Attenuation of the Tip-Clearance Flow in a Pump-Jet Propulsor by Thickening and Raking the Tips of Rotor Blades: A Numerical Study

Cavitation inception and radiated noise of the pump-jet propulsor are closely related to the strength of vortical flows existing in the clearance between rotor-blade tips and duct surface, which is ultimately determined by blade geometry close to the tip. A method is proposed for weakening the tip-clearance flow in pump-jet propulsors by increasing section thickness and rake of rotor blades from 95% tip-radius to the tip. The impacts of the proposed tip-modification method are investigated via numerical simulations by solving the Reynolds-Averaged Navier-Stokes equations with the SST k-omega turbulence model. When the rotor-tip geometry is modified reasonably, the open-water performance of the pump-jet propulsor is barely influenced, though the efficiency of rotor decreases due to the reduction in thrust loading and lift-to-drag ratio close to the tip. Due to rotor/stator interactions, the thrust and torque of a rotor blade as well as pressures at the core of tip-leakage vortex (TLV) fluctuate at multiples of rotor shaft frequency times stator blade number. When the rotor tips are modified, the fluctuation amplitudes of thrust and torque are reduced by more than 35%, the formation of TLV is delayed, the strength of TLV is weakened, and the pressure fluctuation amplitudes at TLV core are almost reduced by 50% at dominating frequencies.