Influence of GI configurations and wall thermal effects on flow structure and pollutant dispersion within urban street canyons

Abstract

Numerous evidence has shown that within the semi-enclosed street canyon, traffic emissions and the secondary pollutants are prone to accumulating in poorly ventilated areas, which is a serious threat to people's health. Among many factors affecting the flow field and pollution diffusion characteristics in street canyon, the effects of wall thermal buoyancy caused by solar radiation coupled with the aerodynamics and shading effect of different forms of green infrastructures (GIs) have not been paid enough attention. This study conducted the numerical simulations validated by wind-tunnel experiments to investigate the influence of different GIs & wall heating (WH) conditions on flow and dispersion within urban street canyons. Eight GIs and four WH configurations are considered. The validation results indicate that SKE-SWFs have better prediction accuracy. Numerical results show that the green belt has little effect on flow and dispersion within street canyon, while other GIs will cause increased pollution on the leeward side to varying degrees, depending on the planting location and volume of tree crown. Under the WWH, the RSTs will lead to the reverse flow at the canyon bottom, so that the pollutants will completely turn to the windward side. Conversely, the LSTs can promote the migration of pollutants to the canyon center, and has little impact on two side walls and pedestrian respiration planes, therefore, the LSTs is suggested to be the optimal tree planting configuration. This study can provide technical guidance for the optimization design of urban green facilities to achieve well-targeted control of local air quality.