Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session LE: Control of Instabilities |
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Chair: Brent C. Houchens, Rice University Room: 003A |
Monday, November 24, 2008 3:35PM - 3:48PM |
LE.00001: ABSTRACT WITHDRAWN |
Monday, November 24, 2008 3:48PM - 4:01PM |
LE.00002: A realistic model of a wall-transpiration actuator for boundary layer control Nils Tilton, Luca Cortelezzi Experimental studies of boundary layer control using continuously distributed wall-suction usually implement suction by applying a pressure gradient to a layer of porous material via an underlying plenum chamber. Theoretical studies, however, usually neglect the penetration of fluid into the porous layer and plenum chamber by forcing the base flow and velocity perturbations to vanish at the interface with the porous layer. We present a realistic model of a wall-transpiration actuator which implements suction through a fluid saturated, rigid, homogeneous, isotropic, porous layer stretched over a semi-infinite plenum chamber. We test our model on the asymptotic suction boundary layer (ASBL) and perform a linear stability analysis. We take account of the full coupling between the flow fields in the boundary layer, porous layer, and plenum chamber using boundary conditions derived by Ochoa-Tapia and Whitaker (Int. J. Heat Mass Transfer, Vol. 38, 1995, pp 2635-2646). We illustrate the impact of wall-permeability, porous layer thickness, and the plenum chamber on the critical Reynolds number and the stability of the Tollmien-Schlichting wave. We use our model to find the optimal operating conditions of an ASBL which minimize the skin friction drag and power required to apply the suction. [Preview Abstract] |
Monday, November 24, 2008 4:01PM - 4:14PM |
LE.00003: Global mode analysis of the stabilization of bluff-body wakes by base bleed E. Sanmiguel-Rojas, A. Sevilla, C. Mart\'Inez-Baz\'an Base bleed is a simple and well-known means of stabilizing the wake behind slender bodies with a blunt trailing edge. In the present research, we investigate the global instability properties of the laminar-incompressible flow using a spectral domain decomposition method to perform the global stability analysis. In particular, we describe the flow instability characteristics as a function of the Reynolds number, Re=$\rho W_{\infty}D/\mu$, and the bleed coefficient, defined as the bleed-to-freestream velocity ratio, $C_b=W_b/W_{\infty}$, where $D$ is the diameter of the body, $\rho$ and $\mu$ the density and viscosity of the free stream, respectively. A first stationary bifurcation for, Re $\simeq$ 364, is found, and a second oscillatory bifurcation for, Re $\simeq$ 598, with a Strouhal number, St= 0.105, both for the most unstable azimuthal mode $|m|$= 1. We also report the existence of a critical bleed coefficient to stabilize both the first, $C^*_{b1}=C^*_{b1}(Re)$, and the second, $C^*_{b2}=\, C^*_{b2}(Re)$, bifurcations such as $C^*_{b1}>C^*_{b2}$ for the range of Reynolds number under study, $0 \leq Re \leq 2000$. For $Re > \,600$ the same kind of bifurcations are found for the azimuthal modes $|m|$= 2 and $|m|$= 3, which exhibit similar behaviors as the $|m|$= 1 mode with respect to the critical bleed coefficient. [Preview Abstract] |
Monday, November 24, 2008 4:14PM - 4:27PM |
LE.00004: Flow control by combining radial pulsation and rotation of a cylinder in uniform flow H. Oualli, S. Hanchi, A. Bouabdallah, M. Gad-el-Hak Flow visualizations and hot-wire measurements are carried out to study a circular cylinder undergoing simultaneous radial pulsation and rotation and placed in a uniform flow. The Reynolds number is in the range of 1,000--22,000, for which transition in the shear layers and near wake is expected. Our previous experimental and numerical investigations in this subcritical flow regime have established the existence of an important energy transfer mechanism from the mean flow to the fluctuations. Radial pulsations cause and enhance that energy transfer. Certain values of the amplitude and frequency of the pulsations lead to negative drag (i.e.\ thrust). The nonlinear interaction between the Magnus effect induced by the steady rotation of the cylinder and the near-wake modulated by the bluff body's pulsation leads to alteration of the omnipresent K\'{a}rm\'{a}n vortices and the possibility of optimizing the lift-to-drag ratio as well as the rates of heat and mass transfer. Other useful applications include the ability to enhance or suppress the turbulence intensity, and to avoid the potentially destructive lock-in phenomenon in the wake of bridges, electric cables and other structures. [Preview Abstract] |
Monday, November 24, 2008 4:27PM - 4:40PM |
LE.00005: Harmonically forced enclosed swirling flow J.M. Lopez, F. Marques, T.T. Lim, Y.D. Cui The response of steady state flows in a cylinder driven by a harmonically modulated rotating endwall is investigated experimentally and numerically. Three dynamic regimes are identified. For very low forcing frequency, the synchronous flow approaches quasi-static adjustment, and for very large forcing frequencies the oscillations are localized in the boundary layers on the cylinder. These localized wall oscillations drive the synchronous flow in the interior to the underlying axisymmetric steady basic state. The third regime occurs for forcing frequencies in the range of the most dangerous axisymmetric Hopf eigenfrequencies, with the 1:1 resonances leading to greatly enhanced oscillation amplitudes localized in the axis region where the flow manifests vortex breakdown recirculation zones. By comparing the spatio-temporal structure of the junction vortices produced by the modulations in this range of frequencies with the vorticity eigenfunctions responsible for the self-sustained oscillations in the unmodulated problem, we have identified the mechanism responsible for the large amplitude pulsations of the vortex breakdown recirculations on the axis at mean rotation rates well below critical for the self-sustained vortex breakdown oscillations. An important consequence of this study is that to achieve a strong resonant effect, it is not sufficient to only consider the temporal characteristics of the flow state, but that the imposed forcing must also match the spatial characteristics. [Preview Abstract] |
Monday, November 24, 2008 4:40PM - 4:53PM |
LE.00006: System identification and model-based control of two-dimensional cavity oscillations Simon Illingworth, Aimee Morgans, Clarence Rowley Direct numerical simulations are used to characterize the resonant instabilities in two-dimensional compressible flow over a rectangular cavity. Specifically, by first using a dynamic phasor model to stabilize the flow, the cavity's linear open-loop transfer function is determined. The transfer function's input and output consist of a body force at the cavity leading edge and a pressure measurement on the trailing edge wall respectively. The transfer function found allows comparison with and validation of a linear model of the cavity. The empirical transfer function is also used to design a model-based feedback controller, useful for reducing oscillations at a a single operating point, or as a starting point for an adaptive controller. Numerical simulations of the closed-loop system show that the model-based controller successfully stabilizes the cavity flow. [Preview Abstract] |
Monday, November 24, 2008 4:53PM - 5:06PM |
LE.00007: Adjoint Analysis of a Compressible Channel Flow Laia Moret-Gabarro, Patricia Cathalifaud, Christophe Airiau We present an adjoint analysis of a compressible channel flow using Direct Numerical Simulation. The final aim of this study is to build a tool to perform control of the aerodynamic noise. The adjoint equations are derived from the 2D unsteady compressible Navier-Stokes equations, and are computed backward in time. Both systems are discretized using a 6th order compact scheme in space and 4th order Runge-Kutta scheme in time. Appropriate wall boundary conditions are derived and validated for the adjoint system. We perform sensitivity analysis by applying different kinds of forcing to the adjoint equations, where the resulting field shows the forcing of the direct system required to obtain a given effect. In this study, we are interested in finding which perturbation creates higher noise levels at the core flow (i.e. in a position far from the wall) and how to reduce it. This test case is the first step to perform adjoint analysis of more complex wall-bounded flows, and to perform optimal open-loop control to reduce noise. [Preview Abstract] |
Monday, November 24, 2008 5:06PM - 5:19PM |
LE.00008: Reduced-Order Estimator-Based Feedback Control of Transitional Channel Flow Milos Ilak, Clarence Rowley Reduced-order models obtained using empirical balanced truncation and balanced proper orthogonal decomposition (BPOD) are used for feedback control of transitional channel flow. The models are developed for linearized flow and the controllers designed for the models are then applied to full DNS simulations. Both localized body forces and wall blowing/suction are used as actuation. Low-order computationally efficient estimators based on the models are designed for all cases, and the state estimates are computed from measurements of wall shear or velocity inside the channel. As a measure of controller performance, we demonstrate that transition energy thresholds for the nonlinear evolution of certain classes of standard perturbations, including localized perturbations and optimals at different wavenumbers, are successfully increased by applying our feedback controllers. [Preview Abstract] |
Monday, November 24, 2008 5:19PM - 5:32PM |
LE.00009: Reduced order models for control of fluids using the Eigensystem Realization Algorithm Zhanhua Ma, Sunil Ahuja, Clarence Rowley We present a computational algorithm for model reduction of high-dimensional fluid simulations based on the Eigensystem Realization Algorithm (ERA), a method often used for system identification of vibrating systems. Our goal is to obtain models that capture the underlying flow physics and, at the same time, are useful for control design. For that purpose, we consider a system whose output is the velocity field in the entire computational domain. For such a large number outputs, ERA is intractable, so we use a technique called output projection, which involves reducing the number of outputs by projecting them onto the most energetic POD modes of the impulse response of the system. The presented algorithm involves a simple snapshot-based procedure commonly used for POD or balanced POD. The resulting models are equivalent to those obtained using balanced POD, but the algorithm involved requires only $O(n)$ inner products as compared to $O(n^2)$ for balanced POD, and does not need any adjoint simulations as required for balanced POD, thus resulting in large computational savings. We apply this technique to 2D flows past a flat plate at a low Reynolds number, and obtain reduced order models of the flow linearized about stable and unstable steady states. [Preview Abstract] |
Monday, November 24, 2008 5:32PM - 5:45PM |
LE.00010: Modelling for Feedback Control of Skin Friction Drag in Algebraic Growth Bryn Jones, Eric Kerrigan, Ahmed Naguib, Jonathan Morrison We address the following problem: given spanwise arrays of wall- mounted shear-stress sensors at upstream and downstream locations, obtain accurate estimates of the flow field above an array of actuators located between the sensors. The accuracy of these estimates is of crucial importance in the design of any closed-loop drag reduction controller. To achieve satisfactory estimates we employ feedback from the sensors in conjunction with a dynamic model, based on that of Luchini (2000), describing perturbation evolution within a laminar boundary layer. The novelty of this work lies in the derivation of a state-space model of sufficiently low order to enable Kalman filter synthesis. Rather than obtaining a reduced- order model via numerical methods such as balanced truncation (Zhou, Doyle, Glover; 1996), we employ a series of approximations based on the results of Andersson, Berggren et al. (1999), to derive a low-order model analytically. A Kalman filter is synthesised and tested on the algebraic growth region of the DNS of Zaki (2005). Despite the use of a low-order model and significant free-stream turbulence, the results demonstrate good performance of the filter. [Preview Abstract] |
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