The impact of semi-open street roofs on urban pollutant exposure and pedestrian-level thermal comfort in 2-D street canyons

Abstract

Semi-open street roofs(SOSRs) have been confirmed to provide solar shading for pedestrians, usually improving outdoor thermal comfort but weakening urban ventilation in three-dimensional(3-D) urban districts. However, these positive and negative impacts have rarely been quantified together. Particularly, their impacts in two-dimensional(2-D) street canyons have not been investigated.Accordingly, by conducting computational fluid dynamic(CFD) simulations coupling turbulence and radiation processes, this study investigates the integrated influences of SOSRs on both ventilation/pollutant exposure risks and pedestrian-level outdoor thermal comfort in 2-D full-scale street canyons(aspect ratios AR = H/W = 1, 3, 5; W = 24 m). Isothermal and non-isothermal cases with three solar angles(LST0900, LST1200, LST1500) are considered. The pedestrian-level maximum normalized velocity(Vpedmax/Vinlet) and physiological equivalent temperature(PET) are analyzed for ventilation and thermal comfort assessment.SOSRs have an overall negative impact on present 2-D streets. Without SOSRs, only one main clockwise vortex exists when H/W = 1 and H/W = 3. Compared to the isothermal cases, LST0900 and LST1200 always strengthen this main vortex, while LST1500 weakens it slightly. The SOSRs may produce some secondary near-ground clockwise and counter-clockwise vortexes, which significantly reduce Vpedmax/Vinlet, i.e. from 0.33–0.44 to below 0.07 when H/W = 1 and 0.063–0.13 to 0.049 when H/W = 3. They also increase pedestrian-level PET by 0.9–10.4 °C(H/W = 1) and 0.1–4 °C(H/W = 3). When H/W = 5, two main vortexes appear in the isothermal case inducing much smaller Vpedmax/Vinlet of 0.004 and solar radiation significantly raises the Vped/Vinlet to 3–25 times without SOSRs. The impacts of SOSRs are not as significant as those at H/W = 1 and 3. Overall, the SOSRs impact should be assessed before their application, especially in 2-D streets.