Hydrodynamic performance of a surface-piercing hydrofoil with differing oblique angle: A numerical study

Oblique surface-piercing hydrofoils are used widely to impart lift to high-speed surface vehicles, and their lift and drag, and hence working efficiency, are affected by their oblique angle and velocity. Reported here is a numerical study of the ventilation and hydrodynamic performance of a surface-piercing hydrofoil with an oblique angle, using the two-phase interFoam solver in OpenFOAM to simulate the hydrofoil processes. The results show that two main regimes occur when the surface-piercing hydrofoil moves in a stable manner, i.e., fully wetted and fully ventilated (the latter comprising tip-vortex-induced ventilation and perturbation-induced ventilation), which are affected by the oblique angle. At low velocity, increasing the oblique angle does not change the ventilation regime but does improve the lift-to-drag ratio of the hydrofoil. At high velocity, as the oblique angle increases, the hydrofoil changes from tip-vortex-induced ventilation to fully wetted, and the lift-to-drag ratio is also increased. In particular, when the oblique angle reaches 30°, perturbation-induced ventilation occurs and the hydrofoil stalls. A phase diagram of the ventilation regime at different values of the Froude number and oblique angle is presented. Given that surface-piercing hydrofoils impart lift to high-speed surface vehicles, an oblique angle of 25° is recommended as being suitable for hydrofoils within the parameters discussed in the paper. These findings support the engineering applications of surface-piercing hydrofoils.