A Study on the Acoustic Radiation Behavior of a Semi-Submerged Cylindrical Shell in Shallow Water Using Boundary Element Method

Acoustic radiation behavior of a flexible cylinder floating on the free surface of a finite depth water is investigated and numerical predictions are presented together with experimental results. A semi-submerged cylinder is considered while it is excited with an impulsive force in radial direction. First, to calculate acoustic radiation from structures vibrating on the free surface, the Helmholtz integral equation (HIE) is employed. Effects of the free surface and finite water depth are introduced by using waveguide Green’s function where a chain image-source method is used for derivation. The acoustic field computations are carried out within the boundary element (BE) framework, enabling the evaluation of radiated pressures by discretizing only the fluid-structure interface. As a test case, acoustic pressures radiating from a sphere pulsating on the free surface of a shallow water are calculated and compared with finite element (FE) solutions. It is observed that results of the numerical method are in good agreement with FE solutions. Moreover, acoustic radiation from a semi-submerged cylinder is investigated. The cylinder problem is first solved numerically. Also, an experiment is performed where acoustic pressure is measured at the same point as the numerical model. In the experiment, a semi-submerged cylinder is excited with a hammer hit and acoustic pressure is recorded by a hydrophone. The results obtained with the mathematical model are compared with the data obtained from the experiment. It is observed that, although the predicted pressure values slightly differ from those of experimental data, the acoustic pressure peaks occur at frequencies which are compatible with the wet frequencies of the cylinder, as expected. In general, the results obtained with the vibro-acoustic mathematical model are in general agreement with the finite element solutions and experiment.