Effect of eccentric axial excitation from the propulsion system on the sound radiation of a submarine hull

Abstract

A model to describe the low frequency dynamic and acoustic responses of a submarine hull subject to a harmonic propeller shaft excitation is presented. The submarine is modelled as a fluid-loaded, ring stiffened cylindrical shell with internal bulkheads and end caps. The stiffeners are introduced using a smeared approach. The bulkheads are modelled as circular plates and the end closures as truncated conical shells. The propeller introduces a harmonic axial force that is transmitted to the hull through the shaft. It results in excitation of the accordion modes only if the force is symmetrically distributed to the hull. Otherwise the excitation can be modelled as the sum of an axisymmetric axial force plus a moment applied to the edge of the hull to take into account the eccentricity of the force. This leads to excitation of the higher order circumferential modes that can result in high noise signature. Structural and acoustic responses are presented in terms of frequency response functions of the axial and radial displacements and directivity patterns for the radiated sound pressure. Results for the case of purely axisymmetric excitation and the case in which an eccentricity is introduced are compared.