Numerical Simulation Of The Air Environment In The Street Canyon At The Crossroads Of Urban Roads With Viaducts

The ever-increasing level of urbanization in China and the increase in the number of motorized vehicles have been increasing year by year,resulting in great traffic congestion and a large number of vehicle exhaust emissions.Most cities construct viaducts at crossroads to alleviate the traffic congestion in the city.The built viaduct further hinders the spread of automobile exhaust in the street canyon.The crossroads at the crossroads of the city are densely constructed and variously shaped,forming a street.The complex and changeable structure of the canyon is not conducive to the normal cleaning and sucking effect of the incoming air.The traffic volume of vehicles is extremely large.Affected by the traffic lights at the ground intersections,a large number of stagnant vehicles in the period will emit high concentrations of automobile exhaust.The crossroads including the viaducts at the crossroads serve as important urban transportation hubs and prosperous business spaces for residents’ activities.Due to the combined effect of the micro-atmospheric environment in the street canyon and excessive exhaust emissions,the formationof high-concentration pollutants on the ground floor of the street will result in the accumulation of pollutants.Physical health and normal travel cause injury.In view of the above situation,this article starts the study as follows:Firstly,we measured the spatial parameters of a selected street intersection with a typical elevated viaduct,and obtained the indexes of the road and building dimensions in the design of the street canyon model.The statistics of the traffic conditions and the vehicles passing through the site were obtained.This provides a reference for the design of the surface source intensity of the model.Then using the three-dimensional CFD simulation method,using FLUENT6.3 numerical simulation calculation software,using the conservation of mass equation,momentum conservation equation,standard k-ε equation and component transport equation,and substituting into the calculation model after grid independent assessment,Discuss the air flow field,velocity distribution,turbulence intensity and diffusion of pollutant concentration at the crossroads with viaducts under different wind direction conditions.Using the same calculation method and numerical equations,the comparison between the equal-size sink experiment and the wind tunnel experiment verified the rationality of the air flow field and pollutant diffusion of the calculation model.The numerical simulation results show that when the incoming wind direction is parallel to the direction of the main street including the viaduct,the incoming stream is contracted below the upper approach bridge to form a lowwind velocity vortex,resulting in a high concentration of CO deposit.Along with the extension of the horizontal section of the viaduct,the CO concentration under the viaduct gradually decreases,which changes the concentration of pollutants in the second half of the street.The concentration of CO at the pedestrian’s breath height above the viaduct remains extremely high,almost twice the height of the auxiliary road.The horizontal vortex at the corner of the crossroads will bring high concentration of CO on the main road into the auxiliary road.When the incoming wind direction is perpendicular to the direction of the main street including the viaduct,the blocking of the viaduct changes the wall climbing effect of the pollutants on the leeward surface of the main road,and the CO on the windward side and the leeward side of the main road underneath does not easily diffuse,maintaining a high concentration;At the height of the pedestrian’s breath above the main road viaduct,the clockwise clean flow in the valley allows the CO concentration on the windward side to be the lowest,and the concentration of CO on the leeward side and the viaduct center is extremely high.Regardless of whether the incoming wind direction is parallel or perpendicular to the viaduct,the CO under the main road viaduct is difficult to diffuse upwards and maintains a high concentration due to the blocking effect of the elevated bridge deck in the street valley;the pollutants on the leeward side of the street are not easily cleaned by the flowing wind and the concentration isrelatively high.The intersection of the horizontal vortex and the upward vortex at the corner of the crossroads has a contaminant concentration that is significantly higher than other parts of the street,and the upward trend is obvious.Pedestrians should avoid long-awaiting at the corners of the viaducts,street leewards,and crossroads to minimize damage to humans from pollutants in the street valleys.