Layer formation and relaminarisation in plane Couette flow with spanwise stratification*

We investigate the dynamics of stably-stratified plane Couette flow at high Reynolds number with gravity oriented in the spanwise direction. We observe near-wall layering and associated new mean flows in the form of large scale spanwise-flattened streamwise rolls. The layers exhibit the expected buoyancy scaling l_z ~ U/N  where U is a typical horizontal velocity scale and N is the buoyancy frequency. We associate the new coherent structures with a stratified modification of the well-known large scale secondary flow in (unstratified) plane Couette flow and find that the possibility of the transition to sustained turbulence is controlled by the relative size of this buoyancy scale to the spanwise spacing of the streaks.