Magnetohydrodynamic Turbulence Decay Under the Influence of Uniform or Random Magnetic Fields

We perform direct numerical simulations of decaying magnetohydrodynamic turbulence subject to initially uniform or random magnetic fields. We investigate the following features: (i) kinetic–magnetic energy exchange and velocity field anisotropy, (ii) action of Lorentz force, (iii) enstrophy and helicity behavior, and (iv) internal structure of the small scales. While tendency toward kinetic–magnetic energy equi-partition is observed in both uniform and random magnetic field simulations, the manner of approach to that state is very different in the two cases. Overall, the role of the Lorentz force is merely to bring about the equi-partition. No significant variance anisotropy of velocity fluctuations is observed in any of the simulations. The mechanism of enstrophy generation changes with the strength of the magnetic field, and helicity shows no significant growth in any of the cases. The small-scale structure (orientation between vorticity and strain-rate eigenvectors) does not appear to be influenced by the magnetic field.