Numerical Study Of Thermal Envoironment, Flow Patterns And Pollutant Dispersion In Street Canyons

Urbanization rapidly spread across the world, it has made urban environment quality become an important issue for the majority. As a a basic element of complex urban built environment, urban street canyon has significant affect on both mesoscale and microscale enivironment. It is the source zone of motor vehicle exhaust, the main occupied zone of urban residents, and also the source zone of indoor fresh air. Thus, it is necessary to have a better understanding of environmental characteristics in street canyons. Which could benefit not only urban and indoor environment improving, but also pollutant removal and building energy consumption reducing, more importantly, ensuring safe and health environment for pedestrians.The charcteristics, diurnal variation and seasonal difference of thermal environment, flow patterns and pollutant dispersion under clear day condition are investigated, the flow patterns under cloudy and windy day condtions are disscussed using computational fluid dynamics(CFD) approach based on a unit street canyon model.First, the formation process of thermal environment and flow patterns in street canyons are invesitigated based on governing equations. Two heat sources of street canyon environment are presented as meteorological environment and artifical environment. Four force sources as gravity, thermal effect, wind pressure and drag force are presented as well. Numerical results demonstrate that distribution conditions of street temperature significantly alters street canyon flow structure and pollutant dispersion. Comparing with commonlly used uniform distribution assumption, uneven temperature distrbution may improve prediction accuracy of wind-buoyancy-driven flow in street canyons.Then, numerical simulations are performed at four representative time events(morning, noon, afternoon and night) under summer and winter condition. The results suggest: 1) strongest swirl intensity and biggest ground-level air velocity could be observed as buoyancy force has the same direction with mechancial wind; 2) while mechancial motion is oppsite with buoyancy forces, several vortices might present in the street canyon; 3) air motion in the street canyon could be weakened by stable thermal stratification when surface temperatures are lower than ambient air temperature. Seasonal comparison demonstrate that the surface temperature among street canyon facets is more evenly distributed, the diurnal variation of surafce temperature, flow patterns and air velocity at ground-level is smaller under winter condition. Moreover, the peak value of air exchange rate is found at noon under summer condition, while it appears at morning under winter condition.Three characteristic parameters are proposed to quantitatively describe pollutant dispersion in street canyons, as mass percent of near-wall-zone, mass percent of near-ground-zone and mass percent of inhabitant-zone. The numerical results of these three parameters under clear day condition reveal that the mass percent of near-wall-zone gradually increased from morning to night, while the mass percent of near-ground-zone is essentially unchanged during daytime(moring, noon and afternoon) and keep only half of the value at night when bulk Richardson number Rb =-0.87.A new street canyon model is proposed by considering the optical effects(i.e. scattering and absorbing) of haze-fog on radiation heat transfer in street canyons. The impact of haze-fog on thermal environment, flow patterns, and pedestrian thermal comfort are invesitigated under four different haze-fog conditions. The numerical results suggest that the surface temperature among street canyon facets becomes more evenly distributed and pedestrian thermal comfort becomes lower under heavy haze-fog condtions.Lastly, the diurnal variation of surface temperature and flow patterns under non-clear day conditions is investigated. As the result of smaller incident global solar radiation and direct solar radiation ratio, thermal environment and flow patterns in the street canyon are observed significantly different under clear and non-clear day conditions. It demonstrates that the conclusions based on clear day assumptions can not be used in non-clear days.The modeling method proposed by this study can improve the prediction accuracy of thermal environment, flow patterns and pollutant dispersion in street canyons and benefit other researches on diurnal and seasonal variation of street canyon environment.