Impingement of buoyancy-driven flows at a stratified interface

Abstract

Laser-induced fluorescence (LIF) and particle-image velocimetry (PIV) are used to study both thermals and plumes impinging on a stratified interface. Data are obtained for a central slice of the flow near the stratified interface. Both the thermal and plume are generated by releasing fresh water at the bottom of a tank filled with two layers of salt water of different densities. Thermals and plumes are studied at Reynolds numbers ranging from 3,000 to 8,000, above the value for the mixing transition, a Schmidt number of about 600, and Richardson numbers from 1 to 22. The Richardson and Reynolds numbers are based on the thermal or plume characteristics (size and vertical velocity) before impingement and the initial density difference across the interface. Laser-induced fluorescence (LIF) is used to determine the maximum penetration height, rebound distance and lateral spreading velocity. The vorticity results obtained from the PIV data reveal the vortical structure near impingement. When the thermal impinges upon the stratified interface, a baroclinic eddy generated at the interface appears to merge with eddies comprising the thermal itself to form a vortex ring. This ring remains near the interface, moving mainly along the lateral or horizontal direction away from the region of impingement. These results suggest that lateral transport is significant for thermals impinging on stratified interfaces, and that ignoring such transport may greatly underestimate overall transport and mixing in such flows.