Large-Eddy Simulation of Flow and Pollutant Transport in Urban Street Canyons with Ground Heating

Li, Xian‐Xiang; Britter, Rex; Koh, Tieh‐Yong; Norford, Leslie K.; Liu, Chun‐Ho; Entekhabi, Dara; Leung, Dennis Y.C.

doi:10.1007/s10546-010-9534-8

Cited by 98 publications

(63 citation statements)

References 37 publications

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“…Li et al [54,55] have performed LES studies of flow and pollutant dispersion in street canyons of h/b = 0.5, 1, and 2 with ground heating (Ri ranging from −0.5 to −2.9), where "ground heating" is the term used for heating or cooling of the air near the ground by heat transfer to or from the ground. Their results showed that the ground heating significantly enhanced the mean flow, turbulence, and turbulent pollutant flux inside street canyons, but weakened the shear layer at the roof level.…”

Section: Flow Turbulence and Scalar Fieldmentioning

confidence: 99%

“…On the other hand, the profile of turbulent heat/pollutant flux can vary depending on the location of the source. For the cases with flux/pollutant source at the ground level, the flux values usually have two maxima: near the source and at the roof level (see [54,55]). However, above the roof level, the transport processes of these fluxes are quite similar under near-neutral conditions [65].…”

Section: Scalar Transfermentioning

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 Turbulence and Scalar Fieldmentioning

confidence: 99%

Section: Scalar Transfermentioning

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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“…A two-dimensional (2D) street canyon is a simplification of an actual urban geometry. Numerous studies have focused on the impact of building characteristics (such as building heights, the shape roof of building, and the density of the building group) on the diffusion of pollutants in idealized and modeling urban street canyons (Li et al, 2008;Huang et al, 2009;Li et al, 2010;Buccolieri et al, 2011;Scargiali et al, 2011;Hang et al, 2012;Addepalli and Pardyjak, 2013;Scungio, 2013;Huang et al, 2015). Scungio et al (2015a) have used the detached eddy simulation to study the turbulent flow in isolated street canyons of different aspect ratios.…”

Section: Introductionmentioning

confidence: 99%

“…Li et al (2010Li et al ( , 2014 have investigated the flow field and pollutant dispersion characteristics inside urban street canyons with ground heating at different Richardson (Ri) numbers. Increasing ground temperature substantially enhanced the mean flow, turbulence, and pollutant flux inside the street canyons, but weakened the shear at the roof level.…”

Section: Introductionmentioning

confidence: 99%

Modeling the Airflow and Particle Dispersion in Street Canyons under Unsteady Thermal Environment with Sinusoidal Variation

Mercola

Wang

Deng

2017

Aerosol Air Qual. Res.

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Unstable temperature stratification conditions have a considerable influence on pollutant diffusion inside street canyons. In this study, we tried to model the air flow and particle dispersion in street canyons under unsteady thermal environment. The sinusoidal variation was used to model the atmosphere temperature on the basis of the solar radiation cycle that occurs over a day. The two-dimensional model of step-up building layouts was applied as the research object and an RNG turbulence model was applied to study the dynamic characteristics of instantaneous airflow, the dimensionless air exchange rate (ACH) and turbulent kinetic energy (TKE) in the different canyons. A series of numerical simulations were performed for different Richardson numbers (Ri). The results demonstrated that the stream function within the street canyons exhibited a periodic shift over a day, and the flow morphology gradually evolved from paralleled bilateral vortexes into a row of vortexes, particularly when the ground temperature increased. Moreover, the local PM concentrations at different times were obtained, and they were affected by the flow field patterns. The total PM mass in the street canyon decreased as the air velocity increased. Furthermore, the dimensionless ACH and TKE exhibit a noticeable fluctuation with time at higher Ri and stronger buoyancy. As the Ri decreases, the enhanced forced convection causes the ACH and TKE to remain constant over time. From these results, inhabitants should be advised to adopt preventative measures aimed at the PM pollution according to the time and location where they live.

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“…Recently, Li et al (2008a) studied momentum and pollutant transport in deep street canyons of aspect ratio 3 and 5, which are commonly found in dense compact cities across the world. Apart from isothermal conditions, the number of large-eddy simulations investigating the effects of unstable stratification on the microscale climate in street canyons is increasing (Cai 2009;Li et al 2009Li et al , 2010Park and Baik 2009). …”

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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Large-Eddy Simulation of Flow and Pollutant Transport in Urban Street Canyons with Ground Heating

Cited by 98 publications

References 37 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

Modeling the Airflow and Particle Dispersion in Street Canyons under Unsteady Thermal Environment with Sinusoidal Variation

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

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