Contribution of membrane wave reradiation to scattering from finite cylindrical steel shells in water

Abstract

Measured levels of acoustic backscattering from an insonified thin cylindrical steel shell in water (measured in a frequency range corresponding to 5<ka<15) show loci in frequency-angle space along which are discrete areas that represent high levels of backscattering. Using an approximate theoretical model in which shell membrane waves are explicitly represented, it is shown that the high levels of backscattering are due to supersonic membrane waves that are established on the shell. The loci of peaks are associated with combinations of frequency and angle at which a plane wave incident upon a similar infinite shell generates a resonant membrane mode of the infinite shell. The locations of the discrete high level spots along the loci correspond to those axial wave numbers that, together with the corresponding circumferential wave number, define resonant modes of the finite shell. The level of backscattered pressure is high at these locations because, at these frequency-angle combinations, the particular resonant mode is driven by a trace-matched incident wave. The analysis is given for a simply supported shell, but the nature of the approximations make the approach applicable to other homogeneous end conditions.