TURBULENT MIXING IN CROSS SHEARED STRATIFIED FLOW

Cross sheared stratified (CSS) flow, in which the horizontal streamwise and spanwise basic sheared flow components interact with each other in a stratified environment, are studied with direct numerical simulation. The cross shear ratio xi, defined as ξ = Dv0/Du0 where Du0 and Dv0 are the respective velocity changes across the heared/stratified layer in the streamwise and spanwise directions, is introduced for CSS flow. The coherent structures of CSS flow are examined for ξ = 0 ~ 1 and Ri = 0.01 ~ 0.2, where Ri is the bulk Richardson number. It is found that in a weakly stratified environment with Ri = 0.01, a significant increase of the magnitude of the mixedness thickness dm is observed from its time series for CSS flow compared to that for parallel sheared stratified flow. Three CSS instability modes, i.e. , a 'streamwise dominant' mode, a 'balanced' mode and a 'spanwise dominant' mode, are identified, which differ from each other with different interaction behaviors between the spanwise 'eddy wrap' structures and the classic streamwise 'cat eye' eddies of the Kelvin-Helmholtz instability. The results also show that in a strongly stratified environment with Ri=0.1-0.2, the eddy-featured coherent structures progressively decay into the wave-like structures, and the normalized mixedness thickness dq /dq ,Ri=0.01 is found to decrease as an exponential function of increasing Ri at the turbulence stage.