Mechanisms of canonical Kelvin-Helmholtz instability suppression in magnetohydrodynamic flows

Abstract

The stabilizing influence of the streamwise magnetic field on Kelvin-Helmholtz (KH) instability is well known. We perform numerical simulations over a wide range of magnetic field strengths to clearly describe the mechanisms through which the stabilization of KH instability is achieved. KH instability evolution is known to be characterized by the stages of (i) linear precursor-vortex development; (ii) precursor-vortices merger and rollup into the primary vortex; and (iii) development of secondary-vortex bands and the nonlinear asymptotic stage. Our simulations exhibit the KH instability disruption mechanisms as a function of magnetic field strength. At strong magnetic field strengths, rapid harmonic velocity-magnetic exchange causes the precursor vortices to continually wind and unwind. Thus the perturbation development does not proceed beyond the first stage. At intermediate magnetic field strengths, the harmonic interaction permits the monotonic development of precursor vortices but prevents merger or primary vortex formation. When the magnetic field is weak, hydrodynamic mechanisms prevail at the first and second stages. The magnetic field only disrupts the nonlinear asymptotic KH growth stage due to the onset of “resistive instability” in the secondary bands.