High-speed magnetohydrodynamic jets: Contrasting turbulence suppression mechanisms due to compressibility and magnetic field

The goal of this study is to contrast the turbulence suppression mechanisms due to compressibility and imposed (streamwise) magnetic field in high-speed magnetohydrodynamic (MHD) planar jets. Rapid distortion analysis of shear flows reveals that the local (point-wise) suppression effect of compressibility is directly proportional to mean shear, where as, the suppression due to streamwise magnetic field is inversely proportional to mean shear. The corresponding suppression parameters are the gradient Mach number (M-g) and the ratio of Alfven-to-shear frequencies (R-A). To examine these contrasting manners of suppression due to compressibility and magnetic field, we perform direct numerical simulations of planar jets in different parameter regimes. We investigate the evolution of turbulent kinetic energy in different regions of the jet: (i) inflection region of high M-g, low R-A; (ii) outer ambient-fluid region of low M-g and high R-A and (iii) core region of low M-g and high R-A. It is demonstrated that compressibility has a dominant inhibiting effect in the inflection region, where as, magnetic field effects are highest in the outer region. All the three regions of the jet are suppressed, when both the suppression mechanisms act in combination. The effect of wavevector orientation on the individual suppression mechanisms is also examined. Both compressibility and magnetic field result in a predominant suppression of streamwise wavevectors, while spanwise wavevectors are unaffected.