(LNM-5月22日 9:00）Extremely large velocity gradients in high-Reynolds number flows
Extremely large velocity gradients in high-Reynolds number flows
报告人: Professor Alain Pumir
Laboratory of Physics, ENS-Lyon, France
The observation of large velocity gradients is a characteristic features of turbulent flows, the more so as the Reynolds number increases. How do these large velocity gradients form, is a multi-facetted 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.
Alain Pumir obtained his PhD in 1983 (Thesis advisor: Yves Pomeau). From 1984 to 1986, he was a Postdoc at Cornell University (Advisor: Eric Siggia). He has been in CNRS since 1983 and worked at Ecole Normale Supérieure in Paris, then, from 1993 till 2008, at the “Institut Nonlinéaire de Nice”, University of Nice. From 2008, he is at Laboratory of Physics, ENS-Lyon. Prof. Pumir has broad research interest on nonlinear dynamics and pattern formation, fluid turbulence, and biophysics. He has (co-)organized several European scale research activities, including European network "Stirring and Mixing", COST action "Particles in Turbulence", and the FP7 project "EuHIT". He is on the editorial boards of Nonlinearity (1996 - present), Phys. Rev. E (2009 - 2014), and J. Phys. A (2011 - present). He is an APS fellow, and was awarded the Humboldt Prize and the "Chevalier de la Legion d'Honneur" (one of the highest honorary distinction by the French government).