Backscattered acoustic wave from a bent finite dimension sheet

Abstract

A marine current turbine is made of a boss and several blades. An acoustic scattering method is appropriate to detect defects in this machine. The acoustic backscattering from a blade identical to a finite plane plate has previously been studied. The flight times of acoustic echoes are calculated taking into account the quasi Lamb waves in a plate. In order to connect with the reality, the shape of the chosen blade is a bent finite dimension sheet. To simplify the study, a half cylindrical shell obtained cutting a tube in two parts is used. This target, immersed in water, is excited with short impulses in a plan perpendicular to its surface with incidence angles between 0 to 360°. Successively, the convex part and the concave part of the half tube are excited. The backscattering signals are compared with those which are obtained from an air-filled cylindrical shell immersed in water. The flight times of the different acoustic signals are identified taking into account the circumferential guided waves in the shell. Spectra are computed with a Fourier transform and the resonances are identified. The convex part and the concave part do not give same results. This experimental result will be explained. © 2012 Acoustical Society of America.