Acoustic scattering from a fluid-loaded cylindrical shell with discontinuities: Single plate bulkhead

Abstract

The far-field acoustic scattered pressure from a fluid-loaded cylindrical shell with an internal plate bulkhead is determined from knowledge of the junction forces between the shell and the internal structure, for the circumferential modes n equals 0 and 2. The n equals 2 case is representative of results, such as the presence of compressional and shear helical waves, for which n is greater than zero. The junction forces are determined using a mobility (impedance) matrix approach, which matches the response of the shell to that of the plate bulkhead at the junction, together with an input of the shell-free response under the influence of obliquely incident acoustic waves. For n equals 0, if the angle of incidence of the acoustic wave is less than approximately 16 deg, for a steel shell in water, the scattered pressure has significant contributions from the supersonic longitudinal axial waves in the shell. For greater angles of incidence, the scattered pressure has comparable contributions from the axial waves and the junction radial load. The tangential shear waves do not contribute in the n equals 0 case. For n greater than 0, if the angle of incidence is less than approximately 30 deg, then the scattered pressure is dominated by the contributions from the supersonic longitudinal axial and tangential shear waves, above the cut-on frequency. Below the cut-on frequency of these waves, the far-field scattered pressure is dominated by the component associated with the radial junction force. For angles of incidence greater than approximately 30 deg, the radial junction force and the shear and longitudinal waves equally contribute. There is, however, a difference in the radiation characteristics of these components. While the contribution from the radial force has a dipole directivity pattern, the contributions from the shear and longitudinal waves have a preferred direction of propagation, since both of these waves are supersonic. The contribution from the junction moment is always insignificant when compared to the other components. Finally, the results show that for very low frequencies, the in-plane response of the plate bulkhead is very small and the junction between the plate and the shell behaves similar to a radially pinned junction. As the frequency increases, the internal structure has in-plane resonances and the radial influence of the bulkhead is at a minimum. At these frequencies, the contribution from the radial forces to the far-field pressure is minimal.