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 CJ: Flow Control II |
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Chair: Philippe Lavoie, University of Toronto Room: Long Beach Convention Center 201A |
Sunday, November 21, 2010 1:00PM - 1:13PM |
CJ.00001: Feedback Flow Control of a Periodically Pitching Airfoil Andrew Lombardi, Patrick Bowles, Thomas Corke, Eric Matlis A method for detecting incipient flow separation using plasma actuators is presented. The detection scheme relies upon the receptivity of the flow to unsteady disturbances that are introduced by the flow actuator. The receptivity to the unsteady disturbances is heightened as the flow approaches the separation limit, and subsequently can be detected downstream. This is demonstrated on a dynamically pitching airfoil that progresses through a dynamic stall cycle. A plasma actuator is located at the leading edge and pulsed at a frequency that is optimal to re-attach the flow. A pressure sensor monitors the unsteady pressure disturbances on the suction-side of the airfoil. Short-time Fourier analysis is used to capture the time-frequency behavior of the pressure sensor time series. Simultaneous flow visualization using a high-speed camera aid in elucidating the fluid response to the actuator input. The method not only provides a precursor for flow separation, but also an indicator when conditions exist where active re-attachment control is no longer needed. A closed-loop, feedback control scheme based on this is demonstrated. [Preview Abstract] |
Sunday, November 21, 2010 1:13PM - 1:26PM |
CJ.00002: Aerodynamic Flow Control using Distributed Active Bleed John M. Kearney, Ari Glezer The aerodynamic effects of large-area air bleed that is driven through surface openings by pressure differences across a lifting airfoil and regulated by addressable, arrays of integrated louvers have been investigated in wind tunnel experiments. Time-dependent interactions between the bleed and cross flows alter the apparent aerodynamic shape of the lifting surface and consequently the distributions of aerodynamic forces and moments. The lift and pitching moment can be significantly altered over a wide range of angles of attack from pre- to post-stall by independently-controlled bleed near the leading (LE) and trailing (TE) edges. While TE bleed effects nearly-linear variation of the pitching moment with minimal changes in lift, LE bleed leads to large variations in lift and pitching moment with minimal drag penalty. Phase-locked PIV shows the effects of the bleed on the flow on the suction surface and in the near wake. Supported by AFOSR [Preview Abstract] |
Sunday, November 21, 2010 1:26PM - 1:39PM |
CJ.00003: Numerical Simulations of Natural and Actuated Flow over a 3D, Low-Aspect-Ratio Airfoil Guillaume Br\`es, David Williams, Tim Colonius Numerical simulations of the unsteady flow over a low-aspect-ratio, low Reynolds number semi-circular planform wing are performed using Lattice Boltzmann method. The simulations exactly match the flow conditions and the detailed geometry from previous wind-tunnel experiments, including the flow actuators installed internally along the leading edge of the wing. To reproduce the pulsed-blowing actuation used in the experiment, a single pulsed square wave forcing is imposed in the simulations as a mass flow boundary condition in the actuators. Three angles of attack, with the active flow control both on and off, are investigated. For both mean and unsteady lift and drag, the numerical simulations show good agreement with the experiments. In particular, the transient increase in lift after the forcing is turned off is well captured in the simulations. Both PIV measurements and transient numerical results indicate that this behavior is associated with the advection of large vortical structures generated by the flow actuation at the leading edge. [Preview Abstract] |
Sunday, November 21, 2010 1:39PM - 1:52PM |
CJ.00004: Aerodynamic Flow Control of a Maneuvering Airfoil Daniel P. Brzozowski, John Culp, Ari Glezer The unsteady aerodynamic forces and moments on a maneuvering, free-moving airfoil are varied in wind tunnel experiments by controlling vorticity generation/accumulation near the surface using hybrid synthetic jet actuators. The dynamic characteristics of the airfoil that is mounted on a 2-DOF traverse are controlled using position and attitude feedback loops that are actuated by servo motors. Bi-directional changes in the pitching moment are induced using controllable trapped vorticity concentrations on the suction and pressure surfaces near the trailing edge. The dynamic coupling between the actuation and the time-dependent flow field is characterized using simultaneous force and velocity measurements that are taken phase-locked to the commanded actuation waveform. The time scales associated with the actuation process is determined from PIV measurements of vorticity flux downstream of the trailing edge. Circulation time history shows that the entire flow over the airfoil readjusts within about 1.5~$T_{CONV}$, which is about two orders of magnitude shorter than the characteristic time associated with the controlled maneuver of the wind tunnel model. This illustrates that flow-control actuation can be typically effected on time scales commensurate with the flow's convective time scale, and that the maneuver response is only limited by the inertia of the platform. Supported by AFSOR. [Preview Abstract] |
Sunday, November 21, 2010 1:52PM - 2:05PM |
CJ.00005: Active Flow Control on a Low Reynolds Number Wing Matthew Munson, Morteza Gharib Control of vortex formation has been shown to be a critical mechanism in some forms of animal flight. Flapping motions create advantageous flow structures which play a role in enhancing lift and increasing maneuverability. Active flow control may be capable of providing similar influence over vortex formation processes in fixed wing flight at small Reynolds numbers. Steady and pulsed mass injection strategies through simple slot actuators are used to explore the open-loop response of the flow around a simple low-aspect ratio wing. Flow dynamics and vortex formation will be quantitatively visualized with DPIV and flow forces will be simultaneously measured with a six-component balance. [Preview Abstract] |
Sunday, November 21, 2010 2:05PM - 2:18PM |
CJ.00006: Low Reynolds Number Flow Dynamics of a Thin Airfoil with an Actuated Leading Edge using Direct Numerical Simulation Kevin Drost, Sourabh Apte Direct numerical simulations are performed to investigate the effect of a movable leading edge on the unsteady flow at high angles of attack over a flat, thin airfoil at Reynolds number of 14700 based on the chord length. The leading edge of the airfoil is hinged at one-third chord length allowing dynamic variations in the effective angle of attack through specified oscillations (or flapping). A fictitious-domain based finite volume approach [(Apte et al. (JCP 2009)] is used to compute the flow over an airfoil with a flapping leading edge on a fixed background mesh. Cases were run at 20 degrees angle of attack to study the drag and lift characteristics with sinusoidal flapping of the leading edge about the hinge over a range of reduced frequencies ($k=\pi f c/U_{\infty}$ = 0.57- 5.7). It is shown that high-frequency low amplitude actuation of the leading edge significantly alters the leading edge boundary-layer and vortex shedding and increases the mean lift- to-drag ratio. The concept of an actuated leading-edge flap has potential for development of control techniques to stabilize and maneuver low-Reynolds number micro-air vehicles in response to unsteady perturbations. [Preview Abstract] |
Sunday, November 21, 2010 2:18PM - 2:31PM |
CJ.00007: Interaction of a circumferentially varying stator row upstream of a propeller in a uniform flow John Farnsworth, Michael Amitay, David Beal, Stephen Huyer A propuslor capable of producing maneuvering forces in all directions effectively eliminates the need for additional control surfaces. Side forces can be generated by the propeller through the variation of the inflow swirl velocity to a conventional propeller. These control forces are generated based on the same geometric principles as a helicopter swash-plate. Instead of cyclically adjusting the propeller blade pitch angle, the relative pitch angle of a fixed pitch propeller is cyclically altered through a pre-swirled inflow generated by an upstream stator row. Wind tunnel and water tunnel experiments were conducted where surface static pressure, forces and moments, and stereoscopic PIV measurements were conducted on a simplified propulsor model. From these measurements a better understanding of the fluidic interactions of the complex propeller system was achieved. [Preview Abstract] |
Sunday, November 21, 2010 2:31PM - 2:44PM |
CJ.00008: Experimental investigations on the drag reduction mechanism of outer-layer vertical blades array Inwon Lee, Nam Hyun An, Kwing-So Choi, Ho Hwan Chun An experimental assessment has been made of the drag reducing efficiency of the outer-layer vertical blades, which were first devised by Hutchins (2003). The local skin friction reduction of the blades was reported to reach as much as 30{\%}. In the present study, a series of drag force measurements in towing tank has been performed toward the assessments of the total drag reduction efficiency of the outer-layer vertical blades. A maximum 9.6{\%} of reduction of total drag was achieved. The scale of blade geometry is found to be weakly correlated with outer variable of boundary layer. In addition, detailed flow field measurements have been performed using time resolved PIV with a view to enabling the identification of drag reduction mechanism. The comparison of real-time turbulence structure observed in the xz-plane in the outer layer revealed that the blades array disturbs the growth of the large-scale structures in the spanwise direction. [Preview Abstract] |
Sunday, November 21, 2010 2:44PM - 2:57PM |
CJ.00009: Benefits of Active Flow Control for Wind Turbine Blades Guannan Wang, Basman Elhadidi, Jakub Walczak, Mark Glauser, Hiroshi Higuchi In this talk, the blade element momentum model is used to design a wind turbine and examine the benefit of active flow control. The results suggest that either the overall operational range of the wind turbine could be effectively enlarged by 80{\%} with the same rated power output or the rated output power could be increased by 20{\%} while maintaining the same level of operational range when the control is on. The optimal location for the actuator is found to be on the outboard of the blade beyond half of the radius. In light of these encouraging results and based on our earlier NACA 4412 flow control studies, a characteristic airfoil (e.g. DU-96-W-180) is being tested in a new anechoic wind tunnel facility at Syracuse University to determine the airfoil lift and drag characteristics with appropriate flow control while exposed to large scale flow unsteadiness. In addition, the effects of flow controllers on the noise spectrum of the wind turbine will be also assessed and measured in the anechoic chamber. [Preview Abstract] |
Sunday, November 21, 2010 2:57PM - 3:10PM |
CJ.00010: Characterizing rotor stator interaction (RSI) using CFD and experimentally obtained wake flow fields Morten Kjeldsen, Pal H.E. Finstad, Roger E.A. Arndt RSI is a major reason for noise and vibration, and reduced performance of turbomachinery. The stationary cascade upstream of the impeller stage is a source of variations in velocity due to angular momentum transfer, creating a cascade blade-to-blade variation. In addition a number of secondary flow fields due to boundary layer dynamics, such as wake flows, emerge from the cascade. At UMN a number of TR PIV fields have been captured downstream of a hydrofoil in liquid water, c=81mm and Re,c= (5 to 8)e5, for different AoAs and for selected passive flow control techniques. The wake trailing the foil is characterized by swirling structures, albeit far from regular shedding. One line of analysis of the captured wake flow fields has been to characterize the structures by a statistical averaged energy analysis over the structures. A second approach has been to use the experimentally obtained data as input in CFD analysis of the impingement of the wake on a rotating vane. Both the procedure and results are described. [Preview Abstract] |
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