Experimental and Numerical Investigation of a Complex Submerged Structure. Part I: Modal Analysis

The purpose of the following paper is to demonstrate the abilities and limitations of commercial Finite Element (FEM) and Boundary Element (BEM) packages, concerning the dynamical properties and the low frequency sound radiation of a complex submerged structure under free field conditions. The investigated model consists of two cylinders with different radii, connected by a conical section and covered with hemispherical end caps. It contains ring stiffeners, flanges and rectangular frames. Since the interest was mainly in whole body vibrations of the structure and its corresponding underwater sound radiation, a modal approach was chosen throughout the whole investigations. Besides, it was expected, that the excitation at the resonance frequencies of the structure would yield a better signal to noise ratio in the experiments. Thus, part I of the paper deals with the experimental and numerical modal analysis. The experiments were carried out in a Norwegian fjord, suspending the model from the research vessel PLANET. Two different approaches were tried for the numerical calculations of eigenmodes and natural frequencies of the submerged body. The FEM code ANSYS was used to set up a pure FEM model, consisting of structural shell elements, three dimensional fluid elements, and absorbing elements at the boundary of the spherical fluid mesh. The second approach was a coupled FEM-BEM solution, combining ANSYS and the BEM code SYSNOISE. Two models with different grades of complexity were used with the FEM-BEM approach. Both methods were tested on a spherical shell, where analytical solutions are available. A comparison of measured and computed modes and eigenfrequencies will be given. Part II of the paper covers the low frequency sound radiation of the structure on the basis of measured and calculated directivity diagrams. The sound pressure was measured by means of a linear vertical hydrophone array at a distance of about 60m to the model. The structure was excited by an internal shaker at various eigenfrequencies and rotated about an axis, perpendicular to its axis of symmetry. The calculated coupled modes (FEM-BEM) of the submerged model provided the velocity boundary conditions for the BEM computation of the radiated sound field [1].