Abstract
The dispersion of motor vehicle exhaust gas was computed using the k-ε model in a built-up residential area of two story buildings. The near neutral atmospheric stability and wind direction perpendicular to the street were adopted as the model conditions. The computed concentrations were compared to the observed SF6 concentrations from a dispersion experiment. The dispersion domain was within 150 m from the street. We investigated the size of the upstream domain (driver region) required for generating the turbulence intensity that agreed with the observations. The driver region of 250 m produced a good agreement in the turbulence intensity between the computed value and the observation at a z=15 m height, although we used an estimated exponent value for the vertical wind profile as the upstream boundary condition. A sensitivity analysis was done for agreement between the computed and observed concentrations when we varied the number of grid points within the street canyons. This analysis showed that more than 10 grids in the vertical and span-wise directions of the street canyons produced a good agreement. An accurate simulation of the turbulence intensity and a high-resolution simulation of the flow and vortices within the street canyons yielded a good agreement between the observed and computed concentrations. The motor vehicle exhaust gas is dispersed by the mechanical turbulence generated by buildings near the grid points in urban areas. Therefore, it is possible to approximate the turbulence generated upstream, far from the dispersion area, by the upstream boundary condition. However, the turbulence generated near the grid points should be computed at a high resolution.
