Experiments on collapsing turbulent regions in stratified fluids

Abstract

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 width of the experimental tank was used as the turbulence source. Initially, the patch grows rapidly, as in an unstratified fluid, until the buoyancy forces arrest its vertical growth. Thereafter, the patch collapses to form horizontally propagating intrusions at its equilibrium density level. The fluid lost from the patch into the intrusion is replenished by return currents generated at the top and bottom edges of the patch. The nose of the intrusion propagates with a constant average speed (‘initial spreading regime’) determined mainly by the horizontal pressure gradient forces and the resistance induced by upstream propagating, low-frequency, columnar internal waves. Although the intrusion propagation speed is independent of viscous effects, they cause the development of a slug of fluid pushed ahead of the intrusion. When this slug reaches the endwall, strong upstream blocking occurs, causing the intrusion 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 slowly growing in the blocked regime. At large times (t) both the patch and the ‘fully blocked’ intrusion begin to grow vertically with a power law of the form t^1/5. A simple mixing model is advanced to explain this observation. Various turbulent and internal-wave parameters pertinent to collapsing patches were also measured, and their properties were compared with those of non-collapsing patches.