A conceptual model of turbulent dynamics over groups of buildings

A basic understanding of turbulent flow characteristics over groups of buildings is currently lacking. As a consequence, the interpretation of measured data from field experiments in urban areas is often not straightforward. Moreover, recent interest in urban air quality has made it important to understand unsteady turbulent processes on small spatial and time scales. Data from high resolution numerical modelling (DNS and LES) has been used to elucidate the dominant turbulent processes that occur in urban areas, and to document the effect of building heterogeneities such as variable building heights. Comparisons are made with wind-tunnel and field measurements. The results indicate that turbulent eddies generated by the edges of buildings are important in mixing within the urban canopy. In particular, shear layers produced at the rooftops of buildings impact intermittently upon buildings further downstream and drive a recirculation in front of the buildings. Hence, they may play an important role in ventilation within the building canopy. When buildings have variable heights, the shear layers are on average less strong, but there are locally intense shear layers associated with buildings that are taller than average. Within the building canopy, eddies shed off the vertical edges of buildings are rotated by the mean shear. Above the buildings momentum transport is dominated by large hairpin-shaped eddies, which produce elongated streamwise low momentum regions. These low momentum regions, which might be associated with higher scalar concentrations, evolve on a timescale of several minutes. A composite conceptual model is proposed based upon these findings.