A Study On The Influence Of Wind Direction And Building Offset On Airflow And Pollutant Dispersion In Intersections

Motor vehicle exhaust emission has become a primary source of air pollution in our country and around the world.Further study of airflow and pollutant dispersion in intersection has an important practical significance.The method combining wind tunnel experiments and CFD simulations is adopted in this article to study the airflow and pollutant diffusion in intersection under the influence of wind flow and layout of buildings.In this study,three-dimensional numerical model was built to simulate airflow movement and contaminant distribution by using the three dimensional incompressible flow homogenization navier-stokes equation,the Realizable k-? epsilon turbulence model and the pollutants convection diffusion equation.The simulation results from FLUENT was compared with the experimental data from the environmental wind tunnel laboratory in the Hamburg university in Germany and the results of numerical simulation done by Sabatino to verify the accuracy of the model parameters.According to the validated model,the numerical simulation for seven wind flows(?= 0 °?15 °?30 °?45 °?60 °?75 °?90 °)and seven building layouts of different offsets(?y = 0 m?0.01 m?0.02 m?0.03 m?0.04 m?0.05 m?0.06 m)have been done in this article.The results show that wind direction and intersection offset have a certain influence on air flux,ventilation rate,the flow field distribution and pollutant concentration distribution.For intersection of symmetrical building,the influence law of the wind direction is as follows:(1)The change of air flux in the four streets intersection has the obvious relationship with wind direction.With the wind angle increasing,the air flux in the third street increases but it decreases in the first street and in the second and fourth street it increases firstly and then decreases subsequently.The change of air flux in the first and third street and the second and fourth street is symmetrical,respectively.(2)The change of air flux in the center of intersection has a direct relationship with wind direction.When the angle of wind direction ? = 45 °,ventilation rate of the center location in intersection would be the most,it is the most benefit to the pollutant diffusion in intersection.The ventilation rate is the least when wind angle ?= 0 ° and 90 °,they are not benefit to the spread of pollutant.(3)The variation of flow field will rise with the change of wind direction in four streets,which affects the spread of pollutant in streets.With wind angle increasing,the pollutant concentration in the second street increases at first and then decreases,the pollutant concentration of the third street is increasing while in the fourth street is always reducing.For intersection with wind angle ? = 0 °,the influence law of the offset value ?y is as follows:(1)The variation of offset ?y has no influence on air flux in the first street,but in the second and third street it increases and in the fourth street which decreases firstly and increases subsequently with offset ?y in intersection increasing.(2)With the increasing of offset ?y,ventilation rate in the center of intersection also increases gradually,with the reduction of pollutant concentration.When the offset ?y = 0.06 m(the maximum offset in this paper),the effect of pollutant diffusion in the center location of intersection is the best.(3)The effect of offset ?y is very small to the flow field in the first and fourth street,but offset ?y has a large impact on the flow field in the second and third street,especially the third street.With the addition of offset ?y,the spread of pollutant would turn from the fourth street to the third street,which makes the distribution of pollutant concentration change.The pollutant concentration in the third street would increase gradually,but the rate of increase has been weakened.Dual source pollutant distribution system was adopted in the wind tunnel experiment with five different wind angle(? = 0 °?30 °?45 °?60 °?90 °)and three different building offset value(?y = 0 m?0.03 m?0.06 m).The pollutants distribution patterns under different working condition were obtained through the experiment.The results of numerical simulation and wind tunnel experiments were qualitatively consistent by analysis of numerical simulation results and pollutant distribution patterns,which verifies the correctness of numerical simulation results and the feasibility and reliability of numerical simulation.The results of this research have great significance to guide urban road traffic planning,urban road intersection layout and the arrangement of the test point position in environmental monitoring.