Study On Vibration And Acoustic Radiation From (Laminatated Composite) Cylindrical Shells

The vibration and acoustic radiation of cylindrical shells compose an important part of the studies of submerged marine structures. In the past few decades, a lot of investigations were dedicated to these related subjects, three of which are of great importance and will be concluded as follows:(1) The interaction between the structure and its surrounding acoustic medium constitutes a feedback loop, i.e., the vibration of the structure may induce the perturbance of the surrounding acoustic medium, due to which the vibration energies are thus radiated outwards, in turn the surrounding acoustic medium may react to structure in terms of radiation damping as well as added inertia, and so on. In the past several decades, more attention was paid to questions such as how to analyze the vibrational and acoustic characteristics of this type of coupled systems, however, the characteristics of the interactive forces between the strucuture and the entrained acoustic medium, as well as those between the substructures have not been adequately addressed. Besides, the questions such as to what extent they affect the vibration and acoustic radiation of the systems are very instructive for better understanding the underlying mechanisms of the related problems.(2) The interaction between the shell and its internal structures plays an important role in the vibrational and acoustic characteristics of the overall structure. Structural waves may be reflected, diffracted and even converted into other types of waves at the junctions between them, the resultant effects of which may lead to the change in resonance frequencies for the waves carried in the shell. Subsequently, the vibrational responses of the overall structure are inevitably different from those of the shell without the internal structures, and the presence of which would embody themselves in the acoustic signals of the overall structure. Due to the above reasons, further investigaitons towards the correspongding problems are still necessary to be conducted.(3) Composite materials are often chosen to construct the cylindrical shells due to their excellent properties, e.g., they can offer lower weight and higher strength and stiffness than most metallic materials. The problem of the vibration and acoustic radiation of laminated composite shells is a little more complex than that of uniform isotropic shells, moreover, questions such as what an important role do their peculiar material properties play in their vibration and acoustic radiation have not been adequately addressed.In many engineering fields, especially in ship and ocean engineering, the above mentioned subjects are of great interests for design engineers, and researchers as well. Due to the above reasons, the following investigations have been conducted in this thesis, which stem from the primary problem of the interaction between (composite) shell-structure and acoustic fluid.(1) The combination of Lagrange's equation and Lagrange's multiplier is employed to develop the governing equations of the vibration of the submerged finite cylindrical shell with internal plate. Generalized constraint equations are derived for the hinged and rigid joints between the plate and the shell, which stem from the boundary conditions in elasticity theories for the plates and shells. Generalized pressure on the finite shell due to the acoustic medium is described based on the Neumann boundary condition in terms of Green's function.(2) Acoustic radiation from a point-driven, fluid-loaded, infinite laminated composite cylindrical shell which is reinforced by doubly periodic rings is investigated theoretically. The theory is based on the classical laminated composite shell theory, the Helmholtz equation, and the boundary conditions at the shell-fluid interface as well as at the junctions between the shell and the rings. The rings interact with the shell only through normal forces. The solution for the radial displacement in wave number domain is developed by using Mace's method for an infinite flat plate. The stationary phase approximate is then employed to find the expression for the far field pressure. Numerical results are presented for discussion of the effects of lamination schemes, Poisson's ratios, and ply angles on the far field acoustic radiation, moreover, the helical spectrum technique is employed to analyze the influences of the internal periodic rings and the ply angles of the laminated composite shells on their vibrational characteristics.(3) Analytical expressions are derived for the vibration and far-field acoustic radiation from a fluid-loaded, laminated composite cylindrical shell which is excited by three types of mechanical drives: point-, ring-, and uniformly distributed, local surface forces along in-plane and radial directions. The present work is based on the classical laminated composite shell theory (CLCST), the Helmholtz wave equation, and boundary conditions at the fluid-shell interfaces, as well as at infinity. Fourier transform is used for solving the responses of the shell, and the stationary phase approximate is employed to find the expression for the far-field acoustic pressure. Through numerical results for the acoustic radiation from the shells with different types of mechanical drives, the coupling effects of the extensional and bending waves in the laminated composite shells are discussed.(4) Asymptotic expressions are derived for the fluid loading of an infinite cylindrical shell which is periodic with its axial direction, and for Hankel functions with sufficient large arguments, they are found equal to those for the fluid loading of an infinite flat plate. Comparisons of wave impedance are made between the theoretical and asymptotic results for an infinite cylindrical, which may reveal the characteristics of the fluid loading of an infinite cylindrical shell and the intrinsic relationships between the mechanism of the acoustic radiation from the shell and from the plate. This will lend themselves to better understanding the underlying physical mechanisms of fluid loading on plates and shells.(5) The coupling effects on two concentric cylindrical shells with periodic annular plates as well as entrained fluid are investigated theoretically. A new formula for the reactive forces by the annular plates is derived on the presumption that only quasi-longitudinal waves exist in the annular plates. Comparisons of numerical results for infinite two-walled cylindrical shell to measured data for a cabin model are in good agreement. Moreover, numerical analysis for the characteristics of the loadings due to the fluid, the annular plates are presented for physical insight of the mechanism of the vibration of the two shells. The helical spectrum technique is employed to analyze the influences of the periodic rings and the annular plates on their vibrational characteristics of the inner shell. Finally, the effects of the fluid and the annular plates between the two shells on their acoustic radiation are discussed.