Comparison of isothermal and isentropic stratification effects on single-mode compressible Rayleigh–Taylor instability

Rayleigh–Taylor instability (RTI) occurs at the perturbed interface between fluids with different densities, where the acceleration and density gradient are in opposite directions. RTI is observed in natural flows, such as atmospheric and geological phenomena, and in engineering applications, including the mixing stages of ramjets, scramjets, and Inertial Confinement Fusion (ICF). We investigate the coupled effects of compressibility, controlled by the background stratification strength and type, and large molar mass differences between the mixing fluids through Direct Numerical Simulations (DNS) of two-dimensional, single-mode RTI by solving the fully compressible multi-species Navier–Stokes equations. Specifically, we compare the effects of initially isothermal versus initially isentropic stratification across a wide range of Atwood numbers. Our earlier results for isothermal stratification show that, at low Atwood numbers, increasing stratification suppresses the growth of the mixing layer, eventually leading to a molecularly well-mixed state. In contrast, we will show that the suppressive effect of stratification is significantly weaker in RTI under initially isentropic stratification. We will also present that at higher Atwood numbers (A > 0.3), RTI growth persists even under strong isothermal stratification and is further enhanced in the isentropic case.