The Research On Vibration And Sound Radiation Of Partially Submerged Elastic Structure

Most of the marine structures and ships, are partially submerged in water, which can be regarded as partially submerged elastic structures. While researching on the vibro-acoustic characteristics of these structures, it is necessary to consider the effect of the free surface of the fluid. However, the previous studies mainly focus on the structure in the infinite fluid, omitting the influence of the fluid's free surface. When more and more attention was paid to the vibration and noise of ships, the study on vibro-acoustic characteristics of partially submerged elastic structure is of great significance to contro l the vibration and noise of these structures.At first, The vibro-acoustic coupling characteristics of the plate which was horizontal submerged in the certain depth under the water, was studied by adopting the thin plate theory to establish the dynamics equation of the all sides simply supported plate, using modal superposition method to give out the vibration displacement, and utilizing hermhotz- huygens radiation integral to describe the underwater radiation sound field of the plate. The free surface of the water led to the anti-symmetric boundary condition, so the method of "mirror" was used to construct the half space green's function, combined with the velocity boundary conditions on the coupling surface of the acoustic and solid medium, and the expression of the radiation sound power of the horizontal submerged and simply supported plate that was close to the water surface. To calculate the near surface horizontal submerged plate's radiated acoustic power, and compare with the results based on FEM/BEM numerical method, good agreement of both results proved the correctness of the two methods. Then the influence of the distance between the plate and the free surface on the radiation sound field of the plate was discussed.For the partially submerged plate, the "free surface" effect was simulated by a n anti-symmetry plane set. The vibration and sound radiation characteristics of partially submerged cantilever plate were analyzed by utilizing the finite element method combined with the indirect boundary eleme nt method. The natural frequencies and mode shapes of the cantilever plate at different immersion depths were calculated firstly, and compared with the values of an existing literature, which verified the correctness of the method. Then, the “half- immersed” plate's vibro-acoustic characteristics were calculated. The impact of different load positions and immersion depths on the vibro-acoustic characteristics of the plate were discussed as well. Studies had shown that the natural frequency of the immersed p late was decreased with the increase of immersion depth. The load position would influenced the sound power level of the plate, and the influence was related to the frequency. The plate's sound power level was increased with the increase of immersion depth. The peak of acoustic radiation power curve in low frequency stage has a tendency to move to low frequency, with the increase of immersion depth.The surface ship can be thought of as the combinations of the partially submerged plate and shell structure. The underwater radiation noise caused by mechanical vibration was the main noise for the ship low in frequency. In this paper, the ship cabins were calculated by finite element method combined with the indirect boundary element method. Considering the influence of the free surface, the radiated noise was calculated with the running of different equipments in different cabins. According to the comparison analysis of these different operating conditions, the radiation noise level of different equipments were grasped when the equipment ran separately, the main source of power equipments in the cabins were obtained as well, which would provide reference in order to decrease the radiation of the mechanical equipment noise.