Research On Response Mechanism Of Street Canyons Pollutant Diffusion Law To Building Structure And Ambient Wind

As one of the basic units of urban form,urban street canyons carry important urban transportation functions and life functions.However,the emission of traffic and sudden traffic accidents have caused urban street canyons to face serious air pollution problems.Therefore,it is of great practical significance to study the law of pollutant diffusion in urban streets canyons.In this paper,numerical simulations are conducted for two types of street canyons,the uplift type and different building height ratios.According to the numerical simulation results,the influences of the parameters of the uplift type and different building height ratios and the ambient wind on the flow field structure and pollutant diffusion law in the street canyon are discussed.The results of the simulation analysis are summarized as follows:The uplifting effect will significantly change the structure of the flow field in the street canyon.Compared with the ideal street canyon,the three types of uplifting street canyons differ in the number of vortices,the relative position of the vortices and the direction of rotation of the vortices.The three types of uplifting street canyons eventually formed a relatively stable flow field structure under the action of uplift and ambient wind.The increase of the lifting height,the increase of the ambient wind speed and the increase of the height of the main structure of the building will accelerate the formation of a stable flow field structure.The first two types of elevated street canyons form a stable structure of 3 vortexes,and the latter type of elevated street canyons form a stable structure of 2 vortices.And the height of the main structure of the building has a vertical stretching effect on the vortex in the street canyon.Compared with ideal street canyons,the three types of elevated street canyons have substantially changed the distribution of pollutant concentration in the street canyons.Increasing ambient wind speed is conducive to the discharge of street canyon pollutants.Under the effect of thermal buoyancy plumes,when the height of the windward building is the same,the size of the vortex increases with the increase of the ambient wind speed.And at the same ambient wind speed,it increases with the height of the windward building.The critical re-entrainment velocity has a certain mathematical law with the height of the windward building.Finally,according to the correlation between the critical re-entrainment velocity of high-temperature smoke and the height ratio of different buildings,a prediction model of critical re-entrainment velocity is derived.