Influence of propulsion shafting longitudinal vibration on the excitation force and vortex dynamics characteristics of pump-jet propulsor

Pump-jet propulsors are inevitably affected by shafting vibrations in actual operating conditions, especially the longitudinal bearing force, which is the main inducing factor of hull acoustic radiation. In the present study, considering the working conditions of the uniform inflow and longitudinal vibration motion of the propulsion shafting, using the improved delay detached eddy simulation, the influence of the propulsion shafting longitudinal vibration on the excitation force and vortex dynamics characteristics of the pump-jet propulsor is investigated under design conditions. The results indicate that the longitudinal vibration modulates the excitation force characteristics of each component of the pump-jet propulsor. Within the calculated range of magnitudes and frequencies, the thrust and torque fluctuations are proportional to the longitudinal vibration magnitude, and their magnitudes have a quadratic relationship with the longitudinal vibration frequency. Additionally, longitudinal vibration directly affects the short-wave instability of the tip clearance vortex by changing the spiral-to-spiral distance, resulting in a more disordered shape of the wake field, especially at low longitudinal vibration frequencies. The wake field always exhibits characteristics of the longitudinal vibration frequency, indicating an energy generation mechanism for the longitudinal vibration frequencies induced by the propulsion shafting longitudinal vibration. •The pump-jet propulsor considers the propulsion shafting longitudinal vibration.•The relationship between longitudinal vibration magnitude and frequency changes and excitation force is discussed.•The influence of longitudinal vibration on the vortex dynamics of the pump-jet propulsor is studied.•An energy generation mechanism for the longitudinal vibration frequencies is revealed.