Turbulence characteristics and organized motion in a suburban roughness sublayer

Oikawa, Susumu; Yang, Meng

doi:10.1007/bf00712122

Cited by 84 publications

(58 citation statements)

References 37 publications

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“…Similar Reynolds stress profiles were observed by Oikawa and Meng (1995) at the outer edge of a suburban area (prevailing winds coming from the direction of the built-up area). The profiles peaked at a level 1.5 times the average building heightH .…”

Section: Introductionsupporting

confidence: 77%

Mean Flow and Turbulence Characteristics in an Urban Roughness Sublayer

Klein

Rotach

2004

Boundary-Layer Meteorology

215

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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Section: Introductionsupporting

confidence: 77%

Mean Flow and Turbulence Characteristics in an Urban Roughness Sublayer

Klein

Rotach

2004

Boundary-Layer Meteorology

215

122

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“…Similarly, a field campaign in Sapporo found that the contributions from the inward and outward interactions are about the same in the shear layer (6h < z < 7h; Oikawa and Meng 1995). In contrast to Rotach (1993), the ejections are found to contribute slightly more than the sweeps.…”

Section: Vertical Momentum Flux U Wmentioning

confidence: 89%

“…In contrast to Rotach (1993), the ejections are found to contribute slightly more than the sweeps. The vertical momentum flux determined in Oikawa and Meng (1995), Macdonald et al (2002) current LES are compared in Fig. 11.…”

Section: Vertical Momentum Flux U Wmentioning

confidence: 99%

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Large-Eddy Simulation of Flow and Pollutant Transports in and Above Two-Dimensional Idealized Street Canyons

Cheng

Liu

2011

Boundary-Layer Meteorol

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A large-eddy simulation (LES) model, using the one-equation subgrid-scale (SGS) parametrization, was developed to study the flow and pollutant transport in and above urban street canyons. Three identical two-dimensional (2D) street canyons of unity aspect ratio, each consisting of a ground-level area source of constant pollutant concentration, are evenly aligned in a cross-flow in the streamwise direction x. The flow falls into the skimming flow regime. A larger computational domain is adopted to accurately resolve the turbulence above roof level and its influence on the flow characteristics in the street canyons. The LES calculated statistics of wind and pollutant transports agree well with other field, laboratory and modelling results available in the literature. The maximum wind velocity standard deviations σ i in the streamwise (σ u ), spanwise (σ v ) and vertical (σ w ) directions are located near the roof-level windward corners. Moreover, a second σ w peak is found at z ≈ 1.5h (h is the building height) over the street canyons. Normalizing σ i by the local friction velocity u * , it is found that σ u /u * ≈ 1.8, σ v /u * ≈ 1.3 and σ w /u * ≈ 1.25 exhibiting rather uniform values in the urban roughness sublayer. Quadrant analysis of the vertical momentum flux u w shows that, while the inward and outward interactions are small, the sweeps and ejections dominate the momentum transport over the street canyons. In the x direction, the two-point correlations of velocity R v,x and R w,x drop to zero at a separation larger than h but R u,x (= 0.2) persists even at a separation of half the domain size. Partitioning the convective transfer coefficient T of pollutant into its removal and re-entry components, an increasing pollutant re-entrainment from 26.3 to 43.3% in the x direction is revealed, suggesting the impact of background pollutant on the air quality in street canyons.

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“…the review by Britter and Hanna, 2003), but little is currently known about the unsteady flow dynamics. It is now believed that unsteady effects due to organized or coherent structures in urban turbulence must be at least as important, in an order of magnitude sense, as the mean flow (Oikawa and Meng, 1995;Feigenwinter and Vogt, 2005). Yet we still do not have a conceptual model of the unsteady flow, let alone a detailed quantitative understanding that might lend itself to future parameterizations.…”

Section: Introductionmentioning

confidence: 99%

Unsteady dynamics and organized structures from DNS over an idealized building canopy

Coceal

Dobre

Thomas

2007

Intl Journal of Climatology

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Abstract:A numerical study is performed to elucidate the dominant turbulent processes that occur in urban areas. Comprehensive data from direct numerical simulations (DNS) over idealized three-dimensional arrays of buildings are analysed to study the unsteady and organized aspects of the turbulent flow. The accuracy of the DNS is evaluated by comparing turbulence statistics with a high quality wind-tunnel dataset. The simulation results are studied using flow visualization as well as statistical methods including quadrant analysis, space-time two-point correlations and conditional averaging. Three regimes of the flow are identified. First, the rough wall flow above the buildings has turbulent organized structures that resemble the 'hairpin vortices' and 'low momentum regions' that are well known to occur in the turbulent boundary layer over smooth walls. These hairpin-like vortices contribute dominantly to vertical momentum transport. Secondly, shear layers develop over the tops of the buildings and shed structures that may sometimes impinge upon downstream buildings and drive a robust recirculation within the building canopy. These unsteady canopy-top shear layers and their interaction with the larger eddies above provide important mechanisms for coupling with the flow within the canopy. Thirdly, the flow within the building canopy is the result of complicated interactions between the above and eddies shed off the vertical edges of the buildings, and their distortion caused by impacting buildings. Mean flow patterns around the buildings are important and lead to significant dispersive stresses. Implications for scalar transport and dispersion are briefly discussed.

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Turbulence characteristics and organized motion in a suburban roughness sublayer

Cited by 84 publications

References 37 publications

Mean Flow and Turbulence Characteristics in an Urban Roughness Sublayer

Mean Flow and Turbulence Characteristics in an Urban Roughness Sublayer

Large-Eddy Simulation of Flow and Pollutant Transports in and Above Two-Dimensional Idealized Street Canyons

Unsteady dynamics and organized structures from DNS over an idealized building canopy

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