Flow pattern and pollutant dispersion over three dimensional building arrays

• Accomplishment of LES investigation of flow and pollutant dispersion in building arrays.
• Discovery of five identifiable flow patterns which is closely related to building density.
• Exploration of the relationship between flow patterns and pollutant dispersion.

The flow pattern and pollutant dispersion in urban canopies is investigated by large eddy simulation of flow over an array of cubes. It had been found that the pattern of flow over an isolated cubical obstacle can be characterized by an external wake of horseshoe vortex around the lower part of the windward face and an internal wake of recirculation cavity leeward. The width of the external wake Wex and the size of the internal wake Lin in the isolated roughness flow are used as key parameters to determine the wake effects on the flow and dispersion in the canopy of the same roughness height h in the isolated roughness flow. Flow patterns are categorized into five types based on the packing density as a result. The five types of urban canopy flow are introduced as (1) Isolated roughness flow when the lateral building interval WL is much greater than Wex and the streamwise building interval WS is much greater than Lin; (2) External wake interference flow when WL is less than Wex while WS is greater than Lin; (3) Internal wake interference flow when WS is in the same order of the size of Lin; (4) Skimming flow when WL is less than Wex and WS is less than Lin (5) Street network flow when WL and WS are much less than the Wex and Lin respectively. 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 case, approximated as the flow around an isolated roughness element, to a high packing density case, resembled as the network flow.