Thibault Flinois

Research

Feedback Control of Bluff Body Wakes for Drag Reduction

I am interested in using system identification, model reduction, and feedback control techniques to understand and modify the dynamics of bluff body wakes, in particular for drag reduction and wake stabilisation purposes.

My first area of focus is the contol of three-dimensional wakes using methods that are readily applicable in practical situations. Specifically, I study rectilinear bodies whose trailing edge has a sharp corner (and therefore a fixed separation point) and which are located close to a flat surface using Large Eddy Simulations. These bodies are intended to represent the simplified geometry of a generic road vehicle. The dynamics of the wake are modified by using synthetic jets located near the rear of the body. These jets apply blowing/suction of air at amplitudes and frequencies that are dictated by a feedback controller in order to reduce the large form drag experienced by the body.

I am also developing system identification techniques, which are computationally tractable even for large unstable systems, and mainly based on system snapshots or output measurements. These methods are used to obtain low-order balanced models of the linearised input-output dynamics of complex (potentially very large) systems, about an unstable equilibrium state. H-infinity loop-shaping or other control design techniques are then used to stabilise the system, based on the identified models. These methods also provide useful information for selecting actuators and sensors.

A third approach I have worked on (while at Caltech) is adjoint-based receding-horizon optimisation (or model predictive control, MPC), where optimal control waveforms are obtained using gradient information, resulting from adjoint simulations. These gradients can be calculated even for fully nonlinear and unsteady flows and can unveil unsuspected and very effective control mechanisms. As the time taken by this procedure can often be restrictive, I have also focused on developing techniques to speed up the convergence of the method, by parallelising parts of the optimisation algorithm.

Education and Research Experience

Visiting Student Researcher, California Institute of Technology, USA, January - September 2013

MEng (Distinction) Aerospace and Aerothermal Engineering, University of Cambridge, UK, 2011

BA cantab (First Class) Mechanical Engineering, University of Cambridge, UK, 2011 (MA cantab 2014)

Journal Articles

• T. L. B. Flinois, A. S. Morgans, P. J. Schmid, Projection-free approximate balanced truncation of large unstable systems (2015), Physical Review E, 92, 023012 (DOI).
• T. L. B. Flinois, T. Colonius, Optimal control of circular cylinder wakes using long control horizons (2015), Physics of Fluids, 27, 087105 (DOI).
• T. L. B. Flinois, A. S. Morgans, Feedback control of unstable flows: a direct modelling approach using the Eigensystem Realization Algorithm (under review by the Journal of Fluid Mechanics).
• T. L. B. Flinois, A. S. Morgans, Linear feedback control of the wake of a three-dimensional bluff body (in preparation).

Conference Presentations and Proceedings

• T. L. B. Flinois, A. S. Morgans, Feedback control of unstable flows based on the Eigensystem Realisation Algorithm, 6th symposium on Global Flow Instability and Control, Crete, Greece, 29 September, 2015 (Abstract).
• T. L. B. Flinois, A. S. Morgans, Linear feedback control of bluff body wakes, Imperial College Aeronautics Research Colloquium, London, UK, 6 May, 2015 (Abstract).
• T. L. B. Flinois, P. J. Schmid, A. S. Morgans, Approximate balanced truncation for large unstable systems, 67th Annual Meeting of the American Physical Society's Division of Fluid Dynamics, San Francisco, USA, 23-26 November, 2014 (Abstract).
• A. S. Morgans, J. A. Dahan, T. L. B. Flinois, Feedback control for reducing the pressure drag of bluff bodies terminated by a backward-facing step, UKACC 10th International Conference on Control, Loughborough, UK, 8-11 July, 2014 (DOI).
• T. L. B. Flinois, A. S. Morgans, Feedback control of the wake of a three-dimensional blunt bluff body. 66th Annual Meeting of the American Physical Society's Division of Fluid Dynamics, Pittsburgh, USA, 24-26 November, 2013 (Abstract).
• T. L. B. Flinois, A. S. Morgans, Feedback control of the wake of a three-dimensional blunt bluff body. Southern California Flow Physics Symposium, California Institute of Technology, USA, 13 April, 2013.

 Awards and Certificates

Royal Aeronautical Society Aerospace Speakers' Travel Grant, 2014.

Imperial College Trust Travel Grant, 2014.

Faculty of Engineering Graduate Teaching Assistant Optional Certificate, 2014.

Doctoral Training Award: Ph.D. studentship, 2011.

Bill Browne Fund Prize for Final Year Project and examination results, 2011.

Churchill College Prize Scholarship 2011 and 2010.

Collaborators and Co-authors

Dr. Aimee S. Morgans, Reader in the Department of Aeronautics, Imperial College London (supervisor).

Prof. Peter J. Schmid, Chair in Applied Mathematics, Imperial College London.

Prof. Tim Colonius, Department of Mechanical and Civil Engineering, California Institute of Technology.