Hydrodynamic Forces Acting on the Three Dimensional Body Advancing on the Free Surface (1 st Report)

Abstract

The previous papers, the firt and the second reports, described a new method to calculate the hydrodynamic sway force and yaw moment acting on a ship with forward-speed oscillating on the free surface, and verified its validity by comparing the hydrodynamic derivatives predicted by the present method with the experimental ones in both cases of a flat plate and a Wigley model.In this paper, the method is applied furthermore to predict the hydrodynamic derivatives of ships with realistic hull shapes, namely, a tanker ship, a Series 60 model, and a container ship. In case of a container ship, in particular, the hydrodynamic derivatives related to roll as well as sway and yaw were computed, and then compared with those obtained by model experiments. As the result, it is shown that the present method predicts very well the effects of ship's forward-speed, which have never been successfully explained by the so-called Strip Method, on the hydrodynamic derivatives.In the previous papers, Sommerfeld's radiation condition, that is to say, ∂φ/∂t+C∂φ/∂l=0 was imposed at the open boundary which is placed at a distance sufficiently far from the ship in order to limit the fluid region to be treated in the numerical calculation. In this paper, however, the condition is replaced by a more relaxed condition ∂φ/∂t+C∂φ/∂l=E in which E denotes the numerical error of the left-hand term of the above-stated Sommerfeld's radiation condition.By making use of the relaxed condition instead of the exact radiation condition, it becomes possible to make the waves, which are generated on the free surface and propagate toward the outside region, pass smoothly through the open boundary.