Extremely Large Velocity Gradients in High-Reynolds Numbers Flows, Dr. Alain Pumir
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June 28, 2017 - 12:00pm to 1:00pm
The observation of large velocity gradients is a characteristic feature of turbulent flows, the more so as the Reynolds number increases. How do these large velocity gradients form, is a multi-faceted question. On the one hand, I will discuss recent work on the yet-to-be-answered question of singularity formation in the fluid equations. One of the important observations is that, as vorticity increases (is stretched), the flow becomes locally 2-dimensional, which tends to slow down further growth. I will present and analyze a possible scenario to overcome this difficulty, based on a cascade of instabilities. On the other hand, I will discuss results from direct numerical simulations of turbulent flows, focusing on the structure of the velocity gradient tensor, conditioned on large values of strain or vorticity. Numerical results clearly show that flow regions with large vorticity fluctuations tend to be effectively 2-dimensional. I will discuss the effect of the Reynolds number.