Impacts of building-height variability on turbulent coherent structures and pollutant dispersion: Large-eddy simulations

In built-up urban areas, there are numerous buildings of various shapes and heights, and their collective impacts on urban flow and air quality are of great concern. Here, we numerically examine the impacts of building-height variability on turbulent coherent structures above an array of square buildings and pollutant dispersion using a large-eddy simulation model. Three simulations are conducted: no building-height variability case (σ/H = 0, where σ is the standard deviation of building heights and H is the average building height), small building-height variability case (σ/H = 0.25), and large building-height variability case (σ/H = 0.5). In the latter two cases, buildings of two different heights are alternately arranged. Pollutants are released from street level. As the building-height variability increases, the average pedestrian-level pollutant concentration decreases and its spatial variability becomes large. The building-height variability greatly influences turbulence intensity and vertical turbulent momentum and pollutant fluxes. The streamwise-elongated low- and high-speed flow structures appear in all three cases, and they are wider in the spanwise direction when there is building-height variability. Strong turbulent fluctuations seem to affect the spanwise extension of the high- and low-speed flow structures. Quadrant analysis shows that stronger ejection and sweep events are more frequent with increasing building-height variability. The regions of ejection (sweep) well coincide with those of pollutant ejection (pollutant sweep). When there is building-height variability, the streaky structures that appear also in the fields of instantaneous vertical turbulent momentum flux are somewhat wider in the spanwise direction.