Optimization of Numerical Models for Environmental Dynamics in Closed Water Basins

湖泊、内湾等封闭水域，由于缺乏与外部淡水的水交换，水质问题严重。因此，开发一种可靠的、能够定量预测流动和污染动力学行为的数值模型是非常重要和迫切的要求。从实用和经济的角度来看，数值方法似乎对这一目的是有效的。然而，由于数值模式具有由数值格式、湍流模型、污染源、汇和输送等模块组成的多重结构，而且每个模块都需要对实际现象进行简化，因此要大幅提高数值模式的精度并非易事。到目前为止，针对每个模块提出了许多模型，但模型的性能还没有得到充分的阐明，本研究的目的是优化封闭流域环境动力学数值模型，使其不仅可以定性地预测流量和污染，而且可以定量地预测污染。本研究可分为基础研究和应用研究两部分。在基本部分，对各种数值格式、湍流模型和污染物输运模型进行了比较，重点考察了守恒性、数值粘度和计算时间。在应用部分，以实用的封闭水域为目标。我们研究了不同空间尺度的区域，如琵琶湖(日本最大的湖泊)、新知湖和中泉湖(咸水湖泊与河流相连)、AGO海湾和地下区域。通过这些应用数据，可以说明数值模式的优势和不足。

Closed water basins such as lakes, inner bay, etc., have serious problem on water quality due to the lack of water exchange with outer fresh water. It is therefore important and urgent require to develop a reliable numerical model, which can quantitatively predict behavior of flow and contamination dynamics. Numerical approach seems to be effective for such purpose from practical and economic points of view. However, it is not easy to increase dramatically the accuracy of numerical model because the model have a multiplex structure composed of many modules, i.e., numerical scheme, turbulence models, source, sink and transportation of contamination, etc. and each module requires simplification of real phenomena. So far many models were proposed for every module though the model performances have not been sufficiently elucidated yet.The objective of this research is optimization of the numerical model for environmental dynamics in closed water basins, which can predict flow and contamination not only qualitatively but also quantitatively. The research can be decomposed into two parts, namely, the basic part and the application part. In the basic part, various numerical schemes, turbulence models and contamination transport models were comparatively examined focusing on conservation, numerical viscosity and CPU time. In the application part, practical closed water areas were targeted. We studied various areas with different spatial scales, such as, Lake Biwa (largest lake in Japan), Lake Shinji and Lake Nakaumi (brackish lakes connected with a river), Ago Bay and underground area. Through these application data, we could elucidate advantage and shortcoming of the numerical models