City breathability in medium density urban-like geometries evaluated through the pollutant transport rate and the net escape velocity

Hang, Jian; Wang, Qun; Chen, Xieyuan; Sandberg, Mats; Wei, Zhu; Buccolieri, Riccardo; Sabatino, Silvana Di

doi:10.1016/j.buildenv.2015.08.002

Cited by 118 publications

(42 citation statements)

References 58 publications

(89 reference statements)

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“…is parameter has already been investigated in other works [62,[72][73][74][75][76][77] for 2D ows, while useful insights into the e ects of building height variations for arrays of 3D buildings have recently been reported in [78][79][80]. e additional analysis we provide here focuses on the combined e ect of the variability of both building heights and AR.…”

Section: E Ect Of Building Height Variationsmentioning

confidence: 95%

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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Section: E Ect Of Building Height Variationsmentioning

confidence: 95%

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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“…Engineering flows over rough surfaces are commonly used as the analytical platforms to enrich our fundamental understanding of urban atmospheric boundary layer (ABL) problems [5,6]. Typical applications include wind engineering for the built environment [7,8], particulate matter (PM) in street canyons [9], city breathability [10,11], and pedestrian wind comfort/safety [12] as well as guideline formulation [13]. Unlike their smooth-surface counterparts, the aerodynamic resistance induced by rough surfaces on turbulent boundary layers (TBLs) is less sensitive to the Reynolds number Re (= Uh/ν; where U is the characteristic velocity scale of flows, h the characteristic length scale of roughness elements and ν the kinematic viscosity).…”

Section: Introductionmentioning

confidence: 99%

Street-Level Ventilation in Hypothetical Urban Areas

2017

Atmosphere

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Street-level ventilation is often weakened by the surrounding high-rise buildings. A thorough understanding of the flows and turbulence over urban areas assists in improving urban air quality as well as effectuating environmental management. In this paper, reduced-scale physical modeling in a wind tunnel is employed to examine the dynamics in hypothetical urban areas in the form of identical surface-mounted ribs in crossflows (two-dimensional scenarios) to enrich our fundamental understanding of the street-level ventilation mechanism. We critically compare the flow behaviors over rough surfaces with different aerodynamic resistance. It is found that the friction velocity u τ is appropriate for scaling the dynamics in the near-wall region but not the outer layer. The different freestream wind speeds (U ∞ ) over rough surfaces suggest that the drag coefficient C d (= 2u 2 τ /U 2 ∞ ) is able to characterize the turbulent transport processes over hypothetical urban areas. Linear regression shows that street-level ventilation, which is dominated by the turbulent component of the air change rate (ACH), is proportional to the square root of drag coefficient ACH ∝ C 1/2 d . This conceptual framework is then extended to formulate a new indicator, the vertical fluctuating velocity scale in the roughness sublayer (RSL) w RSL , for breathability assessment over urban areas with diversified building height. Quadrant analyses and frequency spectra demonstrate that the turbulence is more inhomogeneous and the scales of vertical turbulence intensity w w 1/2 are larger over rougher surfaces, resulting in more efficient street-level ventilation.

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“…The overall flow rate across boundaries (sides and street top) and the local mean age of air are used to discuss influence of building packing density. Recently, Hang et al [17] further discussed city breathability in medium-density urban-like geometries using pollutant transport rate and net escape velocity.…”

Section: Assessment Of Urban Spatial Ventilationmentioning

confidence: 99%

“…Compared to traditional parameters of wind velocity, NEV reflects the effective and net contaminant transport and dilution velocity, which relate not only to the wind speed but also to the flow reversal. Hang et al [17] further adopted the parameter of net escape velocity to access the influence of city size, building height variations and wind direction for the entire pedestrian volume (throughout the z = 0-2 m volume). By assuming that the pollutant source is generated homogeneously in the studied regional space, the net escape velocity of this space can be calculated using Equation (1) [17]:…”

Section: Ventilation Performance Of Regional Spacementioning

confidence: 99%

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Improving Residential Wind Environments by Understanding the Relationship between Building Arrangements and Outdoor Regional Ventilation

et al. 2017

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This paper explores the method of assessing regional spatial ventilation performance for the design of residential building arrangements at an operational level. Three ventilation efficiency (VE) indices, Net Escape Velocity (NEV), Visitation Frequency (VF) and spatial-mean Velocity Magnitude (VM), are adopted to quantify the influence of design variation on VE within different regional spaces. Computational Fluid Dynamics (CFD) method is applied to calculate VE indices mentioned above. Several residential building arrangement cases are set to discuss the effect of different building length, lateral spacing and layouts on four typical space patterns under wind directions oblique or perpendicular to the main (long) building facade. The simulation results prove that NEV, VF and VM are useful VE indices, which can reflect different features of flow pattern in studied regional domains. Preliminary parametric studies indicate that wind direction might be the most important factor for improving spatial ventilation. When the angle between main building facade and wind direction is more than 30 • , ventilation of different exterior spaces could improve evidently. When wind direction is perpendicular to main building façade, decreasing building length can increase NEV of the middle space by 50%, while decreasing lateral spacing would decrease NEV of the intersection space by 35%.

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City breathability in medium density urban-like geometries evaluated through the pollutant transport rate and the net escape velocity

Cited by 118 publications

References 58 publications

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

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

Street-Level Ventilation in Hypothetical Urban Areas

Improving Residential Wind Environments by Understanding the Relationship between Building Arrangements and Outdoor Regional Ventilation

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