Numerical Study Of Wind Field And Pollutant Dispersion In Valley Cities

The complicated terrain of valley cities can significantly affect the diffusion of air pollutants and form unique pollutant diffusion processes and pollutant distributions.With the development of industry and the acceleration of urbanization process,air pollution has become more and more serious.Recent years,valley cities receive much attention from many scholars because of their particular wind fields and pollutant distributions.However,most of these studies concentrated on the pollutant distributions but limited on the relationship between complex terrain,wind fields and pollutant dispersion in valley cities.Therefore they are difficult to guide general air pollution forecast and reduction of air pollutants in valley cities in which the relationship are of key importance.In theory,CFD methods have unique advantages in the study of wind fields and pollutant dispersion in valley cities.At the same time,it is rarely to see relevant applications.In this paper,a CFD model,which was expected to discuss the relationship between complex terrain,wind fields and pollutant diffusion in valley cities,was developed to study the surface wind fields and pollutant dispersion in Lanzhou City.By analyzing the simulation results,the reliability and validity of the CFD methods in the study of wind fields and pollutant dispersion in complex terrain valley cities were revealed,while suggestions on air pollution control in valley cities were put forward.This paper also provides valuable and more precise reference for the applications of CFD methods to air pollution control,air pollution forecasting,industrial area layout,etc.in valley cities.Firstly,a complex terrain mesh model was drawn.For every 200m×200m grids in the whole study area,the average altitudes were extracted using Google earth elevation extraction software.Digital terrain of study area was then obtained by converting these altitudes to coordinates and importing them in Gambit software to generate the mesh for the CFD model.Adjacent points in Gambit were connected by journal file written in program,and volume cells were built on the basis of faces mesh.Secondly,according to the simulation results of wind fields,the relationship between terrain and wind fields was analyzed.The wind fields,which were used to summarize the typical wind fields characteristics and the interaction between the terrain and the wind fields,were simulated using the large eddy model.In addition,refined terrain-modified wind fields were obtained by the CFD method,which proves that it is reliable to describe and study the refined surface wind fields in valley cities using CFD methods.Finally,according to the simulation results of pollutant diffusion,the relationship between terrain,wind fields and pollutant dispersion was analyzed,which was expected to be used to put forward suggestions on air pollution control in valley cities.The pollutant dispersion was described by the mixture multiphase model basing wind fields,and the validity of the proposed approach was verified by the good agreement between the simulated distributions of air pollutant in Lanzhou City and those shown in the satellite photo.It is found that the distributions of pollutant in the terrain of the valley cities are diversified by the complex terrain.Combined with the simulation results,it is easy to find that the impact of special wind fields on the dispersion of pollutant should be considered in the process of industrial layout.These results proved the reliability and advantages of the CFD method in the study of the wind fields and pollutant dispersion in valley cities.At the same time,the methods used in this paper provide valuable and more precise reference for the applications of CFD methods to air pollution control,air pollution forecasting,industrial area layout,etc.in valley cities.