Study On The Law Of Migration And Diffusion Of River Pollutants Based On Random Walk Theory

Intensifying water pollution,declining water quality,and water ecological deterioration has become one of the important factors for restricting the sustainable development of regional economy,water environment management is imminent,but the diffusion regularity of pollutants into the water is an important theoretical base of water environmental pollution improvement and relieving ecological deterioration,as well as an important issue which urgently needs to study.This paper based on the pollutant diffusion regularity in inland river as the research object,by the application of layering and striping flow model,weighted auto-upwind scheme random walk model and the unsteady flow model in the flow field simulation,as well as weighted auto-upwind scheme Monte Carlo method and particle tracking method in pollutant diffusion simulation,the typical examples and the research instance of flow field and pollutant diffusion regularity are studied,respectively,and pollutant diffusion regularity of Wenyu River downstream is obtained through the comparative analysis.Layering and striping flow model is established,and the study area are layered and striped based on the energy equation,by converting the 3-D problems into 2-D and finally converting into 1-D,thus to avoid the computational complexity of 3-D flow field.The research of flow field simulation of real samples Wenyuhe Reservoir is conducted based on the compare of the typical example calculation results and the measured data;Due to compute grid can be encrypted freely,therefore artificial control the calculation precision to ensure the rationality of the calculation results.Weighted auto-upwind scheme random walk model is established using 9,27 point weighted auto-upwind scheme Monte Carlo method to disperse and solve the convection diffusion equation,by contrast,the method has the advantage of the distribution of the weight coefficient,the skew up-wind considerations,etc;Applied to the typical example 2-D and 3-D numerical calculation with high Reynolds number,that results show that the method maintains the characteristics of auto up-wind and skew up-wind,Reynolds number can work to much and not make the equation degrade into simple up-wind,the problem of calculating false turbulent flow and laminar flow can be solved by increasing the viscosity in the past,and the method is easy to solve the problem of 3-D.On the basis of the research on the case river of median water year(P=50%)steady flow and unsteady flow numerical calculation,the weighted auto-upwind scheme random walk model established is applied in the analysis and calculation of flow field distribution near outlet under the integrated sewage river of steady flow and unsteady flow of Wenyu River downstream,providing data basis for the study of the migration of pollutants diffusion.Carried out pollutant migration and diffusion simulation based on two-step process Lagrangian Particle Tracing Method,respectively the raphe jet pollution emissions in the static fluid and turbulent fluid,as well as the side pollution discharge are simulated.Flow field distribution is given first with finite difference method,and then to simulate convection-diffusion phenomenon with Particle Tracing Method.The stability of the model and the rationality of simulation results are verified by the above typical example research.It is an important method for depth study of diffusion mechanism as the pollutant migration and diffusion process can be simulated by this method which follows the thought of the theoretical mechanics.Taking Wenyu River downstream as a research instance and all flow field distribution results by the flow field calculation model as the data base.The typical pollutants as COD and NH3-N near the comprehensive sewage river outfall for steady flow and unsteady flow conditions are simulated based on the weighted auto-upwind scheme Monte-Carlo Method and Particle Tracing Method.Then summarize the methods of the pollutant migration and diffusion under different conditions,get the rule of pollutant migration and diffusion of the river in different water conditions.Research results show that the unsteady flow of discharge increase gradually can make the downstream pollutant migration and diffusion distance decreases,and greatly alleviate the pollution condition of downstream river channel.Development flow field simulation calculation based on layered strips flow model,weighted auto-upwind scheme Random Walk Model and unsteady flow model can provide the data base for the pollutant migration and diffusion simulation.And the river pollutant migration and diffusion simulation based on weighted auto-upwind scheme Monte-Carlo Method and two-step process Lagrangian Particle Tracing Method contribute to a deeper understanding of pollutants diffusion mechanism and the interaction mechanism with flow field,the results more reflect the physical essence of diffusion.Therefore,the river pollutant migration and diffusion random simulation study in different water conditions based on random walk theory not only can provide a basis for improving the status of downstream river pollution,and can provide theoretical support for reservoir operation and environmental management and decision,also has the positive significance for enhancing the environmental and ecological science level and pushing the river ecological sustainable development.