Spanwise variations in nominally two-dimensional rough-wall boundary layers

Reynolds, Ryan T.; Hayden, Paul; Castro, Ian P.; Robins, Alan

doi:10.1007/s00348-006-0243-5

Cited by 39 publications

(36 citation statements)

References 15 publications

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“…The latter induces regions of high and low momentum (HMPs and LMPs respectively). Such patterns are also in agreement with the behaviour found by Reynolds et al (2007) for flow over regular cubical roughness. The mean-flow heterogeneity in both cases were here found to be in correspondence of the periodic spanwise roughness spacing.…”

Section: Effect Of Surface Morphology On the Roughness Sublayersupporting

confidence: 90%

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: Effect Of Surface Morphology On the Roughness Sublayersupporting

confidence: 90%

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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“…The only possible exception that we found in the literature was the experimental study by Reynolds et al (2007), which observed secondary flows over various repeating surface patterns with periods smaller than the boundary layer thickness. They explained that these structures appeared to be Klebanoff-type modes associated with transition that were organised by the periodicity of the surface roughness.…”

Section: Conditions For Large-scale Secondary Flowsmentioning

confidence: 62%

“…Mejia-Alvarez & Christensen (2013) and observed low-and high-momentum pathways occupied by large secondary swirling motions in the mean flow over the irregular surface topology of a replica of a damaged turbine blade. Reynolds et al (2007) observed similar secondary flows form over repeating arrays of cubes in a windtunnel and Wang & Cheng (2006) observed the formation of these secondary flows in open channel flow with longitudinal bedforms consisting of smooth and pointed ridges. Similar structures were also observed by Nugroho et al (2013) over a surface with converging-diverging riblets, which guided the flow into highmomentum pathways near the wall and in turn created large secondary swirling motions.…”

Section: Introductionmentioning

confidence: 81%

Effects of spanwise spacing on large-scale secondary flows in rough-wall turbulent boundary layers

2015

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Large-scale secondary flows can sometimes appear in turbulent boundary layers formed over rough surfaces creating low-and high-momentum pathways along the surface (Barros & Christensen 2014). We experimentally investigate the dependence of these secondary flows on surface/flow conditions by measuring the flows over streamwise strips of roughness with systematically-varied spanwise spacing. We find that the large-scale secondary flows are accentuated when the spacing of the roughness elements is roughly proportional to the boundary layer thickness, and do not appear for cases with finer spacing. Cases with coarser spacings also generate δ-scale secondary flows with tertiary flows in the spaces in between. These results show that the ratio of the spanwise length scale of roughness heterogeneity to the boundary layer thickness is a critical parameter for the occurrence of these secondary motions in turbulent boundary layers over rough walls.

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“…Thirdly, this latter difficulty is exacerbated by the presence of longitudinal structures in the flow, whose spanwise wavelength is an integral multiple of the spanwise wavelength of the roughness itself. Reynolds et al (2007) identified these structures in the case of cube roughness and showed that the consequent spanwise inhomogeneities only fall to amplitudes similar to those present in smooth-wall boundary layers once h/(δ − d) < 0.05. There is no reason to doubt that similar effects will occur for other roughness types.…”

Section: Resultsmentioning

confidence: 94%

Rough-wall boundary layers: mean flow universality

Castro¹

2007

J. Fluid Mech.

169

133

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Mean flow profiles, skin friction, and integral parameters for boundary layers developing naturally over a wide variety of fully aerodynamically rough surfaces are presented and discussed. The momentum thickness Reynolds number Re θ extends to values in excess of 47 000 and, unlike previous work, a very wide range of the ratio of roughness element height to boundary-layer depth is covered (0.03 < h/δ < 0.5). Comparisons are made with some classical formulations based on the assumption of a universal twoparameter form for the mean velocity profile, and also with other recent measurements. It is shown that appropriately re-written versions of the former can be used to collapse all the data, irrespective of the nature of the roughness, unless the surface is very rough, meaning that the typical roughness element height exceeds some 50 % of the boundary-layer momentum thickness, corresponding to about h/δ > 0.2.

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Spanwise variations in nominally two-dimensional rough-wall boundary layers

Cited by 39 publications

References 15 publications

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

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

Effects of spanwise spacing on large-scale secondary flows in rough-wall turbulent boundary layers

Rough-wall boundary layers: mean flow universality

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