| With the rapid development and extensive application of computer and numerical simulation technology,using the computational fluid dynamics(CFD)method to study the diffusion of pollutants in urban blocks has become an efficient and convenient means of research.Focus on the pollution issues in street canyons,the characteristics of pollutants can not be regard because it is the conclusive factor to the diffusion in street canyons.Different from the normal airborne pollutant who can be taken by the wind through the urban then be released,dense pollutants are limited by the characteristics of density and weight which caused the lack of following,so it has a great significance to research the diffusion pattern of dense pollutants in street canyons.Therefore,with the use of CFD,dispersion and accumulation of dense pollutant inside the street canyons with various building heights were numerically investigated,concerning separately with the source of dense pollutant in and out of the street canyons,and investigate the relationship between pollutant stratifications in pedestrian level and five canyon deviations,0.0,0.2,0.4,0.6,and 0.8,as well as to explore the relationship between the distribution of gaseous pollutants in the pedestrian area and the staggered ratio of buildings in the neighborhood.Firstly,mercury vapour(Hg)source who located outside the urban blocks steady influenced by the rural wind and diffuse in the streets were simulated,particularly,correlations between building height variation and pedestrian level pollutant accumulation were established depending on the discrete numerical simulations.Numerical simulations indicate that high altitude Hg pollutants were inclined to accumulate in the region of pedestrian level due to the enhanced street canyon building height variations,Hg concentration in the region of pedestrian level therefore significantly increased.Furthermore,street canyons of peak Hg concentrations usually were sequentially allocated along the wind flows,maximum one mostly closing the upwind side.Hg fluxes in the street canyons of high-rise buildings backward were observed to be significantly higher than those in the street canyons of low-rise buildings backward.Secondly,a explode of toluene(c7h8)in the centre of the street canyons enforced by the urban wind and diffuse in the streets were simulated combined with time,to explore the diffusion of c7h8 and the relationship between the time and urban construction.It is found that: the time of what the c7h8 use to arrive in the downwind street in uniform-height-buildings is longer than the change-height-buildings;When the building blocks are inconsistent,the concentration of c7h8 in the streets of the higher construction leeward is generally higher than the streets located on the leeward side of the lower buildings;When the building is uniform,the concentration of c7h8 in each street decreases with time,but when the building height is inconsiste2 nt,the toluene in the downwind street will even re-gather back to the pedestrian area;After a period of time,the concentration of toluene in the street where the burst was located was the highest concentration of toluene in the building height deviation of 0.4,followed by the building with a deviation of 0.6and 0.8,and then the deviation of 0.2 and 0.In the end of this paper,PIV was used to observe the flow field in the water tank model which is a scaling-down model to the real street canyon,the experimental results verifies the accuracy of CFD in studying the airflow organization problem in urban neighborhoods.Based on the numerical simulation of steady state and unsteady state,this paper analyzes the diffusion law of heavy density gaseous pollutants in the urban area where the buildings are scattered,and the results are provided for the treatment of heavy pollution pollutants in urban blocks The theoretical basis is conducive to different levels of pollution by the location of the region and determine the level,has a certain practical significance. |
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