PHENOMENOLOGY FOR THE DECAY OF ENERGY-CONTAINING EDDIES IN HOMOGENEOUS MHD TURBULENCE

We evaluate a number of simple, one-point phenomenological models for the decay of energy-containing eddies in magnetohydrodynamic (MHD) and hydrodynamic turbulence. The MHD models include effects of cross heli city and Alfvenic couplings associated with a constant mean magnetic h eld, based on physical effects well-described in the literature. The a nalytic structure of three separate MHD models is discussed. The singl e hydrodynamic model and several MHD models are compared against resul ts from spectral-method simulations. The hydrodynamic model phenomenol ogy has been previously verified against experiments in wind tunnels, and certain experimentally determined parameters in the model are sati sfactorily reproduced by the present simulation. This agreement suppor ts the suitability of our numerical calculations for examining MHD tur bulence, where practical difficulties make it more difficult to study physical examples. When the triple-decorrelation time and effects of s pectral anisotropy are properly taken into account, particular MHD mod els give decay rates that remain correct to within a factor of 2 for s everal energy-halving times. A simple model of this type is likely to be useful in a number of applications in space physics, astrophysics, and laboratory plasma physics where the approximate effects of turbule nce need to be included. (C) 1995 American Institute of Physics.