Excitation force mechanisms in a composite pump-jet propulsor under stationary-structure variations

Minimizing the excitation force of a composite pump-jet propulsor is critical for suppressing propulsion-system noise and enhancing the acoustic stealth of the platform. When the fluid–structure interaction (FSI) deformation of the composite rotor interacts with variations in structural parameters, the resulting excitation force becomes highly nonlinear. To address this issue, a steady-state FSI algorithm is employed for the pre-deformation design of the composite rotor, and a transient FSI algorithm is adopted to investigate the influence of three parameters—stator angle, duct length, and tip clearance—on the rotor excitation force. The results show that: (1) the stator angle directly affects the rotor excitation force by altering the inflow angle of attack. A larger stator angle increases the time-averaged rotor thrust, which in turn amplifies the excitation force amplitude at the stator blade-passing frequency. (2) The duct length indirectly influences the rotor excitation force by changing the internal flow rate and consequently the wake fraction of the inflow field. Variations in duct length introduce two competing mechanisms: increasing the duct length suppresses radial flow diffusion in the mid-to-low radius region, thereby raising the local flow rate; conversely, the accompanying increase in wall friction expands the turbulent wake region at higher radii, reducing the local flow rate. The interplay between these effects produces a synchronous V-shaped variation in flow rate, pressure distribution, wake fraction, and excitation force amplitude as the duct length changes. (3) The tip clearance affects the rotor excitation force by modifying the turbulence characteristics within the clearance. Changes in clearance height generate two competing mechanisms: a larger clearance intensifies turbulent mixing between the tip-leakage flow and the mainstream, which decreases the time-averaged thrust and thereby reduces the excitation force amplitude; however, it also enhances the phase coherence of excitation forces across different radial sections, which amplifies the overall excitation force amplitude. The net excitation-force response is determined by the competition between these two effects. (4) Replacing the metal rotor with a composite rotor while retaining the baseline geometry yields superior excitation-force characteristics without altering the stator angle, duct length, or tip clearance, and maintains comparable propulsive efficiency. For the specific pump-jet geometry and operating condition investigated in this work, the present study elucidates the mechanisms by which composite rotors suppress excitation forces and provides a theoretical foundation for the engineering design of low-noise composite pump-jet propulsors.