Interfacial instabilities and thermo-hydrodynamic mechanisms in high-temperature vapour-ventilated supercavitation

This study experimentally investigates the interfacial instabilities and thermo-hydrodynamic mechanisms of supercavities ventilated by high-temperature condensable vapour. Using high-speed imaging with an AI-based segmentation model, we establish a flow regime map that delineates the supercavitation envelope, identifying a distinct rough supercavity regime associated with the interaction between phase change and turbulence. A key finding is the non-monotonic cavity-length response of condensable vapour ventilation, in which high-flow regression is governed by a condensation-assisted source–sink imbalance, and differs mechanistically from the leakage/closure response observed in air ventilation. Specifically, condensation-induced mass loss acts as an internal sink that can exceed the vapour supply and cause cavity regression. While the cavity diameter remains inertially constrained, its length is highly sensitive to these thermodynamic effects. Spatiotemporal analysis indicates a coupled thermo-hydrodynamic route for the interfacial instability. The observed instability is initiated by thermodynamic self-excitation driven by droplet impingement, under-expanded jet expansion, and contact condensation within the ventilation influence region. These disturbances are convected downstream through a Kelvin–Helmholtz-type shear-layer process. The upper–lower asymmetry of the cavity fluctuations is consistent with gravity-related selection, which preferentially amplifies long-wave perturbations on the upper interface through a Rayleigh–Taylor-type route, and damps fluctuations on the lower interface. These findings extend classical cavitation theory into regimes dominated by rapid phase change, providing new insights into thermo-hydrodynamic coupling in multiphase flows.