Modelling and control of coherent turbulent structures in aerodynamics and aeroacoustics

Despite the intrinsic complexity of turbulent flows, large-scale coherent structures are present in turbulence, and are related to aerodynamic drag and sound radiation, for example. This presentation shows methods to model such coherent structures, and approaches to control flows in order to reduce drag and noise radiation. Linear models will be considered first, with coherent structures modelled as dominant responses from the linearised Navier-Stokes system. Such dominant modes compare favourably with coherent structures educed from experimental or numerical data, and allow the proposition of changes to the system aiming at drag or noise reduction; moreover, such linearised models allow the formulation of estimation and control problems. Non-linear reduced-order models (ROMs) are then obtained by the Galerkin projection of the full governing equations onto a basis of modes obtained from the linearised system. Quantitative agreement with reference statistics is obtained for a number of canonical flow configurations. Finally, a ROM for turbulent Couette flow is used to devise a control method aiming at turbulence suppresion. The obtained strategy is seen to relaminarise turbulence in direct numerical simulations, showing that ROMs so obtained have promising applications in optimisation and control.