Studies of turbulence structure using well-resolved simulations with and without effects of a magnetic field

Abstract

This thesis presents results from a large-scale computational study motivated to advance understanding of turbulence structure in isotropic turbulence as well as in magnetohydrodynamic (MHD) turbulence at low magnetic Reynolds number. Direct numerical simulations (DNS) are performed using state-of-the-art massively parallel computers with the care in the choice of the simulation parameters so that the small scales are adequately resolved and the large scales are well contained in the simulation domains. Results of isotropic turbulence provide clarifications not only on the topological features of the small scale motions that take large amplitudes, but also on the values of cancellation exponent which quantifies the sign oscillation characteristics. For topics in MHD turbulence, a central theme is the anisotropy development from initial conditions that are either isotropic, or those that contain some degree of anisotropy resulting from axisymmetric contraction. Scalar mixing in MHD turbulence is also studied briefly, with or without a mean scalar gradient.