Tilt angle dependence of backscattering enhancements from organ pipe modes of open water-filled cylinders: Measurements and models

Abstract

A simple target for simulating narrow low-frequency resonances of cylinders is an open metal pipe completely filled with water. We have previously described how the high-Q organ-pipe modes having a pressure node near each end are easily observed in backscattering experiments with small cylinders [C. F. Osterhoudt and P. L. Marston, J. Acoust. Soc. Am. 110, 2773 (2001)]. The resonance occurs because of the strong reflection of internal acoustic waves from the open ends of the pipe [H. Levine and J. Schwinger, Phys. Rev. 73, 383–406 (1948)]. In the present research, the dependence of the backscattering amplitude on the orientation of the cylinder is measured and modeled. The tilt angle dependence is affected by the symmetry of the organ pipe mode. An approximation was also developed for the backscattering amplitude at high Q resonances based on energy conservation, reciprocity, and the optical theorem. While this analysis applies to cylinders suspended in water away from boundaries, the organ-pipe modes studied may be useful for investigating scattering processes for buried or partially buried cylinders. [Research supported in part by ONR.]