Mean-field transport in stratified and/or rotating turbulence
by
Karl-Heinz Rädler(AIP, Potsdam)
→
Europe/Stockholm
122:026
122:026
Description
The large-scale magnetic fields of stars and galaxies are often described using mean-field dynamo theory. At moderate magnetic Reynolds numbers, the transport coefficients which define the mean electromotive force can be determined from simulations. This applies analogously also to the mean-field theory of passive scalar transport. In the case of axisymmetric turbulence, that is, turbulence with only one preferred direction, magnetic transport is governed by nine coefficients and passive scalar transport by four coefficients. All these coefficients are determined for turbulence in the presence of either rotation or density stratification or, if they are aligned to each other, of both. The kinematic problem is considered in which the magnetic field does not act back on the flow. The test-field method is used where transport coefficients are determined by solving a set of equations with properly chosen mean magnetic fields or mean scalars. The method is adapted to mean fields which may depend on all thee space coordinates. Anisotropy of turbulent diffusion is found to be moderate in spite of rapid rotation or strong density stratification. Contributions to the mean electromotive force determined by the symmetric part of the gradient tensor of the magnetic field turn out to be important. In stratified rotating turbulence, the alpha effect is strongly anisotropic, suppressed along the rotation axis on large length scales, but strongly enhanced at intermediate length scales. Also the Omega x J effect is enhanced at intermediate length scales. The turbulent passive scalar diffusivity is typically almost twice as large as the magnetic turbulent diffusivity.
