Impact of moving traffic on pollutant transport in street canyons under perpendicular winds: A CFD analysis using large-eddy simulations

Abstract

• One-way traffic has stronger influences on pollutant dispersion than two-way traffic • The influence of traffic motion is noticeable under a low approaching wind speed • At U 10 = 2 m/s, the net pollutant flow rate at the side can be increased by 14 times • Traffic density plays a much less role than traffic direction and wind speed • Considering traffic motion is essential for accurate predictions of urban pollutant In urban street canyons, wind and vehicle motions jointly influence the pollutant dispersion. The present study systematically investigated the impact of (i) traffic direction, (ii) traffic density, and (iii) approaching wind speed on the mechanism of pollutant dispersion. Large-eddy simulations were performed for a street canyon with an aspect ratio of unity under two-lane traffic flow. Results show that two-lane traffic moving in the opposite direction insignificantly affects the mean pollutant concentrations inside the canyon, with a change in the net pollutant flow rate by less than 7%. However, traffic moving in the same direction strongly modify the local mean pollutant concentration, especially under a low approaching wind speed. Under the wind with a 10-meter speed of 2 m⋅s −1 , one-way traffic plays a dominant role in determining the concentration pattern, shifting the region with the maximum pollutant concentration from the center to the corner downstream of the traffic direction. The absolute net pollutant outflow rate at the canyon's lateral opening downstream of the traffic direction is increased by about 14 times, which is caused by the outflow convective pollutant flux. Findings here suggest numerical studies that neglect traffic motion can erroneously predict the local pollutant concentration in street canyons.