Effects of apparent mass on the radiated sound power from fluid-loaded structures

Prediction of radiated noise from underwater vehicles due to propeller excitation is of significant interest to maritime defence industries. In mathematical and numerical modelling, the pressure hull of an underwater vehicle is usually modelled with additional mass to achieve neutral buoyancy in water. However, the hull can also be regarded as a buoyant structure surrounded by free-flood spaces whose boundaries are acoustically transparent when flooded. In this work, apparent relative mass describes the ratio of the apparent mass of a longitudinal cylinder section to the mass of the water displaced by that section, where apparent indicates the fact that the effective structural mass is a function of frequency. The overall apparent mass and the apparent sectional mass distribution have an important effect on sound radiation. A rigid cylindrical hull with hemispherical ends is initially examined. Departures from dipole behaviour for a rigid body are shown to depend on apparent relative mass, cylinder length and direction of excitation. The analysis is then extended to higher frequencies and to a representative submarine pressure hull with stiffeners, bulkheads and added mass. The contributions of the rigid body and flexible modes to the radiated sound power for axial and transverse excitation cases are presented.