Recent progress in CFD modelling of wind field and pollutant transport in street canyons

Li, Xian‐Xiang; Liu, Chun‐Ho; Leung, Dennis Y.C.; Lam, K. M.

doi:10.1016/j.atmosenv.2006.04.055

Cited by 332 publications

(169 citation statements)

References 93 publications

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“…Additionally the effort to produce operational, but also simple (appropriate) and transparent modelling tools advances rapidly. The computational fluid dynamics (CFD) technique in its many forms provides a wide range of technical options for investigation [9]. While RANS based models still have a role to play, the use of LES modelling can explicitly resolve atmospheric flows of various scales ranging from an individual roughness element to an entire boundary layer [10].…”

Section: Introductionmentioning

confidence: 99%

“…While RANS based models still have a role to play, the use of LES modelling can explicitly resolve atmospheric flows of various scales ranging from an individual roughness element to an entire boundary layer [10]. Various urban flow and dispersion models have been developed based on CFD to simulate the complicated wind pattern and pollutant transport [9]. The success of these models in predicting street-canyon ventilation depends on physically accurate descriptions of turbulence and dispersion within and above the street, particularly in light of the suggestion of Louka et al [11] that ventilation is mediated by inherently intermittent flushing events driven by instabilities in the shear layer.…”

Section: Introductionmentioning

confidence: 99%

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Transport processes in and above two-dimensional urban street canyons under different stratification conditions: results from numerical simulation

Britter

Norford

2014

Environ Fluid Mech

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Thermal stratification (neutral, unstable and stable) plays an important role in determining the transport processes in and above urban street canyons. This paper summarizes the recent findings of the effect of thermal stratification on the transport of momentum, heat, and pollutants in the two-dimensional (2D) urban street canyons in the skimming flow regime. Special attention is paid to the results from large-eddy simulations (LESs), while other experimental and numerical results are referred to when necessary. With increasing Richardson number, Ri, the drag coefficient of the 2D street canyon as felt by the overlying atmosphere decreases in a linear manner. Under neutral and stable stratification, a nearly constant drag coefficient of 0.02 is predicted by the LESs. Under unstable stratification, the turbulent pollutant transport is dominated by organized turbulent motions (ejections and sweeps), while under stable stratification, the unorganized turbulent motion (inward interactions) plays a more important role and the sweeps are inhibited. The unstable stratification condition also enhances the ejections of turbulent pollutant flux, especially at the leeward roof-level corner, where the ejections dominate the turbulent pollutant flux, outweighing the sweeps. With increasing Ri, both the heat (area active scalar source) and pollutant (line passive scalar source) transfer coefficients decrease towards a state where the transfer coefficients become zero at Ri ≈ 0.5. It should be noted that, due to the limit of the 2D street canyon configuration discussed in this paper, great caution should be taken when generalising the conclusions drawn here.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Transport processes in and above two-dimensional urban street canyons under different stratification conditions: results from numerical simulation

Britter

Norford

2014

Environ Fluid Mech

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“…The Reynolds-averaged Navier-Stokes (RANS) equations with the k − ε turbulence model and the large-eddy simulation (LES) are mostly used in the community. The CFD studies for street canyons, including their merits and key findings, are summarized elsewhere (Vardoulakis et al 2003;Li et al 2006). Unlike field or laboratory measurements, the variables in CFD are calculated in the form of spatio-temporal functions, and CFD is often used to examine the fundamental transport mechanisms since idealized conditions can be fully implemented in sensitivity tests.…”

Section: Introductionmentioning

confidence: 99%

Large-Eddy Simulation of Flow and Pollutant Transports in and Above Two-Dimensional Idealized Street Canyons

Cheng

Liu

2011

Boundary-Layer Meteorol

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A large-eddy simulation (LES) model, using the one-equation subgrid-scale (SGS) parametrization, was developed to study the flow and pollutant transport in and above urban street canyons. Three identical two-dimensional (2D) street canyons of unity aspect ratio, each consisting of a ground-level area source of constant pollutant concentration, are evenly aligned in a cross-flow in the streamwise direction x. The flow falls into the skimming flow regime. A larger computational domain is adopted to accurately resolve the turbulence above roof level and its influence on the flow characteristics in the street canyons. The LES calculated statistics of wind and pollutant transports agree well with other field, laboratory and modelling results available in the literature. The maximum wind velocity standard deviations σ i in the streamwise (σ u ), spanwise (σ v ) and vertical (σ w ) directions are located near the roof-level windward corners. Moreover, a second σ w peak is found at z ≈ 1.5h (h is the building height) over the street canyons. Normalizing σ i by the local friction velocity u * , it is found that σ u /u * ≈ 1.8, σ v /u * ≈ 1.3 and σ w /u * ≈ 1.25 exhibiting rather uniform values in the urban roughness sublayer. Quadrant analysis of the vertical momentum flux u w shows that, while the inward and outward interactions are small, the sweeps and ejections dominate the momentum transport over the street canyons. In the x direction, the two-point correlations of velocity R v,x and R w,x drop to zero at a separation larger than h but R u,x (= 0.2) persists even at a separation of half the domain size. Partitioning the convective transfer coefficient T of pollutant into its removal and re-entry components, an increasing pollutant re-entrainment from 26.3 to 43.3% in the x direction is revealed, suggesting the impact of background pollutant on the air quality in street canyons.

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“…a helical flow. Both field and wind tunnel experiments, as well as numerical models, have shown that pollutant concentrations are greatest on the leeward side (or the trailing edge) of a 2D, unity aspect ratio street canyon, when the above-roof winds have a strong component perpendicular to the canyon axis (Boddy et al, (2005a) and reviews by Li et al (2006) and Vardoulakis et al (2003)). The combined influence of dispersion due to the in-canyon mean flow circulation, and continual vehicular emissions within the street, contributes to the in-canyon pollutant concentrations.…”

Section: Introductionmentioning

confidence: 99%

The influence of background wind direction on the roadside turbulent velocity field within a complex urban street

Smalley

Tomlin

Dixon

et al. 2008

Quart J Royal Meteoro Soc

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ABSTRACT:The turbulent velocity field within a complex urban street in the city of York, United Kingdom was measured over a one-month period, with data coverage over a wide range of background wind directions, θ ref (where θ ref = 0°is relative to the street axis, and angles increasing clockwise). Within the street, a persistent mean-flow cross-street circulation exists for 15°≤ θ ref < 165°in addition to possible flow convergence for 240°≤ θ ref < 300°. The magnitude of the in-street normalised turbulent kinetic energy (TKE) is dependent on the type of predominant in-street mean-flow structures. During conditions that correspond to mean-flow cross-street circulation, the TKE is approximately twice the magnitude on the windward side compared with the leeward side. For nearly all wind directions, and on both sides of the street, the TKE is approximately constant with height for 0.4 < z/H < 0.8. There is evidence that the in-street TKE increases with background TKE when other meteorological influences are relatively constant. For background wind directions free from mean flow convergence, the least variability in the sector-averaged turbulence data occurs when the TKE is normalised by the in-street mean wind speed, rather than the background wind speed. The two-point cross-correlation of the verticalvelocity component fluctuations on the windward side is at least 0.6 between the mid and upper anemometers. The two-point cross-correlation between cross-street (same height) vertical-velocity component fluctuations is negative and non-negligible during mean-flow circulation, which indicates possible cross-street coherence in the turbulent velocity field. The turbulent Reynolds stress anisotropy tensor, which provides an indication of the level of TKE redistribution between the components, and the overall level of turbulence anisotropy, is discussed with reference to the mean-flow structures within the street.

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Recent progress in CFD modelling of wind field and pollutant transport in street canyons

Cited by 332 publications

References 93 publications

Transport processes in and above two-dimensional urban street canyons under different stratification conditions: results from numerical simulation

Transport processes in and above two-dimensional urban street canyons under different stratification conditions: results from numerical simulation

Large-Eddy Simulation of Flow and Pollutant Transports in and Above Two-Dimensional Idealized Street Canyons

The influence of background wind direction on the roadside turbulent velocity field within a complex urban street

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