Israel J Wygnanski

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• Oberleithner, K., Paschereit, C. O., & Wygnanski, I. (2014). On the impact of swirl on the growth of coherent structures. Journal of Fluid Mechanics, 741, 156-199.
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  Abstract: Spatial linear stability analysis is applied to the mean flow of a turbulent swirling jet at swirl intensities below the onset of vortex breakdown. The aim of this work is to predict the dominant coherent flow structure, their driving instabilities and how they are affected by swirl. At the nozzle exit, the swirling jet promotes shear instabilities and, less unstable, centrifugal instabilities. The latter stabilize shortly downstream of the nozzle, contributing very little to the formation of coherent structures. The shear mode remains unstable throughout generating coherent structures that scale with the axial shear-layer thickness. The most amplified mode in the nearfield is a co-winding double-helical mode rotating slowly in counter-direction to the swirl. This gives rise to the formation of slowly rotating and stationary large-scale coherent structures, which explains the asymmetries in the mean flows often encountered in swirling jet experiments. The co-winding single-helical mode at high rotation rate dominates the farfield of the swirling jet in replacement of the co-and counter-winding bending modes dominating the non-swirling jet. Moreover, swirl is found to significantly affect the streamwise phase velocity of the helical modes rendering this flow as highly dispersive and insensitive to intermodal interactions, which explains the absence of vortex pairing observed in previous investigations. The stability analysis is validated through hot-wire measurements of the flow excited at a single helical mode and of the flow perturbed by a time-and space-discrete pulse. The experimental results confirm the predicted mode selection and corresponding streamwise growth rates and phase velocities. © 2014 Cambridge University Press.
• Wygnanski, I., Tewes, P., & Taubert, L. (2014). Applying the Boundary-Layer Independence Principle to Turbulent Flows. JOURNAL OF AIRCRAFT, 51(1), 175-182.
• Wygnanski, I., Tewes, P., & Taubert, L. (2014). Applying the boundary-layer independence principle to turbulent flows. Journal of Aircraft, 51(1), 175-182.
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  Abstract: Velocities measured in turbulent boundary layers over yawed flat plates confirmed that the mean velocity profiles normal to the leading edge are proportional to the velocity profiles parallel to it, with a proportionality constant depending on the yaw angle. This turned out to be the necessary and sufficient condition to make the wall stress components normal and parallel to the leading edge also proportional in the same manner, thus reaffirming the boundary-layer independence principle for turbulent and laminar flows alike. Reinterpretation of old experiments thus changed the mantra stating, "the independence principle does not apply to turbulent flow", thus providing a new insight into three-dimensional boundary-layer flows on yawed, high-aspect-ratio wings. It explains the prevalence of attached spanwise flow near the trailing edges of such wings, and it provides a rationale for turbulence modeling on them. Furthermore, it indicates the direction along which active separation control should take place. © 2013 by the American Institute of Aeronautics and Astronautics, Inc.
• Chen, C., Seele, R., & Wygnanski, I. (2013). Flow Control on a Thick Airfoil Using Suction Compared to Blowing. AIAA JOURNAL, 51(6), 1462-1472.
• Chen, C., Seele, R., & Wygnanski, I. (2013). Flow control on a thick airfoil using suction compared to blowing. AIAA Journal, 51(6), 1462-1472.
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  Abstract: The effects of steady slot suction as a means of controlling separation on a blunt elliptical airfoil were investigated and compared to the effects of steady blowing applied to the same airfoil previously investigated. Slots whose width approaches 1% of the chord and are located downstream of the natural separation location generate the highest lift for a given dimensionless mass-flow coefficient CQ. This holds true whether the flow separates near the leading edge due to high incidence or near the trailing edge due to the bluntness of the airfoil and is contrary to the observations made when blowing was used. Lift generated by suction through a moderately wide slot located downstream of the natural separation line depends only on CQ, but if the slot is located upstream of the separation line it depends on Reynolds number as well. Contrary to separation control by blowing no deleterious effects were observed at low levels of suction regardless of the suction-slot location. Adding a wedge to the blunt trailing edge of this airfoil enhances the effectiveness of suction, whereas making blowing less effective at low momentum inputs thus increasing the gap between these two approaches to flow separation.
• Lucas, N., Doty, M., Taubert, L., & Wygnanski, I. (2013). Reducing the noise emanating from a twin jet nozzle using flexible filaments Topics in Flow Control. Guest editors J.P. Bonnet and L. Cattafesta. Experiments in Fluids, 54(4).
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  Abstract: A twin jet was tested in anechoic facilities at the University of Arizona and NASA Langley Research Center to determine the effectiveness of flexible filaments in jet noise reduction. Results were strongly dependent on filament diameter and material, the most effective of which was found to be Tex 800 Kevlar. In the best configurations, the filaments consistently eliminated screech tones and reduced overall sound pressure level by 3 dB or more. Additionally, broadband shock noise was diminished by more than 5 dB over certain audible frequency ranges. Larger-scale tests run at NASA showed comparable reductions in overall sound pressure level and broadband shock-associated noise. © 2013 Springer-Verlag Berlin Heidelberg.
• Lucas, N., Doty, M., Taubert, L., & Wygnanski, I. (2013). Reducing the noise emanating from a twin jet nozzle using flexible filaments. EXPERIMENTS IN FLUIDS, 54(4).
• Phillips, E., & Wygnanski, I. (2013). Use of Sweeping Jets During Transient Deployment of a Control Surface. AIAA JOURNAL, 51(4), 819-828.
• Phillips, E., & Wygnanski, I. (2013). Use of sweeping jets during transient deployment of a control surface. AIAA Journal, 51(4), 819-828.
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  Abstract: Unsteady aerodynamic loads were measured on a NACA 0021 airfoil during rapid flap deflections. Typical dynamic-stall effects like an overshoot in lift, drag, and pitching moment have been observed in the absence of active flowcontrol.Finalflap-deflection angles and the rate of their deflections were varied at a constant angle of attack.Stall prevention or delay was achieved by fluidic actuators that generated discrete sweeping jets over the upper surface of a deflected simple flap. Static experiments proved the effectiveness of these actuators in preventing flow separation, thus increasing the lift and decreasing the drag of the airfoil. Dynamic tests proved their efficacy in dynamic stall. A significant gain in lift and a substantial reduction in drag were achieved within a short time period,even when the flap deflection was high. © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Seele, R., Graff, E., Lin, J., & Wygnanski, I. (2013). Performance enhancement of a vertical tail model with sweeping jet actuators. 51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 2013.
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  Abstract: Active Flow Control (AFC) experiments performed at the Caltech Lucas Adaptive Wall Wind Tunnel on a 12%-thick, generic vertical tail model indicated that sweeping jets emanating from the trailing edge (TE) of the vertical stabilizer significantly increased the side force coefficient for a wide range of rudder deflection angles and yaw angles at free-stream velocities approaching takeoff rotation speed. The results indicated that 2% blowing momentum coefficient (Cμ) increased the side force in excess of 50% at the maximum conventional rudder deflection angle in the absence of yaw. Even Cμ = 0.5% increased the side force in excess of 20% under these conditions. This effort was sponsored by the NASA Environmentally Responsible Aviation (ERA) project and the successful demonstration of this flow-control application could have far reaching implications. It could lead to effective applications of AFC technologies on key aircraft control surfaces and lift enhancing devices (flaps) that would aid in reduction of fuel consumption through a decrease in size and weight of wings and control surfaces or a reduction of the noise footprint due to steeper climb and descent.
• Chen, C., Seele, R., & Wygnanski, I. (2012). Separation and Circulation Control on an Elliptical Airfoil by Steady Blowing. AIAA JOURNAL, 50(10), 2235-2247.
• Chen, C., Seele, R., & Wygnanski, I. (2012). Separation and circulation control on an elliptical airfoil by steady blowing. AIAA Journal, 50(10), 2235-2247.
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  Abstract: The application of blowing over a thick elliptical airfoil is considered for the purpose of identifying the hierarchy of additional parameters affecting the flow, such as slot width, its location, and orientation, in addition to the mass and momentum flow that were considered as being significant in the past. When a single jet emanates from a narrow slot at a prescribed location, the incremental lift coefficient scales mostly with the momentum coefficient (C μ). For wider slots, the slot width has to be considered as an independent parameter even when the jet velocity is much higher than the freestream. The lift increment depends strongly on the slot location when the latter is near the natural separation location of the flow, but it is independent of Reynolds number. Deleterious effects of steady blowing on C L were observed for low momentum inputs. For wider slots located upstream of the natural separation location, the drag is sensitive to Reynolds number and it is not solely affected by C μ. Flowfield measurements are used to explain some of the observations made. © 2012 EUROPEAN MOLECULAR BIOLOGY ORGANIZATION.
• Oberleithner, K., Paschereit, C. O., Seele, R., & Wygnanski, I. (2012). Formation of turbulent vortex breakdown: Intermittency, criticality, and global instability. AIAA Journal, 50(7), 1437-1452.
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  Abstract: This study provides quantitative insight into the formation of vortex breakdown and the onset of global instability in a turbulent swirling jet. A water jet is guided through a rotating honeycomb that imparts the rotational motion, passed through a contraction, and discharged into a large water tank. The flow states evolving at increasing swirl are mapped out via time-resolved particle image velocimetry. The experimental results scale properly with the swirl number based on the axial momentum flux when the commonly used boundary-layer approximations are omitted. The instantaneous velocity field reveals that vortex breakdown occurs intermittently at a wide range of swirl numbers before it appears in the mean flow. At this intermittent state, the evolving breakdown bubble oscillates heavily between two streamwise locations where the vortex core is subcritical. Upon further increasing the swirl, the breakdown oscillations decay and a region of reversed flow appears in the mean flowfield. The formation of this socalled axisymmetric breakdown state is accompanied by a supercritical-to-subcritical transition of the inflowing vortex core. The reversed flow region is found to grow linearly with increasing swirl until the flow undergoes a supercritical Hopf bifurcation to a global single-helical mode, and vortex breakdown adopts a spiral shape. The global mode shape is extracted from the particle image velocimetry snapshots by means of proper orthogonal decomposition and Fourier analysis. The present experiment reveals that, at gradually increasing swirl, the jet first transitions to an axisymmetric breakdown state that remains globally stable until a critical swirl number is exceeded. This sequence of flow states agrees well with the transient formation of vortex breakdown observed in laminar flows. Copyright © 2012 by Kilian Oberleithner.
• Seele, R., Graff, E., Gharib, M., Taubert, L., Lin, J., & Wygnanski, I. (2012). Improving rudder effectiveness with sweeping jet actuators. 6th AIAA Flow Control Conference 2012.
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  Abstract: The application of active flow control on a vertical tail of a typical twin engine aircraft was investigated. Sweeping jets installed into the rudder surface were used and their effect was assessed by force measurements, flow visualization and local pressure distributions. The airfoil forming the tail is a NACA 0012 with a rudder using 35% of its chord. The tests were carried out at the Lucas Wind Tunnel at the California Institute of Technology at representative Reynolds numbers of up to Re=1.5 million. Multiple flap deflections and spanwise actuator configurations were tested resulting in an increase of up to 50-70% in side force depending on the free stream velocity and momentum input. © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Vatsa, V. N., Koklu, M., Wygnanski, I. L., & Fares, E. (2012). Numerical simulation of fluidic actuators for flow control applications. 6th AIAA Flow Control Conference 2012.
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  Abstract: Active flow control technology is finding increasing use in aerospace applications to control flow separation and improve aerodynamic performance. In this paper we examine the characteristics of a class of fluidic actuators that are being considered for active flow control applications for a variety of practical problems. Based on recent experimental work, such actuators have been found to be more efficient for controlling flow separation in terms of mass flow requirements compared to constant blowing and suction or even synthetic jet actuators. The fluidic actuators produce spanwise oscillating jets, and therefore are also known as sweeping jets. The frequency and spanwise sweeping extent depend on the geometric parameters and mass flow rate entering the actuators through the inlet section. The flow physics associated with these actuators is quite complex and not fully understood at this time. The unsteady flow generated by such actuators is simulated using the lattice Boltzmann based solver PowerFLOW®. Computed mean and standard deviation of velocity profiles generated by a family of fluidic actuators in quiescent air are compared with experimental data. Simulated results replicate the experimentally observed trends with parametric variation of geometry and inflow conditions.
• McVeigh, M. A., Nagib, H., Wood, T., & Wygnanski, I. (2011). Full-Scale Flight Tests of Active Flow Control to Reduce Tiltrotor Aircraft Download. JOURNAL OF AIRCRAFT, 48(3), 786-796.
• McVeigh, M. A., Nagib, H., Wood, T., & Wygnanski, I. (2011). Full-scale flight tests of active flow control to reduce Tiltrotor aircraft download. Journal of Aircraft, 48(3), 786-796.
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  Abstract: The vertical force, or download, acting on the airframe of current tiltrotor configurations during hover amounts to approximately 10% of rotor thrust, or about 6000 lb for the V-22. Various mechanical means have been experimentally tried to reduce this penalty, but none has been implemented, largely because of mechanical complexity. This paper describes the research conducted on the application of active flow control to the problem, since this technique may offer a solution without large weight penalties and unacceptable complexity. The research was conducted as part of the Defense Advanced Research Projects Agency Micro Adaptive Flow Control program. The work culminated in June 2003, when the NASA/U.S. Army/Bell XV-15 tiltrotor aircraft was used to successfully demonstrate the effectiveness of active flow control in reducing airframe download during hover. The wing flaps were fitted with zero-mass-flow actuators that periodically injected/removed air in the flap upper surface boundary layer through slots from the interior of the flap. The active flow control was effective in delaying flow separation from the flap, which reduced the download on the wings. The flight tests were the culmination of extensive laboratory experiments on two-dimensional models and on a powered full-span 16%-scale model of the XV-15 aircraft. The XV-15 flight tests confirmed the laboratory findings by successfully reducing the download measured in hover by as much as 14%, demonstrating that the aerodynamic principles of active flow control can be applied to full-scale air vehicles. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Oberleithner, K., Sieber, M., Nayeri, C. N., Paschereit, C. O., Petz, C., Hege, H. -., Noack, B. R., & Wygnanski, I. (2011). Three-dimensional coherent structures in a swirling jet undergoing vortex breakdown: Stability analysis and empirical mode construction. Journal of Fluid Mechanics, 679, 383-414.
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  Abstract: The spatio-temporal evolution of a turbulent swirling jet undergoing vortex breakdown has been investigated. Experiments suggest the existence of a self-excited global mode having a single dominant frequency. This oscillatory mode is shown to be absolutely unstable and leads to a rotating counter-winding helical structure that is located at the periphery of the recirculation zone. The resulting time-periodic 3D velocity field is predicted theoretically as being the most unstable mode determined by parabolized stability analysis employing the mean flow data from experiments. The 3D oscillatory flow is constructed from uncorrelated 2D snapshots of particle image velocimetry data, using proper orthogonal decomposition, a phase-averaging technique and an azimuthal symmetry associated with helical structures. Stability-derived modes and empirically derived modes correspond remarkably well, yielding prototypical coherent structures that dominate the investigated flow region. The proposed method of constructing 3D time-periodic velocity fields from uncorrelated 2D data is applicable to a large class of turbulent shear flows. © 2011 Cambridge University Press.
• Petz, C., Hege, H. -., Oberleithner, K., Sieber, M., Nayeri, C. N., Paschereit, C. O., Wygnanski, I., & Noack, B. R. (2011). Global modes in a swirling jet undergoing vortex breakdown. Physics of Fluids, 23(9).
• Seele, R., Chen, C., Bhamburkar, C., & Wygnanski, I. (2011). Some effects of blowing, suction and trailing edge bluntness on flow separation from thick airfoils; computations & measurements. 29th AIAA Applied Aerodynamics Conference 2011.
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  Abstract: The relative effectiveness of blowing and suction in controlling separation and circulation is assessed in the present paper. This paper focuses on low momentum input from the leading and trailing edge regions and the effects of a sharp and round trailing edges. Blowing is most effective when it is applied near but upstream of the separation location for the narrowest slot possible. The effectiveness of suction improves when the slot is located near but downstream of the natural separation location and it is as wide as possible. The effects of adding different trailing edge shapes on separation control were also explored. Most of the data presented was taken at incompressible speeds and at Re < 2.5*105. © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Tewes, P., Wygnanski, I., & Washburn, A. E. (2011). Feedback-Controlled Forcefully Attached Flow on a Stalled Airfoil. JOURNAL OF AIRCRAFT, 48(3), 940-951.
• Tewes, P., Wygnanski, I., & Washburn, A. E. (2011). Feedback-controlled forcefully attached flow on a stalled airfoil. Journal of Aircraft, 48(3), 940-951.
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  Abstract: Active maintenance of attached flow at natural poststall conditions requires a small intervention, relative to the one needed to force a separated flow to reattach under the same conditions. Experiments with slot suction applied near the leading edge of a stalled airfoil revealed a hysteresis of lift and drag that depends on the level of suction. This offers an opportunity to keep the flow attached at minimum input levels while guaranteeing that flow separation will not be allowed to occur. A simple approach was adopted that uses a rapidly responding pressure sensor located near the leading edge or in the interior reservoir of the airfoil for feedback control. The proposed controller used a prescribed pressure coefficient to keep the flow attached. Since a dimensionless pressure coefficient is required for this purpose, two similar sensors were installed in the pitot-static tube that monitored the freestream velocity. The impact of the time delay on the stability of the controller was briefly discussed and accounted for. The robustness of the controller was demonstrated under varying freestream velocities. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Woszidlo, R., & Wygnanski, I. (2011). Parameters governing separation control with sweeping jet actuators. 29th AIAA Applied Aerodynamics Conference 2011.
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  Abstract: Parameters governing separation control by sweeping jet actuators are investigated experimentally on a generic "Multiple Flap Airfoil" (MFA). Neither the flow rate nor the momentum input is found to be a sole parameter governing the lift for varying distance between adjacent actuators. However, the product of the mass flow coefficient and a square root of the momentum coefficient collapses the lift onto a single curve regardless of the actuator spacing. Surface flow visualization on the flap suggests the formation of counterrotating pairs of streamwise vortices caused by the interaction of neighboring jets. The actuation intensity required to attach the flow increases with increasing distance from the flap shoulder and increasing flap deflection. No obvious dependence of the ideal actuation location on flap deflection, angle of attack, or actuation intensity is found within the tested range. Comparisons between experimental and numerical results reveal a strong dependence on the thickness of the last flap segment at its hinge. In absence of this geometrical effect potential flow solution appears to be a suitable predictor for the obtainable lift. The flap size affects the achievable lift, the accompanying drag, the required flap deflection, and actuation intensity. By controlling separation the range of achievable lift coefficients is doubled without significant penalty in drag even when considering a safety margin for the maximum applicable incidence. © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Wygnanski, I., Tewes, P., Kurz, H., Taubert, L., & Chen, C. (2011). The application of boundary layer independence principle to three-dimensional turbulent mixing layers. JOURNAL OF FLUID MECHANICS, 675, 336-346.
• Wygnanski, I., Tewes, P., Kurz, H., Taubert, L., & Chen, C. (2011). The application of boundary layer independence principle to three-dimensional turbulent mixing layers. Journal of Fluid Mechanics, 675, 336-346.
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  Abstract: Turbulent mixing layers emanating from slanted trailing edges or nozzles evolve in a manner that is explainable by applying the independence principle to boundary layer flows. Although measurements downstream of a planar chevron splitter plate validate the concept, the intent of this short article is to re-examine the broader ramifications of this observation. Turbulent boundary layer growth on a yawed flat plate is re-examined as is the attached flow direction near the trailing edge of a highly swept-back wing. © 2011 Cambridge University Press.
• Chen, C., Seele, R., & Wygnanski, I. J. (2010). On the comparative effectiveness of steady blowing and suction used for separation and circulation control on an elliptical airfoil. 40th AIAA Fluid Dynamics Conference.
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  Abstract: The relative effectiveness of blowing and suction in controlling separation and circulation is assessed in the present paper. The paper focuses on low momentum input where suction is much superior to blowing in generating lift and decreasing drag. Blowing is mostly deleterious at levels of input that result in the jet velocity being comparable to the free stream's velocity and this paper attempts to explain this effect. Furthermore, the significance of slot width, slot location and the bluntness of the trailing edge are discussed. The effectiveness of suction is generally improved by widening the slot, moving it downstream and reducing the bluntness of the trailing edge. The effectiveness of blowing is best realized when the slot is located near the natural separation point for the narrowest slot possible. There is a threshold input of momentum where blowing becomes more effective than suction. Most of the data presented was taken at incompressible speeds and at Re < 2.5*10 5. © 2010 by the American Institute of Aeronautics and Astronautics, Inc.
• Phillips, E., Woszidlo, R., & Wygnanski, I. (2010). The dynamics of separation control on a rapidly actuated flap. 5th Flow Control Conference.
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  Abstract: Unsteady aerodynamic loads on a NACA 0021 airfoil during rapid flap deflections were investigated. Typical dynamic stall effects like an overshoot in lift, drag, and pitching moment have been observed. Flap deflection angle and speed were varied while the angle of attack remained constant at α = 0°. To prevent stall and accompanying detrimental behavior, fluidic actuators generating discrete sweeping jets were integrated into the simple flap of the airfoil. Initial experiments at various angles of incidence but constant flap deflections proved that these actuators can prevent flow separation, increase lift, and decrease the drag of the airfoil. Dynamic tests revealed that sweeping jet actuators can also eliminate dynamic stall effects. A significant gain in lift and a substantial reduction in drag were achieved within a short period of time, even for high flap deflection angles of 30°. © 2010 by Elisa Phillips.
• Tewes, P., Taubert, L., & Wygnanski, I. (2010). On the use of sweeping jets to augment the lift of a λ-wing. 28th AIAA Applied Aerodynamics Conference.
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  Abstract: Super-critical airfoils that are optimized for high speed subsonic flight require complex auxiliary high-lift systems for take-off and landing. A lambda wing model, based on such an airfoil, but containing simple flaps augmented by sweeping jet actuators, was constructed and tested. The purpose of these tests was to assess the efficacy of this method of separation control on a realistic wing configuration. Force and pressure measurements were carried out on this wing as well as surface flow visualization that used tufts and china clay. The strength of this actuation was altered and its effects were assessed. The orientation of the actuators was also altered for the outboard flap. The first flap had actuators aligned with the free stream while the second one had them parallel to the leading-edge that was swept back at 40°. The actuation from the second set of flaps turned out to be more effective because it affected only the decelerating flow component and no momentum was wasted on span-wise flow. These observations reaffirmed the ideas embedded in the boundary layer "independence principle" for large aspect ratio swept back cylinders. © 2010 by Philipp Tewes.
• Woszidlo, R., Nawroth, H., Raghu, S., & Wygnanski, I. J. (2010). Parametric study of sweeping jet actuators for separation control. 5th Flow Control Conference.
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  Abstract: In this paper, studies of separation control over a generic Multiple Flap Airfoil (MFA) using sweeping jet actuator arrays are presented. These jets, exiting from millimeter-scale nozzles, oscillate from side-to-side in a sweeping manner similar to windshield wipers and are henceforth referred to as sweeping jet actuators. Different flap sizes and flap deflections up to 45° were investigated in the experiments. The MFA, with integrated rows of sweeping jet actuators at several chordwise locations on the flaps, enabled an extensive variation of geometrical and fluid dynamical parameters to study separation control. The effect of flap size, actuator location and actuation parameters on the lift and drag coefficients of the airfoil are discussed. © 2010 by Rene Woszidlo, Surya Raghu, and Israel J. Wygnanski.
• Mabe, J. H., Calkins, F. T., Wesley, B., Woszidlo, R., Taubert, L., & Wygnanski, I. (2009). Single dielectric barrier discharge plasma actuators for improved airfoil performance. Journal of Aircraft, 46(3), 847-855.
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  Abstract: The applicability of single dialectic barrier discharge plasma actuators for use as active flow control devices, capable of enhancing the performance of airfoils, was assessed in this investigation. Measurements were carried out on two thick airfoils with simple flaps: a NACA0021 and an airfoil that is similar to those commonly used on tiltrotor aircraft The chord length of the airfoils was approximately 0.3 and 0.25 m, respectively, and the span was approximately 0.6 m. They were both tested in the same wind tunnel with a test section of 0.6 × 1.1 m. Freestream velocities varying from 5 to 15 m/s were tested, corresponding to chord Reynolds numbers ranging between 0.8 × 10 5 and 3 × 10 5. The lift, moment, and form drag were obtained from the pressure distributions over the airfoil's surface, and the total drag was calculated from a wake survey. The range of incidence angles α varied from -4 deg
• Seele, R., Tewes, P., Woszidlo, R., McVeigh, M. A., Lucas, N. J., & Wygnanski, I. J. (2009). Discrete Sweeping Jets as Tools for Improving the Performance of the V-22. JOURNAL OF AIRCRAFT, 46(6), 2098-2106.
• Seele, R., Tewes, P., Woszidlo, R., McVeigh, M. A., Lucas, N. J., & Wygnanski, I. J. (2009). Discrete sweeping jets as tools for improving the performance of the V-22. Journal of Aircraft, 46(6), 2098-2106.
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  Abstract: Experiments aimed at delaying flow separation through discrete jets pointing in the direction of streaming and sweeping side to side along the span were conducted on a V-22 airfoil with and without deflected trailing-edge flaps. The results indicated substantial drag reduction and lift increase at moderately low inputs of mass and momentum. Additional experiments were carried out on a semispan V-22 wing/nacelle combination, and they too provided an increase in lift-to-drag ratio L/D of approximately 60% (although active flow control was applied to the wing only). The effectiveness of the sweeping jets on reducing the download force acting on a V-22 full-span powered model in hover was also examined. A 29% reduction in download was realized using the embedded sweeping jets, corresponding approximately to a 2000 lb increase in hover lift.
• Taubert, L., & Wygnanski, I. (2009). Preliminary experiments applying active flow control to a 1/24 ^th scale model of a semi-trailer truck. Lecture Notes in Applied and Computational Mechanics, 41, 105-113.
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  Abstract: Preliminary experiments were carried out to investigate possible benefits of using a relatively new type of fluidic actuators (Raman & Raghu [1]) in combination with attached aft bodies to reduce the drag on a standard semi-trailer truck. The actuators generate oscillating jets that cause the formation of streamwise vortices that enhance the entrainment of the shear layer significantly. Taking into account the potential feasibility of any add-ons to a trailer, seven different bodies with simple geometries were chosen for this investigation. © 2009 Springer-Verlag Berlin Heidelberg.
• Woszidlo, R., Taubert, L., & Wygnanski, I. (2009). Manipulating the Flow over Spherical Protuberances in a Turbulent Boundary Layer. AIAA JOURNAL, 47(2), 437-450.
• Woszidlo, R., Taubert, L., & Wygnanski, I. (2009). Manipulating the flow over spherical protuberances in a turbulent boundary layer. AIAA Journal, 47(2), 437-450.
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  Abstract: Means of controlling the flow over a large spherical protuberance were examined. The role of suction around the base of the protuberance in reducing or even eliminating the necklace vortex created by the protuberance was considered. In the absence of suction, this vortex was lifted by the low base pressure existing behind the protuberance into the wake, thus affecting the turbulence level along its path. Large vortex generators placed upstream of the protuberance were able to delay local separation of the flow over the protuberance, thus affecting the symmetry of the wake and the level of turbulence on one side or the other. Observations made using flow visualization were supplemented by hot-wire measurements. The experiments were carried out at low speed at Reynolds numbers that did not exceed 3 × 10 5. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Zakharin, B., Kit, E., & Wygnanski, I. (2009). On a turbulent mixing layer created downstream of a "λ" notch simulating one wavelength of a chevron nozzle. Flow, Turbulence and Combustion, 83(3), 371-388.
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  Abstract: Measurements were carried out in a turbulent mixing layer formed downstream of a splitter plate, that had a Λ-shaped trailing edge. The results revealed that the center of the mixing layer shifts toward the high-speed flow while its sides bend toward the low speed stream at larger distances from the splitter plate. This suggests the existence of a counter rotating streamwise eddies that dominate the flow and substantially increase its level of turbulence relative to the classical plane mixing layer. The change in the orientation of the vorticity, emanating from a chevron nozzle, decreases the susceptibility of the flow to spanwise uniform periodic excitation relative to a classical plane mixing layer. © 2009 Springer Science+Business Media B.V.
• Chen, C., Zakharin, B., & Wygnanski, I. J. (2008). On the parameters governing fluidic control of separation and circulaton. 46th AIAA Aerospace Sciences Meeting and Exhibit.
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  Abstract: Active Flow Control (AFC) was experimentally investigated on an elliptic airfoil to improve our understanding of the flow mechanisms and identify the parameters, governing fluidic control of separation and circulation. Constant blowing, suction and Zero Mass Flux Forcing (ZMFF) were applied. The design of the model enables varying slot-widths, their locations and even orientations without resorting to machining, in addition to other parameters, such as free stream velocity, mass flow through the slots, flow control amplitude and frequency. Wedges attached to the trailing edge of the ellipse modified the shape and sharpness of the trailing edge thus changing the Kutta condition. It was found that for the constant blowing the increment of the lift coefficient scales with the momentum coefficient (Cμ), provided that the slot width is small. For wider slots the empirical correlation is proposed. A deleterious effect of steady blowing on CL was observed at low C μ. An imposed Kutta condition, even when carried out by a small protuberance has a significant effect on the lift, generated by stronger blowing. Suction is much more effective than blowing at low levels of C μ. Neither Cμ , nor volume flow coefficient, C Q, provides universal relation for suction. ZMFF is most effective in reattaching separated flow but the traditional does not provide the universal scaling for ZMFF. Much higher level of input was required to attach separated flow than to keep the flow attached. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc.
• Kit, E., & Wygnanski, I. (2008). On periodically excited turbulent mixing layer created downstream of a plane Chevron partition. Physica Scripta T, T132.
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  Abstract: The flow in a turbulent mixing layer resulting from the merger of two parallel, different velocity streams, created downstream of a 'Chevron'-shaped jagged partition was simulated numerically on the basis of experiments published in 2007. A small flap that was hinged at the trailing edge of the partition could oscillate at a prescribed frequency, and induce regular oscillations in the flow. The latter regulated the large eddy structure that was amenable to phase-locked data acquisition revealing the large spanwise vortices that were generated by Kelvin-Helmholtz instability and streamwise vortices that were triggered by the chevron and were enhanced by a secondary instability in the flow. These, being locked in phase, were mapped by using particle image velocimetry. Numerical simulation of the equations of motion was then carried out in order to reveal the most unstable mechanisms leading to the generation of the streamwise vortical structure. The simulation started by assuming the flow to be two-dimensional (2D) and allowing the large spanwise eddies to develop temporally. At a prescribed time (or the state of development of the large spanwise rolls) the 2D computation was frozen and 3D simulation initiated. The latter exhibited typical evolution of translative instability, which bent the large spanwise structures and stretched some of them to create streamwise vorticity. Bulging of the spanwise eddies was also observed, but the bulging instability is a slower process than the bending one. The results of the simulations compare well to experiments and provide some understanding of this complex interaction. © 2008 The Royal Swedish Academy of Sciences.
• Lucas, N., Taubert, L., Woszidlo, R., Wygnanski, I., & McVeigh, M. A. (2008). Discrete sweeping jets as tools for separation control. 4th AIAA Flow Control Conference.
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  Abstract: Experiments aimed at delaying flow separation through discrete jets pointing in the direction of streaming and sweeping side-to-side along the span were conducted on two airfoils, a NACA0015 and a V-22 airfoil with and without deflected trailing edge flaps. The results indicated substantial drag reduction and lift increase at moderately low inputs of mass and momentum. Additional experiments were carried out on a semi-span V-22 wing-nacelle combination and they too provided an increase in L/D of approximately 60%, even with the presence of nacelle drag and induced drag. The effectiveness of the sweeping jets on reducing the download force acting on a V-22 powered model in hover was also examined. A 29% reduction in download was realized using the embedded sweeping jets. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc.
• Zakharin, B., & Wygnanski, I. J. (2008). Forced Separation and Reattachment of Flow to Glauert Laminar Airfoil Section II. JOURNAL OF AIRCRAFT, 45(6), 1991-2000.
• Zakharin, B., & Wygnanski, I. J. (2008). Forced separation and reattachment of flow to Glauert Laminar Airfoil Section II. Journal of Aircraft, 45(6), 1991-2000.
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  Abstract: The transient process of flow separation and reattachment to a concave surface was investigated experimentally on the rear ramp of a modified Glauert Laminar Airfoil Section II, The measurements were carried out at low incompressible speeds using miniature time-resolved piezoresistive pressure transducers (Endevco model). These measurements were supplemented by flow visualization and by exploratory use of a particle image velocimeter. It was observed that the concavity of the surface impeded flow reattachment, and therefore a much higher momentum coefficient was required to partially attach the flow to a concave surface than to a straight one when both surfaces were supposed to turn the flow by the same amount. Streamwise vortices were observed to exist on the concave ramp of the modified Glauert Laminar Airfoil Section II, and thus the interaction of streamwise and span wise vortices and their impact on flow control are discussed. Lift augmentation by periodic excitation is mainly achieved by the entrainment of the upstream flow into the pulsating jet. For the on-off application of control, the transient time normalized by the time of flight between the actuator and the trailing edge varies between 15 and 20 over the upper and lower surfaces and between 5 to 15 over the ramp. The possibilities of gradually controlling the airfoil performance are also considered, and the phase delays of the surface-pressure perturbation are discussed. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Zakharin, B., Tewes, P., Chen, C., Wygnanski, I. J., & Washburn, A. E. (2008). The Utility of hysteresis for closed-loop control applications that maintain attached flow under natural post stall conditions on airfoils. 4th AIAA Flow Control Conference.
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  Abstract: Much higher control input is required to attach separated flow than to keep an attached flow from separating under natural, post stall conditions. The experiments with slot suction applied near the leading-edge, revealed a hysteresis of lift and drag coefficients that depend on the level of suction. This offers an opportunity to keep the flow attached at minimal input levels, while guarantying that flow separation will be not be allowed to occur. A simple approach was adopted that uses a fast response pressure sensor located near the leading-edge of the airfoil for feedback control. Since a pressure coefficient is required for this purpose, two additional quick responding sensors were installed in the Pitot tube that measures the free stream velocity. The proposed controller used a prescribed pressure coefficient to keep the flow attached. The impact of the time delay on the stability of the controller is briefly discussed and accounted for. The robustness of the controller was demonstrated under varying free stream velocities. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc.
• Kit, E., Wygnanski, I., Friedman, D., Krivonosova, O., & Zhilenko, D. (2007). On the periodically excited plane turbulent mixing layer, emanating from a jagged partition. Journal of Fluid Mechanics, 589, 479-507.
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  Abstract: The flow in a turbulent mixing layer resulting from two parallel different velocity streams, that were brought together downstream of a jagged partition was investigated experimentally. The trailing edge of the partition had a short triangular 'chevron' shape that could also oscillate up and down at a prescribed frequency, because it was hinged to the stationary part of the partition to form a flap (fliperon). The results obtained from this excitation were compared to the traditional results obtained by oscillating a two-dimensional fliperon. Detailed measurements of the mean flow and the coherent structures, in the periodically excited and spatially developing mixing layer, and its random constituents were carried out using hot-wire anemometry and stereo particle image velocimetry. The prescribed spanwise wavelength of the chevron trailing edge generated coherent streamwise vortices while the periodic oscillation of this fliperon locked in-phase the large spanwise Kelvin-Helmholtz (K-H) rolls, therefore enabling the study of the inter- action between the two. The two-dimensional periodic excitation increases the strength of the spanwise rolls by increasing their size and their circulation, which depends on the input amplitude and frequency. The streamwise vortices generated by the jagged trailing edge distort and bend the primary K-H rolls. The present investigation endeavours to study the distortions of each mode as a consequence of their mutual interaction. Even the mean flow provides evidence for the local bulging of the large spanwise rolls because the integral width (the momentum thickness, Θ), undulates along the span. The lateral location of the centre of the ensuing mixing layer (the location where the mean velocity is the arithmetic average of the two streams, y0), also suggests that these vortices are bent. Phase-locked and ensemble-averaged measurements provide more detailed information about the bending and bulging of the large eddies that ensue downstream of the oscillating chevron fliperon. The experiments were carried out at low speeds, but at sufficiently high Reynolds number to ensure naturally turbulent flow. © 2007 Cambridge University Press.
• Mabe, J. H., Calkins, F. T., Wesley, B., Woszidlo, R., Taubert, L., & Wygnanski, I. (2007). On the use of single dielectric barrier discharge plasma actuators for improving the performance of airfoils. Collection of Technical Papers - 37th AIAA Fluid Dynamics Conference, 1, 293-309.
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  Abstract: The applicability of Single Dialectic Barrier Discharge (SDBD) plasma actuators for use as active flow control devices, capable of enhancing the performance of airfoils, was assessed in this investigation. Measurements were carried out on two thick airfoils with simple flaps, a NACA0021 and an airfoil similar to those commonly used on tilt-rotor aircraft. The chord length of the airfoils was approximately 0.3 and 0.25 meters respectively and the span was approximately 0.6 meters. They were both tested in the same wind tunnel with a test section of 0.6 meters × 1.1 meters. Free-stream velocities varying from 5 m/s to 15 m/s were tested, corresponding approximately to chord Reynolds numbers ranging between 0.8×105 & 3×105. The lift, moment and form drag were obtained from the pressure distributions over the airfoil's surface while the total drag was calculated from a wake survey. The range of incidence angles, α, varied from -4° < α < +20° and flap deflections, δf, of 0° and 15° were tested. The location of the actuation was also altered. Two data sets are presented: one where the actuator was placed at approximately 5% of the chord and the other where it was located just upstream of the flap shoulder at a chord location corresponding to about 75%. The momentum input of the SDBD plasma actuators was measured with a hot wire and was in good agreement with previously published results. The input momentum is very weak and is not sufficient to prevent separation at Reynolds numbers greater than 100,000. The SDBD plasma actuators used in this study may only provide sufficient momentum to be effective at very low Reynolds numbers, such as those appropriate to micro-air-vehicles. Under special circumstances their passive presence on the surface may trip the boundary layer making it more resistant to separation, but in those cases a proper roughness strip or vortex generators may delay separation more effectively.
• Naim, A., Greenblatt, D., Seifert, A., & Wygnanski, I. (2007). Active control of a circular cylinder flow at transitional reynolds numbers. Flow, Turbulence and Combustion, 78(3-4), 383-407.
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  Abstract: Active and passive control of flow around a circular cylinder, at transitional Reynolds numbers was investigated experimentally by measuring cylinder surface pressures and wake velocity profiles. Two- and three-dimensional passive boundary layer tripping was considered and periodic active control using piezo-fluidic actuators was introduced from a two-dimensional slot that was nearly tangential to the cylinder surface. The slot location was varied circumferentially by rotating the cylinder and this facilitated either upstream- or downstream-directed actuation using sinusoidal or modulated wave-forms. Separation was controlled by two distinct methods, namely: by forcing laminar-turbulent transition when applied at relatively small angles (30-60°) from the forward stagnation point; and by directly forcing the separated shear-layer at larger angles. In the latter case, actuation produced the largest load changes when it was introduced at approximately 90° from the forward stagnation point. When the forcing frequency was close to the natural vortex-shedding frequency, the two frequencies "locked-in" creating clear and persistent structures. These were examined and categorized. The "lock-in" effect lowered the base pressure and increased the form-drag whereas delaying separation from the cylinder did the opposite. © Springer Science+Business Media B.V. 2007.
• Schmalzel, M., Varghese, P., & Wygnanski, I. (2007). Steady and oscillatory flow control tests for Tilt Rotor Aircraft. Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 95, 190-207.
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  Abstract: The purpose of this manuscript is to address one of the many questions plaguing the application of fluidic active flow control for performance enhancement over wings and airplanes. Specifically, what mode of Active Flow Control (AFC) is most effective; steady suction, steady blowing, or a periodic variation of both? The tilt rotor model is chosen because it represents very demanding requirements over a wide range of incidence angles, α, varying from -90°
• Han, G., Zhou, M. d., & Wygnanski, I. (2006). On streamwise vortices and their role in the development of a curved wall jet. Physics of Fluids, 18(9).
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  Abstract: The streamwise growth of longitudinal vortices in a curved wall jet was investigated experimentally, mostly through Particle Image Velocimetry (PIV). This growth was partially achieved through amalgamation of neighboring vortices having the same sense of rotation and partially due to centrifugal instability, thus the average spacing of the vortices far downstream was independent of the protuberances (micro vortex generators) that initiated them at the slot lip. The spacing of these protuberances along the span established only the preferred distance from the nozzle where vortex amalgamations occurred. The streamwise vortices initiated spanwise undulations in the mean flow creating a radial component of vorticity that when examined at a given distance from the surface was found to be inviscidly unstable, and could enhance further this component of vorticity. A strong coupling was observed between plane periodic excitation of the nozzle-flow and the small stationary protuberances that were placed at the nozzle in the presence of curvature. This coupling enhanced the coherence and strength of the longitudinal vortices far downstream from the nozzle, while in the absence of curvature these structures simply decayed. © 2006 American Institute of Physics.
• Taubert, L., & Wygnanski, I. (2006). Investigation of the near wake of a swept back cylinder with forcing. Collection of Technical Papers - 3rd AIAA Flow Control Conference, 2, 813-819.
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  Abstract: The means of controlling vortex shedding from a yawed cylinder at different sweep back angles 0°, 45° and 60°, are currently investigated at the University of Arizona. Periodic excitation emanating from two slots located symmetrically at different circumferential locations was used in this case. Pressure measurements on the surface and velocity measurements in the near wake were carried out at various spanwise locations along the cylinder. The velocity field was mapped by using a three-dimensional Particle Image Velocimeter (PIV).
• Wesley, B., Zakharin, B., & Wygnanski, I. J. (2006). Several aspects of active flow control on modified GLAS II Airfoil. Collection of Technical Papers - 3rd AIAA Flow Control Conference, 1, 109-124.
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  Abstract: Several aspects of flow separation control on the modified Glauert's Laminar Airfoil Section 11 (GLAS 11) were investigated experimentally. The control, via zero-net mass flux oscillatory blowing from a spanwise slot was considered. The present investigation attempts to quantify the control efficacy of the method and to study the dynamical effects of reattachment and separation, as a prerequisite for closed-loop control. The possible mechanisms of the control and their effect on the flow dynamics are discussed. The significance of the ramp curvature was also of concern. Since the control of the separation over a wall-mounted hump, having a shape that is similar to the upper loft of the GLAS II airfoil, was extensively investigated recently, the present experiment focuses on the differences between the flow over such a hump and over an airfoil. A temporal response of the flow to changes in actuation is considered as well.
• Wygnanski, I. J. (2006). A century of active control of boundary layer separation: A personal view. IUTAM Symposium on One Hundred Years of Boundary Layer Research, 129, 155-165.
• Wygnanski, I. J. (2006). A century of active control of boundary layer separation: A personal view. Solid Mechanics and its Applications, 129, 155-165.
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  Abstract: Active control of separation involves a multitude of parameters that are closely coupled and make the complete understanding of the flow difficult if not impossible. Since clear technological advantages are derived from its utilization we should endeavor to define these parameters and assess their relative significance. The present paper is an attempt in this direction. © 2006 Springer, Printed in the Netherlands.
• Han, G., Zhou, M. D., & Wygnanski, I. (2005). Some Observations of the Coanda Effect. Engineering Turbulence Modelling and Experiments 6, 545-553.
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  Abstract: This chapter elaborates some observations of the Coanda effect. A wall jet flowing over a circular cylinder has replaced the tail rotor on NOTAR type helicopters preventing autorotation. This application is in the presence of an external stream whereupon the Coanda effect becomes synonymous with super-circulation. The "Coanda Effect" is extensively investigated at the University of Arizona, where the initial purpose of the investigation is to determine the effects of streamline curvature on the mean flow and on its turbulence intensities. Moreover, the investigation focuses on the centrifugal instability and the generation of the large stream-wise vortices that eventually lead to the understanding of the separation mechanism of the wall jet from the convex surface. This chapter explores their evolution in the direction of streaming. It is observed that these vortices meander, coalesce, and increase in scale as they move downstream, while the width of the flow and the equivalent turbulent Gortler number also increases. © 2005 Elsevier B.V. All rights reserved.
• Margalit, S., Greenblatt, D., Seifert, A., & Wygnanski, I. (2005). Delta wing stall and roll control using segmented piezoelectric fluidic actuators. Journal of Aircraft, 42(3), 698-709.
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  Abstract: The separated flow around a balance-mounted, 60-deg sweptback, semispan delta wing with a sharp leading edge was controlled using zero-mass-flux periodic excitation from a segmented leading-edge slot. Excitation was generated by cavity-installed piezoelectric actuators operating at resonance with amplitude modulation (AM) and burst mode (BM) signals being used to achieve reduced frequencies (scaled with the freestream velocity and the root chord) in the range from script O sign (1) to script O sign (10). Results of a parametric investigation, studying the effects of AM frequency, BM duty cycle and frequency, excitation amplitude, location of the actuation along the leading edge, and optimal phase difference between the actuators, as well as the Reynolds number, are reported and discussed. Balance data were supplemented by upper surface static pressure measurements and particle image velocimetry (PIV) data. Order unity reduced-frequency modulation of the high-frequency carrier wave increased the normal force generated by the delta wing most effectively. BM with a duty cycle that was as low as 5% was more effective than the amplitude-modulated signal with larger peak excitation velocity and an order of magnitude larger momentum input. PIV data suggest that excitation enhances the momentum transfer across the shear layer, downstream of the original vortex breakdown location, generating a streamwise vortex the size of which is commensurate with the local wing span.
• Schmalzel, M., Varghese, P., & Wygnanski, I. (2005). Download alleviation on a V-22 model having a simple flap used in conjunction with periodic excitation, suction and blowing. SAE Technical Papers.
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  Abstract: Download measurements were carried out on a V-22 airfoil whose Fowler flap and the associated flap follower were replaced by a simple flap. Steady and periodic suction and blowing as well as purely periodic excitation were applied to the flap in order to alleviate the download. The dependence of the download alleviation on flap's slot location, the method of actuation, the level of momentum input and the frequency of the periodic actuation were investigated. Steady blowing at low momentum coefficients proved to be ineffective. The possible ramifications of adding a flat Kruger flap were also considered. Based on this data the download alleviation was measured on a three-dimensional, 1/10th scale model of the airplane whereupon the parametric studies carried out in two dimensions were confirmed. The use of suction in conjunction with a Kruger flap reduced the download on the model by 38% while weak periodic excitation in the absence of a Kruger flap reduced it by 16%. Copyright © 2005 SAE International.
• Darabi, A., & Wygnanski, I. (2004). Active management of naturally separated flow over a solid surface. Part 1. The forced reattachment process. Journal of Fluid Mechanics, 105-129.
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  Abstract: The forced reattachment of flow to an inclined flat surface, simulating a simple flap, was investigated experimentally. The transition from a separated to an attached state of the flow was initiated by an abrupt change in the frequency and the amplitude of periodic perturbations emanating from a slot at the flap shoulder. The excitation parameters determined the total duration of the reattachment process. Minimum reattachment time occurred at an optimal excitation frequency of Fopt+ ≈ 1.5, which was independent of amplitude and flap inclination. The control over the process was achieved by enhancing large spanwise vortices in the flow. Spatial amplification of consecutive vortices induces mean transport of fluid away from the flap surface which causes the main stream to reattach. The time scales of the excitation are at least an order of magnitude smaller than the typical reattachment times. © 2004 Cambridge University Press.
• Darabi, A., & Wygnanski, I. (2004). Active management of naturally separated flow over a solid surface. Part 2. The separation process. Journal of Fluid Mechanics, 131-144.
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  Abstract: The controlled separation of flow from an inclined straight flap at high inclination angles was investigated experimentally. The separation process was initiated by an abrupt change in the excitation emanating from a slot at the flap shoulder. A complete cessation of the actuation resulted in formation of a large vortex above the flap akin to the familiar 'dynamic stall vortex' (DSV) seen over oscillating airfoils in pitch. The DSV temporarily increased the aerodynamic load over the flap before it dropped to its low separated value. The duration of this overload decreased as the flap inclination increased. The use of periodic excitation during separation slowed down the rate of separation and changed its character depending on the amplitude and the frequency used. Forcing separation by switching the excitation to a high frequency (3 < F+ < 8) reduced or even eliminated the increase in flap loading that is associated with the DSV. A switch to low frequencies (F+ < 1) extended the duration of separation and increased the transient overload during the initial stage of the process. © 2004 Cambridge University Press.
• Halfon, E., Nishri, B., Seifert, A., & Wygnanski, I. (2004). Effects of elevated free-stream turbulence on actively controlled separation bubble. Journal of Fluids Engineering, Transactions of the ASME, 126(6), 1015-1024.
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  Abstract: The effects of elevated free-stream turbulence (FST) on natural and periodically excited separation bubbles were examined experimentally at low Reynolds numbers. The bubble was formed at the leading edge of a flat plate and the FST level was altered by placing a grid across the flow at different locations upstream of the plate. The mixing across the separated shear-layer increased due to the elevated FST and the two-dimensional periodic excitation, flattening, and shortening the bubble. Periodic excitation at frequencies that were at least an order of magnitude lower than the initial Kelvin-Helmholtz instability of the separated shear-layer were very effective at low FST. The fundamental excitation frequency and its first harmonic were amplified above the bubble. High frequency excitation (F + ≈ 3, based on the length of the natural low FST bubble that served as a reference baseline) affected the flow near the leading edge of the bubble but it rapidly decayed in the reattachment region. Lower frequencies (F + ≈ 1) were less effective and they decayed at a slower rate downstream of reattachment. An increase in FST level reduced the net effect of the periodic excitation on mixing enhancement and subsequent reattachment process. This was probably due to a destructive interference between the nominally 2D excitation and the random, in space and in time, FST. High FST is known to reduce the spanwise coherence in free shear layers [e.g., Chandrasuda, C., Mehta, R. D., Weir, A. D., and Bradshaw, P., 1978, "Effects of free-stream turbulence on large structures in turbulent mixing layers," J. Fluid Mech., 85, pp. 693-704] and therefore also the effectiveness of the current control strategy, this not withstanding 2D periodic excitation accelerated the reattachment process and the recovery rate of the attached boundary layer, reducing its momentum loss. Copyright © 2004 by ASME.
• McVeigh, M. A., Kiedaisch, J., Nagib, H., Stalker, A., Wood, T., & Wygnanski, I. (2004). Model & full scale tiltrotor download reduction tests using active flow control. Annual Forum Proceedings - American Helicopter Society, 1, 24-32.
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  Abstract: The paper describes the experimental development and full-scale demonstration of the use of periodic excitation (Active Flow Control or AFC) to reduce the airframe download on hovering tiltrotor aircraft. The initial 2-D wind tunnel experiments on a wing and flap model are described. Based on positive results from these tests, a full span, scaled, powered model of the Bell XV-15 tiltrotor was built and tested and the data used to prepare for full-scale flight test evaluation on the XV-15 aircraft. The powered model results are also described. In parallel with the model tests, a design effort was conducted to build and qualify actuators for the XV-15 flight test and to prepare the aircraft for their installation. The paper details the flight test program that successfully demonstrated the use of AFC as an effective means of reducing hover download.
• Neuendorf, R., Lourenco, L., & Wygnanski, I. (2004). On large streamwise structures in a wall jet flowing over a circular cylinder. Physics of Fluids, 16(7), 2158-2169.
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  Abstract: This paper describes an experimental search for large streamwise vortices in a turbulent wall jet flowing over a convex surface. The existence of these vortices resulting from a centrifugal instability was suspected but never previously observed in this flow. They eluded detection because they meander across the span, as they become larger, with increasing distance from the nozzle. A particle image velocimeter, used in air, enabled the mapping of these vortices establishing their circulation and their evolution in the direction of streaming. Although the longitudinal structures educed do not contribute to mean spanwise distortion of the flow, they are strong enough to augment the Reynolds stresses that increase the rate of spread of the jet and its turbulent intensities. Since the temporal resolution of the instrument is not yet adequate to track the motion of large vortices, the turbulent characteristics surrounding a pair of educed counter-rotating vortices were observed in a frame of reference that is statistically stationary relative to the vortex cores. © 2004 American Institute of Physics.
• Seifert, A., Greenblatt, D., & Wygnanski, I. J. (2004). Active separation control: An overview of Reynolds and Mach numbers effects. Aerospace Science and Technology, 8(7), 569-582.
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  Abstract: Separation control, by nominally two-dimensional periodic excitation, was studied experimentally by the authors and co-workers at Reynolds numbers ranging from 3 × 10 4 to 4 × 10 7, including compressibility effects. The tests demonstrated that active control using oscillatory flow excitation can effectively delay flow separation from, and reattach separated flow to, aerodynamic surfaces at various flight conditions. At Reynolds number below 105, where transition does not occur naturally and cannot be passively forced, active separation control may be the only effective method for delaying separation and generating useful lift. The essence of active separation control relies on exploiting instabilities that are inherent in the flow, generally requiring relatively small amplitude excitation. Effective excitation frequencies generate one to four vortices over the controlled region at all times, irrespective of Reynolds number, and perturbations should preferably be amplified over the region that is susceptible to separation. Periodic excitation is vastly superior to steady blowing in terms of performance benefits and eliminates abrupt flow responses, which are undesirable from a control point of view. The effects of compressibility in the absence of shocks are weak and undesirable effects accompanying separation, such as vortex-shedding and buffet, can be significantly reduced or completely eliminated. Separation resulting from shock-wave/boundary-layer interaction can be ameliorated, providing that excitation is introduced upstream of separation. © 2004 Published by Elsevier SAS.
• Stalker, A., Cerchie, D., Cullen, L., & Wygnanski, I. (2004). Using periodic perturbations for download alleviation on tilt-rotor airplane models in hover. 2nd AIAA Flow Control Conference.
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  Abstract: This paper covers the design improvements of the tilt-rotor download test fixture at the University of Arizona and the study of using active flow control and Kruger flaps for download alleviation on a tiltrotor aircraft. The investigation was done using two 3-D aircraft type; a 16.7% scale XV-15 and a 10.5% scale V-22 Osprey models. The study focused on the tilt-rotor in hover, a condition where approximately 10% of the total thrust is lost due to aerodynamicdrag on the airframe. Of this total download the wing accounts for a little more than 50%. Active flow control was used as a method to prevent or delay normal separation at the leading and trailing edges of the wings. This would decrease the download, which would effectively increase the payload capacity of the aircraft. The 3-D experiments follow successful demonstrations in two-dimensional wind tunnel testing. The intention of the 3-D setup was to validate the results of the 2-D testing and then lead to a flight demonstration of the technology on the XV-15 aircraft. Also, the 3-D setup incorporated the use of internal actuators to provide active flow control, an improvement over some initial studies. The 3-D configuration was a key step as a method to prove the concept in an unsteady, three-dimensional flow field that would be experienced in actual flight. The use of active flow control was able to demonstrate a download reduction of almost 15% on both models using an active trailing edge. When combined with an active leading edge the download reduction was approximately 30%. The subsequent flight demonstration of an active flap confirmed the validity of these results and shows promise for future study. The discussion of the experiment design and results now follows. © 2004 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Taubert, L., & Wygnanski, I. (2004). Control of vortex shedding from a cylinder at different sweep back angles. 2nd AIAA Flow Control Conference.
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  Abstract: The control of vortex shedding from a yawed cylinder at different sweep back angles 0°, 45° and 60°, is currently investigated at the University of Arizona. Active flow control is applied on the cylinder in the form of oscillatory, zero mass flux excitation through two slots located symmetrically on the circumference. Surface and wake pressure measurements are carried out, as well as application of flow visualization and Particle Image Velocimetry. © 2004 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Wygnanski, I. (2004). The variables affecting the control of separation by periodic excitation. 2nd AIAA Flow Control Conference.
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  Abstract: A series of flow control experiments were carried out in order to reconcile some of the observations made about enhancement of lift by periodic excitation. The relative merits of flow control were assessed in comparison with conventional design techniques to establish the best strategy to utilize flow control. The frequency of choice, its effect on the pressure distribution, and the optimum location of the actuation and its amplitudes are specifically discussed. The results of periodic excitation were also compared to results generated using steady blowing or suction. © 2004 by Wygnanski.
• Darabi, A., Stalker, A., McVeigh, M., & Wygnanski, I. (2003). The rotor wake above a tiltrotor airplane-model in hover. 33rd AIAA Fluid Dynamics Conference and Exhibit.
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  Abstract: The mean and time-dependent rotor-wake flow over a XV-15 tiltrotor model in hover was studied using stereoscopic particle-image velocimetry. Results from the full-span model were compared to those obtained from a semispan model simulated using an image-plane. Additional data were measured in the wakes of single and twin rotors without the airframe present, with and without an image plane. Rotor wake contraction was considerably reduced over the semi-span model when compared to the full-span case. This is attributed to pressure increases above the fuselage due to the elimination of the lateral flow fluctuations by the solid image plane. In the absence of the image-plane, the correlation between pairs of tip vortices from opposing rotors was negative (i. e. vortices were moving in the same direction) near the centerline of the aircraft. In the absence of the airframe model, the wake flow of two side-by-side rotors was almost indistinguishable from that observed on a single rotor in conjunction with an image plane. © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Greenblatt, D., & Wygnanski, I. (2003). Effect of leading-edge curvature on airfoil separation control. Journal of Aircraft, 40(3), 473-481.
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  Abstract: Separation control on NACA 0012 and NACA 0015 airfoils was compared under incompressible conditions, using leading-edge periodic excitation, in order to assess the effect of leading-edge curvature. Both lift and moment coefficients were considered to compare and analyse control effectiveness. In contrast to the relatively mild NACA 0015 trailing-edge stall, NACA 0012 stall was dominated by a leading-edge bubble-bursting mechanism that gave rise to alternating intervals of partial attachment and separation, but with no regular frequency. Low-amplitude excitation downstream of the bubble enhanced poststall lift and significantly attenuated the associated unsteadiness. In general, larger momentum coefficients were required for NACA 0012 separation control due to the large centrifugal acceleration of the flow around the leading edge. Because of the different stalling characteristics, relatively high- and low-excitation frequencies were effective for the NACA 0012 and NACA 0015 airfoils, respectively. However, the combination of high-excitation amplitudes with relatively low frequencies was effective on the NACA 0012, and this was believed to be associated with the large harmonic content of the evolving perturbations.
• Cullen, L. M., Han, G., Zhou, M. D., & Wygnanski, I. (2002). On the role of longitudinal vortices in turbulent flow over a curved surface. 1st Flow Control Conference.
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  Abstract: Experiments were conducted in two turbulent flows with streamline curvature, a convex wall-jet and a concave boundary layer. Two-point spatial crosscorrelation measurements separated in the spanwise direction clearly reveal the existence of large-scale longitudinal vortices in both experiments. Vortex generators (VGs) were used to enhance the streamwise vortices. The streamwise development of spanwise wavelength indicates these structures are related to the centrifugal or Görtler-type instability caused by the curvature. Surface pressure measurements show the vortices influence flow separation, which has implications for the future development of strategies for separation control and mixing enhancement. © 2002 by the American Institute of Aeronautics and Astronautics Inc. All rights reserved.
• Cullen, L., Nishri, B., Greenblatt, D., & Wygnanski, I. (2002). The effect of curvature on boundary layers subjected to strong adverse pressure gradients. 40th AIAA Aerospace Sciences Meeting and Exhibit.
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  Abstract: Two separate but complimentary experiments were conducted in turbulent boundary layers subjected to severe adverse pressure gradients: one developed over a flat surface, while the other had streamwise concave surface curvature. Measurements of surface pressures, as well as streamwise mean and fluctuating velocities,' established that gross characteristics of the boundary layer such as shape factor and skin friction coefficient were comparable. Detailed profile measurements normal to the wall, demonstrated that the most tangible evidence of the effect of curvature is that turbulence quantities do not scale with local boundary layer integral length scales, while on flat surfaces they do. On the flat surface, the measurement of streamwise cross-correlations showed that the spanwise wavelength of non-stationary three-dimensional structures did not increase. In contrast, curvature brought about an increase in the cross-stream wavelength. This behavior is consistent with the existence of a weak centrifugal or GOrtler-type instability mechanism of the type predicted by stability theory. The main conclusion was that concave surface curvature, whose effect is generally considered to be secondary to that of a strong adverse pressure gradient, plays a very significant role in the behavior of flow approaching separation. © 2002 by the American Institute of Aeronautics and Astronautics Inc. All rights reserved.
• Darabi, A., & Wygnanski, I. (2002). On the transient process of flow reattachment by external excitation. 1st Flow Control Conference.
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  Abstract: The transient reattachment process of a turbulent flow to an inclined flat surface was investigated experimentally. The process was initiated by periodic excitation that was introduced in a step-wise fashion at the hinge of the flap where the flow normally separated. Reattachment of the separated flow is essentially a constant-rate process whose slope depends upon the frequency and amplitude of the excitation and the flap deflection. Exception to this is an adverse surge in the flap aerodynamic loading immediately following the onset of the excitation. The duration of the surge scales with the flight time of the first induced vortex over the entire flap and it is unrelated to the nature of the oscillations. Overall reattachment time attains a minimum at reduced frequencies around F+=1.5. This optimal frequency is independent of the amplitude. The entire process is dominated by large spanwise vortices that were initiated by the excitation and amplified by the shear layer. It was demonstrated that as the coherent structures grow they contribute to net transport of fluid directed away from the surface that causes favorable pressure difference across the reattaching flow. © 2002 by the American Institute of Aeronautics and Astronautics Inc. All rights reserved.
• Greenblatt, D., & Wygnanski, I. (2002). Effect of leading-edge curvature and slot geometry on dynamic stall control4. 1st Flow Control Conference.
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  Abstract: The control of dynamic stall by periodic excitation was studied on NACA 0012 and 0015 airfoils under incompressible conditions, by means of twodimensional leading-edge excitation slots. Timeresolved surface pressure measurements were phaseaveraged and integrated to yield aerodynamic coefficients, and total drag was reduced from a wake survey. The dynamic bubble bursting mechanism, evident on the statically stalling NACA 0012 airfoil, was evident under dynamic conditions and its burst rate was controlled by the pitch-rate. Stall could be controlled by "trapping" the bubble upstream of the excitation slot location. In contrast, the NACA 0015 trailing-edge stall was controlled by a qualitative improvement in the pressure recovery. NACA 0012 dynamic stall was significantly more severe, typically requiring higher excitation amplitudes for effective control of the moment excursions. Furthermore, different reduced frequency ranges were found to be effective for the different airfoils. NACA 0015 control effectiveness was not proportional to Cμ, possibly due to the existence of centrifugal instabilities in the leading-edge region. © 2002 by the American Institute of Aeronautics and Astronautics Inc. All rights reserved.
• Greenblatt, D., & Wygnanski, I. (2002). Effect of leading-edge curvature on separation control: A comparison of two NACA airfoils. 40th AIAA Aerospace Sciences Meeting and Exhibit.
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  Abstract: Static stall and its control by periodic excitation were studied under incompressible conditions on a NACA 0012 airfoil, where excitation was achieved by means of independently employed two-dimensional leading-edge slots, inclined at 45° and 90° to the chord respectively, as well as a slot located in the airfoil aft region. In contrast to trailing-edge type stall of the NACA 0015, the NACA 0012 stall was controlled by a bubblebursting mechanism, giving rise to intervals of alternating partial-attachment and separation, but with no regular frequency. Separation was controlled by attaching the flow downstream of the bubble without eliminating the bubble itself. The combination of low excitation amplitudes with relatively high frequencies was found to be effective for controlling stall, as was the combination of high amplitudes with low frequencies. Large harmonic content at the higher amplitudes was believed to be responsible for this apparent anomaly. The 45° leading-edge slot was more effective for controlling stall than the 90° slot and excitation near the leading-edge was more effective than excitation from the aft portion of the airfoil. Lift augmentation resulting from generation of a "virtual surface", in the absence of natural separation, was found to be considerably inferior to lift augmentation that exploits separation control. © 2002 by the American Institute of Aeronautics and Astronautics Inc. All rights reserved.
• Grife, R., Darabi, A., & Wygnanski, I. (2002). Download reduction on a three dimensional V-22 model using active flow control. 1st Flow Control Conference.
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  Abstract: This study focuses on the use of active flow control to reduce hover download on a 10.5% scale V- 22 powered model. In hover mode, approximately 10% of the total thrust is lost due to drag on the airframe. Two sources of download were investigated: impingement of downwash onto the wing and the fountain flow between the rotors. The download from the wing and fountain contribute 58% and 17.5% of the download respectively1. Encouraged by the results of two-dimensional active flow control testing2,3, which indicated significant download reductions on the V-22 airfoil, similar configurations were constructed for test and evaluation on a complete three-dimensional model of the V-22. Download measurement showed that a 15% reduction in download could be achieved with an oscillatory jet injected into the flow on the upper flap surface. A preliminary investigation using threedimensional PIV was performed in an effort to better understand the tiltrotor flow characteristics. The PIV data indicates that semi-span tiltrotor testing with an image plane has major effects on the global flow. © 2002 by the American Institute of Aeronautics and Astronautics Inc.
• Halfon, E., Nishri, B., Seifert, A., & Wygnanski, I. (2002). Effects of elevated free-stream turbulence on active control of a separation bubble. 1st Flow Control Conference.
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  Abstract: Effects of elevated free-stream turbulence (FST) on natural and periodically excited separation bubble were studied experimentally, with relevance to lowpressure turbine blades at low Reynolds numbers. A bubble was formed at the leading edge of a flat plate and the FST level was altered by placing a grid across the flow at different locations upstream of the plate. The mixing across the separated shear-layer increased due to both elevated FST and Two-dimensional periodic excitation, flattening and shortening the bubble. Periodic excitation at frequencies that were at least an order of magnitude lower than the dominant shear-layer instability were very effective at low FST. The amplitudes of the fundamental excitation frequency and its harmonic were amplified over the bubble. High frequency excitation (F+≈z3, based on the length of the baseline low FST bubble) had a major effect close to separation, while the excitation fluctuating momentum rapidly decayed in the reattachment region. Low frequency excitation, that generated waves comparable to the length of the baseline bubble (F+≈I) were less effective and their magnitude decayed at a slower rate downstream of reattachment. An increase in the level of the FST reduced the net effect periodic excitation had on the mixing enhancement and subsequent reattachment process, probably due to a destructive interference between the nominally 2D excitation and the random, in space and time, FST reducing the span wise coherence and therefore the effectiveness of the current control strategy. However, even at the reduced effectiveness of 2D periodic excitation at elevated FST, it accelerated the reattachment process and the recovery rate of the attached boundary layer, enhancing the boundary layer resistance to repeat separation and reducing its momentum loss further downstream. © 2002 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Kjellgren, P., Cerchie, D., Cullen, L., & Wygnanski, I. (2002). Active flow control on bluff bodies with distinct separation locations. 1st Flow Control Conference.
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  Abstract: Active flow control (AFC) studies are currently in progress at The University of Arizona utilizing both computational and experimental efforts aimed at optimizing the parameters affecting pressure drag. The two-dimensional wind tunnel tests and numerical models use existing tilt-rotor airfoils in their representative aircraft hover orientation to the flow as the test articles. Zero mass-flux oscillatory blowing through slots at various locations on the airfoil achieves significant reduction in download. For the numerical simulations, a semi-implicit finite element solver with unstructured mesh capability was used for the threedimensional, time-dependent flow fields. Two large eddy simulation (LES) turbulence models were used; the constant and also the dynamic coefficient Smagorinsky models. Various numerical resolutions for the forcing slot as well as the computational domain were tested. The airfoil configuration, actuators and slot parameters affecting the interaction of the forced shear layer with the downstream wake continue to be investigated. The results show the impact of each of these control parameters on the flow field and the resulting download. Planned studies will further refine this understanding with the goal of achieving a significant download reduction using the optimum or minimum actuator requirement. © 2002 by the American Institute of Aeronautics and Astronautics,Inc. All rights reserved.
• Margalit, S., Greenblatt, D., Seifert, A., & Wygnanski, I. (2002). Active flow control of a delta wing at high incidence using segmented piezoelectric actuators. 1st Flow Control Conference.
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  Abstract: The separated flow around a 60° degrees sweep, semi-span delta wing at high angle-of-attack was controlled using zero-mass-flux periodic excitation, generated by cavity-installed Piezo-electric actuators. The excitation emanated from a segmented slot at the sharp leading edge. Normal and tangential forces, together with pitching and rolling moments, were measured by means of a four-component balance. The boundary layer on the wind tunnel wall, upstream of the delta wing, was removed using suction, with little effect on the forces and moments measured on the semi span model. Amplitude modulation (AM) and burst mode (BM) signals were used to generate reduced frequencies (scaled with the free stream velocity and the root chord) in the range O(1) to O(10) relative to the high resonance frequency of the actuators that is of O(100) based on the same scaling. A parametric investigation, studying the effects of AM frequency, BM duty cycle and frequency, excitation momentum, its location along the leading edge and the optimal phase between the actuators as well as the Reynolds number, is reported and discussed. Upper surface pressures and PIV data supplements the force and moment data. The comprehensive data-set indicates that frequencies of O(1) are most effective for increasing the normal force generated by the delta wing. Burst mode with a duty cycle as low as 5% was more effective than amplitude modulated signal with the same peak velocity but an order of magnitude larger momentum input. Based on the current findings it is not shown that the enhanced performance is related either to delay of vortex breakdown or to vortex enhancement prior to breakdown, but perhaps to a quasi-2D mechanism, enhancing the momentum transfer across the shear layer and generating a streamwise vortex of size commensurable to the local wing span.
• Taubert, L., & Wygnanski, I. (2002). Generic bluff bodies with undetermined separation location. 1st Flow Control Conference.
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  Abstract: The parameters governing forced attachment of flow to a flat, inclined surface, were determined by Nishri1. The addition of convex curvature is investigated presently using the circular cylinder as a model. In both flows the forcing consisted of two-dimensional, periodic oscillations emanating from a narrow slot. Naturally the flow separates from the surface of a smooth circular cylinder around 70° from the leading stagnation point when the Reynolds number is approximately 40,000. Periodic excitation from a slot located some 40° further downstream from the natural separation location altered very significantly the pressure distribution on that surface. On the opposite side of the cylinder neither the location of separation on the opposite side nor the pressure distribution was substantially affected, but a major change in the base pressure was observed. The cylinder started to lift and the typical vortex shedding from the cylinder was altered if not entirely eliminated. Experiments were carried out using both pressure measurements and particle image velocimetry (PIV). Numerical simulations were also carried out using an unstructured mesh finite element method with dynamic and constant coefficient Smagorinsky large eddy simulation (LES) turbulence models. © 2002 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Greenblatt, D., & Wygnanski, I. (2001). Dynamic stall control by periodic excitation, Part 1: NACA 0015 parametric study. Journal of Aircraft, 38(3), 430-438.
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  Abstract: A parametric study was undertaken to investigate the effect of periodic excitation (with zero net mass flux) on a NACA 0015 airfoil undergoing pitch oscillations at rotorcraft reduced frequencies under incompressible conditions. The primary objective of the study was to maximize airfoil performance while limiting moment excursions to typical prestalled conditions. The incidence angle excursions were limited to ±5 deg, and a wide range of reduced excitation frequencies and amplitudes were considered for 0.3 × 106 ≤ Re ≤ 0.9 × 106 with various flap deflections an excitation locations. Significant increases in maximum lift and reductions in drag were attained while containing the moment excursions. Oscillatory excitation was found to be far superior to steady blowing, which was even detrimental under certain conditions, and flap-shoulder excitation was found to be superior to leading-edge excitation.
• Greenblatt, D., & Wygnanski, I. J. (2001). Use of periodic excitation to enhance airfoil performance at low Reynolds numbers. Journal of Aircraft, 38(1), 190-192.
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  Abstract: The use of periodic excitation to enhance airfoil performance at low Reynolds numbers was demonstrated. For this purpose, experiments were performed in Tel Aviv University's Meadow-Knapp closed-wind-tunnel. It was found that the effectiveness of the method depends on the effectiveness and efficiency of actuators to supply prescribed excitation.
• Greenblatt, D., Neuburger, D., & Wygnanski, I. (2001). Dynamic stall control by intermittent periodic excitation. Journal of Aircraft, 38(1), 188-189.
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  Abstract: The principle of dynamic separation and attachment control was demonstrated on a stationary generic flap by periodic excitation. The dynamic separation and attachment were dependent upon excitation frequency and amplitude, flap detection angle, flap length, and inflow conditions. A large-time-scale disparity between the excitation frequencies and airfoil pitching frequencies with respect to rotorcraft was observed.
• Greenblatt, D., Nishri, B., Darabi, A., & Wygnanski, I. (2001). Dynamic stall control by periodic excitation, Part 2: Mechanisms. Journal of Aircraft, 38(3), 439-447.
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  Abstract: Dynamic flow separation and its control over a stationary deflected surface are used to demonstrate the timescale disparity between the process of dynamic stall, which is dominated by the dynamic stall vortex (DSV), and the excitation-induced large coherent structures that effect its control. Appreciation of this disparity provided a framework for analyzing dynamic stall control on a NACA 0015 airfoil, where leading-edge excitation had effectively eliminated the DSV and significantly attenuated trailing-edge separation. Within this framework, a comparison of static and airfoil phase-locked dynamic pressure data acquired in the vicinity of maximum incidence (α ≈ 25 deg) revealed that chordwise pressure distributions were independent of the airfoil pitching frequency and that the generation and advection of LCSs were not significantly affected by the dynamic airfoil pitching motion. Furthermore, disparities between static and dynamic data diminished as the excitation frequency increased relative to the airfoil pitching frequency. Oscillations of the aerodynamic coefficients induced by the excitations were negligibly small but served to regulate airfoil cycle-to-cycle disparities typical of the baseline poststall regime.
• Hites, M., Nagib, H., Bachar, T., & Wygnanski, I. (2001). Enhanced performance of airfoils at moderate mach numbers using zero-mass flux pulsed blowing. 39th Aerospace Sciences Meeting and Exhibit.
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  Abstract: Oscillatory wall-jets were introduced through spanwise slots along a flapped NACA 0015 airfoil to establish lift augmentation and drag reduction by the unsteady forcing of the separated flow. Pressure coefficient distributions, lift coefficients, and wake velocity profiles, to determine the drag coefficient, were measured over the test-section speed range of 25m/s < U∞ < 140m/s in the NDF. The present results demonstrated for the first time (Kites; 1997) the effectiveness of the oscillatory blowing technique as a separation control scheme at moderate Mach numbers, which exhibit compressibility effects. It is encouraging that lift-enhancement was observed over the entire range 0.1 < M < 0.4, even with the small amount of unsteady blowing applied in these experiments. As a result of the pulsed blowing, the lift coefficient increased by as much as 80%. Maximum pressure coefficients of nearly -5.0 for A/ = 0.4 experiments indicated the flow was supercritical near the leading edge of the airfoil, whereas it was not before the application of oscillatory blowing. The improvement in lift coefficient was found to be sensitive to the forcing frequency, even at the higher Mach numbers. Measurements at low angles of attack with a 20° flap showed that low amplitude pulsed blowing (0.02%) from the flap provided a 27% increasing in lift while steady blowing from the flap was detrimental to lift even at blowing coefficients as high as 3.5%. Oscillatory blowing with coefficients between 0.01% and 0.02%, based on RMS velocity, was shown to yield substantially better performance than steady blowing with Cμ in the range 0.5% to 3.5%. In is estimated that steady blowing of at least 10% would be required to reach the same levels of lift coefficient seen with the oscillatory blowing.
• Likhachev, O., Neuendorf, R., & Wygnanski, I. (2001). On streamwise vortices in a turbulent wall jet that flows over a convex surface. Physics of Fluids, 13(6), 1822-1825.
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  Abstract: Flow visualization and correlation measurements revealed the existence of large streamwise vortices in a turbulent wall jet that is attached to a circular cylinder. These coherent structures were not to be found near the nozzle, nor were they artificially triggered, thus the vortices could be a product of centrifugal instability. The existence and scale of this large-scale coherent motion were corroborated by stability analysis applied to the measured mean flow. © 2001 American Institute of Physics.
• Nagib, H., Kiedaisch, J., Greenblatt, D., Wygnanski, I., & Hassan, A. (2001). Effective flow control for rotorcraft applications at flight mach mumbers (invited). 15th AIAA Computational Fluid Dynamics Conference.
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  Abstract: Pulsating zero-mass flux jets introduced from spanwise slots at various locations on the upper surface of an oscillating VR-7 airfoil model are shown to be effective in controlling lift, moment and drag coefficients over the range of Mach numbers from 0.1 to 0.4. This control is demonstrated over a wide range of mean angles of attack of the oscillating airfoil from light to deep stall conditions. Maintaining the nondimensional frequency and amplitude of the forcing unchanged results in comparable modifications of the aerodynamic coefficients throughout this Mach number range. Therefore, it appears that this active-flow control technique is only limited by the ability to generate the adequate forcing conditions at the higher Mach numbers required for applications such as rotorcraft. © 2001 by the American Institute of Aeronautics and Astronautics Inc.
• Zhou, M. D., & Wygnanski, I. (2001). The response of a mixing layer formed between parallel streams to a concomitant excitation at two frequences. Journal of Fluid Mechanics, 441, 139-168.
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  Abstract: Simultaneous excitation of a turbulent mixing layer by two frequencies, a fundamental and a subharmonic, was investigated experimentally. Plane perturbations were introduced to the flow at its origin by a small oscillating flap. The results describe two experiments that differ mainly in the amplitudes of the imposed perturbations and both are compared to the data acquired while the mixing layer was forced at a single frequency. Conventional statistical quantities such as: mean velocity profiles, widths of the flow, turbulent intensities, spectra, phase-locked velocity and vorticity fields, as well as streaklines were computed. The rate of spread of the flow under concomitant excitation at the two frequencies was much greater than under a single frequency, although it remained dominated by two-dimensional eddies. The Reynolds stresses and turbulence production are associated with the deformation and orientation of the large coherent vortices. When the major axis of the coherent vortices starts leaning forward on the high-speed side of the flow, the production of turbulent energy changes sign (i.e. becomes negative) and this results in the flow thinning in the direction of streaming. It also indicates that energy is extracted from the turbulence to the mean motion. Resonance phenomena play an important role in the evolution of the flow. A vorticity budget showed that the change in mean vorticity was mainly caused by the nonlinear interaction between coherent vorticities. Nevertheless, the locally dominant frequency scales the mean growth rate, the inclination and distortion of the mean velocity profiles as well as the phase-locked vorticity contours.
• Elsberry, K., Loeffler, J., Zhou, M. D., & Wygnanski, I. (2000). An experimental study of a boundary layer that is maintained on the verge of separation. Journal of Fluid Mechanics, 423, 227-261.
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  Abstract: A boundary layer maintained as close as possible to separation over an extended distance was produced, in accordance with the concept of Stratford. The resulting layer was two-dimensional in the mean, had nearly a constant shape factor of 2.5 and approximately linear streamwise growth of its integral length scales. The flow exhibited a definite non-equilibrium character, indicated by the different scales required for collapse of the mean velocity and turbulence intensity profiles. It was also very sensitive to the thickness of the upstream boundary layer. External excitation was imposed for diagnostic purposes and as a tool for delaying separation. The oscillatory momentum level of c(μ) ~ 0.1% was tested for its ability to increase the skin friction c(f) at the prescribed geometry. Various frequencies, corresponding to the Strouhal number 0.008 < fθ 0/U(ref) < 0.064, were used for the free stream reference velocity of U(ref) = 15 m s -1 and for two different inflow conditions. Notable increase (close to 60%) in c(f) was observed at higher frequencies that did not undergo maximum amplification. The increase in c(f) was accompanied by a reduction in the boundary layer thickness and in the shape factor H. The latter decreased in one case from 2.5 to 2.1. The overall turbulence level in the boundary layer decreased due to the addition of plane external perturbations.
• Greenblatt, D., & Wygnanski, I. J. (2000). Control of flow separation by periodic excitation. Progress in Aerospace Sciences, 36(7), 487-545.
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  Abstract: This paper presents a review of the control of flow separation from solid surfaces by periodic excitation. The emphasis is placed on experimentation relating to hydrodynamic excitation, although acoustic methods as well as traditional boundary layer control, such as steady blowing and suction, are discussed in order to provide an appropriate historical context for recent developments. The review examines some aspects of the excited plane mixing-layer and shows how its development lays the foundation for a basic understanding of the problem. Flow attachment to, and separation from, a deflected flap is then shown to be a paradigm for isolating controlling parameters as well as understanding the basic mechanisms involved. Particular attention is paid to separation control on airfoils by considering controlling parameters such as optimum reduced frequencies and excitation levels, performance enhancement, efficiency, reduction of post-stall unsteadiness, compressibility and other important features. Additional topics covered include excitation of separation bubbles, control and exploitation of diffuser flows, three-dimensional effects, the influence of longitudinal curvature and possible applications to unmanned air vehicles. The review closes with some recent developments in the control and understanding of incompressible dynamic stall, specifically illustrating the control of dynamic stall on oscillating airfoils and identifying the crucial time-scale disparity between dynamic stall and periodic excitation.
• Guy, Y., Morrow, J. A., Mclaughlin, T. E., & Wygnanski, I. (2000). Parametric investigation of the effects of active flow control on the normal force of a delta wing. 38th Aerospace Sciences Meeting and Exhibit.
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  Abstract: The efficacy of active flow control in delaying vortex breakdown and enhancing the lift characteristics of a 70°-sweep delta wing is experimentally investigated in a low-speed wind tunnel at the USAF Academy. Periodic blowing and suction with zero net mass flux is applied at the leading edge of the wing. The pressure distribution over the upper surface of the wing is measured at a freestream velocity of 4.3 m/s, corresponding to a chord Reynolds number of 2.1×105. A parametric study is conducted, aimed at investigating the effect of periodic flow excitation on the pressure distribution on the upper surface of the wing. In particular, the normal force is computed and optimum values of key control parameters are established. The momentum coefficient of the flow excitation is varied from 0 to 0.004 and the nondimensional frequency is varied from 0 to 3.5. Pressure distribution on the upper surface of the wing is measured at angles of attack of 20° to 40° and the pressure is integrated to yield the normal force coefficient. It is found that the periodic flow excitation delays wing stall and greatly increases the normal force at angles of attack where stall would have occurred otherwise. At a constant momentum coefficient, the effect of the flow excitation is maximized at a nondimensional frequency of 1.38. At a constant frequency, an almost asymptotic increase of the normal force is observed as the momentum coefficient increases. The effect of the periodic flow excitation reaches its maximum at a momentum coefficient of 0.004 approximately. These results are consistent with results that were obtained in previous investigations. A maximum increase of 38% in the normal force is obtained at an angle of attack of 40o at the test conditions, relative to the unforced case. A 10o delay of the stall angle is achieved.
• Guy, Y., Morrow, J. A., Mclaughlin, T. E., & Wygnanski, I. (2000). Velocity measurements on a delta wing with periodic blowing and suction. 38th Aerospace Sciences Meeting and Exhibit.
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  Abstract: Velocity distribution over a 70°-sweep delta wing with and without periodic flow excitation is experimentally investigated in a water tunnel at the USAF Academy. Periodic blowing and suction with zero net mass flux is applied at the leading edge of the wing. The velocity across the leading edge vortex is measured at angles of attack from 30° to 45°. All tests are performed at a freestream velocity of 0.12 m/s, corresponding to a chord Reynolds number of 3.3*104. It is found that periodic flow excitation significantly delays vortex breakdown. The efficacy of the periodic flow excitation in delaying vortex breakdown increases as the angle of attack increases. The vortex breakdown location is delayed by 0.25 chordlength at an angle of attack of 30° and by as much as 0.5 chordlength at an angle of attack of 45°. It is also found that periodic flow excitation increases the core velocity after the onset of natural breakdown. This is consistent with the results that were obtained in previous wind tunnel experiments with a similar, scaled-up wing. The increased velocity indicates a decreased pressure, hence a lift increase can be anticipated at angles of attack where vortex breakdown occurs without flow excitation.
• Han, G., Tumin, A., & Wygnanski, I. (2000). Laminar-turbulent transition in Poiseuille pipe flow subjected to periodic perturbation emanating from the wall. Part 2. Late stage of transition. Journal of Fluid Mechanics, 419, 1-27.
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  Abstract: Transition in a fully developed circular pipe flow was investigated experimentally by introducing periodic perturbations. The simultaneous excitation of helical modes having indices m = ±1, ±2 and ±3 was chosen. The experiments revealed that the late stage of transition is accompanied by the formation of streaky structures that are associated with peaks and valleys in the azimuthal distribution of the streamwise velocity disturbance. The breakdown to turbulence starts with the appearance of spikes in the temporal traces of the velocity. Spectral characteristics of these spikes and the direction of their propagation relative to the wall are similar to those in boundary layers. Analysis of the data suggests the existence of a high-shear layer in the instantaneous velocity profile. Additional experiments in which a very weak, steady flow was added locally to the periodic axisymmetric perturbation were also carried out. These experiments resulted in the generation of a single peak in the azimuthal distribution of the disturbance amplitude. The characteristics of the transition process (spikes, vortical pattern etc.) within this peak were similar to ones observed in the helical excitation experiments. Based on these results one may conclude that late stages of transition in a pipe flow and in a boundary layer are similar. The present report is part of an ongoing investigation that was initiated by Eliahou, Tumin and Wygnanski (1998a).
• Likhachev, O., Quintana, D., & Wygnanski, I. (1999). On the stability of a laminar wall jet with heat transfer. Applied Scientific Research (The Hague), 62(2), 137-162.
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  Abstract: The hydrodynamic stability of a low speed, plane, non-isothermal laminar wall jet at a constant temperature boundary condition was investigated theoretically and experimentally. The mean velocity and temperature profiles used in the stability analysis were obtained by implementing the Illingworth-Stewartson transformation that allows one to extend the classical Glauert solution to a thermally non-uniform flow. The stability calculations showed that the two unstable eigenmodes coexisting at moderate Reynolds numbers are significantly affected by the heat transfer. Heating is destabilizing the flow while cooling is stabilizing it. However, the large-scale instabilities associated with the inflection point of the velocity profile still amplify in spite of the high level of the stabilizing temperature difference. The calculated stability characteristics of the wall jet with heat transfer were compared with experimental data. The comparison showed excellent agreement for small amplitudes of the imposed perturbations. The agreement is less good for the phase velocities of the sub-harmonic wave and this is attributed to experimental difficulties and to nonlinear effects.
• Likhachev, O., Quintana, D., & Wygnanski, I. (1999). On the stability of a laminar wall jet with heat transfer. Flow, Turbulence and Combustion, 62(2), 137-162.
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  Abstract: The hydrodynamic stability of a low speed, plane, non-isothermal laminar wall jet at a constant temperature boundary condition was investigated theoretically and experimentally. The mean velocity and temperature profiles used in the stability analysis were obtained by implementing the Illingworth-Stewartson transformation that allows one to extend the classical Glauert solution to a thermally non-uniform flow. The stability calculations showed that the two unstable eigenmodes coexisting at moderate Reynolds numbers are significantly affected by the heat transfer. Heating is destabilizing the flow while cooling is stabilizing it. However, the large-scale instabilities associated with the inflection point of the velocity profile still amplify in spite of the high level of the stabilizing temperature difference. The calculated stability characteristics of the wall jet with heat transfer were compared with experimental data. The comparison showed excellent agreement for small amplitudes of the imposed perturbations. The agreement is less good for the phase velocities of the sub-harmonic wave and this is attributed to experimental difficulties and to nonlinear effects. © 1999 Kluwer Academic Publishers.
• Neuendorf, R., & Wygnanski, I. (1999). On a turbulent wall jet flowing over a circular cylinder. Journal of Fluid Mechanics, 381, 1-25.
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  Abstract: The effect of surface curvature on the development of a two-dimensional wall jet was investigated experimentally. A comparison was made between a wall jet flowing around a circular cylinder and its plane equivalent. Velocity surveys and surface pressure measurements in the curved wall jet suggest the existence of two primary regions of interest. The first region, ranging from the end of the potential core to an approximate angular position of θ = 120°, is characterized by a constant surface pressure and a self similarity of the mean flow. The second region is marked by an adverse pressure gradient leading to separation around θ = 230°. The rate of spread of this flow, even in the initial region, is much higher than in the plane wall jet and so are the levels of turbulence and Reynolds stress. The dominant lengthscale in this flow is the radius of curvature R and the dominant velocity scale is the square root of the kinematic jet momentum divided by the radius of curvature. Entrainment of ambient fluid which causes the jet to adhere to the curved surface is also the main reason for its separation which is preceded by a rapid rate of spread of the flow leading to the failure of the boundary-layer approximation.
• Seifert, A., Bachar, T., Wygnanski, I., Kariv, A., Cohen, H., & Yoeli, R. (1999). Application of active separation control to a small unmanned air vehicle. Journal of Aircraft, 36(2), 474-477.
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  Abstract: The wind tunnel experiments on an unmanned air vehicle (UAV) are described to which active separation control was added to improve its performance and to exercise roll authority in an unconventional manner. The four stages of the project was subdivided into four stages which include: oscillatory and steady blowing; self contained oscillatory blowing system development; system installation on the UAV; and flight tests of the UAV with active separation control.
• Darabi, A., Nishri, B., & Wygnanski, I. (1998). On the interaction between steady and oscillatory addition of momentum in the control of separation. Proceedings of the 1998 Thirteenth Australasian Fluid Mechanics Conference, Melbourne, Australia, 13-18 December 1998 (Volume 2), 695-698.
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  Abstract: The addition of weak, steady blowing to periodic perturbations of comparable momentum had detrimental effect on the control of separation over a deflected flap. The strength of the steady blowing had to surpass a certain threshold in order to overcome the damage. The purpose of this study was to investigate this effect and attempt to explain it. The experiments were carried out over a deflected, straight flap. Instantaneous pressure measurements were made over the entire surface while concomitantly two components of velocity were measured above it by using a particle image velocimeter (PIV).
• Eliahou, S., Tumin, A., & Wygnanski, I. (1998). Laminar-turbulent transition in Poiseuille pipe flow subjected to periodic perturbation emanating from the wall. Journal of Fluid Mechanics, 361, 333-349.
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  Abstract: Transition in fully developed circular pipe flow was investigated experimentally by the introduction of periodic perturbations. The simultaneous excitation of the azimuthal periodic modes m = +2 and m = -2 was chosen for detailed analysis. The experiments were carried out at three amplitudes. At the smallest amplitude the disturbances decayed in the direction of streaming. At intermediate input amplitude the disturbances amplified initially but then decayed with increasing distance downstream. Their growth was accompanied by the appearance of higher harmonics. At still higher amplitudes transition occurred. A mean velocity distortion corresponding to an azimuthal index of m = 4 was observed at the intermediate and at the higher levels of forcing When four stationary jets were introduced through the wall to emulate a similar mean velocity distortion, transition was observed at smaller amplitudes of forcing at modes ±2. Thus, weak longitudinal vortices provide an added instability needed to generate a secondary disturbance which, in turn, amplifies the steady vortical structures introduced by the jets. Such vortices may also be generated through the interaction of time-periodic helical modes.
• Naveh, T., Seifert, A., Tumin, A., & Wygnanski, I. (1998). Sweep effect on parameters governing control of separation by periodic excitation. Journal of Aircraft, 35(3), 510-512.
• Nishri, B., & Wygnanski, I. (1998). Effects of periodic excitation on turbulent flow separation from a flap. AIAA Journal, 36(4), 547-556.
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  Abstract: The effects of periodic perturbations on delaying separation or promoting reattachment of initially separated flow were experimentally investigated. The leading parameters affecting the flow are the flap deflection, the input momentum, and its reduced frequency. The sensitivity of the flow to the imposed oscillations depends on its initial state, and this leads to hysteresis with respect to changes in any of the aforementioned parameters. For example, the most effective frequency required to attach the flow to the surface is much lower than the one required to prevent its separation. The amplitude needed to force reattachment may be an order of magnitude larger than the amplitude required to prevent separation at a given inclination of the flap. Nevertheless, periodic forcing is much more effective than steady blowing for boundary-layer control.
• Seifert, A., Eliahu, S., Greenblatt, D., & Wygnanski, I. (1998). Use of piezoelectric actuators for airfoil separation control. AIAA Journal, 36(8), 1535-1537.
• Quintana, D. L., Amitay, M., Ortega, A., & Wygnanski, I. J. (1997). Heat transfer in the forced laminar wall jet. Journal of Heat Transfer, 119(3), 451-459.
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  Abstract: The mean and fluctuating characteristics of a plane, unsteady, laminar, wall jet were investigated experimentally for a constant wall-temperature boundary condition. Temperature and streamwise velocity profiles, including the downstream development of the thermal and hydrodynamic boundary layer thicknesses, were obtained through simultaneous hot and cold wire measurements in air. Even at relatively low temperature differences, heating or cooling of a floor surface sufficiently altered the mean velocity profile in the inner, near-wall region to produce significant effects on the jet stability. Selective forcing of the flow at the most amplified frequencies produced profound effects on the temperature and velocity fields and hence the time-averaged heat transfer and shear stress. Large amplitude excitation of the flow (up to 2 percent of the velocity measured at the jet exit plane) at a high frequency resulted in a reduction in the maximum skin friction by as much as 65 percent, with an increase in the maximum wall heat flux as high as 45 percent. The skin friction and wall heat flux were much less susceptible to low-frequency excitation.
• Quintana, D. L., Amitay, M., Ortega, A., & Wygnanski, I. J. (1996). Heat transfer in the forced laminar wall jet. American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD, 330, 105-110.
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  Abstract: The mean flow and stability characteristics of a plane, laminar wall jet were investigated experimentally for a constant wall temperature boundary condition. The streamwise mean velocity and temperature profiles and the downstream development of the hydrodynamic and thermal boundary layer thicknesses were obtained through simultaneous hot and cold wire measurements. Even at relatively low temperature differences, heating or cooling of the surface sufficiently altered the mean velocity profile in the inner region to produce significant effects on the jet stability. Selective forcing of the flow at the most amplified frequencies produced profound effects on the velocity and temperature fields and hence the time-averaged shear stress and heat transfer. Large amplitude excitation of the flow at high frequencies resulted in a reduction in the maximum skin friction by as much as 65% with an increase in the maximum wall heat flux as high as 45%. The skin friction and wall heat flux were much less susceptible to low frequency excitation.
• Seifert, A., Darabi, A., & Wygnanski, I. (1996). Delay of airfoil stall by periodic excitation. Journal of Aircraft, 33(4), 691-698.
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  Abstract: It was recently demonstrated that oscillatory blowing can delay separation from a symmetrical airfoil much more effectively than the steady blowing used traditionally for this purpose. Experiments carried out on different airfoils revealed that this flow depends on many parameters such as, the location of the blowing slot, the steady and oscillatory momentum coefficients of the jet, the frequency of imposed oscillations, and the shape and incidence of the particular airfoil. In airfoils equipped with slotted flaps, the flow is also dependent on the geometry of the slot and on the Reynolds number in addition to the flap deflection that is considered as a part of the airfoil shape. The incremental improvements in single element airfoil characteristics are generally insensitive to a change in Reynolds number, provided the latter is sufficiently large. The imposed oscillations do not generate large oscillatory lift nor do they cause a periodic meander of the c.p.
• Zhou, M. D., Heine, C., & Wygnanski, I. (1996). The effects of excitation on the coherent and random motion in a plane wall jet. Journal of Fluid Mechanics, 310, 1-37.
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  Abstract: Three components of velocity fluctuations were measured in a plane turbulent wall jet which was modulated periodically by a sinusoidal pressure fluctuation in its settling chamber. The experiment was carried out in a closed-loop wind tunnel in the absence of an external stream at Reynolds number Rej = Ujb/v = 6900 and Strouhal number Stj = fb / Uj = 9.5 × 10-3, where b is the width of the slot from which the jet emerges at an efflux velocity Uj. A detailed comparison is provided with similar measurements made in a natural, unexcited turbulent wall jet. One of the purposes of this experiment was to establish the kinetic energy transfers which take place in the wall jet under controlled perturbations. More specifically, we were interested in determining the interactions occurring between the steady mean flow, the coherent eddies and the 'random' turbulent fluctuations. We used the triple decomposition of the equations of motion as suggested by Hussain (1983) and quickly observed that the usefulness of this decomposition depends on the definition of coherent motion, which is ambiguous in the presence of phase jitter. Two such definitions were considered and the results are discussed in the experimental case-study provided. An attempt is made to define quantitatively the intensities of the coherent motion in externally excited, wall-bounded flows. It is a case-study and not a parametric investigation aimed at maximizing the effects of period oscillations on the wall jet.
• Paschereit, C. O., Wygnanski, I., & Fiedler, H. E. (1995). Experimental investigation of subharmonic resonance in an axisymmetric jet. Journal of Fluid Mechanics, 283, 365-407.
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  Abstract: A resonant subharmonic interaction between two axisymmetric travelling waves was induced in the shear layer of an axisymmetric jet by controlled sinusoidal perturbations with two frequencies separated by one octave. The influence of initial parameters at the nozzle was also investigated. The results show that most of the energy for the resonant growth of the subharmonic comes directly from the mean flow. Fundamental wave acts as a catalyst enhancing the rate of energy transfer from the mean flow to the subharmonic.
• Seifert, A., & Wygnanski, I. J. (1995). On turbulent spots in a laminar boundary layer subjected to a self-similar adverse pressure gradient. Journal of Fluid Mechanics, 296, 185-209.
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  Abstract: An experimental investigation concerned with the characteristics of a turbulent spot propagating in a laminar boundary layer subjected to a self-similar adverse pressure gradient was conducted. Some small differences in the normalized shape of the undisturbed velocity profile were found to have a major influence on the spreading rate of the spot at comparable Reδ*. The rate of spread of the spot was found to be most dramatically affected by the pressure gradient. The interaction of the spot with the wave packet was enhanced by the adverse pressure gradient.
• Seifert, A., Zilberman, M., & Wygnanski, I. (1994). On the simultaneous measurements of two velocity components in the turbulent spot. Journal of Engineering Mathematics, 28(1), 43-54.
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  Abstract: Measurements of spanwise and streamwise velocity components in a turbulent spot artificially initiated in a Blasius boundary layer are described. A special hot-wire rake consisting of 8 `V' shaped arrays was built for this purpose. This multi-probe rake also enabled one to align the individual realizations in both streamwise and spanwise directions and to form more representative ensemble-averages near the tip of the spot. The data reveals the existence of a strong spanwise component of velocity which attains its maximum value at the tip of the spot and is centered around an elevation equivalent to the displacement thickness of the unperturbed boundary layer. This perturbation velocity resembles a wave which follows the leading interface of the spot and may therefore be amenable to analysis.
• Paschereit, C. O., Fiedler, H. E., & Wygnanski, I. (1993). On the influence of initial parameters on subharmonic resonance in an axisymmetric jet. Array.
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  Abstract: Hot wire measurements in an axisymmetric jet are presented. A resonant subharmonic interaction between the two axisymmetric travelling waves was induced in the shear layer of the jet by controlled sinusoidal perturbations with two frequencies separated by one octave. The phase averaging technique was applied to assess the evolution of the coherent waves. By Fourier decomposing the phase averaged data it was possible to obtain the magnitude as well as the phase of the measured signal. In agreement with theory (Cohen & Wygnanski 1987, Michalke 1984) it was found that wherever the excited waves are non-dispersive they may interact in a manner that enhances the amplification rate of the subharmonic wave. (from Authors)
• Siefert, A., Bachar, T., Koss, D., Shepshelovich, M., & Wygnanski, I. (1993). Oscillatory blowing: A tool to delay boundary-layer separation. AIAA journal, 31(11), 2052-2060.
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  Abstract: The effects of oscillatory blowing as a means of delaying separation are discussed. Experiments were carried out on a hollow, flapped NACA 0015 airfoil equipped with a two-dimensional slot over the hinge of the flap. The flap extender over 25 % of the chord and was deflected at angles as high as 40 deg. The steady blowing momentum coefficients could be varied independently of the amplitudes and frequencies of the superimposed oscillations. The modulated blowing was a major factor in improving the performance of the airfoil at much lower energy inputs than was hitherto known.
• Zhou, M. D., & Wygnanski, I. (1993). Parameters governing the turbulent wall jet in an external stream. AIAA journal, 31(5), 848-853.
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  Abstract: Mean velocity distributions in a plane, turbulent, and fully developed wall jet embedded in a uniform stream were measured for a variety of initial velocity ratios and Reynolds numbers. It was determined that the bulk of the flow is self-similar, provided the maximum velocity in the jet is twice as large as the freestream velocity. The normalized velocity profile depends on two velocity scales and on two length scales that, in turn, depend on the momentum flux at the nozzle, the viscosity, and the initial velocity ratio between the jet and the freestream defined by R ≡ (Uj - U∞)/(Uj + U∞). The width of the nozzle that was commonly used to reduce these data has no part in the similarity considerations. The approximate self-similarity may be used to estimate the skin friction that is otherwise determined with considerable difficulty.
• Katz, Y., Horev, E., & Wygnanski, I. (1992). The forced turbulent wall jet. JOURNAL OF FLUID MECHANICS, 242, 577-609.
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  Abstract: The effects of external two-dimensional excitation on the plane turbulent wall jet were investigated experimentally and theoretically. Measurements of the streamwise component of velocity were made throughout the flow field for a variety of imposed frequencies and amplitudes. The present data were always compared to the results generated in the absence of external excitation. Two methods of forcing were used: one global, imposed on the entire jet by pressure fluctuations in the settling chamber and one local, imposed on the shear layer by a small flap attached to the outer nozzle lip. The fully developed wall jet was shown to be insensitive to the method of excitation. Furthermore, external excitation has no appreciable effect on the rate of spread of the jet nor on the decay of its maximum velocity. In fact the mean velocity distribution did not appear to be altered by the external excitation in any obvious manner. The flow near the surface, however, (i.e. for 0 Y "SUP +" 100) was profoundly different from the unforced flow, indicating a reduction in wall stress exceeding at times 30%. The production of turbulent energy near the surface was also reduced, lowering the intensities of the velocity fluctuations. External excitation enhanced the two-dimensionality and the periodicity of the coherent motion. Spectral analysis and flow visualization suggested that the large coherent structures in this flow might be identified with the most-amplified primary instability modes of the mean velocity profile. Detailed stability analysis confirmed this proposition though not at the same level of accuracy as it did in many free shear flows. (A)
• Paschereit, C. O., Oster, D., Long, T. A., Fiedler, H. E., & Wygnanski, I. (1992). Flow visualization of interactions among large coherent structures in an axisymmetric jet. Experiments in Fluids, 12(3), 189-199.
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  Abstract: Interactions between large coherent structures are visualized with both schlieren photography in two air jets and dye photography in a water jet. The density difference needed for the schlieren technique is provided by an electrically heated wire ring surrounding the jet. External forcing with either single axisymmetric, single non-symmetric, combined axisymmetric or combined non-symmetric modes was applied. It was found that forcing the jet with a pair of different spinning modes leads to azimuthal distortions of the mean flow. This observation confirms and explains existing hotwire data. Simultaneous excitation with two axisymmetric modes may produce structures of higher modes or even cause structurally undistinguishable development. Streamwise structures are observed both in the unforced jet and in the axisymmetrically forced jet. They do not seem to be caused by a Görtier instability from the concave curvature of the conventional nozzle, since they were also found in a jet flow from a specially designed nozzle with only convex contraction surface. © 1992 Springer-Verlag.
• Wygnanski, I., Katz, Y., & Horev, E. (1992). On the applicability of various scaling laws to the turbulent wall jet. J. FLUID MECHANICS, 234, 669-690.
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  Abstract: Measures the spatial distribution of the mean velocity in a two-dimensional turbulent wall jet for a variety of nozzle Reynolds numbers. Shows that the bulk of the flow is self-similar and depends on momentum flux (or nozzle), and fluid viscosity and density. Notes that this type of self-similarity enables determination of the skin friction. The 'law of the wall' applies only to the viscous sublayer. The Reynolds stress in the inviscid, inner portion of the flow is not constant. The outer scaling law (Coles' law of the wake) is verified for the inviscid inner portion of the wall jet. Notes the logarithmic velocity distribution cannot be derived making the usual assumptions. (from Authors)
• Marasli, B., Champagne, F. H., & Wygnanski, I. J. (1991). On linear evolution of unstable disturbances in a plane turbulent wake. Physics of Fluids A, 3(4), 665-674.
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  Abstract: The evolution of sinuous perturbation waves in the turbulent wake of a flat plate is investigated. The Strouhal number of the perturbations is chosen so that the waves remain amplified over the entire range of measurements. Detailed comparisons between linear stability theory and the phase-averaged measurements of the coherent velocity field are presented. Initially, before significant amplification of the perturbation amplitude occurs, the agreement between the linear theory and the measurements is good. The measured amplitude and phase distributions of the streamwise and lateral components of the coherent or wave-induced velocity field as well as the coherent Reynolds stress show excellent agreement with their linear theory counterparts. The coherent Reynolds stress, which is generated through a nonlinear interaction between the fundamental mode and the mean flow, augments the turbulent Reynolds stress causing the spreading rate of the wake to increase. However, this nonlinear interaction does not affect the shape of the mean velocity profile in the early stages of amplification. The linear theory predictions deteriorate with increasing downstream distance because of nonlinearity and the stronger interaction with the turbulent field as the neutral point of the perturbation is approached. For the unforced now, the peak in the measured spectrum of the cross-stream (turbulent) velocity fluctuations at any downstream location (in the far wake) corresponds to the local neutral frequency from linear, spatial stability theory for inviscid, parallel flow. © 1991 American Institute of Physics.
• Glezer, A., Katz, Y., & Wygnanski, I. (1989). On the breakdown of the wave packet trailing a turbulent spot in a laminar boundary layer. Journal of Fluid Mechanics, 198, 1-26.
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  Abstract: The evolution of two oblique wave packets trailing a transitional spot in a laminar boundary layer was investigated in order to determine the extent of the interaction between the packets and the spot. The experimental investigation, carried out on two slightly different laminar boundary layers characterized by Falkner Skan constants of β = 0 and β = 0.2, revealed that very small pressure gradients can have significant effects on the stability of the laminar boundary layer and the rate at which it is contaminated by a turbulent spot. Some simple, novel statistical procedures for treating the data were developed and were used to accentuate the understanding of the physical processes governing transition to turbulence.
• Glezer, A., Wygnanski, I., & Gu, X. (1989). Amplitude-modulated excitation of a turbulent mixing layer. Physics of Fluids A, 1(6), 1007-1020.
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  Abstract: The evolution of a momentary, spanwise-uniform disturbance in a plane mixing layer was studied experimentally. Since interaction between the disturbance and the two-dimensional (primary) instability of the mean flow is unavoidable, excitation by a low-level, two-dimensional, time-harmonic carrier wave train provided a clear phase reference. The disturbance was generated by a lower frequency pulsed amplitude modulation of the excitation waveform. The duration of the modulating pulse was equal to the carrier wave period, and the degree of modulation was either 2 or 4. A demodulation technique was used to discriminate between the response to the modulating pulse and the harmonic excitation. This technique decomposes the response into a family of modal two-dimensional wave packets, allowing detailed study of the disturbance, and, in particular, the propagation, amplification, and some of the nonlinear aspects of the behavior of its leading modal components. The fundamental packet is advected with the mean velocity of the two streams. Its streamwise extent and dominant frequencies remain virtually unchanged with downstream distance. The passage of the disturbance is accompanied by a spatial and temporal change in the momentum thickness of the harmonically excited flow. Cross-stream distributions of the streamwise velocity perturbation within the disturbance are similar to those of the harmonically excited flow at streamwise stations having the same momentum thickness. High turbulence levels, not prevalent in the harmonically excited shear layer, are detected within the disturbance and suggest the possibility of transient mixing enhancement. © 1989 American Institute of Physics.
• Katz, Y., Nishri, B., & Wygnanski, I. (1989). The delay of turbulent boundary layer separation by oscillatory active control. Physics of Fluids A, 1(2), 179-181.
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  Abstract: The flow outside a solid wedge that abruptly diverges at an angle of 18° was investigated experimentally. A turbulent boundary layer, which separated at the discontinuity and turned into a mixing layer downstream of it, reattached as a result of harmonic excitation at the apex of the wedge. Preliminary results indicate that this might be an effective way to delay separation of turbulent and laminar boundary layers. © 1989 American Institute of Physics.
• Marasli, B., Champagne, H., & Wygnanski, I. J. (1989). Modal decomposition of velocity signals in a plane, turbulent wake. Journal of Fluid Mechanics, 198, 255-273.
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  Abstract: The Orr Sommerfeld equation admits two solution modes for the two-dimensional plane wake. These are the sinuous mode with antisymmetric streamwise fluctuations and the varicose mode with symmetric streamwise fluctuations. The varicose mode is often ignored because its amplification rates are considerably less than those of the sinuous mode. An experimental investigation of the varicose mode in a two-dimensional turbulent wake was undertaken to determine if this mode of instability agrees as well with linear stability theory, as did the sinuous mode in previous experiments (Wygnanski, Champagne & Marasli 1986). The experiments demonstrated that, although it is possible to generate a nearly pure symmetric disturbance wave, it is very difficult to do, because the flow is very sensitive to the slightest asymmetries which might be present in the experiments. These asymmetries are preferentially amplified, resulting in the eventual distortion of an initially prominent symmetric wave.
• Cohen, J., & Wygnanski, I. (1987). EVOLUTION OF INSTABILITIES IN THE AXISYMMETRIC JET. PART 1. THE LINEAR GROWTH OF DISTURBANCES NEAR THE NOZZLE.. Journal of Fluid Mechanics, 176, 191-219.
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  Abstract: The modal distributions of coherent structures evolving near the nozzle of a circular jet are considered. The effects produced on the instability modes by transverse curvature, flow divergence, inhomogeneous inflow conditions, and the detailed shape of the mean velocity profile, are investigated both theoretically and experimentally. Linear stability analysis applied to a thin shear layer surrounding a large-diameter jet (i. e. a jet whose diameter is large in comparison with a typical width of the shear layer) indicates that many azimuthal modes are equally unstable. An increase in the relative thickness of the shear layer limits the number of unstable modes, and only one helical mode remains unstable at the end of the potential core. The linear model used as a transfer function is capable of predicting the spectral distribution of the velocity perturbations in a jet. This provides a rational explanation for the stepwise behavior of the predominant frequency resulting from a continuous increase in the jet velocity.
• Cohen, J., & Wygnanski, I. (1987). EVOLUTION OF INSTABILITIES IN THE AXISYMMETRIC JET. PART 2. THE FLOW RESULTING FROM THE INTERACTION BETWEEN TWO WAVES.. Journal of Fluid Mechanics, 176, 221-235.
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  Abstract: Leading nonlinear interactions generated by waves externally superimposed on an axisymmetric jet are considered theoretically, and the results verified experimentally. The mean flow in the jet loses its axial symmetry whenever the jet is excited simultaneously by two different azimuthal modes of the the same frequency. Subharmonic resonance occurs in this flow whennever the conditions warrant such an occurrence, generating azimuthal modes which may not have been present otherwise in this flow. Some of these resonance conditions are explored.
• Wygnanski, I. J., & Petersen, R. A. (1987). COHERENT MOTION IN EXCITED FREE SHEAR FLOWS.. AIAA journal, 25(2), 201-213.
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  Abstract: The paper explores the application of the inviscid instability approach to externally excited turbulent free shear flows at high Reynolds numbers. After qualitative descriptions of coherent structures, the following topics are covered: application of the linear stability theory to large coherent structures in free turbulent shear flows; nonlinear evolution of instabilities in free shear flows; the 'preferred mode' concept; and vortex pairing in turbulent mixing layers. Finally, experimental results concerning control of free turbulent shear layers are presented.
• Wygnanski, I., Champagne, F., & Marasli, B. (1986). ON THE LARGE-SCALE STRUCTURES IN TWO-DIMENSIONAL, SMALL-DEFICIT, TURBULENT WAKES.. Journal of Fluid Mechanics, 168, 31-71.
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  Abstract: A systematic study of two-dimensional, turbulent, small-deficit wakes was carried out to determine their structure and the universality of their self-preserving states. Various wake generators, including circular cylinders, a symmetrical airfoil, a flat plate, and an assortment of screens of varying solidity, were studied for a wide range of downstream distances. The results indicate that the normalized characteristic velocity and length scales depend on the initial conditions, while the shape of the normalized mean velocity profile is independent of these conditions or the nature of the generator. The normalized distributions of the longitudinal turbulence intensity, however, are dependent on the initial conditions. Linear inviscid stability theory, in which the divergence of the mean flow is taken into account, predicts quite well the amplification and the transverse distributions of amplitudes and phases of externally imposed sinuous waves on a fully developed turbulent wake generated by a flat plate.
• Gaster, M., Kit, E., & Wygnanski, I. (1985). LARGE-SCALE STRUCTURES IN A FORCED TURBULENT MIXING LAYER.. Journal of Fluid Mechanics, 150, 23-39.
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  Abstract: The large-scale structures that occur at moderately high Reynolds numbers have been modelled by linear inviscid stability theory incorporating first-order corrections for slow spatial variations of the mean flow. The perturbation stream function for a spatially growing time-periodic travelling wave has been numerically evaluated for the measured linearly diverging mean flow. In an accompanying experiment periodic oscillations were imposed on the turbulent mixing layer by the motion of a small flap at the trailing edge of the splitter plate that separated the two uniform streams of different velocity. The results of the numerical computations are compared with experimental measurements. For comparisons made on a purely local basis, agreement in both the amplitude and phase distribution across the mixing layer is excellent. Comparisons on a global scale revealed, less accuracy.
• Shemer, L., Kit, E., & Wygnanski, I. (1985). On the impedance of the pipe in laminar and turbulent pulsating flows. Experiments in Fluids, 3(4), 185-189.
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  Abstract: The response of a facility, consisting of a valveless reciprocating pump, a large settling chamber and a long straight smooth pipe, to a periodic change in the volume was analysed. The impedance of the pipe was estimated in both laminar and turbulent flow regimes under otherwise identical flow conditions. A good agreement with theory was obtained for the laminar flow. The estimate of the pipe impedance from the experimental data in turbulent flow was based on the momentum equation as well as on the measured resonant frequency of the system. These independent methods show that the inertance of the pipe has a qualitatively different behavior in laminar and turbulent flow regimes. © 1985 Springer-Verlag.
• Shemer, L., Kit, E., & Wygnanski, I. (1985). On the impedance of the pipe in laminar and turbulent pulsating flows.. EXP. FLUIDS, 3(4 , 1985, p.185-189.).
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  Abstract: The response of a facility, consisting of a valveless reciprocating pump, a large settling chamber and a long straight smooth pipe, to a periodic change in the volume was analysed. The impedance of the pipe was estimated in both laminar and turbulent flow regimes under otherwise identical flow conditions. A good agreement with theory was obtained for the laminar flow. The estimate of the pipe impedance from the experimental data in turbulent flow was based on the momentum equation as well as on the measured resonant frequency of the system. These independent methods show that the inertance of the pipe has a qualitatively different behaviour in laminar and turbulent flow regimes. (A)
• Shemer, L., Wygnanski, I., & Kit, E. (1985). PULSATING FLOW IN A PIPE.. Journal of Fluid Mechanics, 153, 313-337.
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  Abstract: Turbulent and laminar pulsating flows in a straight smooth pipe are compared at identical frequencies and Reynolds numbers. Velocities at the exit plane of the pipe and pressure drop along the pipe were measured simultaneously. The introduction of the periodic surging had no significant effect on the time-averaged quantities, regardless of the flow regime. The time-dependent components at the forcing frequency, represented by a radial distribution of amplitudes and phases, are qualitatively different in laminar and turbulent flows. A normalization procedure is proposed which relates phase-locked turbulent flow parameters in unsteady flow to similar time-averaged quantities. A simple eddy-viscosity model described by a complex function can account for 'memory' of turbulence and explain the different phase distribution in laminar and turbulent flows.
• Samet, M., Wygnanski, I., & Ginav, S. (1984). Development of a probe for measuring turbulence in three dimensional flows.. Array.
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  Abstract: A new hot wire technique was developed and used for the simultaneous measurement of the three instantaneous velocity components in a turbulent swirling jet issuing into a coflowing stream. The probe includes four wires slanted at 45DEGREES in a double vee funnel configuration. Each vee funnel shaped is perpendicular to the other. A fully developed turbulent pipe flow, at Reynolds number (Re) of 50,000, was measured to evaluate the overall performance of the four wire system. Measurements were carried out at various radial locations 138 pipe diameters downstream from the entrance. This data obtained is compared with the classical results of Laufer (1954) and with the measurements of Wyganski and Champagne (1973) taken at an identical Re. New information concerning the distribution of the UW Reynolds stress product in the pipe flow is presented. Measurements at several axial locations in a turbulent swirling jet issuing into a coflowing stream were conducted. The swirl number and the velocity ratio were 0.265 and 0.2 respectively. At each axial location, the radial survey covered both sides of the centre line. Some of these results are compared with measurements of Samet which were obtained under similar flow conditions and at the same flow facility with a precalibrated directional sensitive five tube pressure probe.
• Wygnanski, I., & Champagne, F. (1984). ON LARGE COHERENT STRUCTURES IN TWO-DIMENSIONAL TURBULENT WAKES.. Array, 403-409.
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  Abstract: A systematic study of turbulent, small-deficit, planar wakes behind various wake generators was carried out to investigate the structure and the universality of their self-preserving states. The results indicate that the normalized characteristic velocity and length scales depend on the initial conditions while the shape of the normalized mean velocity profile is independent of initial conditions or the nature of the wake generator. The normalized distributions of the longitudinal turbulence intensity, however, are dependent on the initial conditions. Linear, inviscid stability theory in which the divergence of the mean flow was taken into account predicts quite well the amplification and the transverse distributions of amplitudes and phases of externally imposed sinuous waves on a turbulent wake generated by a flat plate.
• Wygnanski, I. (1983). ON TURBULENT SPOTS.. Array, 390-400.
• Wygnanski, I. (1983). On turbulent spots.. Array.
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  Abstract: In order to simplify the study of large eddies in turbulent boundary layers a laminar boundary layer was perturbed at a given point. This gave a moving turbulent 'spot' which was completely repeatable. The lifetime, and interactions of the spots were then easily studied using hot wire anemometers. The authors conclude that the turbulent spot (which can occur naturally at surface imperfections etc.) is a basic element of a turbulent boundary layer, that the eddies retain their relative locations as the spot grows and that a synthetic boundary layer produced by spots differs from the natural layer produced randomly and may have some engineering applications. Some discussion of the paper is included. Discussion of the paper is included. (K.R.)
• Oster, D., & Wygnanski, I. (1982). FORCED MIXING LAYER BETWEEN PARALLEL STREAMS.. Journal of Fluid Mechanics, 123, 91-130.
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  Abstract: The effect of periodic two-dimensional excitation on the development of a turbulent mixing region was studied experimentally. Controlled oscillations of variable amplitude and frequency were applied at the initiation of mixing between two parallel air streams. The frequency of forcing was at least an order of magnitude lower than the initial instability frequency of the flow in order to test its effect far downstream. The effect of the velocity difference between the streams was also investigated in this experiment. It was determined that the spreading rate of the mixing layer is sensitive to periodic surging even if the latter is so small that it does not contribute to the initial energy of the fluctuations. Oscillations at very small amplitudes tend to increase the spreading rate of the flow by enhancing the amalgamation of neighboring eddies, but at higher amplitudes the flow resonates with the imposed oscillation. Amalgamation of large coherent eddies is resumed beyond the resonance region, but the flow is not universally similar. Refs.
• Weisbrot, I., Einav, S., & Wygnanski, I. (1982). The nonunique rate of spread of the two-dimensional mixing layer. Physics of Fluids, 25(10), 1691-1693.
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  Abstract: The rate of spread of a plane, incompressible, turbulent shear layer was determined experimentally. The velocity ratio was constant, while the actual velocities were altered. It was established that the divergence of the flow with downstream distance was not uniquely determined by the velocity ratio in the range of variables considered. It is believed that an instability is responsible for this behavior. © 1982 American Institute of Physics.
• Wygnanski, I., Zilberman, M., & Haritonidis, J. H. (1982). ON THE SPREADING OF A TURBULENT SPOT IN THE ABSENCE OF A PRESSURE GRADIENT.. Journal of Fluid Mechanics, 123, 69-90.
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  Abstract: Velocity measurements in the plane of symmetry of a turbulent spot are reported. The number of data points taken at various streamwise locations was adequate to map the ensemble-averaged flow field in a spot at a given instance. These results are compared with velocities taken in laboratory coordinates, whereupon it is shown that the flow field in the spot depends either on the distance from its origin or on the time elapsed from its initiation. The two variables are related so that the flow may be transformed into either a time- or a space-independent problem. The dependence of the spot on the Reynolds number and on the surrounding laminar boundary layer is established. The effects of these parameters on the shape of the ensemble-averaged spot, its size, characteristic celerities, and relative rate of entrainment, are discussed. The present results indicate that a similarity approach based on ensemble-averaged data is severely limited. Refs.
• Wygnanski, I. (1981). The effects of Reynolds number and pressure gradient on the transitional spot in a laminar boundary layer.. Array, ed., Berlin, Fed. Rep. Germany, Springer-Verlag, 1981, Session 3-Experiments, p.304-322..
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  Abstract: The flow associated with the transitional spot is discussed in view of the possible relationship between the spot and the large coherent structure existing in a fully turbulent boundary layer. A brief review of the published results is given, yet the paper stresses new data. The entrainment of non-turbulent fluid into the spot is calculated on the plane of symmetry, taking into account the growth of the spot in the direction normal to the boundary. The effects of Reynolds number on the growth of the spot is discussed, as well as some effect of favourable pressure gradient. It was observed that the spot consists of an orderly array of eddies which give rise to the streaky structure in the spanwise direction. (A)
• Rubin, Y., Wygnanski, I., & Haritonidis, J. H. (1980). Further observations on transition in a pipe.. Array, eds., Berlin, Fed. Rep. Germany, Springer, Verlag, 1980, Session 1, p.17-26..
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  Abstract: Fully developed Poiseuille flow in a pipe was artificially disturbed at x/D 400 and 1700 Re 4000. Puffs and slugs generated by the disturbance were identical to the structures observed when the flow in the inlet region underwent transition (Wygnanski and Champagne 1973). It was established that a slug which has all the attributes of a fully developed turbulent pipe flow is generated by the coalescence of puffs. The puff, which seems to contain a small number of toroidal eddies appears to be a fundamental coherent structure in a fully developed turbulent pipe flow. Artificially generated successions of puffs, which were allowed to interact, closely resembled a slug. The evolution of a slug from puffs was thus established.
• Wygnanski, I., Haritonidis, J. H., & Kaplan, R. E. (1979). ON A TOLLMIEN-SCHLICHTING WAVE PACKET PRODUCED BY A TURBULENT SPOT.. J Fluid Mech, 92(pt 3), 505-528.
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  Abstract: Experimental investigations in the region following the passage of an isolated turbulent spot in a laminar boundary layer reveal the existence of a pair of oblique wave packets. These packets are swept at an angle of approximately 40 degree , and exhibit frequency and wave speed characteristics in agreement with predictions made for oblique Tollmien-Schlichting waves. No waves exist near the centerline of the spot. Several observations of the breakdown of this ordered motion into a new turbulent spot are shown. This breakdown is accompanied by the appearance of an intense shear layer inclined to the wall. Refs.
• Wygnanski, I., Oster, D., Fiedler, H., & Dziomba, B. (1979). On the perseverance of a quasi-two-dimensional eddy-structure in a turbulent mixing layer.. Journal of Fluid Mechanics, 93(2), 325-337.
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  Abstract: Strong external disturbances were introduced into a mixing layer in order to test the formation of the quasi two-dimensional ccoherent eddies and their survival under less than ideal conditions. Velocity and temperature correlation measurements, flow visualization, and the simultaneous use of a large number of sensors suggest that these eddies are very stable in the range of Reynolds numbers considered and they persevere in spite of the external buffeting imposed. -from Authors English
• Gutmark, E., Wolfshtein, M., & Wygnanski, I. (1978). PLANE TURBULENT IMPINGING JET.. Journal of Fluid Mechanics, 88(pt 4), 737-756.
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  Abstract: This paper presents an experimental study of the turbulent structure on the center-line of a two-dimensional impinging jet. The mean velocity, turbulent stresses, triple velocity products and temporal derivatives were measured and the energy balances for the three fluctuating components were calculated. The results indicate a selective stretching of vortices in the direction in which the streamlines spread near the wall, causing anisotropy in this region. The distribution of energy among various frequences was found from spectral measurements. These measurements revealed the existence of a neutral frequency above which the energy was attenuated by viscous dissipation and below which it was augmented by a vortex-stretching mechanism. Refs.
• Oster, D., Wygnanski, I., Dziomba, B., & Fiedler, H. (1978). On the effect of initial conditions on the two dimensional turbulent mixing layer.. IN: STRUCTURE AND MECHANISMS OF TURBULENCE, (PROC. SYMP. ON TURBULENCE, BERLIN, FED. REP. GERMANY, AUG.1-5, 1977), 1 , H. Fiedler, Ed., Berlin, Fed. Rep. Germany, Springer-Verlag, 1978/-.
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  Abstract: It was observed that a very small sinusoidal disturbance, which is placed at the initiation of mixing of two parallel streams has a mayor effect on the spreading rate of the mixing layer. The large eddy structure within the layer is very regular in spite of the fact that the flow is fully turbulent and the Reynoldsnumber is of the order of 10SUB6. The spreading rate of the mixing layer is thus directly related to the growth of the large coherent vortices. The distance, in the direction of streaming, over which the spread of the mixing layer can be controlled, depends on the frequency of forcing. Doubling the initial rate of spread of the mixing layer is not only feasible but can be easily attained. (A)
• Wygnanski, I., & Chih-Ming, H. (1978). Note on the prong configuration of an x-array hot-wire probe. Review of Scientific Instruments, 49(6), 865-866.
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  Abstract: The x-array hot-wire probe with prongs perpendicular to the mean flow is found to be unacceptable for turbulence measurements.
• Zilberman, M., Wygnanski, I., & Kaplan, R. E. (1977). Transitional boundary layer spot in a fully turbulent environment. Physics of Fluids, 20(10), S258-S271.
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  Abstract: A spark was used to initiate and mark in time a turbulent spot in an initially laminar boundary layer. This marked spot of turbulence merged and interacted with the natural turbulent boundary layer generated by a row of spherical trips. By using a digital technique to align individual spot signatures, thus correcting for variations in the transit time to a given measurement station, a structure was tracked over a streamwise extent of 70 average turbulent boundary-layer thicknesses. The scale of the structure is of the order of 10δ in the streamwise direction becoming 2-3δ in the interface region of the boundary layer and is less than 4δ in the spanwise direction, in spite of the fact that no spanwise alignment was performed. The structure is characterized by a convection speed of 0.9 U ∞. It exhibits features in detailed agreement with those at the outer region of the turbulent boundary layer (interface region) and is consistent with existing two- and three-point space-time cortelations. Copyright © 1977 American Institute of Physics.
• CHAMPAGNE, F. H., PAO, Y. H., & WYGNANSKI, I. J. (1976). ON THE TWO-DIMENSIONAL MIXING REGION.. Journal of Fluid Mechanics, 74 , PT. 2, MARCH 23, 1976.
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  Abstract: AN EXPERIMENTAL INVESTIGATION OF THE TWO-DIMENSIONAL INCOMPRESSIBLE MIXING LAYER WAS CARRIED OUT.THE MEASUREMENTS PROVIDE NEW INFORMATION ON THE DEVELOPMENT OF THE MEAN AND TURBULENT FIELDS TOWARDS A SELF-PRESERVING STATE AND ON THE HIGHER-ORDER STATISTICAL CHARACTERISTICS OF THE TURBULENT FIELD.THE RELEVANCE OF INITIAL CONDITIONS TO THE DEVELOPMENT OF THE FLOW IS DISCUSSED IN THE LIGHT OF BOTH PRESENT AND PREVIOUS DATA.MEASUREMENTS OF SPECTRA, PROBABILITY DENSITIES AND MOMENTS TO EIGHTH ORDER OF ALL THREE VELOCITY-COMPONENT FLUCTUATIONS AT VARIOUS TRANSVERSE POSITIONS ACROSS THE FLOW WERE CARRIED OUT USING AN ON-LINE DIGITAL DATA ACQUISITION SYSTEM.THE PROBABILITY DENSITY DISTRIBUTIONS OF THE DERIVATIVE AND THE SQUARED DERIVATIVE OF THE LONGITUDINAL AND LATERAL VELOCITY FLUCTUATIONS WERE ALSO DETERMINED.DIRECT MEASUREMENTS OF MOMENTS TO EIGHTH ORDER OF THE VELOCITY DERIVATIVES WERE ATTEMPTED AND ARE DISCUSSED IN THE LIGHT OF THE SIMULTANEOUSLY MEASURED HISTOGRAMS.THE PROBLEMS IN OBTAINING HIGHER-ORDER STATISTICAL DATA ARE CONSIDERED IN SOME DETAIL.ESTIMATES OF THE INTEGRAL TIME SCALE OF MANY OF THE HIGHER-ORDER STATISTICS ARE PRESENTED.THE HIGH WAVENUMBER STRUCTURE WAS FOUND TO BE LOCALLY ANISOTROPIC ACCORDING TO BOTH SPECTRAL AND TURBULENT VELOCITY-GRADIENT MOMENT REQUIREMENTS.HIGHER-ORDER SPECTRA TO FOURTH ORDER OF THE LONGITUDINAL VELOCITY FLUCTUATIONS WERE MEASURED AND ARE DISCUSSED.FINALLY THE LOGNORMALITY OF THE SQUARED LONGITUDINAL AND LATERAL VELOCITY-DERIVATIVE FLUCTUATIONS WAS INVESTIGATED AND THE UNIVERSAL LOGNORMAL CONSTANT MU WAS EVALUATED.(A.)
• Champagne, F. H., Pao, Y. H., & Wygnanski, I. J. (1976). ON THE TWO-DIMENSIONAL MIXING REGION.. Journal of Fluid Mechanics, 74(pt 2), 209-250.
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  Abstract: An experimental investigation of the two-dimensional incompressible mixing layer was carried out. The relevance of initial conditions to the development of the flow is discussed in the light of both present and previous data. Measurements of spectra, probability densities and moments to eighth order of all three velocity-component fluctuations at various transverse positions across the flow were carried out using an on-line digital data acquisition system. The probability density distributions of the derivative and the squared derivative of the longitudinal and lateral velocity fluctuations were also determined. Direct measurements of moments to eighth order of the velocity derivatives were attempted and are discussed in the light of the simultaneously measured histograms. The problems in obtaining higher-order statistical data are considered in some detail. Estimates of the integral time scale of many of the higher-order statistics are presented. The high wave-number structure was found to be locally anisotropic according to both spectral and turbulent velocity-gradient moment requirements. The measurements provided new information on the development of the mean and turbulent fields towards a self-preserving state and on the higher-order statistical characteristics of the turbulent field.
• Gutmark, E., & Wygnanski, I. (1976). PLANAR TURBULENT JET.. Journal of Fluid Mechanics, 73(pt 3), 465-495.
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  Abstract: This investigation was undertaken in order to extend the available information on the two-dimensional self-preserving jet. Results of hot-wire measurements in a plane incompressible jet are reported. The flow was found to be self-preserving beyond x/d greater than 40 and measurements were made up to x/d equals 120. The quantities measured include mean velocities, turbulence intensities, and third- and fourth-order terms, as well as two-point correlations and the intermittency factor. Conditional sampling techniques were used to obtain exclusively data within the turbulent zone of the jet. The results are compared with previous investigations.
• Zilberman, M., Wygnanski, I., & Kaplan, R. E. (1976). Transitional boundary layer spot in a fully turbulent environment.. PHYS. FLUIDS, 20(10 , Part II, Oct. 1977), eds., New York, U.S.A., Am. Inst. of Physics, 1976).
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  Abstract: A spark was used to initiate and mark in time a turbulent spot in an initially laminar boundary layer. This marked spot of turbulence merged and interacted with the natural turbulent boundary layer generated by a row of spherical trips. By using a digital technique to align individual spot signatures, thus correcting for variations in the transit time to a given measurement station, a structure was tracked over a streamwise extent of 70 average turbulent boundary-layer thicknesses. The scale of the structure is of the order of 10SUPg in the streamwise direction becoming 2-3SUPg in the interface region of the boundary layer and is less than 4SUPg in the spanwise direction, in spite of fact that no spanwise alignment was performed. The structure is characterized by a convection speed of 0.9USUP. It exhibits features in detailed agreement with those at the other region of the turbulent boundary layer (interface region) and is consistant with existing two-and three-point space-time correlations. (A)
• Wygnanski, I., Sokolov, M., & Friedman, D. (1975). ON TRANSITION IN A PIPE - 2. THE EQUILIBRIUM PUFF.. Journal of Fluid Mechanics, 69(pt 2), 283-304.
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  Abstract: Conditionally sampled hot-wire measurements were taken in a pipe at low Reynolds numbers (2700 greater than Re greater than 2000) corresponding to the onset of turbulence as a result of a large perturbation in the flow. This type of transition gives rise to a turbulent puff which maintains itself indefinitely at around Re equals 2200. The structure of puffs was investigated in some detail and was found to be very different from the structure of fully developed turbulent pipe flow. Nevertheless, it is independent of the character of the disturbance which created it. The purpose of the study was to gain some insight into the mechanism of transition in a pipe.
• WYGNANSKI, I., SOKOLOV, M., & FRIEDMAN, D. (1974). ON TRANSITION IN A PIPE; PART II: THE EQUILIBRIUM PUFF. Array, 1974-.
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  Abstract: CONDITIONALLY SAMPLED HOT WIRE MEASUREMENTS WERE TAKEN IN A PIPE AT LOW REYNOLDS NUMBERS BETWEEN 2700 AND 2000 CORRESPONDING TO THE ONSET OF TURBULENCE AS A RESULT OF A LARGE PERTURBATION IN THE FLOW.THIS TYPE OF TRANSITION GIVES RISE TO A TURBULENT PUFF WHICH MAINTAINS ITSELF INDEFINITELY AROUND RE # 2200.THE STRUCTURE OF THE PUFF WAS INVESTIGATED IN SOME DETAIL AND WAS FOUND TO BE VERY DIFFERENT FROM THE STRUCTURE OF THE FULLY DEVELOPED TURBULENT PIPE FLOW, NEVERTHELESS IT IS INDEPENDENT OF THE CHARACTER TO THE DISTURBANCE WHICH CREATED IT.THE PURPOSE OF THE STUDY WAS TO GAIN SOME INSIGHT INTO THE MECHANISM OF TRANSITION IN A PIPE.(A) (MICROFICHE)
• Champagne, F., & Wygnanski, I. (1971). An experimental investigation of coaxial turbulent jets. International Journal of Heat and Mass Transfer, 14(9), 1445-1464.
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  Abstract: The flow field generated by two coaxial jets was investigated experimentally with hot-wire anemometers. The area ratio between the external and internal nozzle was varied as well as the velocity issuing from each of the nozzles. The distribution of the mean velocities, turbulence intensities, and shear stresses were determined for the various cases. The development of the flow field and its approach to a self-preserving state is discussed. The Reynolds numbers based on the nozzle diameters varied from 0 to 10s and the velocities were low enough that the flow can be considered incompressible. © 1971.
• WYGNANSKI, I. (1971). Some preliminary observations on transition in a pipe. Isr J Technol, 9(1-2), 105-114.
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  Abstract: Conventional and conditionally sampled hot- wire measurements were taken in a pipe at Reynolds numbers corresponding to the onset of turbulence. It was observed that the laminar velocity profile is distorted in an axisymmetric fashion with increasing Re. At Re greater than 4000 the increase of Re has no significant effect on the shape of the laminar velocity profile when turbulent slugs appear. Some gross characteristics of turbulent slugs were examined. It was found that a typical slug is symmetrical provided that it originates in the pipe and is not triggered by an obstacle at the inlet.
• Wygnanski, I., & Gutmark, E. (1971). Lateral motion of the two-dimensional jet boundaries. Physics of Fluids, 14(7), 1309-1311.
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  Abstract: Two intermittency signals on each side of a two-dimensional jet were correlated to determine the manner in which boundaries move relative to one another. With some minor reservations it is concluded that each boundary moves independently.
• WYGNANSKI, I. (1970). SOME PRELIMINARY OBSERVATIONS ON TRANSITION IN A PIPE. Array.
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  Abstract: Conventional and conditionally sampled hot-wire measurements were taken in a pipe at Reynolds numbers corresponding to the onset of turbulence. The laminar velocity profile is distorted in an axisymmetric fashion with increasing Re. At Re more than 4000 the increase of Re has no significant effect on the shape of the laminar velocity profile even when turbulent slugs appear. Some gross characteristics of turbulent slugs were examined. It was found that a typical slug is symmetical (the shape of the leading edge is the same as the trailing edge) provided that it originates in the pipe and it is not triggered by an obstacle at the inlet.
• WYGNANSKI, I., & FIEDLER, H. E. (1970). TWO-DIMENSIONAL MIXING REGION. Journal of Fluid Mechanics, 41(pt 2), 327-361.
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  Abstract: The two-dimensional incompressible mixing layer was investigated by using constant-temperature, linearized hot wire anemometers. The measurements were divided into three categories-the conventional average measurements; time-average measurements in the turbulent and the nonturbulent zones; ensemble average measurements conditioned to a specific location of the interface. The turbulent energy balance was constructed twice, once using the conventional results and again using the turbulent zone results. Some differences emerged between the two sets of results. It appears that the mixing region can be divided into two regions, one on the high velocity side which resembles the outer part of a wake and the other on the low velocity side which resembles a jet. The binding turbulent-non- turbulent interfaces seem to move independently of each other. There is a strong connection between the instantaneous location of the interface and the axial velocity profile. Indeed the well known exponential mean velocity profile never actually exists at any given instant. In spite of the complexity of the flow the simple concepts of eddy viscosity and eddy diffusivity appear to be valid within the turbulent zone.
• Wygnanski, I. (1970). Swirling axisymmetrical laminar jet. Physics of Fluids, 13(10), 2455-2460.
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  Abstract: A similarity solution for the complete Navier-Stokes equations describing the flow of an incompressible laminar swirling jet was obtained. The solution clearly shows the effect of swirl on the other velocity components, and it is valid for all degrees of swirl. It thus differs from the previous solutions which were of the perturbation type.
• WYGNANSKI, I., & FIEDLER, H. (1969). SOME MEASUREMENTS IN THE SELF-PRESERVING JET. Journal of Fluid Mechanics, 38(pt 3), 577-612.
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  Abstract: The axisymmetric turbulent incompressible and isothermal jet was investigated by use of linearized constant-temperature hot- wire anemometers. It was established that the jet was truly self- preserving some 70 diam downstream of the nozzle and most of the measurements were made in excess of this distance. The quantities measured include mean velocity, turbulence stresses, intermittency, skewness and flatness factors, correlations, scales, low-frequency spectra and convection velocity. The rms values of the various velocity fluctuations differ from those measured previously as a result of lack of self-preservation and insufficient frequency range in the instrumentation of theprevious investigations. It appears that Taylor's hypothesis is not applicable to this flow, but the use of convection velocity of the appropriate scale for the transformation from temporal to spatial quantities appears appropriate. The energy balance was calculated from the various measured quantities and the result is quite different from the recent measurements of S. Sami, which were obtained 20 diam diwnstream from the nozzle. In light of these measurements some previous hypotheses about the turbulent structure and the transport phenomena are discussed.
• Wygnanski, I. (1969). The axisymmetric laminar jet in an infinite stream. Chemical Engineering Science, 24(4), 763-769.
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  Abstract: The analysis for the flow obtained when an axisymmetrical jet of incompressible fluid discharges into a uniform stream is presented. A solution of the boundary layer equations for the velocity distribution is given in terms of joined coordinate expansions. Some constants appearing is the expansion derived for large values of the coordinate, can be determined from the joining process. This is so because these constants depend on the upstream velocity profile and would otherwise remain unknown. An expansion of the same form will apply to the small deficit axisymmetric wake. Each of the expansions presented is entirely self-consistent to the order considered. © 1996.
• WYGNANSKI, I., & FIEDLER, H. E. (1968). JETS AND WAKES IN TAILORED PRESSURE GRADIENT. Physics of Fluids, 11(12), 2513-2523.
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  Abstract: Similarity solutions of the boundary-layer equations representing the flow of jets in an external stream and tailored pressure gradients were obtained. These solutions apply to jets in coflowing and counterflowing streams. A number of analytical solutions not previously published were obtained. Of particular interest are the solutions for small-increment jets which imply that a gaussian velocity profile exists far downstream from the origin of the jet, even in arbitrary pressure gradient.