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Double-diffusive behaviour at high and low Prandtl number

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Date and time:Ìý

Tuesday, April 5, 2011 - 4:30pm

Abstract:Ìý

Double-diffusive instabilities can occur in fluids which have a stable density stratification, but where the density depends on more than one component. This is for example the case in the ocean, where the density of water depends on its temperature as well as its salinity. When the two components diffuse at different rates, two different types of double-diffusive instabilities can occur depending on the relative stratifications of each individual component. ÌýIn tropical regions of the ocean, hot and salty water typically lie on top of colder and fresher water. ÌýThe temperature is stably stratified, while salinity is unstably stratified. This leads to the "fingering" instability. In polar regions, cold and fresh water typically lie on top of hotter and saltier water. This leads to an oscillatory type of convection. ÌýDouble-diffusive instabilities can also occur in astrophysics, and in particular in the interiors of stars and planets.Ìý

In this talk, I will review both types of double diffusive instabilities and present new results in both geophysical and astrophysical contexts. I will begin by discussing the results of numerical experiments designed to measure the transport properties of both types of double-diffusive convection, in both the astrophysical and geophysical regimes. I will then demonstrate how this small-scale instability can lead to the formation of structures on much larger scales, and what implications this may have for the global evolution of a double-diffusive system. Finally, I will present various applications of our findings to explain oceanic and astrophysical observations.