Numerical Analysis Of Vibro-acoustic Characteristics Of Underwater Damped Composite Shells

Prediction and control of the structural vibration and noise is an especially important subject for both civil ships and naval vessels. It is significant theoretically and practically to study the influence of the fluid characteristics on the underwater structural vibration and that of the fluid boundary conditions on the vibro-acoustic characteristics of underwater structures, because underwater vibro-acoustic characteristics differ greatly from air vibro-acoustic characteristics mainly due to the loading effect of the surrounding heavy fluid. Besides, to study the vibration, acoustic radiation and sound transmission of underwater damped composite shells is especially important for the prediction of vibration and noise and the design of sound-absorbing layer and the safety of submarines. This dissertation addresses a detailed study on numerical analysis of vibro-acoustic characteristics for underwater elastic structures and damped composite shells. The major contributions and conclusions are as follows.Two different fluid-structure interaction equations are established, based on the incompressible fluid and the compressible one respectively, and the influence of the fluid compressibility on natural characteristics and vibration responses is discussed. Then, the influence of computation of vibration responses for underwater structures based on the compressible or incompressible fluid on the acoustic radiation, and several factors related to the influence of the fluid compressibility are studied. The results show that the fluid-structure interaction responses obtained with the consideration of the fluid compressibility differ a lot from those without the consideration of the fluid compressibility for structures in the full-space fluid domain. Especially, near the resonant frequencies, the vibration responses are remarkably reduced with the consideration of the fluid compressibility, and the acoustic radiation can also be impacted by different responses with/without the consideration of the fluid compressibility. Therefore, it can be safely concluded that the vibration responses must be calculated based on the compressible fluid in order to calculate the acoustic radiation for structures in the full-space fluid domain.The fluid-structure interaction equation is established for bodies in the half-space fluid domain, especially sitting on the infinite plane, based on the coupled FEM/BEM theory. Then, the effects of the free surface and the rigid plane on the natural frequencies, vibration responses and the acoustic radiation for underwater structures are discussed. Next, several relative factors including the plate thickness, the structure damping and the distance between the body and the infinite plane are studied. Finally the influence of the fluid compressibility on the structural vibration is further discussed. The results of this research show that the effects of the free surface and the rigid plane on the structural vibration and the acoustic radiation are related to the structural stiffness, the distance between the body and the infinite plane and the frequency of the external force, etc. It is found that the influence of the fluid compressibility is not significant for the structures near the free surface with small stiffness because the structural-acoustic radiation efficiency is very small at low frequencies, but forstructures with big stiffness the fluid compressibility cannot be ignored even if the structures are vibrating near the free surface.The boundary integral equation for the acoustic radiation and the coupled FEM/BEM vibration equation are established for structures in shallow water. The results show that the shallow water can influence the natural frequencies and vibration responses of underwater structures evidently under the condition of small water depth. It is also found that the shallow water impacts on the acoustic radiation much more seriously in a remarkably complex way than on the vibration responses. For example, the water depth can still have an influence on the sound power e...