The Impact Of Traffic Tidal Flow On The Pollutant Dispersion Within Street Canyon

Understanding the law of pollutant dispersion within urban boundary layer is helpful to reasonable urban planning and the control of urban atmospheric pollution.The traffic tidal flow?TTF?phenomenon has become an important part of urban traffic congestion in the rush hour,with uneven traffic induced impact?TII?and uneven pollutant source.Focusing on the TTF,we discuss the correlation of solar radiation,ambient wind,traffic induced impact.As a result,it no only provide a further understanding of the problem of TTF,but also enrich related research of pollutant dispersion.By field measurement,we obtained the typical distribution of TTF.We modelled the TTF within the street canyon using the computational fluid dynamics?CFD?,discussing the influence of uneven pollutant source,uneven TII,ambient wind speed,solar radiation.Firstly,the correlation of ambient wind and TII were investigated.It is concluded that when 4m/s of ambient wind speed was only investigated,the total traffic volume determines the distribution of pollutants rather than the non-uniform pollutant source.The mean dimensionless concentrations of pedestrian layer pollutants on the upwind side are 1.5greater than that on the downwind side.When the tidal flow ratio changed from 1/4 or 4/1 to 4/4,the increasing non-dimensional concentration on the upwind side increases by 0.4 compared with the leeward side.When the 4m/s ambient wind and non-uniform TII were considered together,the dimensionless concentration of the upwind side pollutants was still at least 0.3 higher than the downwind side.When the tidal flow ratio changed from 4/4 to 1/4,the TII of upwind side was changed,which contributed to dilution of pollutants on the upwind side but is not conducive to the dilution of pollutants on the downwind side,so that the dimensionless concentration of the mean pollutants on the upwind side decreases by 0.6 and the downwind side increases by 0.4;when the tidal flow ratio changes from 4/4 to 4/1,the TII is not conducive to the dilution of pollutants on the upwind and downwind side,so that the mean non-dimensional concentration of pollutants in the pedestrian layer on the upwind side and the downwind side increases by 0.1 and 0.5;when the ambient wind speed is1m/s and the tidal flow ratio is 2/4,the mean concentration of pollutants on the downwind side is 0.3 higher than that on the upwind side.The distribution of pollutants is dominated by the inhibition of non-uniform traffic induced convection;when the ambient wind speed is 8m/s,the distribution of pollutants is dominated by ambient wind,and under different traffic tidal flow,the change of concentration at the pedestrian level is about 0.1.Secondly,we discuss the correlation of ambient wind,TII,solar radiation further.It is concluded that when the ambient wind speed of 4m/s was only considered,the contribution of the solar radiation thermal effect and ambient wind to flow structure near ground are much larger than that of traffic-induced convection.The influence of solar radiation on the diffusion of pollutants is through the effect of convection in the street canyon rather than the effect of turbulent kinetic energy;the influence of solar radiation on the distribution of pollutants is greater than the effect of TII;when the ambient wind speed is 4 m/s,at the evening peak of any season,the concentration of pollutants near the ground is at least 0.7 higher than the morning peak in the same total traffic volume.The concentration of pollutants at the pedestrian layer of summer morning peak is the lowest,ranging from 1.2 to 1.8,while that concentration of winter evening peak is the highest,ranging from 1.8 to 4.2.At U₁₀=1m/s and 4m/s,the air convection and traffic induced by the solar thermal effect were the main factors determining the distribution of pollutants within the street canyon.The extreme concentration of pollutants in winter is at least 1.8 higher than that in summer,and the concentration of evening peak is at least 0.8 higher than that of the early morning peak;at U₁₀=8m/s,the concentrations in upwind side is slightly 0.3 higher than that in downwind side in winter,while that is basically consistent in another season,since the distribution of pollutant was dominated by ambient wind.Focusing the traffic tidal flow,we firstly investigated the correlation of solar radiation,ambient wind and traffic induced impact to provide thought and method for studying the pollutant dispersion within street canyon.