COMPARATIVE STUDY ON NUMERICAL SIMULATION OF FLOW AROUND A VERY LARGE FLOATING STRUCTURE

未来对海洋空间利用的需求将越来越大。大部分浅于20米的沿海地区已被填海用于工业和住宅用途，未来有必要利用更深的地区。日本目前正在进行一项利用海洋空间的大型浮式结构的可行性研究项目。浮式构筑物在某些方面优于填海造岛，例如对海洋环境的影响较小。然而，由于其规模与现有的规模相比是巨大的，因此在施工前必须对海湾内的这种结构进行环境评估。本文研究了大型浮体湾内流动与扩散的数值模拟方法。通过对比计算，讨论了方法的有效性和准确性。(1)采用有限差分法（ADI法和Leap-Frog法）和有限元法在二维平面上对具有大型浮体结构的矩形湾的流动和扩散进行了模拟比较。(2)对同一问题采用了多级方法，并与二维计算结果进行了比较。(3)将潮汐池浮体结构模型的流场可视化与计算结果进行了比较。以东京湾某大型浮式结构为例，进行了二维和三维仿真。(4)比较了二维模型和多级模型的潮流和剩余潮流。(5)比较了海湾内河流COD（化学需氧量）的扩散规律。研究结果总结如下：(1)二维结果等价于多层解的纵向积分。(2) adi -法在数值稳定性上优于Leap-Frog法，其时间步长较长。而跳蛙法的编程要比adi法简单得多，两者的数值精度几乎相同。(3)大型浮体结构对海湾内水流和扩散的影响小于填海造岛。(4)多级模型比二维模型更能在较短的计算时间内得到COD的稳定解。少

Needs for ocean space utilization will be increased more and more in future. Large part of coastal area shallower than 20m has been reclaimed to utilize for the industrial and residential purpose and the utilization of deeper area is necessary in future. A feasiblity research project on large-scale floating structures for ocean space utilization is now being conducted in Japan. Floating structures are superior to a reclaimed island in some aspects such as having less impact on the ocean environment. Because the scale is huge compared with those which now exist, however, environmental assessment of such a structure in a bay is necessary before construction. This cooperative study deals with the numerical methods used for the simulation of the flow and the diffusion of a bay with a large-scale floating structure. The validation and the accuracy of the methods are discussed by the comparative calculations. The studies covered during two years are summarized as follows :(1) Simulations of … More the flow and diffusion of a rectangular bay with a large-scale floating structure is compared by the finite difference methods (ADI method and Leap-Frog method) and the finite element method in two-dimensional plane.(2) A multi-level method is applied for the same problem and the result is compared with two-dimensional calculations.(3) Visualization of the flow around a model of the floating structure in a tidal tank is compared with the calculations. As an example of more practical case, two-and three-dimensional simulations are conducted for a large-scale floating structure in Tokyo bay.(4) Tidal currents and the residual currents by two-dimensional methods and a multi-level model are compared.(5) The diffusion patterns of COD (Chemical Oxygen Demand) from rivers inside the bay are compared by the methods.Results obtained from these studies are summarized as follows :(1) Two-dimensional results are equivalent to the vertical integration of the multi-level solution.(2) ADI-method is superior to Leap-Frog method in the numerical stability, it can take longer time-steps. Programming of Leap-Frog method, however, is much easier than ADI-method and the numerical accuracy is almost same between them.(3) A large-scale floating structure causes less effects on the flow and the diffusion in the bay than a reclaimed island.(4) A multi-level model is more effective to obtain the steady solution of COD in a short computational time than two-dimensional methods. Less