Research On The Diffusion Law Of Traffic Pollution In Urban Street Valleys

Vehicle exhaust is one of the major sources of urban air pollution,which has brought negative impact on the urban ecological environment.In particular,in the micro-environment of urban street canyon where dense high-rise buildings are distributed on both sides of the road,the characteristics of high traffic volume and weak ventilation capability can easily cause local accumulation of the pollutants,which intensifies the health hazards to surrounding pedestrians.Rational city layout and effective transportation planning and control is one of the effective means to solve this problem by improving and enhancing the dispersion and dilution capacity of pollutants in the street canyons,which requires the premise of insight into the internal flow field and the dispersion rules of pollutants in the street canyons under various environmental characteristics.Although a lot of related work has been done by domestic and foreign researchers,it has not met the accurate prediction for the distribution of pollutants under various environmental factors.Therefore,this paper aims to further improve the insight of the inner flow characters and dispersion rules of vehicle esxhaust in the scale of urban street canyon based on the achievement from the predecessors and to provide the basises for the future urban planning,traffic control and related environmental policies as well as the reference for the validation of prediction model for vehicle exhaust dispersion in the urban street canyons.Specifically,this paper studied both the static and dynamic factors that influence on the vehicle exhaust as well as flow characters in the street canyon.Those static factors include:ambient wind speed,aspect ratio(AR),the symmetry of street(ASF),building aspect ratio(HB)and building roof configuration(R).Those dynamic factors include:the vehicle speed and vehicle driving conditions.In the study of the static factors in the street canyon,the famous wind tunnel experimental data was first used as a basis to build the numerical analysis model which validated and modified using the experimental result data,at the same time the selection of the turbulence model was completed.Then,five ambient wind speeds,five types of aspect ratios(AR),five types of street symmetry(ASF),and five kinds of building aspect ratio(HB)and 47 types of roof configuration(R)and their layout were simulated.It was found that(1)the increase of ambient wind speed contributes to the proliferation and dilution of pollutants inside the street canyon;also,(2)the dispersion and dilution capacity of pollutants is significantly enhanced with the inccerase of AR;furthermore,(3)when the ASF is less than 1,the the canyon with step-up geometry is more conducive to the air and pollutants exchange at the top of the street canyon;at the same time,the symmetrical street canyon(ASF=1)has a better pollutant dilution than the asymmetrical street canyon.Moreover,the stronger asymmetry of street canyon corresponds to weaker pollutant dilution capabilities;in addition,(4)with the increase of HB,the exchange of air and pollutants at the top of the street canyon is facilitated,while effect on dilution of pollutants discharged from the bottom of the street canyon is weak;besides(5)when the cross-sections formed by the upwind and downwind buildings and their roofs appear to diverge from the inside of the street canyon to the outside,it will be conducive to the outward dispersion and dilution of pollutants from street canyon while it has the opposite effect when the cross-section interior shrinks to the outside.From a comprehensive perspective that is most conducive to the air exchange and pollutant dispersion of street canyon,the optimal roof configurations of windward and leeward buildings are pointed roof and flat roof respectively;finally,(6)among the four static environmental factors of wind speed,AR,ASF and HB,when it comes to the degree of influence on the flow field and pollutant dispersion in street canyon,the sequence decreasingly is wind speed,AR,ASF and HB.In the study of the dynamic factors of street canyon,the two-dimensional numerical model was first established using reference data,and the model was verificated using the theoretical formula.Then,the influence of five vehicle driving speeds and six kinds of vehicle driving conditions on the near-surface flow field and the dispersion of pollutants in the street canyons were studied by using dynamic grid technology,and the following conclusions were obtained:(1)for the vehicle speed,the large speed corresponds to the unfavorable dispersion condition for vehicle exhast in the near-surface layers of the street canyon.(2)For the vehicle driving conditions,it was found that the deceleration process of the vehicle is more conducive to the dilution and dsipersion of pollutants in the near-surface layer of the street canyon.