Backscattering enhancements by partially exposed cylindrical shells due to reflected flexural waves at air–water interfaces

Abstract

For smooth curved objects in contact with a flat interface, the propagation paths of supported surface elastic waves can be altered by the interface, thereby enhancing the characteristics of the backscattered echo returns. In this study, backscattering experiments are conducted on an empty stainless steel cylindrical shell that breaks through an air–water interface and is insonified from below at grazing incidence. The shell has a thickness-to-radius ratio of 5%. The frequencies of interest fall within the vicinity of and below the coincidence frequency, which corresponds to a ka range of 13.8–20.1. The experimental results demonstrate that, for the partially exposed cylindrical shell, the flexural waves can traverse the interface and circumnavigate the shell multiple times. The flexural waves experience mode transitions upon passing through the interface, existing as a0− waves below the interface and a0 waves above the interface. Simultaneously, the flexural waves undergo partial reflection each time they pass through the interface, guiding them to follow a reverse path and generating a periodic series of echo returns, which leads to the enhancement of backscattering. The shift in resonance frequency in relation to the changing exposure is analyzed and predicted using the phase matching theory.