Investigation of non-uniform waterjet pump inflow for a range of ship speeds

Numerical simulations are used to generate a velocity field entering the pump of a waterjet propelled ship so that the level of non-uniformity in axial velocity can be evaluated. Non-uniform axial velocity entering the pump causes unsteady loading of the rotor blades resulting in noise, 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. The ship speeds in this investigation correspond to Froude numbers between 0.34 and 0.84 and Reynolds numbers between 3.6 × 10⁸ and 9.0 × 10⁸. The pump shaft speed and flow rate are determined via calm water resistance data from experiments and the pump curves for the axial flow pump modeled. The numerical simulations are steady single phase and do not include the free surface. In all of the numerical simulations the hull geometry is kept at a fixed draft and on 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 non-dimensional wake entering the pump exhibits a mean axial velocity component that varies proportionally to nozzle velocity ratio 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 axial velocity. It is also found that this hull and pump configuration resulted in the pump operating at a fixed pump flow coefficient for all ship speeds investigated. The result is that the average rotor section inflow angles are also constant over the speed range. This is ideal for a rotor with a fixed pitch distribution because the mean angle of attack on a given rotor section will be independent of ship speed.