Nonlinear diffraction loads by bottom-mounted and truncated circular cylinders were simulated in a 3-dimensional wave basin. The linearized free surface condition was imposed on the mean free surface, while body boundary condition was exactly satisfied on the instantaneous wetted surface. Open boundary condition was treated by introducing an artificial damping and potential stretching. The imposed conditions were tested by simulating an Airy wave for long duration and proven to be efficient. The present scheme was verified by simulation of a linear wave load. It was extended to the computation of the diffraction loads acting on the cylinders by applying the exact boundary conditions on the body surface with linearized free surface conditions. The diffraction loads were found to contain higher order harmonics up to 3rd order.
Diverse efforts have been made to predict the wave loads on the cylinders. Linear diffraction theory was treated by Havelock(1940) for deep water and generalized by MacCamy & Fuchs (1954) lor the various depth. In order to overcome the limitation of the linear theory, nonlinear prediction by perturbation in frequency domain or time domain has been developed, for instance, by Chakrabati (1975), Molin (1979), Eatock Taylor & Hung (1987), Kim &Yue (1989) and Isaacson &Cheung (1992). However, these approaches do not account for the extreme wave environment which may exist in the nature. The fully nonlinear wave problem was advanced by Isaacson (1982), Lin (1990) and Yang & Ertekin (1992). Simulation of wave field due to surface piercing structure is rather complicated, especially under the nonlinear circumstances. The present numerical simulation for nonlinear loads acting on the bottom-mounted and truncated circular cylinder was accomplished by applying the exact body boundary condition but linear free surface condition. Numerical simulation of wave loading is in general affected by treatment of the open boundary.
