Large-eddy simulation of flow field and pollutant transport inside urban street canyons of hight aspect ratios

A large-eddy simulation (LES) the with one-equation subgrid-scale (SGS) model was developed to investigate the flow field and pollutant dispersion characteristics inside street canyons of high aspect ratio. A 1/7th power-law wall model was implemented near rigid walls to mitigate the demanding near-wall resolution requirements in LES. This LES

model had been extensively validated against experimental results for street canyons of aspect ratio 1 and 2 before it was applied to the street canyon of aspect ratio 3. A

ground-level passive pollutant line source was used to simulate vehicular emission. Three vertically-aligned primary recirculations were formed in the street canyon of aspect ratio 3. The magnitude of the mean velocity near the ground level was only 0.5% of the freestream velocity, which makes the pollutant emitted at the ground level very difficult to be diluted. High pollutant concentration and variance were found near buildings where wind moves

upward. It was found that the velocity fluctuation, the pollutant concentration and variance were all closely related to the interactions between the primary recirculations and/or the free surface layer.