Influence of Building Areal Density and Roof Shape on the Wind Characteristics Above a Town

Rafailidis, Stylianos

doi:10.1023/a:1000426316328

Cited by 154 publications

(101 citation statements)

References 19 publications

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“…The experimental results agree reasonably well with each other in the near-surface region such that the data available in literature [56,57] fall within the range of current wind-tunnel measurements of k-type flows. A notable discrepancy is observed for z over 0.5δ (in the outer layer) that is attributed to the dissimilar modeling configuration in different wind-tunnel settings.…”

Section: Wind Speed Profilessupporting

confidence: 82%

“…A notable discrepancy is observed for z over 0.5δ (in the outer layer) that is attributed to the dissimilar modeling configuration in different wind-tunnel settings. The TBL thickness δ in [57] is around 8h, more shallow than that of the current wind-tunnel measurements (12.8 h ≤ δ ≤ 16 h) by over 30%, leading to a thinner near-surface region.…”

Section: Wind Speed Profilescontrasting

confidence: 68%

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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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Section: Wind Speed Profilessupporting

confidence: 82%

Section: Wind Speed Profilescontrasting

confidence: 68%

Street-Level Ventilation in Hypothetical Urban Areas

2017

Atmosphere

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“…In a similar study, Rafailidis (1997) still observed pronounced peaks in Reynolds stress and turbulence intensity profiles above an array of idealized street canyons for particular roof shapes. Kastner-Klein et al (2001) analyzed mean flow and turbulence data measured in wind-tunnel models of idealized street canyons in comparison with data from corresponding field studies, and found qualitative similarity between wind-tunnel flow characteristics and their atmospheric counterparts.…”

Section: Introductionmentioning

confidence: 72%

Mean Flow and Turbulence Characteristics in an Urban Roughness Sublayer

Klein

Rotach

2004

Boundary-Layer Meteorology

212

122

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Abstract. In this study, a detailed model of an urban landscape has been re-constructed in the wind tunnel and the flow structure inside and above the urban canopy has been investigated. Vertical profiles of all three velocity components have been measured with a Laser-Doppler velocimeter, and an extensive analysis of the measured mean flow and turbulence profiles carried out. With respect to the flow structure inside the canopy, two types of velocity profiles can be distinguished. Within street canyons, the mean wind velocities are almost zero or negative below roof level, while close to intersections or open squares, significantly higher mean velocities are observed. In the latter case, the turbulent velocities inside the canopy also tend to be higher than at street-canyon locations. For both types, turbulence kinetic energy and shear stress profiles show pronounced maxima in the flow region immediately above roof level.Based on the experimental data, a shear-stress parameterization is proposed, in which the velocity scale, u s , and length scale, z s , are based on the level and magnitude of the shear stress peak value. In order to account for a flow region inside the canopy with negligible momentum transport, a shear stress displacement height, d s , is introduced. The proposed scaling and parameterization perform well for the measured profiles and shear-stress data published in the literature.The length scales derived from the shear-stress parameterization also allow determination of appropriate scales for the mean wind profile. The roughness length, z 0 , and displacement height, d 0 , can both be described as fractions of the distance, z s − d s , between the level of the shear-stress peak and the shear-stress displacement height. This result can be interpreted in such a way that the flow only feels the zone of depth z s − d s as the roughness layer. With respect to the lower part of the canopy (z < d s ) the flow behaves as a skimming flow. Correlations between the length scales z s and d s and morphometric parameters are discussed.The mean wind profiles above the urban structure follow a logarithmic wind law. A combination of morphometric estimation methods for d 0 and z 0 with wind velocity measurements at a reference height, which allow calculation of the shear-stress velocity, u * , appears to be the most reliable and easiest procedure to determine mean wind profile parameters. Inside the roughness sublayer, a local scaling approach results in good agreement between measured and predicted mean wind profiles.

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“…Roof geometry effects such as pitched roofs or asymmetric canyons (stepup or step-down notch) will also not be addressed here. Many previous investigations [25,[29][30][31] have demonstrated a significant influence of roof geometry on canyon circulation and the associated integral statistics of the flow. The shear layer at roof level is elevated and modified so that the canyon vortex may not form at all [25].…”

Section: Flow and Transport At Street Scalementioning

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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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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Influence of Building Areal Density and Roof Shape on the Wind Characteristics Above a Town

Cited by 154 publications

References 19 publications

Street-Level Ventilation in Hypothetical Urban Areas

Street-Level Ventilation in Hypothetical Urban Areas

Mean Flow and Turbulence Characteristics in an Urban Roughness Sublayer

Transport processes in and above two-dimensional urban street canyons under different stratification conditions: results from numerical simulation

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