Effects of height-asymmetric street canyon configurations on outdoor air temperature and air quality

This paper investigates the effects of height-asymmetric street canyon configurations on air temperature and air quality at the pedestrian level using the ANSYS Fluent® software. The study concerns the situation with a subtropical city where there is a predominant wind direction (as is the case in, e.g., Hong Kong) and where the direction of that wind is perpendicular to the street canyon, since this is the worst-case from air pollution and overheating point of view. In particular, this North-South oriented street has been studied with the realistic solar irradiance at two different sun directions, corresponding to morning (08:00) and afternoon (16:00) hours, respectively. Two step-up and two step-down North-South oriented street canyons are considered under two different incoming wind speeds (high and low). The corresponding ratios of upwind and downwind building heights are = 1/3, 2/3 and 3/1, 3/2, respectively. The results demonstrated that for the step-up canyon, a higher upwind building was found to produce a hotter air temperature only at a low wind speed and polluted more severely at both high and low wind speeds, compared with its lower upwind building counterpart. In contrast, for the step-down canyon, a higher downwind building was found to produce cooler air temperatures at both high and low wind speeds and accumulated more pollutants only at a low wind speed, compared with its lower downwind building counterpart. On the other hand, at the high wind speed, both air quality and thermal environment were better in the step-up canyon than in the step-down canyon. However, at the low wind speed, the air quality was higher in the step-down canyon than the step-up canyon, while the step-up canyon still provided better thermal environment than the step-down canyon. Moreover, a Richardson number (Ri) for the asymmetric street canyons is defined for the evaluation of the buoyancy force versus the inertial force. When |Ri| > 20, the flow field was mainly dominated by natural convection, and an increase of |Ri| resulted in an increase in the air temperature and a decrease in the pollutant concentration. In contrast, when |Ri| < 20, the flow field was dominated by forced convection, and the variation of |Ri| had an insignificant influence on air quality and air temperature. The simulated pollutant concentration and thermal environment results were further processed to obtain optimization guidelines for a north-south asymmetric canyon in the city centers of Hong Kong via the application of multivariate regression analysis with a group of dimensionless parameters. These guidelines will facilitate the renewal of north-south asymmetric street canyons while enhancing air quality and lowering air temperature by serving as a reference for architects.