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 $\alpha$ define regions where distinct flow regimes are either locally or globally stable. To map the boundary between these stability regions, the parameter space $(\alpha,Fr)$ was systematically surveyed by increasing $\alpha$ until the onset of ventilation, while maintaining a constant $Fr$. Two simplified model hydrofoils were examined: a semi-ogive profile 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 $Fr$ ranging from $0.5$ to $2.5$. Ventilation occurred spontaneously for all test conditions as $\alpha$ 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 $Fr$, where the inception angle of attack takes a negative trend. Base ventilation was observed only for the semi-ogive profile but 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 $\alpha$ 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.