Optimization of Numerical Models for Environmental Dynamics in Closed Water Basins

湖泊、内湾等封闭水域，由于缺乏与外界淡水的水交换，水质问题严重。因此，迫切需要建立一个可靠的数值模型来定量预测流动和污染动力学行为。从实际和经济的角度来看，数值方法似乎是有效的。然而，由于模型具有由许多模块组成的多重结构，即数值方案、湍流模型、污染源、汇和输送等，并且每个模块都需要对真实现象进行简化，因此要大幅度提高数值模型的精度并不容易。到目前为止，每个模块都提出了许多模型，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的数值模型，使其既能定性又能定量地预测流量和污染。研究可分为基础部分和应用部分两部分。在基础部分，比较研究了各种数值格式、湍流模型和污染输运模型，重点研究了守恒、数值粘度和CPU时间。在应用部分，针对实际封闭水域。我们研究了不同空间尺度的不同区域，如琵琶湖（日本最大的湖泊）、真司湖和中海湖（与河流相连的咸淡湖泊）、阿古湾和地下区域。通过这些应用数据，我们可以阐明数值模型的优缺点

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