Effect of a Shaft Resonance Changer in the Presence of Fluid Forces

Abstract

The excitation of a submarine hull by the propeller through fluctuating fluid and shaft forces is investigated. The forces are due primarily to the operation of the propeller in a non-uniform wake and occur in the low frequency range. The resulting propeller pressure field can be represented by dipoles which are normal to and along the propeller axis, with the origin at the propeller hub. The hub forces act in the opposite direction to the fluid forces and are modified in transmission to the thrust block. Both fluid and shaft forces excite vibration of the hull. The axisymmetric vibration associated with accordion modes is a powerful source of sound radiation, so this work focuses on the effect of the axial propeller forces. A simplified axisymmetric model of a submarine hull has been developed using the finite element method to represent the behaviour of the structure and the boundary element method to represent the properties of the fluid domain. The model includes a rigid conical section at the aft end of the pressure hull to represent the free-flood structure that supports the aft propeller shaft bearing. This is connected to a dynamic model of the pressure hull itself. It is shown that the conical tail section plays an important role in hull excitation through the fluid. A resonance changer can be used to attenuate the vibration transmission through the propeller shaft, but not the excitation via the fluid. In this paper it is shown how the overall performance of the resonance changer is influenced by the fluid forces.