Impacts of wind flow across street-side building gaps on traffic pollutant dispersion at pedestrian level with different block heights

Urban neighborhoods, which contains numerous buildings and gaps between the buildings, are usually simplified as a single air-impermeable block in numerical simulations of urban air environments. Such a simplification reduces the computational costs but underestimates the pollutant removal capacity of the street, because the high-speed wind flow through the building gaps causes a flushing effect on pollutant distribution in the street. However, the influences of the gap flow on traffic pollutant dispersion remain unclear. Therefore, a four-way street network surrounded by four neighborhoods is assumed as a representative urban area, and the characteristics of the gap flow and its correlation with building height (H/W = 0.5-2, W = 30 m) are investigated numerically. The results show that the flow pattern within the street depends on the relative intensity of the flow across the gaps and the parallel bulk flow along the street. For lower (H/W = 0.5) or higher (H/W = 2) building heights, the gap flow is strong enough to break through the barrier of the bulk flow, and then enters the street to dilute the pollutants, causing the concentrations in the leeward region of the street to be lower than those in the windward region. Opposite results are found for medium building heights (H/W = 0.75-1.5), where the flow in the street is dominated by the parallel flow itself. The results also indicate that there exists a critical height, H/W = 1.5, below which the air environment is good in both the streets and neighborhoods.

Automated summary

The pollutant removal capacity of urban neighborhoods is often underestimated due to the flushing effect of high-speed wind flow through building gaps. The characteristics of gap flow and its correlation with building height affect pollutant dispersion.