Non-Fickian diffusion and tau approximation from numerical turbulence

Details Non-Fickian diffusion and tau approximation from numerical turbulence Non-Fickian diffusion and tau approximation from numerical turbulence

2003-06-25

arxiv.org/abs/astro-ph/0306521v3

Phys.Fluids 16:1020-1027,2004

Astronomy and Astrophysics

Astrophysics

10 pages, 7 figs, Physics of Fluids (final version)

Scientific paper

10.1063/1.1651480

Evidence for non-Fickian diffusion of a passive scalar is presented using direct simulations of homogeneous isotropic turbulence. The results compare favorably with an explicitly time-dependent closure model based on the tau approximation. In the numerical experiments three different cases are considered: (i) zero mean concentration with finite initial concentration flux, (ii) an initial top hat profile for the concentration, and (iii) an imposed background concentration gradient. All cases agree in the resulting relaxation time in the tau approximation relating the triple correlation to the concentration flux. The first order smoothing approximation is shown to be inapplicable.

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