Large-eddy simulation of vortex streets and pollutant dispersion behind high-rise buildings

Understanding turbulent flow and pollutant dispersion in urban areas is one of the important problems in urban meteorology and environmental fluid mechanics. In this study, we examine turbulent flow and pollutant dispersion in a densely built-up area in Seoul, South Korea, using the parallelized large-eddy simulation model (PALM). In particular, we focus on vortex streets and associated pollutant dispersion behind high-rise buildings. The turbulence recycling method is used to produce inflow profiles. Vortices are generated near the high-rise buildings and propagate downstream forming vortex streets behind the high-rise buildings. To investigate characteristics of the vortex streets, spectral and correlation analyses are performed. The spectral analysis reveals that vortices have a non-dimensional vortex shedding frequency of 0.1–0.2, and this periodicity is weakened due to the influence of other buildings. The correlation analysis shows that vortices appear frequently in regions of negative pressure perturbation. The vertical turbulent momentum fluxes induced by ejections and sweeps largely contribute to the total vertical turbulent momentum flux downstream of the high-rise buildings. Especially, ejections in the wake region are stronger compared to other regions because ejections are induced by vortices near the top of the high-rise buildings. It is found that pollutant dispersion is interrupted by both low-rise and high-rise buildings. Strong updraughts behind the high-rise buildings transport pollutant upward and increase the mean pollutant concentration at upper levels. Vortices forming the vortex streets play a role in pollutant mixing in such a way that the vortices eject air of high pollutant concentration from the wake region behind the high-rise buildings and entrain air of low pollutant concentration into the wake region. The mixing by vortices is verified by the correlation between vorticity and pollutant concentration.