of the Hydrodynamics Laboratory at the University of Arizona. The structure and dynamics of the laminar separation bubble were found to depend strongly on the aforementioned parameters. As the flow velocity is reduced, at very low flow velocities the bubble is seen to undergo a drastic change in geometry, resulting in bubble bursting. An attempt is made in this work at understanding the physics of bubble bursting. For certain flow conditions, strong vortex shedding near the reattachment region of the bubble was observed, which is a characteristic behaviour of short bubbles. High-resolution spatio-temporal PIV measurements were made to analyze the formation and breakdown of these flow structures.
• Meng, F., Gross, A., & Fasel, H. F. (2013). Computational fluid dynamics investigation of solar chimney power plant. 43rd Fluid Dynamics Conference.
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  Abstract: A direct numerical simulation of the three-dimensional unsteady flow in a 1:33 scale model of the the Manzanares solar chimney power plant in zero load condition (negligible pressure drop across turbine) was carried out. Instantaneous flow visualizations for the collector show transverse rolls near the collector inlet and longitudinal rolls about halfway into the collector. Based on earlier linear stability theory investigations for plane two- dimensional channel flow, the observed structures are attributed to a Rayleigh-Bénard- Poisuille instability. The present simulations also indicate separation from the bottom and top wall of the chimney inlet and downstream of the turbine mount. The buoyancy-driven flow in the chimney is fully turbulent.
• Salemi, L. C., & Fasel, H. (2013). Linearized Navier-Stokes simulation of the spatial stability of a hypersonic boundary layer in chemical equilibrium. 43rd Fluid Dynamics Conference.
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  Abstract: For certain hypersonic flight conditions, high temperature effects, also known as "real gas effects", become important. In this context, chemical equilibrium refers to a flow where the time scales associated with the chemical reactions are much shorter than the characteristic time scales associated with the fluid dynamics. In this paper, the hydrodynamic stability of the compressible boundary layer in chemical equilibrium was investigated using linearized Navier-Stokes simulations including a simplified chemical equilibrium model. Adequate knowledge of the possible hydrodynamic instabilities present for hypersonic chemical equilibrium flows, is essential for the design of aircraft and engines that fly at such speeds. A high-order-accurate Linearized Navier-Stokes code was developed to investigate the stability of hypersonic boundary layers in chemical equilibrium. Towards this end, the conservation equations for a three-dimensional viscous compressible laminar chemical equilibrium flow subjected to infinitesimal disturbances were derived. These equations were discretized in space using 6th and 4th order finite differences in the streamwise and wall-normal directions, respectively, and integrated in time using a 4th order Runge Kutta scheme. For model verification, test cases for a flat plate were compared with DNS and LST reference data, and presented very good agreement for growth rates and eigenfunctions. The new Linearized Navier-Stokes method enables quantification of the instabilities, offers a cost effective way to investigate the linear instability regime, and the influence of compressibility and high temperature effects.
• Sivasubramanian, J., & Fasel, H. F. (2013). Direct numerical simulation of controlled transition in a boundary layer on a sharp cone at Mach 6. 51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 2013.
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  Abstract: Direct Numerical Simulations are performed to investigate the laminar-turbulent tran- sition in a boundary layer on a sharp cone at Mach 6. The motivation for this research is to make a contribution towards understanding the nonlinear stages of transition and the final breakdown to turbulence in hypersonic boundary layers. Towards this end, three breakdown mechanisms were considered, namely, the second-mode fundamental (K-type), subharmonic (N-/H-type) and oblique breakdown. The simulations were carried out for the laboratory conditions of the hypersonic transition experiments conducted at Purdue University. Several small and medium scale simulations were carried out to explore the parameter space for fundamental and subharmonic resonance. These simulations indi- cated that for the chosen experimental conditions, the fundamental resonance was much stronger than subharmonic resonance. Subsequently a set of highly resolved fundamental and oblique breakdown simulations were performed. The nonlinear interactions observed during the breakdown process are discussed in great detail in this paper. A detailed de- scription of the flow structures that arise due to these nonlinear interactions is provided and an analysis of the skin friction and heat transfer is also presented. These controlled transition simulations clearly demonstrate that both mechanisms, fundamental and oblique breakdown, may indeed be viable paths to complete breakdown to turbulence in hypersonic boundary layers at Mach 6. © 2013 by the authors.
• Alexander, J., Blake, S., Clasby, B., Shah, A. J., Horne, C. V., Horne, J. V., Dianics, J., & Fasel, H. F. (2012). Machine vision and autonomus integration into an unmanned aircraft system. Proceedings of the International Telemetering Conference, 48.
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  Abstract: The University of Arizona's Aerial Robotics Club (ARC) sponsored two senior design teams to compete in the 2011 AUVSI Student Unmanned Aerial Systems (SUAS) competition. These teams successfully designed and built a UAV platform in-house that was capable of autonomous flight, capturing aerial imagery, and filtering for target recognition but required excessive computational hardware and software bugs that limited the systems capability. A new multi-discipline team of undergrads was recruited to completely redesign and optimize the system in an attempt to reach true autonomous real-time target recognition with reasonable COTS hardware. © International Foundation For Telemetering, 2012.
• Brehm, C., & Fasel, H. F. (2012). Direct numerical simulations of steady and unsteady stenotic flows. 50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition.
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  Abstract: In this paper, the incompressible Navier-Stokes equations are solved to study the effect of axisymmetric stenosis on the laminar-to-turbulent transition process in steady and unsteady stenotic flows. Prior research studies1-7 have demonstrated that a strong relationship exists between sites of disease and hemodynamic parameters such as wall shear stress. Thus, there is great practical value in quantifying blood flow velocity and pressure fields for the diagnosis and treatment of cardiovascular diseases. Our studies demonstrate a strong dependence of the transitional flow through a stenosed artery model on the Reynolds number, the degree of stenosis, the peak velocity to mean velocity ratio, and the non-dimensional frequency parameter. This paper explores the role of each parameters and identifies which of these (flow and geometry specific) parameters are critically important to describe the severity of a stenosis. The underlying physical mechanisms are highly complex involving three-dimensional, pulsatile flows at the onset of turbulence. Linear stability analysis for both steady and unsteady axisymmetric base flows is employed in order to get a better understanding of the physical mechanisms. The stability analysis results can be closely linked to the direct numerical simulation results presented in this paper. Copyright © 2012 American Institute of Aeronautics and Astronautics, Inc.
• Brehm, C., Hader, C., & Fasel, H. F. (2012). Novel immersed boundary/interface method for the compressible navier-stokes equations. 50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition.
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  Abstract: An extension of the immersed interface method developed by Brehm and Fasel1-3 to the compressible Navier-Stokes equations is presented. The extension of the immersed interface/boundary method1,2 contains some modifications to the original approach in order to incorporate characteristics of the compressible Navier-Stokes equations. The van Leer flux splitting approach utilized in the compressible Navier-Stokes solver was considered in the design of the finite difference stencils in the vicinity of the immersed boundary. The extensive stability investigations of the immersed scheme confirm that a stable immersed boundary treatment is achieved. The simulation results for a Mach 6 boundary layer flow over a flat plate and a porous wall validate the proposed immersed boundary method for the compressible Navier-Stokes equations. Copyright © 2012 by the authors.
• Chetan, S. J., & Fasel, H. (2012). Experimental investigation of the structure and dynamics of laminar separation bubbles. 50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition.
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  Abstract: This work is an experimental investigation of the dynamics of the laminar separation bubbles, which are typically present on the suction side of lifting surfaces at a large angle of attack. The separation bubble was generated on a flat plate by an adverse pressure gradient induced by The adverse pressure gradient was generated by using an inverted wing with a NACA 643-618 airfoil mounted above the flat plate. Using Particle Image Velocimetry (PIV), a parametric study of the effect of the upstream flow velocity and the induced pressure gradient on the mean flow topology and the unsteady behavior of the separation bubble was carried out in the low-speed water tunnel of the Hydrodynamics Laboratory at the University of Arizona. The topology of the laminar separation bubble, and in particular the unsteady flow dynamics, were found to be strongly dependent on these parameters. For certain conditions, strong vortex shedding near the reattachment region of the bubble was observed, which is a characterisc behavior of short bubbles. High-resolution spatio-temporal PIV measurements were made to analyze the formation and breakdown of these flow structures. The frequency of vortex shedding was determined from Fourier analysis of the time series of the velocity fluctuations. The non-dimensionalised frequencies were found to be nearly independent of the Reynolds number for the range of Reynolds numbers investigated here. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc.
• Fasel, H. F. (2012). Growth and breakdown of a wave packet into a turbulent spot in a cone boundary layer at mach 6 jayahar sivasubramanian1. 50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition.
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  Abstract: Direct Numerical Simulations are performed to investigate the growth and breakdown of a wave packet into a turbulent spot in a sharp cone boundary layer at Mach 6. In order to understand the natural transition process in hypersonic cone boundary layers, the flow was forced by a short-duration (localized) pulse. The pulse disturbance developed into a three-dimensional wave packet which consisted of a wide range of disturbance frequencies and wave numbers. The flow parameters for the simulations are based on the experimental conditions of the Boeing/AFOSR Mach 6 quiet-flow Ludwieg Tube at Purdue University. First, the linear development of the wave packet was studied by forcing the flow with a low-amplitude pulse (0.001% of the free-stream velocity). The dominant waves within the resulting wave packet were identified as the second-mode two-dimensional disturbance waves. In addition, weaker first-mode oblique waves were also observed on the lateral sides of the wave packet. In order to investigate the weakly nonlinear transition regime, medium-amplitude pulse disturbances (0.5% of the free-stream velocity) were introduced. The response of the flow to the medium-amplitude pulse disturbances indicated the presence of a fundamental resonance mechanism. Lower secondary peaks in the disturbance wave spectrum were identified at approximately half the frequency of the high-amplitude frequency band, which would be an indication of a subharmonic resonance mechanism. Strong peaks were also observed for low-wave-number second-mode oblique waves, which indicate a possible presence of an oblique breakdown mechanism. Finally, in order to identify more clearly which of these mechanisms ultimately leads to turbulent breakdown, a simulation with a higher forcing amplitude (5% of the free-stream velocity) was performed. The resulting strongly nonlinear wave packet eventually leads to localized patches of turbulent flow (turbulent spots). The disturbance wave spectrum indicates that both second-mode fundamental resonance and oblique breakdown mechanisms may be the dominant mechanisms for the investigated flow. Both mechanisms may play a role in the natural transition process for a cone boundary layer at Mach 6. Copyright © 2012 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
• Gross, A., & Fasel, H. F. (2012). Flow control for NREL S822 wind turbine airfoil. AIAA Journal, 50(12), 2779-2790.
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  Abstract: The flow over a National Renewable Energy Laboratory S822 wind turbine airfoil was simulated for a chord Reynolds number of 100,000 and an angle of attack of 5 deg. These conditions approximately match the blade element conditions at 80% radius of a 2-m-diameter turbine operating at 300 rpm. Simulations of the uncontrolled flow show boundary-layer separation on the suction side, which is consistent with University of Illinois at Urbana-Champaign experimental data. Active flow control has the potential to locally (and on demand) reduce the unsteady loads on individual turbine blades during nonnominal operation, thereby increasing turbine life. In addition, flow control may help lower the cut-in wind speed. Unsteady flow control for reducing the suction side separation using pulsed vortex generator jets, flip-flop jets, and plasma actuators were evaluated. Actuation frequencies of 2.5 and 5 (normalized with freestream velocity and axial chord length) and blowing velocities of 0.01, 0.1, and 1 (normalized with the freestream velocity) were investigated. It was found that blowing ratios as low as 0.1 were already sufficient for eliminating the suction side separation, resulting in a more than four times increase of the lift-to-drag ratio. The high effectiveness and efficiencyis traced back tohydrodynamic instabilities that lead to a downstream growth of the forced disturbances. Too high actuator amplitudes resulted in early disturbance saturation, which made the control inefficient. Copyright © 2012 by the authors. Published.
• Gross, A., & Fasel, H. F. (2012). Numerical investigation of separation control for wing sections. 6th AIAA Flow Control Conference 2012.
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  Abstract: Numerical simulations of the unsteady separated flow over wing sections at flight Reynolds numbers are challenging. Direct numerical simulations which resolve all scales of the fluid motion promise the highest degree of fidelity but are computationally very expensive. Even large-eddy simulations can be prohibitively expensive because of the high grid resolution required near the walls. Hybrid turbulence models employ Reynolds-averaged Navier-Stokes near the walls and adjust the turbulence model contribution away from the walls according to the local physical grid resolution. In this paper hybrid simulations based on a one-equation renormalization group model are presented for a modified NACA643-618 airfoil at a chord Reynolds number of 1 million and for 10, 20, and 30 degrees angle of attack. For these conditions a short laminar separation bubble develops near the leading edge and the flow separates turbulent from the suction side downstream of the leading edge bubble. For 10 degrees angle of attack the lift and drag data obtained from the simulations are in good agreement with XFoil predictions and active flow control by harmonic blowing through a spanwise slot upstream of the turbulent separation is investigated. © 2012 by the authors.
• Gross, A., Fasel, H. F., Friederich, T., & Kloker, M. J. (2012). Numerical investigation of rotational augmentation for S822 wind turbine airfoil. Wind Energy, 15(8), 983-1007.
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  Abstract: Direct numerical simulations were carried out for an S822 wind turbine blade section at a chord Reynolds number of Re = 100, 000 and an angle of attack of α = 5°. Results for a stationary non-rotating blade section compare favorably with wind tunnel data by the University of Illinois at Urbana-Champaign and XFoil predictions. By adding volume forcing terms to the right-hand side of the Navier-Stokes equations, the Coriolis and centrifugal accelerations resulting from blade rotation are modeled in the blade section simulations. Blade rotation is shown to delay separation especially near the hub, resulting in a lift increase of up to 100% and a drag reduction. The simulations provide insight into a physical mechanism that offers an explanation for the lift increase observed for rotating blade sections when compared with stationary blade sections, which is commonly referred to as rotational augmentation. Rotation is shown to lead to a radial velocity component toward the blade tip in areas where the velocity is substantially different from its free-stream value, such as near the stagnation point and especially in the separated flow region, and to the appearance of stationary and traveling crossflow vortices. A linear stability theory analysis that compares favorably with the simulation data provides proof that the primary instabilities are of a mixed type, including both a two-dimensional mode (Tollmien-Schlichting and Kelvin-Helmholtz type) and a stationary and unsteady crossflow mode. The crossflow instabilities accelerate transition, leading to separation delay, lift increase and drag reduction. This effect is very pronounced at 20% blade radius and still present at 80% radius. Because periodicity conditions were applied in the spanwise direction, the present results provide an explanation for rotational augmentation that is not based on the transfer of fluid from the inboard region toward the blade tip ('centrifugal pumping'). For the low Reynolds number conditions considered here, crossflow instabilities, which destabilize the flow leading to earlier transition and a separation delay, may contribute to rotational augmentation. Copyright © 2012 John Wiley & Sons, Ltd.
• Gross, A., Jagadeesh, C., & Fasel, H. F. (2012). Numerical investigation of three-dimensional separation on axisymmetric bodies at angle of attack. 50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition.
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  Abstract: The understanding of three-dimensional separation is lagging behind the understanding of two-dimensional separation due to its greater complexity and the small number of universal or canonical flow problems. Three-dimensional separation occurs, for example, when low aspect ratio devices such as submarines and torpedoes are operated at large angles of attack, . Two low aspect ratio geometries, the DARPA Suboff bare hull geometry and a hemisphere-cylinder geometry were investigated. Numerical simulations and water tunnel experiments for the DARPA Suboff bare hull geometry which is a prototypical submarine shape, show little flow separation at = 30deg and for Reynolds numbers based on diameter of Re = 10, 000 and Re = 20, 000. A hemisphere-cylinder geometry was derived by replacing the Suboff forebody with a hemisphere. Simulations and water tunnel experiments were carried out for Re = 2000 and 5000 and for = 10deg and 30deg. For = 10deg a large separation bubble that is shedding is observed on the leeward side. For = 30deg two counter-rotating leeward vortices appear and shedding is suppressed. Proper orthogonal decomposition and Fourier analysis in time are employed for investigating the unsteady fluid dynamics. Conclusions are drawn with respect to the mean flow topology and possibly relevant hydrodynamic instabilities. Copyright © 2012 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
• Hosseinverdi, S., & Fasel, H. F. (2012). Investigation of transition and separation in the presence of free-stream turbulence using direct numerical simulations. 50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition.
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  Abstract: Flow separation for a flat plate boundary layer in the presence of free-stream turbulence (FST) is investigated using Direct Numerical Simulations (DNS) based on the incompressible Navier-Stokes equations. To induce separation on the flat plate, a suction/blowing velocity distribution is applied along the upper boundary of the computational domain. The suction/blowing velocity distribution is chosen such that the resulting downstream pressure gradient closely matches that of accompanying water-tunnel experiments where the bubble was generated by an airfoil-shape displacement body in the free stream4, 23. First, two-and three-dimensional DNS were performed without free-stream turbulence. Flow transition cannot occur, of course, for 2-D DNS. However, transition to turbulence was also not observed in the 3-D simulation and the bubble was much longer than in the experiments. In the water-tunnel experiments free-steam turbulence, albeit small, is present of course, which will accelerate transition, and as consequence, reduce the size of the separation bubble. Therefore it can be concluded that the background disturbances in our numerical simulation, which originate from round-off and truncation errors, were not sufficient to reproduce "realistic" separation bubbles. To investigate the effect of free-stream turbulence, a numerical model for generating isotropic grid turbulence is employed for our 3-D simulations. For a simulated low level of free-stream turbulence (0.08%), the separation length is significantly reduced in comparison to the undisturbed (no FST) case and good agreement with the experimental data is obtained. Based on detailed analysis of the timedependent flow field and linear stability theory (LST) calculations, indications are that the dominant mechanism causing transition in the bubble is due to an inviscid Kelvin-Helmholtz instability of the inflectional velocity profile in the separated flow region. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Hosseinverdi, S., Balzer, W., & Fasel, H. F. (2012). Direct numerical simulations of the effect of free-stream turbulence on 'long' laminar separation bubbles. 42nd AIAA Fluid Dynamics Conference and Exhibit 2012.
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  Abstract: Laminar separation bubbles on a flat plate boundary layer in the presence of free-stream turbulence (FST) were investigated by means of Direct Numerical Simulations (DNS). A suction/blowing velocity distribution was applied along the free-stream boundary of the computational domain to induce separation on the flat plate. For numerically generating free-stream turbulence, isotropic grid turbulence, which is obtained from a superposition of eigenmodes from the continuous spectrum of Orr- Sommerfeld and Squire Equations, was introduced at the inflow boundary. The effect of the spanwise extent of the computational domain was investigated by carrying out computations with two different spanwise domain widths. The main characteristics of the separation bubble, such as the bubble length and the skin-friction distribution were very similar for both spanwise domain sizes. However, for the wider domain the 2-D "rollers" were modulated in the spanwise direction and broke up earlier than for the narrow domain. For the narrow domain counter-rotating streamwise vortices appeared ("braids"). Detailed numerical simulations were performed to investigate the effect of the free-stream turbulence energy spectrum. It was found that the transition location was essentially independent of the integral length scale of the free-stream turbulence. Also, the dependence of the separation length on the integral length scale was found to be very weak for the range of length scales considered in our studies. In contrast, the separation length was significantly reduced already for relatively low free-stream turbulence intensity (0.1%) when compared to the baseline case with zero FST. When the FST was increased further the length and height of the bubble continued to decrease. Instantaneous flow field visualizations revealed that the spanwise coherence of the dominant 2D structures was weakened with increasing FST intensity. In addition, proper orthogonal decomposition (POD) analyses of the instantaneous flow data revealed that streamwise "vortical" structures became dominant for high FST levels. Based on a detailed analysis of the time-dependent flow field and a comparison between DNS results and linear stability theory (LST) calculations, it was found that for free-stream turbulence intensities up to 2%, transition in the bubble was still due to an inviscid (Kelvin-Helmholtz) instability of the inflectional velocity profile in the separated flow region, and not due to nonlinear bypass mechanisms. © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Sivasubramanian, J., & Fasel, H. F. (2012). Nonlinear stages of transition and breakdown in a boundary layer on a sharp cone at mach 6. 50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition.
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  Abstract: Direct Numerical Simulations (DNS) are performed to investigate the nonlinear transition mechanisms of a boundary layer on a sharp cone at Mach 6. The flow parameters used in the simulations are based on the experimental conditions of the Boeing/AFOSR Mach 6 quiet-flow Ludwieg-tube at Purdue University. The main objective of the present research is to explore which nonlinear breakdown mechanisms may be dominant in a broad band "natural" disturbance environment and then to perform controlled transition simulations of these mechanisms. Towards this end, a "natural" transition scenario was modeled and investigated by generating wave packet disturbances. These wave packet simulations provided strong evidence for a possible presence of fundamental and subharmonic resonance mechanisms in the nonlinear transition regime. However, the fundamental resonance was much stronger than the subharmonic resonance. In addition to these two resonance mechanisms, the wave packet simulations also indicated the possible presence of oblique breakdown mechanism. To gain more insight into the nonlinear development we performed controlled transition simulations of these mechanisms. These simulations demonstrated that both fundamental breakdown and oblique breakdown may indeed be a viable path to complete breakdown to turbulence in hypersonic cone boundary layers at Mach 6. Copyright © 2012 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
• Brehm, C., & Fasel, H. F. (2011). Biglobal stability analysis as an initial value problem for a stalled airfoil. 41st AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: A biglobal stability approach formulated as an initial value problem is used to study the occurrence of biglobal modes for the flow around a NACA0015 airfoil at a high angle of attack. For the stability analysis of the NACA0015 airfoil both steady and unsteady base flows are considered. An important focus of the paper is on elaborating the differences between the stability characteristics of steady and unsteady base flows. The connection between experimentally observed spanwise periodically occurring three-dimensional separated regions (also referred to as stall cells when occurring on aircraft wings at high angles of attack) and unstable biglobal modes is explored. The occurrence of these three-dimensional separated flow regions can have a detrimental effect on the controllability of the airplane at deep stall conditions. In this paper results from direct numerical simulations of the flow past a NACA0015 airfoil at angle of attack, α = 18°, and Re = 1, 000 are presented. The relevance of the linearly unstable biglobal modes for the fully three-dimensional flow field will be discussed. © 2011 by the authors.
• Brehm, C., Koevary, C., Dackermann, T., & Fasel, H. F. (2011). Numerical investigations of the influence of distributed roughness on blasius boundary layer stability. 49th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition.
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  Abstract: It is well known that surface roughness can have a strong impact on the laminar-turbulent transition process. Whereas for isolated roughness some basic understanding of the physical mechanisms promoting transition has been gathered, the relevant physical mechanisms driving the transition process in the presence of distributed roughness are still far from understood. In this paper the influence of two-dimensional distributed wall roughness on the Blasius boundary layer stability is investigated. Distributed surface roughness is simulated by using an immersed boundary technique. This methodology facilitates the investigation of a large parameter range, regarding size, spacing and shape of the roughness elements, which is relevant for the characterization of distributed wall roughness. Some simulation results are compared to experimental results by Gaster.1 Moreover, it will be demonstrated that these parameters (roughness height, spacing and shape) strongly affect the linear stability characteristics of the boundary layer flow. For example, it was found that small amplitude disturbances experience stronger amplitude growth for sinusoidal roughness elements than for roughness elements with rectangular shape. In addition to investigating the effect of two-dimension roughness on the primary instability we also studied the effect on secondary instabilities. Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc.
• Brehm, C., Reize, F., & Fasel, H. F. (2011). Stability investigation of axisymmetric stenotic flows. 41st AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: The early stages of laminar-turbulent transition in stenotic flows is studied by solving the full Navier-Stokes equations in cylindrical coordinates. For the linear stability analysis and the fully nonlinear direct numerical simulations, both steady and unsteady inflow velocity profiles are considered. The initial linear growth of small three-dimensional disturbances in axisymmetric stenotic basic flows for different Reynolds numbers and degrees of stenosis is investigated by considering the linearized Navier-Stokes equations in disturbance flow formulation. By introducing pulse disturbances different types of instability mechanisms can be analyzed such as biglobal instabilities, transient growth, and convective wave-like instabilities. All three types of instability mechanisms have been found to be relevant for the transition process in stenotic flows. Furthermore, nonlinear direct numerical simulations are employed to investigate which of the different instability mechanisms identified in the linear stability analysis are most relevant for the transition process. © 2011 by the authors.
• Fasel, H. (2011). Machine vision and autonomous integration for an unmanned aircraft system. Proceedings of the International Telemetering Conference, 47.
• Gross, A., & Fasel, H. F. (2011). Flow control for wind turbine airfoil. ASME 2011 5th International Conference on Energy Sustainability, ES 2011, 2051-2060.
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  Abstract: The flow over a NREL S822 wind turbine airfoil was simulated for a chord Reynolds number of 100,000 and an angle of attack of 5deg. These conditions approximately match the blade element conditions at 80% radius of a 2m diameter turbine operating at 300rpm. A simulation of the uncontrolled flow with steady approach flow conditions shows boundary layer separation on the suction side which is consistent with University of Illinois at Urbana-Champaign experimental data. Active flow control has the potential to locally (and on demand) reduce the unsteady loads on individual turbine blades during non-nominal operation, thereby increasing turbine life. In addition, flow control may help lower the cut-in wind speed. Unsteady flow control for reducing the suction side separation using pulsed vortex generator jets, flip-flop jets, and plasma actuators were evaluated. It was found that very low actuation amplitudes were already sufficient for eliminating the suction side separation. The high effectiveness and efficiency is traced back to hydrodynamic instabilities that lead to a downstream growth of the forced disturbances. Too high actuator amplitudes resulted in early disturbance saturation which made the control inefficient. Copyright © 2011 by ASME.
• Gross, A., & Fasel, H. F. (2011). Numerical investigation of passive separation control for airfoil at low Reynolds number conditions. 41st AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: Direct numerical simulations were employed for investigating two different passive flow control strategies for a modified NACA 643-618 airfoil at a chord based Reynolds number of Re=64,200 and an angle of attack of α = 8.64deg. For these conditions the laminar boundary layer separates from the suction side resulting in a loss of lift and a drag increase. Distributed roughness elements with a roughness Reynolds number of Rek = 446 that were mounted near the leading edge and a scalloped leading edge with serration amplitudes of 5% and 0.5% of the chord were considered. Both strategies reduce flow separation and enhance performance. The flow physics are, however, different. The roughness elements are large enough to induce local flow separation resulting in high frequency shedding. The shedding results in an accelerated transition of the separated boundary layer. For the scalloped leading edge with 5% serration amplitude, laminar separation bubbles are situated in the leading edge troughs. The turbulent wedges that originate from these bubbles coalesce near mid-chord. For a serration amplitude of 0.5%, the separation line is deformed in the spanwise direction in a manner that is reminiscent of stall cells. © 2011 by the authors.
• Gross, A., & Fasel, H. F. (2011). Numerical investigation of separation for airfoils. 41st AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: Separation deteriorates airfoil performance and is generally undesirable. In this paper turbulent separation from two different airfoils is investigated. The first airfoil is a modified NACA 643-618 airfoil. At a chord based Reynolds number of Re=322,000 and for an angle of attack of α = 13.2deg a laminar separation bubble forms near the leading edge and the flow separates turbulent at about mid-chord. For these conditions "coarse" grid direct numerical simulations and unsteady hybrid simulations based on a one-equation renormalization group turbulence model and the filter-based Reynolds-averaged Navier- Stokes model were carried out. The direct simulation was found to suffer from insufficient grid resolution. Results obtained from the hybrid simulations provide an adequate match with the experimental reference data at much reduced computational cost. A procedure for "seeding" turbulence velocity fluctuations in areas where the grid resolution increases in the streamwise direction was found to have no effect on the mean flow data. The second airfoil is a generic laminar airfoil. For Re=2,630,000 and α = 23deg the flow separates near quarterchord. Unsteady Reynolds-averaged Navier-Stokes simulations show a curved separation line which is indicative of stall cells. Attempts to explain the spanwise wavelength of the unsteady and steady spanwise flow structures remained unsuccessful. © 2011 by the authors.
• Gross, A., Balzer, W., & Fasel, H. F. (2011). Investigation of boundary-layer separation for lifting surfaces. Proceedings - 2010 DoD High Performance Computing Modernization Program Users Group Conference, HPCMP UGC 2010, 35-44.
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  Abstract: Flow separation from lifting surfaces such as airfoils is undesirable as it deteriorates performance. For example, when airfoils that are designed for large Reynolds numbers are operated at smaller off-design Reynolds numbers, laminar separation can occur. Laminar separation typically leads to transition and reattachment. Transition is influenced by factors, such as free-stream turbulence and wall roughness. Transition and reattachment affect the circulation and, thereby, separation itself. We are employing computational fluid dynamics for investigating the fundamental mechanisms of separation and transition for lifting surfaces. Using highly-resolved direct numerical simulations, we are investigating fundamental aspects of separation and transition in the presence of free-stream turbulence for canonical separation bubbles. In parallel, we are carrying out hybrid turbulence model simulations of an entire airfoil at a larger chord Reynolds number. The combined approach will advance both physical understanding and modeling capabilities, and thus provide a solid platform for the development of separation control strategies for practical applications. © 2011 IEEE.
• Gross, A., Jacobi, R., & Fasel, H. (2011). Investigation of three-dimensional internal and external flow separation. Proceedings - 2010 DoD High Performance Computing Modernization Program Users Group Conference, HPCMP UGC 2010, 45-53.
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  Abstract: Flow separation is always three-dimensional despite the fact that most of the past research has focused on two-dimensional separation. The three-dimensional character of separation is particularly relevant when low-aspect ratio geometries are considered. Separation is often associated with unsteadiness, which is caused by large coherent structures that are a consequence of hydrodynamic instability mechanisms of the mean-flow. We are employing direct numerical simulations for investigating the highly-complex flow physics of three-dimensional laminar separation bubbles. The introduction of pulse disturbances allows us to probe the instability mechanisms. In parallel, we are also employing hybrid turbulence models for simulations of the turbulent flow through a square-duct, and for the Stanford University asymmetric diffuser experiments. By advancing the understanding of the fundamental mechanisms governing three-dimensional separation and by devising modeling strategies for high-Reynolds number flows, we are laying the foundation that may lead to better predictive tools and to separation control devices for practical applications. © 2011 IEEE.
• Hader, C., & Fasel, H. F. (2011). Numerical investigation of porous walls for a Mach 6.0 boundary layer using an immersed boundary method. 41st AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: Temporal direct numerical simulations (TDNS) were carried out for a Mach 6.0 boundary layer on a 2-D porous wall. The porous wall was implemented with an immersed boundary scheme. For validation purposes results were compared with results obtained with an analytical porous wall model. A grid convergence study was conducted to confirm that the grid resolution was adequate. A parameter study revealed that the geometric dimensions for which the porous coating most effectively attenuates the growth of disturbances in the boundary layers. For the optimal parameter setting significant stabilization of the temporal growth of the instability waves is observed. The optimal cavity depth for different numbers of pores per disturbance wavelength was found to remain approximately the same. The present study showed that porous coatings can be modeled with an immersed boundary scheme. © 2011 by the authors.
• Laible, A. C., & Fasel, H. F. (2011). Numerical investigation of hypersonic transition for a flared and a straight cone at Mach 6. 41st AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: Hypersonic boundary layer stability and transition for two different cone geometries at Mach 6 are studied using direct numerical simulation (DNS). The two geometries are (i) the flared cone investigated in the NASA Langley Test Chamber Facility1 and (ii) its straight (non-ared) counterpart. We present a detailed comparison between these geometries with respect to the base flow, i.e. the undisturbed laminar flow, and the linear stability regime. Furthermore, a parameter study regarding secondary fundamental instability was performed for each geometry. Then a wave which experienced a high secondary growth rate was used to initialize numerical simulations deep into the transitional regime. The resulting nonlinear process, which can be considered to be a 'classical' fundamental (K- type) breakdown, is analyzed in detail. By comparing the results for the two different geometries, qualitative and quantitative insight into the hypersonic transition process for flared cones is obtained. In particular the question how strong the nonlinear transition process is altered by the cone flare is discussed. © 2011 by the authors.
• Postl, D., Balzer, W., & Fasel, H. F. (2011). Control of laminar separation using pulsed vortex generator jets: Direct numerical simulations. Journal of Fluid Mechanics, 676, 81-109.
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  Abstract: Direct numerical simulations (DNS) are employed to investigate laminar boundary layer separation and its control by pulsed vortex generator jets (VGJs), i.e. by injecting fluid into the flow through a spanwise array of small holes. Particular focus is directed towards identifying the relevant physical mechanisms associated with VGJ control of low-Reynolds-number separation, as encountered in low-pressure turbine applications. Pulsed VGJs are shown to be much more effective than steady VGJs when the same momentum coefficient is used for the actuation. From our investigations we have found that the increased control effectiveness of pulsed VGJs can be explained by the fact that linear hydrodynamic instability mechanisms are exploited. When pulsing with frequencies to which the separated shear layer is naturally unstable, instability modes are shown to develop into large-scale, spanwise coherent structures. These structures provide the necessary entrainment of high-momentum fluid to successfully reattach the flow. © 2011 Cambridge University Press.
• S., C., A., D., & Fasel, H. F. (2011). Direct numerical simulation of complete transition to turbulence via oblique breakdown at Mach 3. Journal of Fluid Mechanics, 674, 5-42.
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  Abstract: A pair of oblique waves at low amplitudes is introduced in a supersonic flat-plate boundary layer at Mach 3. Its downstream development and the concomitant process of laminar to turbulent transition is then investigated numerically using linear-stability theory, parabolized stability equations and direct numerical simulations (DNS). In the present paper, the linear regime is studied first in great detail. The focus of the second part is the early and late nonlinear regimes. It is shown how the disturbance wave spectrum is filled up by nonlinear interactions and which flow structures arise and how these structures locally break down to small scales. Finally, the study answers the question whether a fully developed turbulent boundary layer can be reached by oblique breakdown. It is shown that the skin friction develops such as is typical of transitional and turbulent boundary layers. Initially, the skin friction coefficient increases in the streamwise direction in the transitional region and finally decays when the early turbulent state is reached. Downstream of the maximum in the skin friction, the flow loses its periodicity in time and possesses characteristic mean-flow and spectral properties of a turbulent boundary layer. The DNS data clearly demonstrate that oblique breakdown can lead to a fully developed turbulent boundary layer and therefore it is a relevant mechanism for transition in two-dimensional supersonic boundary layers. © 2011 Cambridge University Press.
• S., C., Laible, A. C., & Fasel, H. F. (2011). Numerical investigation of wave packets in a mach 3.5 cone boundary layer. AIAA Journal, 49(1), 67-86.
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  Abstract: Transition initiated by a wave packet in a cone boundary layer at Mach 3.5 has been investigated using linear stability theory and direct numerical simulations. Disturbances have been introduced into the boundary layer by pulsing the wall-normal velocity through a hole on the cone surface. The present study can be divided into three parts. In the first part, the linear development of a wave packet is studied in detail. The second part focuses on the identification of possible, asymmetric resonance triads for the most dominant oblique instability waves of the wave packet. New triads have been found that have not yet been reported for a supersonic boundary layer. These triads might explain some major findings in the third part of the present work, which focuses on the weakly nonlinear development of a wave packet that was generated by a large-amplitude pulse. The initial disturbance development of this wave packet remains still linear, while farther downstream nonlinear wave interactions alter the shape and the disturbance spectrum of the packet. The results of the third part suggest that oblique breakdown might be the strongest nonlinear transition mechanism for a supersonic boundary layer.
• S., C., Wernz, S., & Fasel, H. F. (2011). Numerical investigation of the nonlinear transition regime in a Mach 2 boundary layer. Journal of Fluid Mechanics, 668, 113-149.
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  Abstract: The transition process in a supersonic flat-plate boundary layer at Mach 2 is investigated numerically using linear stability theory (LST) and direct numerical simulations (DNS). The experimental investigations by Kosinov and his co-workers serve as a reference and provide the physical conditions for the numerical set-up. In these experiments, the weakly nonlinear regime of transition was studied. This led to the discovery of asymmetric subharmonic resonance triads, which appear to be relevant for transition in a Mach 2 boundary layer. These triads were composed of one primary oblique wave of frequency 20kHz and two oblique subharmonic waves of frequency 10kHz. While the experimentalists have focused on this new breakdown mechanism, we have found that the experimental data also indicate the presence of another mechanism related to oblique breakdown. This might be the first experimental evidence of the oblique breakdown mechanism in a supersonic boundary layer. With the simulations presented here, the possible presence of oblique breakdown mechanisms in the experiments is explored by deliberately suppressing subharmonic resonances in the DNS and by comparing the numerical results with the experimental data. The DNS results show excellent agreement with the experimental measurements for both linear and nonlinear transition stages. Most importantly, the results clearly show the characteristic features of oblique breakdown. In addition, we also investigated the subharmonic transition route using LST and DNS. When forcing both the subharmonic and the fundamental frequencies in the DNS, a subharmonic resonance mechanism similar to that in the experiments can be observed. © 2010 Cambridge University Press.
• Shams, E., Gross, A., & Fasel, H. (2011). Performance analysis of solar chimneys of different physical scales using CFD. ASME 2011 5th International Conference on Energy Sustainability, ES 2011, 2147-2156.
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  Abstract: The available power of a solar chimney power plant (SCPP) is investigated based on axi-symmetric simulations that are carried out with Ansys Fluent. Different combinations of tower height and collector radius are investigated. The simulations show that the available power is proportional to the product of tower height and collector area. This scaling law holds for a wide parameter range, even for relatively disparate ratios of tower height and collector diameter. Three different turbulence models are employed for the calculations of the large power plants. Results obtained with a Reynolds stress model are found to be most accurate. In addition, experiments with an instrumented laboratory scale model are conducted. Temperature measurements obtained from this model are found to compare well with the simulation data. Copyright © 2011 by ASME.
• Sivasubramanian, J., & Fasel, H. F. (2011). Numerical investigation of laminar-turbulent transition in a cone boundary layer at Mach 6. 41st AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: Laminar-Turbulent transition in a hypersonic cone boundary layer is investigated using Direct Numerical Simulations (DNS). The flow parameters used in the simulations are based on the experimental conditions of the Boeing/AFOSR Mach 6 quiet-flow Ludwieg Tube at Purdue University. The main objective of the present research is to determine which nonlinear mechanisms may be dominant in a broad band "natural" disturbance environment and then to perform controlled transition simulations of these mechanisms. Towards this end, a "natural" transition scenario was modeled and investigated by generating wave packet disturbances. These wave packet simulations provided strong evidence for a possible presence of fundamental and subharmonic resonance mechanisms in the nonlinear transition regime. However, the fundamental resonance was much stronger than the subharmonic resonance. To gain more insight into the nonlinear development we performed controlled transition simulations of these mechanisms. We found that the strength of fundamental resonance is strongly influenced by the wave angle of the secondary oblique wave pair. For a conical geometry this issue is even more complicated than for a flat-plate as the wave angle of a disturbance wave changes in the downstream direction. Therefore, first we carried out a parameter study to find the most strongly resonating oblique wave pair. Then a set of highly resolved controlled fundamental (K-type) breakdown simulations was performed using the most strongly resonating oblique wave pair as secondary waves. These simulations demonstrated that fundamental breakdown may indeed be a viable path to complete breakdown to turbulence in hypersonic cone boundary layers. © 2011 by the authors.
• Sivasubramanian, J., & Fasel, H. F. (2011). Transition initiated by a localized disturbance in a hypersonic flat-plate boundary layer. 49th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition.
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  Abstract: Direct Numerical Simulations (DNS) were performed to investigate transition initiated by a localized disturbance in a hypersonic flat-plate boundary layer. In order to model a natural transition scenario, the boundary-layer was forced by a short duration (localized) pulse through a hole on the flat-plate. The pulse disturbance developed into a three- dimensional wave packet which consisted of a wide range of disturbance frequencies and wave numbers. First, the linear development of the wave packet was studied by pulsing the flow with a low amplitude (0:001% of the freestream velocity). The dominant waves within the resulting wave packet were identified as two-dimensional second mode disturbance waves. Hence the wall{pressure disturbance spectrum exhibited a maximum at the spanwise mode number κ = 0. The spectrum broadened in downstream direction and the lower frequency first mode oblique waves were also identified in the spectrum. However, the peak amplitude remained at κ = 0 which shifted to lower frequencies in the downstream direction. In order to investigate the nonlinear transition regime, the °ow was pulsed with a higher amplitude disturbance (5% of the freestream velocity). The developing wave packet grows linearly at first before reaching the nonlinear regime. The wall pressure disturbance spectrum confirmed that the wave packet developed linearly at first. The response of the flow to the high amplitude pulse disturbance indicated the presence of a fundamental resonance mechanism. Lower amplitude secondary peaks were also identified in the disturbance wave spectrum at approximately half the frequency of the high amplitude frequency band, which would be an indication of a subharmonic resonance mechanism. The disturbance spectrum indicates however, that fundamental resonance is much stronger than subharmonic resonance. Copyright © 2011 by the authors.
• Balzer, W., & Fasel, H. F. (2010). Direct numerical simulation of laminar boundary-layer separation and separation control on the suction side of an airfoil at low Reynolds number conditions. 40th AIAA Fluid Dynamics Conference.
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  Abstract: Laminar flow separation from lifting surfaces at low Reynolds number conditions can result in significant performance losses for aircraft with small geometric dimensions and is therefore a limiting factor in research areas involving unmanned flight technology and aircraft design. We investigated separation and transition of the flow on the suction side of a NACA 643 - 618 airfoil at a chord Reynolds number Rec = 64, 200 and two angles of attack, α = 8.64° and α = 13.85°, respectively. For a simplified model geometry we conducted highly-resolved direct numerical simulations (DNS) using a high-order accurate numerical code that solves the incompressible Navier-Stokes equations for general orthogonal grids. Results for the uncontrolled, "natural" flow are presented and compared to available experimental data and numerical simulations of the entire airfoil. For the larger angle of attack, α = 13.85°, a shallow leading-edge separation bubble develops that exhibits clear differences to the more typically observed separation starting from the aft section of the airfoil at lower angles of attack. In a side-by-side comparison we will elaborate on these differences in an effort to understand the relevant physical mechanisms that govern the flow field in both scenarios. This understanding is necessary in order to design and improve flow control strategies aimed at preventing or delaying flow separation. We also present DNS results of separation control using high-amplitude, two-dimensional blowing and suction through a narrow spanwise slot. It is demonstrated that flow control can lead to a significant performance increase but can also cause performance degradation when the control parameters are chosen inappropriately. © 2010 by the authors.
• Balzer, W., & Fasel, H. F. (2010). Numerical investigation of the effect of free-stream turbulence on laminar boundary-layer separation. 40th AIAA Fluid Dynamics Conference.
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  Abstract: Direct numerical simulations (DNS) are employed to investigate the effect of free-stream turbulence (FST) on laminar boundary-layer separation and separation control. To model the effects of FST, a numerical method first proposed by Jacobs1 for the generation of isotropic grid turbulence is used. For a laminar separation bubble on a flat plate, which was investigated in wind-tunnel experiments by Gaster,2 it is shown that using low levels of FST in numerical simulations can improve the agreement between experiments and DNS. The characteristics of the fluctuations inside the boundary layer prior to separation agree very well with those reported in experiments and other numerical simulations. Even for the highest turbulent intensity in the free stream (2.5%), which we investigated, the boundary layer is found to separate from the surface and undergo transition to turbulence in the separated shear layer above the wall. In addition, time-dependent flow analysis and comparison between DNS results and linear stability theory reveal that the linear shear-layer instability mechanism (Kelvin-Helmholtz instability) is not bypassed but can be detected in all cases. DNS of active flow control using harmonic blowing and suction through a narrow spanwise slot is shown to significantly reduce separation and delay flow transition. For increased levels of FST, however, the high-amplitude, two-dimensional disturbance waves introduced by the forcing are seen to interact with three-dimensional disturbances inside the boundary layer which results in an accelerated transition process when compared to the uncontrolled case. Copyright © 2010 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc.
• Brehm, C., & Fasel, H. F. (2010). A non-staggered immersed interface method for solving the incompressible Navier-Stokes equations. 40th AIAA Fluid Dynamics Conference.
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  Abstract: For simulations involving highly complex geometries, as they occur in many fields of science and engineering, the process of generating a high-quality grid is extremely time-consuming. Specifically, for flows containing moving boundaries, the considerable advantage of the immersed interface/boundary method becomes evident. While good results, both qualitative and quantitative, have been obtained, most current schemes rely on low-order corrections in the vicinity of boundary/interface. The objective of the paper is to present a newly developed immersed interface method for solving the incompressible Navier-Stokes equations with moving boundaries in primitive variable formulation on non-staggered grids. This time-explicit immersed interface method is based on a local Taylor series expansion at irregular grid points whereby numerical stability is enforced through a local stability condition. A matrix stability analysis of the full discretization matrix was used to rigorously analyze the impact of different boundary discretizations on the overall stability of the numerical scheme. To validate the immersed incompressible Navier-Stokes solver, two different physically relevant flows are simulated. The two-dimensional flow around a circular cylinder placed in a uniform free-stream is simulated in the range ReD = 20 to ReD = 200 covering the range of steady and unsteady flow regimes. Computations of Tollmien-Schlichting waves in a two-dimensional channel flow are presented for a sub-critical and super-critical case, challenging the near wall accuracy of current schemes. Finally, an analytical test function approach demonstrates the full capability of the present immersed Navier-Stokes solver with moving boundaries. © 2010 by the authors.
• Brehm, C., & Fasel, H. F. (2010). Novel immersed interface method based on local stability conditions. 40th AIAA Fluid Dynamics Conference.
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  Abstract: A novel immersed interface method is presented which is based on a local Taylor series expansion at irregular grid points whereby numerical stability is enforced through a local stability condition. In the past, various immersed interface/boundary methods were developed by solely considering the order of the local truncation error at the irregular grid points. The numerical stability of these schemes was demonstrated in a global sense by applying either a matrix stability analysis or considering a number of different test cases. None of these schemes used a concrete local stability condition to derive the stencils at irregular grid points for advection-diffusion type equations. This paper will show the derivation of stencil coefficients at irregular grid points for the one dimensional advection-diffusion equation. The advection-diffusion equation may be viewed as a simple model equation for the incompressible Navier-Stokes equations. This paper will demonstrate that the local stability constraints can be justified in a global setting as long as the DFL-number stability limit is not reached. This paper will also present an extension of the one-dimensional immersed interface method to the two-dimensional case. In addition, the novel immersed interface method will be extended with a capability which allows for moving boundaries. Finally, several numerical sample problems as well as numerical matrix stability analysis for advection-diffusion type equations will prove the full operability of this novel immersed interface method. © 2010 by the authors.
• Gross, A., & Fasel, H. F. (2010). Active flow control for NACA 6-series airfoil at Re = 64,200. AIAA Journal, 48(9), 1889-1902.
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  Abstract: Earlier research for the NACA 643-618 airfoil has shown that lift-curve and stall behavior change noticeably when the chord Reynolds number is reduced significantly below its design value. For the same airfoil and a Reynolds number based on chord of Re = 64,200, separation control by pulsed vortex generator jets and a two-dimensional volume forcing that was motivated by a plasma actuator were investigated. Pulsed vortex generator jets with moderate jet velocities and the two-dimensional volume forcing were found to introduce coherent spanwise structures that are highly effective for separation control. A linear stability analysis of the time-averaged flow showed that span wise disturbances are amplified by an instability mechanism, which explains why both control strategies are highly efficient Angled pulsed vortex generator jets with large jet velocities were found to introduce streamwise vortices and to result in earlier transition. Although the control is highly effective, it is not very efficient, as streamwise vortices are not supported by the flow. © 2010.
• Gross, A., & Fasel, H. F. (2010). Hybrid RANS/LES simulations of turbulent channel and diffuser flows. 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition.
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  Abstract: The turbulent flow in a square-duct at a bulk Reynolds number of 10,000 was simulated using the flow simulation methodology and filter-based Reynolds-averaged Navier-Stokes hybrid turbulence models. Simulations were carried out for 3 different grid resolutions to determine how well the hybrid models adapt to changes in the grid resolution. A comparison of instantaneous flow fields, bulk velocities, and profiles of resolved, unresolved, and total turbulence kinetic energy is provided. The hybrid turbulence model that delivered the best results for the square-duct flow was then employed for simulations of the Stanford asymmetric diffuser. The diffuser flow is physically more complex and features a turbulent separation bubble. Copyright © 2010 by the authors.
• Gross, A., & Fasel, H. F. (2010). Modification of ninth-order weighted essentially nonoscillatory scheme for mixed subsonic/supersonic flow. AIAA Journal, 48(11), 2698-2701.
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  Abstract: A study was conducted to demonstrate a modification to the high-order-accurate weighted essentially nonoscillatory (WENO) scheme implementation. The motivation behind this effort was to have a high-order-accurate discretization for the convective fluxes that worked well for subsonic and supersonic flows. A procedure was discussed for constructing high-order-accurate upwind schemes. The researchers suggested reconstructing the flow variables at the left (L) and right side (R) of the i +1/2 interface using high-order-accurate WENO approximations and to employ the Roe scheme. The viscous fluxes were computed using a shifted control volume approach with fourth-order accuracy, and the solution was integrated in time with a second-order-accurate implicit Adams-Moulton scheme. The ghost cell approach was applied at all boundaries to solve the problem in place of one-sided difference stencils and truncated WENO schemes.
• Gross, A., & Fasel, H. F. (2010). Numerical investigation of separation for airfoils at low Reynolds numbers. 40th AIAA Fluid Dynamics Conference.
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  Abstract: The present study is concerned with the Aeromot 200S Super Ximango motor glider for which we built two 1:5 scale dynamically scaled models. For a two-dimensional section of its wing, which has a modified NACA 643-618 airfoil, we computed the unsteady time-dependent flow for two chord Reynolds numbers, 64,200 and 322,000. At Re=64,200, the wing tip Reynolds number at model takeoff speed, most of the turbulent energy spectrum can be captured using direct numerical simulations and turbulence modeling is not required. For α = 8.64deg laminar separation occurs near the maximum thickness resulting in a considerable performance loss. As the angle of attack is increased a leading edge bubble forms. The turbulent boundary layer downstream of the bubble is more resistant to separation resulting in a considerable performance recovery. For even higher angles of attack the leading edge bubble "bursts" and performance is once again lost. At Re=322,000, the model cruise Reynolds number based on mean aerodynamic chord, computer limitations prohibit direct numerical simulations and necessitate turbulence modeling. We employed filter-based Reynolds-averaged Navier-Stokes for simulations at an angle of attack of 13.2deg. The flow again separates near the maximum thickness location. In a separate simulation we show how performance can partially be recovered by harmonic blowing through a spanwise slot near the leading edge of the airfoil. © 2010 by the authors.
• Gross, A., & Fasel, H. F. (2010). Numerical investigation of supersonic flow for axisymmetric cones. Mathematics and Computers in Simulation, 81(1), 133-142.
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  Abstract: An existing curvilinear finite-volume code with robust shock-capturing scheme was modified to allow for simulations of supersonic flow for axisymmetric cone geometries. It is shown how for an axisymmetric coordinate system the convective and viscous flux derivatives in the circumferential direction reduce to a y-momentum equation source term. The advantage of this approach over an axisymmetric code is that the governing equations and the discretization do not need to be changed. This paper provides a detailed derivation of the axisymmetric source terms from the full Navier-Stokes equations. Results are shown for a sharp and a blunt cone for approach flow Mach numbers of M = 3.5 and M = 7.99. © 2010 IMACS.
• Gross, A., & Fasel, H. F. (2010). Numerical simulation of low-pressure turbine blade separation control. AIAA Journal, 48(8), 1582-1601.
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  Abstract: Low-pressure turbines are a common and important element of many modern jet engines. Any performance improvement will lead to net savings in aircraft operating costs. At low operating Reynolds numbers or for reduced stage solidities laminar separation from the constituent blades can noticeably deteriorate overall engine performance. Under such conditions active control of laminar separation may eliminate or reduce associated losses resulting in increased engine performance.Ahigh-order-accurate finite volume Navier-Stokes code for investigating separation control for the PackB blade geometry at a Reynolds number based on axial chord of 25,000 was employed. For this Reynolds number laminar separation was observed in the experiments. First, a grid resolution study for the uncontrolled flow was carried out, which shows grid convergence for our simulations. Then separation control by pulsed vortex generator jets was investigated. It is shown how this control results in an earlier transitioning of the flow and the introduction of spanwise coherent structures. The increased wall-normal mixing associated with these two mechanisms results in an effective separation control. An even more efficient separation control can be accomplished by harmonic blowing through a slot. The astounding effectiveness of the latter control scheme is attributed to an amplification of the disturbance input through a hydrodynamic instability mechanism and to the suppression of three-dimensional structures, which weaken the spanwise structures. Finally, separation control by streamwise vortices that were generated by volume forces was investigated. As the flow does not amplify such structures the energy input required for accomplishing an effective control was found to be large compared with the other flow control techniques. Because the structures are steady they can, however, likely be generated with passive devices such as vortex generators. Copyright © 2010 by the authors.
• Gross, A., Fasel, H. F., Friederich, T., & Kloker, M. J. (2010). Numerical investigation of S822 wind turbine airfoil. 40th AIAA Fluid Dynamics Conference.
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  Abstract: We carried out direct numerical simulations of the flow past a two-dimensional S822 wind turbine blade section at a chord Reynolds number of Re=100,000 and an angle of attack of α = 5deg. Without blade rotation the separated boundary layer "rolls up" into spanwise vortices which then "break up" into smaller scale structures leading to transition to turbulence. Simulation results compare favorably with XFoil predictions and wind tunnel data by the University of Illinois at Urbana Champaign. By adding volume forcing terms to the right-hand-side of the Navier-Stokes equations the effect of blade rotation can be simulated. Blade rotation is shown to result in a radial velocity component towards the blade tip in areas where the velocity is substantially different from its free stream value, such as near the stagnation point and especially in the separated flow region. The spanwise velocity component makes the flow crossflow unstable resulting in stationary and traveling crossflow vortices. A linear stability theory analysis which compares favorably with the simulation data provides proof that the primary instabilities are of mixed type, including both a two-dimensional mode (Tollmien-Schlichting and Kelvin-Helmholtz type) and a stationary and unsteady crossflow mode. The crossflow instability results in an earlier transition and a separation delay, lift increase, and drag reduction. This effect is more pronounced at 20% than at 80% blade radius. Because we apply periodicity conditions in the spanwise direction our results provide an explanation for rotational augmentation that is not based on the transfer of fluid from the inboard region towards the blade tips ("centrifugal pumping"). Instead, we argue that rotational augmentation, at least for low Reynolds number flows, is a direct consequence of crossflow instabilities which destabilize the flow leading to earlier transition and separation delay. © 2010 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc.
• Gross, A., Kondaraju, S., & Fasel, H. F. (2010). Numerical investigation of separation control for wind turbine airfoil. ASME 2010 4th International Conference on Energy Sustainability, ES 2010, 2, 895-904.
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  Abstract: We simulated the flow over the NREL S822 wind turbine airfoil for a chord Reynolds number of 100,000 and an angle of attack of 5deg. These conditions approximately match the blade element conditions at 80% radius of a 2m turbine that is operating at 300rpm. A simulation of the uncontrolled flow with steady approach flow conditions shows flow separation on the suction side which is consistent with University of Illinois at Urbana-Champaign experimental data. We also investigated the effect of a time-dependent approach flow similar to the conditions encountered during yawed turbine operation. Because the blade rotation frequency is much lower than the frequencies associated with the blade aerodynamics we increased the frequency of the approach flow fluctuations. Nevertheless, the time-dependent inflow was found to have little effect on the separated flow. Finally, we investigated separation control using a plasma actuator located near the maximum thickness on the suction side of the airfoil. Pulsed actuation was found to be very efficient and effective as the disturbance input is amplified by the flow. © 2010 by ASME.
• Koevary, C., Laible, A., Mayer, C., & Fasel, H. F. (2010). Numerical simulations of Controlled transition for a sharp circular cone at Mach 8. 40th AIAA Fluid Dynamics Conference.
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  Abstract: The linear and nonlinear development of disturbance waves in an axisymmetric boundary layer on a sharp circular cone at Mach 8 are investigated by numerical solution of the full 3D, time dependent, compressible Navier-Stokes equations. Disturbances are introduced by wall-normal blowing and suction near the upstream boundary of the computational domain. Small amplitude disturbances are introduced to study their linear stability behavior and the results are compared with Linear Stability Theory (LST) for validation. These results are also used to guide parameter selection for simulations of nonlinear transition phenomena. In particular, for simulations with large amplitude 2D disturbances, fundamental resonance can be observed. This resonance leads to nonlinear amplification of 3D modes and the classical aligned A-vortex pattern as the boundary layer begins to transition to turbulence. Simulations of oblique breakdown are also performed and the two transition mechanisms are compared. Copyright © 2010 by The Authors. Published by the American Institute of Aeronautics and Astronautics, Inc.
• Kremheller, A., & Fasel, H. (2010). Water tunnel experiments of three-dimensional separation bubbles on a flat plate. 40th AIAA Fluid Dynamics Conference.
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  Abstract: Three-dimensional laminar separation bubbles generated on a flat plate by a three-dimensional displacement body were investigated in a water tunnel for a Reynolds number range of ReD = 2,000 to ReD = 30,000 based on displacement body thickness, D. The separation bubble was generated on a flat plate to exclude complex surface curvature effects. Boundary layer suction was applied on the surface of the displacement body in order to prevent flow separation from the displacement body. Several displacement bodies with different aspect ratios (Λ = 0.5 - 3) were used to allow for a variation of the spanwise extent of the imposed pressure gradient. The influence of the pressure gradient on the separation bubble was also investigated by changing the distance of the displacement body from the flat plate. Velocity vector field measurements using a Particle Image Velocimetry (PIV) system as well as dye flow visualizations were employed for investigating the physical mechanisms governing the dynamics of the three-dimensional separation bubble. With these techniques we were able to identify the reverse flow region in the bubble and obtained limiting streamline patterns. The topologies of the separation bubbles are documented for different Reynolds numbers and different pressure gradients. For a Reynolds number of ReD = 5,000 and an aspect ratio of Λ = 0.5 the separation bubble and the wake were found to be steady without any shedding. For higher Reynolds numbers the bubble was found to shed vortical structures which possibly resulted from an inviscid Kelvin-Helmholtz instability. These instabilities of the separated shear layer in combination with a slowly varying reverse flow magnitude were found to likely be the cause of a so-called bubble "breathing" which manifests itself as an intermittent change of the dimensions and intensity of the separation bubble. The streamline pattern of the three-dimensional separated region for a higher aspect ratio (Λ = 2) was found to be similar to that of low aspect ratio wings at large angles of attack (close to stall). © 2010 by the American Institute of Aeronautics and Astronautics, Inc.
• Mayer, C. S., Fasel, H. F., Choudhari, M., & Chang, C. (2010). Detailed comparison of DNS with PSE for oblique breakdown at Mach 3. 40th AIAA Fluid Dynamics Conference.
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  Abstract: Oblique breakdown in a supersonic flat-plate boundary layer is investigated using Direct Numerical Simulations (DNS) and Parabolized Stability Equations (PSE). This paper constitutes an extension to our previous studies of the complete transition regime of oblique breakdown. In these studies, the flow was assumed to be symmetric in the spanwise direction. A new DNS has been performed where the symmetry condition was removed. This simulation demonstrates that the "classical" oblique breakdown mechanism initialized by two symmetric instability waves with equal disturbance amplitudes loses its symmetry late in the turbulent stage for a low-noise environment. Hence, for the streamwise extent of the computational domain in our studies, the symmetry condition is justified. Furthermore, new data from a longer time average of the original symmetric simulation of oblique breakdown (CASE 3) are discussed. These data verify that a converged time average is reached. The final part of the paper focuses on a comparison of PSE results obtained from NASA's LASTRAC code to the DNS results. This comparison corroborates that the nonlinear PSE approach can successfully predict transition onset and that despite the large amplitude forcing used to introduce the oblique mode disturbances in the DNS, the latter constitutes a generic reference case for oblique breakdown at Mach 3 and, therefore, can be used to validate reduced order models for the full transition zone. Copyright © 2010 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc.
• Radi, A., & Fasel, H. F. (2010). Experimental investigation of laminar separation bubbles on a flat plate. 40th AIAA Fluid Dynamics Conference.
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  Abstract: Similar to the wind tunnel experiments by Gaster,3 an invertedly mounted airfoil was used to create laminar separation bubbles on a flat plate. The present experiments were carried out in a water tunnel. Suction was applied on the airfoil to prevent separation from its surface. The bubble dimensions were large enough, and the naturally occurring frequencies low enough, to allow highly resolved temporal and spatial PIV measurements. The influence of the momentum thickness Reynolds number at separation (Reθs) was investigated. The natural shedding frequencies and the bubble size varied with Reθs. Two-dimensional disturbances were introduced into the upstream boundary layer with a vibrating ribbon. The separation bubble was found to be highly susceptible to two-dimensional forcing. When forced with frequencies close to the natural shedding frequency, the bubble length could be decreased by as much as 50%. Forcing of this frequency apparently exploits the natural instability mechanism of the separated shear layer. This enforces the development of vortical structures ("rollers") which are highly effective in bringing high-momentum fluid into the separated region. This leads to an early reattachment of the flow. Using phase-locked PIV measurements, the formation, downstream movement and breakdown of these unsteady flow structures was investigated. The growth of disturbances, which lead to these structures, was studied. A comparison of the amplification rates and disturbance amplitude profiles with Linear Stability Theory showed good agreement. Long separation bubbles (Gaster3) were observed in certain cases. These bubbles were mainly steady, showing only weak, intermittent shedding activity. The reattachment of long bubbles occurs gradually, stretching over a relatively long streamwise extend. Short bubbles are characterized by a more abrupt reattachment, caused by the formation of vortical structures (rollers) in the reattachment region. © 2010 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc.
• Sivasubramanian, J., & Fasel, H. F. (2010). Direct Numerical Simulation of a turbulent spot in a cone boundary-layer at Mach 6. 40th AIAA Fluid Dynamics Conference.
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  Abstract: Highly resolved spatial Direct Numerical Simulations (DNS) were performed to investigate the growth and breakdown of a localized disturbance into a turbulent spot in a sharp cone boundary - layer at Mach 6. The flow parameters used in the simulations are based on the experimental conditions of the Boeing/AFOSR Mach 6 quiet - flow Ludwieg Tube at Purdue University.1 In order to model a natural transition scenario, the boundary - layer was pulsed through a hole on the cone surface. The pulse disturbance developed into a three-dimensional wave packet which consisted of a wide range of disturbance frequencies and wave numbers. The dominant waves within the resulting wave packet were identified as two - dimensional second mode disturbance waves. In addition, weaker oblique waves were observed on the lateral sides of the wave packet. The developing wave packet grows linearly at first before reaching the nonlinear regime and eventually leads to localized patches of turbulent flow (turbulent spot). The wall - pressure disturbance spectrum showed strong secondary peaks at the fundamental frequency for larger azimuthal wave numbers. This development indicates that fundamental resonance might be the dominant nonlinear mechanism for a cone boundary - layer at Mach 6. The flow structures within the turbulent spot were studied in detail and general features of the spot were analyzed. Copyright © 2010 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc.
• Sivasubramanian, J., & Fasel, H. F. (2010). Numerical investigation of boundary-layer transition initiated by a wave packet for a cone at Mach 6. 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition.
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  Abstract: Direct Numerical Simulations are performed to investigate transition initiated by a wave packet in a sharp cone boundary layer at Mach 6. In order to understand the natural transition process in hypersonic cone boundary layers, the flow was pulsed through a hole on the cone surface to generate a wave packet which consisted of a wide range of disturbance frequencies and wave numbers. The flow parameters for the simulations are based on the experimental conditions of the Boeing/AFOSR Mach 6 quiet-flow Ludwieg Tube at Purdue University. 1 First, the linear development of the wave packet was studied by forcing the flow with a low amplitude pulse (0.001% of the freestream velocity). The dominant waves within the resulting wave packet were identified as the second mode two-dimensional disturbance waves. In addition, weaker first mode oblique waves were also observed on the lateral sides of the wave packet. In order to investigate the weakly nonlinear transition regime, medium amplitude pulse disturbances (0.5% of the freestream velocity) were introduced. The response of the flow to the medium amplitude pulse disturbances indicated the presence of a fundamental resonance mechanism. Lower secondary peaks in the disturbance wave spectrum were identified at approximately half the frequency of the high amplitude frequency band for azimuthal mode numbers k c±55, which would be an indication of a sub-harmonic resonance mechanism. Finally, in order to identify more clearly which of these mechanisms ultimately leads to turbulent breakdown, a simulation with a higher forcing amplitude (5% of the freestream velocity) was performed. The developing strongly nonlinear wave packet eventually leads to localized patches of turbulent flow (turbulent spots). In these nascent turbulent spots various known properties of mature turbulent spots could be identified. Copyright © 2010 by the authors.
• Terzi, D. V., Mayer, C., & Fasel, H. (2010). The late nonlinear stage of oblique breakdown to turbulence in a supersonic boundary layer. IUTAM Bookseries, 18, 415-420.
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  Abstract: Oblique breakdown to turbulence was initiated by low amplitude forcing in a laminar flat-plate boundary layer at Mach three. The growth of the instability waves was investigated using spatial Direct Numerical Simulations (DNS). Excellent agreement with theory was obtained in the linear stage corroborating that the entire transition process from the linear regime to the final breakdown was captured. A fully turbulent flow was reached demonstrating that this transition scenario is a viable path to turbulence. Key events in the late nonlinear stage of breakdown are studied in detail. © 2010 Springer Science+Business Media B.V.
• Zengl, M., Terzi, D. V., & Fasel, H. (2010). Numerical investigation of subharmonic resonance triads in a Mach 3 boundary layer. IUTAM Bookseries, 18, 445-450.
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  Abstract: The possibility of a subharmonic-resonance triad in the transition process for a flat-plate boundary layer at Mach three was demonstrated using Direct Numerical Simulations (DNS). The parameters controlling such a resonance triad were investigated in detail. Finally, the interaction of this triad with the so-called 'oblique breakdown' scenario was studied. To this end, the mechanisms were invoked separately and in combination. By itself the oblique breakdown leads to a faster growth of instability waves than the subharmonic resonance. However, in a combination with a subharmonic-resonance triad, the dominant modes of the oblique breakdown experience a slower growth, whereas those of the subharmonic resonance are more amplified. Overall, the interaction promotes transition and the rise in skin friction and the drop in shape factor are moved upstream when compared to the oblique breakdown. © 2010 Springer Science+Business Media B.V.
• Balzer, W., Gross, A., & Fasel, H. F. (2009). Control of boundary-layer separation for lifting surfaces. Department of Defense Proceedings of the High Performance Computing Modernization Program - Users Group Conference, HPCMP-UGC 2009, 37-45.
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  Abstract: The lack of understanding of most of the relevant physical mechanisms when applying flow control limits the prospects of successfully transitioning flow-control technologies into real flight vehicles. Successful control of boundary-layer separation for lifting surfaces promises major performance gains especially when large laminar runs are desired in order to minimize the skin-friction drag. We systematically explore the fundamental1 mechanisms of the interaction of separation and transition that are relevant for effective and efficient flow control applications. Toward this end, we are employing computational fluid dynamics (CFD) for investigating active flow control for a NACA 643-618 airfoil at a chord Reynolds number ReC=64,200 and various angles-of-attack. CFD results are compared to wind/water tunnel experiments carried out at the University of Arizona. For simulations of the entire wing section we are using a high-order-accurate finite volume code based on the compressible Navier-Stokes equations. For very highly resolved DNS which focus exclusively on the separated region on the suction side of the wing, we are employing a higher-order-accurate compact finite difference code based on the incompressible Navier-Stokes equations in vorticity-velocity formulation. These simulations are set up to fully resolve the flow field and enable us to reveal some of the intricate physical mechanisms associated with unsteady separation and transition, flow instabilities, and active flow control. © 2010 IEEE.
• Gross, A., & Fasel, H. F. (2009). Active flow control for airfoil at low Reynolds numbers. 39th AIAA Fluid Dynamics Conference.
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  Abstract: Scaled model flight research has the potential of revolutionizing general aviation by reducing the number of flight tests required during the design and certification phase of airplanes. For dynamic scaling the Froude number has to be kept constant resulting in Reynolds number dissimilarity. The present study is concerned with the airfoil of the Aeromot 200S Super Ximango motor glider for which we built a dynamically scaled model (1:5). For a two-dimensional section of its wing, which has a modified NACA 643-618 airfoil, we investigated the Reynolds number dependence of the flow in the wind tunnel. The lift curve and the stall behavior were found to change noticeably when going from the full size cruise to the model cruise conditions. We simulated the flow for Reynolds numbers based on chord of Re=64,200 and 322,000. At Re=64,200, a sufficient part of the energy spectrum is resolved and turbulence modeling is not required. For this Reynolds number we investigated separation control by pulsed vortex generator jets and a plasma actuator. At the higher Reynolds number, Re=322,000, computer limitations prohibit direct numerical simulations and necessitate turbulence modeling. In the second part of the paper different hybrid turbulence modeling approaches are discussed and tested in 2-D calculations which allow us to identify the most promising approach for later 3-D simulations. Copyright © 2009 by the authors.
• Gross, A., & Fasel, H. F. (2009). Hybrid RANS/LES simulations of turbulent channel flow. 47th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition.
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  Abstract: For the simulations discussed in this paper two geometries are considered: A square-duct channel flow at a Reynolds number based on bulk velocity and hydraulic diameter of 10,000 (627 based on friction velocity and hydraulic diameter) and a 1:3.33 rectangular channel flow at a Reynolds number based on bulk velocity and channel height of 10,000 (909 based on friction velocity and hydraulic diameter). Reynolds-averaged Navier-Stokes (RANS) and hybrid turbulence model simulations based on the flow simulation methodology (FSM), filter-based RANS, and partially-averaged Navier-Stokes simulations were carried out. In addition, for the square-duct flow, as a reference, we also carried out direct numerical simulations (DNS). A new FSM contribution function is proposed and tested. It is also shown how hybrid RANS/LES models can be extended to allow for "turbulence seeding" ("backscatter") in regions of vanishing model contribution and energy transfer to the unresolved scales in regions with increasing model contribution. A comparison of the mean flow data obtained with the different methods for three different grid resolutions showed that FSM and filter-based RANS with variable filter width consistently yielded the best bulk velocity and velocity profile predictions. It remains to be investigated if these methods perform equally well for more complex flows, such as separated diffuser flows. Copyright © 2009 by the authors.
• Jacobi, R., Gross, A., & Fasel, H. (2009). Numerical investigation of three-dimensional separation in internal and external flows. Department of Defense Proceedings of the High Performance Computing Modernization Program - Users Group Conference, HPCMP-UGC 2009, 96-105.
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  Abstract: For Navy relevant geometries, separation of wall bounded flows is a highly complex phenomenon. Because of the relatively high Reynolds numbers involved, separation is always associated with considerable unsteadiness. This unsteadiness is caused by large coherent structures that are a consequence of hydrodynamic instability mechanisms of the mean flow. In addition, due to the shape of underwater vehicles (submarines, torpedoes, low aspect ratio lifting or control surfaces) the separation is three-dimensional (3D). The combination of three-dimensionality and unsteadiness results in a highly complex time-dependent topology of the separated region. In a combined numerical/experimental effort, we are studying laminar separation bubbles in external flows. For these simulations, we employ highly-resolved direct numerical simulations (DNS) to obtain a deeper understanding of the various physical mechanisms governing separation, transition, and reattachment of 3D bubbles. Ultimately, such understanding may pave the way for the development of effective and efficient strategies for preventing separation for practical applications. We are also evaluating hybrid turbulence models for high Reynolds number flows. In particular, we describe DNS, Reynolds-Averaged Navier-Stokes (RANS), and hybrid simulations of a turbulent square duct flow. Based on these simulations we decided on two hybrid strategies for simulating the asymmetric diffuser experiments that were conducted at Stanford University by J. Eaton et al. The first mean flow results look very encouraging. If successful, this research will result in hybrid models that are suitable for a wide variety of flow topologies and Reynolds numbers. © 2010 IEEE.
• Laible, A. C., Mayer, C. S., & Fasel, H. F. (2009). Numerical investigation of transition for a cone at Mach 3.5: Oblique breakdown. 39th AIAA Fluid Dynamics Conference.
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  Abstract: The transition process initiated by one or multiple pairs of oblique waves is studied for a 7°-cone using direct numerical simulation (DNS). The simulations were performed under the experimental conditions of the NASA Langley Mach 3.5 Quiet Tunnel. In particular, we investigated two scenarios: (i) the transition process initiated by a single pair of oblique waves with azimuthal mode number kc = 12 and (ii) the transition process initiated by first a single and then by multiple pairs of oblique waves with azimuthal mode number kc = 32. The first case (kc = 12) corresponds to planned experiments at NASA Langley with a controlled disturbance input, whereas the latter case (kc = 32) attempts to mimic 'natural' transition by superimposing waves with a broad frequency spectrum (but only a single azimuthal wave number) at the in flow. The amplitudes of these waves correspond to their N{factors (eN- method) obtained from precursor simulations of the linear regime. Qualitative similarities and differences have been documented. In particular, the intermittency resulting from the superposition of waves with a broad frequency spectrum causes the transition location to vary in time. Moreover, the results in this paper indicate that if transition is initiated by multiple pairs of oblique waves with a broad frequency spectrum, nonlinear mechanisms are enhanced when compared to the case, where transition was initiated by a single pair of oblique waves. Therefore, in the time average, the transition location for the case with multiple pairs of oblique waves is located upstream of the transition location for the case with a single pair of oblique waves. Copyright © 2009 by the authors.
• Mayer, C. S., A., D., & Fasel, H. F. (2009). DNS of complete transition to turbulence via oblique breakdown at Mach 3: Part II. 39th AIAA Fluid Dynamics Conference.
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  Abstract: The final stage of transition to turbulence in a supersonic flat-plate boundary layer at Mach 3 is investigated using Direct Numerical Simulations (DNS). A set of six simulations was performed to study the development of a pair of oblique waves from their initiation at low amplitudes to final breakdown to turbulence. In a previous paper (part I of this study), the linear and early nonlinear regimes of this so-called "oblique breakdown scenario" were discussed. The focus of the present paper (part II) is on the early turbulent regime, i.e. the region of the flow downstream of the maximum of skin friction. In this region, the flow is shown to have lost periodicity in time and to exhibit the typical mean flow and spectral properties of a turbulent boundary layer as known from the literature and theory. The DNS data clearly demonstrate that oblique breakdown can lead to a fully developed turbulent boundary layer and therefore may be a relevant mechanism for practical applications. authors.
• Mayer, C. S., Laible, A. C., & Fasel, H. F. (2009). Numerical investigation of transition initiated by a wave packet on a cone at Mach 3.5. 39th AIAA Fluid Dynamics Conference.
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  Abstract: Transition initiated by a wave packet in a cone boundary layer at Mach 3:5 has been investigated using LST and DNS. Disturbances have been introduced into the boundary layer by pulsing the wall-normal velocity through a hole on the cone surface. The computational setup is very close to experiments by Corke et al.1 and Matlis.2 The present study can be divided into three parts. In the first part, the linear development of a wave packet is studied in detail. Disturbance spectra in the frequency{azimuthal mode number plane based on wall-pressure amplitudes and time envelopes of the disturbance signal are discussed. The second part of the present study focuses on the identification of possible, asymmetric resonance triads for the most dominant oblique instability waves of the wave packet. New triads have been found that have not yet been reported for a supersonic boundary layer. These triads might explain some major findings in the third and final part of the present work, which focuses on the weakly nonlinear development of a wave packet that was generated by a large amplitude pulse. The initial disturbance development of this wave packet remains still linear, while farther downstream nonlinear wave interactions alter the shape and the disturbance spectrum of the packet. The disturbance spectrum from this study and the results of other investigations performed in parallel (Laible et al.3) suggest that oblique breakdown might be the strongest nonlinear transition mechanism for a supersonic boundary layer. Copyright © 2009 by the authors.
• Plogmann, B., Mack, S., & Fasel, H. F. (2009). Experimental investigation of open- and closed-loop control for airfoil under low Reynolds number conditions. 39th AIAA Fluid Dynamics Conference.
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  Abstract: Airfoils operating under low Reynolds number conditions are susceptible to laminar separation, which can cause significant performance losses. The flow field around a two-dimensional, slightly modified NACA 643 - 618 airfoil was investigated experimentally for a wide range of angles of attack (between -12° and 21°). At chord Reynolds numbers of Re = 64, 200 and Re = 137, 000 and at moderate angles of attack, the airfoil suffers severe performance losses due to large regions of laminar separation on its suction side. Open-and closed-loop flow control strategies were employed to eliminate or at least to significantly reduce laminar separation. When successful, the control resulted in a significant lift recovery and drag reduction. Closed-loop control was found to be equally effective as an optimized open-loop control. The main advantage of the proposed easy-to-implement feedback controller is that it automatically adjusts the unsteady actuation to changing free-stream conditions. Copyright © 2009 by the authors.
• Sivasubramanian, J., Mayer, C. S., Laible, A. C., & Fasel, H. F. (2009). Numerical investigation of wavepackets in a hypersonic cone boundary layer at Mach 6. 39th AIAA Fluid Dynamics Conference.
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  Abstract: The linear and weakly nonlinear transition regime of a hypersonic boundary layer on a sharp cone with circular cross section is investigated using Direct Numerical Simulations (DNS). In a natural transition scenario a broad disturbance spectrum is excited by free-stream disturbances leading to complex wave interactions. Therefore, in order to understand the natural transition process in hypersonic cone boundary layers, the flow was pulsed through a hole on the cone surface to generate a wavepacket with a wide range of disturbance waves. First, a two-dimensional DNS of a linear wavepacket was performed and results are compared to linear stability theory (LST). A good agreement was found to exist between DNS and LST results. We then performed DNS of a linear and a weakly nonlinear three-dimensional wavepacket. High frequency second-mode two-dimensional waves and low amplitude first-mode oblique waves were found to be present in the linear wavepacket. The high amplitude waves in the nonlinear wavepacket are modulated in the azimuthal direction due to weakly nonlinear interactions. A broad high amplitude frequency band was identified in the spectrum of the weakly nonlinear wavepacket, indicating fundamental resonance. Furthermore, secondary peaks were identified at approximately half the frequency of the high amplitude frequency band, which may be a consequence of subharmonic resonance. Copyright © 2009 by the authors.
• Terzi, D. v., Sandberg, R. D., & Fasel, H. F. (2009). Identification of large coherent structures in supersonic axisymmetric wakes. Computers and Fluids, 38(8), 1638-1650.
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  Abstract: Direct numerical simulation data of supersonic axisymmetric wakes are analysed for the existence of large coherent structures. Wakes at Ma = 2.46 are considered with results being presented for cases at Reynolds numbers ReD = 30, 000 and 100,000. Criteria for identification of coherent structures in free-shear flows found in the literature are compiled and discussed, and the role of compressibility is addressed. In particular, the ability and reliability of visualisation techniques intended for incompressible shear-flows to educe meaningful structures in supersonic wakes is scrutinised. It is shown that some of these methods retain their usefulness for identification of vortical structures as long as the swirling rate is larger than the local compression and expansion rates in the flow field. As a measure for the validity of this condition in a given flow the 'vortex compressibility parameter' is proposed which is derived here. Best 'visibility' of coherent structures is achieved by employing visualisation techniques and proper orthogonal decomposition in combination with the introduction of artificial perturbations (forcing of the wake). The existence of both helical and longitudinal structures in the shear layer and of hairpin-like structures in the developing wake is demonstrated. In addition, elongated tubes of streamwise vorticity are observed to emanate from the region of recirculating flow. © 2009 Elsevier Ltd.
• Brehm, C., Mack, S., Gross, A., & Fasel, H. F. (2008). Investigations of an airfoil at low Reynolds number conditions. 4th AIAA Flow Control Conference.
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  Abstract: The low-Reynolds number aerodynamics of airfoils that were designed for higher Reynolds numbers can be surprisingly different from what one might expect. The intricate interplay of separation and transition can substantially affect airfoil performance at lower than de-sign Reynolds number. We employed computational fluid dynamics for investigating the natural (uncontrolled) flow field around the NACA 643 - 618 airfoil at a chord Reynolds number of 64,000 for different angles of attack. Airfoil performance at moderate angles of attack is reduced considerably due to a massive trailing edge separation. At high angles of attack a leading edge separation bubble is formed which was found to transition the flow and reduce the trailing edge separation resulting in a lift recovery. We also investigated open-loop control by blowing and suction through a slot. We found that by exploiting hydrodynamic instability of the baseflow the control could be made more effective. The final objective is to develop closed-loop controllers for airfoils that will help recover the full size performance at lower model Reynolds-number conditions with minimal control effort. Copyright © 2008 by the authors.
• Fasel, H. F., Balzer, W., & Gross, A. (2008). Investigation of separation control for low-pressure turbines using CFD. ICAS Secretariat - 26th Congress of International Council of the Aeronautical Sciences 2008, ICAS 2008, 6, 220-226.
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  Abstract: Separation from low-pressure turbine (LPT) blades at low operating Reynolds numbers can significantly degrade performance. We investigated separation control by steady and pulsed vortex generator jets (VGJs) as well as harmonic blowing through a spanwise slot using computational fluid dynamics (CFD). The detailed fluid dynamics were explored in fully resolved direct numerical simulations (DNS) for a flat and curved plate model geometry under LPT conditions. The most promising AFC control schemes were then validated for the full LPT blade geometry. Steady VGJs were found to generate streamwise vortices. Our simulations have shown that pulsed VGJ actuation leads to an earlier transition to turbulence and the generation of spanwise coherent structures. In both cases, separation is controlled by an increased entrainment of freestream fluid. The stunning effectiveness of pulsed VGJs is explained by a hydrodynamic instability mechanism which amplifies spanwise instability modes. The authors confirm that they, and/or their company or institution, hold copyright on all of the original material included in their paper. They also confirm they have obtained permission, from the copyright holder of any third party material included in their paper, to publish it as part of their paper. The authors grant full permission for the publication and distribution of their paper as part of the ICAS2008 proceedings or as individual off-prints from the proceedings.
• Gross, A., & Fasel, H. F. (2008). CFD for investigating active flow control (invited). 4th AIAA Flow Control Conference.
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  Abstract: Computational fluid dynamics (CFD) can be employed for investigating the relevant physical mechanisms associated with active flow control (AFC). High fidelity simulations provide insight into why particular AFC applications are effective and efficient. The paper starts out with a categorization of different CFD approaches according to the flow properties and available computer resources. Then, several examples of research from our CFD laboratory are presented where we successfully employed CFD for investigating AFC techniques. These examples demonstrate the capabilities and limitations of CFD when investigating AFC. For example, Reynolds-averaged Navier-Stokes may be employed for investigating steady flow control (such as a high Reynolds number Coanda wall jet for circulation control). For unsteady flow control (such as separation control by pulsed vortex generator jets) the flow structures that are relevant for the effectiveness of the actuation have to be resolved. Copyright © 2008 by the authors.
• Gross, A., & Fasel, H. F. (2008). High-order-accurate numerical method for complex flows. AIAA Journal, 46(1), 204-214.
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  Abstract: A numerical method employing high-order-accurate (higher than third) upwind discretizations for solving the compressible Navier-Stokes equations on structured grids is discussed. The inviscid fluxes are computed by a procedure based on a weighted essentially nonoscillatory interpolation of the characteristic variables and the Roe scheme. Application of the numerical method to a number of test cases of increasing complexity, that are prototypical for several of the key aspects of practical flows, demonstrates the accuracy and robustness of the method even when computing on distorted curvilinear grids. Significant reductions in computer time are possible when a second-order- accurate implicit Adams-Moulton scheme is employed for time integration. The combination of implicit time integration and high-order-accurate spatial discretization is shown to lead to significant savings in compute time as the grid resolution requirement is lowered and the time step can be increased.
• Gross, A., & Fasel, H. F. (2008). Multi-block Poisson grid generator for cascade simulations. Mathematics and Computers in Simulation, 79(3), 416-428.
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  Abstract: High quality computational grids can greatly enhance the accuracy of turbine and compressor cascade simulations especially when time-dependent results are sought where vortical structures are convected through the computational domain. A technique for generating periodic structured grids for cascade simulations based on the Poisson equations is described. To allow for more complex geometries, the grid can be divided into individual zones or blocks. The grids are generated simultaneously in all blocks, assuring continuity of the grid lines and their slopes across the zonal boundaries. Simple geometric rules can be employed for enforcing orthogonality at block boundaries. The method results in grids with low grid distortion by allowing both, block boundaries and grid points on physical boundaries, to move freely. Results are presented for a linear turbine and a linear compressor cascade. © 2008 IMACS.
• Gross, A., & Fasel, H. F. (2008). Numerical investigation of flow separation in an asymmetric diffuser. 46th AIAA Aerospace Sciences Meeting and Exhibit.
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  Abstract: A number of important technical applications rely on diffusers where the flow is decelerated in the presence of an associated adverse pressure gradient, resulting in rapid boundary layer growth, potentially flow separation, and unsteadiness (both, small and large scale turbulent motion). Simulations of high Reynolds number flows with strong adverse pressure gradient and flow separation are computationally challenging because standard turbulence models are difficult to calibrate for such flows and simulations that resolve all scales of the turbulent motion can become prohibitively expensive. For validation purposes we computed two well documented turbulent channel flow cases using steady Reynolds-averaged Navier-Stokes. In collaboration with J. Eaton at Stanford University we then started simulations of an asymmetric diffuser at an inflow Reynolds number based on channel height and bulk velocity of 10,000 using Reynolds-averaged Navier-Stokes, direct numerical simulations, and a hybrid turbulence modeling approach, the flow simulation methodology. The capability of the various approaches to accurately predict time-averaged properties of the flow are discussed. Although at this point none of the different approaches is entirely satisfactory, the current results provide valuable hints and insights of how to proceed with such flow simulations so that more reliable results can be obtained. Copyright © 2008 by the authors.
• Gross, A., & Fasel, H. F. (2008). Numerical investigation of low-pressure turbine separation control. 4th AIAA Flow Control Conference.
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  Abstract: Low-pressure turbines are a common element of many modern jet engines. Flow separation from the suction side of the constituent blades at low Reynolds number conditions can noticeably deteriorate overall engine performance. Under such conditions active control of laminar separation may eliminate or reduce associated losses resulting in increased engine performance. We investigated separation control for the PackB LPT blade geometry at a Reynolds number based on axial chord of 25,000 where laminar separation was observed in the experiments. For our investigations we performed numerical simulations using a higher-order-accurate finite-volume compressible Navier-Stokes code developed in our laboratory. A grid resolution study for the uncontrolled flow indicated grid convergence for our simulations. Pulsed vortex generator jets are shown to result in an earlier transitioning of the flow and successful separation control. The jet amplitude was found to influence the intensity of spanwise coherent structures downstream of the actuator location. An even more efficient separation control can be accomplished by harmonic blowing through a slot (or alternatively plasma actuators). The astounding effectiveness of the latter control scheme is attributed to the suppression of three-dimensional structures which weaken the spanwise structures. Transition was also delayed by streamwise vortices, which were introduced by volume forces. As the flow does not amplify such structures the energy input required for obtaining streamwise vortices of sufficient strength was found to be more than two orders of magnitude larger than for the harmonic blowing through a slot. This, however, is not a real concern as such structures can likely be generated with passive devices such as vortex generators. Copyright © 2008 by the authors.
• Gross, A., & Fasel, H. F. (2008). Strategies for simulating flow through low-pressure turbine cascade. Journal of Fluids Engineering, Transactions of the ASME, 130(11), 1111051-11110513.
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  Abstract: Laminar separation on the suction side of low-pressure turbine blades at low Reynolds number operating conditions deteriorates overall engine performance and has to be avoided. This requirement affects the blade design and poses a limitation on the maximum permissible blade spacing. Better understanding of the flow physics associated with laminar separation will aid in the development of flow control techniques for delaying or preventing flow separation. Simulations of low-pressure turbine flows are challenging as both unsteady separation and transition are present and interacting. Available simulation strategies have to be evaluated before a well-founded decision for the choice of a particular simulation strategy can be made. With this in mind, this paper provides a comparison of different flow simulation strategies: In particular, "coarse grid" direct numerical simulations, implicit large-eddy simulations, and simulations based on a hybrid turbulence modeling approach are evaluated with particular emphasis on investigating the dynamics of the coherent structures that are generated in the separated flow region and that appear to dominate the entire flow. It is shown that in some instances, the effect of the dominant coherent structures can also be predicted by unsteady Reynolds-averaged Navier-Stokes calculations. Copyright © 2008 by ASME.
• Gross, A., Balzer, W., & Fasel, H. F. (2008). Active separation control for lifting surfaces at Low-Reynolds number operating conditions. 2008 Proceedings of the Department of Defense High Performance Computing Modernization Program: Users Group Conference - Solving the Hard Problems, 9-17.
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  Abstract: The low-pressure turbine (LPT) stage is a common element of many modern jet engines. Its performance at cruise conditions is of great economical importance. Low-Reynolds number conditions and high blade loading can result in laminar separation from the suction side and performance degradation. For external aerodynamics problems, such as airfoils, low-Reynolds number conditions and large angles of attack can similarly lead to laminar flow separation, and in the worst case complete stall. Successful control of separation from lifting surfaces at such detrimental conditions promises significant savings in operating expenses and improved safety. We are employing computational fluid dynamics (CFD) for investigating passive and active flow control for lifting surfaces. Because of the many design parameters available for flow control strategies, such as the spacing and their dimensions, a trial and error optimization is ill-fated. It has been convincingly demonstrated in the past that through a deepened understanding of the underlying fundamental physical mechanisms the effectiveness of flow control can be greatly improved. In addition, an improved understanding will likely result in entirely new and innovative flow control devices and strategies. Using CFD we investigated separation control using vortex generator jets (VGJs) and harmonic blowing through a slot for a typical LPT blade. With pulsed and harmonic VGJs and for moderate blowing ratios as well as for harmonic blowing through a slot the generation of spanwise coherent structures that were amplified by the flow appeared to be the dominant mechanism for separation control. When the forcing amplitude was increased, the two-dimensional coherence was reduced and turbulent mixing appeared to become the more dominant mechanism. The hole spacing was found to have little effect as long as it was smaller than the length of the separated flow region. © 2008 IEEE.
• Gross, A., Balzer, W., & Fasel, H. F. (2008). Numerical investigation of low-pressure turbine flow control (Invited). 38th AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: The low-pressure turbine is an important element of modern jet engines. Any performance improvement will lead to net savings in aircraft operating expenses. Experiments showed that separation from low-pressure turbine blades can be controlled by steady and pulsed vortex generator jets and plasma actuators. This paper summarizes numerical simulations that were carried out at the computational fluid dynamics laboratory at the University of Arizona. A two-pronged approach was taken. i) The entire low-pressure turbine blade was studied using a versatile but computationally less efficient multi block code. ii) Separation bubbles under low-pressure turbine conditions on a flat and a curved plate were investigated with a high-order-accurate research code. The first approach allows for simulations of the uncontrolled flow and for investigating active flow control techniques for the entire blade. The grid resolution especially near the actuators is, however, not sufficient for these simulations to be true direct numerical simulations. For the second approach all length scales down to the dissipative length scales were resolved. These well resolved high-fidelity simulations allow in-detail investigations of the relevant physical mechanisms associated with separation control by steady and pulsed vortex generator jets. Copyright © 2008 by the authors.
• Gross, A., Jacobi, R., Wernz, S., & Fasel, H. (2008). Numerical investigation of internal and external three-dimensional flow separation. 2008 Proceedings of the Department of Defense High Performance Computing Modernization Program: Users Group Conference - Solving the Hard Problems, 52-60.
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  Abstract: Separation of wall bounded flows for Navy relevant geometries is a highly complex phenomenon. Due to the shape of underwater vehicles (submarines, torpedoes, low aspect ratio lifting or control surfaces) separation is often three-dimensional (3D). Because of the relatively high Reynolds numbers involved, separation is always associated with considerable unsteadiness. This unsteadiness is caused by large coherent structures that are a consequence of hydrodynamic instability mechanisms of the mean flow. The combination of threedimensionality and unsteadiness results in a highly complex time-dependent topology of the separated region. We are simulating 3D separation bubbles in internal (diffuser) and external flows. The diffuser flow simulations are conducted in collaboration with experiments at Stanford University by J. Eaton. The flow is turbulent and difficult to access by direct numerical simulations (DNS). The objective here is to develop, validate, and advance appropriate hybrid turbulence modeling capabilities that will lower the computational expense associated with such simulations. In a combined numerical/experimental effort we are also studying laminar separation bubbles in external flows. For these simulations we are employing highly resolved DNS. The objective here is to obtain high-fidelity flow data that will be analyzed to obtain a deeper understanding of the various physical mechanisms governing separation, transition, and reattachment of 3D bubbles. Ultimately, such understanding may pave the way for the development of effective and efficient flow control strategies for preventing separation in practical applications. © 2008 IEEE.
• Gross, A., Pearman, C., Kremer, R., Napier, B., Gosla, C., Kurz, A., Mack, S., Brehm, C., Heine, B., Radi, A., Marovic, B., Retzko, S., Feindler, N., Zickler, B., Fasel, H. F., & Osbrink, A. (2008). 1/5 scale model of aeromot 200S SuperXimango for scaled flight research. Collection of Technical Papers - AIAA Applied Aerodynamics Conference.
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  Abstract: Within the framework of a NASA STTR program, we designed and developed a 1/5 dynamically scaled model of the Aeromot (AMT) 200S motor glider ("SuperXimango"). This model is one element in a "tool box" for scaled model flight research that also includes wind/water tunnel experiments, aerodynamic design tools, and computational fluid dynamics. This 1/5 dynamically scaled model of the Aeromot 200S SuperXimango is awaiting flight testing which will allow for a validation and verification of the scaling by comparison with full scale flight test data. An important criterion in the design and development of the flight model is its ability to serve as a versatile platform for developing and testing of novel technologies. Therefore, great emphasis was placed on a modular design and construction so that certain modules can be easily exchanged.
• Heine, B., Mack, S., Kurz, A., Gross, A., & Fasel, H. F. (2008). Aerodynamic scaling of general aviation airfoil for low Reynolds number application. 38th AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: Meaningful scaled model flight research requires models that are dynamically and aerodynamically scaled. While dynamic scaling assures that the flight dynamics scale (such as turn rates etc.), aerodynamic scaling assures that the scaled airplane, especially the airfoil, has the same aerodynamic properties (such as aerodynamic coefficients, derivatives, stall behavior etc.) as the full size airplane or airfoil despite the change in Reynolds number associated with the change in length scales, air speeds, and air properties. The present study is concerned with the Aeromot 200S Super Ximango motor glider for which a dynamically scaled 1:5 scale model was built. We investigated aerodynamic scaling for a two-dimensional section of its wing which has a modified NACA 643-618 geometry. For this airfoil, wind tunnel data and airfoil analysis code predictions were found to change noticeably when going from the full size cruise to the model cruise conditions. We investigated two different approaches for obtaining aerodynamic similarity between the 1:5 scale and the full size airfoil at cruise conditions: i) Forced transition with a trip wire and ii) modifying the airfoil geometry. Copyright © 2008 by the authors.
• Jacobi, R., Wernz, S., & Fasel, H. F. (2008). Numerical investigation of localized separation induced by a three-dimensional pressure gradient. 38th AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: Steady and unsteady separation on a flat-plate generated by a localized three-dimensional pressure gradient has been studied with Direct Numerical Simulations for a range of flow parameters. The fundamental characteristics of this type of flow have been analyzed and three distinctly different topologies were found. Fundamental features described for three-dimensional separated flows in general as well as for the flow over a hemisphere-cylinder in particular could be reproduced. The closed separation topologies are all very similar near the line of separation. One pattern in particular displays features that closely resemble the flow over a cylinder with a hemispheric cap. In addition to the closed separation patterns, we also found an open separation topology that may not have been documented before. Copyright © 2008 by the authors.
• Laible, A. C., Mayer, C. S., & Fasel, H. F. (2008). Numerical investigation of supersonic transition for a circular cone at mach 3.5. 38th AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: The Direct Numerical Simulation (DNS) of the transition process in a supersonic boundary layer from the laminar to the turbulent state significantly challenges existing numerical codes. High-order accurate methods are commonly used to improve the accuracy of simulations and thus reduce the number of required grid points. In this paper the development of a high-order code which is tailored towards stability and nonlinear transition simulations over a circular cone is discussed. A thoroughly conducted validation is presented. In particular, small amplitude disturbances are introduced to study the linear wave amplification (eigenbehavior). The results are compared to Linear Stability Theory (LST). Moreover, a three-dimensional stability diagram - in the downstream-frequency-azimuthal mode domain (Rx - F - k) - is extracted from these calculations and analyzed. Finally, the possible occurrence of oblique breakdown is highlighted by performing simulations with continuously forced finite{amplitude disturbances. Copyright © 2008 by the authors.
• Mack, S., Brehm, C., Heine, B., Kurz, A., & Fasel, H. F. (2008). Experimental investigation of separation and separation control on a laminar airfoil. 4th AIAA Flow Control Conference.
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  Abstract: The flow field around a two-dimensional slightly modified NACA 64 3 - 618 airfoil was investigated for two chord Reynolds numbers, Re = 64, 200 and Re = 137, 000. Surface- pressure distributions and total aerodynamic forces were measured for angles of attack in the range of -12° to 20°. Experimental results were compared to numerical results obtained by Direct Numerical Simulations and calculations using the two-dimensional design code XFOIL. Additionally, acoustic measurements and flow-visualizations were performed to study the transition and the separation behavior of the flow for various angles of attack. For certain angles of attack, separation bubbles developed which had a strong influence the airfoil performance. Using the combined approach, experimental and numerical, a deeper insight into the underlying physical mechanisms and into the separation and transition behavior of the boundary layer around the airfoil was obtained. Based on these results, passive and active flow-control strategies were investigated. The flow-control strategies aimed at improving the overall airfoil performance by preventing or at least reducing regions of separated flow. Copyright © 2008 by we authors.
• S., C., & Fasel, H. F. (2008). Investigation of asymmetric subharmonic resonance in a supersonic boundary layer at Mach 2 using DNS. 46th AIAA Aerospace Sciences Meeting and Exhibit.
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  Abstract: The transition process in a supersonic at-plate boundary layer at Mach 2 is inves- tigated numerically using linear stability theory (LST) and direct numerical simulations (DNS). Experimental investigations by Kosinov et al. serve as a reference and provide the physical conditions for the numerical setup. In these experiments, the weakly nonlinear regime of transition was studied resulting in the discovery of asymmetric subharmonic resonance triads composed of one primary oblique wave of frequency 20 kHz and two oblique subharmonic waves of frequency 10 kHz. The experimentalists concluded that during the transition process, specific subharmonic resonance triads are selected depending on the am- plitude ratio between fundamental and subharmonic disturbances. With the simulations presented in this paper, the subharmonic transition route is studied in detail. A similar subharmonic resonance mechanism as observed in the experiments is also visible in our numerical simulations. Additionally, several other resonance triads can be identified using LST and DNS. In our simulations, the selection process for a specific triad is not influenced by the amplitude ratio of fundamental and subharmonic disturbances. In contrast to the experiments, the DNS results show that the phase relation between disturbances of both frequency plays a more crucial role. Copyright © 2008 by the authors.
• S., C., A., D., & Fasel, H. F. (2008). DNS of complete transition to turbulence via oblique breakdown at mach 3. 38th AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: A pair of oblique waves at low amplitudes is introduced in a supersonic at-plate boundary layer. Its downstream development and the concomitant process of laminar to turbulent transition is then investigated numerically using Direct Numerical Simulations (DNS). In the present paper, first the linear regime is studied in great detail. Comparisons to linear stability theory clearly determine that the so-called "oblique breakdown" mechanism is initiated. The focus of the second part is the nonlinear regime. It is shown how the spectrum is filled up by nonlinear interactions, what flow structures arise and how these structures locally break down to small scales. Finally, a logarithmic region of the mean streamwise velocity profile is formed indicating the beginning of a fully turbulent flow region. The simulations demonstrate that oblique breakdown is a viable path to turbulence. Copyright © 2008 by the authors.
• Sivasubramanian, J., & Fasel, H. F. (2008). Numerical investigation of supersonic axisymmetric wakes with active and passive flow control. 4th AIAA Flow Control Conference.
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  Abstract: The base drag of axisymmetric bodies at supersonic speeds make a substantial contribution to the total drag. We employed computational fluid dynamics for investigating transitional supersonic axisymmetric wakes at a freestream Mach number of M = 2.46. The objective of this research is to study how various active and passive flow control techniques affect the base drag and to understand how the mean flow properties are altered by these techniques. Simulations were carried out for a Reynolds number based on diameter of Re D = 100, 000. To lower the grid resolution requirements and save computer time we employed a hybrid turbulence model, the flow simulation methodology. We investigated flow control mechanisms that alter the near wake by introducing axisymmetric and longitudinal perturbations in the approach boundary layer. We also investigated passive control using steady bleed jets. Copyright © 2008 by the authors.
• Balzer, W., Gross, A., & Fasel, H. F. (2007). Active flow control of low-pressure turbine separation. Department of Defense - Proceedings of the HPCMP Users Group Conference 2007; High Performance Computing Modernization Program: A Bridge to Future Defense, DoD HPCMP UGC, 73-82.
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  Abstract: Operating low-pressure turbines (LPT) at off-design conditions or considering more aggressive designs can lead to laminar separation on the suction side of the LPT blades resulting in significant turbine and overall engine performance losses. In these instances, performance improvements may be achieved with active flow control (AFC). In an extensive experimental research program at the Air Force Research Laboratory (AFRL) at Wright-Patterson AFB, Dr. R.B. Rivir and co-workers systematically investigated the benefits of AFC with steady and pulsed vortex generator jets (VGJs) for a linear PakB LPT cascade. Pulsed VGJs were found to be very effective in mitigating separation. We are employing two in-house computational fluid dynamics (CFD) research codes for investigating the physical mechanisms associated with AFC for LPT geometries. For simulations of the entire LPT blade, a high-order-accurate finite volume code based on the compressible Navier-Stokes equations is used. Data from direct numerical simulations (DNS) with up to 19.4 million grid points of a PakB blade at Re=25,000 are compared with experimental data. For our fully resolved DNS that focus exclusively on the separated flow region, we are employing a high-order-accurate compact finite difference code based on the incompressible Navier-Stokes equations in vorticity-velocity formulation. Here, we study a separation bubble on a flat plate and on a curved wall model-geometry under LPT conditions. These simulations enable us to identify the intricate physical mechanisms associated with unsteady separation, transition, flow instabilities, and active control using VGJs. In particular, our simulation results provide an explanation for the stunning effectiveness of pulsed VGJs for separation control. © 2007 IEEE.
• Gross, A., & Fasel, H. F. (2007). Characteristic ghost-cell boundary condition. AIAA Journal, 45(1), 302-306.
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  Abstract: A method is proposed based on the boundary conditions that deals with the variables in ghost cells where the resulting flux differences satisfy the nonreflecting conditions. The boundary conditions were implemented into a compressible mixed finite volume/finite difference code in which the convective terms were discretized with a fifth-order accurate scheme based on a weighted essentially nonoscillatory (WENO) extrapolation of the characteristic variables. The state variables at the boundaries need to be prescribed in ghost cells before the governing equations were advanced in time. Velocities and temperatures were extrapolated at the outflow boundary, assuming zero second derivatives and the static pressure was prescribed. The characteristic boundary condition was found to be more transparent for outgoing waves and vortical structures, resulting in a faster relaxation of the flow to the ambient conditions.
• Gross, A., & Fasel, H. F. (2007). Control-oriented proper orthogonal decomposition models for unsteady flows. AIAA Journal, 45(4), 814-827.
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  Abstract: Controller development in the relatively young field of closed-loop (or feedback) flow control is constrained by the lack of proper models for the description of the dynamics and response of flows to a control input (actuation). If a set of ordinary differential equations could be derived that describes the unsteady flow with sufficient accuracy and that also models the effect of the actuation, conventional control theory tools could be employed for controller design. In this paper, reduced-order models based on a Galerkin projection of the incompressible Navier-Stokes equations onto a proper orthogonal decomposition modal basis are described. The model coefficients can be modified or calibrated to make the model more accurate. An error-minimization technique is employed to obtain the coefficients that describe how the control input enters the model equations. These models work well in the vicinity of the design operating point. Composite models are constructed by combining modes from different operating points. This approach results in more versatile models that are valid for a larger range of operating conditions.
• Gross, A., & Fasel, H. F. (2007). Investigation of low-pressure turbine separation control. Collection of Technical Papers - 45th AIAA Aerospace Sciences Meeting, 9, 6300-6314.
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  Abstract: Low-pressure turbines are a common element of many modern jet engines. Flow separation from the suction side of the constituent blades at low Reynolds number conditions can noticeably deteriorate overall engine performance. Two separation control strategies for an aggressive low-pressure turbine blade that was designed for an integrated flow control were investigated numerically using a hybrid turbulence modeling approach: Pulsed vortex generator jets are shown to result in an earlier transitioning of the flow and successful separation control. An even more efficient separation control can be accomplished by harmonic blowing through a slot. The astounding effectiveness of the latter control scheme is attributed to the suppression of three-dimensional structures. Instead, the flow is dominated by strong spanwise coherent structures that very effectively reduce separation.
• Gross, A., & Fasel, H. F. (2007). Self-adaptive closed-loop control of low-reynolds number laminar separation. Collection of Technical Papers - AIAA Applied Aerodynamics Conference, 1, 303-314.
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  Abstract: Closed-loop control of laminar separation was investigated numerically for a low-pressure turbine blade and an airfoil at relatively low operating Reynolds-numbers. Open-loop investigations show that within a certain frequency range disturbances are amplified. This can be exploited for making the control more effective. The amplification is attributed to a hydrodynamic instability of the base flow. A simple closed-loop controller, where a downstream pressure signal is fed back to an upstream actuator, is shown to be as effective as an optimized open-loop controller. The simple closed-loop controller can be improved by continuously adjusting the controller parameters during the run time of the simulation using a minimization algorithm that minimizes a desired objective such as drag over lift and control effort. This allows for an optimization of the gain and phase of the feedback resulting in a better "synchronization" or coupling of the actuation and the flow dynamics.
• Husmeier, F., & Fasel, H. F. (2007). Numerical investigations of hypersonic boundary layer transition for circular cones. Collection of Technical Papers - 18th AIAA Computational Fluid Dynamics Conference, 1, 289-305.
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  Abstract: This paper focuses on secondary instability mechanisms of high-speed boundary layers over cones with a circular cross section. Hypersonic tran7sition investigations at Mach 8 are performed using Direct Numerical Simulations (DNS). At wind-tunnel conditions, these simulations allow for comparison with experimental measurements to verify fundamental stability characteristics. According to Linear Stability Theory (LST) the boundary layer is most unstable to second-mode two-dimensional waves. Our investigations indicate that secondary instability mechanisms involving two-dimensional waves appear to be of lesser importance in the nonlinear stages of breakdown to turbulence in spite of the high amplification rates of the two-dimensional primary disturbances. Instead, second-mode oblique waves at small wave angles, which are almost as amplified as second-mode two-dimensional waves, dominate the nonlinear behavior.
• S., C., Wernz, S., & Fasel, H. F. (2007). Investigation of oblique breakdown in a supersonic boundary layer at Mach 2 using DNS. Collection of Technical Papers - 45th AIAA Aerospace Sciences Meeting, 17, 11526-11542.
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  Abstract: The early nonlinear stages of transition in a supersonic boundary layer at Mach 2 are investigated using spatial Direct Numerical Simulations (DNS). The computational setup matches earlier experimental studies by Kosinov et al., where transition was triggered by localized forcing leading to the development of a wedge-shaped wave packet. While the focus of these experiments has been on a new breakdown mechanism, called asymmetric subharmonic resonance, our interpretation of the experimental data indicates the presence of another, possibly competing mechanism, which exhibits the characteristics of an oblique breakdown. If confirmed, this would be the first experimental evidence of the oblique breakdown mechanism in a supersonic boundary layer. With the simulations presented here, the possible presence of this breakdown mechanism in the experiments is explored by deliberately suppressing subharmonic resonances in the DNS and by comparing the numerical results with the experimental data. The DNS results show excellent agreement with the experimental measurements for both linear and nonlinear transition stages. Most importantly, our results clearly show characteristic features of the oblique breakdown mechanism as observed in our earlier numerical investigations.
• Sivasubramanian, J., Sandberg, R. D., A., D., & Fasel, H. F. (2007). Numerical investigation of transitional supersonic base flows with flow control. Journal of Spacecraft and Rockets, 44(5), 1021-1028.
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  Abstract: Drag reduction by means of flow control is investigated for supersonic base flows at M = 2.46 using direct numerical simulations and the flow simulation methodology. The objective of the present work is to understand the evolution of coherent structures in the flow and how flow control techniques can modify these structures. For such investigations, simulation methods that capture the dynamics of the large turbulent structures are required. Direct numerical simulations are performed for transitional base flows at ReD = 30,000. Because of the drastically increased computational cost of direct numerical simulations at higher Reynolds numbers, a hybrid methodology (flow simulation methodology) is applied to simulate base flows with flow control at ReD = 100,000. Active and passive flow control techniques that alter the near wake by introducing axisymmetric and longitudinal perturbations are investigated. A detailed analysis of the dynamics of the resulting turbulent structures is presented.
• Wernz, S., & Fasel, H. F. (2007). Nonlinear resonances in a laminar wall jet: Ejection of dipolar vortices. Journal of Fluid Mechanics, 588, 279-308.
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  Abstract: Nonlinear mechanisms leading to the ejection of dipolar vortices from a laminar wall jet are being investigated using highly accurate Navier-Stokes simulations. With a set of well-defined numerical experiments for a forced Glauert wall jet, the nonlinear resonant interaction between the large-amplitude harmonic disturbance and a small-amplitude wave packet is systematically explored using two-dimensional simulations. Generated by a small-amplitude pulse, the wave packet experiences rapid resonant growth in the subharmonic part of its spectrum resulting in vortex mergings and, ultimately, the ejection of a pair of counter-rotating vortices from the wall jet. This two-dimensional subharmonic instability, if not mitigated by competing three-dimensional instabilities, can lead to the detachment of the entire wall jet from the surface. As shown using three-dimensional direct numerical simulations, vortex ejection still occurs in a forced transitional wall jet if the two-dimensional wave packet can reach a large amplitude level upstream of the region of three-dimensional turbulent breakdown. Movies are available with the online version of the paper. © 2007 Cambridge University Press.
• Brehm, C., Gross, A., & Fasel, H. F. (2006). Closed-loop control of low-pressure turbine laminar separation. Collection of Technical Papers - 3rd AIAA Flow Control Conference, 1, 522-534.
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  Abstract: Laminar separation from the suction side of low-pressure turbine (LPT) blades can significantly degrade engine efficiency. If laminar separation could be controlled, aerodynamic performance could be maintained over a wider operating range. Also, new and more aggressive stage designs with reduced solidity (less blades) would become possible. One example is the new L1M LPT blade, which was designed to allow for an increased aerodynamic loading by applying active now control (AFC) in a continuous fashion. Motivated by this and other success stories where open-loop flow control was shown to result in dramatic performance improvements, the general focus has shifted towards closed-loop control which promises even greater gains and a greater robustness and capability to adjust to changing operating conditions. However, a fully satisfactory methodology for designing robust and efficient closed-loop controllers for fluids problems has not been devised yet. This paper summarizes different approaches for investigating control of flow separation from the L1M blade using 2-D numerical simulations. A parameter study with open-loop control by harmonic wall normal blowing upstream of the separation was conducted to determine the optimum forcing parameters. We believe that by exploiting flow instability mechanisms the flow control can be made more efficient. It was also demonstrated how a proportional differential (PD) controller with self-adjusting parameters can be employed successfully for closed-loop control. Even better closed-loop controllers will likely become possible if a real-time prediction of the now dynamics over a reasonably broad parameter range became available. Galerkin models perform well for one given operating point but do not generalize well. Here, neural networks were explored for making real-time predictions of the unsteady separated L1M now field. The resulting models are shown to be both accurate and robust and to generalize well.
• Fasel, H. F. (2006). Instability and transition in boundary layers: Direct numerical simulations. Solid Mechanics and its Applications, 129, 257-267.
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  Abstract: Abstract: The fundamental aspects of DNS for investigating transition in boundary layers are discussed. Emphasis of this paper is on the socalled spatial simulation model. Several key examples of successful applications of DNS for advancing the understanding of transition physics are presented. © 2006 Springer, Printed in the Netherlands.
• Fasel, H. F., & Postl, D. (2006). Interaction of separation and transition in boundary layers: Direct numerical simulations. Fluid Mechanics and its Applications, 78, 71-88.
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  Abstract: The role of hydrodynamic instability mechanisms in the presence of laminar boundary layer separation is investigated by means of Direct Numerical Simulations. In a series of simulations involving generic laminar separation bubbles we show that the "natural" onset of unsteadiness (i.e. the development of visible vortex shedding) is not necessarily caused by an absolute/global instability. Our results indicate that the entrainment of high-momentum fluid required to "close" the separation bubble is primarily provided by 2-D or "2-D coherent" structures, which are a consequence of the (inviscid) hydrodynamic instability of the separated shear layer. In a series of highly resolved simulations for a flat-plate boundary layer subjected to low-pressure turbine blade conditions, we demonstrate that this natural instability mechanism (with respect to two-dimensional disturbances) can be exploited for effective control of separation using pulsed vortex generator jets. © 2006 Springer.
• Fasel, H. F., A., D., & Sandberg, R. D. (2006). A methodology for simulating compressible turbulent flows. Journal of Applied Mechanics, Transactions ASME, 73(3), 405-412.
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  Abstract: A flow simulation Methodology (FSM) is presented for computing the time-dependent behavior of complex compressible turbulent flows. The development of FSM was initiated in close collaboration with C. Speziale (then at Boston University). The objective of FSM is to provide the proper amount of turbulence modeling for the unresolved scales while directly computing the largest scales. The strategy is implemented by using state-of-theart turbulence models (as developed for Reynolds averaged Navier-Stokes (RANS)) and scaling of the model terms with a "contribution function." The contribution function is dependent on the local and instantaneous "physical" resolution in the computation. This physical resolution is determined during the actual simulation by comparing the size of the smallest relevant scales to the local grid size used in the computation. The contribution function is designed such that it provides no modeling if the computation is locally well resolved so that it approaches direct numerical simulations (DNS) in the fine-grid limit and such that it provides modeling of all scales in the coarse-grid limit and thus approaches a RANS calculation. In between these resolution limits, the contribution function adjusts the necessary modeling for the unresolved scales while the larger (resolved) scales are computed as in large eddy simulation (LES). However, FSM is distinctly different from LES in that it allows for a consistent transition between RANS, LES, and DNS within the same simulation depending on the local flow behavior and "physical" resolution. As a consequence, FSM should require considerably fewer grid points for a given calculation than would be necessary for a LES. This conjecture is substantiated by employing FSM to calculate the flow over a backward-facing step and a plane wake behind a bluff body, both at low Mach number, and supersonic axisymmetric wakes. These examples were chosen such that they expose, on the one hand, the inherent difficulties of simulating (physically) complex flows, and, on the other hand, demonstrate the potential of the FSM approach for simulations of turbulent compressible flows for complex geometries. Copyright © 2006 by ASME.
• Gross, A., & Fasel, H. F. (2006). Coanda wall jet calculations using one- and two-equation turbulence models. AIAA Journal, 44(9), 2095-2107.
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  Abstract: The wall jet over a convex surface, the Coanda wall jet, has many potential technical applications, such as circulation control of airfoils. Reynolds-averaged Navier-Stokes is a potentially very powerful tool for Coanda wall jet calculations. A number of common linear eddy-viscosity turbulence models were used for Reynolds-averaged Navier-Stokes calculations of a turbulent Coanda wall jet experiment Calculations were also carried out with an explicit algebraic stress model. The differences in the computed results were shown to be large depending on what turbulence model was employed. The k-ω model was then employed for three-dimensional Reynolds-averaged Navier-Stokes calculations where the downstream evolution of streamwise vortices in the turbulent wall jet was studied for various spanwise wavelengths and forcing amplitudes. It was found that a centrifugal instability of the turbulent mean flow may lead to the streamwise growth of longitudinal vortices. Beyond a certain disturbance amplitude threshold the relative phase of the vortices had a noticeable impact on the amplification rates and the mean flow characteristics were affected by the added wall normal momentum exchange.
• Gross, A., & Fasel, H. F. (2006). RANS, URANS, and LES of Coanda wall jet flows. Collection of Technical Papers - 36th AIAA Fluid Dynamics Conference, 2, 1243-1258.
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  Abstract: Coanda wall jets in technical applications are almost always turbulent. Since reliable prediction tools are not available, wind/water tunnel tests are indispensable for validating any new design. Still, complete confidence in Coanda wall jet applications can only be obtained if the intricate fluid dynamics are understood. Ongoing research efforts, both experimentally and numerically, aim at broadening our understanding of the physical mechanisms involved in Coanda wall jet flows. This paper explores the feasibility of several Reynolds averaged Navier-Stokes (RANS) turbulence models for calculating a Coanda wall jet over a circular cylinder in a quiescent ambient. Neither one of the models captures all aspects of the flow correctly. It is shown how a finite ambient velocity affects both, jet separation location and generated aerodynamic forces. The grid resolution requirements can make direct numerical simulations (DNS) of turbulent Coanda wall jet flows unaffordable. Hybrid turbulence modeling approaches such as detached eddy simulation (DES) and the flow simulation methodology (FSM) combine the advantages of DNS, large eddy simulation (LES), and RANS. The dynamics of the large scale coherent structures are captured while the small scale and more homogeneous turbulence motion is modeled. Using 3-D FSM it is shown how streamwise vortices generated at the nozzle exit can significantly alter the wall jet characteristics.
• Gross, A., & Fasel, H. F. (2006). Reduced order models for closed-loop control of time-dependent flows. Collection of Technical Papers - 44th AIAA Aerospace Sciences Meeting, 22, 16872-16893.
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  Abstract: Controller development in the relatively young field of closed-loop (or feed back) flow control is constrained by the lack of proper models for the description of the dynamics and response of flows to a control input (actuation). If a set of ordinary differential equations could be derived that describes the unsteady flow with sufficient accuracy and that also models the effect of the actuation, control theory tools could be employed for controller design. In this paper reduced order models (ROMs) based on a Galerkin projection of the incompressible Navier-Stokes equations onto a proper orthogonal decomposition (POD) modal basis are described. The model coefficients can be modified or calibrated to make the model more accurate. An error-minimization technique is employed to obtain the coefficients that describe how the control enters the model equations. These models work well in the vicinity of the design operating point. Composite models are constructed by combining POD modes from different operating points. This approach results in more versatile ROMs that are valid for a larger range of operating conditions.
• Laible, A., Valsecchit, P., & Fasel, H. (2006). Numerical investigation of secondary centrifugal instabilities in the Coanda wall jet. Collection of Technical Papers - 44th AIAA Aerospace Sciences Meeting, 15, 10861-10873.
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  Abstract: In a wall jet over a convex surface (Coanda flow), the mean streamwise velocity profile may be unstable with respect to a centrifugal instability mechanism. Small initial perturbations can therefore grow to finite-amplitude counter-rotating longitudinal vortices (Gört1er vortices). These vortices may also become unstable (secondary instability) and develop increasing distortions in the streamwise direction. Here, a numerical investigation of the secondary instability mechanisms has been carried out using temporal simulations and solving the full Navier-Stokes equations for incompressible flows. Using the temporal model, global flow effects are neglected, thus allowing to focus on local phenomena. Also, by adjusting the downstream extent of the computational domain, the primary 2D instability modes (viscous and inviscid) resulting from the wall jet velocity profile can be selectively suppressed. Different stages of the growth of laminar Görtier vortices in combination with the two-dimensional wall jet profile have been considered as base flows for the investigation of secondary instability. Two secondary modes previously predicted by theory could be observed. The amplification rate of the sinuous mode was found to be predominant at earlier stages of the Görtier vortex growth, whereas the varicose mode dominates the development farther downstream. Furthermore, it was found that the primary instabilities in the streamwise direction can support the amplification of the varicose mode up to large amplitudes. Finally, a comparison was made with previous studies for boundary layer flow over concave wall.
• Larignon, B., Weraz, S., Goldstein, D. B., & Fasel, H. F. (2006). Numerical investigation of vortex onset in supersonic Taylor-Couette flow. Journal of Thermophysics and Heat Transfer, 20(3), 536-543.
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  Abstract: The compressible flow between concentric cylinders, in the case where the inner cylinder rotates and the outer one is at rest, has been computed using a pseudospectral/finite difference method. The onset of turbulence in rarefied supersonic gap flows for flywheel applications, and particularly the impact of surface roughness on the development of Taylor vortices, has been explored. An immersed boundary method using an external force field was employed to model the surface roughness. The Navier-Stokes code was validated against results from the literature for a supersonic wide-gap flow. To validate the immersed boundary method, the rotor wall was replaced by a smooth offset wall modeled with an external force field. Very good agreement with the analytical solution for steady Couette flow was achieved. The growth of Taylor vortices in a narrow gap with smooth walls was studied first. Then, the external force field was used to create a riblet on the rotor to investigate the influence of the roughness of the rotor on the flow and, more particularly, on the onset of the instability. The most interesting feature is the early appearance of vortices for Taylor numbers where the flow with smooth walls is in the subcritical regime.
• Postl, D., & Fasel, H. F. (2006). Direct numerical simulation of turbulent flow separation from a wall-mounted hump. AIAA Journal, 44(2), 263-272.
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  Abstract: A high-order-accurate numerical method for solving the incompressible Navier-Stokes equations in general orthogonal coordinates is presented. The method is applied to a test case of the NASA Langley Computational Fluid Dynamics Validation Workshop 2004, a turbulent flow over a wall-mounted hump geometry. Results of direct numerical simulations (DNS) for the unforced flow as well as for a case with steady suction are presented and compared to the available experimental data. The DNS predictions are shown to agree well with the experiments, except in the vicinity of the experimental reattachment locations. The simulations predict slightly longer recirculation regions for both the unforced and the controlled case. The results presented in this work suggest that, with the rapidly increasing computational resources of modern supercomputers such as the Cray XI, DNS is becoming a viable alternative to the use of turbulence models for investigating complex turbulent flows at moderately high Reynolds numbers. Copyright © 2005 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Sandberg, R. D., & Fasel, H. F. (2006). Direct numerical simulations of transitional supersonic base flows. AIAA Journal, 44(4), 848-858.
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  Abstract: Transitional supersonic base flows at M = 2.46 are investigated using direct numerical simulations. Results are presented for Reynolds numbers based on the cylinder diameter ReD = 3 × 104-1 × 105. As a consequence of flow instabilities, coherent structures develop that have a profound impact on the global flow behavior. Simulations with various circumferential domain sizes are conducted to investigate the effect of coherent structures associated with different azimuthal modes on the mean flow, in particular on the base pressure, which determines the base drag. Temporal spectra reveal that frequencies found in the axisymmetric mode can be related to dominant higher modes present in the flow. It is shown that azimuthal modes with low wave numbers cause a flat base pressure distribution and that the mean base pressure value increases when the most dominant modes are deliberately eliminated. Visualizations of instantaneous flow quantities and turbulence statistics at ReD = 1 × 105 show good agreement with experiments at a significantly higher Reynolds number. For these investigations, a high-order-accurate compressible Navier-Stokes solver in cylindrical coordinates developed specifically for this research was used.
• Sandberg, R. D., & Fasel, H. F. (2006). Investigation of supersonic wakes using conventional and hybrid turbulence models. AIAA Journal, 44(9), 2071-2083.
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  Abstract: Transitional and turbulent supersonic wakes behind axisymmetric bodies with a blunt base are investigated numerically using state-of-the-art Reynolds averaged Navier-Stokes models and the flow simulation methodology. The centerpiece of the flow simulation methodology is a strategy to provide the proper amount of modeling of the subgrid scales. This is accomplished by a "contribution function" which locally and instantaneously compares the smallest relevant scales to the local grid size. The turbulence closures chosen are a state-of-the-art wall-distance free explicit algebraic stress model, or a standard K-ε model for comparison. Axisymmetric Reynolds averaged Navier-Stokes and fully three-dimensional flow simulation methodology calculations are performed on various computational grids for wakes at M = 2.46 for several Reynolds numbers. The data obtained from all simulation strategies are compared with available direct numerical simulation results for the transitional cases and to experimental results at the highest Reynolds number investigated. Of particular interest is the performance of commonly used compressibility corrections and modifications to closure-coefficients specifically derived for high-Reynolds number flows. The ability of the flow simulation methodology to reproduce flow structures found in direct numerical simulations is scrutinized and a reason for the failure of Reynolds averaged Navier-Stokes calculations to correctly predict the base pressure-distribution is given.
• Sandberg, R. D., & Fasel, H. F. (2006). Numerical investigation of transitional supersonic axisymmetric wakes. Journal of Fluid Mechanics, 563, 1-41.
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  Abstract: Transitional supersonic axisymmetric wakes are investigated by conducting various numerical experiments. The main objective is to identify hydrodynamic instability mechanisms in the flow at M = 2.46 for several Reynolds numbers, and to relate these to coherent structures that are found from various visualization techniques. The premise for this approach is the assumption that flow instabilities lead to the formation of coherent structures. Three high-order accurate compressible codes were developed in cylindrical coordinates for this work: a spatial Navier-Stokes (N-S) code to conduct direct numerical simulations (DNS), a linearized N-S code for linear stability investigations using axisymmetric basic states, and a temporal N-S code for performing local stability analyses. The ability of numerical simulations to exclude physical effects deliberately is exploited. This includes intentionally eliminating certain azimuthal/helical modes by employing DNS for various circumferential domain sizes. With this approach, the impact of structures associated with certain modes on the global wake-behaviour can be scrutinized. Complementary spatial and temporal calculations are carried out to investigate whether instabilities are of local or global nature. Circumstantial evidence is presented that absolutely unstable global modes within the recirculation region co-exist with convectively unstable shear-layer modes. The flow is found to be absolutely unstable with respect to modes k> 0 for ReD > 500 and with respect to the axisymmetric mode k = 0 for ReD > 100 000. It is concluded that azimuthal modes k = 2 and k = 4 are the dominant modes in the trailing wake, producing a 'four-lobe' wake pattern. Two possible mechanisms responsible for the generation of longitudinal structures within the recirculation region are suggested. © 2006 Cambridge University Press.
• Sivasubramanian, J., & Fasel, H. F. (2006). LES and DES of high Reynolds number, supersonic base flows with control of the near wake. Proceedings - HPCMP Users Group Conference, UGC 2006, 80-88.
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  Abstract: The drag associated with supersonic base flows is of critical importance for the design of aerodynamic bodies, such as missiles and projectiles. The base drag which accounts for a significant part of the total drag, that may be reduced by means of active and passive control of the near wake. There is evidence that large (turbulent) coherent structures evolve in these flows and strongly influence the mean flow. Therefore, in order to understand the dynamics of coherent structures in the wake and how flow control mechanisms modify these structures, numerical simulations were conducted. We performed Large-Eddy Simulations (LES) based on the Flow Simulation Methodology (FSM) for a Reynolds number of ReD = 100,000 and Mach number M = 2.46 using a high-order accurate research code, which was developed at the University of Arizona. Flow control mechanisms that alter the near wake by introducing axisymmetric and three-dimensional perturbations, thus emulating active and passive flow control were investigated. We also studied supersonic base flows at Reynolds number ReD = 3,300,000 and Mach number M = 2.46 using Detached-Eddy Simulations (DES). These investigations were performed using the commercial CFD-code Cobalt. In addition, for the same Reynolds number, we investigated Passive flow control using afterbody boat-tailing. Our results are compared to available experimental data. © 2006 IEEE.
• Sivasubramanian, J., Sandberg, R. D., A., D., & Fasel, H. F. (2006). Numerical investigation of flow control mechanisms for drag reduction in supersonic base-flows. Collection of Technical Papers - 44th AIAA Aerospace Sciences Meeting, 15, 10775-10784.
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  Abstract: Supersonic axisymmetric base flows are prototypical for flows behind projectiles and missiles. There is evidence that large (turbulent) coherent structures exist in these flows and strongly influence the mean flow. The objective of the present investigation is to evaluate if and how successful passive flow control techniques modify the large structures and the mean flow. To this end, numerical simulations using two hybrid RANS/LES methods, the Flow Simulation Methodology (FSM) and Detached Eddy Simulations (DES), were employed to investigate the supersonic base flow at ReD = 3.3 × 10 6 and Ma = 2.46. As a passive control method, boat-tailing is studied. The obtained mean flow data and the turbulent coherent structures are compared to available experimental results and to simulations without flow control.
• Sivasubramanian, J., Sandberg, R. D., A., D., & Fasel, H. F. (2006). Numerical investigation of transitional supersonic base flows with flow control. Collection of Technical Papers - 44th AIAA Aerospace Sciences Meeting, 8, 5685-5696.
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  Abstract: Drag reduction by means of flow control is investigated for supersonic base flows at Mach number M = 2.46 using Direct Numerical Simulations (DNS) and the Flow Simulation Methodology (FSM). The objective of the present work is to understand the evolution of coherent structures in the flow and how flow control techniques modify these structures. For such investigations, simulation methods that capture the dynamics of the large turbulent structures are required. For transitional base flows at ReD = 30,000 DNS are performed. Due to the drastically increased computational cost of DNS at higher Reynolds numbers, a hybrid RANS/LES method (FSM) is applied to simulate base flows with flow control at ReD = 100,000. Active and passive flow control techniques that alter the near-wake by introducing axisymmetric and longitudinal perturbations are investigated. A detailed analysis of the dynamics of the resulting turbulent (coherent) structures is presented.
• Wernz, S., & Fasel, H. F. (2006). Numerical investigation of transition mechanisms influencing the development of turbulent wall jets. Collection of Technical Papers - 36th AIAA Fluid Dynamics Conference, 4, 2442-2457.
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  Abstract: The transitional region downstream of the nozzle exit in wall jets over plane and curved surfaces at high Reynolds numbers (Rej=10,000) is investigated using three-dimensional Navler-Stokes simulations. The transition process is crucial for the development of the turbulent wall jet since it leaves a strong signature farther downstream. During transition, energetic vortical structures (spanwise, streamwlse) develop from hydrodynamlc instabilities and propagate into the turbulent now, while strongly influencing the meannow characteristics, e.g., the spreading rate, the skin-friction, and ultimately the separation location. Smaller-sized Navier-Stokes simulations are employed for investigating the influence of various now parameters, such as the shape of the jet profile at the nozzle exit, the disturbance level, and also the effect of wall curvature on the overall development of the vortical structures and the mean now. For some flow configurations, the transition mechanisms are investigated in greater detail using Direct Numerical Simulations with a grid-resolution that is fine enough for capturing the transition mechanisms during the later stages of the breakdown. The simulations show that the development of spanwise vortices during wall-jet transition leads to the formation of braid vortices that loop around the spanwise vortices and appear as streamwise vortices in the time-averaged now. In the presence of wall curvature (Coanda wall jet), the strength of the spanwise vortices is strongly reduced and streamwise vortices develop instead from a centrifugal Görtler-tpye instability.
• Wernz, S., Ringwald, H., & Fasel, H. F. (2006). Numerical investigation of instabilities in three-dimensional skewed shear layers. Collection of Technical Papers - 3rd AIAA Flow Control Conference, 2, 1041-1056.
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  Abstract: Vortex development in skewed three-dimensional shear layers has been investigated using Direct Numerical Simulations (DNS). A highly accurate incompressible Navier-Stokes code for computing wall-bounded shear flows has been adapted to the present flow geometry and has been validated for select test cases. As a prototypical laminar base flow for the calculations, a spatially developing Blasius shear layer with a cross-flow component is employed. Due to the inflection point in the skewed velocity profile, inflectional instabilities arise leading to the development of streamwise or oblique vortical disturbances from small-amplitude perturbations depending on the free-stream skew angles. In experiments, these disturbances develop into interesting patterns of vortices with the merging of neighboring streamwise aligned vortices for some configurations, or diamond shaped vortex patterns for others. In the present study, the initial linear growth of the vortical disturbances is studied first using a linearized version of the Navier-Stokes code and simulation results are compared with results from Linear Stability Theory (LST). Small-amplitude disturbances experience rapid linear growth in downstream direction and reach large amplitudes within a short streamwise distance. The nonlinear development and interaction of the vortical disturbances into oblique vortices is studied using the fully nonlinear version of the Navier-Stokes code. Special focus is on the mechanisms leading to the merging of streamwise vortices in symmetric skewed shear layers.
• Gross, A., & Fasel, H. F. (2005). Numerical investigation of low-pressure turbine blade separation control. AIAA Journal, 43(12), 2514-2525.
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  Abstract: Laminar separation on the suction side of low-pressure turbine (LPT) blades at low operating Reynolds numbers can degrade overall engine efficiency and impose limitations on the flight envelope. In wind-tunnel experiments it was shown that laminar separation can be controlled by pulsed vortex generator jets. This active-flow-control technology could be transferred to real flight hardware with more confidence if the physical mechanisms involved in the control were better understood. Here, calculations of a linear LPT cascade at a Reynolds number based on axial chord of 2.5 × 104 are presented and compared to experimental data. Good agreement was observed between numerical and experimental results, except in the separated region near the trailing edge. In two-dimensional calculations separation was controlled by pulsed blowing through a slot upstream of the flow separation location. The blade pitch was then increased by 25% to obtain a larger region of separated flow. Again, using pulsed blowing through a slot, the separation could be controlled, and an increase of 19% in the time-averaged ratio of lift and drag was achieved.
• Gross, A., & Fasel, H. F. (2005). Turbulence modeling for low pressure turbine blades. 35th AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: Laminar flow separation from Low Pressure Turbine (LPT) blades at low operating Reynolds numbers can limit the operating envelope of jet engines. After separating from the suction side of the blade, the flow transitions to turbulence and may or may not reattach. Many intriguing questions about the interaction between separation and transition still remain unanswered, complicating any turbulence modeling effort. When the Reynolds number is low enough, the flow lends itself to Direct Numerical Simulations (DNS). Two-dimensional Reynolds-Averaged Navier-Stokes (RANS) calculations promise to be a low cost alternative to DNS. They may also allow for computations at Reynolds numbers too high for DNS. Turbulence modeling of transitional flows is still an unresolved issue. In this paper, different turbulence models will be employed for predicting the separating and transitional flow on an LPT blade. In addition, results from an Implicit Large Eddy Simulation (ILES) and a Flow Simulation Methodology (FSM) simulation are presented. The numerical results are compared with experimental data. © 2005 by the author(s).
• Husmeier, F., S., C., & Fasel, H. F. (2005). Investigation of transition of supersonic boundary layers at mach 3 using DNS. 43rd AIAA Aerospace Sciences Meeting and Exhibit - Meeting Papers, 9337-9349.
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  Abstract: Several breakdown scenarios to turbulence of a supersonic flat-plate boundary layer with and without an adverse pressure gradient (APG) at Mach 3 are investigated using Direct Numerical Simulations (DNS). The effects of APG on an oblique breakdown and on "classical" fundamental and subharmonic breakdown mechanisms are studied. For the Mach number under investigation the boundary layer is most unstable with respect to a first mode three-dimensional wave according to Linear Stability Theory (LST). Therefore, "oblique" secondary instability mechanisms were investigated where both primary and secondary disturbance waves are three-dimensional. These breakdown mechanisms are scrutinized in order to gain insight with respect to their relevance for practical applications.
• Larignon, B., Wernz, S., Goldstein, D. B., & Fasel, H. F. (2005). Numerical investigation of vortex onset in supersonic Taylor-Couette flow. 43rd AIAA Aerospace Sciences Meeting and Exhibit - Meeting Papers, 12963-12973.
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  Abstract: The compressible flow between concentric cylinders, in the case where the inner cylinder is rotating and the outer one is at rest, has been computed using a pseudo-spectral/finite difference method. The objective of this research has been to explore the onset of turbulence in rarified supersonic gap flows for flywheel applications. Of particular interest is the impact of surface roughness on the development of Taylor vortices. For modeling the surface roughness, an immersed boundary method using an external force field was employed. The Navier-Stokes code was validated against results from the literature for a supersonic wide-gap flow. For validating the immersed boundary method, the rotor wall was replaced by a smooth offset wall modeled with an external force field. Very good agreement with the analytical solution for steady Couette flow was achieved. For reference, the growth of Taylor vortices in a narrow gap with smooth walls was studied first. Then, the external force field was used to create a riblet on the rotor in order to investigate the influence of the roughness of the rotor on the flow and more particularly on the onset of the instability in the gap. The most interesting feature is the early appearance of vortices for Taylor numbers where the flow with smooth walls is in the subcritical regime.
• Linnick, M. N., & Fasel, H. F. (2005). A high-order immersed interface method for simulating unsteady incompressible flows on irregular domains. Journal of Computational Physics, 204(1), 157-192.
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  Abstract: Immersed boundary methods and immersed interface methods are becoming increasingly popular for the computation of unsteady flows around complex geometries using a Cartesian grid. While good results, both qualitative and quantitative, have been obtained, most of the methods rely on low-order corrections to account for the immersed boundary. The objective of the present work is to present, as an alternative, a high-order modified immersed interface method for the 2D, unsteady, incompressible Navier-Stokes equations in stream function-vorticity formulation. The method employs an explicit fourth-order Runge-Kutta time integration scheme, fourth-order compact finite-differences for computation of spatial derivatives, and a nine-point, fourth-order compact discretization of the Poisson equation for computation of the stream function. Corrections to the finite difference schemes are used to maintain high formal accuracy at the immersed boundary, as confirmed by analytical tests. To validate the method in its application to incompressible flows, several physically relevant test cases are computed, including uniform flow past a circular cylinder and Tollmien - Schlichting waves in a boundary layer. © 2004 Elsevier Inc. All rights reserved.
• Sandberg, R. D., & Fasel, H. (2005). Direct numerical simulations of transitional supersonic base flows. 43rd AIAA Aerospace Sciences Meeting and Exhibit - Meeting Papers, 9569-9580.
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  Abstract: Investigations of transitional supersonic base flows at M = 2.46 using Direct Numerical Simulations (DNS) are discussed. As a consequence of flow instabilities, coherent structures evolve that have a profound impact on the global flow behavior. Simulations with various circumferential domain sizes are performed to investigate the effect of coherent structures associated with different azimuthal modes on the mean flow. It is shown that the mean base pressure increases when the most dominant modes are deliberately eliminated. Results are presented for Reynolds numbers between 30,000 and 100,000. The results obtained from the DNS at ReD = 100,000 are compared to experiments at ReD = 3,300,000. For these investigations, a high-order accurate compressible Navier-Stokes solver in cylindrical coordinates was developed.
• Terzi, D. V., Sandberg, R. D., Sivasubramanian, J., & Fasel, H. F. (2005). High accuracy DNS and les of high reynolds number, supersonic base flows and passive control of the near wake. Department of Defense High Performance Computing Modernization Program: Proceedings of the HPCMP Users Group Conference 2005, 2005, 119-127.
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  Abstract: Supersonic axisymmetric base flows are prototypical for flows behind projectiles and missiles, For these flows, drag reduction can be achieved by means of passive control of the near wake. Thereby, large (turbulent) coherent structures play a dominant role. The objective of the present investigation is to elucidate if and how successful passive flow control techniques modify these structures. To this end, first Direct Numerical Simulations (DNS) for a Reynolds number of ReD = 100,000 and Mach number of Ma =2.46 were performed using a high-order accurate and highly parallelized research code which was developed at the University of Arizona. Thereby, roughly 52 million grid points were employed. The DNS data serve to visualize typical structures of the unsteady flow field and to verify that the use of less computational costly RANS/LES methods is applicable for this flow. Two of these methods, the Flow Simulation Methodology (FSM) and Detached Eddy Simulations (DES), were then employed to investigate the supersonic base flow at ReD =3.3 × 106 and Ma = 2.46 using between 460,000 and seven million grid points. For the DES, the commercial CFD-code Cobalt was employed. This unstructured grid solver allowed then to perform simulations with boat-tailing. The obtained mean flow data are compared to available experimental results. © 2005 IEEE.
• Valsecchi, P., Terzi, D. V., & Fasel, H. (2005). Numerical investigation of shear-layer instabilities using temporal and spatial DNS. 43rd AIAA Aerospace Sciences Meeting and Exhibit - Meeting Papers, 13007-13017.
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  Abstract: A comparative investigation of shear-layer instability mechanisms in wall-bounded flows is performed by employing both spatial and temporal direct numerical simulations. In this study, two complex flows are considered, the transitional backward-facing step flow and the turbulent wall-jet over plane and curved surfaces (Coanda flow). For the temporal simulations, (mean) profiles that are obtained from the spatial simulations are used as base flows. Temporal simulations neglect any spatial evolution of both the base flow and the disturbances and can therefore be used to investigate the local stability behavior. First the primary Kelvin-Helmholtz type instability of the shear layer is addressed. For secondary instabilities (subharmonic and fundamental), the investigation aims to determine the dominant instability mechanisms responsible for the occurrence of the typical flow structures that are observed in the spatial simulations. The complimentary nature of the temporal simulations provides crucial information for the interpretation of the spatial simulation results.
• Wernz, S., Gross, A., & Fasel, H. F. (2005). Numerical investigation of coherent structures in plane and curved wall jets. 35th AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: Turbulent wall jets over convex surfaces (Coanda flows) and turbulent plane wall jets are investigated using 3-D Navier-Stokes simulations. By comparing the two flows, the effect of wall-curvature on the development of coherent structures that are present in the flow field can be isolated. These coherent structures, namely large spanwise "rollers" and longitudinal G̈ortler-type vortices, are likely to influence the development of the Coanda wall jet along the surface of a cylinder and its eventual separation from the cylinder surface. The simulations show that both types of vortical structures are naturally amplified in the Coanda flow. When forced, the longitudinal structures reach amplitudes large enough to suppress the formation of spanwise rollers. In contrast, for the plane wall jet, longitudinal vortices form only in response to forcing and they exhibit a weaker growth in downstream direction. As a consequence, they are unable to prevent the spanwise rollers from developing. This indicates that the G̈ortler-type centrifugal instability does contribute to the growth of the longitudinal structures in the Coanda flow. The flow fields are analyzed using Proper Orthogonal Decomposition (POD) to extract time-dependent coherent structures from the flow. Progress in the development of the Flow Simulation Methodology (FSM) for computing the entire Coanda flow geometry is also discussed. © 2005 by the author(s).
• Gross, A., & Fasel, H. F. (2004). Active control of separation for low-pressure turbine blades. 2nd AIAA Flow Control Conference.
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  Abstract: At low Reynolds number conditions the flow over Low-Pressure Turbine (LPT) blades can become susceptible to laminar separation. In general, laminar separation results in high losses of aerodynamic efficiency and has to be avoided. In experiments performed at the Air Force Research Laboratory, separation on the suction side of a LPT blade was successfully controlled by application of pulsed Vortex Generator Jets (VGJs). A transfer of VGJs to real flight vehicles may allow for wider flight envelopes or more aggressive designs. We have carried out numerical simulations in order to shed some light on the physical mechanisms involved in Active Flow Control (AFC) of laminar LPT separation. The flow in the LPT cascade of the experiments was simulated at the design blade spacing and at a 25% larger blade spacing. From 2-D calculations we could demonstrate that the separation could be controlled successfully by pulsed blowing through a slot. The effectiveness of pulsed blowing could be attributed to the generation of strong spanwise coherent structures that increased the wall normal momentum transfer. © 2004 by the authors.
• Gross, A., & Fasel, H. F. (2004). Numerical investigation of streamwise coherent structures in a turbulent coanda wall jet. 34th AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: Coanda wall jets are obtained by tangential blowing over convex surfaces. For sufficiently large jet velocities, the flow quickly transitions to turbulence. Turbulent Coanda wall jets remain attached to the surface for relatively large distances downstream of the nozzle. This can be exploited for technical applications, such as the tail rotor less NOTAR helicopter or for circulation control (augmentation) of airfoils. The dynamics of the turbulent flow structures are oi interest since they are likely to influence the jet properties (jet spreading and associated jet velocity decay and ultimately jet separation location). Of particular interest here are the energetic large scale coherent structures that significantly contribute to the wall normal mixing and hence the jet spreading. These structures appear to be the consequence of instabilities of the turbulent mean flow. The turbulent mean velocity profile of the wall jet has an inflection point, thus giving rise to a shear layer instability, and as a consequence spanwise coherent structures develop that travel in the downstream direction. The outer layer of the wall jet is unstable with respect to a Gortler-type centrifugal instability mechanism leading to streamwise coherent structures. In the present paper results from steady Reynolds Averaged Navier-Stokes (RANS) calculations and unsteady simulations based on our Flow Simulation Methodology (FSM) of a turbulent Coanda wall jet are presented. The k-uj turbulence model and an Explicit Algebraic Stress Model (EASM) were employed. Using steady RANS calculations, the amplification of streamwise coherent structures was investigated. The streamwise structures were forced at both linear and non-linear amplitudes. At small, linear disturbance amplitudes the disturbance amplitude growth rates were consistent with Linear Stability Theory (LST) results. Resonance mechanisms appeared to play a role when the streamwise structures were forced with larger, non-linear amplitudes. The dynamics of the unsteady flow structures were explored in time accurate FSM simulations. When the spanwise extent of the computational domain was made deliberately very narrow the streamwise structures could be suppressed. As a consequence of this, the spanwise structures gained in strength, leading to an earlier separation of the wall jet. © 2004 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc.
• Israel, D. M., Postl, D., & Fasel, H. F. (2004). A flow simulation methodology for analysis of coherent structures and flow control. 2nd AIAA Flow Control Conference.
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  Abstract: The Flow Simulation Methodology (FSM) is evaluated for the case of a separated flow subject to control by oscillatory forcing. The geometry chosen is the hump geometry from the NASA Langley CFD Validation Workshop 2004. In addition to the FSM, Direct Numerical Simulation (DNS) results are also presented. Simulation data for the uncontrolled flow, control by steady suction, and control by unsteady forcing all agree very well with the experimental data for both DNS and FSM. In addition, the ability of the FSM to produce accurate results over a wide variety of model Alter widths is demonstrated. Finally, the relative strengths of the two approaches (FSM and DNS) are compared. © 2004 by the American Institute of Aeronautics
• Postl, D., Gross, A., & Fasel, H. F. (2004). Numerical investigation of active flow control for low-pressure turbine blade separation. AIAA Paper, 8666-8678.
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  Abstract: In the present paper, active flow control (AFC) for low-pressure turbine (LPT) applications is investigated using two separate computational approaches. In the first approach, a boundary layer on a flat plate is subjected to the same streamwise pressure gradient as measured on the suction side of a Pak-B LPT blade at a Reynolds number of 25,000 (based on axial chord). The relevant mechanisms for AFC using steady and pulsed vortex generator jets (VGJs) are investigated by direct numerical simulations. Results indicate two main mechanisms associated with VGJ control: free stream momentum entrainment due to streamwise structures for steady, angled VGJs and early (by-pass) boundary layer transition for pulsed VGJs. Two-dimensional simulations show that the natural instability of the separated shear layer can effectively be exploited for the purpose of flow control. In the second approach, the entire flow through a linear LPT cascade is investigated. Both two- and three-dimensional simulations are presented and compared to experimental data. At a Reynolds number of 25,000 and a blade spacing of 88% axial chord, good agreement is observed between numerical and experimental results, except in the separated region near the trailing edge. The separated flow through a LPT cascade with a blade spacing of 110% axial chord is controlled using pulsed blowing through a slot upstream of the separation location. Results indicate that with AFC, an increase of 30% in the time-averaged ratio of lift over drag can be achieved.
• Sandberg, R. D., & Fasel, H. F. (2004). Application of a new flow simulation methodology for supersonic axisymmetric wakes. AIAA Paper, 4105-4114.
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  Abstract: The Flow Simulation Methodology (FSM) is employed to investigate the wake behind axisymmetric bodies with a blunt base at supersonic speeds. The centerpiece of the FSM is a strategy to provide the proper amount of modelling of the subgrid scales. This is accomplished by a "contribution function" which locally and Instantaneously compares the smallest relevant scales to the local grid size. The underlying compressible Navler-Stokes code in cylindrical coordinates employs high-order accurate finite differences and a high-order accurate axis treatment. The code also Incorporates fully three-dimensional transport equations for turbulent kinetic energy and turbulent dissipation including compressible extensions and a state-of-the-art Reynolds stress model. Axisymmetric RANS calculations were performed for wakes at Re D = 60,000 and M = 2.46, employing both the standard K-ε model (STKE) and an explicit Algebraic Stress Model (EASM α) in order to evaluate the performance of each. Results of FSM calculations using the EASM α are compared to DNS results and to steady RANS calculations.
• Sandberg, R. D., & Fasel, H. F. (2004). High-accuracy DNS of supersonic base flows and control of the near wake. Proceedings - Department of Defense High Performance Computing Modernization Program Users Group Conference, UGC 2004, 80-88.
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  Abstract: Large-scale numerical simulations of axisymmetric, supersonic base flows were conducted at various Reynolds numbers. Direct Numerical Simulations (DNS) were employed to investigate the hydrodynamic stability behavior of the near-wake region. As a consequence of physical flow instabilities, large coherent structures evolve that have a significant impact on the mean flow wand and are responsible for a considerable amount of base-drag. It is demonstrated that the deliberate exclusion or reinforcement of certain helical modes can lead to a rise in bas -pressure and thus decrease the drag of a blunt body at supersonic speed. For these investigations, a high-order accurate compressible Navier-Stokes solver in cylindrical coordinates with high parallel efficiency was developed and employed on the SGI Origin 3900 shared memory complex at the ERDC MSRC. In addition to providing vital insight into the physical mechanisms in supersonic base flows, the DNS results are intended for use as benchmark data for the development of a Flow Simulation Methodology (FSM) for high Reynolds number turbulent flows. © 2004 IEEE.
• Sandberg, R. D., & Fasel, H. F. (2004). Instability mechanisms in supersonic base flows. AIAA Paper, 5803-5814.
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  Abstract: The hydrodynamic stability behavior of the near-wake is investigated using Direct Numerical Simulations (DNS) for a supersonic axisymmetric baseflow at M=2.46. As a consequence of the instability, coherent structures evolve that influence the mean flow behavior. Results are presented for two different Reynolds numbers, Re D = 30, 000 and Re D = 60,000. For these investigations, a high-order accurate compressible Navier-Stokes solver in cylindrical coordinates was developed. Simulations with various domain-sizes were performed to investigate the instability behavior to different azimuthal modes.
• Wernz, S., & Fasel, H. F. (2004). Numerical investigation of forced Coanda wall jets. 34th AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: Forced wall jets over a convex wall (Coanda flows) are being investigated using 3D Navier-Stokes simulations. The focus of our numerical investigations is to study the development and interaction of large coherent structures in the turbulent Coanda flow. These coherent structures, namely large spanwise "rollers" and longitudinal Gortler-type vortices, can greatly influence the development of the Coanda wall jet along the surface of a cylinder and its eventual separation from the cylinder surface. In our numerical simulations of the Coanda flow over a cylinder segment, selective forcing is employed to either enhance or suppress both types of coherent structures. The effect of the selective forcing on the spreading and decay rate of the jet is then studied. It is found that both forcing of the spanwise structures and forcing of the streamwise structures increases the streamwise velocity decay of the wall jet, while only the forcing of spanwise structures significantly enhances the spreading rate. © 2004 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc.
• Wernz, S., & Fasel, H. F. (2004). Numerical investigation of instability mechanisms in laminar and turbulent wall jets. AIAA Paper, 7008-7017.
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  Abstract: Secondary instability in forced laminar and turbulent wall jets is investigated using two-dimensional Navier-Stokes simulations. For modeling the turbulent flow, the Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach is employed in which the turbulent mean flow and the time-dependent large coherent structures are computed time-accurately while the effect of small-scale random turbulent motion is modeled using an Explicit Algebraic Stress Model (EASM). Several parameter studies are presented indicating that subharmonic resonance may be the cause for the vortex pairing observed in experiments of forced and unforced wall jets.
• Fasel, H. F., A., D., & Sandberg, R. D. (2003). A methodology for simulating compressible turbulent flows. Proceedings of the ASME/JSME Joint Fluids Engineering Conference, 1 C, 1887-1894.
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  Abstract: A Flow Simulation Methodology (FSM) is presented for computing the time-dependent behavior of complex compressible turbulent flows. The development of FSM was initiated in close collaboration with C. Speziale (then at Boston University). The objective of FSM is to provide the proper amount of turbulence modelling for the unresolved scales while directly computing the largest scales. The strategy is implemented by using state-of-the-art turbulence models (as developed for RANS) and scaling of the model terms with a "contribution function". The contribution function is dependent on the local and instantaneous "physical" resolution in the computation. This "physical" resolution is determined during the actual simulation by comparing the size of the smallest relevant scales to the local grid size used in the computation. The contribution function is designed such that it provides no modelling if the computation is locally well resolved so that it approaches a DNS in the fine-grid limit and such that it provides modelling of all scales in the coarse-grid limit and thus approaches an unsteady RANS calculation. In between these resolution limits, the contribution function adjusts the necessary modelling for the unresolved scales while the larger (resolved) scales are computed as in traditional LES. However, FSM is distinctly different from LES in that it allows for a consistent transition between (unsteady) RANS, LES, and DNS within the same simulation depending on the local flow behavior and "physical" resolution. As a consequence, FSM should require considerably fewer grid points for a given calculation than would be necessary for a traditional LES. This conjecture is substantiated by employing FSM to calculate the flow over a backward-facing step at low Mach number and a supersonic, axisymmetric baseflow. These examples were chosen such that they expose, on the one hand, the inherent difficulties of simulating (physically) complex flows, and, on the other hand, demonstrate the potential of the FSM approach for a wide range of compressible flows.
• Linnick, M. N., & Fasel, H. F. (2003). A high-order immersed boundary method for unsteady incompressible flow calculations. 41st Aerospace Sciences Meeting and Exhibit.
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  Abstract: Immersed boundary methods are becoming increasingly popular for the computation of unsteady flows around complex geometries using a Cartesian grid. While good results, both qualitative and quantitative, have been obtained, many of the methods rely on low-order corrections to account for the immersed boundary. The objective of the present work is to present a high-order immersed boundary method for the 2-D, unsteady, incompressible Navier-Stokes equations in stream function-vorticity formulation. The method employs an explicit Runge-Kutta (second or fourth order) time integration scheme, fourth-order compact finite-differences for computation of spatial derivatives, and a nine-point, fourth-order compact discretization of the Poisson equation for computation of the stream function. Corrections to the finite-difference stencils are used to maintain high formal accuracy at the immersed boundary, as confirmed by analytical tests. To validate the method in its application to incompressible flows, several physically relevant test cases are computed, including uniform flow past a circular cylinder and Tollmien-Schlichting waves in a boundary layer. © 2003 by Mark N. Linnick.
• Postl, D., Gross, A., & Fasel, H. F. (2003). Numerical investigation of low-pressure turbine blade separation control. 41st Aerospace Sciences Meeting and Exhibit.
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  Abstract: Separation control mechanisms in low-pressure turbine (LPT) applications are investigated numerically. Two-dimensional simulations of boundary layer separation on the suction side of a turbine blade in a linear cascade are presented and compared to experimental data. Active flow control by means of pulsed blowing is employed and its impact on the separation behavior is discussed. Fundamental separation control mechanisms associated with steady and pulsed vortex generator jets (VGJs) are investigated by direct numerical simulations of a separating boundary layer under conditions comparable to those in low Reynolds number LPT applications. The vortex generator jets are employed in two configurations, vertical as well as angled with respect to the freestream direction. Our results confirm experimental findings that pulsed jet blowing is more effective than steady jet blowing. In addition, pulsed VGJs that are injected vertically are shown to achieve the greatest reduction in the extent of boundary layer separation. © 2003 by the authors Published by the American Institute of Aeronautics and Astronautics, Inc.
• Sandberg, E. D., & Fasel, H. F. (2003). A flow simulation methodology for compressible turbulent axisymmetric wakes. 41st Aerospace Sciences Meeting and Exhibit.
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  Abstract: A new Flow Simulation Methodology (FSM) is employed to investigate the wake behind axisymmetric bodies with a blunt base at supersonic speeds. The centerpiece of the FSM is a strategy to provide the proper amount of modelling of the subgrid scales. This is accomplished by a "contribution function" which locally and instantaneously compares the smallest relevant scales to the local grid size. The underlying compressible Navier-Stokes code in cylindrical coordinates employs high-order accurate finite differences and a high-order accurate axis treatment. The code also incorporates fully three-dimensional transport equations for turbulent kinetic energy and turbulent dissipation including compressible extensions and a state-of-the-art Reynolds stress model. FSM calculations are performed for M = 2.46 and ReD = 30,000 and are compared to DNS data obtained with essentially the same code. In addition, the FSM calculations are compared to steady RANS calculations using the standard K - ε model (STKE) and an explicit Algebraic Stress Model (ASM). Preliminary results for axisymmetric RANS calculations at M = 2.46 and ReD = 3,300,000 are shown. For this Reynolds number, the focus was on testing the turbulence model that would later be used for FSM calculations. © 2003 by the authors.
• Wernz, S., Valsecchi, P., Groß, A., & Fasel, H. F. (2003). Numerical investigation of turbulent wall jets over a convex surface. 33rd AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: In a numerical effort, wall jets over a convex wall (Coanda flows) are being investigated using direct numerical simulations (DNS) based on the spatial as well as on the temporal model. The focus of our numerical investigations is the development and interaction of large coherent structures in the turbulent Coanda flow. As observed in experiments, large spanwise "rollers" and longitudinal Gortler-type vortices can greatly influence the development of the jet along the surface and its eventual separation from the surface. From our spatial DNS of the Coanda flow over a cylinder segment, we found that both types of coherent structures can coexist. The presenence of these structures was also confirmed by preliminary temporal DNS of a narrow streamwise flow segment in the Coanda flow. The temporal DNS is capable of capturing coherent structures that are generated by local flow instabilities at significantly reduced computational expense. © 2003 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc.
• Fasel, H. F. (2002). Numerical investigation of the interaction of the Klebanoff-mode with a Tollmien-Schlichting wave. Journal of Fluid Mechanics, 450, 1-33.
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  Abstract: Direct numerical simulations (DNS) of the Navier-Stokes equations are used to investigate the role of the Klebanoff-mode in laminar-turbulent transition in a flat-plate boundary layer. To model the effects of free-stream turbulence, volume forces are used to generate low-frequency streamwise vortices outside the boundary layer. A suction/blowing slot at the wall is used to generate a two-dimensional Tollmien-Schlichting (TS) wave inside the boundary layer. The characteristics of the fluctuations inside the boundary layer agree very well with those measured in experiments. It is shown how the interaction of the Klebanoff-mode with the two-dimensional TS-wave leads to the formation of three-dimensional TS-wavepackets. When the disturbance amplitudes reach a critical level, a fundamental resonance-type secondary instability causes the breakdown of the TS-wavepackets into turbulent spots.
• Fasel, H. F., Seidel, J., & Wernz, S. (2002). A methodology for simulations of complex turbulent flows. Journal of Fluids Engineering, Transactions of the ASME, 124(4), 933-942.
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  Abstract: The fundamental aspects of the flow simulation methodology (FSM) for computing turbulent shear flow were analyzed. It was found that the turbulent flow physics depends on the interaction between outer coherent structures and near-wall boundary layer. The centerpiece of FSM was to provide proper amount of modeling of the subgrid scales. The results show the potential of FSM for calculating complex turbulent flows.
• Israel, D. M., & Fasel, H. F. (2002). Numerical investigation of turbulent separation control using periodic disturbances. 40th AIAA Aerospace Sciences Meeting and Exhibit.
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  Abstract: The new Flow Simulation Methodology (FSM) proposed by Speziale (1997) is applied to the separation control experiments of Seifert and Pack (1999). By smoothly ramping between RANS and LES based on the local flow conditions, the FSM allows simulations which capture the unsteady nature of the shear layer without requiring LES resolution in the boundary layer. This makes the FSM ideal for investigations of Active Flow Control (AFC). In the experiments a model simulating the upper surface of a 20% thick airfoil was mounted on the side of a wind tunnel. The flow over this airfoil separates at ~ 64% chord if no control is applied. This model was used to investigate sweep and compressibility effects over a range of Reynolds numbers. The goal of our ongoing research is to implement and validate the FSM in an high-order accurate CFD code capable of simulating these experiments. In this paper we demonstrate the FSM for the experimental geometry with a fully turbulent boundary layer at the domain inflow. The calculations show that the FSM does allow the formation of unsteady structures in the separated region. © 2002 by American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Israel, D. M., Fasse, E. D., & Fasel, H. F. (2002). Numerical simulation of closed loop active flow control of separation. 1st Flow Control Conference.
• Terzi, D. v., & Fasel, H. F. (2002). A new flow simulation methodology applied to the turbulent backward-facing step. 40th AIAA Aerospace Sciences Meeting and Exhibit.
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  Abstract: For the flow simulation methodology (FSM) of Speziale and Fasel, turbulent stresses and heat fluxes are computed using a state-of-the-art RANS turbulence model, but where the model contribution is scaled according to a contribution function which depends on the physical resolution of the computation. In flow regions (and instants of time) with high physical resolution only the smallest (unresolved) scales of motion are modelled while the larger structures are computed directly (similar to LES), whereas in regions of very coarse resolution an unsteady RANS simulation is recovered. The methodology thus may be classified as a RANS-LES hybrid or better, a unified approach to RANS and LES. In order to evaluate the potential of this methodology, it is applied to the unsteady flow over a backward-facing step at ReH = 3000 and Ma = 0.25, with laminar separation and turbulent reattachment. The results are compared to DNS, LES, and unsteady RANS. © 2002 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Wernz, S., Seidel, J., & Fasel, H. F. (2002). Numerical investigation of turbulent wall jets. 40th AIAA Aerospace Sciences Meeting and Exhibit.
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  Abstract: The evolution of large scale, coherent structures in turbulent wall jets is investigated numerically using our Flow Simulation Methodology (FSM). Previous experimental research has shown that these structures significantly change the mean Reynolds shear stress distribution and also the skin friction. The combination of the coherent motion in the shear layer region of the wall jet and the turbulent boundary layer underneath pose a unique challenge for turbulent simulations. We use FSM to investigate the effect of these large coherent structures, both in the unsteady Reynolds Averaged Navier-Stokes (URANS) limit and in the Large Eddy Simulations (LES) range. In addition, Direct Numerical Simulations (DNS) will be used to show the emergence of large structures with strong spanwise coherence during wall jet transition. The computational results are scrutinized to investigate the applicability and performance of FSM for this complex turbulent flow. © 2002 by American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Israel, D. M., & Fasel, H. F. (2001). Numerical investigation of compressibility effects on active control of boundary layer separation. 15th AIAA Computational Fluid Dynamics Conference.
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  Abstract: Active flow control (AFC) as a method of separation control was investigated for laminar flow over a curved wall. Experimental research has demonstrated that AFC using periodic suction and blowing is an extremely efficient way to suppress or weaken separation. However, there is a lack of accurate computational data for this flow. We applied CFD to investigate the experimental geometry of Seifert and Pack (1999), which consists of the top surface of an airfoil mounted on the side wall of the 0.3-meter cryogenic wind tunnel at NASA Langley Research Center. We used an high order finite-difference code to simulate laminar flow at a Reynolds number of Rec = 104 and Mach numbers of 0.25 and 0.65. The effects of forcing frequency and amplitude were investigated, as well as effects of compressibility. © 2001 by American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Seidel, J., & Fasel, H. F. (2001). Numerical investigations of heat transfer mechanisms in the forced laminar wall jet. Journal of Fluid Mechanics, 442, 191-215.
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  Abstract: The effect of high-amplitude forcing on a laminar wall jet over a heated flat plate is analysed. Highly accurate direct numerical simulations (DNS) are used to investigate the dominant transport mechanisms. When forcing is applied, the skin friction is reduced markedly and the wall heat transfer is increased. Detailed examination of the unsteady flow field showed that the concepts of eddy viscosity and eddy thermal diffusivity, usually applied to turbulent flows, can be applied to the analysis of unsteady laminar flows to explain the effect of highly unsteady phenomena.
• Siegel, S. G., & Fasel, H. F. (2001). Effect of forcing on the wake drag of an axisymmetric bluff body. 39th Aerospace Sciences Meeting and Exhibit.
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  Abstract: The wake of an axisymmetric bluff body was investigated using water tunnel experiments. The parameters common to all investigations were a Reynolds number of 1000 or 1500 based on the body diameter, and a boundary layer thickness entering the body base of 30 % of the base diameter. Harmonic forcing was accomplished using eight individual piston pump actuators providing blowing and suction disturbances into the boundary layer close to the body base. The flow field was evaluated using flow visualization, single wire hot film anemometry, and direct drag force measurements. Due to the low velocities and small model cross sectional areas, the resulting drag forces are difficult to measure. A high precision balance was developed to measure the drag force. The balance is able to measure forces in the Millinewton range with Micronewton resolution. While the ±1 modes are dominant in the natural wake, the associated vortices are nonperiodic in time and irregular in size and orientation. When forcing the ±1 modes it was possible to lock their frequency, phase and orientation to the forcing over a relatively large frequency range. Within the lock-in range, the wake drag increased by up to 40 %. The mean flow of the wake was axisymmetric. © 2001 by Stefan G. Siegel.
• Terzit, D. v., Linnickf, M. N., Seidelf, J., & Fasel, H. F. (2001). Immersed boundary techniques for high-order finite-difference methods. 15th AIAA Computational Fluid Dynamics Conference.
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  Abstract: The use of immersed boundary techniques for Cartesian grid methods is becoming increasingly popular for predicting flows with complex geometries. It has been demonstrated that for cases where accuracy near immersed boundaries is not crucial, existing methods are sufficiently accurate. However, if near-wall accuracy is paramount, immersed boundary techniques for high-order methods can not be used without corrections. This assertion is corroborated by computing the flow over a backward-facing step at low Reynolds number and for Tollmien-Schlichting waves in a flat-plate boundary layer using second-order and fourth-order finite-difference methods. The immersed boundary technique is extended to the compressible Navier- Stokes equations, and both the compressible and incompressible Navier-Stokes equations are employed to evaluate immersed boundary techniques. In the compressible code, the same difference operator is applied to all derivatives computed, thus more clearly demonstrating the effects of the various corrections for the immersed boundary technique. The methods of Goldstein et al. [1] and Mohd- Yusof [2] serve as prototypical immersed boundary techniques. A correction method is suggested which eliminates the need for ad hoc adjustments and allows for the efficient use of high order finite difference methods. Its performance is compared to other recently published methods. © 2001 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Meitz, H. L., & Fasel, H. F. (2000). A compact-difference scheme for the navier-stokes equations in vorticity-velocity formulation. Journal of Computational Physics, 157(1), 371-403.
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  Abstract: This paper presents a new numerical method for solving the incompressible, unsteady Navier-Stokes equations in vorticity-velocity formulation. The method is applicable to spatial simulations of transitional and turbulent boundary layer flows. It is based on a compact-difference discretization of the streamwise and wall-normal derivatives in Cartesian coordinates. A Fourier collocation approach is used for the spanwise derivatives. Important new features of the numerical method are the use of nonequidistant differences in the wall-normal direction; the use of split-compact differences in the streamwise direction; a new, fast iteration for a semi-implicit time integration of the wall-normal diffusion terms; and an improvement of the buffer domain technique to prevent reflections of waves at the outflow boundary. Results of test calculations are presented to verify the improvements obtained by the use of these new techniques. © 2000 Academic Press.
• Zhang, H. L., Bachman, C. R., & Fasel, H. F. (2000). Reynolds-averaged Navier-Stokes calculations of unsteady turbulent flow. 38th Aerospace Sciences Meeting and Exhibit.
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  Abstract: In this study, a combination of the unsteady incompressible Navier-Stokes equations in vorticityvelocity formulation and the Algebraic Stress Model (ASM) of Gatski and Speziale (1996) is employed for Unsteady Reynolds Averaged Navier-Stokes (URANS) calculations of turbulent boundary layer flows. The Navier-Stokes equations are solved using a fourth-order compact difference scheme in space and a fourth-order Runge-Kutta method in time. The highly accurate numerical method greatly reduces the possibility of contamination of the results by second-order artificial dissipation from the numerical schemes. A flat plate boundary layer subjected to a strong adverse pressure gradient with laminar separation and turbulent reattachment is investigated. Performing URANS calculations for this Aow, we found that unsteady vertical structures remain in the flow field despite the large "effective eddy viscosity" produced by the turbulence model (ASM). This is due to the fact that a special function is used in this turbulence model such that the eddy viscosity is strongly coupled with the unsteady flow structures. For comparison, URANS calculations were also carried out employing the standard k - ∈ model, where in contrast no unsteady vertical structures were found in the flow field. For further comparison, results from 2-D "Direct Numerical Simulation (DNS)" and 3-D Large-Eddy Simulation (LES) using the standard Smagorinski model are also presented and discussed. © 1998 by American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Dratler, D. I., & Fasel, H. F. (1996). Spatial evolution of a monochromatically forced flat-plate wake. AIAA Journal, 34(11), 2299-2305.
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  Abstract: Nonlinear disturbance development in a two-dimensional, incompressible, spatially developing wake behind a thin flat plate aligned parallel to a uniform freestream is investigated by direct numerical integration of the Navier-Stokes equations. For the numerical integration, finite difference methods together with an alternating direction implicit/Adams-Bashforth time integration scheme are employed. The wake is harmonically forced at the inflow boundary at the frequency of maximum amplification predicted by linear stability theory. The response to this monochromatic forcing includes a disturbance component at the forcing frequency that grows very rapidly before saturating a short distance downstream. This saturation can be predicted qualitatively from a linear stability analysis of the distorted mean flow. Farther downstream, the disturbance energy is concentrated in the fundamental disturbance, the second harmonic, and the mean-flow distortion component. At large amplitudes, a Kármán vortex street forms. Variations in forcing strength do not alter the qualitative behavior of the forced wake. The results of these simulations compare well with both linear stability theory and experimental measurements.
• Rist, U., & Fasel, H. (1995). Direct numerical simulation of controlled transition in a flat-plate boundary layer. Journal of Fluid Mechanics, 298, 211-248.
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  Abstract: The three-dimensional development of controlled transition in a flat-plate boundary layer is investigated by direct numerical simulation (DNS) using the complete Navier-Stokes equations. The numerical investigations are based on the so-called spatial model, thus allowing realistic simulations of spatially developing transition phenomena as observed in laboratory experiments. For solving the Navier-Stokes equations, an efficient and accurate numerical method was developed employing fourth-order finite differences in the downstream and wall-normal directions and treating the spanwise direction pseudo-spectrally. The present paper focuses on direct simulations of the wind-tunnel experiments by Kachanov et al. (1984, 1985) of fundamental breakdown in controlled transition. The numerical results agreed very well with the experimental measurements up to the second spike stage, in spite of relatively coarse spanwise resolution. Detailed analysis of the numerical data allowed identification of the essential breakdown mechanisms. In particular, from our numerical data, we could identify the dominant shear layers and vortical structures that are associated with this breakdown process.
• Kral, L. D., & Fasel, H. F. (1994). Direct numerical simulation of passive control of three-dimensional phenomena in boundary-layer transition using wall heating. Journal of Fluid Mechanics, 264, 213-254.
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  Abstract: A numerical model is presented for investigating control of the three-dimensional boundary-layer transition process. Control of a periodically forced, spatially evolving boundary layer in water is studied using surface heating techniques. The Navier-Stokes and energy equations are integrated using a fully implicit finite difference/spectral method. The Navier-Stokes equations are used in vorticity-velocity form and are coupled with the energy equation through the viscosity dependence on temperature. Passive control of small amplitude two-dimensional waves and three-dimensional oblique waves is numerically simulated with either uniform or non-uniform wall heating applied. Both amplitude levels and amplification rates are strongly reduced with heating applied. Comparison is made with parallel and non-parallel linear stability theory and experiments. Control of the early stages of the nonlinear breakdown process is also investigated using uniform wall heating. Both control of the fundamental and subharmonic routes to turbulence are investigated. For both breakdown processes, a strong reduction in amplitude levels and growth rates results. In particular, the high three-dimensional growth rates that are characteristic of the secondary instability process are significantly reduced below the uncontrolled levels.
• Rempfer, D., & Fasel, H. F. (1994). Dynamics of three-dimensional coherent structures in a flat-plate boundary layer. Journal of Fluid Mechanics, 275, 257-283.
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  Abstract: An investigation is presented that analyses the energy flows that are connected to the dynamical behaviour of coherent structures in a transitional flat-plate boundary layer. Based on a mathematical description of the three-dimensional coherent structures of this flow as provided by the Karhunen-Loeve procedure, energy equations for the coherent structures are derived by Galerkin projection of the Navier-Stokes equations in vorticity transport formulation onto the corresponding basis of eigenfunctions. In a first step, the time-averaged energy balance - showing the energy flows that support the different coherent structures and thus maintain the fluctuations of the velocity field - is considered. In a second step, the instantaneous power budget is investigated for the particularly interesting case of a coherent structure providing a prime contribution to the characteristic spike events of the transitional boundary layer. As this structure shows a strong variation in energy, the question about which mechanisms cause these variations is addressed. Our results show that the occurrence of a spike must be attributed to an autonomous event and cannot be interpreted as just an epiphenomenon of the passage of a Λ-vortex.
• Rempfer, D., & Fasel, H. F. (1994). Evolution of three-dimensional coherent structures in a flat-plate boundary layer. Journal of Fluid Mechanics, 260, 351-375.
• Fasel, H., Thumm, A., & Bestek, H. (1993). Direct numerical simulation of transition in supersonic boundary layers: oblique breakdown. American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FED, 151, 77-92.
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  Abstract: Laminar-turbulent transition phenomena in compressible flat-plate boundary layers are investigated by direct numerical simulations using the complete Navier-Stokes equations for three-dimensional compressible flows. The numerical method allows for investigations of spatially developing, three-dimensional disturbance waves in a growing two-dimensional boundary layer. In this paper, results from simulations for two different types of breakdown to turbulence in a flat-plate boundary layer at M∞ =1.6 are presented.
• Kloker, M., Konzelmann, U., & Fasel, H. (1993). Outflow boundary conditions for spatial Navier-Stokes simulations of transition boundary layers. AIAA journal, 31(4), 620-628.
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  Abstract: For numerical simulations of the spatially evolving laminar-turbulent transition process in boundary layers using the complete Navier-Stokes equations, the treatment of the outflow boundary requires special attention. The disturbances must pass through this boundary without causing reflections that would significantly alter the flow upstream. In this paper, we present various methods to influence the disturbed flow downstream of the region of interest, such that the disturbance level at the outflow boundary is significantly reduced, and hence the possibility of reflections is minimized. To demonstrate the effectiveness of the various techniques to alter the disturbance flow near the outflow boundary, the fundamental breakdown of a strongly decelerated boundary layer is simulated. Our results show that the most effective method is to spatially suppress the disturbance vorticity within a so-called 'relaminarization zone.' The suppression of the disturbance vorticity is gradually imposed within this zone by means of a weighting function. The enforced decay of the disturbance vorticity leads to a practically complete dissipation of any fluctuating component. Most importantly, this technique causes only a negligible upstream effect. The 'relaminarized' boundary-layer flow then passes through the outflow boundary without significant reflections.
• Rempfer, D., & Fasel, H. (1993). The dynamics of coherent structures in a flat-plate boundary layer. Applied Scientific Research, 51(1-2), 73-77.
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  Abstract: From the data of a direct numerical simulation the three-dimensional coherent structures of an incompressible, spatially evolving flat-plate boundary layer have been calculated using the POD method. By Galerkin projection of the Navier-Stokes equations onto the corresponding system of eigenfunctions then a low-dimensional model of the flow in the form of a system of ODE's has been derived. In a region of the boundary layer just beyond the spike stages of transition this system displays deterministic chaos that has been quantified by determining its Lyapunov exponents. © 1993 Kluwer Academic Publishers.
• Kral, L. D., & Fasel, H. F. (1991). Numerical investigation of three-dimensional active control of boundary-layer transition. AIAA journal, 29(9), 1407-1417.
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  Abstract: A numerical model is developed for the investigation of boundary-layer transition control of spatially evolving instability waves. Active control of a periodically forced boundary layer in an incompressible fluid is simulated using surface heating techniques. The Navier-Stokes and energy equations are solved using a fully implicit finite-difference/spectral method. Temperature perturbations are introduced locally along finite heater strips to directly attenuate instability waves in the flow. A feedback control loop is employed in which a downstream sensor is used to monitor wall shear stress fluctuations. Active control of small-amplitude two-dimensional and three-dimensional disturbances is shown. Both wave reinforcement and wave attenuation are demonstrated. Active control of the early stages of the nonlinear fundamental breakdown process is also numerically investigated. The high three-dimensional growth rates that are characteristic of the secondary instability process are significantly reduced using either two-dimensional or three-dimensional control inputs to the heater strips. A receptivity study of the processes by which the localized temperature perturbations generate instability waves in the boundary layer is made. It is shown that the boundary layer is more receptive to narrow heater strips in that they maximize the amplitude level of the disturbances in the boundary layer.
• Fasel, H. F., Rist, U., & Konzelmann, U. (1990). Numerical investigation of the three-dimensional development in boundary-layer transition. AIAA journal, 28(1), 29-37.
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  Abstract: A numerical method for solving the complete Navier-Stokes equations for incompressible flows is introduced that is applicable for investigating three-dimensional transition phenomena in a spatially growing boundary layer. Results are discussed for a test case with small three-dimensional disturbances for which detailed comparison to linear stability theory is possible. The validity of our numerical model for investigating nonlinear transition phenomena is demonstrated by realistic spatial simulations of the experiments by Kachanov and Levchenko for a subharmonic resonance breakdown and of the experiments of Klebanoff et al. for a fundamental resonance breakdown.
• Laurien, E., & Fasel, H. (1988). NUMERICAL INVESTIGATION OF THE ONSET OF CHAOS IN THE FLOW BETWEEN ROTATING CYLINDERS.. Zeitschrift fur angewandte Mathematik und Mechanik, 68(5), t311-t313.
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  Abstract: The flow in the gap between concentric circular cylinders, the inner cylinder rotating, exhibits distinct unsteady stages after the steady Taylor-vortex flow is established. The flow can be either time periodic with one or two traveling waves superposed on this state or 'chaotic', i. e. nonperiodically oscillating in time. When the rotation speed OMEGA is increased from a stage with two traveling waves one of the waves disappears and chaos sets in. The authors investigate this process theoretically assuming cylinders of infinite length and periodic flow in axial and azimuthal directions for a small gap r//i/r//0 equals 0. 877, r//i and r//0 being the inner and outer cylinder radii. 5 Refs.
• Fasel, H. (1984). INVESTIGATION OF NONLINEAR TRANSITION MECHANISMS IN BOUNDARY-LAYER FLOWS BY NUMERICAL SOLUTION OF THE NAVIER-STOKES EQUATIONS.. Array, 170-175.
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  Abstract: The present investigations are based on a direct numerical approach, i. e. , bypassing the formulation of eigenvalue problems as is customary in hydrodynamic stability theory. Rather, a rectangular integration domain A-B-C-D is selected whereby the disturbances are introduced at the upstream boundary A-D. The reaction of the flow to the disturbances introduced at A-D is then determined by numerical solution of the complete Navier-Stokes equations for two-dimensional incompressible flow. Therefore, the downstream development of the disturbances can be studied. 6 refs.
• Fasel, H. (1984). NUMERICAL SIMULATION OF NONLINEAR GROWTH OF WAVE PACKETS IN A BOUNDARY LAYER.. Array, 31-37.
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  Abstract: The nonlinear amplification process of two-dimensional wave packets in a laminar, incompressible boundary layer on a flat plate is investigated numerically using the complete Navier-Stokes equations. The results of our calculations display several distinct nonlinear phenomena that are similar to those observed in experimental investigations of three-dimensional wave packets.
• Fasel, H., & Booz, O. (1984). NUMERICAL INVESTIGATION OF SUPERCRITICAL TAYLOR-VORTEX FLOW FOR A WIDE GAP.. Journal of Fluid Mechanics, 138, 21-52.
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  Abstract: For a wide gap and large aspect ratios, axisymmetric Taylor-vortex flow has been observed in experiments up to very high supercritical Taylor (or Reynolds) numbers. This axisymmetric Taylor-vortex flow was investigated numerically by solving the Navier-Stokes equations using a very accurate implicit finite-difference method. The emphasis of the present paper is placed upon displaying and elaborating the details of the flow field for large supercritical Taylor numbers. Spectral analysis of the flow variables indicates that the number of harmonics contributing substantially to the total solution increases sharply when the Taylor number is raised. In the axial plane, boundary layers develop along the inner and outer cylinder walls while the flow in the core region of the Taylor cells behaves in an increasingly inviscid manner. Refs.
• Williams, D. R., Fasel, H., & Hama, F. R. (1984). EXPERIMENTAL DETERMINATION OF THE THREE-DIMENSIONAL VORTICITY FIELD IN THE BOUNDARY-LAYER TRANSITION PROCESS.. Journal of Fluid Mechanics, 149, 179-203.
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  Abstract: The vortex loop observed in flow-visualization studies of boundary-layer transition has been investigated by mapping the instantaneous velocity and vorticity fields. All three velocity components have been measured with hot-film anemometers at numerous grid points in a measuring volume centered on the location where the vortex loop appears in flow-visualization studies. The instantaneous vorticity field has been computed from the velocity field, and the vortex loop is revealed in the longitudinal component of vorticity. The loop propagates downstream at approximately the primary disturbance wavespeed. The fluid in the outer part of the boundary layer travels faster, and flows over the loop. This forms the inflexional high-shear layer, which breaks down into the hairpin vortices. The magnitude of the vorticity in the high-shear layer is actually about three times larger than that in the loop. These two regions of vorticity are distinguished by the direction of the instantaneous vorticity vectors.
• Eppler, R., & Fasel, H. (1980). IUTAM Symposium on Laminar-Turbulent Transition held in Stuttgart, Fed. Rep. Germany on September 16-22, 1979.. Array.
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  Abstract: Individual papers are abstracted elsewhere.
• Eppler, R., & Fasel, H. (1980). IUTAM Symposium on Laminar-Turbulent Transition, held in Stuttgart, Fed. Rep. Germany on September 16-22, 1979.. Array.
• Fasel, H., & Bestele, H. (1980). Investigation of nonlinear, spatial disturbance amplification in plane Poiseuille flow.. Array, eds., Berlin, Fed. Rep. Germany, Springer-Verlag , 1980, Session 4, p.173-185..
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  Abstract: the numerical solution, are examined. (from authors abstract) Nonlinear effects of spatial disturbance amplification in plane Poiseuille flow are studied numerically by direct solution of the Navier-Stokes equations for two-dimensional, incompressible flows. Time-wise periodic disturbances are introduced at the inflow boundary of a rectangular integration domain. The reaction of the flow, i.e., the propagation of these disturbances in downstream direction, is calculated by use of an implicit finite-difference method for the solution of the complete, unsteady Navier-Stokes equations. Results of calculations with large disturbance amplitudes are discussed for a supercritical and a subcritical case. Reynolds-stress distributions and certain energy distributions, which can both be obtained from
• Hama, F. R., Williams, D. R., & Fasel, H. (1980). Flow field and energy balance according to the spatial linear stability theory of the Blasius boundary layer.. Array, eds., Berlin, Fed. Rep. Germany, Springer-Verlag, 1980, Session 2, p.73-85..
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  Abstract: In search of physical processes which control the boundary-layer instability, instantaneous streamlines and instantaneous total vorticity fields have been mapped out. One of the purposes of these demonstrations is to dispel a widespread myth that a voriticity maximum is attached to the critical layer. There is, however, a simple and potentially important mechanism by which the total vorticity peaks appear inside the flow field. Based upon the energy equations for the spatial stability analyses, contributing terms have been evaluated. It is particularly emphasized that the damping is primarily due to the negative energy production by the Reynolds stress rather than the viscous dissipation. Moveover, the pressure-velocity correlation term is found to have significant contributions to the energy balance. (A)
• Fasel, H., & Booz, O. (1977). An implicit finite difference method for the solution of the Navier Stokes equations for unsteady flows between rotating coaxial cylinders.. Array.
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  Abstract: In this paper an implicit finite difference method for the solution of the complete Navier Stokes equations is discussed. The method is applicable for the investigation of steady and unsteady flows between coaxial cylinders. Due to their superior numerical stability characteristics over explicit methods preference was given to an implicit method. The intended investigation of laminar flow instability, which exists in this case for larger Reynolds numbers, places stringent requirements concerning the numerical stability on a numerica'method. Here, essential parts of the numerical method are described and finally some typical results of steady and unsteady flow calculations are given. (A)
• Fasel, H., Bestek, H., & Schefenacker, R. (1977). NUMERICAL SIMULATION STUDIES OF TRANSITION PHENOMENA IN INCOMPRESSIBLE, TWO-DIMENSIONAL FLOWS.. AGARD Conf Proc, 14. 1-14. 8.
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  Abstract: Results of stability and transition studies which were obtained by employing an implicit finite difference method for the solution of the complete Navier-Stokes equations are discussed. Emphasis is placed on the discussion of investigations of some aspects of stability and transition for which treatment with other theoretical approaches becomes extremely difficult. In one investigation discussed in this paper, the reaction of a flat-plate boundary layer to periodic disturbances of finite amplitude is studied in order to gain insight into the nonlinear, two-dimensional development of the transition process. In another effort, the numerical model is applied to the investigation of stability and transition in plane Poiseuille flow with special consideration of a nonlinear, spacewise disturbance amplification. A third topic of discussion involves a study of the effects of a single roughness element (backward-facing step) on stability and transition in a boundary layer flow.
• Fasel, H., Bestek, H., & Schefenacker, R. (1977). Numerical simulation of transition phenomena in incompresible, two-dimensional flows.. IN: PROC. CONF. LAMINAR-TURBULENT TRANSITION (LYNGBY, DENMARK : MAY 2-4, 1977), NEUILLY-SUR-SEINE, FRANCE, ADVISORY GROUP AER, 14-1.
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  Abstract: In this paper preliminary results of stability and transition studies are discussed which were obtained by employing an implicit finite difference method for the solution of the complete Navier-Stokes equations. Emphasis is placed on the discussion of investigations of some aspects of stability and transition for which treatment with other theoretical approaches becomes extremely difficult. In one investigation discussed in this paper the reaction of a flat-plate boundary layer to periodic disturbances of finite amplitude is studied in order to gain insight into the nonlinear, two-dimensional development of the transition process. In another effort the numerical model is applied to the investigation of stability and transition in plane Poiseuille flow with special consideration of nonlinear, spacewise disturbance amplification. A third topic of discussion involves a study of the effects of a single roughness element (backward-facing step) on stability and transition in a boundary layer flow. (A)
• Fasel, H. (1976). INVESTIGATION OF THE STABILITY OF BOUNDARY LAYERS BY A FINITE-DIFFERENCE MODEL OF THE NAVIER-STOKES EQUATIONS.. Journal of Fluid Mechanics, 78(pt 2), 355-383.
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  Abstract: The stability of incompressible boundary-layer flows on a semi-infinite flat plate and the growth of disturbances in such flows are investigated by numerical integration of the complete Navier-Stokes equations for laminar two-dimensional flows. Forced time-dependent disturbances are introduced into the flow field and the reaction of the flow to such disturbances is studied by directly solving the Navier-Stokes equations using a finite-difference method. An implicit finite-difference scheme was developed for the calculation of the extremely unsteady flow fields which arose from the forced time-dependent disturbances. A demonstration of the suitability of the numerical method for the investigation of stability and the initial growth of disturbances is presented for small periodic perturbations. Refs.
• Fasel, H. (1975). Difference Method for Computation of Navier-Stokes Equations for Two-Dimensional, Laminar Flows in Incompressible Media.. Dtsch Luft Raumfahrt Forschungsber DLR-FB 75-32, 67-90.
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  Abstract: A survey of finite difference methods for the numerical solution of the Navier-Stokes equations is presented. Several versions of the governing equations are assessed with regard to their possible advantages and disadvantages for finite difference methods. Problematical aspects and difficulties arising from the development and application of finite difference methods are discussed in more detail. Characteristic properties of explicit and implicit methods and consequences thereof for the numerical calculation of fluid flows are examined.

Proceedings Publications

• Barraza, B., Gross, A., Leinemann, M., Hader, C., & Fasel, H. F. (2024). Transition Model for Second Mode and Crossflow Instabilities in Hypersonic Flow. In AIAA SCITECH 2024 Forum.
• Frisch, A. G., Cotnoir, A., Borgmann, D., Hosseinverdi, S., Little, J. C., & Fasel, H. F. (2024). Investigation of Cross-Flow in the Presence of a Laminar Separation Bubble. In AIAA SCITECH 2024 Forum.
• Hader, C., & Fasel, H. F. (2024). Numerical Investigation of Hypersonic Boundary-Layer Transition for an Ogive Geometry. In AIAA SCITECH 2024 Forum.
• Threadgill, J. A., Hader, C., Singh, A., Tsakagiannis, V., Fasel, H. F., Little, J. C., Lugrin, M., Bur, R., Chiapparino, G., & Stemmer, C. (2024). Scaling and Transition Effects on Hollow-Cylinder/Flare SBLIs in Wind Tunnel Environments. In AIAA SCITECH 2024 Forum.
• Tsakagiannis, V., Hader, C., & Fasel, H. F. (2024). Numerical investigations of laminar-turbulent transition for a hollow cylinder flare at Mach 5. In AIAA SCITECH 2024 Forum.
• Barraza, B., Gross, A., Haas, A. P., Hader, C., & Fasel, H. F. (2023). Machine-Learning-Based Transition Prediction for Hypersonic Boundary Layers with Crossflow. In AIAA SCITECH 2023 Forum.
• Barraza, B., Gross, A., Leinemann, M., Hader, C., & Fasel, H. F. (2023). Local Transition Model for Crossflow Instability in High-Speed Boundary-Layers. In AIAA AVIATION 2023 Forum.
• Flood, J. J., Hader, C., Skora, A., Fasel, H. F., & Craig, S. A. (2023). Experimental measurements and numerical investigations of boundary-layer instabilities on a Mach 5 hollow cylinder. In AIAA SCITECH 2023 Forum.
• Hader, C., & Fasel, H. F. (2023). Numerical investigation of the laminar-turbulent boundary-layer transition for a circular cone at Mach 5: wind tunnel and flight conditions. In AIAA SCITECH 2023 Forum.
• Hartman, A., Fasel, H. F., Wernz, S., Haas, A. P., & Hader, C. (2023). Numerical Investigations of Hypersonic Boundary-Layer Stability for a Blunt Flat Delta Wing. In AIAA AVIATION 2023 Forum.
• Hosseinverdi, S., & Fasel, H. F. (2023). Delay of Separation and Transition for a Laminar Airfoil Using Active Flow Control. In AIAA AVIATION 2023 Forum.
• Leinemann, M., Haas, A. P., Hader, C., & Fasel, H. F. (2023). Direct Numerical Simulations of the Nonlinear Boundary Layer Transition Regime on a Blunt Swept Flat Plate at Mach 6. In AIAA AVIATION 2023 Forum.
• Singh, A., Hader, C., Threadgill, J. A., Fasel, H. F., & Little, J. C. (2023). Schlieren Visualization of Controlled Disturbances in Mach 5 Flow Over a Hollow Cylinder Flare. In AIAA SCITECH 2023 Forum.
• Tsakagiannis, V., Hader, C., & Fasel, H. F. (2022).

  Nonlinear wave packet simulation for a cone at Mach 10 using a GPU accelerated pseudo-spectral scheme

  . In AIAA AVIATION 2022 Forum, AIAA 2022-3341.
• Woodward, M., Tian, Y., Lin, Y. T., Mohan, A. T., Hader, C., Fasel, H. F., Chertkov, M., & Livescu, D. (2023). Data-Driven Mori-Zwanzig: Reduced Order Modeling of Sparse Sensors Measurements for Boundary Layer Transition. In AIAA AVIATION 2023 Forum.
• Woodward, M., Tian, Y., Mohan, A. T., Lin, Y. T., Hader, C., Livescu, D., Fasel, H. F., & Chertkov, M. (2023). Data-Driven Mori-Zwanzig: Approaching a Reduced Order Model for Hypersonic Boundary Layer Transition. In AIAA SCITECH 2023 Forum.
• Adrian, G., Hosseinverdi, S., Little, J., & Fasel, H. F. (2022).

  Unsteady Behavior of a Laminar Separation Bubble Subjected to Wing Structural Motion

  . In AIAA SCITECH 2022 Forum, AIAA 2022-2331.
• Bahrainirad, L., Hosseinverdi, S., & Fasel, H. F. (2022).

  Laminar-Turbulent Transition in a Swept Low-Speed Boundary Layer,

  . In AIAA AVIATION 2022 Forum, AIAA 2022-3633.
• Barraza, B., Gross, A., Haas, A., Hader, C., Fasel, H. F., & Barraza, B. F. (2022). Machine-Learning-Based Amplification Factor Transport Equation for Hypersonic Boundary-Layers. In AIAA AVIATION 2022.
• Berry, S., Semper, M., Riha, A., Mullen, D., Reed, H., & Fasel, H. F. (2022).

  Development of the BOLT II Roughness Experiment for Flight

  . In AIAA SCITECH 2022 Forum., AIAA 2022-0347.
• Borgmann, D., Hosseinverdi, S., Little, J., & Fasel, H. F. (2022).

  Active Control of Boundary-layer Transition in Laminar Separation Bubbles

  . In AIAA SCITECH 2022 Forum, AIAA 2022-2433.
• Haas, A., Hader, C., & Fasel, H. F. (2022).

  Numerical Investigation of Cross-Flow Instability for a Supersonic Swept Wing with a Biconvex Airfoil

  . In AIAA SCITECH 2022 Forum..
• Hader, C., & Fasel, H. F. (2022). Numerical Investigation of Boundary-Layer Transition for a slender cone at Mach 6 initiated with Random Disturbances. In AIAA 2022-0602. AIAA SCITECH 2022 Forum, AIAA 2022-0602.
• Hader, C., & Fasel, H. F. (2022). Flow control using steady blowing and suction strips in a Mach 6 Boundary Layer on a Flared Cone: “Natural” Transition. In AIAA AVIATION 2022 Forum, AIAA 2022-3339.
• Hartman, A., Hader, C., & Fasel, H. F. (2022). Numerical investigation of the effects of wall heating and cooling on the nonlinear transition stages for a sharp cone at Mach 6. In AIAA SCITECH 2022 Forum, AIAA 2022-0601.
• Hosseinverdi, S., Grille, A., Fasel, H. F., & Little, J. (2022). Investigation of Transition and its Active Control in Separation Bubbles for a Wing Section at Re=200k: DNS, Theory, and Experiments. In AIAA SCITECH 2022 Forum, AIAA 2022-2329.
• Hosseinverdi,, S., & Fasel, H. F. (2022).

  High-Fidelity Versatile Incompressible Flow Solver for Direct Numerical Simulations and Linear Stability Investigations

  . In AIAA SCITECH 2022 Forum., AIAA 2022-1199.
• Hurworth, A., Hader, C., & Fasel, H. F. (2022). Direct Numerical Simulations of Hypersonic Boundary-Layer Transition for a Sharp Cone at Mach 10. In AIAA SCITECH 2022 Forum, AIAA 2022-0945.
• Leinemann, M., Hader, C., & Fasel, H. F. (2022).

  Numerical Investigation of Boundary-Layer Transition initiated by Random Disturbances for a Flat Plate at Mach 6  

  . In AIAA SCITECH 2022 Forum, AIAA 2022-1580.
• Meersman, J., Hader, C., & Fasel, H. F. (2022).

  Numerical Investigation of Hypersonic Boundary-Layer Transition for an Ogive Cone

  . In AIAA SCITECH 2022 Forum, AIAA 2022-0946.
• Stevens, S., Hader, C., Fasel, H. F., & Stevens, S. F. (2022). Numerical investigation of the nonlinear transition stages for a sharp cone at Mach 10. In AIAA AVIATION 2022 Forum, AIAA 2022-3554.
• Subramanya, S. F. (2022).

  Direct Numerical Simulations of Laminar-Turbulent Transition for Transonic Boundary Layers initiated by Random Disturbances

  . In AIAA SCITECH 2022 Forum, AIAA 2022-2431.
• Subramanya, S., Hader, C., & Fasel, H. F. (2022).

  Direct Numerical Simulations of Laminar-Turbulent Transition for Transonic Boundary Layers initiated by Random Disturbances 

  . In AIAA SCITECH 2022 Forum, AIAA 2022-2431.
• Threadgill, J., Hader, C., Craig, A., Flood, J., Fasel, H. F., Little, J., & Threadgill, J. F. (2022). Fin-induced Shock Boundary Layer Interactions on a Flat Plate and Hollow Cylinder at Mach 5. In AIAA SCITECH 2022 Forum, AIAA 2022-1816..
• Fasel, H. F., Tumin, A., & Sivasubramanian, J. (2016, June). The Reynolds Number Effect on Receptivity to a Localized Disturbance in a Hypersonic Boundary Layer. In AIAA Paper No. 2016-4246.
• Balthazar, M., Gross, A., & Fasel, H. F. (2014, January 13-17). Experimental Investigation of Flows with Three-dimensional Separation. In 52nd AIAA Aerospace Sciences Meeting.
• Brehm, C., Hader, C., & Fasel, H. F. (2014, January 13-17). A High-order Immersed Interface Method for Compressible Flows. In 52nd Aerospace Sciences Meeting.
• Fasel, H. F., Sivasubramanian, J., & Laible, A. (2014, September 8-12). Numerical Investigation of Transition in a Flared Cone Boundary Layer at Mach 6. In IUTAM-ABCM Symposium on Laminar-Turbulent Transition.
• Gross, A., & Fasel, H. F. (2014, January 13-17). Hybrid Turbulence Model Simulations of Internal and External Flows. In 52nd AIAA Aerospace Sciences Meeting.
• Hader, C., Brehm, C., & Fasel, H. F. (2014, June 16-20). Numerical Investigation of Transition Delay for Various Controlled Breakdown Scenarios in a Mach 6 Boundary Layer Using Porous Walls. In 7th AIAA Theoretical Fluid Mechanics Conference.
• Hass, A. P., Gross, A., & Fasel, H. F. (2014, June 16-20). Numerical Investigation of the Three-dimensional Vertical Flow for an Ellipsoid Model. In 32nd AIAA Applied Aerodynamics Conference.
• Salami, L., Fasel, H. F., Wernz, S., & Marquart, E. (2014, June 16-20). Numerical Investigations of Wavepackets in a Hypersonic HIgh-enthalpy Boundary Layer on a 5 deg Sharp Cone. In 7th AIAA Theoretical Fluid Mechanics Conference.
• Salemi, L., Gross, A., Fasel, H. F., Wernz, S., & Marquart, E. (2014, January 13-17). Linearized Navier-Stokes Calculations of the Spatial Stability of a Hypersonic Boundary Layer on a 5^o Sharp Cone with High Temperature Effects. In 52nd Aerospace Sciences Meeting.
• Shirzad, H., & Fasel, H. F. (2014, September 8-12). Laminar-turbulent Transition in a Laminar Separation Bubble in the Presence of Free-stream Turbulence. In IUTAM-ABCM Symposium on Laminar-Turbulent Transition.

Presentations

• Bailey, M. (2019, September). Numerical investigation of the nonlinear transition stages in a high enthalpy boundary layer,. IUTAM Symposium on Laminar-Turbulent Transition. London, UK: IUTAM.
• Fasel, H. F. (2019, July). Investigation of Laminar-Turbulent Transition for Transonic Boundary Layers Using Advanced Computational Tools. ARO Fluid Dynamics Program Review. Colorado Springs: Army Research Office.
• Fasel, H. F. (2019, July). Numerical Investigations of the Nonlinear Transition Stages in Hypersonic Boundary Layers for Navy Relevant Mach Numbers and Model Geometrics. ONR Program Review for High-Speed Flows. Boulder, CO: Office of Naval Research.
• Fasel, H. F. (2019, June). Direct numerical simulations for understanding complex flow phenomena. AIAA Aviation Conference. Dallas, TX: AIAA.
• Fasel, H. F. (2019, September). Investigation of laminar-turbulent transition using high-fidelity direct numerical simulations. German Symposium on Hypersonic Transition. Munich, Germany.
• Hader, C. (2019, July). Numerical Investigation of Nonlinear Transition Stages in Hypersonic Boundary Layers for Wind-Tunnel and Free-Flight Conditions. AFOSR Workshop on High-Speed Flows and Turbulence. Boulder, CO: Airforce office of Scientific Research.
• Hader, C. (2019, September). DNS of natural transition in high-speed boundary layers. IUTAM Symposium on Laminar Turbulent Transition. London, UK: IUTAM.
• Hosseinverdi, S. (2019, September). Towards understanding of natural boundary-layer transition via random excitation. IUTAM Symposium on Laminar-Turbulent Transition. London, UK: IUTAM.
• Little, J. (2019, July). Wing Sweep, Structural Motion and Their Effect on Separation and Transition - Simulations, Wind Tunnel and Flight Experiments. AFOSR Program Review for low speed flows and Flow Control. Colorado Springs: Air Force Ofiice of Scientific Research.
• Agate, M., Pande, A., Little, J., & Gross, A. (2018, June). Agate, M., Pande, A., Little, J.C., Gross, A., and Fasel, H.F., “Active flow control of the laminar separation bubble on an oscillating airfoil near stall. 56th AIAA Aerospace Sciences Meeting, AIAA-2018-2049, 2018. 56th AIAA Aerospace Sciences Meeting. Atlanta: AIAA.
• Fasel, H. F. (2018, January). Role of Klebanoff modes in active control of laminar separation bubbles.. 13th ERCOFTAC SIG 33 Workshop, Progress in Flow Instability, Transition and Control. Paraty, Brazil: ERCOFTAC.
• Fasel, H. F. (2018, July). Numerical Investigation of Nonlinear Boundary Layer transition. AFOSR/ONR High-Speed Research Review. Washingtion DC: AFOSR/ONR.
• Fasel, H. F. (2018, june). Numerical investigation of boundary-layer transition and separation using controlled disturbance input. Invited talk. 48th AIAA Fluid Dynamics Conference. Atlanta: AIAA.
• Gross, A., & Little, J. (2018, january). 1. Gross, A., Little, J. & Fasel, H. F. Numerical investigation of shock wave turbulent boundary layer interactions. 2018 AIAA Aerospace Sciences Meeting, AIAA 2018-1807.. 2018 AIAA Aerospace Sciences Meeting. Orlando: AIAA.
• Meersman, J., & Hader, C. (2018, june). Meersman, J. A., Hader, C. & Fasel, H. F. Hypersonic boundary-layer transition: Comparison of the fundamental resonance breakdown for a flared and straight cone at Mach 6. 2018 Fluid Dynamics Conference, AIAA 2018-3851. AIAA 2018. Atlanta: AIAA.
• Fasel, H. F. (2016, April). Direct Numerical Simulations of Nonlinear transition for a flared cone. AVT-NATO workshop on Second Mode hypersonic Transition. South Bend, IN: Notre Dame.
• Fasel, H. F. (2016, August). DNS of stability and transition for hypersonic boundary layers. AFOSR Conference on high speed flows and turbulence. Washington, DC.
• Fasel, H. F. (2016, July). Effect of structural motion on the flow physics of transition an separation. AFOSR Conference on Flow Physics and Control. Arlington, VA.
• Fasel, H. F. (2016, October). Investigation of transition in high-speed and high enthalpy flows. CTS Workshop on Experimental and Numerical investifation of Boundary Layer Transition. Tullahoma, TN.
• Fasel, H. F. (2016, October). Stability and transition investigations of High-Enthalpy flows. AVT-NATO - 240 Congress. Avila, Spain.
• Fasel, H. F., Little, J. C., & Gross, A. (2016, June). Impact of Structural Motion on Separation and Separation Control: Integrated Investigation using Numerical Simulations, Theory, Wind-tunnel and Free-flight Experiments. AFOSR/ARO Review Meeting. Arlington, VA: AFOSR/ARO.
• Little, J. C., Fasel, H. F., & Gross, A. (2016, June). Investigation of 3D Shock-Boundary Layer Interaction: A Combined Approach using Experiments, Numerical Simulations and Stability Analysis. AFOSR/ONR Review Meeting. Arlington, VA: AFOSR/ONR.
• Fasel, H. F. (2014, July 29-31). Investigation of Boundary-layer Separation: Its Control. AFOSR Flow Interactions and Control Workshop. Arlington, VA: AFOSR.
• Fasel, H. F. (2014, June 19). Investigation of High-speed Boundary-layer Stability and Transition Using Direct Numerical Simulations. Seminar. Stuttgart, Germany.
• Fasel, H. F. (2014, March 19-21). Direct Numerical Simulation of Laminar-turbulent Transition for a Cone at M-6. Symposium on Nonlinear Stability Theory: From Weakly Nonlinear Theory to the Verge of Turbulence. London, UK: Imperial College.