T<scp>URBULENT</scp> F<scp>LOWS OVER</scp> R<scp>OUGH</scp> W<scp>ALLS</scp>

Jiménez, Javier

doi:10.1146/annurev.fluid.36.050802.122103

Cited by 1,255 publications

(1,001 citation statements)

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“…Figure 2(b) shows the well-established empirical relationship between drag and λ F . This has been observed in previous studies on rough-wall boundary layers as reported by Grimmond & Oke (1999) and Jimenez (2004). However, the effect of plan solidity on bulk quantities is not clear.…”

Section: Introductionsupporting

confidence: 68%

“…These variations follow the dotted and dot-dashed lines in figure 2(a). The different cases were designed on the basis of previous studies that show the location of the peaks in y 0 at λ P = 0.27 and λ F = 0.15 (Grimmond &Oke 1999 andJimenez 2004). Progressive repositioning of the downstream roughness elements in the sheltered regions of the upstream obstacles, has allowed us to achieve variations in plan solidity at fixed frontal solidity.…”

Section: Roughness Morphologymentioning

confidence: 99%

“…Nevertheless, while rough-walls are of great importance, they are much less understood than their smoothwall counterpart (Jimenez 2004). It is well established that, compared to the law of the wall for smooth-walls, any rough surface morphology results in a downward shift in the logarithmic portion of the velocity profile.…”

Section: Introductionmentioning

confidence: 99%

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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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Section: Introductionsupporting

confidence: 68%

Section: Roughness Morphologymentioning

confidence: 99%

See 1 more Smart Citation

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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“…Dependent on whether k is based on S a , S q or S z,5×5 , the ratio δ/k varies between ≈37 and 6. Jiménez (2004) states that universal behaviour in the outer layer should be observed for cases where δ/k > 40. We will see later that S z,5×5 is close to the equivalent sand-grain roughness of the surfaces and thus appears to be a suitable measure for k. The resulting δ/k = 6 indicates that we are in a regime where a certain degree of non-universal behaviour can be expected, but this regime is of high practical relevance, since in many examples of turbulent flow over rough surfaces δ/k is significantly lower than 40 (Sadeh, Cermak & Kawatani 1971;Bogard, Schmidt & Tabbita 1998;Wood et al 2010).…”

Section: Methodsmentioning

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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“…García-Mayoral & Jiménez [8] identified drag-reduction regimes, gathering in one previously proposed theories [14,4,9,10]. For low Reynolds number or small riblets, effects are well-characterized by ∆ h. When size or Reynolds increases, break-down mechanisms, based on Kelvin-Helmotz instabilities, cancel out the beneficial viscous effect.…”

Section: Introductionmentioning

confidence: 96%