Séminaire de Mécanique d'Orsay

Le Jeudi 7 décembre 2017 à 14h00 - Salle de conférences du LIMSI

Optimal mixing in stratified plane Poiseuille flow

Florence Marcotte
DAMTP et BP Institute, Institut Jean le Rond d'Alembert

Mixing is an ubiquitous process in environmental flows. In order to better understand the mechanisms involved in achieving well-⁠mixedness of a stratified flow, we investigate the optimisation of mixing in stratified, 2D Poiseuille flow by means of a fully nonlinear direct-⁠adjoint looping method, where the initial perturbation leading to maximal mixing efficiency over a given time horizon is determined. A recent study by Foures et al. (2014) has demonstrated that the widely-⁠used approach consisting in maximizing the time-⁠averaged kinetic energy surprisingly results in largely sub-⁠optimal suppression of a passive scalar variance. By identifying the initial perturbations which either maximize the kinetic energy transient growth or minimize a specific `mix-norm' at a chosen target time, we extend these results to the case of an active scalar. We show how buoyancy modifies the optimal-mixing strategy and, by considering the time evolution of the kinetic energy and potential energy reservoirs, discuss the physical mechanisms involved in achieving efficient irreversible mixing.

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