Impacts of shape and height of building roof on airflow and pollutant dispersion inside an isolated street canyon

Two-dimensional numerical simulations were performed using the FLUENT code to investigate the impacts of the shape and height of building roofs on flow and pollutant dispersion inside an isolated street canyon. The numerical model was validated against wind tunnel measured data. Five different roof shapes (vaulted, trapezoidal, slanted, upward wedge-shaped, and downward wedge-shaped roofs) and three different roof heights (Z(H) / H = 1/6, 1/3, and 1/2, where Z(H) is the roof height and H is the building height) were considered in the simulations. The numerical results revealed that the vortical flow structure and pollutant distribution within an isolated canyon depend significantly on the shape and height of building roofs. A large unique clockwise vortex is established in the canyon, and thus the pollutants accumulate in the leeward side of the canyon for the vaulted and downward wedge-shaped roofs at Z(H) / H = 1/6, 1/3, and 1/2, the trapezoidal roof at Z(H) / H = 1/6 and 1/3, and the slanted and upward wedge-shaped roofs at Z(H) / H = 1/6, while an upper clockwise vortex and a lower counterclockwise vortex are generated, and consequently the pollutants pile up in the windward side of the canyon for the trapezoidal roof at Z(H) / H = 1/2 and the slanted and upward wedge-shaped roofs at Z(H) / H = 1/3 and 1/2. In the one-vortex flow regimes, the vortex center moves upwards and the vortex size expands vertically as the roof height increases for the vaulted, trapezoidal, and downward wedge-shaped roofs. In the two-vortex flow regimes, the size and rotational speed of both the upper clockwise and lower counterclockwise vortices increase with roof height for the slanted and upward wedge-shaped roofs. At each of the three roof heights examined, the lowest pollution level occurs in the canyon with the downward wedge-shaped roof, suggesting that the downward wedged-shaped roof is more beneficial for pollutant removal from the isolated canyon than the vaulted, trapezoidal, slanted, and upward wedge-shaped roofs. Due to the establishment of two-vortex flow structures, the trapezoidal roof at Z(H) / H = 1/2, the slanted roof at Z(H) / H = 1/3 and 1/2, and especially the upward wedge-shaped roof at Z(H) / H = 1/3 and 1/2 are unfavorable for pollutant dispersion and dilution in the canyon compared to the vaulted and downward wedge-shaped roofs.