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

• Published in: The International journal of heat and fluid flow Vol. 85; p. 108625
• Main Authors: Praturi, Divya Sri, Girimaji, Sharath S.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.10.2020
• Subjects: Compressibility; MHD jets; Turbulence suppression; MHD jets; Turbulence suppression; Compressibility
• ISSN: 0142-727X; 1879-2278
• DOI: 10.1016/j.ijheatfluidflow.2020.108625

•Novelty in simulation parameters and setup: The effects of compressibility and magnetic field on the growth of turbulence in planar jets are studied for a wide range of Mach (M) and Alfvén Mach (MA) numbers. The non-ideal (viscous and resistive) effects are included. Random, isotropic initial fluctuations also enable characterization of the effect of wavevector orientation on suppression.•Novelty in approach: This paper is the last part of a systematic study that is aimed at characterizing the individual and combined effects of compressibility and magnetic field on shear flows (references of publications from previous parts are given in the main manuscript). Such an approach leads to the development of closure models that can effectively capture the action of compressibility and magnetic field; and efficient flow control strategies.•Highlights of results reported: It is demonstrated by means of direct numerical simulations that compressibility and magnetic field have contrasting manners of turbulence suppression. Compressibility suppresses the regions of the jet where shear is maximum, where as, magnetic field affects the regions of minimal shear. The resulting suppressed flow field is dominated by spanwise wavevectors. When compressibility and magnetic field act in combination, all the regions of the jet are suppressed. 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 (Mg) and the ratio of Alfvén-to-shear frequencies (RA). 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 Mg, low RA; (ii) outer ambient-fluid region of low Mg and high RA and (iii) core region of low Mg and high RA. 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.