Investigation On Vibro-acoustic Responses Of Propeller-shaft-hull Coupled System And Passive Control Methods

The acoustic stealth of a submarine is crucial for its safety and performance.Since the propeller,shaft and hull are the main components of the submarine,they are also the key factors which affect the dynamic and acoustic characteristics of the submarine.In this thesis,the numerical simulation and experimental method are used to investigate the effects of structural discontinuities,such as mass points,keel and bulkheads,on the sound radiation from a hull.In addition,the vibro-acoustic responses of a propeller-shaft-hull system and corresponding control methods are studied.The main research contents include:(1)The coupled finite element/boundary element method is applied to investigate the acoustic responses of an underwater stiffed axisymmtric hull under the harmonic force excitation and harmonic dipole excitation.The influence of the forward cabin on the acoustic response of the hull is studied by panel contribution method.According to the k-space frequency spectrum,the effects of the mass points and keel on the sound radiation from the hull are also investigated.Additionally,according to the squared normal velocities of the surface and the radiation efficiency of the hull,the effects of the number of the bulkheads on the acoustic characteristics of the hull are studied.(2)A coupled propeller-shaft-hull system is established to study its vibro-acoustic characteristics.The relationship between the vibration of the shaft and the sound radiation from the coupled system is illustrated by the sound power spectrum of the coupled system and the squared velocities of the hull and shaft.To investigate the influence of the shaft vibration on the acoustic response of the shaft-hull system,shaft mode is defined according to the percentage of the shaft's effective modal mass of the coupled system's for a certain mode,and the contribution of a shaft mode to the sound radiation is also investigated.The results show that the stiffness of the stern bearing affects the dynamic characteristics of the system significantly,and the reduction of the stern bearing's stiffness can be used to decrease the sound radiation from the coupled system.For the harmonic force excitation,the vibration energy from the propeller to the hull mainly transmits through the shaft.The equivalent reduced method is adopted to simplify the vibration of an elastic propeller.(3)Two passive control methods,the symmetrical foundation and the vibration isolator,are investigated based on the analysis of the vibro-acoustic responses of the propeller-shaft-hull coupled system.The symmetrical foundation can reduce the sound radiation significantly.The modal acoustic contribution technique is applied to study the noise suppression mechanism of the symmetrical foundation.The results show that the optimal position of the axial vibration isolator is between the shaft and the thrust bearing,and the design of the isolator should consider the relationship between the axial vibration's resonance frequency range of the shaft and the natural frequency of the axial vibration of the hull.(4)The experimental investigation of the shaft-hull coupled system is studied.In air,the dynamic response of the model is measured.In water,the dynamic and acoustic responses of the model are measured.The numerical solutions and the control effect of the symmetrical foundation are validated by the experimental results.The thesis has conducted a series of researches by theoretical and experimental methods for the vibro-acoustic characteristics of the propellershaft-hull coupled system.The influences of the structural discontinues,shaft and propeller on the responses of the coupled system,and the corresponding control methods are studied.A good engineering application is shown in the work.