Direct numerical simulation of salt sheets and turbulence in a double‐diffusive shear layer

Abstract

We describe three‐dimensional direct numerical simulations (DNS) of double‐diffusively stratified flow interacting with inflectional shear. The extreme difference in diffusivity (and thus minimum length scale) between heat, salt and momentum in seawater is replicated for the first time in a three‐dimensional simulation. The primary instability generates salt sheets, which are oriented parallel to the direction of the sheared background flow. Subsequently, two distinct mechanisms of secondary instability combine to lead the flow to a turbulent state. In this state, the effective saline diffusivity is smaller than that calculated by previous investigators for the unsheared case. The Schmidt number is much smaller than unity, indicating that salt sheets are less effective at transporting momentum than is often assumed.