EXPERIMENTS ON COLLAPSING TURBULENT REGIONS IN STRATIFIED FLUIDS

Laboratory experiments were carried out to investigate the properties of a collapsing turbulent patch generated within a linearly stratified fluid by a sustained energy source and its long-time evolution in the presence of lateral boundaries. An oscillating grid spanning the widt h of the experimental tank was used as the turbulence source. Initiall y, the patch grows rapidly, as in an unstratified fluid, until the buo yancy forces arrest its vertical growth. Thereafter, the patch collaps es to form horizontally propagating intrusions at its equilibrium dens ity level. The fluid lost from the patch into the intrusion is repleni shed by return currents generated at the top and bottom edges of the p atch. The nose of the intrusion propagates with a constant average spe ed ('initial spreading regime') determined mainly by the horizontal pr essure gradient forces and the resistance induced by upstream propagat ing, low-frequency, columnar internal waves. Although the intrusion pr opagation speed is independent of viscous effects, they cause the deve lopment of a slug of fluid pushed ahead of the intrusion. When this sl ug reaches the endwall, strong upstream blocking occurs, causing the i ntrusion to decelerate ('blocked regime'); the intrusion nose, however , eventually reaches the endwall. The thickness of the patch is found to be approximately constant during the initial spreading regime and s lowly growing in the blocked regime. At large times (t) both the patch and the 'fully blocked' intrusion begin to grow vertically with a pow er law of the form t(1/5). A Simple mixing model is advanced to explai n this observation. Various turbulent and internal-wave parameters per tinent to collapsing patches were also measured, and their properties were compared with those of non-collapsing patches.