Study On Numerical Simulations Of Hydrodynamics And Pollutant Transport In River And Shallow Lakes

With the acceleration of urbanization and industrialization process, water pollution in rivers and lakes are very severe in our country, which have become the key issues related to the future development of our social and economic. One of the main measures to control the water pollution is to analyze and forecast the changes of water flow and quality by using numerical simulation method, which can provide the necessary technical support for the policy-makers of environmental management. Although many scholars have carried out a large number of theoretical research in the field of water flow and water quality simulation and great progresses have been made, there are still many issues need further research to deal with the problems which include irregular topography of the area, multifarious climatic conditions, numerous pollutant index, complex relationships between different pollutants, complex flow regimes, low computational efficiency, instability in calculation process and so on. Therefore, it is necessary to explor and analyse the hydrodynamic characteristics of flow in rivers and lakes, and develop a high-performance, efficient and stable flow-water quality coupled model.In this thesis, we studied the several key scientific problems of numerical simulation in flow-water quality of rivers and shallow lakes, then developed a high-performance two-dimensional mathematical model based on upwind finite volume method, and chose the large east-lake water system as the research object to analyzed the influence on water pollution transport and transformation processes in connected lakes. Moreover, we derived the corresponding relations between the pollutants concentrations and flow field in the natural conditions and water diversion project. Relevant results can be generalized in the lake water network connectivity project, and these methods have good engineering applications prospects. The main work and innovation includes:(1) In view of the problems such as irregular terrain, weather conditions, water flow patterns, accuracy, computational efficiency and so on, we presented a two-dimensional flow numerical simulation model for rivers and shallow lakes. A weighted surface-depth gradient method which considers the local Froude number is used to improve the adaptable ability, and this method can accurately simulate complex flow regimes such as subcritical, supercritical, transcritical, discontinuities and so on. Meanwhile, we analyzed the negative depth problem caused by flux calculations, and proposed a water flux correction method that can effectively solved the negative depth problem in the process of numerical calculation, then ensured the water amount conservation and improved the stability of numerical simulation which provided accurate flow field support for pollutant transport simulations. (2) Considering the unsynchronized problem of the calculation space-time in the1D-2D coupled numerical model, we proposed a simultaneous solution which based on the Riemann approximate solution and then developed the corresponding1D-2D coupled numerical model for modeling the river flow and plane flow. In this coupled model, the numerical fluxes at the interface of one-dimensional, two-dimensional and the coupling region are calculated by using the approximate Riemann solver. This method got the time synchronization and offered the solutions to accuracy difference occurred in the independent solving process. Therefore, the coupled numerical model achieved the water flow numerical simulation under conditions of rivers and lakes connected.(3) Numerical simulation of flow-transport equations usually faced the problems aboat complex topography, numerical damping effect and numerical oscillation. In order to deal with these problems, we developed a flow-pollutions coulped model for simulating pollutants transport in shallow water flows. In this coulped model, the water flow and concentration of pollutants fluxes were solved by using HLLC-type approximate Riemann solver, which can effectively capture the flow pattern and concentration of pollutants in intermittent conditions and reduces the numerical damped phenomenon. Therefore, the model provided the reliable theoretical support for researching the transport of water pollutants under different hydrodynamic conditions.(4) Considering that the numerical simulation of water flow and pollutants transport in large-scale computing area are faced with the contradiction between the computational efficiency and grid resolution, we improved a grid adaptive dynamic encryption method which can dynamically refine or sparse grids numbers according to the water gradient or pollutants gradient characteristics. This method could improve the space accuracy of numerical simulation and reduced the number of computing cells, and then implement the balance between accuracy and computational efficiency of the numerical model.(5) A CPU multi-threaded parallel computing pattern, by using JAVA language, was proposed for numerical simulation of water flow and water quality. The computational domain is divided through domain decomposition techniques, and virtual cells were introduced to seamlessly connect the different decomposition areas, which ensured the reasonable allocation of CPU resources in the calculation process, and then a multi-threaded parallel computing technology was used to accelerate the process. The parallel computing pattern greatly improved the computational efficiency of the flow-pollutants model, and effectively dealt with the problem which the calculation time is too long in practical projects.(6) The proposed flow-pollutions coulped model was used to analyze the great East Lake area, and carried out the application research of flow and water quality simulation in water environment planning. First, we adopted the measured data to achieve good parameter of the flow-pollutions model, then studied the hydrodynamic properties of connectedlakes, and contrastively analyzed the characteristic of the flow field distribution under the natural conditions and water diversion project, explored the quantitative relationship between the water pollutions variables and flow variables under natural circulation condition, searched the main factors of the water pollutants which affected by water flow, thus revealed the regularities of pollutants transport with the change of the flow field distribution. Finally, we used the water diversion routes, discharge and time variable parameter optimization, in the water diversion project, as the breakthrough, derived the curves between the pollutants concentration and water flow field under the different water diversion conditions, and clarified the relations between water flow, water transport and water quality improvement. These studies can provide an important scientific basis for water environmental planning and the water pollution control programs implementing.