Analysis of the acoustic radiation of an infinite periodically stiffened cylindrical shell near a free surface

The acoustic radiation from an immersed cylindrical shell, periodically stiffened by internal axisymmetric frames, has been studied in the past due to its interest in underwater applications. Particularly, it has been shown that Bloch–Floquet waves induced by the periodic arrangement of stiffeners can lead to significant radiation of the shell in the far field. However, the fluid domain in these studies was generally unbounded, which is not representative of practical applications when the submerged structure is close to the sea surface. This work, therefore, investigates the influence on radiated pressure of a free surface close to a periodically stiffened cylindrical shell immersed in water. The shell is excited by a harmonic point force. The free surface corresponds to a pressure released boundary condition. A semi-analytical model is developed based on a frequency–wavenumber decomposition. The cylindrical shell and stiffeners are represented, respectively, by Flügge's analytical model and finite element models. The radiation impedance of the fluid domain including the free surface is evaluated using the image source method. Radiated pressure results are presented as a function of angle and frequency to study the effects of the free surface, specifically on Bloch–Floquet waves.