Scaling and Numerical Analysis of Nonuniform Waterjet Pump Inflows
von Ellenrieder KD
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Numerical simulations of the velocity fields entering the pump of a waterjet propelled ship have been performed to evaluate the level of nonuniformity in axial velocity. Nonuniform axial velocity entering the pump causes unsteady loading of the rotor blades resulting in noise and vibrations, and can increase the risk of rotor cavitation. The hull form modeled in this study is the R/V Athena, fitted with a waterjet propulsion system. Ship speeds correspond to Froude numbers between 0.34 and 0.84 and Reynolds numbers between 3.6×108 and 9.0×108 . The pump flow rate and shaft speed are determined via calm water resistance data from experiments and the pump curves for the axial flow pump. The numerical simulations are for steady, single phase flow and do not include free surface effects. In all of the numerical simulations, the hull draft is constant and the trim angle is fixed at 0 (even keel). The variation in axial velocity is quantified by total distortion coefficient, harmonic components, and rotor section inflow angles. Using harmonic analysis, it is shown that the nondimensional wake entering the pump exhibits a mean axial velocity component that varies proportionally to nozzle velocity ratio (NVR) and harmonics of constant amplitude and phase, for all ship speeds. Thus, the wake pattern at any ship speed can be approximated using the harmonic content of the wake at one ship speed, and scaling the mean component of axial velocity. It is found that this hull and pump configuration resulted in the pump operating at a fixed pump flow coefficient for all ship speeds simulated. Thus, the average rotor section inflow angles are also constant over the speed range examined, which would be ideal for a rotor with a fixed pitch distribution as the mean angle of attack on a given rotor section will be independent of ship speed.