Transition Pathways Between Hydrodynamic and Magnetohydrodynamic Turbulence in Channel Flow

We present direct numerical simulations of turbulent channel flow of an electrically conducting fluid at friction Reynolds number Reτ = 140 and magnetic interaction parameter N ranging from 0.006 to 0.2, assuming a low magnetic Reynolds number. In this regime, the imposed magnetic field remains unaffected by the fluid motion. First, we investigate three magnetic field orientations (streamwise, spanwise, and wall-normal) across various magnetic strengths. Our results confirm that the wall-normal field most effectively suppresses turbulence, consistent with Lee & Choi (2001, J. Fluid Mech.). For sufficiently strong magnetic fields, the flow becomes laminar in the outer region while near-wall streaky structures retain turbulence. To analyze this anisotropic suppression, we employ a spectral framework that resolves both physical and scale space simultaneously. Additionally, we examine the transition from a purely hydrodynamic turbulent state to a magnetohydrodynamic (MHD) regime and vice versa, focusing on turbulence regeneration following magnetic field removal. We characterize the transition time and pathway between fully developed MHD and HD regimes.