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 relaminarizing flows, the inner and outer regions behave nearly independently. While the inner region turbulence decays monotonically, large-scale restratification, internal waves, and potential energy-driven motions are observed in the outer region. The simulation results are used to construct a physical model of stratified wall-bounded flows. Stable stratification weakens the interaction between the inner and outer regions by decreasing the vertical transport, leading to near-decoupling of the two layers at strong enough stratification. The notion that the disappearance of the log region marks the onset of buoyancy control provides a criterion for estimating the Richardson number delineating the transition from buoyancy affected to buoyancy controlled flows. Data that should be useful for creating parametrizations for prediction of stratified flows are also presented. (C) 2000 American Institute of Physics. [S1070-6631(00)51210-0].