Tip leakage flow structure and cavitation inception in a ducted marine propeller

Cavitation inception and the associated flow structure in the tip region of a ducted propeller are investigated experimentally at varying advance ratios (J) using high-speed imaging and stereoscopic particle image velocimetry (SPIV) measurements in a refractive index-matched facility. At design and higher J values, inception occurs in axially aligned secondary vortices, located between the blade suction side and the tip leakage vortex (TLV), circumferentially after the trailing edge. With decreasing J, the inception shifts first to the TLV, and then along its core towards the leading edge. High-resolution SPIV data follow the evolution of TLV, tip leakage flow, near wake and several secondary vortices. Time-resolved SPIV at 30 kHz enables calculation of all three mean vorticity components, hence capturing axial vortices, and identifies the origin of flow structures. At high J values, inception occurs when quasi-axial vortices are stretched by the circumferential TLV and co-rotating secondary vortices located in the shear layer connecting the TLV to the suction side blade tip. With decreasing J, inception shifts to the TLV and towards the leading edge owing to earlier rollup and higher vortex strength, along with earlier breakup, evidenced by high core turbulence and a decrease in peak vorticity despite an increase in circulation.