Three-dimensionalization of the stratified mixing layer at high Reynolds number

Abstract

Numerical analyses of the stability of a stratified two-dimensional Kelvin-Helmholtz billow against three dimensional disturbances provide a theoretical means of identifying the primary mechanism(s) that induce transition to turbulence. We identify, through fully resolved three-dimensional numerical simulations, secondary modes of instability which have been suggested by recent observations to be responsible for this transition. Our analyses lead us to two primary conclusions. First, as the Reynolds number is increased at a fixed stratification level, the vortex pairing process may be entirely suppressed by the rapid growth of three-dimensional secondary instabilities. Second, the new transition mechanisms identified herein have significant implications for the efficiency of mixing in the turbulent flow that develops subsequent to saturation of the secondary instabilities.