Turbulence and Air Exchange in a Two-Dimensional Urban Street Canyon Between Gable Roof Buildings

Garau, Michela; Badas, Maria Grazia; Ferrari, Simone; Seoni, Alessandro; Querzoli, Giorgio

doi:10.1007/s10546-017-0324-4

Cited by 30 publications

(28 citation statements)

References 61 publications

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“…Previous measurements pointed out that, with squared section buildings, the turbulence in the roughness sublayer (i.e., the region where the flow is influenced by the individual roughness elements) is more intense in the case of wake interference compared to skimming flow (Garau et al 2018). Colour maps shown in Fig.…”

Section: Turbulencementioning

confidence: 92%

“…1. The experimental facility and measuring techniques are the same as those used by Garau et al (2018). However, for the sake of clarity, we recall here the main characteristics of the set-up and procedures.…”

Section: Experimental Set-up and Proceduresmentioning

confidence: 99%

“…Llaguno-Munitxa et al (2017) confirmed the negative role of facade elements on the turbulent mixing above a building array, whereas they observed that complex roof geometries tend to increase the mixing. Numerical simulations described by , and water-channel experiments of Garau et al (2018) highlighted how, in presence of two-dimensional canopy, pitched roofs modify the turbulence structure inside the canyons and deepens the shear layer at their upper interface.…”

Section: Introductionmentioning

confidence: 99%

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On the Flow Past an Array of Two-Dimensional Street Canyons Between Slender Buildings

Badas

Ferrari

Garau

et al. 2019

Boundary-Layer Meteorol

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The flow above idealized, two-dimensional series of parallelepipedal buildings is examined with the aim of investigating how the building width (W) to height (H) aspect ratio affects the turbulence in the roughness sublayer and the ventilation of the underlying street canyons. We compare the case of buildings with a squared section (ARB = W/H = 1.0) with a configuration with slender buildings (ARB = 0.1) both in the case of unit canyon width (D) to height (H) aspect ratio (ARC = D/H = 1) and in the case of ARC = 2. The former corresponds to skimming flow and the latter to wake-interference regime. Measurements are performed in a water channel, measuring velocity on a vertical mid-plane using a particle image velocimetry technique. The mean flow, its second-order turbulence statistics, the exchange fluxes, and the integral time scales investigated, with results showing that slender buildings enhance turbulence production and yield longer integral time scales in the region just above the building roof. Namely, in the skimming-flow and wakeinterference regimes, the maximum vertical velocity variance is more than doubled and increased by 50%, respectively. The combined analysis of the turbulence production fields and the snapshots of the flow during sweep and ejection events demonstrate that the shear layer between the canyon and the external flow is significantly more unstable with slender buildings, mainly because the damping effect of the vertical velocity fluctuations from the flat roof of the upwind building is substantially missing. Consequently, a larger (downstream) portion of the interface is prone to the direct interaction of the external flow structures. The higher turbulence intensity promotes the ventilation at the canyon interface, which is increased by a factor of two in the skimming-flow regime and a factor of 1.26 in the wake-interference regime. In summary, the present experiments show that the effect of the reduced building aspect ratio is particularly significant when the urban canopy consists of narrow canyons. The result is of interest since narrow street canyons are typically bounded by slender buildings in the urban texture of the old European city centres.

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Section: Turbulencementioning

confidence: 92%

Section: Experimental Set-up and Proceduresmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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On the Flow Past an Array of Two-Dimensional Street Canyons Between Slender Buildings

Badas

Ferrari

Garau

et al. 2019

Boundary-Layer Meteorol

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“…However, most of the works are focussed on ARC = 1.0, 2.0, even if sometimes the range is extended down to 0.2 (Li et al, 2008) with the aim to understand flow behaviour in cities where buildings are extremely tall, or up to 6.0 reproducing isolated buildings (see e.g. Garau et al, 2018). Even though two-dimensional street-canyons are widely studied, the effect of the building aspect ratio on street canyon flow and dispersion is still not clear.…”

Section: Introductionmentioning

confidence: 99%

Air exchange in urban canyons with variable building width: a numerical LES approach

Garau

Badas

Ferrari

et al. 2019

IJEP

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The aim of this work is to gain further insight into the role played by the building aspect ratio (ARB, i.e. the ratio of the building width, WB, to the building height, H) and its influence on street canyon flow. We carried out a series of Large Eddy Simulations with arrays of obstacles with different widths, ranging from 0.5 to 2.0, and two canyon aspect ratios (ARC, i.e. the ratio of the canyon width, W, to H) ARC = 0.5 and ARC = 1.0. Experimental data was obtained in a water channel for the corresponding configuration and used to validate numerical simulations. Results were found strongly dependent on the building aspect ratios, with two distinct behaviors identified with respect to the canyon aspect ratio. The residence time decreases with decreasing ARB, irrespective of the canyon aspect ratio, suggesting that ARB and ARC can be optimized to guarantee an optimal street canyon ventilation in urban planning.

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“…Much effort has been done in recent years to analyze urban canopy flows by means of CFD, often using Reynolds-averaged NavierStokes (RANS) simulations of two-dimensional (2D) arrays of buildings. e interest of the scientific community for such a simplified building arrangement is justified by the fact that the 2D array can be considered as an archetype for more complex geometries [10][11][12][13]. Huang et al [14] carried out 2D simulations to investigate the effect of wedge-shaped roofs on the flow in an urban street canyon and found that they have significant influence on the vortex structure and pollutant distribution pattern.…”

Section: Introductionmentioning

confidence: 99%

CFD Analysis of Urban Canopy Flows Employing the V2F Model: Impact of Different Aspect Ratios and Relative Heights

Nardecchia

Bernardino

Pagliaro

et al. 2018

Advances in Meteorology

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Computational fluid dynamics (CFD) is currently used in the environmental field to simulate flow and dispersion of pollutants around buildings. However, the closure assumptions of the turbulence usually employed in CFD codes are not always physically based and adequate for all the flow regimes relating to practical applications. e starting point of this work is the performance assessment of the V2F (i.e., v 2 − f ) model implemented in Ansys Fluent for simulating the flow field in an idealized array of twodimensional canyons. e V2F model has been used in the past to predict low-speed and wall-bounded flows, but it has never been used to simulate airflows in urban street canyons. e numerical results are validated against experimental data collected in the water channel and compared with other turbulence models incorporated in Ansys Fluent (i.e., variations of both k-ε and k-ω models and the Reynolds stress model). e results show that the V2F model provides the best prediction of the flow field for two flow regimes commonly found in urban canopies. e V2F model is also employed to quantify the air-exchange rate (ACH) for a series of two-dimensional building arrangements, such as step-up and step-down configurations, having different aspect ratios and relative heights of the buildings. e results show a clear dependence of the ACH on the latter two parameters and highlight the role played by the turbulence in the exchange of air mass, particularly important for the step-down configurations, when the ventilation associated with the mean flow is generally poor.

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Turbulence and Air Exchange in a Two-Dimensional Urban Street Canyon Between Gable Roof Buildings

Cited by 30 publications

References 61 publications

On the Flow Past an Array of Two-Dimensional Street Canyons Between Slender Buildings

On the Flow Past an Array of Two-Dimensional Street Canyons Between Slender Buildings

Air exchange in urban canyons with variable building width: a numerical LES approach

CFD Analysis of Urban Canopy Flows Employing the V2F Model: Impact of Different Aspect Ratios and Relative Heights

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