Reduction of vertical transport in two-dimensional stably stratified forced shear flows
Résumé
The effect of stable stratification on the vertical transport of passive contaminants in forced, stationary, two-dimensional (2D) and inhomogeneous shear turbulence is investigated numerically. The mean flow consists of several superimposed parallel sheared layers in a stably stratified medium. We find that, as stratification increases, the vertical transport decreases much faster than predicted by mixing length estimates. For the highest stratification, particles vertical dispersion nearly vanishes. The proposed interpretation emphasizes the role of weakly sheared layers where the relative increase of the mean horizontal velocity with respect to the root-mean-square (rms) vertical velocity causes the decrease of the Lagrangian correlation timescale.