Impact of wedge-shaped roofs on airflow and pollutant dispersion inside urban street canyons
Introduction
The dispersion of atmospheric pollutants in urban environments depends on the turbulent airflow around complex building structures. A typical configuration is the so-called urban street canyon, which is formed along a relatively narrow street in a densely built urban area with tall buildings lined up continuously along both sides. Within urban street canyons, the pollutants from motor vehicle exhausts have a direct impact on the health of the drivers, cyclists, pedestrians, vehicle passengers, as well as people living or working in the nearby buildings. Since traffic is accepted to be a major emission source of air pollutants in urban areas, and further increase of city traffic is expected, investigations of dispersion processes in street canyons have become a focal point in environmental research [1].
Full-scale field measurements, experimental and numerical models have all been used widely for studying airflow and pollutant dispersion in street canyons. Xie et al. [2] monitored using an automatic sampling system the spatial distribution of traffic-related pollutants within street canyons in Guangzhou, China. The results showed that average horizontal and vertical profiles of pollutant concentrations within street canyon depended on wind direction at the roof level and leeward average concentrations were about 1 time higher than those observed at the windward side. During the hot summer weather conditions, Niachou et al. [3] conducted an experimental study of temperature and airflow distribution inside an urban street canyon in Athens, Greece. It was revealed that the observed airflow characteristics were associated with the impact of thermal effects mainly induced from ground heating due to the incident solar radiation. Meroney et al. [4] and Kastner-Klein and Plate [5] conducted atmospheric boundary layer wind tunnel studies of pollutant dispersion in street canyons. These experiments included the physical modeling of an isolated street canyon and urban environment with many equally spaced buildings. Kovar-Panskus et al. [6] performed wind tunnel experiments on the influence of solar-induced wall-heating on the flow regime inside an urban street canyon. They found that the heating of the windward-facing wall does appear to have some influence on the generation of a very weak secondary flow close to the ground of the canyon at very lower Froude numbers. Gromke and Ruck [7], [8] carried out wind tunnel studies on the impact of avenue-like tree planting on flow fields and dispersion of traffic exhausts in urban street canyons. The results from wind tunnel measurements showed reduced air exchange between street canyons and the ambient air and larger overall traffic exhaust concentrations were found in street canyons with avenue-like tree planting when compared to the tree-free counterpart.
Recently, computational fluid dynamics (CFD) approaches have been increasingly adopted to simulate pollutant dispersion in street canyons. Using the PHOENICS code, Xie et al. [9] carried out two-dimensional calculations for simulating airflow fields and pollutant concentration distributions in urban street canyons under different wind speeds and canyon configurations. Nazridoust and Ahmadi [10] simulated using the FLUENT code for the dispersion of gaseous and particulate exhaust emissions in different street canyons. Cai et al. [11] applied a large-eddy simulation (LES) model to a street canyon in order to derive the fields of wind, turbulence, scalar concentration, concentration fluctuations, and scalar flux across the roof level. Santiago and Martín [12] investigated the pollutant dispersion inside a street canyon using SLP-2D (street Lagrangian particles). These earlier studies have shown that the CFD approach is capable of reproducing the qualitative features of airflow and pollutant distributions inside street canyons.
As noted above, airflow and pollutant dispersion in street canyons have been extensively studied. These studies focused on the effects of wind direction and speed, building configurations, vehicle-induced turbulence and solar-induced building surface-heating on wind flow and pollutant transport in street canyons. In the investigation of building configuration effect on in-canyon wind flow and air quality, it was found that the roof shapes have a significant influence on in-canyon wind flow and pollutant distribution patterns [9]. The roof shapes considered in the previous experimental and numerical studies mainly included the triangular and flat roofs. As for wedge-shaped roofs, only several cases were examined in the wind tunnel modeling of pollutant dispersion within street canyons [5]. Up to now, the numerical investigations on airflow and pollutant dispersion in a street canyon formed by wedge-shaped roof buildings have not been reported. Therefore, further research work is still necessary to understand the effect of wedge-shaped roofs on wind flow and dispersion of traffic-related pollutants so as to help urban planners and architects to take into account wedge-shaped roof configurations with minimum negative impact on local air quality.
The goal of the present work is to provide numerical simulations of airflow and pollutant dispersion within urban street canyons of different wedge-shaped roof combinations and reveal the impact of wedge-shaped roofs on in-canyon wind flow and pollutant distribution patterns. In order to do so, two-dimensional models of urban street canyon are considered and gaseous pollutants emitted from vehicle exhaust are analyzed. A two-dimensional numerical model based on the Reynolds Averaged Navier–Stokes (RANS) equations coupled with the standard k–ɛ turbulence model and the transport equation for passive pollutant concentration is firstly developed using the FLUENT code [13], and then validated against the experimental data obtained from a detailed wind tunnel study by Meroney et al. [4], [14], [15]. After this, the flow fields and concentration profiles inside urban street canyons of sixteen different wedge-shaped roof combinations are evaluated numerically. The free-stream wind speed as well as, pollutant source strength and building configurations modeling an urban environment are the same for all simulation cases, whereas the wedge-shaped roofs of buildings forming the test canyon containing a line source are altered in the simulations.
Section snippets
Computational street canyon configurations
Fig. 1 shows the two-dimensional urban street canyon configurations, which consists of a sequence of parallel streets. A line source of pollutant (a mixture of ethane and air) with constant emission rate is placed at the center of the test canyon floor to model the emission source from the vehicular exhaust. The three styles of building are employed here. The flat-roofed building has a square cross-section of dimension H. From this basic shape are derived two buildings, of width H, with H/3
CFD model
Computational fluid dynamics (CFD) modeling is based on the numerical solution of the governing airflow and pollutant dispersion equations, which are derived from basic conservation and transport principles: (a) the mass conservation (continuity) equation, (b) the momentum conservation (Navier–Stokes) equation and (c) the transport (convection–diffusion) equation for pollutant concentration.
Discussion on the airflow fields
We use the above verified CFD model to carry out the numerical simulations. In each of the seventeen cases depicted in Fig. 1, the simulation is performed using the boundary conditions, the ambient and pollutant source parameters, which are exactly the same as those applied in the above test case for model validation. In order to investigate the influence of wedge-shaped roofs on airflow and pollutant dispersion inside an urban street canyon we consider the case 1, in which both the upwind and
Conclusions
Based on the Reynolds Averaged Navier–Stokes (RANS) equations coupled with the standard k–ɛ turbulence model and the transport equation for pollutant concentration, a two-dimensional numerical model for evaluating airflow and pollutant dispersion inside an urban street canyon is firstly developed using the FLUENT code, and then validated against the experimental data obtained from a detailed wind tunnel study by Meroney et al. [4], [14], [15]. It was found that the model performance is
Acknowledgements
This work was supported by the National Natural Science Foundation of China under Contract No. 70371011.
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