Abstract:

Typical plastics do not support leaky Rayleigh waves in water because the shear velocity is less than the speed of sound in water. Consequently, backscattering from plastic cylinders is ordinarily small. If the shear-wave attenuation is sufficiently small, there is, however, a mechanism for enhanced backscattering by tilted circular cylinders. At a certain tilt angle, rainbow caustics merge in the meridional plane defined by the incident wave vector and the cylinder's axis. The merged caustics are associated with rays internally reflected off an end of the cylinder but are analogous to a merging of caustics observed [C. M. Mount and P. L. Marston, Light and Color in the Open Air (OSA, Washington, DC, 1997), pp. 14--16] in optical experiments with long plastic fibers. Ultrasonic observations of the backscattering enhancement are reported for a plastic cylinder in water with ka=25. The enhancement mechanism should also apply to liquid cylinders in a thin shell if the speed of sound for the liquid is less than that of water. [Work supported by the Office of Naval Research.]