Effects of cavitation number and pitching frequency on hysteresis characteristics of a pitching hydrofoil

In this study, the effect of cavitation number σ and pitching frequency f∗ on hysteresis characteristics of a pitching hydrofoil, has been investigated. The angle of attack α changed from 5° to 15°, then from 15° to 5° at each pitching period. A pitching 2D (Two-dimensional) Clark-y hydrofoil is analyzed with high-speed video recording for the cavitation pattern. The hysteresis phenomenon is observed in increasing and decreasing of α, and the hysteresis effects changed with σ and f∗. The cavitating flow around the pitching hydrofoil during different conditions are simulated by revised k-ω SST (Shear Stress Transfer) turbulence model and Merkle cavitation model. The calculation results of hysteresis characteristic parameters showed that the decrease of σ leads to the increase of hysteresis effect. As σ decreased, the evolution of cavity vortex is more complex. Especially in the downstroke, there are complex phenomena such as vortex decomposition and merging, which leads to the intensification of hydrodynamic hysteresis effect. There is a nonlinear relationship between the hysteresis characteristic parameters and f∗. As f∗ increases, its influence on the cavitation flow in the downstroke becomes more and more serious. Besides, the evolution form of cavitation vortices also changes, which leads to the change of hysteresis characteristics.