Finger dynamics in rarefaction-driven single-mode Rayleigh–Taylor instability under high-amplitude conditions

This study examines the finger dynamics of high-amplitude, single-mode rarefaction-driven Rayleigh–Taylor instability (RTI) through rarefaction-tube experiments and numerical simulations. Unlike RTI driven by gravity or mechanical acceleration, rarefaction-driven RTI features a distinct wavefront-traversal (WT) stage prior to the linear stage. During the WT stage, finger velocities are decomposed into contributions from the rarefaction flow’s bulk motion and baroclinic-vorticity-induced velocity, and a theoretical framework is developed to quantitatively evaluate these components. Incorporating the WT-stage outcomes into the Layzer-type model (Li et al. 2025 J. Fluid Mech. 1016, A27) resolves the singularity-induced ill posedness at high amplitudes and, with the inclusion of interface stretching, enables accurate prediction of perturbation growth up to the quasi-steady stage. At high amplitudes, the inertial effect provides the fundamental contribution to bubble growth, whereas the curvature effect governs the transition to the quasi-steady state and the stretching effect determines the magnitude of the quasi-steady velocity. Beyond the quasi-steady stage, pronounced bubble deceleration emerges due to strong vorticity transport from the bubble region toward the spike. This transport reduces the interfacial vorticity that sustains bubble motion and thereby results in growth suppression. As the vortices shed from the spike reach the vicinity of the bubble tip, their induced velocity becomes significant, eventually terminating the deceleration and leading to a fluctuating saturated state.