FINITE-ELEMENT METHOD IN FLUID-STRUCTURE INTERACTION DYNAMICS AND APPLICATIONS TO UNDERWATER FRAME STRUCTURES

The purpose of this study is to provide an effective computational method of analysis capable of modeling fluid-solid systems under dynamic loads to calculate the hydrodynamic forces on the solid; then apply the results to investigate the behavior of underwater frame structures subjected to earthquake ground motions. The hydrodynamic forces induced on the solid body oscillating in a liquid are identified to be the drag force and the inertia force. The drag force is found to be negligible for cylindrical shapes when the diameter is in the order of 10 to 20 feet and when the cylinders are subjected to a dynamic motion, with frequencies and amplitudes within the range of frequency content and the amplitudes of an earthquake ground motion. It is shown that cavitation is unlikely to occur for the above conditions.;To study the hydrodynamic forces due to acceleration of a solid body, the Lagrangian finite element approach is employed. An effective finite element program is developed to model both the fluid and solid regions. With the finite element program developed, the hydrodynamic forces due to the small amplitude oscillation of a solid in a fluid is calculated. It is shown that the results of the finite element analysis are in good agreement with other analytical methods. The inertia forces calculated in this study are shown to be independent of the frequency of applied motion and are only a function of acceleration, fluid density, and shape for the range of frequencies in an earthquake ground motion. Thus the surrounding fluid (water) can be replaced by an additional mass which is called the 'added mass'.;The response of a submerged frame structure subjected to an earthquake ground motion is calculated. The effect of submergence is modeled by superimposing the 'added masses' to the real mass of the frame structure. Two analyses are performed; first, the analysis with the added masses obtained by the finite element method, and second, the analysis with the added masses obtained by a 'simplified' method. For the particular structure analyzed in this study, it is found that the member forces using the 'simplified' method are slightly higher than the analysis using the finite element method.