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Accueil > Actualités > Séminaires > Séminaires 2016

Mardi 12 janvier 2016 à 11h00 en salle K118

Erik Lindborg, Département de mécanique du KTH, Stockholm

Titre/Title : Mixing efficiency — Problem Solved ?

Contact :Pierre Augier (équipe MEIGE)

Résumé/Abstract : In this seminar I will first derive an exact expression for the mean square vertical fluid displacements in homogeneous stratified turbulence with constant background stratification. The mean square displacement can be related to the eddy diffusivity of any scalar which is conserved at fluid particles. A series of simulations show that the derived expression is, indeed, correct in the limit of high buoyancy Reynolds number. Simulations of a turbulent passive scalar diffusion confirms that the mean profile of a passive scalar diffuses according to the classical diffusion equation with an eddy diffusivity which conforms to the derived expression. After this, I will discuss the issue of “mixing efficiency” in stratified turbulence and its parametric dependence on the buoyancy Reynolds number and the turbulent Froude number, Fr. It will be argued that the mixing efficiency, to leading order, is a function of the Froude number, and not the buoyancy Reynolds number and that it goes as Fr⁻² for weak stratification (large Fr) and approaches a constant in the limit of small Fr. Results from a series of very high resolution direct numerical simulations of stratified turbulence will be presented. The results are in good agreement with the theoretical arguments.