Stably stratified turbulent channel flows. I. Stratification regimes and turbulence suppression mechanism

Our objective in this study is to study inhomogeneous stratified shear flows using large eddy simulation; stratified pressure-gradient-driven channel flow was selected. The flows can be separated into three regimes: buoyancy affected, buoyancy controlled, and buoyancy dominated flows. The regime boundaries are defined by Richardson and Reynolds numbers based on the friction velocity. Buoyancy affected flows remain actively turbulent and attain a statistical steady state that resembles a lower Reynolds number unstratified flow. Flows in the buoyancy controlled regime are not in equilibrium. In the cases studied, an asymmetry develops with respect to the channel centerline leading to one-sided turbulence. Eventually, the “inactive” half undergoes a transition initiated by the active half and symmetry is restored. At higher Richardson numbers, the flows are buoyancy dominated, the near-wall burst-sweep process is completely disrupted and turbulence production ceases, leading to relaminarization. In relaminar...