Study on effects of fluctuations in the wind direction on pollutant diffusion in urban areas

The effect of varying wind directions on pollutant diffusion from a point source in an urban area have been studied in a wind tunnel experiment, using tracer gas techniques. Most experiments were conducted under neutral conditions of the atmospheric boundary layer. Some cases were investigated in stable and unstable conditions. Diffusion fields on a city model of Hamamatsucho in Tokyo, Japan were simulated in a turbulent boundary layer wind tunnel. The scale of the model was taken as 1:600. Ethylene, C2H4, was used as the tracer gas. A total of nineteen wind directions from west-northwest 292.5degrees to north- northwest 337.5degrees were investigated at eighty-three sampling points located on the model during the diffusion experiments. Flow characteristics were studied by local changes in the wind direction with flag fiber paper, wind speed, and flow visualization. A theoretical prediction method for pollutant diffusion for varying wind directions was applied and the differences between the experiments and the idealized theory were compared. The results obtained indicate that; (a) the local wind direction observed was inconstant and changed by a small wind angle; (b) the gas diffusion for stable conditions was higher than that for the neutral and unstable conditions; and (c) the agreement between the wind tunnel experiment and idealized theory are closed for the purpose of practical applications. The diffusion measured in these experiments may be used to develop and evaluate numerical models to predict the pollutant in an urban area.