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Pollutant Dispersion over Two-dimensional Idealized Street Canyons: a Large-eddy Simulation Approach
| Content Provider | Semantic Scholar |
|---|---|
| Author | Liu, Chun-Ho |
| Copyright Year | 2011 |
| Abstract | A series of two-dimensional (2D) street canyon models with a wide range of building-height-to-street-width (aspect) ratios are employed in this study to elucidate the pollutant transport over idealized urban areas. The large-eddy simulation (LES) is used to resolve the turbulent flows and pollutant transport in the urban boundary layer (UBL) over the street canyons. An area source of uniform pollutant concentration is applied on the ground of the first street canyon to examine the pollutant plume dispersion behaviors over the downstream building roughness elements. The LES results show that, for the street canyon with the pollutant source, the pollutant removal is governed by atmospheric turbulence in both skimming flow and wake-interference regimes. Statistical analysis reveals that the turbulent kinetic energy (TKE) is peaked near the top of the building roughness elements that contributes most to turbulent pollutant removal. The roof-level TKE distribution also demonstrates that the turbulence production is not governed by local wind shear. Instead, the descending TKE from the UBL plays a more important role. In the UBL, the vertical pollutant profiles illustrate self-similarity behaviours in the downstream region. The pollutant disperses rapidly over the buildings, exhibiting a Gaussian-plume shape. Maximum vertical pollutant dispersion coefficient is observed at aspect ratio equal to 1/10. A strong correlation between friction factor and dispersion coefficient is found, implying that the downstream air quality could be improved by increasing the roughness of urban area. Oke (1988) classified the flow patterns in 2D street canyons into three characteristic regimes as function of the building- height-to-street-width aspect (AR) ratio. The skimming flow regime, which is also known as d-type roughness, is in the range of AR > 0.7. In this flow regime, the mean flow from the urban boundary layer (UBL) does not enter into the lower street canyon. Instead, stable and isolated recirculation(s) develops inside the street canyon, resulting in poor air ventilation and pollutant removal. Street canyons of 0.4 < AR < 0.7 fall into wake-interference flow regime. In this flow regime, the mean flow from the UBL could enter the upper portion street canyon but cannot touch the ground. Although the air exchange rate in the wake-interference flow regime is better than that in the skimming flow regime, a stronger pollutant re-entrainment is found which may lead to a higher pollutant concentration within the street canyons. Street canyon of AR < 0.4 is in the isolated roughness regime. In this flow regime, the mean flow from the UBL could reach the ground of the street canyons so the air ventilation and pollutant removal is much better compared with the other two flow regimes. Although the pollutant transport within the street canyon is strongly related to flow regime, the plume in the UBL may not directly related them. To elucidate the relationship between urban roughness parameters and pollutant plume dispersion behaviour, large-eddy simulation (LES) of plume dispersion over idealized 2D street canyons was conducted. METHODOLOGY The open-source CFD code OpenFOAM 1.7.0 (2011) was used in this study. The computational domain, boundary conditions (BCs) and other modeling details are described in this section. The computational domains in this study are based on the LES in Cheng and Liu (2011) with extended domain size and various aspect ratios. The domain bottom consists of repeated, identical street canyons to construct an idealized 2D urban roughness with building height h, building width d (= h), and building separation b in streamwise direction. The domain above the building roof represents the UBL of height H (= 7h). The spanwise domain size is 5h. For the flow field, no-slip BCs are applied on the roughness at the bottom and a free-slip BC is applied on the domain top develop an UBL of thickness δ (= H+h) . Periodic BCs are applied in the horizontal directions in order to simulate the flow over an infinitely large urban area. The flow is driven by a background pressure gradient, which is only applied in the UBL, in the streamwise direction. The flow in the current LES is assumed to be incompressible and isothermal. For the pollutant transport, a constant concentration source Φ is prescribed on the ground of the first street canyon after the inlet. Zero-gradient BCs are applied on other ground surfaces, the building facades, and the top boundary. Zero-pollutant and open BCs are applied at the domain inlet and outlet, respectively, to prevent from the interference of background pollutant concentration and pollutant reflection. The s chematic of computational domain and BCs of the model of unity AR is shown in Figure 1. The geometry and BCs of the current computations are similar to each other. The major differences among those models are the ARs, domain size in the streamwise direction, and the number of street canyons. In this study, the ARs of the models are in the range of 0.1 to 2.0 which is sufficient to cover all of the three flow regimes in 2D street canyons proposed by Oke (1988). A flat model |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://hub.hku.hk/bitstream/10722/165402/1/Content.pdf |
| Alternate Webpage(s) | https://www.harmo.org/Conferences/Proceedings/_Kos/publishedSections/H14-117.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
