POLLUTANT DISPERSION OVER A THREE-DIMENSIONAL BUILDING ARRAY

This paper investigates the flow pattern and pollutant dispersion in urban canopy by large eddy simulation of flow over an array of cubes. It has been found that the flow pattern can be categorized as five type based on the packing density, exemplified by the building separation, in comparison with the scales of flow over a isolated obstacle, characterized by a horseshoe-like external wake and an internal cavity. The five types of urban canopy flow are introduced as (1) Isolated roughness flow when the lateral and longitudinal building separations are much greater than the scales of external wake and internal cavity of flow over isolated roughness element; (2) External wake interference flow when lateral building separation is less than the lateral spacing of external wake of flow over isolated roughness element while the longitudinal building separation is greater than the longitudinal scale of internal cavity of flow over isolated roughness element; (3) Internal wake interference flow when the longitudinal building separation is in the same order of the size of internal cavity of flow over isolated roughness element; (4) Skimming flow when the both lateral and longitudinal building separations are less than the lateral and longitudinal scale of flow over isolated roughness element; (5) Street network flow when both lateral and longitudinal building separations are much less than the lateral and longitudinal scale of flow over isolated roughness element. Results of time-averaging velocity field and pollutant concentration contours are demonstrated for each type of flow patterns. It is concluded that the behavior of flow pattern and pollutant dispersion is governed by the packing density from a very low packing density, approximated as the flow around an isolated roughness element, to a high packing density, resembled as the network flow. INTRODUCTION The pollutant dispersion in urban atmosphere is an important topic in urban environment and has been paid great attention since late last century. For two dimensional Figure 1. 2D street canopy (Oke, 1988) urban canopies Oke (1988) proposed a street canyon model that the flow patterns over 2D street canyon depend on the ratio of the building separation of l to its height h. The flow regime is called isolated roughness if l/h > 2 (Fig.1a), wake interference if l/h ∼ 1 (Fig.1b) and the flow is called skimming when l/h < 1/2 (Fig.1c). Since the pollutant dispersion is closely related to the flow patterns, the classification of flow regimes for 2D street canyon gives great help for understanding pollutant dispersion in real building canopy and casting engineering models (Walton et al., 2002). However in real urban canopy the geometrical configuration is three dimensional and the flow patterns are different from those in 2D street canyon. For instance the flow pattern of 3D isolated roughness, shown in Fig.2 (Hanna et al., 1982), is completely different from the flow pattern over 2D roughness elements (Fig.1a). Results of flow visualization(Martinuzzi & Tropea, 1993) indicate that there exist a horse shoe vortex around the roughness element and a pair of circulating flows behind the roughness element. In this case the pollutant will be entrained into the horse shoe vortex with high concentration if there is a pollutant source at the front of the roughness element. The purpose of this paper is to explore whether flow patterns can be classified to some identifiable regimes in 3D urban building canopy and to investigate the essential feature 1 June 30 July 3, 2015 Melbourne, Australia 9 8D-1