Turbulent entrainment across stable density step structures

Abstract

This paper is a summary of a research on the properties of the turbulent entrainment across stable density-stratified layers, a phenomenon commonly observed in geophysical fluid dynamics. A turbulent environment is created by oscillating grids in quasi-homogeneous layers of slightly different density separated by thin sheets of fluid containing a strong density gradient. The buoyancy flux across thin stable layers is found to be a function of the Richardson and Prandtl numbers. Theoretical and experimental studies are made, leading to a determination of the order of magnitude of the four terms expressing the balance of energy in physical space. It is shown that the turbulent energy flux is of the same order of magnitude as the viscous dissipation of kinetic energy in the quasi-homogeneous layers where the slight density gradient is not altogether negligible because a small but measurable fraction of the turbulent energy flux goes into maintaining the buoyancy flux. Data taken in the stable layer show the simultaneous existence of large internal waves and patchy turbulence. Some properties of these waves as functions of the Richardson number are determined.