Turbulent dispersion of pollutants in urban-type canopies under stable stratification conditions

Abstract

Large eddy simulation is performed for the dispersion of a passive scalar in the turbulent boundary layer over an urban-like roughness surface under neutral and stable stratification. Square arrays of cubical obstacles are used to model the urban canopy with a ground level point source located in front of the obstacle. We design cases for varying building intervals to investigate the relationship between flow patterns and dispersion processes for different plan area densities λp. It has been found (Shen et al. 2015) that flow patterns in a three-dimensional urban canopy can be classified into five basic regimes in neutral stratification: isolated roughness, external wake interference, internal wake interference, skimming flow and streets network. This classification still holds in the presence of moderately stable stratification. In area with large λp, the material released from the point source tends to be trapped by the leeward recirculation and is well-mixed inside the canopy. The mean concentration level within roughness canopies is high due to the reduced advection velocity. In area with small λp, the great part of the material is entrained into the horseshoe vortex wrapping around the obstacle and carried downwind. The material is concentrated in the lower region of the canopy with a relatively higher temporal fluctuation. In the presence of stable stratification, the spread of the plume is reduced and the temporal fluctuation is suppressed as well. For area of large λp, the advective part of the scalar flux is responsible for carrying the material at ground level aloft into the urban canopy layer.