Coupling Effects Of Different Turbulence Mechanisms On Street Canyon Pollutants

Air pollution outside the built environment is a major problem that seriously threatens the health of human beings.However,due to the complex interaction between atmospheric flow and buildings,the dispersion mechanism of traffic pollutants has always been a difficult research topic.The dispersion of urban traffic pollutants is mainly affected by turbulence.The eddy caused by turbulence is a key factor in the exchange of mass,energy and momentum.In the field of building environments,three main mechanisms of turbulence generation affect vehicle pollutant dispersion: Windinduced turbulence(WIT),Thermal-induced turbulence(TIT)and Vehicle-induced turbulence(VIT).At present,many studies have focused on the effects of wind and thermal buoyancy induced by solar radiation,but has little research on the mechanical turbulence caused by vehicle movement,and the research of the three coupling effects is still in the blank.In this master's thesis,the typical urban ideal street canyon was taken as the research object.The dynamic mesh updating method based on Computational Fluid Dynamics(CFD)was used to numerically analyze the mechanical turbulence characteristics produced by vehicle movement.Then,the coupling mathematical model of fluid flow,heat transfer and pollutant dispersion in urban street canyon environment was established under the action of ambient wind,solar radiation and traffic flow induction to calculate the flow field and pollutant concentration distribution;Finally,three different vehicle models were established to analyze the turbulent kinetic energy(TKE)characteristics of different vehicle and wind direction.The main results are as follows:When the vehicle moves,TKE is mainly distributed behind the vehicle and the magnitude of the TKE around the vehicle body far exceeds the case of stationary vehicles.Driving speed has a significant impact on TKE.When the driving speed is 15 m/s,TKE near the tail is about 1.8 times than that of 10 m/s and 6.3 times than that of 5 m/s.Compared with the vehicle's static condition(V=0 m/s),vehicle-induced turbulence can reduce the CO concentration by about 4% at breathing height.In a weak wind environment,the leeward side heating and the ground heating are beneficial to the dispersion of pollutants.Dispersion effect of the ground heating on the pollutants is greater than that heating on the leeward side.Heating on the windward side increases the concentration of pollutants.The movement of the vehicle reduces the average concentration of pollutants by about 4%;Under the action of moderate wind velocity and high wind velocity,the adverse effect of windward side heating is reduced.The effect of vehicle movement on pollutants has also become less significant.Through the analyses of the car,SUV and truck,it is found that the shape of the vehicle had a significant influence on the generation of TKE.The TKE induced by the truck is the largest,and the TKE induced by the car is the smallest due to its more streamlined shape.When the wind direction is consistent with the vehicle movement(wind direction of 0°),the TKE induced at the wake region of the vehicle is the smallest.When the wind direction is opposite to the vehicle movement(wind direction of 180°),the TKE induced at the wake region of the vehicle is the largest and the distribution range is longest.The TKE under 90° wind direction is between 0° and 180°.