Turbulence characteristics within sparse and dense canopies

Piétri, Laurence; Petroff, Alexandre; Amielh, Muriel; Anselmet, Fabien

doi:10.1007/s10652-009-9131-x

Cited by 39 publications

(27 citation statements)

References 41 publications

(121 reference statements)

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“…For the case of λ = 0.25 for the aligned array, the region within the urban canopy is mostly occupied by flow recirculations and the flow within the canopy is almost completely separated from the boundary-layer flow above. This flow pattern is often classified as skimming flow in the literature (Perrier et al 1972;Wieringa 1993;Pietri et al 2009). On the other hand, for the case of λf = 0.028 for the staggered array, the flow within and above the cube array is similar to the semi-smooth turbulent flow conditions within and above a sparse canopy.…”

Section: Flow Patternsmentioning

confidence: 99%

Adjustment of Turbulent Boundary-Layer Flow to Idealized Urban Surfaces: A Large-Eddy Simulation Study

Cheng

Porté‐Agel

2015

Boundary-Layer Meteorol

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Accurate prediction of atmospheric boundary layer (ABL) flow and its interaction with urban surfaces is critical for understanding the transport of momentum and scalars within and above cities. This, in turn, is essential for predicting the local climate and pollutant dispersion patterns in urban areas. Large-eddy simulation (LES) explicitly resolves the large-scale turbulent eddy motions and, therefore, can potentially provide improved understanding and prediction of flows inside and above urban canopies. This study focuses on the validation and the use of a recently-developed LES framework to simulate a turbulent boundary layer flow through idealized urban canopies represented by uniform arrays of cubes. The LES framework is first validated with wind tunnel experimental data. Good agreement between the simulation results and the experimental data are found in the vertical and spanwise profiles of mean velocities and velocity standard deviations at different streamwise locations. Next, the model is used to simulate ABL flows over surface transitions from a flat homogeneous terrain to aligned and staggered arrays of cubes with height h. For both configurations, five different frontal area densities (λ f ), equal to 0.028, 0.063, 0.111, 0.174 and 0.250, are considered. Within the arrays, the flow is found to adjust quickly and shows similar structure of the wake of the cubes after the second row. Above the arrays, an internal boundary layer (IBL) is identified. No significant difference in the depth of the IBL among different cases is observed. The drag exerted by the cubes on the flow (D f ) and the drag coefficients of the cubes (C d ) are calculated explicitly using the LES results. For the downstream cubes, D f is found to increases with decreasing density for both configurations, and larger values of C d are found for the cubes of staggered arrays than those of the aligned arrays with the same λ f . At a downstream location where the flow immediately above the cube array is already adjusted to the surface, the spatially averaged velocity is found to have a logarithmic profile for all the cases. The values of the displacement height (d) are found to increase roughly from 0.65h to 0.9h as λ f increases from 0.028 to 0.25 for both configurations. For the aerodynamic roughness (z 0 ), a maximum value at λ f =0.11 is observed for both configurations. For all the cube densities tested, larger values of z 0 are obtained for the staggered arrays than for the aligned ones. The results of z 0 are discussed and compared with existing theoretical expressions proposed in the literature. The effective mixing length (l m ) within and above different cube arrays are also calculated using the LES results. A local maximum of l m within the canopy is found in all the cases, with values ranging from 0.2h to 0.4h. These patterns are different from those used in existing urban canopy models.

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Section: Flow Patternsmentioning

confidence: 99%

Adjustment of Turbulent Boundary-Layer Flow to Idealized Urban Surfaces: A Large-Eddy Simulation Study

Cheng

Porté‐Agel

2015

Boundary-Layer Meteorol

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“…However, different and often confusing definitions of sparse and dense canopies exist in the literature. Luhar et al (2008) and Pietri et al (2009) define a sparse canopy as one in which the velocity profile approaches that of a turbulent boundary layer, while in a dense canopy the drag discontinuity at the top of the canopy creates an inflection point in the velocity profile. This definition is too general and overlooks intermediate scenarios.…”

Section: Introductionmentioning

confidence: 99%

Dispersive Stresses at the Canopy Upstream Edge

Moltchanov

Bohbot-Raviv

Shavit

2011

Boundary-Layer Meteorol

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The derivation of flow and mass transfer models in canopy and porous media environments involves the spatial-averaging of the flow properties and their subscale equations. The averaging of the momentum equation generates the dispersive stress terms that represent the subscale spatial variations of the unresolved velocity field. While previous studies ignored the dispersive stresses in their flow models, recent evidence indicates that the dispersive stresses may be important. Here we focus our attention on the magnitude of the normal dispersive stresses in the entry region of a 'forest patch', where the in-canopy velocities are large and the longitudinal derivatives do not cancel out. Highly detailed particle image velocimetry measurements, at a temporal and spatial resolution of 5 Hz and 1.4 mm, are obtained inside and around a 1-m long model canopy which consists of transparent vertical cylinders 6 mm in diameter and 74.3 mm high (h). The cylinders are randomly distributed to form a relatively sparse forest patch with a leaf area density of 7.56 m −1 and a fluid volume fraction (porosity) of 0.965. We present results of the double averaged flow properties at three different regions of the forest patch; the upstream edge (x ≈ 0), the fully-developed interior region (x ≈ 10h) and the downstream edge (x ≈ 13h). We find that the normal dispersive stresses around the entry region of the forest patch are significantly larger than the normal Reynolds stresses. An order of magnitude analysis of the relevant terms in the momentum equation indicates that the longitudinal derivatives of the dispersive stresses are of the same order of magnitude as that of the drag force and similar to that of the horizontal convection term. The longitudinal derivatives of the Reynolds stresses are smaller, though cannot be ignored. Comparing these results with the characteristic profiles measured in the fully-developed region indicates that the dispersive stresses, which are generated at the forest patch entrance, decrease along an adjustment region while maintaining their profile shape. We find that the dispersive stresses influence the rate at which momentum penetrates into the 123 334 S. Moltchanov et al. canopy. These observations suggest that under certain flow conditions, dispersive stresses may dominate the momentum balance and therefore must be considered in future canopy and porous media flow models.

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“…The proposed methodology has been tested with experimental measurements in a canopy flow with a simplified geometry, carried out in a laboratory channel of the Department of Civil, Architectural and Environmental Engineering of the University of Naples Federico II (Naples, Italy), since the focus of this paper is on the application of the KC complexity, not on the base flow. However, it should be pointed out that in the last decade, some authors performed measurements both on more realistic geometries and using more advanced techniques, at least 2C laser Doppler velocimetry (LDV) [28,29].…”

Section: Introductionmentioning

confidence: 99%

Randomness Representation of Turbulence in Canopy Flows Using Kolmogorov Complexity Measures

Mihailoviĉ

Mimić

Gualtieri

et al. 2017

Entropy

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Abstract:Turbulence is often expressed in terms of either irregular or random fluid flows, without quantification. In this paper, a methodology to evaluate the randomness of the turbulence using measures based on the Kolmogorov complexity (KC) is proposed. This methodology is applied to experimental data from a turbulent flow developing in a laboratory channel with canopy of three different densities. The methodology is even compared with the traditional approach based on classical turbulence statistics.

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Turbulence characteristics within sparse and dense canopies

Cited by 39 publications

References 41 publications

Adjustment of Turbulent Boundary-Layer Flow to Idealized Urban Surfaces: A Large-Eddy Simulation Study

Adjustment of Turbulent Boundary-Layer Flow to Idealized Urban Surfaces: A Large-Eddy Simulation Study

Dispersive Stresses at the Canopy Upstream Edge

Randomness Representation of Turbulence in Canopy Flows Using Kolmogorov Complexity Measures

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