Dynamics of turbulence strongly influenced by buoyancy

The dynamics of quasi-horizontal motions in a stably stratified fluid have been simulated for Froude numbers of order 1, so that the flows are strongly affected by the stable density stratification, and for a range of Reynolds numbers. It is found that the horizontal scales of the motion grow continuously in time. The vertical scales decrease and the vertical shearing increases with time, maintaining the Richardson number of order 1, as suggested by Lilly [J. Atmos. Sci. 40, 749 (1983)] and Babin [Theor. Comput. Fluid Dyn. 9, 223 (1997)]. Small-scale instabilities and turbulent-like motions are observed to occur in the high shearing regions, while the larger-scale motions appear to evolve somewhat independently of the Reynolds number. The results suggest that the larger-scale, quasi-horizontal motions would be a continuous source of smaller-scale turbulence until the local Reynolds number drops below a critical value, which is estimated. Finally, a Froude number based upon a vertical differential scale and used in previous scaling arguments and theories is estimated in terms of other parameters. (C) 2003 American Institute of Physics.