High frequency acoustic backscattering enhancements for finite cylindrical shells in water at oblique incidence.

摘要

The scattering of sound by finite cylindrical shells in water is investigated. Both experimental and theoretical examinations are performed which include comparisons of measured backscattering enhancements with a quantitative ray theory and an approximate partial wave series solution. Broadband backscattering experiments are carried out for the full range of tilt angles (measured with respect to the cylinder's axis) to identify regions of enhanced backscattering from moderately thick-walled steel cylindrical shells (having thickness-to-radius ratios of 7.6% and 16.3%). It is found that the backscattering is significantly enhanced when conditions arise such that a generalization of a leaky Lamb wave is launched on the shell. This surface guided wave reflects off the shell truncation, which is flat and perpendicular to the cylinder axis, and reradiates into the backscattering direction. In addition to various helical ray contributions, particularly large enhancements are observed for a ray propagating along the cylinder's meridian. The meridional ray enhancement for the generalization of the lowest order antisymmetric leaky Lamb wave is observed over a large range of tilt angles, nearing end-on incidence in the coincidence frequency region. Coupling conditions are determined which locate the various enhancements in frequency-angle space. High-frequency narrowband backscattering experiments are used to make quantitative measurements of enhancement amplitudes, which agree well with theoretical predictions. For highly oblique tilt angles the meridional ray amplitudes are several times greater than the specular reflection amplitude for scattering from a rigid sphere of like radius. Both air-filled and water-filled cylindrical shells are examined. An abrupt drop in the backscattered meridional ray amplitude is observed at very high frequencies. This drop is the result of the next antisymmetric leaky Lamb mode and mode conversion upon reflection at the cylinder's end. Simplified calculations display aspects of this threshold behavior. In addition experimental procedures are described for obtaining transient and wide bandwidth backscattering data using a PVDF sheet source in water for frequencies extending up to 1 MHz.