Effects of Atwood number and isothermal stratification strength on multi-mode three-dimensional compressible Rayleigh–Taylor instability

A stronger fundamental understanding of compressible and non-Boussinesq turbulence in the Rayleigh–Taylor instable (RTI) configuration aims to significantly improve our ability to predict the rate of turbulent mixing that occurs in various types of flows, improving simulations of supersonic-to-hypersonic propulsion and combustion, stellar atmosphere dynamics, High-Energy-Density (HED) hydrodynamics as found in confinement fusion processes, and more general multiphase flows. In this study, we present preliminary results from Direct Numerical Simulations (DNS) of the three-dimensional, multi-mode compressible Rayleigh–Taylor instability initially under isothermal stratification. We solve the multi-species, fully compressible Navier–Stokes equations using an Adaptive Mesh Refinement (AMR) method, systematically varying compressibility effects through the isothermal Mach number (Ma) in the range of 0.15 to 0.75, and examining variable-density effects by different Atwood numbers (At = 0.1, 0.3, and 0.5). Our results demonstrate that, at low Atwood number (At = 0.1), an increase in the isothermal Mach number completely suppresses the growth of the RTI mixing layer. Whereas, at higher Atwood numbers (At = 0.3 and 0.5), the suppression effect from compressibility is weakened, and the RTI mixing layer continues to grow throughout the simulation duration. We discuss these findings in detail, including quantitative assessments of the mixing layer growth, mixedness and the energetics of the RTI mixing layer.