Prediction and analysis of the far-field directivity of line-spectrum noise induced by stator-rotor interaction in a pump-jet propulsor

Noise level is one of the key metrics to measure the overall performance of underwater vehicles, of which the line-spectrum noise induced by stator-rotor interaction in pump-jet propulsors is an important element. In this study, the hydrodynamic and acoustic performances of the ‘Suboff’ model equipped with a pump-jet propulsor consisted of a front-stator and a 7-bladed rotor are investigated. Unsteady simulations in the presence of a non-uniform wake are conducted to obtain the hydrodynamic characteristics and the unsteady pressure fluctuations on the blade surface. The present acoustics prediction method considers the loading noise generated by the interaction between the wake and rear rotor. Unsteady pressure loadings on a real rotor blade geometry are considered as the surface acoustic source. The free-field acoustic radiation of the rotating blade is solved using the frequency domain acoustic analogy method. The scattering effect of the duct is considered using boundary integral element method, enabling rapid prediction of the far-field directivity of stator-rotor interaction line-spectrum noise. According to the Tyler-Sofrin theory, sound pressure can be decomposed into the sum of infinity circumferential modes. However, only a finite number of propagable modes exist in the presence of a duct. In this study, the observable modes and far-field directivity of stator-rotor interaction noise in the pump-jet propulsor are investigated for various sailing speeds, and the positions of major sound sources are analyzed through the unsteady pressure difference between the two sides of the rotor blade. This serves as a reference for the integrated hydrodynamic-acoustic optimization design of pump-jet propulsors.