Study On The Diffusion Characteristics Of Vehicle Pollutants In Space Of Irregular Street Canyon

The dense buildings in the city have seriously deteriorated the ventilation capacity in the city.The exhaust pollution from motor vehicles on the streets cannot be diluted out.A large amount of exhaust gas accumulates in the street canyon space,which seriously affects the health of residents and pedestrians on the street.The diffusion of pollutants in urban street canyons is a complex problem,which is induced by turbulence caused by various factors.However,the continuous deepening of the urbanization process has continuously complicated the shape of the street canyon,and it has also made the propagation of pollutants in the street canyon more complicated.These three types of irregular street canyons are targeted for theoretical analysis and numerical simulation research on the propagation law of pollutants.The thesis first based on the numerical simulation(CFD)method to simulate the diffusion of pollutants in the viaduct-street canyon.The synergy of pollutant diffusion when the height,width and environmental wind speed of the viaduct is analyzed,and it is found that changes in the width and height of the viaduct have obvious changes to the synergy of the area under the viaduct.As the width and height of the viaduct continue to increase,the leeward side The synergy of the nearby field is getting worse and worse;when the environmental wind speed is low,the synergy angle near the leeward side of the bottom of the street valley is greater than that near the windward side.When the wind speed increases to 2m/s,the bottom of the canyon is upwind The synergy angle near the side is greater than the synergy angle near the leeward side.However,as the wind speed continues to increase,there is no obvious change in the distribution of the field synergy angle.At the same wind speed,the concentration of pollutants near the leeward side is higher than that of the windward.The change of viaduct width has obvious change for the change of regional synergy under the viaduct.With the continuous increase of viaduct width,the field synergy near leeward side is getting worse and worse;the change of viaduct width has obvious change for the change of regional synergy under the viaduct.When the wind speed increases to 2m / s,the synergy angle near the windward side of the valley bottom is greater than that near the leeward side.However,with the continuous increase of the wind speed,the distribution of the field synergy angle is not obvious,and under the same wind speed,the concentration of pollutants near the leeward side is high on the windward side.Then,the physical model of the overhead layer-street canyon system was established,and the mechanism of pollutant diffusion in the irregular street canyon was analyzed.The results show that when there is an overhead layer,the reason for the decrease in the average concentration of CO along the width direction of the street canyon at Y=1.5 m in the target street canyon is different in the upper and lower street canyons.In the lower street canyon,the reason for the decrease in the average concentration of CO is that most of the CO is carried to the area above the height of1.5 m in the target street canyon;in the upper street canyon,the reason for the decrease in the average concentration of CO is that most of the CO is taked to the upstream area of the target street canyon.Finally,the physical model of the viaduct-turntable-tunnel three-dimensional transportation system is established,and the propagation characteristics of thermal effects on the pollutants in the three-dimensional transportation system are analyzed.The results show that when the sun shines on the wall,it will cause the temperature of the wall to rise,which will heat the temperature of the air near the wall and increase the thermal buoyancy of the air.It is conducive to the flow of air at the bottom to the upper part,so as to drive the upward movement of pollutants.As a result,the pollutants near the heating surface diffuse outward,and the concentration of the pollutants decreases.When the ambient wind is northerly,after adding solar radiation,the heated side will increase the diffusion of air upward and around due to the effect of thermal buoyancy,resulting in a decrease in the strength and length of the pollution zone on the heated side of the turntable.When the environmental wind is a westerly wind,the distribution of pollutants in the pedestrian breathing layer above the turntable is hardly affected by solar radiation.