Numerical Analysis Of Pollutant Dispersion In Street Canyon Based On Porous Medium Model

In recent years,the number of the vehicle of the big cities in China is increasing with the high speed.It brings prosperity to the country economy,but it also contributes much to the air pollution in some of the big cities, According to the results of much reseach, vehiclar exhaust is the main source of air pollution in some big cities. Street canyon means the street and the continuous buildings around the street in the city,the character of the urban street canyon has influence on the pollution dispersion in the street. So the city blocks in the valley of pollutants is particularly urgent research.The research object of this paper is street canyon in the city, introduces the characteristics of the street canyon in the city detailed, including:geometric features,flow field characteristics and convection diffusion characteristics of the pollutant, and analysis the basic theory of the spread of pollutants convection, introduces the convection model of the pollutants in the street canyon. It is a complicated process for the vehiclar exhaust to disperse in street canyon.Many factors may affect this process such as the source intensity,meteorological factors and street geometry and so on.Thie paper used the methods which combination of the numerical simulation and the experimental observation, a appropriate size of the physical mode of the street canyon was built up which was based on the distribution of the roads and buildings. Because of the great amount of caleulation resulted from complexity of buildings besides urban roads, buildings and roads were parameterizedly treatedas Porous medium with momentum conflux under extend design conditions, and on this basis using Gambit software grid partition, combined with city in the wind speed logarithmic atmospheric boundary contour lines, define the right initial condition and boundary conditions, and can meet the requirements of the convergence are iterative coefficient, through the Fluent software iterative calculation get city blocks in the canyon pollutants convection expansionSpeed fields and concentration rate obtained from the simulation show:(1) when the wind is at the same, pollutant diuffsion of urban roads along river was enhaneed with the inerease of traffie density. (2) When the angle between thewind direction and the street direction presents between 0°, the exhaust diffusion was enhanced, and the pollution concentration reduced,but pollutants accumulated down to the wind direction, leading to serious excess of the pollutant coneentration beyond standards in some areas. When the wind direction and the street direetion are vertieal, the part pollutant diuffsion is obviously improved and the partly exeess of the pollutant concentration beyond standards cannot easily be formed, but the influence on buildings along the street becomes greater.In order to verify the correctness of the numerical results, this paper chose the same streets on field test observation, through the streets in the wind and leeward side on different height of each set up four views were observed, in five observation of all the observation points within the concentration of CO measured and get the change rule of the concentration of CO, and field test of measured data and the numerical calculation results were compared, the results found that this paper established mathematical model and the correctness of the numerical calculation.The results of the present paper have developed a better understanding of the mechanism for the convection and dispersion of vehicle pollutants within urban street canyons, which is helpful for the traffic emission control, the air quality control network, and urban street planning.