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

Abstract

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.

Large-eddy simulation of turbulent flows over an urban building array with the ABLE-LBM and comparison with 3D MRI observed data sets

“…The power spectra showed a peak at normalized frequency 0.2-0.3, indicating the vortices are oscillated approximately at this frequency. This frequency is similar to the result reported by Han et al [49] in a LES simulation of urban flows with tall buildings. The coherent vortex motion contributes to the TKE and momentum flux transport of at the lee side of the tall building [49], which is also evident in our flowing TKE and turbulent momentum flux analysis.…”

“…This frequency is similar to the result reported by Han et al [49] in a LES simulation of urban flows with tall buildings. The coherent vortex motion contributes to the TKE and momentum flux transport of at the lee side of the tall building [49], which is also evident in our flowing TKE and turbulent momentum flux analysis. The capability to simulate these coherent vortices in the flow is important in building structure design [50] and in transport and dispersion modeling applications.…”

“…The presence of a Karman vortex street on the lee side of tall buildings has been reported in many references [47][48][49][50][51]. In this MRV test, the center building was tall enough and the Reynolds number based on the bulk velocity and the building width is about 3700, well in the range of the necessary condition for a turbulent Karman vortex street at the lee side.…”

Numerical Simulations of Boundary-Layer Airflow Over Pitched-Roof Buildings

“…Past research in urban atmospheric problems has shown that coupling may exist between atmospheric airflow at the city scale (O(10 km)), neighbourhood scale (O(1 km)), and street scale (O(0.1 km)) (Barlow et al 2017;Fernando 2010). This spatial coupling effect is par-ticularly strong around urban forms such as groups of tall buildings (Han et al 2017;Fuka et al 2018;Hertwig et al 2019), or around steep topographic changes. To understand the importance of coupling effects we require a mapping between urban morphology and flowfield characteristics, such as mean velocity, turbulent statistics, momentum and heat fluxes.…”