Mixing by shear instability at high Reynolds number

Abstract

Shear instability is the dominant mechanism for converting fluid motion to mixing in the stratified ocean and atmosphere. The transition to turbulence has been well characterized in laboratory settings and numerical simulations at moderate Reynolds number—it involves “rolling up”, i.e., overturning of the density structure within the cores of the instabilities. In contrast, measurements in an energetic estuarine shear zone reveal that the mixing induced by shear instability at high Reynolds number does not primarily occur by overturning in the cores; rather it results from secondary shear instabilities within the zones of intensified shear separating the cores. This regime is not likely to be observed in the relatively low Reynolds number flows of the laboratory or in direct numerical simulations, but it is likely a common occurrence in the ocean and atmosphere.