Large-eddy simulation of turbulent transports in urban street canyons in different thermal stabilities

Cheng, Wing-Chi; Liu, Chun‐Ho

doi:10.1016/j.jweia.2010.12.009

Cited by 94 publications

(48 citation statements)

References 21 publications

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“…As a result, the pollutant retention time in the street canyon increases with increasing Ri. We also performed an LES of street canyon flow under stable stratifications by inverting the direction of g. For Ri = 0.19 and 0.37 our results of flow and pollutant are quite similar to those reported in Cheng and Liu [61]. Recently, Xie et al [52] investigated the flow and dispersion in an array of staggered cubes with an approaching flow of different thermal stratifications.…”

Section: Flow Turbulence and Scalar Fieldsupporting

confidence: 77%

“…Uehara et al [47] measured flow and turbulence in a cubic array under unstable, neutral, and stable thermal stability in a wind tunnel. Cheng and Liu [61] performed an LES of flow and dispersion in urban street canyons under unstable, neutral, and stable stratification conditions. The mean wind speed in the street canyon is enhanced under unstable conditions but is weakened under stable conditions.…”

Section: Flow Turbulence and Scalar Fieldmentioning

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 Fieldsupporting

confidence: 77%

Section: Flow Turbulence and Scalar Fieldmentioning

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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“…Increasing ground temperature substantially enhanced the mean flow, turbulence, and pollutant flux inside the street canyons, but weakened the shear at the roof level. In addition to ground heating, Cheng and Liu (2011) have examined the characteristics of flows and pollutant dispersion in 2D urban street canyons with ground cooling conditions at different bulk Richardson numbers. The observed results have shown the stable stratification (ground cooling) slowed primary recirculation in the unique geometry of the 2D street canyons.…”

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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“…While the use of LES for the simulation of the stably stratified conditions has become more common in the recent years, only few studies run LESs of the urban stably stratified atmospheric boundary layer (ABL). Cheng et al (2010) and Tomas et al (2016) studied a stratified flow over arrays of surface mounted boxes. Xie et al (2013) studied the effect of stable stratification on the dispersion conditions over a kilometre-scale domain in London.…”

mentioning

confidence: 99%

Sensitivity of local air quality to the interplay between small- and large-scale circulations: a large-eddy simulation study

Wolf¹,

Esau²,

Reuder

2017

Atmos. Chem. Phys.

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Abstract. Street-level urban air pollution is a challenging concern for modern urban societies. Pollution dispersion models assume that the concentrations decrease monotonically with raising wind speed. This convenient assumption breaks down when applied to flows with local recirculations such as those found in topographically complex coastal areas. This study looks at a practically important and sufficiently common case of air pollution in a coastal valley city. Here, the observed concentrations are determined by the interaction between large-scale topographically forced and local-scale breeze-like recirculations.Analysis of a long observational dataset in Bergen, Norway, revealed that the most extreme cases of recurring wintertime air pollution episodes were accompanied by increased large-scale wind speeds above the valley. Contrary to the theoretical assumption and intuitive expectations, the maximum NO 2 concentrations were not found for the lowest 10 m ERA-Interim wind speeds but in situations with wind speeds of 3 m s −1 . To explain this phenomenon, we investigated empirical relationships between the large-scale forcing and the local wind and air quality parameters. We conducted 16 large-eddy simulation (LES) experiments with the Parallelised Large-Eddy Simulation Model (PALM) for atmospheric and oceanic flows. The LES accounted for the realistic relief and coastal configuration as well as for the large-scale forcing and local surface condition heterogeneity in Bergen. They revealed that emerging local breeze-like circulations strongly enhance the urban ventilation and dispersion of the air pollutants in situations with weak largescale winds. Slightly stronger large-scale winds, however, can counteract these local recirculations, leading to enhanced surface air stagnation.Furthermore, this study looks at the concrete impact of the relative configuration of warmer water bodies in the city and the major transport corridor. We found that a relatively small local water body acted as a barrier for the horizontal transport of air pollutants from the largest street in the valley and along the valley bottom, transporting them vertically instead and hence diluting them. We found that the stable stratification accumulates the street-level pollution from the transport corridor in shallow air pockets near the surface. The polluted air pockets are transported by the local recirculations to other less polluted areas with only slow dilution. This combination of relatively long distance and complex transport paths together with weak dispersion is not sufficiently resolved in classical air pollution models. The findings have important implications for the air quality predictions over urban areas. Any prediction not resolving these, or similar local dynamic features, might not be able to correctly simulate the dispersion of pollutants in cities.

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Large-eddy simulation of turbulent transports in urban street canyons in different thermal stabilities

Cited by 94 publications

References 21 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

Sensitivity of local air quality to the interplay between small- and large-scale circulations: a large-eddy simulation study

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