The transient characteristics of cloud cavitating flow over a flexible hydrofoil

The objective of this paper is to investigate the two-phase flow structure of cloud cavitation over a flexible hydrofoil with particular emphasis on cavitating flow-induced vibration and the cavity shedding dynamics via combined experimental and numerical studies. The experimental studies are conducted in a closed-loop cavitation tunnel using a simultaneous sampling technique and the numerical investigations are performed using a hybrid coupled fluid structure interaction model. Results are presented for a modified NACA66 hydrofoil made of POM Polyacetate and good agreement can be obtained between the numerical and experimental results. The predicted cavitation behaviors, including the cavity growth, break-off and collapse, agree fairly well with the experiment. The vibration velocity goes up with the growth of the attached cavity and the break off of the cloud cavity induces some fluctuations of the vibration velocity. For the cavity shedding dynamics, the cavity partially collapses with the vibration and breaks into several medium-scale cloud cavities accompanied with complex interaction between the counter-rotating vortexes. Good agreement can be obtained between the predicted and measured vibration amplitudes, with the dominant flow-induced vibration frequency in accordance with the cavity shedding frequency and other two vibration frequencies corresponding to the natural frequencies of the first bending and twisting mode respectively. The one-dimensional analytical model is applied to better illustrate the physical mechanism of the cavitating flow induced pressure pulsation. The results show that the transient cavitating behaviors lead to the periodic pressure fluctuation on the hydrofoil. The one-dimensional model can track the main features of the pressure fluctuations, which are mainly derived from the cavity volume acceleration.