NUMERICAL CLARIFICATION OF THE IRREGULAR FREQUENCY PHENOMENON OF TIME-DOMAIN BEM FOR FLOATING BODY HYDRODYNAMICS

摘要

The irregular frequency phenomenon relating to the numerical prediction of hydrodynamic forces on free-surface-piercing floating bodies utilizing transient free surface Green function method is discussed in this paper. Two kinds of boundary integral equations are investigated. It is found that the mixed sources and dipoles distribution model can give stable numerical results without exciting an influence of irregular frequency. In contrast, the source distribution model always exhibits irregular frequency effects in the source strength and the induced fluid velocity. The theoretical solution of the initial-boundary value interior problem that relates to the definition of the source distribution model is obtained recently (Duan, et al 1999). By this means, the influence of irregular frequency can clearly be seen, which provides the basis to explain the numerical unstable phenomenon of the source distribution model. To obtain the fluid velocity without the influence of the irregular frequency, a new source distribution model with redefined interior problem and a numerical scheme for computing the source strength and fluid velocity are proposed. Numerical validations are performed for both two- and three-dimensional bodies, it is shown that the presented method is very stable and accurate, and it is very suitable for the linear and nonlinear hydrodynamic force calculation of offshore structures.