Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session L9: Flow Control IV |
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Chair: Hassan Nagib, Illinois Institute of Technology Room: 312 |
Monday, November 21, 2011 3:35PM - 3:48PM |
L9.00001: Unsteady Aerodynamic Flow Control of a Suspended Axisymmetric Moving Platform Thomas Lambert, Bojan Vukasinovic, Ari Glezer The aerodynamic forces on an axisymmetric wind tunnel model are altered by fluidic interaction of an azimuthal array of integrated synthetic jet actuators with the cross flow. Four-quadrant actuators are integrated into a Coanda surface on the aft section of the body, and the jets emanate from narrow, azimuthally segmented slots equally distributed around the model's perimeter. The model is suspended in the tunnel using eight wires each comprising miniature in-line force sensors and shape-memory-alloy (SMA) strands that are used to control the instantaneous forces and moments on the model and its orientation. The interaction of the actuation jets with the flow over the moving model is investigated using PIV and time-resolved force measurements to assess the transitory aerodynamic loading effected by coupling between the induced motion of the aerodynamic surface and the fluid dynamics that is driven by the actuation. It is shown that these interactions can lead to effective control of the aerodynamic forces and moments, and thereby of the model's motion. [Preview Abstract] |
Monday, November 21, 2011 3:48PM - 4:01PM |
L9.00002: Experimental Study of Plasma Control of an Unstarting Supersonic Flow Seong-kyun Im, Hyungrok Do, Mark A. Cappelli Experimental studies of the control of unstarting supersonic model inlet flows using Dielectric Barrier Discharges (DBD) is demonstrated at Mach 4.7 flow conditions and a static temperature of $\sim $60K and static pressure of $\sim $1kPa. Planar Laser Rayleigh Scattering (PLRS) is used to visualize important flow features, such as boundary layers and shockwaves. Supersonic flow unstart is initiated by injecting mass into model inlet flows of either laminar or tripped turbulent boundary layer flow conditions. DBD discharge actuation of the tripped turbulent flow delays the unstart process, shifting the unstart dynamics closer to what is seen for the laminar boundary layer case. In all studies, a single DBD actuator pair is used, oriented parallel to the freestream flow, generating spanwise disturbances. It is proposed that strong suction flow which brings high momentum freestream flow near exposed electrode can be a mechanism of this actuation. PLRS reveals that this actuation is spatially confined to the regions close to the actuator electrodes, greatly limiting their performance. [Preview Abstract] |
Monday, November 21, 2011 4:01PM - 4:14PM |
L9.00003: High Mach Number Leading-edge Flow Separation Control using AC DBD Plasma Actuators Christopher Kelley, Patrick Bowles, John Cooney, Chuan He, Thomas Corke, Bradley Osborne, Joseph Silkey, Joseph Zehnle Wind tunnel experiments were conducted to quantify the effectiveness of alternating current dielectric barrier discharge flow control actuators to suppress leading-edge stall on a NASA energy efficient transport airfoil at compressible freestream speeds. The objective of this research was to increase lift, reduce drag, and improve the stall characteristics of the supercritical airfoil near stall by flow reattachment at relatively high Mach and Reynolds numbers. In addition, the effect of unsteady (or duty cycle) operation on these aerodynamic quantities was also investigated. The experiments were conducted for a range of Mach numbers between 0.1 and 0.4. corresponding to a Reynolds number range of 560,000 through 2,260,000. Lift, drag, quarter chord moment, and suction side pressures were measured near stall for baseline, steady actuation, and a scan of nondimensional duty cycle frequencies. The results show that the plasma actuators were effective at reattaching the leading-edge separated flow as evidenced by the increase in maximum lift coefficient and stall angle (as much as 2.5 degrees). The experiment also showed that lift was increased the most when the plasma actuator was operated unsteady with a nondimensional frequency of unity. [Preview Abstract] |
Monday, November 21, 2011 4:14PM - 4:27PM |
L9.00004: Interaction of Finite-span Synthetic-jets with a Cross-flow over a Swept Wing Joseph Vasile, Michael Amitay The formation of secondary flow structures due to the interaction of three finite span synthetic jets with a cross-flow was investigated experimentally over a finite sweptback wing (cross-sectional profile of the NACA 4421) at a Reynolds number of 100,000. Stereoscopic PIV data were collected across the three jets in the wing's mid-span section, where the effect of the jets' location, and their blowing ratio were analyzed based on the three-dimensional flow field using time-averaged and phase-averaged statistics. The arrangement of synthetic jets was investigated through the use of varying actuation combinations in order to fully understand the interaction of the three jets with the cross flow. In the present study, an angle of attack of 13.5deg was chosen for the model, in which the boundary layer was attached in the vicinity of the middle synthetic jet and partially separated in the vicinity of the jet closer to the wing tip. The present work confirmed the previous findings of the presence of secondary tilted flow structures. [Preview Abstract] |
Monday, November 21, 2011 4:27PM - 4:40PM |
L9.00005: Reduction of vortex shedding intensity from a cylinder using semi-active flow control Jarle V. Ekanger, Morten Kjeldsen Experiments were performed in the open water channel at the Waterpower Laboratory, NTNU, Norway, with the aim of reducing vortex shedding intensity by semi-active flow control. The test rig consisted of a perforated steel tube lined by a rubber bellows. The holes (d/D=0.6) formed a line at the leading edge, one tube diameter apart. Two flow control modes were attainable; (1) lining being flush with the cylinder wall, and (2) pressurized lining creating leading edge bumps. Upstream flow conditions were monitored, and used as input for the control loop governing the pressure of the lining. A flat metal rod, onto which strain gauges were glued, was positioned in the wake. It was assumed that the motion of the rod corresponded to the velocity components normal to the main flow direction. Thus the motion of the rod described the vortex shedding from the tube. Strouhal numbers were found to be approximately 0.3. It was the assumption that the bumps would disrupt vortex formation and reduce the vortex intensity. Tests showed that the assumption was plausible, with observed intensity reductions of 15-30{\%} for Re$_{D} \quad \sim $ [20000 to 50000]. Plots also appear to show a breakdown of organization in the wake when the tube is in activated mode. It was shown that semi-active control of vortex shedding behind a cylinder is achievable. [Preview Abstract] |
Monday, November 21, 2011 4:40PM - 4:53PM |
L9.00006: Lift and Drag on a NACA0015 Airfoil With Duty Cycle Active Flow Control Pooya Kabiri, Douglas Bohl, Goodarz Ahmadi Active flow control experiments were carried out over a NACA 0015 airfoil with a trailing edge flap. Two arrays of synthetic jet actuators were mounted in the airfoil with one on near the leading edge (0.1c) and the other on the main wing body near the wing/flap interface (0.65c). Characterization of the SJA's showed they produced their highest exit velocities at a frequency of 1100 Hz, which was near the natural frequency of the piezo membranes. When actuated at frequencies corresponding to the flow natural frequencies (10-100Hz) the jets produced no jet velocity. In order to control the flow using a frequency near the flow's natural shedding frequency the synthetic jets were actuated using a forcing frequency near the piezo natural frequency with a duty cycle frequency of 10-1000Hz. Force balance results showed that for a 0\r{ } flap deflection the active flow control delayed stall and lowered drag regardless of the duty cycle frequency. At flap deflections of 20\r{ } and 40\r{ } differences were observed between the continuously forced and duty cycles cases. For these cases continuous forcing increased the stall angle and reduced drag. Duty cycle forcing also delayed stall however it significantly increased drag near the stall AOA even compared to the no forcing case. [Preview Abstract] |
Monday, November 21, 2011 4:53PM - 5:06PM |
L9.00007: Manipulating forces by interactive separation and circulation control Hassan Nagib, Paul Reinhard, Paul Rozier Steady, unsteady and intermittent suction and blowing from localized or distributed slots are used to reveal the physical mechanisms and their interaction in order to manipulate (enhance or reduce) the forces on various aerodynamic bodies and surfaces. Performance under ideal inviscid conditions is used as a standard of performance to compare the outcomes to. While high-lift airfoils were part of the focus, flow over humps which lead to large separation zones was also investigated. Surface pressure measurements, wake surveys and surface visualization were utilized over a wide range of operating conditions in the NDF at IIT. Velocities ranged from $20$ to $110$ m/s ($0.06 < M < 0.31$), corresponding to chord Reynolds numbers from $500,000$ to $3,700,000$, and included a full range of airfoil angles of attack with flap deflections from $10$ to $55$ degrees and various leading edge configurations. Steady suction control was more effective at eliminating the large separation bubble created by the model, requiring a pressure ratio between the applied force and inviscid force of approximately unity, whereas blowing required a two to one ratio. Pulsed suction was superior and enhanced by the operating frequency or duty cycle. Separation control (SC) was modified by the presence of circulation control (CC). Steady-blowing SC near the leading edge reduced the effect of blown-flap CC, whereas steady-suction SC increased the performance gain. [Preview Abstract] |
Monday, November 21, 2011 5:06PM - 5:19PM |
L9.00008: Improvement for efficiency of frictional drag reduction by repetitive bubble injection Yuji Tasaka, Yuichi Murai, Hyun-Jin Park, Yukihiro Tohge Repetitive bubble injection was examined for promoting frictional drag reduction by bubbles based on idea of ``resource allocation'' and voidage waves formed in a horizontal bubbly channel. Gain factor, which is defined as the ratio of degree of the drag reduction by the mean void fraction of bubbles, $\alpha$, showed dramatic improvement on the efficiency of the drag reduction in comparison with continuous bubble injection at the same $\alpha$. This improvement is prominent at small void fractions at which the continuous bubble injections enhance the frictional drag. Simultaneous recording of advecting bubbles with measurements of shear stress profiles at the channel wall indicated that preceding large bubbles provide great drag reduction and following small, low concentration bubbles don't affect the frictional drag. Namely the bubbles as a limited resource are efficiently used for the drag reduction in the repetitive bubble injection. [Preview Abstract] |
Monday, November 21, 2011 5:19PM - 5:32PM |
L9.00009: Transitory Aerodynamic Control of Unsteady Separation George T.K. Woo, Ari Glezer The dynamics of the controlled three-dimensional attachment of transitory stall over an airfoil oscillating in pitch is investigated in wind tunnel experiments using a partial spanwise array of surface-integrated pulsed jet actuators. The actuation has a characteristic time scale that is an order of magnitude shorter than the convective time scale of the base flow. It is shown that the control authority of the single pulse is highly-dependent on the oscillation cycle phase due to the timed interactions between the actuation jets and the evolution of the dynamic stall vortex on the suction surface. The transitory effects of the actuation can be extended and exploited for significant suppression of the dynamic stall by using successive pulsed actuation strategically staged during the cycle. High-resolution phase-locked PIV measurements in the cross stream plane on the suction surface and in the near wake demonstrate that the pulsed actuation sequence can effectively regulate the accumulation (trapping) and shedding of vorticity. The time-modulation of the vorticity fluxes results in significant temporal changes in circulation, and consequently in the measured time-dependent aerodynamic forces and moments. [Preview Abstract] |
Monday, November 21, 2011 5:32PM - 5:45PM |
L9.00010: Experimental Study of the Temporal Nature of an Actively Controlled Three Dimensional Turret Wake Patrick Shea, Mark Glauser Experimental measurements have been performed to characterize the actively controlled wake of a three-dimensional, non- conformal turret which is a bluff body commonly used for housing optical systems on airborne platforms. As a bluff body, turrets can generate strong turbulent flow fields that degrade the performance of the optical systems and the aircraft. Experiments were performed in a low-speed wind tunnel at Syracuse University using particle image velocimetry and dynamic pressure measurements with the objective of developing a better understanding of the spatial and temporal nature of the wake flow field. Active control was achieved using dynamic suction in the vicinity of the turret aperture and was found to have a significant impact on the structure of the wake as well as the temporal characteristics of the flow field. With a better understanding of the wake characteristics, closed-loop, active flow control systems will be developed to help reduce fluctuating loading and aero- optical distortions associated with the turbulent flow field. [Preview Abstract] |
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