Effects of increasing the degree of building height asymmetry on ventilation and pollutant dispersion within street canyons

Rational urban design helps to build sustainable cities with high ventilation capacity and pollutant removal capacity, but the effect of building height on ventilation and pollutant dispersion inside asymmetric canyons has not been fully studied. In this paper, we studied the effect of increasing the degree of building height asymmetry (DBHA) on canyon ventilation and pollutant diffusion in shallow and deep asymmetric street canyons by considering six different building height ratios (BHR = 3/4, 1/2, 1/3, 4/3, 2/1 and 3/1). The results show that increasing the DBHA in asymmetric canyons can improve the ventilation and pollutant removal capacity. For step-up canyons, increasing the downwind building height is very useful to improve ventilation and pollutant removal. For shallow/deep step-up canyons with BHR = 1/3, the air exchange rate (ACH) increased to 211.2% and 380.1% of the flat canyons, respectively. The spatially-average pollutant concentration in the pedestrian zones (leeward K_avg* ang windward K_avg*) decreases significantly with the increase of DBHA, especially for the deep step-up canyon with BHR = 1/3, the leeward K_avg* and windward K_avg* decrease to 15.3% and 3%, respectively. Also, increasing the upwind building height can also improve the ventilation capacity in the step-down canyons. For the deep step-down canyon with BHR = 3/1, the leeward K_avg* and windward K_avg* decreased to 40.6% and 24.1% of the deep flat canyon, respectively. Notably, the ventilation capacity is very low for step-down canyons with BHR = 4/3, and for step-down canyons with BHR ≥ 2/1, the ventilation capacity and pollutant removal capacity increase significantly with the increase of DBHA. Therefore, in urban planning, step-down canyons with BHR = 4/3 should be avoided and designed to satisfy the condition of BHR ≥ 2/1. These findings will be a valuable reference for urban designers to build sustainable cities with high ventilation capacity.