Free surface effects on ventilated cavity evolution around a surface vehicle: Gas leakage, wake dynamics, and drag reduction

Ventilated cavity flow offers a promising approach for reducing the wetted area and hydrodynamic resistance of surface vehicles. However, its overall hydrodynamic performance under free-surface constraints is strongly influenced by cavity instability, wake–wave interaction, and the associated irreversible dissipation mechanisms that remain insufficiently understood. This study employs high-speed visualization experiments and validated numerical simulations to investigate the effects of ventilation coefficient ( C Q ) and immersion depth ( H* ) on cavity evolution, drag reduction, vorticity dynamics, and entropy production around a surface vehicle. The results reveal that increasing C Q elongates the cavity, alters the tail-gas leakage mode, and reduces the total drag coefficient by 28.9%, whereas increasing H* suppresses radial cavity expansion and weakens the drag reduction effect. The free surface strongly affects cavity closure and vortex evolution by promoting Kelvin wake formation and upward gas leakage. Entropy production analysis shows that entropy production associated with turbulent dissipation contributes more than 70% of the total entropy production rate. Moreover, high drag reduction does not necessarily correspond to a large decrease in irreversible dissipation, because pronounced free-surface disturbance generates additional entropy production. These findings provide physical insight into ventilation control and drag-reduction characteristics of surface vehicles operating near the free surface.