Numerical Analysis And Active Control Simulation Of Underwater Structural Acoustic Radiation And Transmission

Numerical analysis and active control of underwater structural acoustic radiation and transmission are very important for achieving quiet submerged structures. First, the design procedures such as simulation-based-design and vibration-and-noise-control-by-design for achieving a quiet submerged structure are based on the numerical analysis of response behavior of the submerged structure. Second, active structural acoustic control has emerged as a very promising method to reduce structural radiated noise at low frequencies and has been found to be complementary to passive control methods, which are good at high frequencies. This dissertation addresses a detailed study of numerical analysis and active control simulation of underwater structural acoustic radiation and transmission. The major contributions of this dissertation are as follows:Based on the boundary element formulation of half-space radiation, the effect of a free surface on acoustic radiation from a sphere is discussed. It is shown that the free surface has significant influence on acoustic radiation when the sphere is near the surface in the low frequency range. The presence of the free surface reduces the acoustic power radiated by the sphere significantly.A computational model of sound transmission from air to water through a stiffened plate or a composite laminates excited by a harmonic oblique incident plane wave and mounted in an infinite baffle is presented. Finite element method (FEM) is used to model the structures. Boundary element method (BEM) based on the Rayleigh integral in the plate surface is employed for the acoustic fluid. The sound transmission losses through the stiffened plate and the composite laminate are discussed with the proposed model. The numerical results show that the influence of the stiffness of the structures on sound transmission from air to water is important only if the structures' stiffness is very large, and the big difference of characteristic acoustic impedance between air and water dominates the sound transmission when the structures' stiffness is small. The influence of different stacking sequences such as symmetric angle-ply, anti-symmetric angle-ply and cross-ply on the sound transmission losses of the laminated composite plates is evaluated too with the proposed model. The results indicate that the laminate stacking geometry may have a significant influence on the sound transmission losses of laminated composite plates and the designers must be careful when deciding on the stacking sequences.-11 -AbstractA vibration modal approach and a radiation modal approach are proposed to analyze acoustic radiation from structures with complex geometries using FEM and BEM. The power radiated by vibration modes is discussed. It is shown that the power radiated by a vibration mode alone is to increase total radiated power and the interaction of different vibration modes can lead to an increase or a decrease or no change in the total radiated power, moreover, control of the vibration modes will be a good way to reduce both vibration and radiated sound as long as the influence of the interaction of the vibration modes on sound radiation is insignificant. In the presented radiation modal approach the CHIEF method is adopted to obtain a radiation mode formulation for extehor acoustic radiation analysis of three-dimensional structures without the nonuniqueness difficulty at critical frequencies. A pulsating cube is involved to demonstrate the radiation modal analysis procedure. The shapes and radiation efficiencies of the radiation modes of the cube are studied.Based on the Lagrangian of a piezoelectric body, a vanational equation of a piezoelectric structure is derived by using Hamilton's principle. Then, a finite element formulation of a laminated plate integrated with piezoelectric layers and viscoelastic layer is developed on the basis of the vanational equation for modeling the coupled electroelastic behavior using the Mindlin plate theory. After then, boundary element formulation based on the R...