Direct Numerical Simulation of Homogeneous Isotropic Cross-Helical MHD Turbulence

Incompressible magnetohydrodynamic (MHD) turbulence with cross helicity excited by a large-scale source is considered. The source is a random external force contributing energy with a controlled level of cross helicity. Based on this approach, high levels of cross helicity can be formed and maintained without using an external constant magnetic field. The force has been implemented in the TARANG code. A series of numerical experiments with constant energy flow and various levels of injection of cross helicity were carried out. The ratio of the cross helicity to the total energy varied in the range from 0 to 0.6. The calculations were carried out at the kinetic Reynolds number R = 2094 and the magnetic Prandtl number Pr m = 1 on a grid of 512 3 nodes. An equal amount of energy per unit of time was introduced into the velocity and magnetic fields. Regardless of the level of cross helicity, the rate of dissipation of magnetic energy was always higher than the intensity of its injection. An analysis of spectral energy fluxes revealed a positive energy flux from the velocity field to the magnetic field. The high levels of cross helicity led to energy accumulation on large scales and to an energy redistribution over scales. The spectral energy densities in the Elsässer variables z ± showed a significant difference in the slopes of the z + and z – spectra. On the curve of the spectral flux z – , a region close to the inertia interval was detected. Qualitative agreement with results obtained earlier using cascade models was shown. Varying the degree of field alignment did not affect the integral or spectral characteristics of the flow. The results obtained allow us to use the proposed force for a detailed study of the formation and dissipation of homogeneous isotropic turbulence with a high level of cross helicity.