AVERAGE CONCENTRATION PROFILES IN AN IDEALIZED URBAN CANOPY BASED ON HIGH RESOLUTION NUMERICAL SIMULATIONS

摘要

The urban canopy formed by the buildings and the surrounding space has a profound impact on the processes in the atmospheric surface layer. For example, if we consider the low level dispersion of pollutants, we have competing mechanisms that increase or decrease the concentrations in the canopy as compared with flat terrain. The decreased average flow in the canopy leads to a decreased dilution and therefore to an increase of the concentrations. On the other hand, the increased turbulence intensity tend to decrease the concentrations. Depending on the situation, as reported by several authors, the concentrations in the canopy can either be lower or greater than the flat terrain value. To investigate this and other issues, we have performed detailed, high resolution, numerical simulations of the flow in and above an idealized urban canopy, using a E-ε turbulence closure model. The idealized canopy is formed by a regular array of obstacles, either cubes or billboards, with different geometrical layout (square and staggered). Different source position relative to the obstacles and different source heights have been investigated but only one example is discussed here. The simulations are performed at full scale, not laboratory scale. The analysis of the flow field and its average properties has been reported in Carissimo and Macdonald (2001) and Carissimo and Tehranian (2001). In this paper we focus on the analysis of the dispersion calculations.