Impact of green walls on ventilation and heat removal from street canyons: Coupling of thermal and aerodynamic resistance

Green walls are considered a sustainable and environmental technology for improving the local urban microclimate. The presented wind tunnel measurements reveal the coupling effect of aerodynamic and thermal resistance of green walls in flat and steep street canyons in a simplified urban neighborhood model. The building and ground surfaces are set to thermal conditions at a series of Richardson numbers (Ri) ranging from 0 to 2.07. Green walls induce flow resistance due to their high skin friction and large volume-specific surface areas, which leads to the formation of a thick boundary layer near green walls, where hot air is trapped. These boundary layers weaken the convective heat transfer from building surfaces to the canyon, leading to an air temperature reduction in the canyon. The lower temperature reduces the strength of buoyancy, which, together with the aerodynamic resistance, results in the reduction of air flow speed, turbulence kinetic energy (TKE), and air ventilation in the canyon. The reduction of air ventilation is more significant in non-isothermal conditions, particularly for the steep canyon, where the air entrainment at the roof level is reduced by 32.4% in the isothermal condition, and air removal is reduced by up to 91.5% in the non-isothermal conditions (Ri of the steep canyon, Ris = 2.07). Green walls alter the flow structures by suppressing the strength of the downward flow, canyon-wide vortical flow, and ascending buoyant flow.