Numerical Study of Green Water on a Freely Moving Object

Green water effect is a highly nonlinear free surface problem. It can cause severe damage on floating marine structures. Therefore, it is of great importance to be able to predict the green water occurrence on the freely-moving floating marine structures and the associated dynamic loads acting on them. In the proposed paper, a solution scheme for solving the green water problem in the numerical wave tank with commercial CFD software FLUENT will be presented. This solution scheme consists of the decompositions of the computational domain, and an automatic updating method for the motion of the moving object. The numerical wave tank is set up by using a plunge-type wave-maker to generate waves and by adding source terms in the dissipation zone to eliminate the reflected waves. The computational domain is divided into dynamic zones, stationary zones, dense rigid zone that is useful in capturing the green water details, and the fine mesh zone near the free surface. The mesh in the vicinity of the moving object (dense rigid zone) is refined to better model the green water problem. The motion response of the object is obtained based upon the solution of the general equations of rigid body motion (6-DOF) and the hydrodynamic forces acting on the object. In order to compare with the existing experimental data, a 2D object is used in the numerical study. The motion of the object is restrained from surge, which means only heave and pitch motions are allowed. The details of the green water occurrences including water running up onto the deck, water propagating on the deck, water impacting the vertical wall, water running up along the vertical wall, wave breaking and water spray have been modeled successfully. The quantitative analysis on the impact load on deck and the upper structure has also been carried out. The comparison of the numerical results and experimental measurements shows that the current numerical solution scheme is robust and reliable in modeling green water problem.