Effects Of Wind Direction On The Airflow And Pollutnat Dispersion Inside A Long Street Canyon With Various Building Layout

In recent years,the high-speed development of economic has led to the rapid development of urbanization and automobileization in China.Moreover,the urban population and motor vehicle ownership are both increasing.Motor vehicle exhausting pollution has become the main pollution source of urban atmospheric pollution.Street canyon is an important part of urban planning and construction,and it is closely related to the life of urban residents.The pollution problem of motor vehicle exhausting in street canyons has become a hotspot in environmental engineering field.Based on foreign and domestic related research,this paper commands numerical simulation and wind tunnel experiment to explore effects of wind direction on the airflow and pollutant dispersion in a long street canyon with different building layouts.It also reveals the diffusion of pollutants and distribution characteristics of concentration in a long street canyon.On the basis of physical model of wind tunnel measured data at Karlsruhe University in Germany,a three-dimensional CFD model based on the incompressible NS equations,k-? turbulence model and the pollutant transportation equation for evaluating the flow and pollutant dispersion was built up and conducted with ANSYS FLUENT 14.0,and then validated against the wind tunnel experiment.The best turbulence model is standard k-? model and the most suitable Sct number is 0.3.Using established CFD model,21 cases were conducted in this paper to investigated impacts of different wind directions(?=0°,15°,30°,45°,60°,75°,90°which ? is the angle between the approaching flow and the street axis.)and the heightto-width of street canyon(H/W=0.5,1,1.5).The computed air velocity vector fields and pollutant concentration contours indicate that the in-canyon airflow and migration of pollutants relate to wind directions and the height-to-width of street canyon strongly:(1)Firstly,effects of wind directions on the in-canyon airflow and pollutant dispersion were studied with a street height-to-width ratio of 1:1(H/W=1).The results show that concentration distribution of pollutants is basically symmetrical about the street axis and pollutants concentrations are higher at the exit to street canyons due to the airflow is mainly parallel to the street axis under ? =0° and 15°.Pollutants accumulate on the leeward side and concentration of pollutants is gradually increasing due to a helical flow structure is established in the street canyon under ?=30°,45°,60°and 75°.When the wind angle is 90°,concentration of pollutants on both sides is lowand concentration distribution of pollutants is symmetrical about the center of street due to the corner eddy.In the street canyon,ACH increases first and then decreases with wind direction increasing,which indicates that a certain wind direction is conducive to the air circulation.(2)Secondly,impacts of the height-to-width of street canyon(H/W=0.5,H/W=1and H/W=1.5)on the in-canyon airflow and pollutant dispersion were studied.The results show that as the height-to-width of street canyon increasing,the velocity of the airflow in the horizontal direction becomes larger and concentration distribution of pollutants decreases in the street canyon under ? =0° and 15° with the maximum concentration at the exit of street canyons.When wind direction angle ranges from 30°to 75°,the leeward side concentration of H/W=1.5 is significantly lower than that of H/W=0.5 due to the center position of the clockwise vortex in the longitudinal section of street is constantly moving up and distribution of in-canyon pollutant concentration field in the pedestrian height is mainly affected by airflow movement which is parallel to the street axis with the increasing of the height-to-width of street canyon.Concentration of pollutants on the leeward side increases with the increasing of wind direction angle.When the wind angle is 90°,the concentration distribution of pollutants is symmetrical about the center of street.As the height-to-width of street canyon increasing,distribution of pollutants becomes wide and air flux decreases in the street canyon which indicates that the increasing of height-to-width of street canyon is unconducive to the air circulation.In this paper,eight representative cases(H/W=0.5 and 1,?=0°,30°,60°,90°)were qualitatively verified by wind tunnel experiment based on a system of instantaneous concentration distribution of pollutant with laser sheet.The calculated results agree reasonablely well with wind tunnel experiment by comparing wind-tunnel experiments with simulated dates.The results of this paper have a certain reference function to urban street planning and air quality prediction in street canyons.