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 D19: Flow Control I |
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Chair: Daniel Bodony, University of Illinois at Urbana-Champaign Room: 322 |
Sunday, November 20, 2011 2:10PM - 2:23PM |
D19.00001: Validated model of arc-filament plasma actuators for control of wall-bounded flows Daniel Bodony, Mahesh Natarajan Plasma actuators based on the electrical arcs between two electrodes have shown promise in controlling high-subsonic and low-supersonic flows. Simulation-based predictions of these flows have often used heuristic models for the effect the plasma has on the flow to be controlled. In this talk we present a two-parameter model of the actuator which combines the unsteady Joule heating induced by the plasma with a thermally perfect model of air. PIV and spectroscopy data are used, in conjunction with simulations, to understand the two parameters and demonstrate how their values are to be determined. The importance of the cavity in which the electrodes are mounted is discussed, as is the role of diffusion. We demonstrate the use of the actuator model by controlling a high-subsonic, separating boundary layer in an S-duct geometry. [Preview Abstract] |
Sunday, November 20, 2011 2:23PM - 2:36PM |
D19.00002: Numerical Study for Separation Control Mechanism of Impulse Actuation SolKeun Jee, Omar Lopez Mejia, Robert Moser A flow control mechanism by which impulse actuation delays flow separation is investigated numerically. The actuation produces a short-duration high-velocity jet, which exploits the sensitivity of separated flow to momentary actuation. Previous experimental and numerical studies have shown that this actuation disrupt the separated region on a stalled airfoil, reattaching the boundary layer. This actuation, which is spatially as well as temporally localized, globally alters the baseline flow over long time (100 times the actuation time). The computations reported here provide detailed flow structure associated with the actuation and the separated flow. The flow modification includes four major stages following an impulse actuation: disruption of the separated region, vorticity extraction from the boundary layer, the reattachment and return to stall. It was hypothesized that the disruption of the separating layer is resulted from interactions with the vortices produced by the actuation. This was tested by artificially introducing similar vortices up- and down-stream of the nominal separation. Results are consistent with the hypothesis and show that a complete disruption of the separated shear layer is required for the desired flow modifications. [Preview Abstract] |
Sunday, November 20, 2011 2:36PM - 2:49PM |
D19.00003: Axisymmetric Synthetic Jets: A Momentum-based Modeling Approach Xi Xia, Kamran Mohseni A dimensional analysis approach is presented in order to offer a unified modeling approch for axisymmetric continuous and synthetic jets. The synthetic jet is injected into a quiescent air environment and the flow field is measured in the far field using hot-wire anemometry. Four parameters are identified to control the far field of the jet. These are: the spatial position, jet width, centerline velocity, and momentum flux. The functional dependency of these parameters is given by dimensional analysis. Experimental data show that in the far field, the spreading rate and decay rate of the jet are constants and the momentum flux is also conserved. Therefore, two nondimensional parameters are proposed to represent the characteristics of the far field of a synthetic jet. Furthermore, the relationship between the two constants are given theoretically and verified experimentally in this study. The results of several continuous jets are also presented for comparison. The validity of this work provides universal model for the far field of both synthetic jets and continuous jets regardless of the configurations of jet actuators or the activation conditions. It can be also concluded from this model that the momentum flux and the virtual eddy viscosity are actually the key parameters controling the characteristics of a synthetic jet far field. [Preview Abstract] |
Sunday, November 20, 2011 2:49PM - 3:02PM |
D19.00004: ABSTRACT WITHDRAWN |
Sunday, November 20, 2011 3:02PM - 3:15PM |
D19.00005: 3D Numerical Analysis of Flow Control on Wind Turbine Blades Onkar Sahni, Ertan Karaismail Wind turbine blades are exposed to unsteady and spatially-varying loadings in a real field. These loadings result in fluctuating structural forces which in turn lead to failure of blades as well as gearbox. In this study, we perform numerical analysis of flow over a wind turbine blade placed in a wind tunnel; where dynamic motions are imposed to the blade in order to emulate scenarios observed in a real field. Furthermore, we also study the effect of active flow control (via synthetic-jets) on unsteady aerodynamic characteristics of the blade under dynamic motions; the idea is to be able to control aerodynamic loads and mitigate failures. Numerical analysis is based on massively parallel simulations using hybrid turbulence models. Comparisons with experimental data will also be included. [Preview Abstract] |
Sunday, November 20, 2011 3:15PM - 3:28PM |
D19.00006: Aerodynamic Performance Enhancement of a Finite Span Wind Turbine Blade using Synthetic Jets Keith Taylor, Chia Min Leong, Michael Amitay Modern wind turbines undergo significant changes in pitch angle and structural loading through a revolution. Recent developments in flow control techniques, coupled with increased interest in green energy technologies, have led to interest in applying these techniques to wind turbines, in an effort to increase power output and reduce structural stress associated with widely varying loading. This reduction in structural stress could lead to reduced operational costs associated with the maintenance cycle. The effect of active flow control on the aerodynamic and structural aspects of finite span blade was investigated experimentally. When synthetic jets were employed the effect on aerodynamic performance and structural vibrations, during static and dynamic pitch conditions, was significant. In order to investigate if the jets can be actuated for less time (reduce their power consumption), they were actuated during only a portion of the pitch cycle or using pulse modulation. The results showed that these techniques result in significant reduction in the hysteresis loop and the structural vibrations. [Preview Abstract] |
Sunday, November 20, 2011 3:28PM - 3:41PM |
D19.00007: Numerical simulation of tandem-cylinder noise-reduction using plasma-based flow control Meng Wang, Ahmed Eltaweel, Flint Thomas, Alexey Kozlov, Dongjoo Kim The noise of low-Mach-number flow over tandem cylinders at $Re_D= 22,000$ and its reduction using plasma actuators are simulated numerically to confirm and extend earlier experimental results. The numerical approach is based on large-eddy simulation for the turbulent flow field, a semi-empirical plasma actuation model, and Lighthill's theory for acoustic calculation. Excellent agreement between LES and experimental results is obtained for both the baseline flow and flow with plasma control in terms of wake velocity profiles, turbulence intensity, and frequency spectra of pressure fluctuations on the downstream cylinder. The validated flow-field results allow an accurate acoustic analysis based on Lighthill's equation, which is solved using a boundary-element method. The effectiveness of plasma actuators for reducing noise is demonstrated. In the baseline flow, the acoustic field is dominated by the interaction of the downstream cylinder with the upstream wake. With flow control the interaction noise is reduced drastically through suppression of vortex shedding from the upstream cylinder, and the vortex-shedding noise from the downstream cylinder becomes dominant. The peak sound pressure level is reduced by approximately 15 dB. [Preview Abstract] |
Sunday, November 20, 2011 3:41PM - 3:54PM |
D19.00008: Identification of frequency lock-on using Koopman spectral analysis Jonathan Tu, Clarence Rowley, Shawn Aram, Rajat Mittal The separated flow past an airfoil at a high angle-of-attack is characterized by the frequencies of the wake, shear layer, separation bubble, and zero-net-mass-flux (ZNMF) actuation (if applied). In certain configurations, ``lock-on'' may occur, in which some or all of these frequencies take on the same value. Previous studies have shown that the presence of lock-on may be related to the effectiveness of ZNMF forcing at a particular frequency. As a model for separated airfoil flows, we analyze the high Reynolds number flow past a finite-thickness flat plate with an elliptical leading edge, inducing separation via a blowing and suction boundary condition rather than angle-of-attack. 3-D large eddy simulations are performed and the resulting data are analyzed using Koopman spectral analysis and proper orthogonal decomposition (POD). Koopman analysis clearly identifies the characteristic flow frequencies and provides corresponding spatial modes. From the spatial support in these modes we determine whether or not lock-on occurs and which structures (wake, shear layer, or separation bubble) are involved. A combination of Koopman and POD analyses shows that ZNMF forcing is most effective when lock-on is achieved in the most energetic modes. [Preview Abstract] |
Sunday, November 20, 2011 3:54PM - 4:07PM |
D19.00009: On the interaction of von Karman shedding and sinusoidal actuation Kunihiko Taira Modifying the dynamics of unsteady separated flow around a bluff body is of engineering importance, as it directly relates to lift and drag on the body. We numerically investigate how the use of sinusoidal flow control input (mimicking synthetic jet actuator) alters the laminar wake dynamics dominated by the von Karman vortex shedding behind a circular cylinder. This study considers the influence of actuator location, forcing amplitude, and forcing frequency, with particular focus on lock-on. Also analyzed is the resulting change in the force experienced by the cylinder to extract useful operating condition for potential applications in feedback control and vibration suppression. [Preview Abstract] |
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