Investigation of scale effects and prediction methodology for low-frequency unsteady broadband forces in pump-jet propulsors operating under submarine hull wake conditions

The vibration and noise radiation of submarine thrusters have become an important research object for the low-noise design of submarines. The unsteady force generated during the operation of the thruster caused by the incoming turbulence is one of the important sources of low-frequency vibration noise in submarines. In this paper, a high-fidelity numerical simulation model of pump-jet propulsion unsteady flow under submarine hull wake condition is constructed based on the improved delayed separation eddy simulation and large eddy simulation (LES) turbulence models. By comparing with the experimental data, it is found that the LES turbulence model forecasts the low-frequency unsteady broadband force with higher accuracy. A method for predicting the low-frequency unsteady broadband force of pump-jet propulsion is formed. Based on this method, the flow field characteristics and low-frequency unsteady broadband forces of the pump-jet thruster under three scaling scales and two advance ratios are calculated. Results indicate that the scaling effect of the pump-jet thruster cannot be neglected: the duct thrust coefficient KT_d decreases with increasing scaling scale, while the stator drag coefficient KT_s, rotor thrust coefficient KT_r, rotor torque coefficient KQ_r, thruster efficiency, amplitude of unsteady force at the first-order lobe frequency, and amplitude of energy integral in the corresponding interval all increase accordingly. Additionally, at identical scaling scales, as inflow velocity rises, both the amplitude of unsteady forces on rotor blades and the amplitude of energy integrals grow significantly.