Classification of resonances and remote aspect‐ratio determination of submerged spheroidal elastic shells

Abstract

Acoustic scattering was studied from submerged, steel, spheroidal shells in their resonance region. Arbitrary angular incidences were considered to analyze the possible aspect‐angle dependence of the resonances. The shells are sufficiently thin so that the “proper backgrounds” of the RST [G. C. Gaunaurd and M. F. Werby, Int. J. Solids Structures 22, 1149–1159 (1986)] can be suitably suppressed, and the resonances can be adequately isolated. It is shown that the shell resonances are caused by surface standing waves propagating at discrete frequencies with an almost constant phase velocity. From this (surface) standing‐wave interpretation (viz., geodesics) accurate resonance locations have been derived. Only two basic vibrational modes seem to exist, one along and the other normal to the axis of symmetry. Thus, only resonances corresponding to these two directions are possible. Furthermore, the fixed resonance locations seem to have amplitudes that change with incidence angle. Hence, this procedure permits the classification of resonances, and it can he exploited to extract the aspect ratio of the spheroidal shell from its returned echo. These points are illustrated with numerous computational examples. [Work supported by NAVELEX and ONR.]