| Atmospheric pollution is one of the main causes of urban air quality deterioration,and urban environmental safety has become an important issue affecting human health.Among them,people are most concerned about the air quality in urban street canyons.Therefore,it is necessary to study the migration and diffusion laws of pollutants in street canyons,and propose corresponding urban planning countermeasures to alleviate air pollution in street canyons and ensure the health and safety of urban residents.Based on the computational fluid dynamics(CFD)technology and the Reynolds-averaged Navier-Stokes(RANS)method,this study investigated the diffusion laws of pollutants under different stability conditions and street canyon structures,especially the effects of different Richardson numbers(Ri_b)on pollutant dispersion under stable conditions,as well as the effects of complex building clusters on pollutant diffusion.The numerical simulation results showed that:1.Atmospheric stability has a significant impact on the diffusion of pollutants around isolated street canyons.Specifically,under stable conditions,the turbulent kinetic energy is small,and the concentration around the street canyon is high;under unstable conditions,the turbulent kinetic energy is large,and the concentration around the street canyon is relatively low.2.Under different Richardson numbers(Ri_b)in stable stratification conditions,the vortices inside the street canyon gradually become larger and move downward as Rib increases.When Ri_bis around 0.25,the flow field structure undergoes significant changes,and the length of the leeward side of the street canyon first increases and then decreases,while the turbulent kinetic energy gradually weakens,the vertical distribution of the plume becomes narrower,and the horizontal transport distance becomes longer.When Ri_bis greater than or equal to 1.0,the plume trajectory behind the street canyon first gradually decreases and then gradually rises.3.Different street canyon aspect ratios and stability conditions significantly affect the dispersion of pollutants around street canyons,under stable conditions,the highest pollution concentration is found in street canyons with an aspect ratio of 1:1,while under unstable conditions,the lowest pollutant concentration is found in street canyons with an aspect ratio of 1:5.The stability conditions have the greatest impact on the change in pollutant concentration in street canyons with an aspect ratio of 1:5,and the least impact on those with an aspect ratio of 1:1.4.Different stability conditions have different impacts on complex building clusters.Under stable conditions,the vertical distribution of pollutants is low,and the horizontal diffusion is far-reaching,with pollutants mainly concentrated in street canyons three to four.Under unstable conditions,the vertical distribution of pollutants is high,and the horizontal diffusion is close,with pollutants mainly distributed in street canyons one to three. |
