Prandtl number effects on mixing in Kelvin-Helmholtz billows

We use DNS to study the effect of Prandtl number on mixing in weakly density stratified Kelvin-Helmholtz (KH) billows at low to moderate Reynolds numbers. The time evolution of the density field of the flow and the mixing are significantly influenced by the Prandtl number. By increasing the Prandtl number, the final amount of mixing increases for Reynolds that are too low to sustain three-dimensional motions. Contrarily, the final amount of mixing decreases with increasing Prandtl number for sufficiently high Reynolds number KH flows that can develop significant three-dimensional motions. Our results indicate a steady increase in the total amount of mixing for buoyancy Reynolds numbers above 7, consistent with the results of Shih et al. (2005). Our measure of mixing efficiency exhibits a decreasing trend with increasing Prandtl number, in agreement with previous studies.