Modulation mechanisms of swept angle on the ventilated flows around surface-piercing hydrofoils

This study systematically investigates the natural ventilation characteristics around surface-piercing hydrofoils using an integrated experimental and numerical methodology, focusing on the modulation mechanisms of the sweep angle (Λ). A comprehensive flow-regime classification is developed for moderate-to-high depth-based Froude numbers (Frh), and three distinct states are identified: tip-vortex-induced ventilation (TVIV), fully wetted flow (FW), and nose ventilation (NV). The stability domains and transition thresholds are quantitatively mapped within the Λ-Frh parameter space. The critical findings demonstrate that the TVIV-to-FW transition is governed by spanwise secondary flow-induced nonlinear vortex interactions. Multiscale vortex fusion drives the intermittent breakdown of secondary tip vortices, forming a tip-locking phenomenon that suppresses TVIV development. This is quantitatively validated through analysis of the circulation distribution and spectral coherence characteristics. Synergistic interactions between attenuated leading-edge adverse pressure gradients and the failure of free-surface sealing trigger the FW-to-NV transition, accompanied by the disappearance of the choking effect. Significantly, the stable FW regime delivers superior hydrodynamic performance across a broad velocity range at moderate values of Λ. Temporal evolution analysis indicates that TVIV takes an order of magnitude longer than NV at identical Frh with both timescales being inversely proportional to Frh. These findings provide critical insights for anti-ventilation design in high-performance marine applications.