Volume 113, Issue 1, January 2003
Index of content:
- STRUCTURAL ACOUSTICS AND VIBRATION 
Acoustic scattering by a cylindrical shell with symmetric line constraints in the heavy fluid-loading limit113(2003); http://dx.doi.org/10.1121/1.1521427View Description Hide Description
A cylindrical shell, modelled using Donnell–Mushtari thin shell theory, is reinforced by two internal rigid plates attached to the shell along lines parallel to the shell axis. A circumferential mode expansion is used to obtain numerical results for the scatteredsound field due to the presence of the reaction forces along the attachment lines. In the heavy fluid-loading limit, which is appropriate for low and mid-frequency ranges for practical underwater structures, asymptotic analysis is presented which allows the peak frequencies in the scattered field due to the reaction forces to be expressed (to leading order) in terms of the geometry and the shell and fluid parameters. These predictions agree well with results obtained by numerically evaluating the infinite sums needed to calculate the reaction forces and hence the scattered field.
Leaky helical flexural wave backscattering contributions from tilted water-filled cylindrical shells113(2003); http://dx.doi.org/10.1121/1.1526471View Description Hide Description
Helical flexural waves on a bluntly truncated tilted water-filled cylindrical steel shell in water are found to give large contributions to the backscattering above the coincidence frequency. The presence of the water inside the shell increases the damping of the leaky wave when short tone bursts are used. The magnitude of the scattering is found by modifying a ray analysis developed for empty shells. When longer bursts are used, some of the internally radiated energy (corresponding to the case of one internal chord) is superposed on the ordinary helical ray backscattering. This occurs as a consequence of the internal excitation of helical rays.
Application of Padé via Lanczos approximations for efficient multifrequency solution of Helmholtz problems113(2003); http://dx.doi.org/10.1121/1.1514932View Description Hide Description
This paper addresses the efficient solution of acoustic problems in which the primary interest is obtaining the solution only on restricted portions of the domain but over a wide range of frequencies. The exterior acoustics boundary value problem is approximated using the finite element method in combination with the Dirichlet-to-Neumann (DtN) map. The restriction domain problem is formally posed in transfer function form based on the finite element solution. In order to obtain the solution over a range of frequencies, a matrix-valued Padé approximation of the transfer function is employed, using a two-sided block Lanczos algorithm. This approach provides a stable and efficient representation of the Padé approximation. In order to apply the algorithm, it is necessary to reformulate the transfer function due to the frequency dependency in the nonreflecting boundary condition. This is illustrated for the case of the DtN boundary condition, but there is no restriction on the approach which can also be applied to other radiation boundary conditions. Numerical tests confirm that the approach offers significant computational speed-up.