Two-dimensional compressible buoyancy-driven shear layers at varying Atwood numbers

Compressibility and compositional variable-density (VD) effects on instability growth within buoyancy-driven shear layers (BSLs) are investigated using direct numerical simulations (DNS). Two-dimensional simulations are performed in a periodic domain consisting of alternating columns of high- and low-density fluids with neutral background stratification. This study extends previous investigations of similar flow configurations from the incompressible regime into compressible flows, focusing on variations in the Atwood number, A, and the isothermal Mach number, Ma. Two distinct flow regimes are observed: an early-time regime characterized primarily by shear instabilities reminiscent of Kelvin–Helmholtz dynamics, during which compressibility and VD effects are minimal; and a subsequent regime driven predominantly by buoyancy, exhibiting similarities to Rayleigh–Taylor instability. In the buoyancy-dominated regime, compressibility suppresses the growth of the mixing layer, enhances mixing asymmetry due to VD effects, and leads to a more mixed mixing layer. Moreover, the suppressive effect of compressibility increases with increasing A. While a greater Atwood number accelerates the flow regime transition, it also intensifies instability suppression at later times, showing the coupled yet competing roles of compressibility and VD effects.