Control of convection-dominated flows

Abstract : In this thesis, a flow control procedure is developed numerically and is then implemented experimentally. The purpose of this procedure is to reduce the amplitude of perturbations in convection dominated flows. To design such a technique three aspects are analyzed in a first part. Since information in convection-dominated flows essentially travel downstream, incoming perturbations are better described by placing sensors upstream. This intuitive idea is studied quantitatively by introducing the concept of visibility length . In addition, a description of the flow dynamics is obtained using system identification techniques. These tools have the advantage of providing models based solely on experimentally accessible data and are therefore directly applicable to real flows. Finally, a feed-forward control approach is found to be most appropriate and a comparison with the classical linear quadratic gaussian technique is presented from numerical and theoretical point of views. In a second part, these three aspects are then taken into account in the design of a feed-forward identification and control procedure, which is then simplified to be more amenable to practical implementations in experiments. In particular, the system impulse responses are first identified, and are then directly used for the computation of the control law. Hence, the technique only relies on simple least-squares minimizations and has the advantage of manipulating quantities that have clear physical meanings, such as perturbation convective speeds and characteristic frequencies. Thus, in a last part, the control procedure is applied experimentally to the quenching of natural disturbances in a plane channel flow at Re = 870. Results show that the magnitude of the signal recorded by the objective sensor can be reduced by up to 45%.
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Fabien Juillet. Control of convection-dominated flows. Fluids mechanics [physics.class-ph]. Ecole Polytechnique X, 2013. English. ⟨pastel-01009463⟩

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