Vortex-cavity interactions in ventilated underwater launches with lateral velocity and waves

Ventilated cavitation is widely used for drag-reduction and stability-enhancement in underwater vehicles. This study presents a numerical investigation of ventilated cavitation during the underwater launch process, accounting for effects of lateral velocity and surface waves. The fluid-structure interaction is resolved using the Boundary Data Immersion Method, and the gas-liquid interface is captured with a Volume of Fluid scheme. Validation against underwater launch experiments and vertical water-tunnel tests confirms the accuracy of predicted cavity evolution and vehicle motion. The shoulder-attached cavity evolves in two distinct stages: pre- and post-ventilation. After ventilation onset, the reduced velocity difference across the cavity suppresses Kelvin–Helmholtz instability, leading to a stabilized interface. Transition from external to internal vortical structures further enhances cavity stability. Under the present lateral velocity conditions, lateral motion breaks flow symmetry: under no lateral velocity, periodic vortex merging induces large-scale shedding and load fluctuations; conversely, lateral motion promotes continuous small-scale shedding on the downstream side, preventing energy accumulation and suppressing large-scale oscillations. These findings reveal the role of vortex-cavity interactions in governing hydrodynamic stability during asymmetric launches.