Freak (Extreme, rogue) waves are extremely large water waves in ocean and may occur all over the world sea area. Such a wave may lead to damage of coastal and offshore structures. Accurate prediction of extreme wave-induced forces and motions is of importance and necessaries for researchers and engineers for the purpose of structure design and disaster prevention. Due to the complexity of nonlinear wave-structure interactions related with distorted free surface and relatively large amplitude of structure response, a great deal of effort is required to investigate the physics. Here, a Computational Fluid Dynamics (CFD) model has been developed to study focused wave impact on a floating structure and validated by a newly designed experiment. Focused waves are generated based on the mechanism of wave focusing in a two-dimensional wave tank. In the experiment, a model of a box-shaped floating body with a small freeboard is adopted in order to easily obtain green water phenomena. The computations are performed by a Constrained Interpolation Profile (CIP)-based Cartesian grid method. The CIP algorithm is adopted as the base scheme to obtain a robust flow solver of the Navier-stokes equation with free surface boundary. An improved THINC scheme (THINC, tangent of hyperbola for interface capturing), the more accurate THINC/SW scheme (THINC with Slope Weighting), is applied as the free surface/ interface capturing method. Main attentions are paid to the three degrees of freedom (3-DOF) body motions, pressure domain around the structure and nonlinear phenomena, such as water on deck. The highly nonlinear wave-structure interactions, including significant body motion and water on deck, are modeled successfully in comparison with experimental measurements. It is concluded that the present model with the aid of the CIP technique can provide with acceptably accurate numerical results on the route to practical purposes
