Impacts Of Dynamic Wind Fields And Amplitude Variations On Air Environment Within Urban Street Canyons

Urban street canyons are one of the important places for people to live,and in recent years,the growth of urban population has led to the increase of both urban building heights and motor vehicle emissions,which will certainly cause harm to human activities.Therefore,many experts and scholars have studied urban street canyons in recent decades.However,the studies have been done under the condition that the incoming wind speed is stable,while the actual wind field is time-varying.The assumption that wind speed does not vary with time not only overestimates the average concentration of pollutants in the street valley,but also has a bias in predicting the spatial distribution of concentrations.Therefore,in order to reduce the risk of urban residents’ exposure to pollutant air and improve the quality of urban habitat,it is necessary to investigate the effect of dynamic wind fields on the air environment within street canyons.In this study,numerical simulations are used to investigate the distribution and time variation of the flow field and pollutant concentration field in the row and staggered street canyon under the dynamic wind field;the variation of the flow field and pollutant concentration field in the street canyon when the amplitude of the dynamic wind field changes and the average wind speed of the incoming flow varies;and finally the comparison with the steady-state wind field to analyze the difference between the two effects on the air environment in the street canyon.In the process of analyzing the dynamic wind field,the distribution and change process of flow field and pollutant concentration in the street canyon are analyzed from typical points,typical lines,typical areas and the whole,respectively.In the comparison with the steady-state wind field,the three concepts of spatial average velocity ratio,pollutant concentration change rate and effective ventilation volume are introduced.The results show that both the dynamic wind field and its amplitude affect the distribution and variation of the flow field within the street canyon,which in turn affects the distribution and variation of pollutant concentrations within the street canyon.The eddy flow in the street valley under the dynamic wind field is a process that changes continuously with time.As the dynamic wind field changes periodically,the airflow velocities at different locations within the street canyon have different degrees of time delay.The degree of time delay of airflow velocity in different areas is from small to large in the windward area,central area and leeward area.The spatially averaged velocity ratio within the street canyon under the dynamic wind field is greater than that of the steady-state wind field.As the amplitude increases,it is favorable to the diffusion of pollutants in the leeward area and unfavorable to the diffusion of pollutants in the windward area.The moment of peak pollutant concentration in the street canyon does not change with the amplitude,but is only related to the location and the mean wind speed at the incoming flow;the peak pollutant concentration appears earlier when the mean wind speed at the incoming flow is higher.Meanwhile,the steady-state wind field overestimates the pollutant concentration in the actual street canyon.