| Rapid growth of urban population growth leads to the shortage of land in the city,resulting in a significant increase in building density;on the other hand,with the increasement of private motor vehicles in the city,there are more exhaust emissions,which makes the urban air pollution problems increasingly serious.Street canyons are the basic structural unit of the urban atmospheric environment,and the air quality in the micro-environment of street canyons directly affects the urban atmospheric environment.Therefore,in recent years,many people have studied the different characteristics of the street-valley environment.However,most of the studies were conducted on isolation canyons,and only a few studies were associated with the influence of the impacts of the distances between the blocking and upstream buildings of street canyons(D)on characteristics of airflow and distribution of pollutant in the street canyons.As blocking buildings often exist in urban area where also the canyons exist,it is necessary to find the effect of different features of D on the air quality in the street canyons and reduce the impacts of traffic pollutants on pedestrians in the street.In this paper,CFD is used to study the influence of the distance(D)on the flow field and the distribution of pollutant concentration,which can provide reference for more accurate prediction of air quality and design principles for more reasonable town planning.The numerical simulation is set for the airflow and pollutant-distribution in the cayons with four typical layouts under the same wind flow profile function when D changes.The horizontal distribution of airflow field and pollutants at several typical positions and the vertical distribution at typical heights are discussed.At the same time,the street canyons are divided into three regions,furthermore the average velocity,turbulence intensity and the dimensionless concentration of pollutants in the space below the height of the buildings beside the street and 2m are counted.The simulation results show that the change of D affects the airflow and the distribution of pollutants in the street.When the canyon buildings are in the determinant layout,the average concentration of the pollutants in the street will increase firstly and then decrease with the increase of D regardless of the relative layout of the upstream buildings,while the change of the zone below 2m is just opposite.When the average concentration of pollutants in the street is the highest,the corresponding value of D is 90 m.It means that the increasement of D(when D < 90m)is unfavorable to the indoor air quality of buildings beside the street,however the increasement of D(when D > 90m)is harmful to people on the street.So,we suggested the distance between the upstream buildings and the canyon buildings needn't to be too long for urban planning.For the staggered layout of the street building,when the relative layout of the upstream buildings is determinant,with the increasement of D,the average concentration of pollutants in the street shows a fluctuating change--Increase first and then decrease and again,the average concentration is minimal when D is infinity;when the relative layout of the upstream buildings is staggered,with the increasement of D,the average concentration decreases firstly and then increases and then decreases again,the average concentration is also minimal when D is infinity.What's more,the numerical results of horizontal distribution of airflow field and pollutants at several typical positions and the vertical distribution at typical heights shows that,in the space below 2m,the pollutant concentration is highest if D is infinity.It suggests that the existence of the upstream buildings is conducive to the the proliferation of pollutants in the near-ground space,conducive to the streets of the valley to improve the breathing environment,which also improve the breathing environment in urban canyons and benefits the pedestrians health. |
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