SIMILARITIES BETWEEN VARIOUS LAMB WAVES IN SUBMERGED SPHERICAL-SHELLS, AND RAYLEIGH-WAVES IN ELASTIC SPHERES AND FLAT HALF-SPACES

A variety of resonance features are studied in the back-scattering cross sections (BSCS) of an air-filled metal spherical shell submerged in water and insonified by a plane cw sound wave. Rayleigh (R) and whispering gallery (WG) waves were originally investigated for vibrational purposes for (flat) half-spaces in contact with vacuum. Lamb waves were originally studied in flat plates also in contact with vacuum. These old findings are generalized to the cases of an elastic spherical shell (o.d./i.d. = 2a/2b) fluid-loaded on both surfaces, and excited by an incident plane wave. The various (leaky-type) Lamb waves present in the shell are shown to reduce to the earlier R/WG waves as a >> 1 >> b and rho-f --> 0. The manner in which each one of these various shell waves manifests itself in the various frequency bands of the shell's BSCS as perceived by a remote sensor is also studied. Dispersion plots for the various phase velocities of the various waves are displayed in very wide (i.e., 0 < ka < 500) bands, and a number of analogies between Lamb and R/WG waves are obtained as the submerged shell becomes a solid sphere (b << a), and vice versa (b less-than-or-similar-to a). The fluid loadings, the finite shell thickness, and the curvatures of the structure all generate novel types of waves in the shell (that manifest their effects in its BSCS) that could have never emerged from earlier models that ignored these effects, and which are analyzed here.