Scattering of an obliquely incident plane acoustic wave by a circular cylindrical shell. Experimental results

To explain the mechanisms of the acoustic scattering of an obliquely incident plane wave by a circular cylindrical shell, it is necessary to know the acoustic propagation in a similar shell of infinite length. Theoretical studies have been published which have shown that these mechanisms are related to the propagation of the guided waves with circumferential modes in the axial direction of the shell. These guided waves can be characterized by a thickness mode (Lamb waves) and by a circumferential mode. This last mode is made up by the phase matching of two helical waves that entwine the shell in the two directions. In this paper, the results and the analysis of measurements of the acoustic pressure scattered by an infinitely long circular cylindrical shell are presented. This paper uses and verifies the theoretical results published in Acustica/Acta-Acustica by N. Veksler et al. [1] ("Scattering of an obliquely incident plane acoustic wave by a circular cylindrical shell. Results of computations"). The measurements are carried out for an aluminium shell with relative thickness h = 1/10 (h = 1 - b/a, b: inner radius, a: outer radius) immersed in water. The range of the nondimensional frequency is 5 < x = ka < 150 (k: wave number of the acoustic wave in water). The plane wave is incident from an oblique angle. The various incidence angles are between 0 degrees and 46 degrees. It is shown that with an increasing of the angle of incidence every helical wave undergoes qualitative and quantitative changes, especially so near the cut-off frequencies.