Origin of acoustic noise of tip clearance vortex cavitation inception and mitigation by Parylene C coating

Tip clearance vortex (TCV) cavitation poses a significant challenge in duct propeller design, as it tends to emerge with increased speed and blade load, resulting in cavitation noise radiation, blade vibration, and rotor erosion. Despite decades of research into the intricate flow field at the rotor's tip, the inception of TCV cavitation remains poorly understood. This study elucidates the initial occurrence of TCV cavitation and acoustic noise radiation under inception conditions, and it's explored that the acoustic noise origins at the inception of leading edge separation vortex (LESV), where LESV is stretching and lifting up to form a low pressure zone at the vortex core. A new approach to postpone LESV inception by Parylene C coating, a flexible film able to resist high shear stress, is systematically investigated on a hydrofoil in a small cavitation water tunnel. It is found that LESV becomes less organized and separation length is increased by up to over twice with Parylene C coated, leading to a pressure resurgence of LESV's core. It indicates the mechanism of LESV inception delay by the coating, with an apparent reduction of acoustic noise radiation. Efficacy of the coating on a duct propeller is further validated in a large cavitation water tunnel, reducing acoustic noise radiation by 26.4 dB at critical cavitation number and increasing the critical rotating speed by up to 8%, which is convincing and indicates a positive general hydrodynamic performance of Parylene C coating.