Naval Architecture Principles - 船舶原理 CHP 5 - Part2.ppt

* 5.7 Form (Eddie-Making) Resistance Previously, we made an assumption that the friction resistance coefficient of a ship (or a model) is the same as that of a smooth flat plate with the same length (Re) wetted surface area; namely, the friction resistance of a ship is the same as that of a flat plate with the same length and wetted surface area. In generally, this assumption is approximately correct. However, a careful investigation has shown that there are differences between the friction resistance of a ship and that of a plate with the same length wetted surface. Usually, the friction resistance of a curved surface object is greater than that of a flat plate with same length wetted surface. Their difference is called the form resistance or form drag. The form drag consists of 3 parts. Eddy-making Resistance; the curvature causes the pressure change along the ship. Due to the viscosity, the pressure change will cause the flow separation from the surface, generate eddies. Energy is fed into eddies, and the resulting resistance is called eddy-making resistance. Main contribution to the form resistance is made by eddy-making resistance. For a low speed ship, it is important to avoid the abrupt change of the hull in order to minimize the eddy-making resistance. 2. The curvature of a ship (or a model) will change the local velocity along the ship. Since the path along a streamline from bow to stern is longer on a shaped body than on a flat plate, the average velocity along a ship V. Thus 3 Interaction between viscous wave-making resistances, which is very complicated. It is a research topic in Marine Hydrodynamic and ship-model test. The increase or decrease of resistance due to the interaction are classified into form drag. Sometimes, some items may be directly classified into wave-making resistance. It is understood now that why the difference between the total resistance coeff. frictional resistance coeff. is called the residual coef