Effect of Advance Ratio on the Flow Structure and Cavitation Inception in the Tip Region of a Ducted Marine Propeller

The flow in the tip region of a ducted marine propeller, with blades made of acrylic, is studied in the JHU refractive index-matched turbomachine facility. The propeller has a tip diameter and gap of 300.6 and 2.1 mm respectively, and Re=1.0-1.2 x 10⁶. Characterization of incipient cavitation is performed using high-speed flow visualization at design and off-design advance ratios (J=V_z/nD), with V_z being the spatially averaged inlet axial velocity, n being the rotor rps, and D, the rotor diameter. At design J=0.85, cavitation inception occurs in secondary vortices aligned perpendicularly to the primary tip leakage vortex (TLV), circumferentially downstream of the blade trailing edge (TE). Conversely, below design flowrate, J=0.68, inception occurs along the primary TLV, in the mid-chord region, circumferentially upstream of the TE. Ensemble-averaged velocity distributions in a series of meridional planes, obtained using stereo-PIV, reveal that with decreasing J there is: (i) an upstream shift in blade loading, (ii) earlier rollup and breakdown of the TLV, as well as entrainment of the opposite sign vorticity away from the endwall. Furthermore, while at J=0.85, the turbulent kinetic energy peaks in the region of secondary and counter-rotating vortices, predominantly owing to interaction of the backward tip leakage flow with the main passage flow, at J=0.68, the peak is in the vicinity of the TLV center. Dominant turbulence production mechanisms for normal Reynolds stresses will also be discussed.