High-frequency backscattering enhancements for thick truncated cylindrical shells in water at oblique incidence

Abstract

Various authors have shown that the backscattering by a finite thin cylindrical shell at oblique incidence is enhanced when resonance conditions are met for the propagation of a leaky wave on the shell [see, e.g., M. L. Rumerman, J. Acoust. Soc. Am. 93, 55–65 (1993)]. To explore the backscattering enhancements relevant to high-frequency detection, impulse response measurements were carried out for slender thick cylindrical shells using a broad bandwidth PVDF sheet source, for tilt angles ranging from broadside to end-on incidence. These measurements reveal large backscattering enhancements beyond the shear wave cutoff angle, which extend to near end-on incidence in the coincidence frequency region. A simple approximation for the phase velocities of several surface waves is used to identify the underlying mechanisms. A broad enhancement feature is shown to result from the axial propagation of the supersonic a0 leaky Lamb wave. The associated meridional ray radiates a backward directed wavefront having a vanishing Gaussian curvature. Also observable are what appear to be resonance loci associated with the helical propagation of the subsonic a0−. [Work sponsored in part by the Office of Naval Research and by the ARL:UT Independent Research and Development Program.]