Numerical investigations of flow and passive pollutant exposure in high-rise deep street canyons with various street aspect ratios and viaduct settings

As a novelty, the two-main-vortex pattern start appearing in full-scale street canyons as H/W changes from 4 to 5, however previous studies using wind-tunnel-scale models (H = 6cm) reported two to five vortexes as H/W = 2-5. This finding is validated by both smoke visualization in scale-model outdoor field experiments (H = 1.2m, W = 0.6m) and CFD simulations of Reynolds number independence. Cases with two main vortexes (H/W = 5-6) experience much larger daily pollutant exposure (~ 103-104 mg/m3/day) than those with single main vortex as H/W = 1-4 (~ 101-102 mg/m3/day). Moreover leeward-side pollutant exposures are much larger than windward-side as H/W = 1-4 while oppositely as H/W = 5-6. Assuming a general population density, the total iF is 485-803 ppm as H/W = 1, 2020-12051 ppm as H/W = 2-4, and 51112-794026 ppm as H/W = 5-6. With a single elevated pollutant source, cases with viaducts experience significantly smaller pollutant exposures than cases without viaducts. Road barriers slightly increase pollutant exposure in near-road buildings with H/W = 1 while reduce a little as H/W = 3 and 5. Two-source cases can experience 2.60-5.52 times pollutant exposure as great as single-source cases.