Turbulence, mixing and Prandtl number effects in stratified plane Couette flows

We report direct numerical simulations (DNS) of stratified plane Couette (SPC) flows in both continuous and layered stratifications for a wide range of Prandtl number Pr from 0.7, 7 to 70. In the continuously stratified set-up, Pr has a significant impact on the momentum and heat fluxes across the flow. Variations of these fluxes then modifies the turbulence characteristics in the way that is consistent with Monin–Okukhov (M-O) similarity theory. The mixing properties in the interior of the flow, however, appear to be independent of Pr . We employ M-O theory and the Osborn (1980) model to formulate scalings for the turbulent diffusivities. These scalings are then verified by DNS and discussed with respect to existing scalings in the literature. In the layered configuration, we investigate the transient mixing of a density interface, where the variation of Pr indeed becomes important.