Numerical evaluations of urban design technique to reduce vehicular personal intake fraction in deep street canyons

Highlights: • Personal intake fraction (P_IF) in 2D street is ~1 and ~10² ppm as H/W = 1–3 and 5. • Cases with H/W = 5 and height variation attain three vortexes and P_IF of 10³ ppm. • Elevated building designs reduce 〈P_IF〉 by 3 or 5 orders as H/W = 5. • Wind catchers break multi-vortex pattern as H/W = 5 and reduce 〈P_IF〉 by 1–2 orders. • Viaducts with one more CO source raise concentration but P_IF only change slightly. -- Abstract: High-rise deep street canyons usually experience poor ventilation and large vehicular pollutant exposure to residents in near-road buildings. Investigations are still required to clarify the flow and dispersion mechanisms in deep street canyons and explore techniques to reduce such large pollutant exposure. By conducting computational fluid dynamics (CFD) simulations validated by wind tunnel data and scale-model outdoor field measurements, we investigate the integrated impacts of aspect ratios, first-floor and second-floor elevated building designs, viaduct settings, height variations and wind catchers on the flow, personal intake fraction (P_IF) of CO (carbon dioxide) and its spatial mean value 〈P_IF〉 in two-dimensional (2D) street canyons. Results show that cases with H/W = 5 experience two counter-rotating vortices, much poorer ventilation and 1–2 orders larger 〈P_IF〉 (43.6–120.8 ppm) than H/W = 1 and 3 (3.8–4.3 and 5.6–5.8 ppm). Moreover, in cases with H/W = 5 the height variation results in three vertically-aligned vortices and much weaker wind, subsequently produces greater 〈P_IF〉 (1402–2047 ppm). To reduce 〈P_IF〉 with H/W = 5, various urban designs are evaluated. The first-floor elevated building design creates more effective ventilation pathways than the second-floor elevated type does and reduces 〈P_IF〉 at H/W = 5 by five orders (1402 to ~0.01 ppm) or two orders (43.6 to ~0.1 ppm) in cases with or without the height variation. However, such reductions at H/W = 1 and 3 are only 76.8%–81.4% and 22.4%–36.2% respectively. Wind catchers destroy the multi-vortex flow pattern as H/W = 5, produce a contra-clockwise main vortex and reduce 〈P_IF〉 by 1–2 orders for cases with or without the height variation.