Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session GJ: Flow Control IV |
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Chair: Dietmar Rempfer, Illinois Institute of Technology Room: Long Beach Convention Center 201A |
Monday, November 22, 2010 8:00AM - 8:13AM |
GJ.00001: Flow Structures and Interactions of a Fail-Safe Actuator Wasif Khan, Yoseph Elimelech, Michael Amitay Vortex generators are passive devices that are commonly used in many aerodynamic applications. In their basic concept, they enhance mixing, reduce or mitigate flow separation; however, they cause drag penalties at off design conditions. Micro vanes implement the same basic idea of vortex generators but their physical dimensions are much smaller. To achieve the same effect on the baseline flow field, micro vanes are combined with an active flow control device, so their net effect is comparable to that of vortex generators when the active device is energized. As a result of their small size, micro vanes have significantly less drag penalty at off design conditions. This concept of ``dual-action'' is the reason why such actuation is commonly called hybrid or fail-safe actuation. The present study explores experimentally the flow interaction of a synthetic-jet with a micro vane in a zero pressure gradient flow over a flat plate. Using the stereo particle image velocimetry technique a parametric study was conducted, where the effects of the micro vane shape, height and its angle with respect to the flow were examined, at several blowing ratios and synthetic-jet configurations. [Preview Abstract] |
Monday, November 22, 2010 8:13AM - 8:26AM |
GJ.00002: Bluff Body Separation Control using Pulsed Actuation George T.K. Woo, Thomas M. Crittenden, Ari Glezer The severing of a separating shear layer using pulsed actuation jets is exploited for separation control over a 3D bluff body by the transitory manipulation of the shedding of the large-scale vortical structures. In the present wind tunnel investigation, actuation is effected by surface-integrated discrete arrays of pulsed (combustion-powered) actuators having a characteristic time scale that is an order of magnitude shorter than the convective time scale of the base flow. High-resolution PIV measurements, taken phase-locked to the actuation, show that the interaction between the pulsed jets and the separated cross flow results in significant streamwise advection of the separation point. The flow response is dominated by disparate time scales between the onset of the actuation and the subsequent relaxation which directly affects the attachment process. Successive actuation can extend the interactions within the attaching boundary layer such that the aerodynamic forces and moments are quasi-steady and significantly enhanced compared to what can be achieved with continuous time-harmonic actuation. Supported by the US Army. [Preview Abstract] |
Monday, November 22, 2010 8:26AM - 8:39AM |
GJ.00003: Interaction of a Finite-span Synthetic-jet and Cross-flow over a Swept Wing Joseph Vasile, Yoseph Elimelech, Michael Amitay An experimental investigation was performed to study the interaction of a single finite-span synthetic jet with the flow over a finite and swept back wing at a Reynolds number of 10$^{5}$ for three angles-of-attack. For the actuation levels, two momentum coefficients were considered, corresponding to two blowing ratios, 0.8 and 1.2. Stereoscopic PIV data were acquired in the vicinity of the synthetic-jet orifice at the wing's mid-span section. The effect of blowing ratio was analyzed using both time and phase averaged statistics. The results show that the flow field in the vicinity of the synthetic-jet orifice becomes three-dimensional and time-dependant and is governed by the superposition of two kinds of flow structures: (1) streamwise structures that are associated with the finite span of the jet (edge vortices), and (2) spanwise flow structures that are generated along the orifice's long axis due to the vortex pairs that are formed by the synthetic jet. Furthermore, an analysis of the flow field showed that the streamwise flow structures are more pronounced while the coherence of the spanwise flow structures is deteriorated within few orifice widths. [Preview Abstract] |
Monday, November 22, 2010 8:39AM - 8:52AM |
GJ.00004: Hybrid Manipulation of Streamwise Vorticity in a Diffuser Boundary Layer Abraham Gissen, Bojan Vukasinovic, John Culp, Ari Glezer The formation of streamwise vorticity concentrations by exploiting the interaction of surface-mounted passive (micro-vanes) and active (synthetic jets) flow control elements with the cross flow is investigated experimentally in a small-scale serpentine duct at high subsonic speeds (up to M = 0.6). Streamwise vortices can be a key element in the mitigation of the adverse effects on pressure recovery and distortion caused by the naturally occurring secondary flows in embedded propulsion systems with complex inlet geometries. Counter rotating and single-sense vortices are formed using conventional passive micro-vanes and active high-power synthetic jet actuators. Interaction of the flow control elements is examined through a hybrid actuation scheme whereby synthetic jet actuation augments the primary vanes' vortices resulting in dynamic enhancement of their strength. It is shown that such sub-boundary layer individual vortices can merge and evolve into duct-scale vortical structures that counteract the inherent secondary flow and mitigates global flow distortion. [Preview Abstract] |
Monday, November 22, 2010 8:52AM - 9:05AM |
GJ.00005: Experimental study of airfoil separation control using synthetic jets Xi Xia, Kamran Mohseni The flow control over an airfoil is studied experimentally in a wind tunnel. Synthetic jets are placed on the top surface of the airfoil as flow actuators. The position and the angle of the jet orifice, together with the frequency and jet strength could be varied in order to adjust the separation or reattachment points on the surface. An Array of hot-film sensors are located on the surface in order to detect the location of the reattachment point. The airfoil is mounted on a 6 d.o.f force balance system to dynamically measure the drag and lift forces on the airfoil. Results from the hot-film sensor array measurement are correlated to the measured drag and lift forces. [Preview Abstract] |
Monday, November 22, 2010 9:05AM - 9:18AM |
GJ.00006: Performance Enhancement of a Wind Turbine Blade using Synthetic Jets Keith Taylor, Chia Leong, Michael Amitay Recent developments in flow control techniques, coupled with increased interest in green energy technologies, have led to interest in applying flow control techniques to wind turbines, in an effort to increase power output and reduce structural stress associated with widely varying loading. A reduction in structural stress could lead to reduced operational costs associated with maintenance. Presented is an investigation into the effect of active flow control on the aerodynamic and structural aspects of a finite-span S-809 airfoil. Synthetic jets are employed in an open loop control scheme to demonstrate the effect on lift, drag, and vibrations of the blade at Reynolds numbers of 110,000 and 220,000. Vibrometer measurements are presented to quantify vibration frequency, and time dependant fluctuations in lift and drag are correlated to tip deflection fluctuations. Static and dynamic pitch conditions are examined, with a sinusoidal pitch profile implemented for dynamic conditions. It is demonstrated that flow control can reduce tip deflection in static conditions, as well as reduce or eliminate hysteresis as the blade dynamically pitches into and out of separation. [Preview Abstract] |
Monday, November 22, 2010 9:18AM - 9:31AM |
GJ.00007: Controlled Dynamic Stall using Pulsed Fluidic Actuation George T.K. Woo, Ari Glezer Controlled attachment of transitory stall over an oscillating airfoil is investigated in wind tunnel experiments using a spanwise array of surface-integrated pulsed jet actuators such that the characteristic actuation time scale is an order of magnitude shorter than the convective time scale of the base flow. Earlier work showed that single-pulse actuation results in a rapid [O(1TCONV)] attachment of the separated flow followed by a slower [O(10TCONV)] detachment. These dynamics are exploited for controlled mitigation of pre- and post-stall dynamics during the oscillation cycle and the interaction between the actuation jets and the evolution of the dynamic stall vortex. The transitory effects of the actuation can be extended and exploited for trapping and regulation of the dynamic stall vorticity concentrations by using staged, multiple actuation pulses during the cycle. These interactions are investigated using high-resolution phase-locked PIV measurements in the cross stream plane (including the near wake). The temporal changes in the vorticity flux results in significant changes in circulation, and consequently in the time-dependent aerodynamic forces and moments. [Preview Abstract] |
Monday, November 22, 2010 9:31AM - 9:44AM |
GJ.00008: Stall Control Simulation with an Impulse Jet Sol Keun Jee, Robert Moser, Omar Lopez An impulse jet is investigated numerically to understand the mechanism by which this jet controls a stalled flow over an airfoil. The DDES (delayed detached eddy simulation) turbulence model is used in this stall control study for a NACA 4415 airfoil at an angle of attack of 20 degree and Reynolds number Re=570,000. An impulse jet, which is applied upstream of the nominal flow separation point, generates vortices that convect downstream, interact with the separating shear layer, dismantle the layer and allow following vortices to propagate along the surface in the separation region. These following vortices shift the separation point aft reattaching the boundary layer, which returns slowly to its initial stall condition, as observed in wind-tunnel experiments. A simple model of the impulse jet actuator used herein is found to be sufficient to represent the global effects of the jet on the stalled flow because it correctly represents the momentum injected into the flow. [Preview Abstract] |
Monday, November 22, 2010 9:44AM - 9:57AM |
GJ.00009: Numerical investigation of the interaction between a finite-span synthetic jet and a cross flow over a swept wing Michel Rasquin, Nicholas Mati, Onkar Sahni, Kenneth Jansen The interaction of a finite-span synthetic jet with the flow over a finite and swept back wing at a Reynolds number of $10^5$ and at low angles of attack is studied by means of parallel adaptive flow simulations. The focus of the work is to explore the details of the flow structures in the vicinity of the synthetic jet, in coordination with experimental studies. Both instantaneous and phase-averaged flow fields are collected for that purpose. It is found that an array of counter-rotating vortical structures formed by the synthetic jet interacts with the cross flow, and develops three-dimensionalities as they are advected downstream. The effect of two blowing ratios (of 0.8 and 1.2) is also explored. In the case of low blowing ratio, coherent vortical structures are found to be dominant. At high blowing ratio, coherent vortical structures breakdown forming random ones. Finally, the predictions of the CFD simulations and the experimental measurements are compared. [Preview Abstract] |
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