W. G. Pritchard Lab Seminar - 109 Boucke Building

	     **December 5, 2001**
	
Is Hydrodynamic Turbulence Really Isotropic and Universal at Small Scales ?

Alain Pumir

Institut Nonlineaire de Nice-CNRS, FRANCE

Abstract:

Hydrodynamic turbulence is one of the most challenging unsolved problems
in the physical sciences, characterized by a very complex motion
involving many length scales. The smallest scales of motion, which are
excited by nonlinear mechanisms ("the cascade") are expected to be
isotropic, and in general, universal i.e. independent of the large scale
properties of the flow.

We review some recent work, and demonstrate that the postulate of local
isotropy is at odds with both numerical and experimental observations, 
in particular concerning the 3rd moment of the correlation function of 
a passive scalar, as well as the velocity field itself. We also present
recent theoretical work based on various simplified models, which allows
us to make explicit predictions for the structure of the three point
correlation function of a passive scalar. The generalization of these 
ideas to the problem of modeling the turbulent velocity fluctuations
will be discussed.