Backscattering by tilted cylindrical shells above the coincidence frequency caused by helical flexural waves

Abstract

Generalized leaky Lamb waves excited by ultrasound incident on cylindrical shells in water produce important backscattering effects. At a certain critical tilt angle, leaky waves launched along the meridian of the cylinder have been shown to lead to large backscattering enhancements [S. F. Morse et al., J. Acoust. Soc. Am. 103, 785–794 (1998)]. At angles less than the critical tilt, these leaky waves follow helical paths and also make significant contributions. In the present work, experiments using tone bursts were performed on a stainless steel shell to investigate the contributions of a0 leaky Lamb modes. The tone bursts were of sufficient duration to superpose helical wave bursts of successive circumnavigations, along with the meridional contribution near the critical tilt, to arrive at a steady state backscattering amplitude for the cylinder. This was compared against an approximate numerical partial wave series solution and a ray theory solution as a function of the tilt angle. The data follow the basic shape of the theoretical curves. However, the experimental amplitudes are more suppressed at higher frequencies resulting from losses due to reflection of leaky waves at the cylinder’s end. [Work supported by the Office of Naval Research.]