Abstract
A wind tunnel study of the flow and diffusion within model urban canopies was conducted. The concentration patterns within and above canopies were investigated in the Part 2 of this study. Diffusion for four different building densitites (ρz=10-40%) was tested. Tracer gas was emitted from the rooftop at the center of a model building. For the low building density, the highest rooftop concentrations were observed upwind of the source. For the high building density, on the other hand, the highest rooftop concentrations were measured downwind of the source. These differences in the rooftop concentration patterns can be explained by the different rooftop flow patterns shown in the Part 1 of this study. The modified Wilson's equation is a better repesentation of the concentration of the rooftop dispersion.
At ground-level, the maximum concentration appeared windward of the downwind building. This suggests that the gas was transported to the downwind building, then impinged the downwind building, and fl owed down to the ground by cavity flow. In this case, the maximum concentration can be approximately estimated by Briggs's dispersion parameter.
