Collapse of turbulence in stably stratified channel flow: a transient phenomenon

The collapse of turbulence in a pressure driven, cooled channel flow is studied by using 3-D direct numerical simulations (DNS) in combination with theoretical analysis using a simple local similarity model. Previous studies with DNS reported a definite collapse of turbulence in case when the normalized surface cooling h/L exceeded a critical value of 0.5. Recent study by the present authors revealed that such collapse occurs when the flow momentum at the onset of cooling is too small to generate sufficient mixing and hence downward heat transport in order to compensate the heat loss at the surface. The same study, however, indicates that in pressure driven flow, ceasing turbulence automatically leads to flow acceleration, which eventually, causes shear to regenerate turbulence again, in contrast to the DNS findings. In the present study this apparent contradiction is solved. It is shown that also in DNS a recovery of turbulence will occur naturally, provided that perturbations of finite amplitude are imposed to the laminarized state and that sufficient time for flow acceleration has lapsed. Likewise, the collapse can be prevented by imposing sufficient momentum to the initial state. For both cases the existence of a turbulent end state can be explained by the integrated budget of turbulent kinetic energy and mean velocity profiles are in close agreement with those predicted by local similarity scaling. As such it is concluded that the collapse of turbulence is a temporary, transient phenomenon for which a universal, critical cooling rate does not exist.