Rarefaction-induced Rayleigh–Taylor growth for a heavy gas layer

The Rayleigh–Taylor (RT) interface evolution of a heavy gas layer driven by rarefaction waves is investigated. Distinct air/SF6/air gas layers with varying thicknesses and phase combinations are examined to assess the influence of initial conditions on interface coupling. Accelerated by rarefaction waves, the first interface exhibits RT-stable evolution, characterized by a sustained oscillation in perturbation amplitude, while the second interface remains RT-unstable, showing continuous perturbation amplitude growth. The RT-stable evolution of the first interface induces dynamic coupling between the two interfaces. Specifically, when the two interfaces are in-phase, dynamic coupling suppresses perturbation growth at both interfaces in the early stage, but enhances the developments of both interfaces in the late stage. Conversely, when the two interfaces are anti-phase, dynamic coupling exerts an opposing influence on interface evolution. By incorporating the time-varying acceleration and density in rarefaction-driven flows, the linear model proposed by Mikaelian [“Rayleigh-Taylor and Richtmyer-Meshkov instabilities in finite-thickness fluid layers,” Phys. Fluids 7, 888–890 (1995)] accurately predicts the early-time amplitude growth in most cases. The bubble and spike Froude numbers for the second interface exceed predictions from the nonlinear models under constant acceleration and density. This discrepancy arises because, in rarefaction-driven flows, the decreasing density reduces the drag acting on the bubble and spike tips, causing them to reach larger asymptotic velocities to balance the drag and buoyancy forces.