Structural and acoustic responses of a submerged vessel

Abstract

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