On the Mechanism of Air Pollutant Removal in Two-Dimensional Idealized Street Canyons: A Large-Eddy Simulation Approach

摘要

Flow resistance, ventilation, and pollutant removal for idealized two-dimensiona (2D) street canyons of different building-height to street-width (aspect) ratios AR are exam ined using the friction factor /, air exchange rate (ACH), and pollutant exchange rate (PCH) respectively, calculated by large-eddy simulation (LES). The flows are basically classified intc three characteristic regimes, namely isolated roughness, wake interference, and skimming flow, as functions of the aspect ratios. The LES results are validated by various experimenta and numerical datasets available in the literature. The friction factor increases with decreasing aspect ratio and reaches a peak at AR = 0.1 in the isolated roughness regime and decrease; thereafter. As with thefriction factor, the ACH increases with decreasing aspect ratio in tht wake interference and skimming flow regimes, signifying the improved aged air remova for a wider street canyon. The PCH exhibits a behaviour different from its ACH counterpar in therange of aspect ratios tested. Pollutants are most effectively removed from the stree' canyon with AR = 0.5. However, a minimum of PCH is found nearby at A R = 0.3, at whicl the pollutant removal is sharply weakened. Besides, the ACH and PCH are partitioned intc the mean and turbulent components to compare their relative contributions. In line with oui earlier Reynolds-averaged Navier-Stokes calculations (Liu et al., Atmos Environ 45:4763-4769, 2011), the current LES shows that the turbulent componentscontribute more to botl ACH and PCH, consistently demonstrating the importance of atmospheric turbulence in tht ventilation and pollutant removal for urban areas.