Measurements in an urban-type boundary layer

Reynolds, Ryan T.; Castro, Ian P.

doi:10.1007/s00348-008-0470-z

Cited by 62 publications

(54 citation statements)

References 38 publications

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“…With increasing roughness height and increasing roughness factor the aspect ratio decreases near the wall towards a more isotropic pattern. This is in line with the observations of Reynolds & Castro (2008) for staggered cube roughnesses where a decrease in the ratio of the streamwise to the spanwise correlation length of the streamwise velocity was observed within the roughness sublayer.…”

Section: Structure Of the Velocity Fieldsupporting

confidence: 80%

Parametric forcing approach to rough-wall turbulent channel flow

Busse

Sandham

2012

J. Fluid Mech.

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The effects of rough surfaces on turbulent channel flow are modelled by an extra force term in the Navier-Stokes equations. This force terms contains two parameters, related to the density and the height of the roughness elements, and a shape function, which regulates the influence of the force term with respect to the distance from the channel wall. This permits a more flexible specification of a rough surface than a single parameter such as the equivalent sand grain roughness. The effects of the roughness force term on turbulent channel flow have been investigated for a large number of parameter combinations and several shape functions by direct numerical simulations. It is possible to cover the full spectrum of rough flows ranging from hydraulically smooth through transitionally rough to fully rough cases. By using different parameter combinations and shape functions it is possible to match the effects of different types of rough surfaces. Mean flow and standard turbulence statistics have been used to compare the results to recent experimental and numerical studies and a good qualitative agreement has been found. Outer scaling is preserved for the streamwise velocity both for the mean profile as well as its mean square fluctuations in all but extremely rough cases. The structure of the turbulent flow shows a trend towards more isotropic turbulent states within the roughness layer. In extremely rough cases spanwise structures emerge near the wall and the turbulent state resembles a mixing layer. A direct comparison with the study of Ashrafian et al. (2004) shows a good quantitative agreement of the mean flow and Reynolds stresses everywhere except in the immediate vicinity of the rough wall. The proposed roughness force term may be of benefit as a wall model for direct and large-eddy numerical simulations in cases where the exact details of the flow over a rough wall can be neglected.

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Section: Structure Of the Velocity Fieldsupporting

confidence: 80%

Parametric forcing approach to rough-wall turbulent channel flow

Busse

Sandham

2012

J. Fluid Mech.

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“…Reynolds and Castro (2008) and Michioka et al (2011) investigated the occurrence of sweeps and ejections in the urban canopy by computing two-point correlations of u. However, the direct correlation between street-canyon ventilation and the flow was not made.…”

Section: The Mean Fluctuating Velocity Field For Street-canyon Ventilmentioning

confidence: 99%

Pollutant Dispersion in Boundary Layers Exposed to Rural-to-Urban Transitions: Varying the Spanwise Length Scale of the Roughness

Tomas

Eisma

Pourquié

et al. 2017

Boundary-Layer Meteorol

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Both large-eddy simulations (LES) and water-tunnel experiments, using simultaneous stereoscopic particle image velocimetry and laser-induced fluorescence, have been used to investigate pollutant dispersion mechanisms in regions where the surface changes from rural to urban roughness. The urban roughness was characterized by an array of rectangular obstacles in an in-line arrangement. The streamwise length scale of the roughness was kept constant, while the spanwise length scale was varied by varying the obstacle aspect ratio l/ h between 1 and 8, where l is the spanwise dimension of the obstacles and h is the height of the obstacles. Additionally, the case of two-dimensional roughness (riblets) was considered in LES. A smooth-wall turbulent boundary layer of depth 10h was used as the approaching flow, and a line source of passive tracer was placed 2h upstream of the urban canopy. The experimental and numerical results show good agreement, while minor discrepancies are readily explained. It is found that for l/ h = 2 the drag induced by the urban canopy is largest of all considered cases, and is caused by a large-scale secondary flow. In addition, due to the roughness transition the vertical advective pollutant flux is the main ventilation mechanism in the first three streets. Furthermore, by means of linear stochastic estimation the mean flow structure is identified that is responsible for street-canyon ventilation for the sixth street and onwards. Moreover, it is shown that the vertical length scale of this structure increases with increasing aspect ratio of the obstacles in the canopy, while the streamwise length scale does not show a similar trend.

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“…The same facility has been used for previous rough-wall studies such as Reynolds & Castro (2008) and Amir & Castro (2011). The tunnel has a working section of 4.5 m in length, with a 0.9 m × 0.6 m cross-section.…”

Section: Experimental Facilitymentioning

confidence: 99%

“…Furthermore, Cheng et al (2007) also argued that for boundary-layer flows over staggered arrays of cubical elements, the ρuv underestimates the surface stress by some 25%. Therefore, a corrected estimate should be used, defined as (Reynolds & Castro 2008),…”

Section: Indirect Estimate Of Skin-frictionmentioning

confidence: 99%

Effects of frontal and plan solidities on aerodynamic parameters and the roughness sublayer in turbulent boundary layers

Placidi

Ganapathisubramani

2015

J. Fluid Mech.

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Citation: Placidi, M. & Ganapathisubramani, B. (2015). Effects of frontal and plan solidities on aerodynamic parameters and the roughness sublayer in turbulent boundary layers. Journal of Fluid Mechanics, 782, pp. 541-566. doi: 10.1017Mechanics, 782, pp. 541-566. doi: 10. /jfm.2015 This is the accepted version of the paper.This version of the publication may differ from the final published version. Experiments were conducted in the fully-rough regime on surfaces with large relative roughness height (h/δ ≈ 0.1, where h is the roughness height and δ is the boundary layer thickness). The surfaces were generated by distributed LEGO TM bricks of uniform height, arranged in different configurations. Measurements were made with both floating-element drag-balance and high-resolution particle image velocimetry on six configurations with different frontal solidity, λ F , at fixed plan solidity, λ P , and vice versa, for a total of twelve rough-wall cases. Results indicate that the drag reaches a peak value λ F ≈ 0.21 or a constant λ P = 0.27, whilst it monotonically decreases for increasing values of λ P for a fixed λ F = 0.15. This is in contrast with previous studies in the literature based on cube roughness that show a peak in drag for both λ F and λ P variations. The influence of surface morphology on the depth of the roughness sublayer (RSL) is also investigated. Its depth is found to be inversely proportional to the roughness length, y 0 . A decrease in y 0 is usually accompanied by a thickening of the the RSL and vice-versa. Proper orthogonal decomposition analysis was also employed. The shapes of the most energetic modes calculated using the data across the entire boundary layer are found to be selfsimilar across the twelve rough-walls cases, however, when the analysis is restricted to the roughness sublayer, differences that depend on the wall morphology are apparent. Moreover, the energy content of the POD modes within the RSL suggest that the effect of increased frontal solidity is to redistribute the energy towards the larger-scales (i.e. larger portion of energy is within the first few modes) whilst the opposite is found for variation of plan solidity. Permanent

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Measurements in an urban-type boundary layer

Cited by 62 publications

References 38 publications

Parametric forcing approach to rough-wall turbulent channel flow

Parametric forcing approach to rough-wall turbulent channel flow

Pollutant Dispersion in Boundary Layers Exposed to Rural-to-Urban Transitions: Varying the Spanwise Length Scale of the Roughness

Effects of frontal and plan solidities on aerodynamic parameters and the roughness sublayer in turbulent boundary layers

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