A Laboratory Study of Localized Boundary Mixing in a Rotating Stratified Fluid

Judith Wells, Ph.D., 2003
Karl Helfrich, Advisor

Oceanic observations indicate that abyssal mixing occurs near rough topography. How locally mixed fluid interacts with the ambient fluid is an open question. Laboratory experiments explore localized vertical boundary mixing within a body of linearly stratified, rotating fluid. A single oscillating bar produces a small region of turbulence along the wall at middepth. Mixed fluid quickly reaches a steady state height set by a turbulent-buoyant balance, independent of rotation.

Initially, the bar is exposed on three sides. Mixed fluid intrudes directly into the interior rather than forming a boundary current. The circulation patterns suggest a model of unmixed fluid being laterally entrained into the turbulent zone. In accord with the model, observed outflux is constant, independent of stratification and restricted by rotation.

Later the bar is laterally confined between two walls, which form a channel opening into the basin. A small percentage of mixed fluid enters a boundary current, which exits the channel. The bulk forms a cyclonic circulation in front of the bar, which blocks the channel and restricts horizontal entrainment. In the confined case, the volume flux of mixed fluid decays with time.