Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session ED: Flow Control III |
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Chair: Tim Colonius, California Institute of Technology Room: 101D |
Sunday, November 22, 2009 4:15PM - 4:28PM |
ED.00001: Development of Reduced-order Models for Feedback Control of Axisymmetric Jets Aniruddha Sinha, Andrea Serrani, Mo Samimy We present the preliminary steps toward development of reduced-order models (ROM) for feedback control of a high-speed and high Reynolds number axisymmetric jet. The control objective is two-fold: attenuation of far-field acoustic radiation, or, enhancement of bulk mixing, using a set of localized arc filament plasma actuators that perturb the initial shear layer of the jet through intense localized Joule heating. The proposed feedback sensing mechanism involves pressure information from the irrotational near-field of the jet. The proposed route for creating the ROM involves collecting PIV data of the jet simultaneously with the pressure measurements, performing Proper Orthogonal Decomposition and Stochastic Estimation to obtain a time- and space-resolved database, and using Galerkin Projection to derive the dynamical model. Here we evaluate the above strategy using a DNS database (Freund, J. B., J. Fluid Mech., 438, 2001, 277--305). The ROMs obtained using various modeling options are simulated and their comparative fidelity are adjudged based on the original simulation results. [Preview Abstract] |
Sunday, November 22, 2009 4:28PM - 4:41PM |
ED.00002: Hybrid Ensemble and Variational Estimation for Chaotic Systems Joseph Cessna, Thomas Bewley An estimate of a system is developed from knowledge of the model of the system along with a time history of noisy measurements. Unlike a filtered result, an estimate conditioned on both past and future measurements is said to be a smoothed estimate. For linear systems, computing the prior smoothed estimate trajectory appropriately has no bearing on the most recent filtered estimate. However, in chaotic systems, the estimate probability distribution is non-gaussian and must be computed through some form of linearization. As a result, one would expect that knowing a prior trajectory more accurately (through smoothing) could be beneficial in improving the most recent estimate via a reinterpretation of past measurements about this new trajectory. Unfortunately, for high-dimensional chaotic systems, typical smoothing approaches such as the Ensemble Kalman Smoother update the prior smooth trajectories upon receipt of a new measurement without altering the most recent filtered estimate. Using a new hybrid approach combining the EnKS with an adjoint variational method, we examine how the smoothed trajectories can be used consistently to improve the most recent filtered estimate. By appropriately defining a cost function, we can use the EnKS estimate as an initial condition for a well-posed variational iteration. It is shown that this technique reduces to the well known optimal Kalman Smoother result in the case of a linear system. [Preview Abstract] |
Sunday, November 22, 2009 4:41PM - 4:54PM |
ED.00003: State estimation of turbulent channel flow Christopher Colburn, Thomas Bewley The challenges involved in state estimation of wall-bounded turbulent flows are considered through computational experiments using a variety of measurement strategies (including wall information only). These experiments are part of a larger ongoing effort to develop high fidelity estimates of turbulent flows using the Ensemble Kalman Filter, a method used widely within the weather forecasting community, and other related estimation strategies. This study, which attempts to quantify rigorously the ``propagation of information'' in turbulent flows, might also help to shed some new light on various ``top-down" versus ``bottom-up'' hypotheses currently being debated in the literature on near-wall and highly-sheared turbulent flows. [Preview Abstract] |
Sunday, November 22, 2009 4:54PM - 5:07PM |
ED.00004: EnVO: a hybrid ensemble/variational adaptive observation algorithm David Zhang, Thomas Bewley Advances in autonomous technology enable the development of Adaptive Observation (AO) strategies, which identify future sensor locations to reduce forecast error. The approaches to AO strategies are mainly divided into two techniques, the ``uncertainty'' based techniques such as the Ensemble Transform Kalman Filter by Bishop et al. 2001 and the ``sensitivity'' based techniques such as the Singular Vector by Palmer et al. 1998. Both approaches have their advantages and disadvantages; hence the combination of both is perhaps ideal. We propose a hybrid ensemble/adjoint AO technique, dubbed Ensemble Variational Observation (EnVO). The EnVO technique uses \emph{both} sensitivity and uncertainty information, and can be applied in such a way as to ensure its optimized sensor trajectories are both feasible and efficient over the time window between the present and forecast time. Integrated statistic results indicates EnVO is able to consistently identifies target sites that produce lower forecast errors. [Preview Abstract] |
Sunday, November 22, 2009 5:07PM - 5:20PM |
ED.00005: Grid-based Bayesian Estimation Exploiting Sparsity for systems with nongaussian uncertainty Thomas Bewley, Ati Sharma We present a new algorithm for Bayesian estimation of nonlinear ODE systems $d{\bf x}/dt={\bf f}({\bf x})$ with finite, nongaussian uncertainty. The algorithm presented represents the evolution of the probability distribution in phase space, $P({\bf x},t)$, discretized on an Eulerian (that is, fixed, Cartesian) grid, and consists of two main steps: (1) Between measurement times, $P({\bf x},t)$ is marched via careful numerical discretization of the PDE governing its evolution using a Godunov method with second-order CTU correction and an MC flux limiter. (2) At measurement times, $P({\bf x},t)$ is updated via Bayes' theorem. The key to the efficiency of the new method is a novel technique for leveraging {\it sparsity} of the probability distribution (that is, leveraging the fact that it is essentially zero almost everywhere in phase space). The absence of a fundamental dependence on a central estimate and the second-order moments of its uncertainty renders the new approach better suited than Kalman-based approaches to nongaussian uncertainty distributions, while the Eulerian discretization of $P({\bf x},t)$ in the new approach avoids the sticky wicket associated with Lagrangian (``particle''-based) discretizations. [Preview Abstract] |
Sunday, November 22, 2009 5:20PM - 5:33PM |
ED.00006: Global Model Reduction for Fluid-Structure Interaction in Flapping Flexible Wings Mingjun Wei, Tao Yang Reduced-order models (ROMs) for fully-coupled fluid-structure interaction problems are desired in many applications (e.g. design of flapping-wing Micro Air Vehicles). Traditional approach is to build ROMs individually for fluid and solid and couple them through the interface. In this work, we suggest an approach to apply model reduction globally on a uniform description of fluid and solid in Eulerian framework. The idea has been made possible by a set of combined fluid-structue equations, where solid properties are presented as extra terms to Navier-Stokes equations. Then, typical Proper Orthogonal Decomposition (POD)/Galerkin projection can be used for model reduction as in most fluid-only problems, with special care of the extra ``solid'' terms. In the example, we show that one can capture most energy by only a few POD modes. More importantly, the leading POD modes show the signatures of both fluid flow and solid structure. [Preview Abstract] |
Sunday, November 22, 2009 5:33PM - 5:46PM |
ED.00007: Application of Proper Orthogonal Decomposition to Disk Wakes Zachary Berger, Aaron J. Orbaker, Mark N. Glauser, Hiroshi Higuchi, Makan Fardad, Rory Bigger This work investigates the effects of flow control on the near wake region of a disk in a water flow utilizing the POD reconstructed time dependent velocity fields. Velocity measurements were collected using time resolved particle image velocimetry (TRPIV) at a Reynolds number of 20,000 based on the disk diameter both with and without control. Since this is a time resolved velocity data base we are able to reconstruct the time dependent velocity field in the wake for baseline and controlled cases using various POD truncations and observe velocity reconstructions. The current interest is in the convergence of the spatial eigenvalues of the baseline and controlled cases for the future development of closed-loop control systems from the perspective of the time dependent velocity field instead of just using the normal energy convergence criteria. [Preview Abstract] |
Sunday, November 22, 2009 5:46PM - 5:59PM |
ED.00008: Frequency-Domain Proper Orthogonal Decomposition of Synthetic-Jet Actuated Flow for Airfoil Control Guy Ben-Dov, Arne J. Pearlstein, Daniel P. Brzozowski, Ari Glezer Using a proper orthogonal decomposition in the \textit{frequency domain}, we construct a forced ODE system for use as a reduced-order model of flow over an actuated airfoil. The time-dependent part of the flow in the wake, attributable to actuation, is linearized about the mean wake flow (generated from time-averaged flow fields obtained from PIV data). The resulting PDE system consists of the continuity and linearized vorticity transport equations. Using instantaneous inlet conditions from several realizations with different actuation, we apply a spectral method to compute the impulse response of the PDEs in the frequency domain over a wide range of frequency. The decomposition allows substitution of the resulting modes into the PDEs, and Galerkin projection to ODEs. Only the forcing terms are frequency-dependent, and they can be parametrized on an actuator-specific basis. A neural network technique is suggested to relate the dynamics (given by the ODEs) to the aerodynamic forces on the airfoil, which will allow the model to be used as the basis of a control system. [Preview Abstract] |
Sunday, November 22, 2009 5:59PM - 6:12PM |
ED.00009: Active Control of Aerodynamic Forces on a Rapidly Maneuvering Airfoil Daniel Brzozowski, John Culp, Ari Glezer The unsteady aerodynamic forces and moments on a rapidly maneuvering free-moving airfoil are investigated in wind tunnel experiments. The airfoil is mounted on a 2-DOF traverse and its trim and dynamic characteristics are controlled using position and attitude feedback loops that are actuated by servo motors. The motion of the airfoil is effected by bi-directional changes in the pitching moment using controllable trapped vorticity concentrations on both the suction and pressure surfaces near the trailing edge that are induced and regulated by hybrid synthetic jet actuators. 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 unsteady flow characteristics induced by the fluidic actuation during a prescribed maneuver are compared with the effects of a simple rigid-body motion of the airfoil when an external torque is used to achieve a similar maneuver. It is shown that the time-dependent aerodynamic forces and induced flow fields in the near wake of the moving airfoil are significantly different, emphasizing the role of the coupling between the flow control actuation and the model's unsteady aerodynamics. [Preview Abstract] |
Sunday, November 22, 2009 6:12PM - 6:25PM |
ED.00010: Effects of Oscillatory Actuation Frequency on Wall-Mounted Hump Flow Jennifer Franck, Tim Colonius A large eddy simulation (LES) is used to explore the effects of high frequency actuation on the natural separation bubble formed by a wall-mounted hump. Low frequency forcing at $F^+\sim O(1)$ has been shown to increase entrainment through regular shedding of large-scale structures. Using a LES technique previously validated on the baseline and controlled flow over the wall-mounted hump geometry, the effectiveness of high frequency actuation is explored and compared with previous investigations. It is found that the high frequency actuation does not produce distinct vortical structures in the separated shear layer, and does not delay the onset of separation. The resulting mean flow is relatively unaltered by the application of high frequency actuation, although the local flow surrounding the actuation location is slightly modified. [Preview Abstract] |
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