Street canyon ventilation and airborne pollutant dispersion: 2-D versus 3-D CFD simulations
Introduction
A ‘street canyon’ refers to a narrow space between buildings that line up continuously along both sides of a street (Li, Liu, Leung, & Lam, 2006). It has a unique climate where micro-scale meteorological processes dominate (Oke, 1988). Pollutants emitted at the ground level considerably deteriorate the local air quality and impose direct impacts on human health. The highest level of pollution and the most outdoor human activities are both concentrated at street canyons, causing the most serious health threat. (Vardoulakis, Fisher, Pericleous, & Gonzalez-Flesca, 2003). The thermal comfort of pedestrians is also related to the street canyon geometries (Chatzidimitriou & Yannas, 2017; Syafii et al., 2017). The pedestrian wind environment and thermal comfort could be improved by intentionally designing the street canyon (Du, Mak, & Li, 2019). Understanding the airflow and pollutant dispersion within the urban street canyon is important to the sustainability of the urban environment.
The wind flows in the street canyons are inherently complex and exhibit a wide range of physical characteristics including large low-speed areas, strong pressure gradients, unsteady flow regions, three-dimensional effects and wakes (Deck, 2005). These wind flow mechanisms are strongly related to geometry configurations and incoming wind directions. The most widely studied cases in the literature are those with wind perpendicular to the street axis because they represent the worst situation for air pollutant dispersion (Li et al., 2006). Under such wind direction, it is reasonable to assume that the street is infinitely long. Then, the original complex 3-D problem could be simplified as a 2-D one.
There are two types of 2D cases in previous studies: pure (only 2 directions are simulated) and quasi 2D (all three directions are simulated for a quasi-infinitely long street canyon using lateral periodic boundary conditions). In the 2-D cases, the most important geometrical feature of a street canyon is the aspect ratio, which is the height (H) of the canyon being divided by the width (W). Oke (1988) suggested that the flow within 2-D street canyon could be described in terms of three regimes depending on the aspect ratio (H/W) (Oke, 1988). From a three-dimensional point of view, the length (B), which usually expresses the road distance between two major intersections of the canyon, represents another important geometrical feature of the street canyon. The airflow in the street ends is characterized by horizontal corner vortices. Soulhac et al. (2009) concluded that the flow and dispersion at the street ends were dominated by a large vertical-axis recirculating vortex, which has an important influence on exchanges between the streets and overlying atmosphere. Carpentieri and Robins (2010) measured the mean and turbulent tracer fluxes within several street intersections in a wind tunnel model of a real urban area located in Central London. They found an increase in turbulent exchange at roof level at the intersections (Carpentieri, Hayden, & Robins, 2012). Their later wind tunnel measurements indicated that complex advective patterns appeared at intersections composed of very simple and regular geometries (Carpentieri, Robins, Hayden, & Santi, 2018). Michioka, Takimoto, and Sato (2014) conducted a series of large-eddy simulations of 3D street canyons with multiple street lengths. Their simulations show that the mean concentration within the canyon decreased with street length B due to stronger lateral dispersion. The DNS (direct numerical simulation) study of Coceal, Goulart, Branford, Glyn Thomas, and Belcher (2014) showed that the complicated flow pattern had a significant influence on dispersion and mixing within the intersection. Based on the wind tunnel measurements, Nosek, Kukačka, Jurčáková, Kellnerová, and Jaňour (2017) calculated the pollution flux (turbulent and advective) at the lateral openings of three different 3D street canyons when the wind was perpendicular and oblique to the along-canyon axis. Their results confirmed that the buildings’ roof-height variability at the intersections plays an important role in the dispersion of the traffic pollutants within 3D canyons.
Riain, Fisher, Martin, and Littler (1998) summarized that the dispersion of gaseous pollutants in a street canyon depended on the air exchange rate at the openings of street canyons, including the roof of the street canyon and street ends. Vardoulakis et al. (2003) subdivided street canyons into short (B/H ≈ 3), medium (B/H ≈ 5) and long canyons (B/H ≈ 7) based on the street length. In relatively short canyons, corner vortices might be strong enough to inhibit the formation of a stable vortex perpendicular to the street in the mid-section. With the increase of street length, this ventilation effect will become less important (Theurer, 1999). Chan, So, and Samad (2001) found that the B/H ratio can also affect the pollutant concentration inside street canyons. Their later study found that the correlation between pollutant concentration and B/H is due to the vortices generated at the street ends (Chan, Au, & So, 2003). Xue and Li (2017) simulated the pollutant dispersion within 3D street canyons and found a maximum pollutant concentration at the symmetry plane and minimum pollutant concentration at street ends. All these important features which are evident in 3-D street canyons are normally neglected in the 2-D airflow and ventilation simulations. In LES studies, although 3-D computation domain is widely used, the streets are usually assumed as infinitely long by using periodic boundary condition at side boundaries to reduce computational cost (Lateb et al., 2016).
In the past two decades, there have been many modeling and experimental studies focusing on 2-D canyon cases (Koutsourakis, Bartzis, & Markatos, 2012; Marciotto & Fisch, 2013; Magnusson et al., 2014; Ngan & Lo, 2016). Previous studies show significant differences in airflow and dispersion between 3-D and 2-D canyons (Nosek et al., 2017; Xue & Li, 2017). However, it is still not clear when and how well the 2-D models could represent the airflow and pollutant dispersion in the 3D scenarios. As many urban design guidelines were based on previous studies with 2D model, it is necessary to find out the differences between 3D and 2D simulations. Additionally, the 2D simulation can extensively reduce the computational resource, especially at LES scenarios. In the near future, the quasi-2D model is expected to be widely used in LES studies. The present paper attempts to identify requirements that the ventilation at 3-D street canyon can be represented by 2-D models. Specifically, the main research questions are:
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Can a 2-D model represent a real 3-D street canyon for street canyon ventilation simulation?
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Is there a minimum street length/height ratio that a 2-D model could represent a 3-D street canyon?
These questions are explored by conducting a series of 3-D simulations with different street lengths and comparing against a corresponding 2-D simulation. The ambient wind is assumed perpendicular to the street direction at 3-D scenarios. Different indicators such as ACH, normalized concentration, retention time are used as metrics to evaluate the ventilation and air pollution dispersion performance. This paper is structured as follows. The details of the model geometries and methodology are given in Section 2. In Section 3, the results are presented by looking at the flow and concentration fields along with multiple ventilation indices. Conclusions are presented in Section 4.
Section snippets
Methodology
The airflow in the urban area is considered as isothermal and the buoyancy effect is neglected. The time-averaged velocity and concentration fields are predicted using the Reynolds-averaged Navier–Stokes equations (RANS). The open source CFD (computational fluid dynamics) codes OpenFOAM v4.0 is used to solve governing equations of fluid dynamics. The data from wind tunnel experiments carried out by Tominaga and Stathopoulos (2011) is used to validate the computational model.
Three-dimensional street canyon airflow
Dispersion within three-dimensional street canyon is heavily influenced by the flow structure. Therefore, we begin by describing the basic flow pattern within a street canyon surrounded by urban buildings and subjected to perpendicular approaching wind, as shown in Fig. 5. Gromke and Ruck (2007) summarized that there are two distinguishable flow characteristics, i.e., vertically rotating (recirculating with the along-canyon axis) canyon vortices and horizontally rotating (recirculating with the
Conclusions
In this study, the differences between 2-D and 3-D RANS simulations on resolving the ventilation at street canyon are investigated. The focus is on identifying the threshold value of street length (B) that 2-D results can well represent real 3-D street canyon. Here the skimming flow regime is considered with two aspect ratios (H/W = 1.0 and 2.0) for their wide adoption in previous studies. Both isolated street canyon (ISC) and multiple street canyon (MSC) configurations are considered. The air
Acknowledgments
The first author (S Mei) wishes to acknowledge the financial support from Wuhan University for funding his academic visit at the University of Reading, UK, where the work was conducted. Prof. Fu-Yun Zhao would like to acknowledge the financial support from the Natural Science Foundation of China (NSFC Grant No. 51778504), Fundamental Research Projects from Shenzhen Government (Grant No. JCYJ20160523160857948), and National Key Research and Development Program of the Ministry of Science and
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