Abstract
Using controlled tests in a wind tunnel, we simulated the pollution of four different street canyons formed by four 3D urban array models. The urban models differed by the geometry of roofs (pitched and flat roofs) and buildings (courtyard and solid buildings).
We simulated traffic pollution from a ground-level source positioned in the middle of the street canyons. We show that the courtyard buildings significantly improve (by a factor of 1.3) the ventilation of the street canyons only in the cases with pitched roofs.
We explain the differences between the ventilation performances of the street canyons by analysing the dynamics of the coherent structures. The buildings at the roof level shed two main vortex structures into the flow.
However, the street canyon with pitched roofs and the courtyard buildings shed more stable structures that collide and penetrate deeper downstream near the wind-facing eaves. Near the pedestrian zone, ventilation is driven by advection, manifested as corner vortices at the street ends and flow convergence from the windward to the leeward side in the middle of the street canyons.
The corner vortices are more pronounced in the courtyard buildings regardless of roof shape, resulting in higher concentrations than solid buildings.