On the ventilation of surface-piercing hydrofoils under steady-state conditions

The present study experimentally investigates the onset of ventilation of surface-piercing hydrofoils. Under steady-state conditions, the depth-based Froude number Fr and the angle of attack α define regions in which distinct flow regimes are either locally or globally stable. To map the boundary between these stability regions, the parameter space (α,Fr) was systematically surveyed by increasing α until the onset of ventilation while maintaining a constant Fr. Two simplified model hydrofoils were examined: a semi-ogive with a blunt trailing edge and a modified NACA 0010-34. Tests were conducted in a towing tank under quasi-steady-state conditions for aspect ratios of 1.0 and 1.5, and for Fr ranging from 0.5 to 2.5. Ventilation occurred spontaneously for all test conditions as α increased. Three distinct trigger mechanisms were identified: nose, tail and base ventilation. Nose ventilation is prevalent at Fr<1.0 and Fr<1.25 for aspect ratios of 1.0 and 1.5, respectively, and is associated with an increase in the inception angle of attack. Tail ventilation becomes prevalent at higher Fr, and the inception angle of attack exhibits a negative trend. Base ventilation was only observed for the semi-ogive profile, but it did not lead to the development of a stable ventilated cavity. Notably, the measurements indicate that the boundary between bistable and globally stable regions is not uniform and extends to significantly higher α than previously estimated. A revised stability map is proposed to reconcile previously published and current data, demonstrating how two alternative paths to a steady-state condition can lead to different flow regimes.