Experimental investigation on tip vortex cavity deformation and flow dynamics using high-speed imaging and laser Doppler velocimetry measurements

Abstract

Cavitation affects engineering applications relating to aquatic operations. Tip vortex cavitation (TVC) leads to several technical problems, such as efficiency loss and noise. Hence, there is a need for a comprehensive understanding of the formation mechanism, shape distortions, and underlying physical phenomena of TVC. The dispersion relation of inertial waves on inviscid cavitating vortices is a valuable tool for predicting various TVC shapes. This study explains the patterns of flow around the tip of an elliptical foil and the cavity shape deformations under different flow conditions through experimental analysis. Experiments are conducted using a National Advisory Committee for Aeronautics hydrofoil in the cavitation tunnel at Chungnam National University. The appearance, development, and shapes of vortex cavitation are closely examined using high-speed imaging technology. There is good agreement between the vortex cavity shapes captured by this high-speed imaging and those derived in previous studies. Using laser Doppler velocimetry (LDV) measurements and analysis through image processing of high-speed images, we compare the vortex core trajectories in cavitating and non-cavitating conditions. There is a good match between the two, demonstrating the feasibility of predicting the flow behaviors around vortex cavities using LDV data. As the noise from the TVC is considered a significant source of underwater radiated noise we also measured sound pressure level, which exhibits abrupt changes at specific cavitation numbers, supporting the occurrence of the singing vortex phenomenon and highlighting its sensitivity to small variations in flow conditions. Furthermore, we obtain precise measurements of the instantaneous TVC diameter under different cavitation numbers, allowing the physical functionality of distinct TVC shapes to be determined. Our results significantly extend the scientific understanding of the flow around TVC and the fundamental causes of TVC distortions.