Speaker
Prof.
Nathan Kleeorin
Description
The effects of compressibility (finite Mach number effect)
and stratification of turbulent fluid flow on mean-field
transport coefficients of magnetic field and passive scalar
(number density of particles and temperature field) are
studied for small and large magnetic Reynolds numbers (or
small and large Peclet and Schmidt numbers) using
correspondingly the quasi-linear approach and the spectral
tau-approach. For small magnetic Reynolds numbers (or small
Peclet numbers) the turbulent diffusion coefficient of both,
magnetic and passive scalar fields coincide and decrease
with increase of the degree of compressibility of turbulent
velocity field (defined as the ratio of the mean square of
divergence of velocity fluctuations to the mean square of
curl of turbulent velocity field). At some value of the
degree of compressibility of turbulent velocity field, the
turbulent diffusion coefficient can be negative, but the
total diffusion coefficient (molecular + turbulent) is
always positive. For large magnetic Reynolds numbers (or
large Peclet numbers) the compressibility of fluid flow
reduces the turbulent diffusion coefficient, but it is
always positive. On the other hand, the compressibility of
an inhomogeneous turbulence causes the pumping velocity of
the passive scalar in the direction of the gradient of the
turbulence intensity, while the density stratification
results in a counter gradient pumping velocity. Final effect
of the gradient or the counter gradient transport depends on
the value of the Mach number. The pumping velocity of the
magnetic field is always counter gradient, and for small
magnetic Reynolds numbers it is standard (determined by the
gradient of the turbulent magnetic diffusion), while for
large magnetic Reynolds numbers this velocity can increase
with compressibility. The comparison with DNS results are
also discussed.
