Rough-Wall Turbulent Boundary Layers

Raupach, M. R.; Antonia, R. A.; Rajagopalan, S.

doi:10.1115/1.3119492

Cited by 1,131 publications

(789 citation statements)

References 90 publications

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“…Well above the roughness both flows must resemble each other at least qualitatively since both are rough wall flows (cf. Raupach et al, 1991). However, the corresponding pictures in the roughness sublayer and canopy layer are very different.…”

Section: Conclusion and Discussionmentioning

confidence: 99%

“…The Reynolds number is Re = 5800 based on the velocity at the top of the domain and the cube height, and the corresponding roughness Reynolds number is Re τ = u τ h/ν = 500. This Reynolds number is in the fully rough regime (Raupach et al, 1991). An alternative definition of roughness Reynolds number (Re * ) = u τ z 0 /ν is sometimes used in the literature (Snyder and Castro, 2002), where z 0 is the roughness length.…”

Section: Numerical Methods and Geometrymentioning

confidence: 99%

“…In terms of turbulence dynamics, there are at least two extensively studied flows in the literature that one could compare with, and they lead to different paradigms. First, one could think of the flow above the buildings as a rough wall boundary layer (Raupach et al, 1991;Jimenez, 2004). Raupach et al (1991) suggested that well above the roughness the turbulence structure should have universal characteristics independent of the nature of the rough surface, and should in fact resemble a smooth wall boundary layer, but this view is not universally accepted.…”

Section: O Coceal Et Almentioning

confidence: 99%

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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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Section: Conclusion and Discussionmentioning

confidence: 99%

Section: Numerical Methods and Geometrymentioning

confidence: 99%

Section: O Coceal Et Almentioning

confidence: 99%

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

Coceal

Dobre

Thomas

2007

Intl Journal of Climatology

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“…However, there is some consensus on the necessity to look at the Reynolds shear stress characteristics. It is outside the scope of this paper to further investigate the best way to define these regions and therefore definitions from previous literature are followed (Raupach et al 1991;Cheng et al 2007;Cheng & Castro 2002b). The upper boundary of the RSL is defined as the location where the spatially-spanwise-averaged wall-normal profiles converge to within 10%.…”

Section: Effect Of Surface Morphology On the Roughness Sublayermentioning

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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“…Pokrajac et al 2007;Florens et al 2013). The roughness sublayer typically extends up to a height of two to five times the roughness height into the flow (Raupach, Antonia & Rajagopalan 1991). The large variation in thickness of the roughness sublayer is due to the influence of both the roughness topography and the flow conditions (Pokrajac et al 2007).…”

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confidence: 99%

Reynolds-number dependence of the near-wall flow over irregular rough surfaces

2016

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The Reynolds-number dependence of turbulent channel flow over two irregular rough surfaces, based on scans of a graphite and a grit-blasted surface, is studied by direct numerical simulation. The aim is to characterise the changes in the flow in the immediate vicinity of and within the rough surfaces, an area of the flow where it is difficult to obtain experimental measurements. The average roughness heights and spatial correlation of the roughness features of the two surfaces are similar, but the two surfaces have a significant difference in the skewness of their height distributions, with the graphite sample being positively skewed (peak-dominated) and the grit-blasted surface being negatively skewed (valley-dominated). For both cases, numerical simulations were conducted at seven different Reynolds numbers, ranging from Re τ = 90 to Re τ = 720. The positively skewed surface gives rise to higher friction factors than the negatively skewed surface in all cases. For the highest Reynolds numbers, the flow has values of the roughness function U + well in excess of 7 for both surfaces and the bulk flow profile has attained a constant shape across the full height of the channel except for the immediate vicinity of the roughness, which would indicate fully rough flow. However, the mean flow profile within and directly above the rough surface still shows considerable Reynolds-number dependence and the ratio of form to viscous drag continues to increase, which indicates that at least for some types of rough surfaces the flow retains aspects of the transitionally rough regime to values of U + or k + well in excess of the values conventionally assumed for the transitionally to fully rough threshold. This is also reflected in the changes that the near-wall flow undergoes as the Reynolds number increases: the viscous sublayer, within which the surface roughness is initially buried, breaks down and regions of reverse flow intensify. At the highest Reynolds numbers, a layer of near-wall flow is observed to follow the contours of the local surface. The distribution of thickness of this 'blanketing' layer has a mixed scaling, showing that viscous effects are still significant in the near-wall flow.

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Rough-Wall Turbulent Boundary Layers

Cited by 1,131 publications

References 90 publications

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

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

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

Reynolds-number dependence of the near-wall flow over irregular rough surfaces

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