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 degrees, 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 degrees is recommended as being suitable for hydrofoils within the parameters discussed in the paper. These findings support the engineering applications of surface-piercing hydrofoils.