A Numerical Study of Flow and Pollutant Dispersion Characteristics in Urban Street Canyons

Abstract

The flow and pollutant dispersion in urban street canyons are investigated using a two-dimensional numerical model with the k?ε turbulent closure scheme. It is shown that the flow field is characterized mainly by the number and intensity of vortices produced in the street canyon. As the street aspect ratio (ratio of the building height to the width between buildings) increases, the number of vortices increases. In the upper-canyon region, the downward motion near the downwind building is stronger than the upward motion near the upwind building, and the turbulent kinetic energy (TKE) is higher near the downwind building than near the upwind building because of stronger wind shears near the downwind building. The TKE budget analysis shows that the shear production is high near the interface between the ambient flow and the street canyon flow and that the turbulent dissipation is also high where the shear production is high. The horizontal advection and diffusion are found to play a crucial role in splitting the vortex into two or more. There is a critical value of the ambient wind speed above which the number and distribution pattern of vortices remain the same regardless of the ambient wind speed. For the given flow fields, two different emission sources (street-level source and advected source) are considered to examine pollutant dispersion in the street canyons that have a one-vortex flow regime and a two-vortex flow regime. Results indicate that the distribution of pollutant concentration in the street canyons during each period of continuous emission and nonemission can be largely explained in terms of the vortex circulation.