Integrated impacts of solar heating and water evaporation on urban airflows and thermal environments in 2D street canyons

Solar heating affects both airflows and thermal environments. However, the combined effects of turbulent mixing, solar heating and water evaporation remain largely unexplored. Therefore, this study used computational fluid dynamics (CFD) simulations to investigate integrated impacts of turbulent mixing, solar radiation, and evaporation of water bodies on airflow and temperature/ water vapor concentration distribution in two-dimensional (2D) street canyons. Standard k-epsilon model, P-1 solar radiation model, and species transport model were employed. We considered street canyons with various aspect ratios (building height/street width, H/W = 1,3,5) and two water layouts under different solar radiative scenarios (isothermal, ground, leeward and wind-ward heating). The water surface of layout A coincides with the street ground, while layout B is 2 m below the ground. As H/W = 1, one clockwise main vortex appears and evaporation from layouts A and B slightly affects the airflow and temperature. However, under windward heating with layout A as H/W = 3, water evaporation changes original two vertically aligned vortices to three vortices with larger vapor concentration increments and larger temperature reductions (similar to 2.5 K) near the water surface. From layout A to B, velocity change over water surface due to water layout (13.38%- 53.00%) dominates vapor concentration over water surface rather than evaporation rate change (0.32%-8.73%) among aspect ratios (H/W = 1, 3, 5).