Experimental and numerical investigation of the impacts of rotor tip-rake on excitation forces of pump-jet propulsors

The tip-clearance flow in a pump-jet propulsor exerts great impacts on the fluctuating pressures and resultant unsteady forces, which are important sources of structural vibrations and radiated noise underwater. The blade geometry close to the tip is an important factor determining the vortex strength in the tip-clearance flow. In the open-water condition, the effects of raking the rotor tips on the duct-surface fluctuating pressures and the resultant unsteady forces acting on different components of the propulsor are investigated via physical model experiments and the numerical solution of Reynolds-averaged Navier-Stokes (RANS) equations coupled with the SST k - ω turbulence model. The measured and simulated results of hydrodynamic pressures are consistent to each other, and the simulated flows help better understand why the fluctuating pressures change with the tip geometry. The strong fluctuations of duct-surface pressures are caused by intensive tip separation vortices. The duct-surface pressure fluctuations are effectively reduced by using the rake distribution near the tip towards blade back side and, for the combination of the five-bladed rotor and the seven-bladed stator, the resultant unsteady horizontal (and vertical) forces acting on the duct and stator are also reduced; while increasing rake leads to negative effect on pressure fluctuations and unsteady horizontal (and vertical) forces acting on all the components of the propulsor.