Numerical investigation of the tip-vortex-induced ventilation formation mechanism for a surface-piercing hydrofoil

Abstract

Recent experiments have demonstrated that tip vortices can trigger the ventilation formation around a surface-piercing hydrofoil. However, the influence of this ventilation on transient flow structures and vortex evolution remains unresolved. This paper numerically investigates the tip-vortex-induced ventilation formation for a surface-piercing hydrofoil at a stalled yaw angle. The predicted unsteady ventilated cavities with tip vortices and pressure-side spray are in reasonable agreement with experimental observations. The ventilation formation process can be divided into three stages: base ventilation, tip-vortex ventilation, and suction-side ventilation. It is indicated that ventilation has a greater impact on the lift coefficient than the drag coefficient. The lift coefficient increases during the base ventilation and tip-vortex ventilation stages due to the expansion of the low-pressure stalled flow, but decreases in the suction-side ventilation stage because of the gradual replacement of this low-pressure region by an aerated cavity. Tip-leakage and tip-separation vortices initially exist independently at the hydrofoil tip, then expand and merge through air ventilation, ultimately forming a strongly stable tip vortex. Furthermore, ventilation promotes vortex generation, with the major contributors being the vortex stretching and baroclinic torque terms.