Energy flux decomposition in magnetohydrodynamic turbulence

In hydrodynamic (HD) turbulence, an exact decomposition of the energy flux across scales has been derived that identifies the contributions associated with vortex stretching and strain self-amplification to the energy flux across scales. Here, we extend this methodology to homogeneous magnetohydrodynamic (MHD) turbulence with both vanishing and strong mean magnetic field. Applied to data from direct numerical simulations, this decomposition allows for an identification of physical processes and for the quantification of their respective contributions to the energy cascade, as well as a quantitative assessment of their multi-scale nature through a further decomposition into single- and multi-scale terms. We find that vortex stretching is strongly depleted in MHD compared with HD, and the kinetic energy is transferred from large to small scales almost exclusively by the generation of regions of small-scale intense strain induced by the Lorentz force. In regions of large strain, current sheets are stretched by large-scale straining motion into regions of magnetic shear. This magnetic shear in turn drives extensional flows at smaller scales. Magnetic energy is transferred from large to small scales predominantly by the aforementioned current-sheet thinning in regions of high strain. This mechanism remains dominant in the strongly anisotropic regime. As a consequence of these results, we present a subgrid-scale turbulence model based on the dominant physical mechanisms that performs better compared to purely dissipative models.