A wind tunnel study on flow instability and pollutant dispersion inside an isolated street canyon

The air quality in street canyons is closely related to the category of the street canyons and approaching wind conditions. Compared to urban street canyon (USC), flow and dispersion in isolated street canyon (ISC) show more instability. This study conducted wind-tunnel experiments to investigate the influence of wind velocity (UH), upwind building width (WB), wind profile (alpha) and turbulence intensity (I) on flow instability and pollutant dispersion characteristics within an ISC. The flow field visualization and quantitative pollutant detection system were applied to capture the flow and diffusion pattern characteristics. Results show that compared to the stable clockwise main vortex in USC, the flow structure in ISC presents the strong instability, and the average concentration in ISC is about half of that in USC. Due to the flow separation vortex, the flow instability in ISC is manifested by the alternation of upper clockwise vortex and lower counterclockwise vortex. That also cause the pollutant accumulate on the windward side. And this particular flow structure in ISC is determined by both building height Reynolds number ReH,crit (= 1.6 x 104) and building width Reynolds bumber ReW,crit (= 2.4 x 104). Moreover, as alpha increases, the clockwise main vortex will gradually dominate in ISC, meanwhile, the increasing turbulence intensity will be conducive to strengthening the air exchange rate and reducing pollutant concentrations. This study will be helpful to further understand the instability characteristics and formation mechanism of flow and dispersion within ISCs.