Theoretical And Experimental Study On Vibration And Sound Characteristics Of Propeller-shaft-shell Coupled Structures

With the ongoing progress and development of the military power of the world, the concealment of submarine has received more and more attention.The study on the vibration and sound radiation of complex underwater structures is to seek an effective way to reduce vibration and noise and gradually becomes one of the important research directions of submarine vibration and noise control. Previous studies focus on the vibration and sound radiation of the uniform structures, and they are hard to meet the current practical engineering. So an accurate and efficient way to establish a complicate propeller-shaft-shell non-uniform structure and carry out related mechanistic study are under consideration. The investigation on the vibration and acoustic radiation of the coupled propeller-shaft-shell structures are carried out in this paper by theoretical calculation and experiment. The influence of non-uniform mass and stiffness of the structure on the structure vibration and acoustic radiation is studied. The main research contents include:In theoretical research, firstly, based on the semi-analytical modified variational method, the domain decomposition method is utilized due to its high accuracy and efficiency. Through this semi-analytical method, the free and forced vibration characteristics of the nonuniform structures are investigated. The nonuniform structures include stiffened shell and propeller-shaft-shell coupled structure. The styles of the stiffened shells include ringer, stringer, orthogonally stiffened rib and double-cycle ringer. In order to analyze the influence of the nonuniformity on the vibration characteristics, the coupled terms between all the circumferential wave numbers, axial and circumferential wave numbers, symmetric and antisymmetric modes are under considerations. Moreover, the characteristics of the coupled modes are analyzed from the perspective of the contribution from the different circumferential wave numbers. Based on this, a more complicated propeller-shaft-shell coupled structure is established. Both rigid and elastic propeller are modeled by lumped mass and spring oscillator, respectively. The shaft is modeled by the displacement function in three directions to consider the axial, transverse and vertical vibration. The hull is a stiffened conical-cylindrical-spherical joined shell and subdivide into several cabins by bulkhead. The shaft and hull is connected by the spring system. The system is established by the spring in both axial and radial directions to model the stern bearing and trust bearing. This complicated coupled structure model has great advantage in the parametric analysis and the mechanism analysis.Secondly, in order to solve a more model, which is closer to the real submarine, a FEM/BEM numerical method is used to predict the radiation characteristics of the complicated underwater structures. The numerical method is carried out by the use of ANSYSTM and LMS Virtual Lab AcousticsTM. Under the excitation of propeller induced longitudinal and lateral forces, vibration and acoustic radiation of submarine are investigated and discussed in detail, especially, the nonuniform structure. The longitudinally force induced by the propeller not only provokes the longitudinal vibration mode but also the bending vibration mode, namely longitudinal and transverse coupling characteristics. For the nonuniform structure, there are always peaks related to the modes of coupled bending vibration mode and local mode, induced by the nonuniformity and the coupling degree depends on the contribution of the nonuniformity. Moreover, the effect of the mass and stiffness nonuniformity on the added mass coefficients is analyzed. The computation process used here is based on the results of the dry mode obtained by ANSYS, and fluid-structure coupling calculation is made via the mode superposition method in the LMS Virtual Lab. Acoustics. This method is good at handling the fluid-structure coupling problem instead of establishing the fluid domain in ANSYS as the traditional method. So the calculation efficiency is very high.In order to investigate the vibration and sound radiation characteristics of the propeller-shaft-hull coupled structure and verify the numerical simulation results, a corresponding large-scale underwater structure experiment is designed and carried out. The resonances of the hull, excited by the force transmitting via the propeller-shaft system, and the radiated sound field due to the vibrating hull are measured by the acceleration sensor and the hydrophone, respectively. The effects of the mass and stiffness nonuniformity and the symmetry of excitation on the vibro-acoustic characteristics of the coupled structures are under consideration to analyze the longitudinal and transversal coupling characteristics. Moreover, the relation between acoustic radiation and vibration modes and transfer mechanism of the excitation from the propeller-shaft system are revealed.