Study On Pollutant Dispersion In Street Canyons Based On The Coupling Effect Of Dynamic Wind Field And Building Roof Angle

With the rapid development of modern society,automobiles have become one of the most basic means of transportation for mankind,and the resulting environmental pollution problems have become more and more serious.Environmental pollution is one of the important factors that endanger human health.The street canyon is an important place for human activities and social interaction,and it is also the main emission area of vehicle exhaust.Optimizing the pollutant diffusion to improve the air quality in street canyons is the focus of this thesis.In this thesis,The Euler-Lagrangian method is adopted to study numerically the flow field and pollutant diffusion in the street canyon,where two wind fields,two-dimensional steady and three-dimensional dynamic wind fields,are considered.Three main contents discussed are:(1)The flow field structure and diffusion of gaseous pollutants in two-dimensional street canyons;(2)The flow field structure and diffusion trajectory of particulate matter(PM)in a two-dimensional street canyon;(3)The flow field structure and pollutant distribution law of gaseous pollutants in the three-dimensional street canyon.It also analyzes in detail the coupling effect of the roof tilt angle and the incoming wind speed on the three-dimensional unsteady flow field and pollutant distribution in a street canyon.The results are as follows:(1)Numerical simulation indicates that gaseous pollutants and PM in the canyons present three basic structures: single clockwise vortex,transition vortex,and double vortex structures,which are identified at increasing roof slopes.The lowest pollutant and PM concentration is achieved in the flow field with a single vortex structure.The concentration of gaseous pollutants and PM reaches the highest value in pedestrian-level areas in the flow field with a transitional vortex structure.Unlike gaseous pollutants,the concentration of PM does not always decrease with increasing altitude,and higher PM concentrations sometimes occur in the mid-level areas of the canyon.It is generally recommended to use a smaller roof tilt angle to discharge gaseous pollutants and PM.(2)The three-dimensional flow field structure and the pollutant distribution,and their changes with the variation of roof tilt angle ? are similar to the two-dimensional model structure.The flow field changes from a single vortex to a double vortex structure,and the pollutants move from the leeward to the windward side.Under the dynamic incoming wind field,the change of incoming wind speed has an important impact on the flow pattern in the street canyon.No matter how the triangular roof angle? changes,if the incoming wind speed increases and then decreases,the flow field and pollutants in the street canyon will always be compressed first and followed by an expansion,the flow field expands out of the canyon eventually as the wind speed decreases.As a result,both the total and pedestrian height level pollutant concentration in the street canyon show a trend of first increasing and then decreasing with the time.According to the changes in the total and local(specify to pedestrian height area)pollutant concentration,it can be concluded that the building canyon with an inclination angle of 25° to 30° has strong ability to cope with the dynamic change of the wind field,and it is the roof angle that can provide a clean and livable room for the residents.