Impacts of unsteady uniform inflow and pitching frequency on the cavitating flow characteristics around a pitching hydrofoil

The effects of unsteady uniform inflow and pitching frequency on cavitating flow characteristics around a Clark-Y pitching hydrofoil were studied by numerical simulation. The hydrodynamic hysteresis characteristics and typical cavitation evolution process during the different conditions are emphasized. The attack angle α of the hydrofoil follows a triangular wave motion trajectory, characterized by a mean incidence angle α₀ of 10° and an amplitude Δα₀ of 5°. The unsteady incoming velocity is characterized and adjusted primarily by the amplitude of the unsteady uniform velocity (ΔU) and the frequency of the unsteady uniform velocity (f). In this study, ΔU is configured to four different levels: 0 m/s, 0.75 m/s, 1.50 m/s, and 3.0 m/s, while f is set at three distinct values: 1 HZ, 2 HZ, and 4 HZ. Additionally, the pitching frequency (f∗) is established at three separate rates: 1 HZ, 2 HZ, and 4 HZ. The cavitating flow around a pitching hydrofoil under various conditions is simulated using a modified k-ω SST(Shear Stress Transfer) turbulence model in conjunction with the Merkle cavitation model. Results showed that the unsteady inflow velocity significantly changes the hysteresis effect and the typical evolution frequency of cavity. Moreover, the phenomenon of rapid secondary fracture and cavity shedding around hydrofoil under the unsteady inflow condition is found. The findings indicated that the impact of ΔU on the hysteresis effect of a pitching hydrofoil is roughly linear. And the f directly affects the formation and residence time of cavity. In addition, the matching relationship between f and f∗ also affects the space-time position of cavitation.