Comparison of urban airflow between solar-induced thermal wall and uniform wall temperature boundary conditions by coupling CitySim and CFD

Abstract

Previous studies investigated the urban wind airflow under the isothermal condition, solar-induced thermal wall and uniform wall temperature boundary conditions. The differences in spatially-averaged airflow properties among those three boundary conditions have not been investigated yet. This study investigated different scenarios to identify: (1) whether the solar-induced non-uniform wall temperatures could be replaced by the uniform wall temperatures (average of all wall temperatures from solar-induced scenario) in the analysis of urban wind airflow and (2) the wind conditions that the wall thermal boundary conditions might be neglected. In this study, urban energy model, CitySim, was employed to calculate the solar-induced walls’ temperatures of an idealized 5 × 5 building array, which were set as thermal boundary conditions in computational fluid dynamic (CFD) simulations. The simulation results were compared to those obtained from cases with uniform wall temperatures and isothermal conditions. The results showed that excluding wall thermal boundary conditions underestimated the spatially-averaged flow properties when reference wind speed (Uref) was lower than 2.0 m s−1 under the designed scenarios and noticeable difference of airflow pattern was observed between isothermal and non-isothermal conditions. The solar-induced wall temperature condition might be replaced by uniform wall temperature condition with Uref of 3.0 m s−1 under parallel wind condition, and with Uref of 5.0 m s−1 under oblique wind conditions. Meanwhile, the airflow structures of the street canyons inside the three-dimensional building array differed significantly from the previous results obtained from isolated street canyon model.