Fluid envelopes of planets and stars support both oscillatory wave motions AND turbulent convective motions (i.e. at the same time), because the density-stratification is rarely purely stable or purely unstable (i.e. but rather a mix of the two). In this talk, with the help of direct numerical simulation results, I will show that the two dynamics are strongly coupled when the stratification strength of the stable layer is moderate or weak, and that they can feed on each other in ways that have not been previously discussed. The most remarkable result that I'll discuss concerns the emergence in the stably-stratified layer of a mean flow that changes direction over long time scales, a phenomenon reminiscent of the quasi-biennial oscillation in Earth's lower tropical stratosphere. I will demonstrate that the generation of the mean flow, which results from nonlinear wave interactions, is strongest when the convectively-generated internal waves are weakly damped at their intrinsic frequency, but strongly dissipated when Doppler-shifted because of the mean flow. A prediction for the effect of the problem parameters on the emergence of the mean flow will be presented.
Dr. Louis-Alexandre Couston joined in the Summer of 2016 Michael Le Bars' team at the Institut de Recherche sur les Phenomenes Hors Equilibre (IRPHE) in Marseille, France, as a postdoctoral researcher for the project FLUDYCO funded by the European Research Council (FLUid DYnamics of planetary COres). From Fall 2011 to Spring 2016, Louis was a graduate student at UC Berkeley, California, USA, where he earned an MSc/PhD in Mechanical/Ocean Engineering under the supervision of Prof. M.-Reza Alam. He was a member of the Theoretical and Applied Fluid Dynamics Laboratory (TAFLab) at Berkeley.