Numerical Simulation Of Pollutant Diffusion And Dispersion At Urban Bus Stop In Wind Conditions

In recent years,the eco-development of modern city is highly promoted.A greener and healthier life style is gradually becoming the common requirements.Due to that,more people choose bus as the prime transport method.Although the widely using buses would decrease the energy consumption and reduce the emission of air pollution,the bus passengers' health would possibly be undermined when waiting at the bus-stop,as exposed to the air pollution without any protection.In this paper,the concentration of CO and exposure levels of the passengers is numerically investigated.The study has significant meaning to understand the diffusion process of the air pollution around the bus-stop.Based on the study in this paper,the arrangement of bus-stop could be optimized to create a healthier life for the passengers.Specifically,the study is unfolded in the following:Simulation investigation is conducted on the bus-stop without bus idling.It turns out the concentration of CO on the bus-stop is corresponding to the background concentration in surrounding environment.The intensity and direction of wind have less influence on the concentration distribution.It is due to the open design of the bus-stop which could not induce the effects of aggregation.The increase on the wind speed would strength the diffusion effects and enlarge CO distribution area.As the wind speeding up,the concentration of CO gradually reaches stable.The variation of wind direction mainly affect the style of CO distribution,which can lead to locally unbalance.The different parking position have different influence on the passenger,and the concentration of CO at breathing height of each passenger would change.If the exhaust pipe was moved from left end to the right end of the bus,the breathing air around the passengers would be less polluted.When two buses park with low speed,the blocking effects of two buses have bad influence on CO diffusion.Besides,the volume of intaked CO is studied under different conditions.The results are very close in the cases of different wind intensity and directions.However,the number of buses parking on the bus-stop would bring great differences.In addition,the volume of CO intaking of the passenger close to the front of the bus would be dominated by the parking position of the bus.When the bus parked by the side of the station,its blocking effect can make road motor vehicle emissions of CO gathered on the platform.The pollutant discharged by the idling bus diffuses and disperses under the influence of the flow field around the building.At the leeward bus stop,CO moves downstream after gathering behind the bus.The CO cluster forms behind bus will move towards to the shelter when the distance between the bus and the building increases.When the bus station is located in the leeward side buildings,CO concentration distribution will changes with the platform and the spacing between buildings.When the bus-stop lies close to the intersection,the most important parameter to decide the concentration of CO is the relative position of bus-stop and surround buildings.In addition,there would be differences of passenger intaking volume and CO concentration around the bus-stop,when the bus-stop locates at the upstream and the downstream of the intersection.If the bus-stop lies at downstream,the intaking volume of CO of waiting passengers at the bus-stop is quite small,whereas it is sensitive the changes of wind direction.By the help of dynamic meshing technique,the bus parking process by the bus-stop is simulated.It could be found that the closer distance between the passenger and exhausting pipe,the maximum concentration of CO at the breathing level is bigger.For the passengers at the far end of the exhausting pipe,the low level concentration of CO would stays for longer time.The longer time the bus parking by the bus-stop,the concentration of CO at the breathing level would be more likely boosting up when the bus drives out.If the exhausting pipe is allocated at the right side,the concentration of CO at all passengers' breathing level would be relatively same with quantitatively increase than the original configuration.In the situation with side wind,the difference on concentration of CO at different passenger's breathing level is smaller.The total intake volume of CO increases by prolonging the idling time,altering the pipe position and introducing the crosswind.