基于NURBS表达的精确自由液面修正

提出了基于三维NURBS表达的精确自由液面修正方法。对船体曲面和甲板进行NURBS表达后,通过对任意倾斜液面下的舱容及容积心的计算以及给定舱室装载量条件下舱室内液体液面的确定,可计算出任意倾斜状态下液体自由流动后产生的倾斜力矩,从而得到精确的静稳性臂修正值。通过对不同舱型的舱室在不同装载量和不同浮态下的倾斜力矩的计算和比较分析,总结规范计算结果和精确解之间存在误差的原因,分析了误差随舱型、装载量和浮态变化的规律。该方法表明了采用精确自由液面倾斜力矩计算的必要性和工程应用价值,并给出在船体设计中如何根据实际情况进行精确自由液面修正的建议。

Precise Free Surface Correction Based on NURBS Representation

Nowadays,the influence of free surface on the stability of ship is usually calculated according to the related criteria such as IMO.Generally,the correction values are considered on the assumption that the tanks with slack liquid are cuboid-shaped with the same sizes and they are half filled with liquid. The correction values may be the same as long as the tanks have the same dimensions and same block coefficients while having no relationship with the actual shapes of tanks.In addition,the correction methods in the related criteria take no account of the effects of floatation on the correction values.So,more correction will lead the stability of ship not to satisfy the requirements of the related criteria.On the contrary,the sailing safety cannot be assured due to the insufficient evaluation of the stability.Researches indicated that differences maybe exist between the correction results by IMO and the precise ones.Because of the above reasons,it is necessary to calculate the influence of free surface on the stability of ship precisely considering the actual loading conditions.In this paper,the method of precise free surface correction based on NURBS representation is presented.Applying the NURBS representation of the hull surface and deck,the hold capacity and the center at arbitrary inclining surface can be calculated and the surface position under the specified load ratio can be determined.So the inclination moment caused by the loose liquid with any floatation can be calculated and further static stability levers be corrected precisely. Through the computation,comparison and analysis of the inclination moment of different shapes of tanks with different load ratio and floatation,the reasons for the difference between the conventional correction results and the precise ones are analyzed and the changing rules of the differences with the shapes of tanks,load and floatation are described.The results of calculation and analysis indicate the necessity and the engineering practicality of the precise correction.Some advices for the practical free surface correction during the ship design are provided.