ASA 127th Meeting M.I.T. 1994 June 6-10

2aSA8. Bragg-modulated hybrid ray-mode acoustics for plane wave sound scattering from a periodically ribbed submerged cylindrical elastic shell.

Leopold B. Felsen

Ronald Shaya

Dept. of Elec. Eng., Polytechnic Univ., Six Metrotech Center, Brooklyn, NY 11201

The ray-acoustically parametrized formal spectral integral solution presented in the companion paper [L. B. Felsen and R. Shaya, preceding paper] is evaluated by asymptotic procedures structured around saddle point methods, contour deformations, and pole residue extractions in the complex spectral wave-number ((nu),(beta)) domains pertaining to the scattering configuration. Saddle points give rise to geometrical reflections, whereas intercepted poles give rise to shell-guided leaky, trapped, and creeping wave fields, both for the bare-shell and rib-induced contributions. The resulting scattered field ray-acoustic algorithm contains (a) specular and Bragg-modulated diffracted ray fields; (b) Bragg-modulated helically traveling shell-guided mode fields that are phase-matched to the fluid; and (c) helically resonant shell-guided modes formed by phase coherent circumnavigations of the modes in (b). The propagation and leakage characteristics of the shell-guided modes are determined by the dispersion equation for the rib-loaded cylindrical shell.