Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session FL: Turbulence and Instability Control I |
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Chair: Mo Samimy, Ohio State University Room: Tampa Marriott Waterside Hotel and Marina Meeting Room 8 |
Monday, November 20, 2006 8:00AM - 8:13AM |
FL.00001: Reduced-order Model Based Feedback Control of Cavity Flows -- Changes in the Flow Characteristics Mo Samimy, Jesse Little, Marco Debiasi, Edgar Caraballo, Andrea Serrani, Xin Yuan We have developed and experimentally implemented reduced-order model based feedback control of subsonic cavity flows. Reduced-order models were developed via Proper Orthogonal Decomposition (POD) using particle imaging velocimetry (PIV) in conjunction with the Galerkin projection of the governing Navier-Stokes equations onto the resulting spatial eigenfunctions. The stochastic estimation technique using simultaneous PIV and surface pressure measurements was used to establish correlation between the flow field and surface pressure. For the implementation of the controller, dynamic surface pressure measurements were used for the estimation of the POD modal coefficients. The reduced-order model was linearized around the equilibrium point and a linear-quadratic optimal controller was designed and implemented in the experiments. The actuator was a compression driver type and its output was channeled through a one millimeter slit spanning the entire width of the cavity leading edge. Models based on one or more flow conditions for a Mach 0.3 cavity flow were developed and used, which suppressed the cavity resonant modes. We will compare and contrast the flow characteristics for the open- and closed-loop controlled flows. [Preview Abstract] |
Monday, November 20, 2006 8:13AM - 8:26AM |
FL.00002: Transitory Separation Control using Pulsed Actuation Dan Brzozowski, Ari Glezer The transitory response of a separated flow over an airfoil to pulsed actuation on time scales that are an order of magnitude shorter than the characteristic convective time scale is investigated experimentally (Re = 570,000). Actuation is effected by momentary [O(1 msec)] pulsed jets that are generated by an array of combustion-based actuators embedded in the surface. The flow field is computed from multiple high-resolution PIV images that are obtained phase-locked to the actuation and allow for continuous tracking of vorticity concentrations. The brief actuation pulse leads to a severing of the separated vorticity layer and the subsequent detachment and shedding of a large-scale clockwise vortex which forms the separated flow domain. The CW severed vorticity layer behind the detached vortex is advected along the surface and has a distinct streamwise edge that begins to roll up. Ultimately, the downstream edge of the surface vorticity layer begins to lift off but partial flow attatchment is maintained for at least 10 convective time scales before the flow relaxes to the separated state. Time-dependent attached flow can be maintained by repetitious pulsed actuation. [Preview Abstract] |
Monday, November 20, 2006 8:26AM - 8:39AM |
FL.00003: Control of dynamic stall using zero-net-mass-flux oscillatory jets Seonghyeon Hahn, Donghyun You, Parviz Moin Recently, the zero-net-mass-flux oscillatory jets or synthetic jets have proven to be a promising tool for controlling various separated flows. The objective of the present study is to explore the applicability of synthetic jets for attaining lift enhancement of a pitching airfoil suffering from dynamic stall. For this purpose, numerical simulations of flow over a sinusoidally pitching NACA0015 airfoil are conducted at the chord Reynolds number of 90,000. The uncontrolled flow in this case shows mild stall and is dominated by a series of small shear-layer vortices from leading-edge separation, where synthetic jets actuated at the $12\%$ chord downstream location from the leading edge lead to mean lift enhancement. At a higher Reynolds number of $3 \times 10^5$, unsteady RANS employing the $v^2$--$f$ model predicts deep stall with the generation of a large-scale leading-edge vortex. In this case, synthetic jets are deficient in controlling the grossly moving separation and necessitate a more sophisticated actuation algorithm. Results from an ongoing large-eddy simulation at $\textrm{Re}=3 \times 10^5$ and the predictive capability of URANS in synthetic-jet control applications will also be discussed. [Preview Abstract] |
Monday, November 20, 2006 8:39AM - 8:52AM |
FL.00004: Large-eddy simulation of flow separation over an airfoil with synthetic jet control Donghyun You, Parviz Moin We have performed large-eddy simulation of separation control for flow over an airfoil and evaluated the effectiveness of synthetic jets as a control technique. The flow configuration consists of flow over a NACA 0015 airfoil at Reynolds number of 896,000 based on the airfoil chord length and freestream velocity. A small slot across the entire span connected to a cavity inside the airfoil is employed to produce oscillatory synthetic jets. Detailed flow structures inside synthetic jet actuator and the synthetic jet/cross-flow interaction have been simulated using an unstructured finite volume large-eddy simulation solver. Simulation results have been compared with the experimental data, and qualitative and some quantitative agreements have been obtained for both uncontrolled and controlled cases in terms of mean pressure coefficients and wake profiles. As in the experiment, the present large-eddy simulation confirms that the synthetic jet actuation effectively delays the onset of flow separation and causes a significant increase in the lift coefficient. [Preview Abstract] |
Monday, November 20, 2006 8:52AM - 9:05AM |
FL.00005: Manipulation of a transitional jet using a single perpendicular synthetic jet David Tamburello, Michael Amitay The effects of the upstream location of a single synthetic jet actuator (i.e., within the main jet nozzle oriented perpendicular to the main jet) on an axisymmetric free jet (Re = 6600) was investigated experimentally using PIV. The synthetic jet was driven at a frequency of 1000Hz (Strouhal number of 0.16) and it's momentum coefficient was varied from 0.005 to 0.16. When the synthetic jet is located near the main jet exit, the flow is both vectored away from the synthetic jet and drawn toward it. These are attributed to both augmentation of the main jet coherent structures and to direct impact of the synthetic jet onto the main jet. Conversely, when the synthetic jet is located farther upstream (i.e. deeper into the nozzle), the main jet is drawn back toward the synthetic jet, which may be caused by a virtual modification of the nozzle shape. These results were compared to those for a steady control jet under the same conditions. [Preview Abstract] |
Monday, November 20, 2006 9:05AM - 9:18AM |
FL.00006: Flow Field Features of the Induced Recirculation Zone using Planar Jet Injection. Kareem Ahmed, David Forliti, Jack Moody The objective of the present work is to study the recirculation bubble induced by injecting a planar jet in to a channel flow. The work is motivated by the desire to provide a flame stabilization environment for ramjet engines without the use of a sudden expansion or bluff-body which incurs drag penalties. A planar jet issuing into a cross flow is used to create a recirculation zone reminiscent of the flow field created downstream of a rearward-facing step configuration which is used in a current ramjet/scramjet systems. The effect of confinement at high momentum flux ratios leads to a saturation of the control of the recirculation bubble size and strength. Comparison to a rearward-facing step flow shows the current method produces higher turbulence levels and larger integral length scales. It is clear that the flow field produced with fluidic injection has potential for flame anchoring and enhancing combustion rates. [Preview Abstract] |
Monday, November 20, 2006 9:18AM - 9:31AM |
FL.00007: A Parametric Study of the Induced Recirculation Zone using Single/Double Planar Jet Injection Jack Moody, David Forliti, Kareem Ahmed Planar jet injection into a cross flow induces a large-scale recirculation zone. The relative importance of momentum and mass flux ratios is experimentally studied using injection slots having different widths. To acquire additional secondary control of the recirculation zone, a second downstream slot is employed with the objective of tailoring the recirculation bubble characteristics. Blowing and suction were explored for the downstream slot location. The control of dominant frequencies is documented for different operating conditions and geometries. The ability to control the mean and turbulent characteristics and vortex shedding frequency through multiple injection jets will aid in improving flame holding, burring rates, and control of combustion instabilities for combustion devices. [Preview Abstract] |
Monday, November 20, 2006 9:31AM - 9:44AM |
FL.00008: Active control of high subsonic jets Jin-Hwa Kim, Jeff Kastner, Yurii Utkin, Igor Adamovich, Mo Samimy Localized arc filament plasma actuators developed at OSU were used for the jet flow control in a Mach 0.9 jet with an exit diameter of 2.5 cm and a Reynolds number of 7.9x10$^{5}$. The azimuthal modes of forcing were m = 0-3, $\pm $1, $\pm $2, and $\pm $4 since only eight actuators were used. The forcing Strouhal number was varied from 0.13 to 3.0. The jet spreading was strongly dependent on the actuation modes at low Strouhal numbers. However, the effects of mode were negligible at a higher forcing Strouhal number. As far as the spread/mixing enhancement is concerned, the most effective forcing was at an approximate Strouhal number of 0.3, which is inline with what is in the literature. The average streamwise velocity field showed that the jet spreading was best at a forcing mode of m = $\pm $1. At a low Strouhal number ranging from 0.13 to 0.4, the decay of the centerline Mach number for m =1, 2, and $\pm $2 was comparable to that for m = $\pm $1. The jet spreading/mixing enhancement was minimal at higher azimuthal modes of m=3 and $\pm $4. In these modes, the centerline Mach number decayed slower than any other modes. The growth and decay of the perturbation, imparted by the plasma actuators, along a lipline of the jet showed a good correlation with the jet spreading. [Preview Abstract] |
Monday, November 20, 2006 9:44AM - 9:57AM |
FL.00009: Dynamic Modeling and Feedback Control of a Synthetic Jet-like Compression Driver Actuator Kihwan Kim, Marco Debiasi, Andrea Serrani, Mohammad Samimy This research is focused on dynamic modeling and feedback control of a synthetic jet-like compression driver actuator, which has been successfully employed for feedback control of subsonic cavity flows. The pressure response of the actuator is linear with distinctive frequency-dependent behaviors. In order to develop the dynamic model of the actuator considering time delay, a subspace-based identification method is implemented using experimental frequency response data. The order of the identified model is reduced by balanced realization. The reduced-order model preserves the key characteristics of the actuator within the frequency band of cavity oscillation where the actuator operates. The feedback control of the actor is aimed at having the pressure response of the actuator track the reference. It benefits the overall cavity control loop by mitigating uncertainties resulted from the actuator dynamics. To cope with the time-delay problem, the Smith-predictor structure is employed in conjunction with a $H_{\infty}$ mixed- sensitivity method. The experimental results of the feedback control will be presented in the conference. [Preview Abstract] |
Monday, November 20, 2006 9:57AM - 10:10AM |
FL.00010: Relaminarization of the vortex structure in a low velocity ratio jet in crossflow Graeme Watson, Lorenz Sigurdson We have explored in a wind tunnel the stabilizing effect of an annular synthetic jet on the vortex structure of an elevated jet in crossflow (JICF) with Reynolds numbers $O$(10$^{3})$. The synthetic jet was coaxial to the elevated JICF to provide added momentum to the flow. It was observed that an initially turbulent JICF can be made steady and relaminarized in its near field$^{\ast }$. This effect was observed for all forcing frequencies up to the limiting operational frequency of the synthetic jet apparatus. A similar relaminarization behavior was also observed for an unforced elevated JICF near a jet to free stream velocity ratio of one. Hot-wire anemometry measurements in the near-field of the synthetic jet were used to determine the momentum flux required for forced relaminarization. A simple model based on an empirical weighting was used to combine the momentum flux from the synthetic jet and JICF flows to determine a unified momentum velocity ratio$^{\ast }$. We compare the momentum velocity ratio at the onset of forced relaminarization of JICFs operating at different initial velocity ratios. \newline $^{\ast }$J. Diep, G. Watson, and L. Sigurdson, ``Concentric synthetic jet control of a low velocity ratio elevated jet in crossflow'', submitted. [Preview Abstract] |
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