Dispersion mechanisms in a street canyon

Caton, François; Britter, Rex; Dalziel, Stuart B.

doi:10.1016/s1352-2310(02)00830-0

Cited by 103 publications

(61 citation statements)

References 17 publications

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“…Within the resulting shear layer Kelvin-Helmholtz instabilities arise and generate vortices that grow while they are advected downstream from the upwind corner (see Fig. 2; also refer to [33]). …”

Section: Flow and Dispersion Under Neutral Stratification Conditionsmentioning

confidence: 99%

“…The concentration of a passive pollutant in a 2D street canyon is governed by three main mechanisms: the source input rate, the advection-diffusion processes inside the canyon, and the transfer at the roof level of the canyon [33]. Of the three mechanisms, the last one has attracted the most research concerns due to its intimate relationship with urban air quality.…”

Section: Flow and Dispersion Under Neutral Stratification Conditionsmentioning

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: Flow and Dispersion Under Neutral Stratification Conditionsmentioning

confidence: 99%

Section: Flow and Dispersion Under Neutral Stratification Conditionsmentioning

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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“…This is not easy as the geometry of real intersections leads to complex threedimensional flows and associated dispersion conditions. In addition to mean flow processes, turbulent fluxes may play a significant role in exchanging pollutants between streets and with the flow above the canopy (Soulhac, 2000;Caton et al, 2003). Despite this, most street canyon and street intersection dispersion modelling studies focus only on mean fluxes, due to the difficulties arising in measuring concentration and velocity fluctuations at the same time and location.…”

Section: Introductionmentioning

confidence: 99%

“…However, they did not try to calculate integral mass fluxes through the interface between the canopy and the region above. Caton et al (2003) applied a similar PTV technique in a water flume in order to validate their model for pollutant mass exchange between the canopy and the region above. Only mean mass fluxes were estimated, though, while no attempts were made to measure turbulent fluxes.…”

Section: Introductionmentioning

confidence: 99%

Wind tunnel measurements of pollutant turbulent fluxes in urban intersections

Carpentieri

Hayden

Robins

2012

Atmospheric Environment

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“…However, in-street turbulence may also be driven by the above-roof turbulence (Kim and Baik, 2003), depending on the above-roof turbulence level. The analytical velocity and concentration model of Caton et al (2003) showed that for flows with high above-roof turbulence levels, the transfer of pollutants between the canyon and flow above depends on the external turbulence. They showed that within the canyon the mean vorticity field and the TKE were largest in magnitude at z/H = 1.…”

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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Dispersion mechanisms in a street canyon

Cited by 103 publications

References 17 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

Wind tunnel measurements of pollutant turbulent fluxes in urban intersections

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

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