Three dimensionalization of the stratified mixing layer

A theoretical analysis of the stability of a stratified, two-dimensional Kelvin–Helmholtz billow against three-dimensional perturbations is presented. This predicts the three-dimensional spectrum to be dominated by a shear aligned convective instability, which is localized in the region surrounding the billow core. The results of a direct numerical simulation of the evolution of the three-dimensional stratified mixing layer fully verify the dominance of this convective mode in the mixing transition. The origin of the streamwise streaks of vorticity, which precede turbulent collapse in a stratified shear layer, is thereby explained.