Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session NO: Separated Flows III |
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Chair: J. Crouch, The Boeing Company Room: Salt Palace Convention Center 251 C |
Tuesday, November 20, 2007 11:35AM - 11:48AM |
NO.00001: Thrust Vectoring Flow Control Using Plasma Actuators Jamey Jacob, Michael Bolitho Thrust vectoring flow control is demonstrated using plasma synthetic jet actuators (PSJA). The PSJA is a geometric variant of a plasma actuator, consisting of a symmetric electrode array that results in a circular region of dielectric barrier discharge plasma. Quiescent flow PIV measurements of the PSJA reveal that the flowfield on actuation resembles that of a zero-mass flux or synthetic jet that is useful for flow control, particularly separation reduction. Like synthetic jets, unsteady pulsed actuator operation results in formation of multiple vortex rings. The output jet momentum is found to be affected by the power input, actuator dimension and pulsing frequency. While increasing the input power increases the maximum jet velocity, an optimum range of pulsing frequencies and actuator dimensions are observed to exist in order to maximize jet momentum. By asymmetrically varying the plasma input parameters, such as frequency, amplitude and duty cycle, it is possible to control the jet angle. Vectoring using high frequency pusling akin to synthetic jets is more effective than vectoring by modifying steady control inputs and differences in control effectiveness are due primarily to the time scales associated with the vortex formation. [Preview Abstract] |
Tuesday, November 20, 2007 11:48AM - 12:01PM |
NO.00002: A Comparison between Plasma Synthetic Jets and Conventional Jets Arvind Santhanakrishnan, Jamey Jacob The flow field of a jet created by an actuator employing a surface dielectric barrier discharge (DBD) is investigated experimentally via PIV measurements, and a comparison of its fluid dynamic characteristics with mechanically driven continuous and synthetic jets is presented. The plasma synthetic jet actuator consists of two electrodes arranged asymmetrically separated by a dielectric material and under an input of high voltage, high frequency AC at ambient conditions of pressure and temperature, a region of DBD plasma is formed starting from the edges of the exposed electrode. In an initially quiescent medium, this plasma region is observed to induce and sustain entrainment of near wall fluid, the volume of which is ejected out from the base of the actuator in the form of a jet. The electrodes are shaped either in the form of annular arrays for rendering a circular jet, or linear arrays for rendering a rectangular jet. Unsteady pulsing of the PSJA at time scales decoupled to the AC input frequency results in a flow field dominated by counter rotating vortical structures similar to conventional synthetic jets. The jet outputs are found to be affected by a variety of system inputs, including the input electrical power, pulsing frequency, and actuator dimension. The effects of varying the above parameters on the self similarity behavior of plasma synthetic jets are examined and compared to mechanically driven jets. [Preview Abstract] |
Tuesday, November 20, 2007 12:01PM - 12:14PM |
NO.00003: Origin of transonic buffet on airfoils. J. Crouch, A. Garbaruk, D. Magidov Transonic airfoils are characterized by a zone of supersonic flow followed by a shockwave, which intensifies with increasing lift coefficient. At sufficiently high values of lift, the flow becomes globally unsteady resulting in dramatic oscillations in the shock position. This buffet onset has been shown to result from global instability (Crouch, Gabaruk {\&} Magidov 2007, J. Comput. Phys. v. 224, p. 924). The stability analysis is based on linear perturbations to steady solutions of the Reynolds-averaged Navier-Stokes equations (RANS), which are required to analyze the flow at the high Reynolds numbers of interest for transonic flow. We briefly describe the global-stability approach, and then apply the stability analysis and unsteady RANS calculations to investigate the origins of the buffet onset. Buffet-boundary predictions from the stability analysis are shown to be in good agreement with experiments. [Preview Abstract] |
Tuesday, November 20, 2007 12:14PM - 12:27PM |
NO.00004: Reduced-order model-based control of unstable steady states in 2D incompressible flow past airfoils at low Reynolds numbers Sunil Ahuja, Clarence W. Rowley The stable flow past 2-D airfoils at high angles of attack and low Reynolds number is periodic vortex shedding. There also exists an unstable steady state, and here we present controllers that stabilize this. Our control design is based on reduced order models of the equations linearized about this steady state. These models are obtained using an extension of an approximate balanced truncation method, valid for unstable systems. The stable and unstable subspaces are first decoupled, and reduced order models are obtained for the dynamics on the stable subspace. Since the dimension of the unstable subspace is small, they are treated exactly, with the corresponding states being the unstable eigenmodes. We derive stablilizing control laws, first assuming full state feedback, and then using reduced order observers using a few surface pressure measurements. The closed-loop models compare well with the nonlinear DNS simulations for small perturbations to the steady state. The control laws have a large domain of stabilization; they suppress the vortex shedding, but the agreement with the model deteriorates for large perturbations. [Preview Abstract] |
Tuesday, November 20, 2007 12:27PM - 12:40PM |
NO.00005: Compressible Flow Simulation of Separation Control over a Wall-Mounted Hump Jennifer Franck, Tim Colonius We investigate compressible, turbulent, separating flow over a wall-mounted hump using large eddy simulation (LES). The geometry is identical to the benchmark experiments of Seifert and Pack (2003). The flow is characterized by a turbulent, unsteady flow separation, recirculation bubble, and reattachment. Various LES models including an implicit LES, a constant coefficient Smagorinsky model and a dynamic coefficient Smagorinsky model are employed and compared against one another. The LES results compare favorably with the experimental data for compressible subsonic Mach numbers. Zero net-mass flux control modeled after the experiments is also included in the simulations, and parameters such as forcing frequency and coefficient of momentum are investigated. Results are also compared with two-dimensional low Reynolds number simulations. Spectral analysis of the unforced turbulent flow indicates the same dominant frequency as the large coherent vortices shed in the two-dimensional flow. The two-dimensional simulations are faster to compute and are used to examine closed-loop control strategies that will ultimately be validated using LES. [Preview Abstract] |
Tuesday, November 20, 2007 12:40PM - 12:53PM |
NO.00006: Experiments on a rectangular cylinder in flutter Zachary Taylor, Gregory Kopp, Roi Gurka The flow mechanisms for the flutter instability of suspended long span bridges are poorly understood. Currently, this disastrous phenomenon is avoided by performing wind tunnel testing on a typical section of the bridge. However, there remain gaps in knowledge between design, the case studies and the underlying fluid dynamic phenomena. To this end, an experimental program -- including Particle Image Velocimetry (PIV) - has been carried out in an open wind tunnel for a rectangular section model with a chord-to-thickness ratio of 7. The position of the model was measured in order to perform phase averaging of the flow field based on the model's oscillation. The presentation will focus on the role of the vortices. These vortices will be identified by proper orthogonal decomposition (POD) of the PIV data as well as by analysis of the two-dimensional velocity gradient tensor. Divergent response with increasing velocity is characteristic of flutter; thus it is expected that energy balances are essential in describing the phenomenon. A description of the kinetic energy and turbulent production terms will be presented as well as the estimation of the dissipation from the experimental data. Preliminary results have suggested that the turbulent kinetic energy production term is crucial in describing some of the flow mechanisms. The correlation between the turbulent production and the vortex formation will be addressed. [Preview Abstract] |
Tuesday, November 20, 2007 12:53PM - 1:06PM |
NO.00007: Unsteady-wall-pressure and velocity measurements in a low-Mach-number flow over a shallow axisymmetric cavity. Ke Zhang, Ahmed Naguib The unsteady floor pressure and velocity were measured in the flow over a cavity with different width-to-depth, and length-to-depth ratios at very low Mach number (M$<$0.1). For turbulent boundary layer conditions at separation, the pressure fluctuations acting on the floor of the cavity were measured using a microphone array, while the associated velocity field was captured employing simultaneous hotwire measurements at the same streamwise and spanwise locations as the wall microphones but at different heights above the cavity floor. A unique aspect of the present study is that it employs an axisymmetric cavity geometry with and without end walls in order to examine the flow in finite-width and azimuthally-uniform cavities respectively. Amongst other findings, the results show that for sufficiently high Reynolds numbers, the cavity width has a significant influence on the establishment/cessation of self-sustained oscillation modes. [Preview Abstract] |
Tuesday, November 20, 2007 1:06PM - 1:19PM |
NO.00008: Shock motion and flow separation. Jean-Paul Dussauge, Sebastien Piponniau, Pierre Dupont, Jean-Francois Debieve In separated shock/boundary layer interactions, the system of shock wave is unsteady. The frequencies of the shock oscillations are, in first analysis, much lower than in the rest of the flow, including the separated zone. This result is discussed in different cases. A compilation of the dominant frequency of the shock motion is recalled, where frequencies are normalized by the upstream velocity and by the length of interaction. This puts the results together with some scatter, but determines more than the right order of magnitude of the dominant frequency. Two particular flow cases are examined: compression ramp flows and shock reflections. It has been proposed (Ganapathisubramani et al. 2006) to explain the low frequencies in compression ramp flows as the action of very large scale eddies (VLSE), on the basis of a strong correlation between VLSE and shock corrugations. However, in shock reflections, such events cannot explain the measured low frequencies. In this case, the explanation is probably in the 3-d structure of the separated bubble or in the onset of global instability in the separated zone. Finally it is shown that VSLE can produce the right frequency range in the compression ramp flow, and therefore that there is no contradiction between the two cases. [Preview Abstract] |
Tuesday, November 20, 2007 1:19PM - 1:32PM |
NO.00009: ABSTRACT WITHDRAWN |
Tuesday, November 20, 2007 1:32PM - 1:45PM |
NO.00010: Investigation of the flow in a rectangular cavity using tomographic and time-resolved PIV Christian Haigermoser, Lukas Vesely, Michele Onorato The unsteady flow in a 2-dimensional cavity at low Reynolds number is investigated experimentally in water. Time-resolved Particle Image Velocimetry (PIV) shows the oscillatory character of the flow and a flapping of the shear layer connected with inflows and outflows. These inflows and outflows are related to instants of high and low cavity drag. The Strouhal number based on the cavity length St$_{L}$ of the fluctuating cavity drag is evaluated to be 0.33. Instead, the vortex shedding frequency and the related Strouhal number appear to be of one order of magnitude higher. It is concluded, that due to the very low flow speed the acoustic pressure waves, which are radiated from the forward facing step when a vortex impinges thereon, are not strong enough to trigger instabilities in the shear layer and no feedback mechanism as proposed first by Rossiter (1964) is present. The 3-dimensionality of the flow was studied using tomographic PIV in a parallel wind tunnel experiment. The scope of this experiment was to identify the origin of the instantaneous 3-dimensional flow. The scales of the 3-dimensional structures in the flow are visualized and quantified using 2-point spacial correlations. [Preview Abstract] |
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