Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session A20: Flow Control: Theory |
Hide Abstracts |
Chair: Clancy Rowley, Princeton University Room: 2008 |
Sunday, November 23, 2014 8:00AM - 8:13AM |
A20.00001: Heuristics for Effective Actuator and Sensor Placement in Feedback Flow Control Kevin Chen, Clarence Rowley Actuator and sensor placement can be just as consequential for the performance of localized feedback flow control as controller design. Yet, effective placement is not well understood, and the use of suboptimal placements is common. We report descriptions and characteristics of effective actuator and sensor placements for optimal flow control. We review optimal placements in the linearized Ginzburg--Landau and Orr--Sommerfeld/Squire models of fluid flow. We then analyze the feedback control of these models by relating physical observations with mathematical tools. Although these tools do not fully predict optimal placements, they do reveal patterns that most or all effective placements share. Most notably, effective actuator--sensor placements provide good authority over unstable modes and transient growth, and avoid large time lags between inputs and outputs. [Preview Abstract] |
Sunday, November 23, 2014 8:13AM - 8:26AM |
A20.00002: Control of streaks induced by free-stream turbulence in incompressible boundary layers: application to a linear model George Papadakis, Liang Lu, Pierre Ricco Active wall-transpiration control of streaks generated within an incompressible boundary layer due to free-stream turbulence is examined. The flow model is based on the linearised unsteady boundary-region (LUBR) equations. The effect of free-stream turbulence appears as explicit forcing of these equations, given by an analytic expression, which is obtained by asymptotic matching with the far field conditions. The presence of the forcing term necessitates the reformulation of the control problem and the re-derivation of its solution. The objective cost function that is minimised comprises the weighted energy of the streak and the actuation. It is shown that the control signal consists of two components, a feed-back part (that depends on the state vector) and a feed-forward part. Explicit equations that provide these two components are derived. The developed method is efficient and has modest memory requirements. Computations with different wavenumbers in the wall normal direction demonstrates the significant effect of forcing for the same initial conditions. The effect of actuation on the perturbation energy and vorticity fields is examined. [Preview Abstract] |
Sunday, November 23, 2014 8:26AM - 8:39AM |
A20.00003: Approximate Balanced Truncation for Large Unstable Systems Thibault Flinois, Aimee S. Morgans, Peter J. Schmid A new snapshot-based extension of approximate balanced truncation to unstable systems that does not rely on the computation of global modes is presented. Applying feedback control to fluid flows often allows reaching the desired goal -- e.g. drag minimisation, suppression of instabilities -- more efficiently than passive and open-loop control, or where these approaches are ineffective. However a low-order approximation of the system's input-output dynamics is often required in order to make controller design and online implementation tractable. Several system identification and model reduction procedures have been developed to this end: one such method that has received much attention is approximate balanced truncation, or balanced POD. It is applicable to stable systems and has recently been extended to unstable systems. This extension is based on the projection of the system onto its stable subspace, but this procedure can become computationally expensive for large systems. Here we show how balanced POD can be applied to unstable systems in a way that scales well even for very large systems, as it is projection-free and does not require computing global modes. We show that this method can be easily implemented by applying it to model systems and other selected flow configurations. [Preview Abstract] |
Sunday, November 23, 2014 8:39AM - 8:52AM |
A20.00004: 4D-Var identification of DMD Reduced-Order Models Gilles Tissot, Laurent Cordier, Bernd R. Noack A reduced-order modelling (ROM) strategy is crucial to achieve model-based control in a wide class of flow configurations. In turbulence, ROMs are mostly derived by Galerkin projection of first principles equations onto the proper orthogonal decomposition (POD) modes. POD is widely used since it extracts from a sequence of data an orthonormal basis which captures optimally the flow energy. Unfortunately, energy level is not necessarily the correct criterion in terms of dynamical modelling and deriving a dynamical system based on POD modes leads sometimes to irrelevant models. In this communication, the Dynamic Mode Decomposition (DMD) as recently proposed by Schmid (JFM 2010) is used to determine the DMD modes. A DMD ROM is then derived by Galerkin projection of the Navier-Stokes equations onto a selected set of optimized-DMD modes. Finally, a four-dimensional variational assimilation approach (4D-Var) is employed to identify the coefficients of the DMD ROM. Essentially, 4D-Var combines imperfect observations, a background solution and the underlying dynamical principles governing the system under observation to determine an optimal estimation of the true state of the system. The methodology is illustrated for a DNS cylinder wake flow at Re=200 and PIV measurements at Re=13000. [Preview Abstract] |
Sunday, November 23, 2014 8:52AM - 9:05AM |
A20.00005: Riccati-based Feedback Stabilization of an Oscillating Vertical Cylinder using a POD Reduced-Order Model Laurent Cordier, Gilles Tissot, Bernd Noack The aim of this communication is to demonstrate the use of Reduced-Order Model (ROM) based on Proper Orthogonal Decomposition (POD) to stabilize the flow over a circular cylinder in the laminar regime (Reynolds number equal to 60). The control is introduced by vertical oscillations of the cylinder, the objective being to determine by linear control the vertical velocity of the cylinder that stabilizes the flow. Since in Fluid-Structure Interaction, the POD algorithm cannot be applied directly, the fictitious domain method of Glowinski et al. (JMF 1999) is implemented where the solid domain is treated as a fluid undergoing an additional constraint. The POD-ROM is then classically obtained by projecting the Navier-Stokes equations on the first POD modes. The cylinder movement is enforced in the POD-ROM through the introduction of Lagrange multipliers. Finally, a Linear Quadratic Regulator framework is used to determine the optimal control law such that the flow is stabilized. [Preview Abstract] |
Sunday, November 23, 2014 9:05AM - 9:18AM |
A20.00006: Numerical approximation of spectrum of the linearized Navier-Stokes operator for flow around an infinite cylinder Jonathan Gustafsson, Sivaguru S. Sritharan Numerical approximations of the spectrum of the Oseen operator and the linearized Navier-Stokes operator for flow around a cylinder in two dimensions have been studied for Reynolds numbers between 2 and 60. By approximating the eigenfunctions with a spectral method featuring basis functions covering the entire exterior domain, it is possible to obtain a numerical approximation to the continuous spectrum and the isolated eigenvalues (point spectrum). The numerical approximation of the spectra agrees with the previous rigorous results by Babenko (1982). That is a parabolic tongue containing the continuous spectra for the Oseen operator and a parabolic tongue containing the continuous spectrum plus a finite number of isolated eigenvalues for the linearized Navier--Stokes operator. The research feature a novel way of selecting location of collocation points. Future work will include control on the surface of the cylinder and examining its effect on both the unstable eigenvalues and the continuous spectrum. [Preview Abstract] |
Sunday, November 23, 2014 9:18AM - 9:31AM |
A20.00007: Model-based design of drag-reducing spanwise wall oscillations Mihailo Jovanovic, Armin Zare We study the model-based design of spanwise wall oscillations for drag reduction in a turbulent channel flow. Our approach selects the optimal period of oscillations by examining the influence of periodic base-flow-modification on Reynolds stresses. These are obtained from the linearized Navier-Stokes equations that are driven by colored-in-time stochastic forcing. Forcing correlations are selected, through a convex optimization procedure, to reproduce the statistical signature resulting from direct numerical simulation of the uncontrolled flow. We show that our analysis reliably predicts the optimal period of wall-oscillations in a simulation-free-manner. This demonstrates the effectiveness of our model-based approach in designing drag reducing wall oscillations and lays ground for utilizing such techniques in other passive or active flow control setups. [Preview Abstract] |
Sunday, November 23, 2014 9:31AM - 9:44AM |
A20.00008: A framework for studying the effect of compliant surfaces on wall turbulence Mitul Luhar, Ati Sharma, Beverley McKeon It has long been recognized that compliant surfaces can serve as passive controllers for turbulent flows. However, the lack of a physics-based, computationally cheap theoretical framework that predicts the effect of compliant surfaces on turbulence has restricted progress towards designing performance-improving walls. To address this gap, we extend the resolvent analysis of McKeon {\&} Sharma (2010, \textit{J. Fluid Mech.}). Under this analysis, the turbulent velocity field is expressed as a linear superposition of propagating modes, identified via a gain-based decomposition of the Navier-Stokes equations. Compliant surfaces, modeled as a complex wall-admittance linking pressure and velocity, affect the gain and structure of these modes. Using a pattern search, we show that walls with unphysical negative damping are required to interact favorably with modes resembling the energetic near-wall cycle, which could explain why previous studies have met with limited success. Our results suggest that positive-damping walls could be effective for modes resembling the so-called very large-scale motions (VLSMs). Since the VLSMs have an organizing influence on smaller-scale turbulence, they may serve as a pathway for compliant walls to affect the entire flow. [Preview Abstract] |
Sunday, November 23, 2014 9:44AM - 9:57AM |
A20.00009: Global eigenfunction based actuation and sensor design for compressible, viscous flows Mahesh Natarajan, Jonathan Freund, Daniel Bodony A method is developed to estimate optimal actuator types and locations for controlling compressible, viscous flows using linear feedback. Based on an analysis of the eigensystem of the linearized compressible Navier-Stokes operator for steady baseflow, the forward and adjoint global modes are used to estimate of where the controller should be placed, and what type of controller (mass, momentum, energy, etc.) it should be. The method is demonstrated using direct numerical simulations of a separated boundary layer in a Mach 0.65 diffuser at different Reynolds numbers. The baseflow is taken as the true steady solution or the time-averaged flow. For sufficiently low Reynolds numbers, global stabilization of the flow is achieved; only partial stabilization is achieved at higher Reynolds numbers. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700