Compressibility Effects on the Rayleigh-Taylor Instability

Rayleigh-Taylor instability (RTI) is important in variety of flows including inertial confinement fusion (ICF). During the coasting stage of ICF, RTI develops between the hot spot and colder surrounding plasma, due to the large temperature and density difference. RTI plays a significant role in the loss of compression and target performance in ICF. In previous works of compressible RTI initial thermal equilibrium is assumed, leading to exponential variation in the background density. Consequently, the effect of compressibility is obscured by that of the background stratification. To isolate compressibility effects, we perform direct numerical simulations (DNS) of RTI with uniform density variation on the two sides of the interface. The speed of sound at the interface is varied and simulations using the gas kinetic method (GKM) are performed for static Mach numbers (M) up to M=0.6. With reference to the thermal equilibrium case, it is shown that compressibility has a destabilizing effect and the instability grows faster with M. This is notably different from shear flows where compressibility inhibits shear layer growth. Compressibility is also shown to increase the bubble-spike asymmetry in RTI flows. The physics underlying these effects is explored by examining the evolution of pressure and dilatational fields and their role on vorticity dynamics.