Numerical Simulation Of 3D Flow And Water Quality On Tidal Reach Of Changjiang River Based On COHERENS

After consulting plenty of literatures about 3D numerical simulation, the methods of tidal current numerical modeling are commented, and the analysis for the development situation of tidal current numerical modeling is made. The numerical simulation of 3D tidal flow and water quality based on COHERENS is built. Because the vertical coordinate is transformed by aσtransformation, the boundary conditions of bottom and surface are simplified. The stress in momentum equation is handled by using Boussinesq assumption. Pressure terms are handled by using static water pressure assumption. In solving the equations Arakawa "C"grid is used in horizontal direction. Gravity wave water level is computed by external mode. Internal mode is used to calculated depth-resolved physical quantities. The external mode uses shorter time step, while the internal mode uses longer one. All horizontal derivatives are expressed in explicit scheme while vertical terms are expressed in implicit scheme. The advection terms are calculated by TVD scheme with supperbee limiting function. The "limiting " nutrient element is nitrogen in this water quality model, which can simulate eight water quality indexs simultaneously, including microplankton carbon, microplankton nitrogen, detrital carbon, detrital nitrogen, ammonia, nitrate nitrogen, dissolved oxygen zooplankton nitrogen.Applying the case of wind induced flow in a closed rectangular basin is to verify the model. The computed result indicates the model can preferably simulate wind induced flow in a closed rectangular basin. The model precision improve over to delamination number in vertical direction.In order to validate the water quality model, four different difference schemes were applied to simulate the concentration distribution in cones container. Four difference schemes contain upwind scheme,Lax-Wendroff scheme,TVD scheme using the superbee limiting function and TVD scheme using the monotonic limiting function. The result of simulation indicates that superbee TVD scheme is also precise scheme of the program, gives the best performance both in preserving the original shape as in maintaining the gradient of the contaminant concentration. Then the superbee TVD is chosen in the model.3D flow and water quality model is applied to predict transport and circulation of pollutant in Changshu tidal river. First, the model was successfully utilized into the simulation for the change of current field according to the different hydrological condition, the simulated values agree well with observed values. Based on current simulation, it simulate the contamination NH3 distribution and predict impact scope and extent of sewage discharge. The contamination concentration distribution is reasonable .The results show that the model is correctly adopted and the parameters chosen also are computed rationally. The works extends the COHERENS application, and provides technical support for studying Pollutants transportation in tidal river.