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Seminar

Title : Topographic control of stratified flows: Blocking, upstream jets, and isolating layers
Name of the speaker : Kraig Winters
Affiliation : Scripps Institution of Oceanography
Laboratory organizer : LOCEAN
Date and time : 18-06-2015 11h00
Location : UPMC - 4 place Jussieu - Paris 5e - salle de réunion LOCEAN, tour 45/55, 4eme étage
Summary :

We consider stratified flows over isolated obstacles in the limit of strong stratification and tall obstacles for which the total kinetic energy of the flow is insufficient to draw all of the fluid up and over the obstacle crest and so the fluid below some height is arrested or blocked. Optimal solutions to the nonlinear, hydrostatic, Boussinesq equations are developed for steady, density-stratified, topographically controlled flows characterized by blocking and upstream influence. These flows are jet-like upstream and are asymmetric and thinning as they accelerate over the crest. A stagnant, uniform-density isolating layer, surrounded by a bifurcated uppermost streamline, separates the accelerated flow from an uncoupled flow above. The flows are optimal in the sense that, for a given stratification, the solutions maximize the topographic rise above the blocking level required for hydraulic control while minimizing the total energy of the flow. Hydraulic control is defined mathematically by the asymmetry of the accelerated flow as it passes the crest. A subsequent analysis of the Taylor-Goldstein equation shows that these sheared, non-uniformly stratified flows are indeed subcritical upstream, critical at the crest, and supercritical in the downslope flow with respect to gravest-mode, long internal waves. The solutions are relevant to oceanic flows over topography, atmospheric flows over mountains, stratified towing experiments, arrested wedge flows and selective withdrawal.