Experimental Characterization of the Unsteady Flow Structure and Cavitation Inception in the Tip Region of a Ducted Marine Propeller

In ducted marine propellers, the tip flow vortices include a primary tip leakage vortex (TLV) and multiple counter- and co-rotating secondary vortices. Near design conditions, flow visualizations show that cavitation inception occurs within co-rotating secondary vortices as they are entrained into the TLV, and presumably stretched by it, circumferentially downstream of the blade trailing edge. Due to the inherent small timescales, intermittency, and limited spatial extent of this phenomenon, its signature is smeared, but still visible, in the ensemble-averaged flow field. Hence, high-speed (30 kHz) stereo-PIV is employed to measure the time-resolved evolution of the flow in the tip region of a two-bladed acrylic propeller in a refractive index-matched facility. The tip diameter and tip gap are 300.6 and 2.1 mm respectively, and Re=1.0-1.2 x 10⁶. Data acquired in meridional planes capture the evolution of the tip leakage flow, TLV roll-up, formation and entrainment of secondary vortices, vortex-wake interaction, entrainment of the endwall boundary layer, interactions with the main passage flow, and eventual vortex breakup. The TLV is a 3D structure with its core surrounded by perpendicularly aligned vorticity originating from the blade wake and from the endwall. These results serve as a guide for the ongoing 3D tomographic PTV, which will be used for calculating the pressure field in the region of cavitation inception. Sample results of the latter will be presented.