Decay of passive scalar fluctuations in homogeneous magnetohydrodynamic turbulence

摘要

We study the decay of passive scalar fluctuations in homogeneous magnetohydrodynamic turbulence, by performing direct numerical simulations in a cubic box. The applied magnetic field is constant and uniform, while the magnetic Reynolds number is much less than 1, hence the quasistatic approximation is used. The effect of the magnetic field on the decay rate of the scalar variance is documented, also in comparison to the hydrodynamic (i.e., nonmagnetic) case. At large times, the scalar variance decays according to an exponential law. Furthermore, the results show that the scalar, which is initially isotropic, develops rapidly an anisotropic state. Its intensity depends on the anisotropy of the flow and on the Schmidt number. The anisotropy of the velocity field is reflected on that of the scalar, as Fourier modes corresponding to wavevectors having a large component parallel to the magnetic field are more attenuated. Anisotropy at large and small scales is analyzed by computing several statistical quantities in physical and spectral spaces. It is found that the scalar is mainly anisotropic at large wavenumbers; an explanation based on the scalar transfer properties is provided. (C) 2008 American Institute of Physics.