Numerical investigation on cavitation and induced noise reduction mechanisms of a three-dimensional hydrofoil with leading-edge protuberances

In this work, a National Advisory Committee for Aeronautics 66 hydrofoil with leading-edge protuberances is designed. The large eddy simulation combined with the Schnerr–Sauer cavitation model is used to obtain a satisfactory result as compared with the experimental measurement, integrating the permeable Ffowcs Williams–Hawkings equation for cavitation-induced noise analysis. It is found that the special leading-edge geometric structure deflects the incoming flow, creating two counter-rotating streamwise vortices at the peak shoulders. These lead to upwash and downwash effects and alter the pressure distribution on the suction side. The low pressure localized in the trough facilitates the advancement of the leading-edge cavitation while severely limiting the spanwise development of the cloud cavity, shortening the cavitation evolution by about 20% and reducing the maximum cavitation volume by about 35%. Analysis using the vorticity transport equation indicates that different vorticity transport equation splitting terms play dominant roles at different stages of cavitation evolution. Although the cavitation induces disturbances in the primary vortex, the effect is limited. Acoustic simulation shows that the bionic structure can reduce the total sound pressure level by 7.8–8.3 dB. The spherical noise reduction is not as effective as expected due to the similar cavitation volume acceleration processes of the two hydrofoils. However, the pressure fluctuation caused by the collapse of the cloud cavity is reduced by cavitation suppression, which reduces the linear noise. In addition, the protuberances suppress the generation of large-scale vortex systems and transform them into smaller ones, resulting in reduced spanwise correlation and coherence of the shedding vortices. This is a critical factor in noise reduction. Finally, we hypothesize that the unstable noise reduction is related to the streamwise vortices in the trough regions. These vortices increase the momentum exchange within the boundary layer, affecting its stability and weakening the acoustic feedback loop.