The effects of lateral entrainment on pollutant dispersion inside a street canyon and the corresponding optimal urban design strategies

Intensive traffic emissions have caused many environmental problems and have a negative effect on public health. With the aim of mitigating these problems, it is essential to figure out how the flow structure affects the pollutant dispersion within the urban canopy. Most previous studies focus on the canopy vortex caused by top entrainment, but few previous studies are aware of the importance of lateral entrainment. By conducting computational fluid dynamic (CFD) simulations validated by wind tunnel data, we investigate the effects of lateral entrainment on pollutant dispersion inside a street canyon. Eight three-dimensional street canyons with various building heights and street lengths are considered. Besides, three optimal design strategies are proposed to improve the air quality by enhancing the lateral entrainment. The results of this analysis demonstrate that lateral entrainment could conditionally reduce the pollutant concentration of low-rise canyons. This reduction, which is affected by lateral entrainment, is confined in a range of approximately 2.5 times the street width from the street ends. In contrast, the lateral entrainment causes a more pronounced reduction in the pollutant concentrations of the high-rise canyons. Besides, all three strategies can considerably facilitate the lateral entrainment, leading to a significant reduction in the cross-section pollutant concentrations (by up to 76%) and therefore a significant reduction in the personal intake fraction P_IF of the residents (by up to 81%).