Study On The Wind Environment And Pollutant Dispersion Of Street Canyons In Three-Dimensional Urban Buildings

In recent years,with the rapid development of China's economy and the acceleration of urbanization,the number of motor vehicles in China has continued to rise,although the increase of motor vehicles has brought a lot of convenience to the people,it have become the main source of air pollution in urban neighborhoods.At the same time,the urban construction land is becoming increasingly tense,and the density of the building complex is greatly increased.Many "street canyons" are formed between tall buildings,which seriously affect the diffusion and dilution of pollutants in the neighborhood.It makes traffic pollutants gather in the streets,and causes great harm to the health of passers-by.Therefore,it is of great significance to study the wind environment and pollutant diffusion law in three-dimensional urban street canyons for improving the air environment of urban blocks and improving the quality of life for urban residents.In this paper,CFD numerical simulation technology is used to analyze the street wind environment and pollutant diffusion in the micro-scale range under the three-dimensional urban building layout.The simulation process selects the standard-k? model and component transport model.First,the air environment of a block under two single factors of building layout and relative height is studied,then further analyze the air environment of the block under the complex architectural pattern of a certain city.The simulation results show that:First of all,in the three urban architectural layouts,the airflow around the buildings in the symmetrical layout is relatively uniform.The airflow in the interlaced layout is more strongly hindered by the building,and the turbulence in the canyon is very strong;Affected by the layout,when there are blocked buildings in the upper reaches,the pollutants mainly gather between the leeward surface and the central surface of the street.When the upstream unobstructed buildings are concentrated,the pollutants mainly gather between the windward and central surfaces of the street;The flow velocity in the first canyonsatisfies the trend(Layout I < Layout II< Layout III),while that in the second canyon satisfies the trend(Layout II < Layout III < Layout I);for the concentration of pollutants in the first canyon satisfies the trend(Layout III<Layout I<Layout II),while that in the second canyon satisfies the trend(Layout II<Layout III<Layout I);The concentration distribution of pollutants in the street canyon is affected by the double factors of flow velocity and vortex flow structure;The average concentration of pollutants in the second street canyon is greater than that in the first street canyon,there are differences in pollutants in different street canyons with the same layout.Secondly,in the four street canyons with different heights of upstream and downstream buildings,the eddy structure and flow velocity in different types of target street canyons are different,which have an impact on the pollutant concentration distribution in target street canyons.Among them,the "concave" shaped street canyon will have pollutants diffusing into the upstream street canyon,while the other three types of Street canyons will not have pollutants diffusing into the upstream street canyon,and pollutants will only spread to the lower canyons;Within the statistical range of each canyon,the air velocity in concave canyon is the lowest,while that in ascending and convex canyon is the highest.For the concentration of pollutants in each street canyon,the concentration of pollutants in the convex street canyon is the highest,while the concentration of pollutants in the descending street canyon is the lowest;Among the four types of street canyons,falling street canyons are most conducive to the spread of pollutants.Finally,for the more complex urban street types,when the wind direction is incident from the north,for the different typical height planes,the contaminants show successively linear?linear+partial cluster?cluster?fractured cluster along the height directions.Controlling the continuous interface of buildings to guide the lateral flow of airflow in street canyons or setting appropriate open space upstream to increase the flow corridors can effectively change the diffusion of pollutants in street canyons;Under the northwest wind,the airflow in four main roads has changed greatly.At 1.6m and 10 m near the ground,the parallel airflow in branch roads turns into more obvious pronounced flow,where the pollutants accumulate in the central location.At the altitudes of 20 m and 30 m,the increase of upstream open space enlarges the windward area of incoming airflow,which makes pollutants diffuse to the east side of block buildings as a whole;The residence time of pollutants in each street is different in different incoming airflow directions,wind direction changes have different effects on the concentration distribution of pollutants in each street.The pollutant diffusion in each canyon is not an isolated system,but an interconnected organic whole.