Wind-tunnel measurements for thermal effects on the air flow and pollutant dispersion through different scale urban areas

Cui, Peng-Yi; Li, Ming-Jia; Tao, Wen‐Quan

doi:10.1016/j.buildenv.2015.12.010

Cited by 91 publications

(30 citation statements)

References 34 publications

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“…Many numerical studies have been conducted for the urban street canyons under unstable and neutral stratification (e. g., Sini et al, 1996;Kim and Baik, 1999;Xie et al, 2006;Li et al, 2008Li et al, , 2010aLi et al, , 2012Dallman et al, 2014;Hang et al, 2016;Cui et al, 2016), while only a few have been done under stable stratification (e. g., Cheng and Liu, 2011b;Xie et al, 2013;Boppana et al, 2014;Li et al, 2015;Tomas et al, 2015). Although stable stratification conditions occur less frequently in urban areas during nighttime than in their rural surroundings due to anthropogenic heat release and the enhanced turbulence by urban structures, the research of stable stratification is still very important from a practical point of view, since the reduced turbulence can lead to strong concentrations of contaminants, and the reduced downward heat flux can result in very low surface temperatures and eventual frost damage in cold regions (Flores and Riley, 2011).…”

Section: Introductionmentioning

confidence: 99%

Effect of stable stratification on dispersion within urban street canyons: A large-eddy simulation

Britter

Norford

2016

Atmospheric Environment

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This study employs a validated large-eddy simulation (LES) code with high tempo-spatial resolution to investigate the effect of a stably stratified roughness sublayer (RSL) on scalar transport within an urban street canyon. The major effect of stable stratification on the flow and turbulence inside the street canyon is that the flow slows down in both streamwise and vertical directions, a stagnant area near the street level emerges, and the vertical transport of momentum is weakened. Consequently, the transfer of heat between the street canyon and overlying atmosphere also gets weaker. The pollutant emitted from the street level 'pools' within the lower street canyon, and more pollutant accumulates within the street canyon with increasing stability. Under stable stratification, the dominant mechanism for pollutant transport within the street canyon has changed from ejections (flow carries high-concentration pollutant upward) to unorganized motions (flow carries high-concentration pollutant downward), which is responsible for the much lower dispersion efficiency under stable stratifications.

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Section: Introductionmentioning

confidence: 99%

Effect of stable stratification on dispersion within urban street canyons: A large-eddy simulation

Britter

Norford

2016

Atmospheric Environment

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“…Nevertheless, the CFD model reproduced the main flow structures such as in-canyon vortices, rooftop recirculation zones, and stagnation-point heights reasonably well. The magnitude and location of the counterclockwise-rotating vortices could affect the dispersion of pollutants emitted from mobile sources in the street canyon (Cui et al 2016). For instance, the counterclockwise-rotating vortex residing at the downwind side in the lower layer in this study could block the transport of pollutants toward the upwind side and increase the pollutant concentrations near the centre of the street bottom by making converging zone there (Park et al 2015).…”

Section: Comparison Of the Simulated Mean Flow Patterns With The Windmentioning

confidence: 85%

“…Relatively few studies have focused on step-up and/or step-down street canyons (Baik et al 2000;Sagrado et al 2002;Huang et al 2009;Gu et al 2011;Pardyjak 2013, 2015;Miao et al 2014;Cui et al 2016;Hayati et al 2019). Some examined experimentally and numerically the characteristics of flow and pollutant dispersion in two-dimensional step-up street canyons by comparison with even-notch and step-down street canyons (Baik et al 2000, Sagrado et al 2002Miao et al 2014).…”

Section: Introductionmentioning

confidence: 99%

“…Gu et al (2011) showed the dependency of the flow patterns in the step-up and step-down street canyons with the uneven building heights based on large-eddy simulations. Cui et al (2016) investigated momentum and heat transfer in outdoor and indoor environments around step-up canyons in a stratified flow using windtunnel experiments and a multiscale physical model. They found that the vortex structure and stagnation point in step-up street canyons depend on the Richardson number.…”

Section: Introductionmentioning

confidence: 99%

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Flow Characteristics Around Step-Up Street Canyons with Various Building Aspect Ratios

Park

Kim

Choi

et al. 2019

Boundary-Layer Meteorol

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We investigate the flow characteristics around step-up street canyons with various building aspect ratios (ratio of along-canyon building length to street-canyon width, and upwind building height to downwind building height) using a computational fluid dynamics (CFD) model. Simulated results are validated against experimental wind-tunnel results, with the CFD simulations conducted under the same building configurations as those in the windtunnel experiments. The CFD model reproduces the measured in-canyon vortex, rooftop recirculation zone above the downwind building, and stagnation point position reasonably well. We analyze the flow characteristics, focusing on the structural change of the in-canyon flows and the interaction between the in-and around-canyon flows with the increase of building-length ratio. The in-canyon flows undergo development and mature stages as the building-length ratio increases. In the development stage (i.e., small building-length ratios), the position of the primary vortex wanders, and the incoming flow closely follows both the upstream and downstream building sidewalls. As a result, increasing momentum transfer from the upper layer contributes to a momentum increase in the in-canyon region, and the vorticity in the in-canyon region also increases. In the mature stage (i.e., large building-length ratios), the primary vortex stabilizes in position, and the incoming flow no longer follows the building sidewalls. This causes momentum loss through the streetcanyon lateral boundaries. As the building-length ratio increases, momentum transfer from the upper layer slightly decreases, and the reverse flow, updraft, and streamwise flow in the in-canyon region also slightly decrease, resulting in vorticity reduction. Keywords Building-length aspect ratio • Computational fluid dynamics model • Development and mature stages • Flow characteristics • Step-up street canyon

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“…For three-dimensional UCL geometries, the plan area index λ p and frontal area index λ f [4][5][6]9,10] are regarded as the key urban parameters to influence the flow and pollutant dispersion in urban areas. In addition, the other factors including overall urban form and ambient wind directions [2,3,[6][7][8][9], building height variations [5,[20][21][22][23][24][25], thermal buoyancy force for weak-wind atmospheric conditions [10,18,[26][27][28][29][30], etc. also play significant roles in the flow and pollutant dispersion in UCL models.…”

Section: Introductionmentioning

confidence: 99%

Impacts of Urban Layouts and Open Space on Urban Ventilation Evaluated by Concentration Decay Method

et al. 2017

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Previous researchers calculated air change rate per hour (ACH) in the urban canopy layers (UCL) by integrating the normal component of air mean velocity (convection) and fluctuation velocity (turbulent diffusions) across UCL boundaries. However they are usually greater than the actual ACH induced by flow rates flushing UCL and never returning again. As a novelty, this paper aims to verify the exponential concentration decay history occurring in UCL models and applies the concentration decay method to assess the actual UCL ACH and predict the urban age of air at various points. Computational fluid dynamic (CFD) simulations with the standard k-ε models are successfully validated by wind tunnel data. The typical street-scale UCL models are studied under neutral atmospheric conditions. Larger urban size attains smaller ACH. Keywords: small open space; air change rate per hour (ACH); concentration decay method; urban age of air; computational fluid dynamic (CFD) simulation

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Wind-tunnel measurements for thermal effects on the air flow and pollutant dispersion through different scale urban areas

Cited by 91 publications

References 34 publications

Effect of stable stratification on dispersion within urban street canyons: A large-eddy simulation

Effect of stable stratification on dispersion within urban street canyons: A large-eddy simulation

Flow Characteristics Around Step-Up Street Canyons with Various Building Aspect Ratios

Impacts of Urban Layouts and Open Space on Urban Ventilation Evaluated by Concentration Decay Method

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