Generation and spreading of a turbulent mixed layer in a rotating, stratified fluid

Abstract

Results are presented from an experimental investigation into the generation and growth of a turbulent mixed layer above and below an oscillating horizontal grid in a rotating stratified fluid. The (square) grid was located at middepth at one end of a long rectangular container, and was confined on three of its four sides by the vertical walls of the container. Flow visualization data are presented to show (1) the rapid initial vertical growth of the mixed layer above and below the grid, (2) the collapse of the mixed layer and the generation of an intrusive flow into the initially undisturbed interior of the fluid, (3) the deflection of the intrusive flow toward the right side wall, and (4) the formation of a boundary current, which circulates around the container before returning mixed fluid to the source region at the grid. Examples of stable and unstable boundary currents are illustrated. Quantitative data are presented to show that the thickness of the mixed layer increases with either decreasing buoyancy frequency, and/or increasing grid frequency, throughout all phases of its development. The influence of background rotation upon the vertical growth of the mixed layer is shown to be negligible during the rapid initial vertical expansion of the layer, though this is not the case in the longer‐term behavior. For this latter phase, the layer thickness increases with increasing rotation rate of the system, under otherwise identical conditions. Measurements of the initial equilibrium thickness hc of the layer show that it scales with the parameter (K/N)½, where K is the grid action, and N is the buoyancy frequency of the stratified fluid. Values of the dimensionless parameter [hc/(K/N)½] show no significant dependence upon the ratio N/2Ω of the buoyancy and inertial frequencies, for a range of values of the latter between 0.4 and 29.7. This behavior is shown to be compatible with the result (Hopfinger, 1987) that shear‐free turbulence is dominated by background rotation only if the Rossby number based on the rms velocity and integrated length scale is less than 0.2.