Simulation Of The Distribution Of Particulate Matter In The Ventilated Street Canyon Covered Elevated Road

The mechanism model of street canyon under elevated coverage has become a very important block form in the urban micro climate.The presence of the air intake(openings arising from discontinuities building in the same side of a road)can affect the flow field and the distribution characteristics of the particles.The width and the distribution of single and double sides of the air intake have different influence on the flow field and particle distribution.In this context,this paper uses three-dimensional CFD numerical simulation method to study the flow and the distribution of particulate matter in the street canyon.By using the DPM model,six types of street canyons(Width Height Ratio is 2.2)are simulated and calculated: unilateral 10 m air intake,unilateral 30 m air intake,unilateral 50 m air intake,bilateral 10 m air intake,bilateral 30 m air intake,bilateral 50 m air intake.At the same time,three criteria were introduced to evaluate the environmental quality of the street canyons: Breathing Surface Concentration(BSC),Effective Ventilation Rate(EVR),Average Residual Time(ART).The characteristics and the distribution characteristics of the flow and the distribution particulate of matter in the six models are analyzed and compared in detail.The results of three metrics in different spatial regions of the six models are calculated.The results show that there is a complete vortex in the canyon of the model of unilateral 10 m air intake,and the center of the vortex is located in Z=30m and Z=170m.The velocity peak is less than 1.4m· s-1.The concentration of pollutants in the leeward side is significantly higher than that in the windward side,and the concentration of contaminants in the area of Z=0-20m?Z=80-120 m and Z=180-200 m in the leeward side(X<10m)is the highest which was more than 400 ?g·m-3.In the model of unilateral 30 m air intake,the velocity inlet flow field is only uniform distribution in the area of close to the leeward side(X<10m),and the large vortex and secondary vortex in the valley are more.The concentration distribution of particles showed two obvious characteristics,and the distribution of the concentration of the breathing surface height(Y=1.5m)in the windward side is uniform.In the model of unilateral 50 m air intake,the interior of the valley will form three distinct vortices(there are three vortices on both side of the axis of air intake(Z=100m))and several small vortices.Above the elevated road,the particles are mainly concentrated on the leeward side and close to the air intake,and the particle distribution is uniform under the elevated road.In the model of bilateral 10 m air intake,large vortex is formatted in the street valley space.The vortex center is located on the central axis(X=20m)of the valley and the distribution of the vortex center is a strip.The maximum concentration of particulate matter is less than 410 ?g·m-3.Along the axis +X,the concentration of particulate matter increases first and then decreases,and the concentration of leeward side is generally higher than that of windward side.In the model of bilateral 30 m air intake,on the two sides of the air intake axis two large vortices are formed(a total of four).The distribution of the particle is not uniform,When X<20m,the concentration of particles will increase with the increase of the distance,and when X>20m,the particle concentration decreases rapidly with the distance.In the model of bilateral 50 m air intake,there are two large vortices in the valley space,and the center of the vortex is small and located in the area of Z=20-30 m and Z=170-180 m.The distribution of the pollutant in the height of the breathing surface is positively correlated with the distance from the building of the leeward side(the increase of X).In the upper and lower parts of the elevated road,the concentration of pollutants in the direction of axis X is small.The concentration distribution of particles in the model of bilateral air intake is lower than that of the model of unilateral air intake.With the increase of the width of the air intake,the effective ventilation rate(EVR)increases gradually.The occurrence frequency(OF)of the different street models in Z = 60 m / 140 m and Z = 40 m / 160 m is maintained between 2.5 and 3,the other areas are all below 2.During the areas of Z = 60 m / 140 m and Z = 40 m / 160 m,the average residual time(ART)is more consistent,and maintains between 2 and 3,and there is a higher ART of the valley in the model of bilateral air intake than the model of the unilateral air intake.At the far end of the air intake(Z<10m/Z>190m),the ART of the model of unilateral air intake is higher than that of the model of bilateral air intake.This feature is contrary to the area near the air intake.In this paper,the model is focused on the street canyons with unilateral air intakes or symmetrical bilateral air intakes on both sides of the street,while the asymmetric canyon model is not studied.In the following study,we can make a further expansion of the model in order to get a more comprehensive study of the flow and pollutant distribution.In this paper,the model is focused on the street canyons with unilateral air intakes or symmetrical bilateral air intakes on both sides of the street,while the asymmetric canyon model is not studied.At the same time,select no tour 30 m of highways under the influence of mouth as the contrast model,analysis the influence of highway,presence of highways,particle concentration increases by about 20%.In municipal planning and construction,when unilateral planning road,planning width in 0 ~ 30 m range,according to actual needs and location choice of the reasonable width without valley particle concentration distribution across the street has a great influence.When planning the width in the 30 m and 50 m range,select a reasonable width of 50 m.