Interfacial evolution and coupled dynamics of underwater supersonic gas jets during structural separation

The separation of a moving structure driven by an underwater supersonic gas jet involves complex fluid-structure interactions, among which the bidirectional coupling mechanism between the unsteady jet dynamics and the moving body remains poorly understood. In this study, a high-fidelity numerical model rigorously validated against experiments is established to investigate the coupling mechanisms under different jet expansion conditions. The results reveal that a self-sustained instability cycle of the jet—necking, bulging, breaking, and back-attack—dominates the pressure pulsations acting on the separating structure surface. Meanwhile, the motion of the separating body, constrained by a narrow radial gap, actively intensifies the jet unsteadiness, thereby establishing a strong bidirectional feedback that significantly controls jet evolution and separation efficiency. To further elucidate the underlying mechanism, an interface entrainment coefficient is introduced to quantify the mass and momentum exchange across the gas-liquid interface, providing a unified scaling for the evolution of jet instability. This study establishes a predictive framework for fluid-structure coupling in underwater supersonic separation systems.