Objectives This study aims to investigate the effects of dynamic variations in rotor tip-clearance on the unsteady forces of a pump-jet propulsor, while analyzing the role of a flexible isolation system in influencing the excitation characteristics of the propulsion system. The findings contribute to optimizing propulsion system design and enhancing submarine stealth performance.

Methods An overset grid technique is applied to construct a numerical model of the SUBOFF submarine with a pump-jet propulsor, coupled with the SST k-ω turbulence model for simulations. Numerical calculations are performed for operating conditions where the rotor's vertical vibration frequency approaches the rotor blade passing frequency. Frequency spectrum analysis is conducted to assess the effects of dynamic tip-clearance variations on fluctuating pressure and excitation forces.

Results The results indicate that dynamic tip-clearance variations significantly affect the three-directional (x, y, z) excitation forces of the rotor and duct, with the vertical excitation force showing the highest amplitude, followed by the lateral and axial forces. Moreover, pulsating pressure on the rotor surface peaks at new branch frequencies, particularly at RF (rotor vibration frequency) and BPF (blade passing frequency). The lateral and vertical excitation forces of the duct are more strongly influenced by tip-clearance variations than those of the rotor.

Conclusions The dynamic changes in rotor tip-clearance induced by the flexible isolation system significantly affect the unsteady force characteristics of the pump-jet propulsor. The findings provide valuable guidance for enhancing the compatibility between the flexible isolation system and the pump-jet propulsor, thus contributing to submarine noise reduction and improved stealth performance.