Comparison of uniform and non-uniform surface heating effects on in-canyon airflow and ventilation by CFD simulations and scaled outdoor experiments

Abstract

Many studies have simplified the thermal buoyancy in urban areas as a uniform surface heating effect. However, a realistic scenario under solar shading conditions corresponds to non-uniform surface heating. Furthermore, effective validations of Computational Fluid Dynamics (CFD) simulations under realistic meteorological conditions are still limited. As a novelty, this study adopted scaled outdoor experimental data to validate CFD simulations. The validated CFD model was then utilized to investigate the flow characteristics with non-uniform surface heating in a full-scale street canyon (building height/street width, H/W = 2; H = 12 m). Moreover, we numerically quantified the differences between the impacts of uniform and non-uniform surface heating on in-canyon airflow and ventilation under various thermal conditions.In the absence of surface heating, there was a single vortex in the street canyon. However, for uniform and non-uniform surface heating, thermal buoyancy resulted in the appearance of three in-canyon vortices. The ventilation differences between these two surface heating conditions increased with thermal buoyancy. When the dynamic force dominated the urban airflow (Richardson number, Ri = 0.1), the flow structures for both surface heating conditions were similar, with a ventilation difference of only 3.77%. When both the dynamic and thermal buoyancy forces were significant (Ri = 3.54), the flow patterns of the two surface heating conditions differed near the windward wall. The corresponding ventilation difference reached 27.36%. This study emphasizes the importance of examining urban flows and ventilation under non-uniform surface heating conditions, particularly when the thermal effects are significant.