Structural and acoustic responses of a submarine hull due to propeller forces

Abstract

The low frequency structural and acoustic responses of a simplified axisymmetric submarine model to fluctuating propeller forces along the submarine axis are investigated. The forces arise from a hydrodynamic mechanism and are transmitted from the propeller to the submarine hull through both the shaft and the fluid. Numerical models have been developed to simulate the strongly coupled structure–fluid interaction of a submerged vessel in the frequency domain. The structure is modelled using the finite element method, so that more complex features such as ring-stiffeners, bulkheads and the propulsion system can be taken into account. A simple, passive vibration attenuation system known as a resonance changer is included in the model of the propeller/shafting system. The surrounding fluid is modelled using the boundary element method. The influence and importance of model parameters such as structural stiffness and fluid loading effects are investigated. Due to the fluctuating propeller forces, the hull is excited by axial structural forces transmitted through the propeller/shafting system as well as by acoustic dipoles, where the dipoles are correlated to the structural forces in strength and direction. The acoustic dipole at the propeller also radiates sound directly to the far field of the surrounding fluid. It is demonstrated that the performance of the RC is negatively influenced at frequencies above the fundamental axial resonance of the hull by the effect of forces transmitted through the fluid. Another problem arises due to increased axial movement of the propeller, when the RC is optimised to minimise excitation of the hull via the propeller shaft. This results in an additional sound field that excites the submarine hull in a similar manner to the fluid forces that arise directly from the hydrodynamic mechanism.