Enhancement of heat removal from street canyons due to buoyant approaching flow: Water tunnel PIV-LIF measurements

Abstract

Heat removal from street canyons to the urban boundary layer above buildings is one of physical processes that may largely determine the (local) urban heat island (UHI) effects. Only few studies show the impact of a buoyant approaching boundary layer flow on heat removal. In this study, we show that heat removal from street canyons is enhanced when the approaching flow is more buoyant as induced by upwind convective heating from a heated ground surface. For this purpose, we perform simultaneous velocity and temperature field measurements in an atmospheric boundary layer water tunnel using particle image velocimetry (PIV) and laser-induced fluorescence (LIF), respectively. Our measurements show that in this case heat removal may be doubled due to reduced suppression at canyon opening. The effectiveness of the heat removal depends highly on the aspect ratio of the street canyons and the distance between street canyon and the place where the buoyant approach flow is produced. Additionally, we show that when the upwind ground surface presents a lower surface temperature, a cooling effect is induced in the immediate downwind street canyon as cooled fluid enters the canyon. The present study suggests that the buoyancy of an approaching boundary layer should be considered in analyzing local UHI and the implications for related UHI mitigation measures are also discussed.