Effects of building setback on thermal comfort and air quality in the street canyon

The concern of mitigating thermal stress and pollutant concentration has increased in recent urban design. Most previous studies have demonstrated the promising potential of integrating building setbacks to simultaneously improve thermal comfort and air quality within street canyons. By conducting computational fluid dynamic (CFD) simulations with the consideration of solar radiation, the effects of both horizontal and vertical building setbacks on outdoor thermal comfort and air quality are simultaneously investigated inside the low-rise and high-rise street canyon. Several design parameters of building setbacks are considered, i.e., the dimensionless height (H-HS/W) and dimensionless width (D-HS/W) for the horizontal setbacks (HS), as well as the dimensionless length (L-VS/L) and dimensionless width (D-VS/W) for the vertical setbacks (VS), where W and L are the street width and length. The results demonstrate that the horizontal building setbacks are advocated within the low-rise street canyon, which simultaneously improves the thermal comfort and air quality. By manipulating its dimensionless vertical cross-section area SHS (increasing S-HS = H-HS/W x D-HS/W) and its dimensionless aspect ratio H-HS/D-HS (lowering HHS/DHS), the average PET can decline by up to 2.1 C and the average pollutant concentration can reduce by up to 61% at the two-side pedestrian level (windward and leeward sides). The vertical building set-backs are more suitable for the high-rise street canyon to create better outdoor environments. The dimensionless horizontal cross-section area SVS (= L-VS/L x D-VS/W) should be as large as possible so that the average PET can decrease by up to 0.7 C and the average pollutant concentration can reduce by up to 35% at the two-side pedestrian level. Finally, an effectiveness index to simultaneously evaluate thermal comfort and air quality is introduced and proves effective.