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Friday 20 Mar 2015Stratified shear turbulence: Application to overflows and gravity currents

Robert Ecke - Los Alamos National Laboratory, USA

Harrison 106 14:00-15:00

Oceanic overflows play an important role in mixing of the ocean owing to the strong turbulence induced by shear as stably stratified water overflows into a deeper region and accelerates down an incline. Prominent examples of such overflows include the dense water flowing out of the Norwegian-Iceland-Greenland Sea via the Denmark Strait and Faroe Bank Channel into the North Atlantic and the Mediterranean Outflow. Field measurements of these overflows yield valuable information but are limited by the large spatial range and the difficulty of the environment. Similarly, numerical simulations contribute important insight into the relevant fluid dynamics of such overflows. I will describe laboratory experiments designed to mimic overflow/gravity-current physics over modest effective length scales where rotation plays a minor role. At the heart of these experiments is the ability to simultaneously measure the velocity and density fields of a vertical cross section with high precision. I will describe two configurations: A) a sloping boundary to explore the unstable regime after the flow accelerates over the sill and B) a horizontal boundary to explore the stability of the flow in the lead up to the sill with respect to varying Richardson number (Ri). At low Ri, Kelvin-Helmholtz instability dominates and the turbulent vertical transport is well described by a Prandtl mixing length description with entrainment coefficients that are consistent with other experimental approaches. At higher Ri, the instability becomes quite intermittent so that typical turbulent averaging schemes may be misleading. Instead, we use a local approach in which the Thorpe Length for each vertical profile is evaluated and grouped according to the degree of overturning. This local measure has some remarkable properties and helps elucidate the nature of instability of turbulent shear flows.

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