Differential Diffusion of Scalars in Sheared, Stratified Turbulence

Abstract

To determine why differential diffusion occurs, rapid distortion theory (RDT) is used. To determine where differential diffusion occurs in the target parameter space, laboratory experiments are employed. RDT confirmed previous explanations that differential diffusion occurs because of excess upgradient flux of the lower molecular diffusivity scalar. RDT also showed that differential diffusion depends on the strength of shear, the density ratio, the molecular diffusivities of the scalars, and the activity of the scalars. Experiments revealed that heat mixes more efficiently across a sheared density interface compared to salt for strongly stratified, weakly turbulent flows. In the presence of differential diffusion, buoyancy flux and mixing efficiency at the interface are functions the density ratio and entrainment laws for a two scalar flow exhibiting differential diffusion agree with laws for single scalar shear flows. Experiments show a weak dependence of differential diffusion on shear compared to RDT and no conclusive density ratio dependence. The theoretical and experimental results agree with past experiments, DNS, and theory. A survey of the literature in oceanography and limnology revealed that differential diffusion can be significant in many parts of the world's oceans and lakes.