Wind tunnel experiments on how thermal stratification affects flow and dispersion within and above urban street canyons

Abstract

We used a thermally stratified wind tunnel to investigate the effects of atmospheric stability on flow and dispersion in street canyons. Using a model that represented city streets with simply shaped block forms, we conducted experiments while varying atmospheric stability across 7 stages from stable (Rb=0.79) to unstable (Rb=0.21). Model scale was assumed to be 1: 300. We used a laser Doppler anemometer (LDA) and a cold wire to measure flow fields within and above street canyons . In addition to mean values of wind speed components and temperatures, we measured turbulence intensity, shear stress, and heat flux distribution. A tracer gas (C2H6) was released from a line source in the street canyon center to measure concentration distribution inside the street canyon and study its ventilation mechanism. Results indicated the following: Cavity eddies that arise in street canyons tend to be weak when the atmosphere is stable and strong when unstable. Owing to flow changes that depends on the atmospheric stability, air pollution concentrations in street canyons are high when the atmosphere is stable and low when unstable. Air pollutants released in street canyons recirculate from canyon tops to interiors, with the proportion varying according to atmospheric stability.