Assessment of pollutant dispersion in urban street canyons based on field synergy theory

Abstract

Vehicle emissions are an important factor in the deterioration of air quality in urban street canyons. To improve the air quality in urban street canyons, it is necessary to have a deep understanding of the mechanism of pollutant dispersion within the urban boundary layer. The transport process of pollutants in street canyons is essentially a convective mass transfer process. In this paper, a computational fluid dynamics (CFD) simulation is conducted to investigate the effects of wind speed, the setting of the viaduct, and the roof shapes of the street canyons on pollutant dispersion. According to the field synergy theory, Sherwood number and field synergy number are used to evaluate the three factors on the diffusion of pollutants in street canyons quantitatively. The results show that the Sherwood number decreases with increased viaduct construction height and decreased wind speed, which is compatible with the observed growth of pollutant fraction in the street canyon. When the pollution source on the viaduct is considered, a less substantial influence of construction height on pollutant diffusion is observed. Among the four types of roofs, upward pitched roofs resulted in the highest pollution, with a pollution concentration degree of 1.6–2.3 times that of flat roofs, followed by double pitched roofs, with a pollution concentration degree of 1.2–2 times that of a flat roof. By the method of analogy analysis and numerical simulation, the field synergy theory has been extended from applications in heat transfer in small-scale confined spaces to those in urban street canyon pollution simulation in large-scale open spaces.