Compressibility effects on mixing layer in Rayleigh-Taylor turbulence

The compressibility effects on the mixing layer are examined in Rayleigh-Taylor (RT) turbulence via direct numerical simulation at a high Atwood number of 0.9 and three typical Mach numbers (0.32, 0.71, and 1). The focus has been on the evolution of the mixing layer and the generation of kinetic energy. Specifically, a novel finding emerges at high Atwood number, where enhanced compressibility with increasing Mach number leads to a mean flow directed opposite to gravity in front of the bubble mixing layer. This mean flow, induced by compressibility, causes the width of the bubble layer in compressible RT turbulence to deviate from the quadratic growth observed in the incompressible case. It is further established that this deviation can be modeled by dilatation within the mean flow region. Moreover, the compressibility significantly influences the generation of global kinetic energy at high Mach numbers. The global kinetic energy of RT turbulence with high compressibility is primarily derived from the conversion of internal energy through pressure-dilatation work, rather than from the conversion of potential energy. It is also revealed that the mean flow leads to the conversion of kinetic energy into potential energy, while the fluctuating flow converts the potential energy into kinetic energy within the mixing layer.