Experimental investigation of enhancements in scattered pressure field of fluid-loaded shell with periodic structure

Abstract

The scattered field from an irregularly ribbed, fluid-loaded plate has been the subject of several recent investigations; one has proposed the existence of dispersive wave phenomena that is due to interference between Bragg diffraction and locally propagating Bloch waves [D. M. Photiadis, J. Acoust. Soc. Am. 91, 1897–1903 (1992)]. Recent measurements at NRL have confirmed the existence of such interference patterns in the scattered pressure field of a fluid-loaded cylinder having nearly periodically spaced ribs in the frequency region of 4.0<ka<30. It is shown that the theory for the two-dimensional case can be applied to the three-dimensional cylinder problem with moderate first-order agreement with experimental data. Possible approaches to better approximations of solving the cylinder problem are proposed and discussed, including using the developed (2-D) plate theory with a derived analytic form of the bending wave dispersion relation from the thin shell equations. The far-field behavior is shown to be determined by the dispersion relation of the free waves on the structure, the rib spacing, the transmission (T) and reflection (R) coefficients of a flexural wave incident upon a rib, and the deviation from periodicity of the ribs. The feasibility of applying this theory to experimental data for the purpose of determining T and R is also considered.