Transition to turbulence in quasi-two-dimensional MHD flow driven by lateral walls

Abstract: This manuscript has been accepted for publication in Physical Review Fluids, see https://journals.aps.org/prfluids/accepted/d5074S28J6b11905012b7cb06505e8f2149dd5f20. This work investigates the mechanisms that underlie transitions to turbulence in a three-dimensional domain in which the variation of flow quantities in the out-of-plane direction is much weaker than any in-plane variation. This is achieved using a model for the quasi-two-dimensional magnetohydrodynamic flow in a duct with moving lateral walls and an orthogonal magnetic field. In this environment, conventional subcritical routes to turbulence, which are highly three-dimensional, are prohibited. To elucidate the remaining mechanisms involved in quasi-two-dimensional turbulent transitions, the magnetic field strength and degree of antisymmetry in the base flow are varied, the latter via the relative motion of the lateral duct walls. Introduction of any amount of antisymmetry to the base flow drives the critical Reynolds number infinite, as the TS instabilities take on opposite signs of rotation, and destructively interfere. However, an increasing magnetic field strength limits interaction between the instabilities, permitting finite critical Reynolds numbers. The transient growth only mildly depends on the base flow, with negligible differences for friction parameters $H \gtrsim 30$. Direct numerical simulations, initiated with random noise, indicate that for $H \leq 1$, supercritical exponential growth leads to saturation, but not turbulence. For higher $3 \leq H \leq 10$, a turbulent transition occurs, and is maintained at $H=10$. For $H \geq 30$, the turbulent transition still occurs, but is short lived, as the turbulent state quickly collapses. In addition, for $H \geq 3$, an inertial subrange is identified, with the perturbation energy exhibiting a $-5/3$ power law dependence on wave number.