Impacts of urban geometry on outdoor ventilation within idealized building arrays under unsteady diurnal cycles in summer

Abstract

Efficient ventilation in urban area can dilute air pollution and remove surplus of heat in summer. The urban wind and thermal conditions are influenced by the interaction between local meteorological condition and urban geometry. This study investigated the impacts of building height, building distance and layout form on outdoor ventilation within 5 × 5 idealized building arrays during five consecutive diurnal cycles by steady and unsteady simulations. By employing the hourly meteorological data of a typical year, the diurnal varying solar-induced walls’ temperature boundary conditions in computational fluid dynamics (CFD) simulations were calculated by CitySim. The steady simulation by using standard k-ε turbulence model and cylindrical computational domain was validated. The mean velocity ratios at pedestrian level (Vr‾), air change rates at roof level (ACHroof) and air change rates of the entire targeted canyon (ACHvol) were employed to evaluate the outdoor ventilation efficiency. The results showed that the steady simulations were not able to unfold the accumulative effect of transient weather conditions on the aforementioned parameters at some specified hours. Increasing building distance generated larger Vr‾ and ACHroof but lower ACHvol. Larger Vr‾ and ACHvol but smaller ACHroof was normally observed in square layouts compared to staggered layouts. The increment in building heights led to larger ACHvol and ACHroof monotonously, while its impact on Vr‾ differed among specific hours. This study presented the importance of considering the transient meteorological conditions and diurnal cycles of atmospheric stability conditions in studying urban wind conditions.