Mark Pastoor

Feedback shear layer control for bluff body drag reduction

Drag reduction strategies for the turbulent flow around a D-shaped body are examined experimentally and theoretically. A reduced-order vortex model describes the interaction between the shear layer and wake dynamics and guides a path to an efficient feedback control design. The derived feedback controller desynchronizes shear-layer and wake dynamics, thus postponing vortex formation. This actuation is tested in a wind tunnel. The Reynolds number based on the height of the body ranges from 23 000 to 70 000. We achieve a 40% increase in base pressure associated with a 15% drag reduction employing zero-net-mass-flux actuation. Our controller outperforms other approaches based on open-loop forcing and extremum-seeking feedback strategies in terms of drag reduction, adaptivity, and the required actuation energy.

Model-based Control of Vortex Shedding Using Low-dimensional Galerkin Models

A model-based flow control strategy is proposed for the suppression of vortex shedding behind a circular cylinder. The control design is based on a hierarchy of low-dimensional Galerkin models of the cylinder wake. These models are constructed from a Karhunen-Loeve decomposition of a simulation without actuation. The key enablers are an additional physical mode in the Karhunen-Loeve approximation and an energy-based control which respects the regime of model validity. The developed control strategy is successfully tested in direct numerical simulations. Copyright  2003 by J. Gerhard, M. Pastoor, R. King, B.R. Noack, A. Dillmann, M. Morzynski and G. Tadmor. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. ∗Research engineer. Corresponding author: phone: ++49-30-314.79574, x24100; fax: x21129; e-mail: johannes.gerhard@tu-berlin.de †Research engineer ‡Professor §Research engineer ¶Professor ‖Professor ∗∗Associate Professor

Feedback Control Applied to the Bluff Body Wake

In the present study the flow around a 2D bluff body with blunt stern is investigated experimentally and theoretically. The goal is to decrease and stabilize drag by active control. Low-dimensional vortex models are used to describe actuation effects on the coherent structures and the pressure field. Open-loop actuation as well as feedback control is applied using robust H ∞-controllers and slope-seeking feedback for a range of Reynolds numbers based on the height from 20 000 to 60 000. As expected, a decreased drag is observed to be related to delayed vortex shedding, i.e. an extended recirculation zone. Intriguingly, a control which mitigates the natural coupling between separating upper and lower shear-layer and the vortex street serves that purpose.

Model-based Coherent-structure Control of Turbulent Shear Flows Using Low-dimensional Vortex Models

In this study, a flow control strategy is presented for manipulating coherent shear-flow structures. As a benchmark problem, the transitional flow around a backward-facing step with local acoustic actuation at the upper edge is chosen. The strategy is based on a hierarchy of low-dimensional vortex models and targets the use of control-theory methods for control design. The hierarchy ranges from an one-vortex model which describes the vortex roll-up and shedding to a vortex-blob model which resolves the formation of Kelvin-Helmholtz vortices and the dynamics of the recirculation zone with a few hundred vortices. The higher-dimensional variants predict well the frequency-dependent reduction of the recirculation zone with harmonic actuation. The Lagrangian vortex models incorporate a continuous production, Copyright  2003 by M. Pastoor, R. King, B.R. Noack, A. Dillmann, and G. Tadmor. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. Research engineer. Corresponding author: phone: ++49-30-314.79574, x24100; fax: x21129; e-mail: mark.pastoor@tu-berlin.de Professor Research engineer Professor Associate Professor merging, and annihilation of vortices, but this hybrid nature excludes the use of many control-theory methods. Hence, oneand two-dimensional Eulerian vortex models employing a remeshing technique are proposed. The dynamics of these Eulerian variants define a continuous evolution in a phase space with fixed dimension and are hence better suited for control-theory and dynamics-systems approaches.

Control, observation and energy regulation of wake flow instabilities

A three-dimensional Galerkin model is used in feedback design to regulate the perturbation kinetic energy in the flow around a cylinder. The objective may vary from stabilization in order to reduce drag to mixing enhancement. The Landau model [H.K. Khalil, 2002] includes an oscillatory state pair and a shift mode, exchanging energy with the mean flow. Given the models simplicity, it is essential to maintain closed-loop dynamics close to the systems natural dynamic range which is represented by an invariant manifold and a natural frequency, adding a design constraint addressed in this note.

Spatiotemporal Waveform Observers and Feedback in Shear Layer Control

Active control enhances shear layer mixing by exciting natural instabilities. Harmonizing the periodic actuation with vortex release can be used to manipulate and improve the mixing effect. This type of feedback requires tracking large shear layer vortices. We propose an observer structure based on a very simple model, representing the collective signal of a set of sensors as a spatiotemporal waveform. The observer tracks the slow drift and abrupt changes in the spatiotemporal phasors (local Fourier coefficients). Benefiting from a much longer time constant than the period of vortex release, it naturally filters spatial and temporal high frequency noise. The discussion is illustrated by simulations and wind tunnel experiments for the flow over a backward facing step, where the design objective is shortening the recirculation bubble. Simulations of the free shear layer are used to illustrate means to track asynchronous vortex merging events.