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  • Boundary-Layer Meteorology manuscript No. (will be inserted by the editor)

    Flow and pollutant transport in urban street canyons of different aspect ratios

    with ground heating

    Xian-Xiang Li · Rex E. Britter · Leslie K. Norford · Tieh

    Yong Koh · Dara Entekhabi

    Received: September 8, 2010

    Abstract A validated large-eddy simulation model was employed to study the effect of the aspect ratio and

    ground heating on the flow and pollutant dispersion in urban street canyons. Three ground heating intensities

    (neutral, weak and strong) were imposed in the street canyons of aspect ratio 1, 2, and 0.5. The detailed

    patterns of flow, turbulence, temperature and pollutant transport were analysed and compared. Of all the cases

    simulated, significant changes of flow and scalar patterns were caused by ground heating in the street canyon

    of aspect ratio 2 and 0.5, while only the street canyon of aspect ratio 0.5 showed change in flow regime (from

    wake interference flow to skimming flow). Ground heating generated strong mixing of heat and pollutant, but

    the normalised temperature inside street canyons was spatially uniform and somewhat insensitive to the aspect

    ratio and heating intensity. The quadrant analysis of pollutant flux showed that sweeps dominated in most of

    the regions within the street canyon, while ejections dominated in the wake of the line source and above the

    street canyon. The ground heating was shown to enhance ejections at the leeward roof-level corner.

    X-X Li

    CENSAM, Singapore-MIT Alliance for Research and Technology, S16-05-08, 3 Science Drive 2, Singapore 117543

    E-mail: lixx@smart.mit.edu

    R.E. Britter

    Department of Urban Studies and Planning, Massachusetts Institute of Technology, Cambridge, MA, USA

    L.K. Norford

    Department of Architecture, Massachusetts Institute of Technology, Cambridge, MA, USA

    T-Y Koh

    School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371

    D. Entekhabi

    Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA

  • 2 Xian-Xiang Li et al.

    Keywords Aspect ratio · Ground heating · Large-eddy simulation · Pollutant dispersion · Urban street

    canyon · Unstable stratification

    1 Introduction1

    With rapid urbanization and city expansion, the sustainable development of the urban environment is now2

    facing many challenges, such as urban heat island effect, deteriorating air quality and severe securities con-3

    cerns (Fernando et al., 2001; Britter and Hanna, 2003; Belcher, 2005). As the typical element of the urban area,4

    the street canyon exhibits a distinct climate where microscale meteorological processes dominate (Oke, 1988).5

    These unique microscale meteorological processes affect not only the local air quality but also the comfort of6

    city inhabitants (Bottema, 1993). Therefore, many research efforts have been devoted to the characteristics of7

    airflow and pollutant dispersion in urban street canyons during the past few decades (Ahmad et al., 2005; Li8

    et al., 2006; Vardoulakis et al., 2003).9

    Located within the roughness sublayer (RSL), the flow in urban street canyon has a strong dependence on10

    the local urban morphology. The flow field inside an urban street canyon is mainly determined by the aspect11

    ratio (AR) defined as building-height-to-street-width ratio (h/b, where h is the building height and b the street12

    width; see Fig. 1). The flow inside street canyons can be classified into different flow regimes according to the AR,13

    i.e. isolated roughness flow (IRF), wake interference flow (WIF) and skimming flow (SF) regimes (Oke, 1988).14

    It was shown that within the SF regime, different ARs resulted in different number of primary recirculations in15

    urban street canyons (Li et al., 2008, 2009). Another factor, the thermal effect (due to solar radiation, release of16

    stored heat, and anthropogenic heat), also has a profound impact on the flow field, and therefore, the pollutant17

    dispersion in street canyons, as demonstrated by many previous studies (Nakamura and Oke, 1988; Ca et al.,18

    1995; Sini et al., 1996; Uehara et al., 2000; Kim and Baik, 2001; Xie et al., 2006).19

    With the development of computer hardware and algorithms, the computational fluid dynamics (CFD)20

    technique has become a popular and powerful tool in urban street canyon research due to its efficiency and21

    relatively low cost (Li et al., 2006). Of the many CFD models, Reynolds-averaged Navier-Stokes equations22

    (RANS) models have provided many insights into the characteristics of the airflow and dispersion in urban23

    street canyons during the past two decades. Recently, large-eddy simulation (LES) has become popular in24

    street-canyon studies mainly owing to its power of handling transient and unsteady turbulent processes (e.g.,25

    Liu and Barth, 2002; Cui et al., 2004; Liu et al., 2004, 2005; Cai et al., 2008; Letzel et al., 2008; Li et al., 2008).26

  • LES of urban street canyons of different AR with ground heating 3

    The LES model developed by Li et al. (2010) was validated against wind-tunnel data and then was applied27

    to study the flow and dispersion inside the urban street canyon of AR 1 with ground heating. With different28

    ground heating intensities, the flow, turbulence and pollutant dispersion exhibited some characteristics that29

    were not present under isothermal conditions. They demonstrated that LES could be an ideal tool to study the30

    thermal effects in street canyons.31

    In this paper, the LES model developed in Li et al. (2010) is employed to study the effect of AR and ground32

    heating on the flow and pollutant dispersion in urban street canyons. The objective is to investigate how the33

    change of urban geometry will affect the flow pattern and pollutant transport when ground heating is present.34

    2 Numerical model and boundary conditions35

    2.1 Numerical model36

    The LES code (Li et al., 2010) developed for incompressible turbulent flow based on a one-equation subgrid-scale37

    (SGS) model is employed in this study. The thermal buoyancy forces are, using the Boussinesq approximation,38

    taken into account in both the dimensionless Navier-Stokes equations and the transport equation for SGS39

    turbulent kinetic energy (TKE). The reference length scale H (the building height of the street canyon of40

    AR 1), the reference velocity scale U (free-stream velocity) and the reference temperature θa (the ambient41

    temperature) are used to make the governing equations dimensionless.42

    The details of the numerical model can be found elsewhere (Li et al., 2008; Li, 2008; Li et al., 2010) and will43

    not be repeated here.44

    2.2 Computational domain and boundary conditions45

    Figure 1 depicts the schematic computational domain used in the current study, which represents a typical46

    street canyon in an idealised manner. The spanwise-homogeneous computational domain consists of a street47

    canyon of height h at the bottom and a free shear layer of height h above the building. The width of the street48

    is b and the length is L.49

    The background atmospheric flow is simulated in the form of a pressure-driven free stream in the free shear50

    layer only. The approaching wind flow is perpendicular to the street axis, which results in a free-stream wind51

    speed U in the streamwise direction. The air flow boundary conditions are set to be periodic in the streamwise52

  • 4 Xian-Xiang Li et al.

    and spanwise directions. No-slip conditions are set at all rigid walls. At the top of the domain, a shear-free53

    boundary condition is assumed.54

    A line source with emission rate Q is located on the ground with a distance xs (= b/2 in this study) from55

    the leeward building. At the inlet, the temperature is set to θa and the pollutant concentration is set to zero56

    (free of pollutant). At the outlet, the convective boundary conditions (Li et al., 2008) are prescribed for both57

    the temperature and pollutant to make sure that they are convected outside the domain and will not enter58

    into the domain again from the inlet. The air temperature at the top is set to ambient temperature θa and the59

    ground level (bottom) maintains a constant temperature θf = θa +∆θ (ground heating). The temperatures at60

    the rigid walls can either be set to a fixed value (ambient temperature θa) or adiabatic (no heat flux at walls);61

    We take the former situation in the present study.62

    2.3 Simulation conditions63

    In this study, three street canyons of different AR, i.e., 0.5 (h