Scattering enhancements for partially exposed cylinders at a free surface caused by Franz waves: Measurements and geometric model

Abstract

Creeping waves on solid cylinders having slightly subsonic phase velocities and large radiation damping are usually described as Franz waves because of their association with complex poles investigated by Franz. For free-field high frequency broadside backscattering in water, the associated echoes are weak due to the radiation damping. It is demonstrated here, however, that for partially exposed solid metal cylinders at a free surface viewed at grazing incidence, the Franz wave echo can be large relative to the specular echo even for ka above 20 when the grazing angle is sufficiently small. The reason is that at small grazing angles and small exposures, the Fresnel zone associated with the specular echo is occluded so that the specular echo is weak [K. Baik and P. L. Marston, IEEE J. Ocean. Eng. 33, 386 (2008)] while the Franz wave is partially reflected at the interface. This has been confirmed from the evolution of echo timing with cylinder exposure and by SAS imaging. In the experiment a solid cylinder was slowly lowered through the free surface into the water. In bistatic scattering a Franz echo can be present with small exposure without the Franz wave reflecting from the free surface. [Supported by ONR.]