Hydrodynamic forces on a semi-displacement ship at high speed

Abstract

A numerical method based on 2D + t theory (two-dimensional plus time dependent theory) was developed to study the steady and unsteady hydrodynamic problems of a semi-displacement ship with round bilge at high forward speed. The ship was forced to oscillate in heave in the unsteady problem. No incident waves were present. In the 2D + t theory, the original three-dimensional (3D) problem was simplified as fully nonlinear two-dimensional (2D) time-dependent problems in cross planes. A boundary element method was used to solve the 2D problems. The non-viscous flow separation from the round bilge of the ship hull was simulated. The pressure on the hull surface was evaluated and the sectional hydrodynamic vertical forces were obtained. In the steady problems, the sectional vertical forces along the ship were calculated when the ship was displaced to different vertical positions. In the unsteady problems, the sectional added mass and damping coefficients along the ship length were evaluated. The present numerical results were compared with published experimental results and existing numerical results. Good agreement was achieved between the present calculations and the experiments, although some discrepancies near the bow and the stern were observed. The three-dimensional effects at those positions could be the reasons for the discrepancies. In the unsteady problem, the interaction between the local steady flow and unsteady flow were automatically included and the nonlinearities in both steady and unsteady flow were considered. The present method can be generalized to the seakeeping problem in which a semi-displacement ship encounters incident waves.