Cavitation induced hysteresis of a pitching hydrofoil near free surface

This paper investigates the hysteresis characteristics of force coefficients of an oscillating hydrofoil in a near-free surface cavitation flow field by utilizing unsteady numerical simulation methods. The study primarily focuses on analyzing the effects of dynamic stall conditions, reduced frequency, cavitation number, immersion depth, and Froude number on the hysteresis curve. By comparing the vortex distribution, volume fraction, velocity streamlines, and fluctuating pressure coefficient in the flow field, the authors also examine the reasons for the differences in the hysteresis curve at the same angle of attack under different conditions. The results suggest that cavitation significantly impacts the fluctuation of the hysteresis curve, mainly due to the shedding and collapse of the cavity on the hydrofoil pressure surface, which results in pressure fluctuations at the trailing edge. This issue can be addressed by reducing the stall angle of attack, Froude number, and increasing the reduced frequency, cavitation number, and immersion depth to slow down cavitation in the flow field, thus reducing the fluctuation of the hysteresis loop. Furthermore, the structure of the cavitation flow field under different conditions is clearly distinguished, and the hysteresis loop experiences obvious fluctuations when there is obvious vortex separation in the flow field and many small cavities remaining above the pressure surface. As the immersion depth decreases and the corresponding Froude number increases, the effect of the free surface becomes stronger, leading to an increase in the free surface wave amplitude. This effect causes the hydrofoil pressure surface to gradually evolve into super-cavitation, and the fluctuation of the hysteresis curve tends to be stable.