Flow and pollutant dispersion at urban street intersections: a large-eddy simulation

Street intersections play an important role in determining flow pattern and pollutant dispersion in the urban canopy. One one hand, traffic flows are disrupted and vehicles are obliged to accelerate and decelerate, thereby increasing emissions. The idling vehicles waiting for traffic light will increase emissions above the level for free-flowing traffic. On the other hand, street intersections are regions where there is significant exchange of pollutants between the connected streets. Therefore, a model for urban area dispersion will not be comprehensive without taking into account the important effects induced by urban street intersections. This geometry can be seen as a basic element of urban areas and the results from this paper may be of interest to urban planners and transportation system designers.

A validated large-eddy simulation (LES) was employed to simulate the wind flow and pollutant dispersion at urban street intersections, where the wind flow exhibits some three-dimensional characteristics and the channelling effect is apparent. A line source was put in the middle of one street at the ground level to simulate the continuous vehicle emission on the road.

Several scenarios with different incoming flow direction were studied. For each scenario, the mean flow, turbulence, and pollutant transport were visualized and analyzed to reveal the associated three-dimensional characteristics, which were absent from two-dimensional street canyon.