Speaker
Prof.
Karl-Heinz Raedler
(Astrophysical Institute Potsdam)
Description
It is demonstrated that the total mean-field diffusivity for
passive scalar transport in a compressible fluid showing
isotropic turbulence may well be smaller than the molecular
diffusivity. This is in full analogy to the old finding
regarding the magnetic mean-field diffusivity in an
electrically conducting turbulently moving compressible
fluid (e.g. Krause and Rädler 1980). For both the passive
scalar and the magnetic case several analytical results on
mean-field diffusivities found within the second-order
correlation approximation are presented as well as numerical
results obtained by the test-field method, which apply
independent of this approximation. Mean-field diffusivities
smaller than the molecular diffusivities may occur in
compressible turbulence for not too large Peclet or magnetic
Reynolds numbers and, in addition, slow variations of the
flow patterns. Analogous results has also been found in
simple model of anisotropic potential-flow turbulence. The
decay of mean fields under the influence of compressible
turbulence is strongly influenced by the so-called memory
effect (Hubbard and Brandenburg 2009), that is, the relevant
diffusivity coefficients depend on the decay rates.