船舶大幅横摇粘性流场多尺度有限体积法研究

Vessels are susceptible to the environment while sailing, which will induce rolling, and the large amplitude rolling is the most dangerous roll motion. The number of the ship damaged due to the large amplitude rolling is about one third of the total amount by statistics. The large amplitude rolling is one of the most difficult problems in the research of seakeeping, for the main reason is the understanding and analysis of the viscous flow field caused by the large amplitude rolling is limited by the present methods and approaches used in the research, which are disabled in capturing the details and analyzing the mechanism of the flow field. Therefore, the prediction of the hydrodynamic force of the ship in large amplitude rolling is short of reliability. The viscous flow field is classified into macro-scale flow field and meso-scale flow field by the multi-scale analysis method in the present research, which are investigated correspondingly. A new finite volume method is developed based on the new theory and adopted as the macro algorithm in order to simplify calculation, while the base and correction function on the meso-scale flow field are formed in order to capture the flow characteristics. Then, these two flow fields are combined with the coefficient. In addition, the investigation on the determination of the separation point, vortex generation mechanism, kinetic characteristic, integral convergence and elimination of the high frequency oscillation of the pressure solution are performed for improving the formed code. At last, a multi-scale finite volume method code is developed, which is more accurate than Discrete Vortex Method and RANS approch in the viscous flow field simulation in large amplitude rolling, and model test with PIV is adopted in order to validate the method.

船舶在外界环境的激励下易发生摇荡运动，而大幅横摇又是摇荡运动中最为危险的一种。据统计，全球每年因大幅横摇而全损的船舶约占全损船舶总数的33%。大幅横摇运动是耐波性研究中最为困难的问题之一，其主要原因是目前的研究方法和手段制约了对于大幅横摇运动所诱导粘性流场的深入认识及了解，导致流场细节缺失，流动机理不清，因而对于大幅横摇船舶水动力的预报缺乏足够精度。本课题基于多尺度分析方法，将流场分为宏观和细观尺度分别研究，基于新理论建立一种新的有限体积法作为宏观算法以减少计算量，同时在细观尺上建立准确描述流动特征的基函数和修正函数，再将两种尺度上的流动以等效系数的形式相耦合。并且，通过针对流动分离点确定、漩涡生成机制和近场运动特性的研究及改善积分收敛性、消除压力解的高频振荡等工作来完善算法，最终形成对于船舶大幅横摇粘性流场模拟准确度高于离散涡和一般RANS方法的多尺度有限体积法，并采用模型试验进行检验。

船舶大幅横摇是一种高度非线性的高雷诺数湍流问题，涉及到自由液面飞溅和漩涡等多尺度复杂流动现象，现有的单一尺度网格计算方法在船舶粘性流场模拟中难以获得精确结果，或者在流场细节精细模拟方面需要承受巨大的计算成本。因此，基于多尺度分析和模拟理论基础，开发适用于船舶大幅横摇粘性非定常复杂多相流的多尺度模拟方法，在船舶水动力基础研究具有十分重要的研究意义。.课题组基于现有多尺度分析理论体系和开源计算平台OpenFOAM特点，初步建立非定常多相流跨尺度普适模型——两重网格耦合计算的twoMeshFoam，通过cavity、damBreak基础算例验证了该初步模型在单相流和两相流模拟中的可行性与不足。同时，采用现有单尺度网格精细模拟方法（LES，重叠网格尺寸达到0.625mm）对S60二维模型开展大幅横摇模拟，对舭龙骨诱导流动分离和漩涡生成的流场特性进行分析，得到一些关于流动分离点判断及涡生成机制的结论，同时基于局部流场漩涡流场特点对舭龙骨附近漩涡近场的演变过程和运动特性开展分析。在此基础上，根据横摇粘性流场和漩涡多尺度特性对上述多尺度模型进行改进，先后解决了流场水气分数耗散失真和数值虚假漩涡问题，形成改进后的twoMeshFoam计算模型。针对单舭龙骨圆柱模型开展多尺度模拟，对比不同横摇周期、幅值和不同舭龙骨高度的漩涡运动规律，进一步得出舭龙骨诱导流动分离、漩涡产生以及漩涡运动特性的普适规律。并且基于单尺度精细模拟与多尺度模拟对比，验证了所采用方法的可行性和有效性，有望进一步改进后应用到三维模型和工程问题中。此外，开展了单舭龙骨圆柱模型的横摇试验，同时得到了宏观层次的模型横摇力矩和微观层次的PIV流场测试信息，通过对比不同横摇工况的力矩和PIV速度及涡量场，验证了多尺度模拟结果所形成的规律。