No description available.

Excitation of the low frequency vibrational modes of a submerged vessel can generate significant radiated noise levels. Vibrational modes of a submarine hull are excited from the transmission of fluctuating forces through the shaft and thrust bearings due to the propeller rotating in an unsteady fluid. The focus of this work is to investigate the structural and acoustic responses of a submarine hull under axial excitation. The submarine hull is modelled as a cylindrical shell with internal bulkheads and ring stiffeners. The cylindrical shell is closed by truncated conical shells, which in turn are closed at each end using circular plates. The entire structure is submerged in a heavy fluid medium. The structural responses of the submerged vessel are calculated by solving the cylindrical shell equations of motion using a wave approach and the conical shell equations with a power series solution. The displacement normal to the surface of the structure in contact with the fluid medium was calculated by assembling the boundary/continuity matrix. The far field radiated sound pressure was then calculated by means of the Helmholtz integral. Results from the analytical model are compared with computational results from a fully coupled finite element/boundary element model. The individual and combined effects of the various influencing factors, corresponding to the ring stiffeners, bulkheads, conical end closures and fluid loading, on the structural and acoustic responses are characterised by examining the contribution by the circumferential modes. It is shown that equally spaced internal bulkheads generate a periodic structure thus creating a grouping effect for the higher circumferential modes, but do not have strong influence on the sound radiation. Stiffeners are found to have an important effect on both the dynamic and acoustic responses of the hull. The contribution of the conical end closures on the radiated sound pressure for the lowest circumferential mode numbers is also clearly observed. This work shows the importance of the bending modes when evaluating the sound pressure radiated by a submarine under harmonic excitation from the propulsion system.

Excitation of the low frequency vibrational modes of a submerged vessel can generate significant radiated noise levels. Vibrational modes of a submarine hull are excited from the transmission of fluctuating forces through the shaft and thrust bearings due to the propeller rotating in an unsteady fluid. The focus of this work is to investigate the structural and acoustic responses of a submarine hull under axial excitation. The submarine hull is modelled as a cylindrical shell with internal bulkheads and ring stiffeners. The cylindrical shell is closed by truncated conical shells, which in turn are closed at each end using circular plates. The entire structure is submerged in a heavy fluid medium. The structural responses of the submerged vessel are calculated by solving the cylindrical shell equations of motion using a wave approach and the conical shell equations with a power series solution. The displacement normal to the surface of the structure in contact with the fluid medium was calculated by assembling the boundary/continuity matrix. The far field radiated sound pressure was then calculated by means of the Helmholtz integral. Results from the analytical model are compared with computational results from a fully coupled finite element/boundary element model. The individual and combined effects of the various influencing factors, corresponding to the ring stiffeners, bulkheads, conical end closures and fluid loading, on the structural and acoustic responses are characterised by examining the contribution by the circumferential modes. It is shown that equally spaced internal bulkheads generate a periodic structure thus creating a grouping effect for the higher circumferential modes, but do not have strong influence on the sound radiation. Stiffeners are found to have an important effect on both the dynamic and acoustic responses of the hull. The contribution of the conical end closures on the radiated sound pressure for the lowest circumferential mode numbers is also clearly observed. This work shows the importance of the bending modes when evaluating the sound pressure radiated by a submarine under harmonic excitation from the propulsion system.

Thesis (M.S.)--University of Delaware, 2006. / Includes bibliographical references.

No description available.

Nonlinear shallow shell equations are derived for a thin shell of revolution having the shape of a narrow segment of a toroidal shell centered at the equator. The equations are derived by considering a cylindrical shell, described by nonlinear Donnell theory, with an initial radial deformation. Linear buckling equations are obtained by perturbing the nonlinear shell equations. The buckling equations are solved for the case of a simple supported equatorial segment of a spherical shell loaded in the axial direction by its own weight. Plots are presented which compare a critical thickness parameter with the results of an elementary approach. The elementary approach assumes that the shell will buckle if the maximum compressive stress is greater than the critical compressive stress for a complete sphere loaded by uniform external pressure. / M.S.

Thin, elastic, shallow shells having uniform thickness and rectangular boundaries are investigated. The boundary conditions are either simply supported or claped, and the shell is subjected to a uniformly distributed load applied over either the full shell area or a central region. The thickness, material properties, edge lengths, and surface area of the shell are specified, and the objective is the determination of the shell shape which will maximize the buckling load. Marguerre's two, coupled, non-linear equations of equilibrium are used to describe prebuckling deformations and stresses. Considering small vibrations about the equilibrium state, two, coupled, linear equations of motion are derived. Subsequently, by recognizing that at buckling the lowest frequency of vibration goes to zero, the buckling equations are obtained. Finally, the Lagrange multiplier technique is employed to formulate an augmented objective function, and the calculus of variations is applied in order to derive the governing set of equations. The resulting system of equations is solved numerically by the finite difference method. Results for shells with various surface areas are presented. For each surface area the investigation is performed on shells having either clamped or simply supported boundary conditions and either a square or a rectangular boundary. The applied uniform load covers either the full shell area or a partial central region. The shell form, buckling load, and buckling modes of the optimal forms are compared with those of the reference form (double sine) having the same surface area, and changes are noted. Also, comparisons with respect to forms, buckling load, and type of buckling are made between the optimal form of a shell subjected to a full uniform load and the optimal form of the same shell subjected to a partial uniform load. In some cases, the buckling load of the optimal form is sensitive to imperfections in the form or in the loading distribution as well as to changes in the design. In these cases, some of the apparent advantages of the optimum form may be diminished. Thus, the frequencies of vibration at buckling, the corresponding buckling modes, and the presence of adjacent equilibrium states are monitored in order to evaluate the sensitivity of the optimal form to imperfections and to design changes. / Ph. D. / incomplete_metadata

Call number: LD2668 .T4 1978 A94 / Master of Science

published_or_final_version / Civil and Structural Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy