Turbulent boundary layers over permeable walls: scaling and near-wall structure

Manes, Costantino; Poggi, Davide; Ridolfi, Luca

doi:10.1017/jfm.2011.329

Cited by 133 publications

(258 citation statements)

References 44 publications

(95 reference statements)

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“…Both these above-mentioned approaches yield a U τ value that is within 5% of each other. This value is also similar to the U τ obtained by assuming it to be the maximum of the Reynolds shear stresses as in other studies in the literature (Manes et al 2011). …”

Section: Indirect Estimate Of Skin-frictionsupporting

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: Indirect Estimate Of Skin-frictionsupporting

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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“…As suggested in figure 2(b), the mean velocity profile over a large range of Re K possesses characteristics of both the impermeable boundary and the highly permeable boundary, with a logarithmic velocity profile above the interface (e.g. Suga et al 2010;Manes, Poggi & Ridolfi 2011a) and an inflection point at the interface (e.g. Goharzadeh, Khalili & Jørgensen 2005;Breugem et al 2006).…”

Section: Flow Characteristics For Re Kmentioning

confidence: 92%

“…The relative intensity of the vertical velocity fluctuations therefore increases while the intensity of the streamwise fluctuations decreases (Breugem et al 2006;Suga et al 2010;Manes et al 2011a), enhancing mass transfer (as seen in figure 1). The observed decrease in streamwise turbulence intensity near the interface is most likely related to a change in turbulent structures in the near-wall region, as Breugem et al (2006) found that the signatures of the large-scale motions, typical for the impermeable boundary, disappear for Re K 1.…”

Section: Flow Characteristics For Re Kmentioning

confidence: 95%

“…Experimental evidence of their existence, however, remains sparse (e.g. Manes et al 2011a), with no experimental evidence for the existence of KH instabilities at the SWI for granular beds. This suggests that an inflectional velocity profile in these systems is a result of the drag induced by the porous medium and not necessarily indicative of an inflectional instability.…”

Section: Flow Characteristics For Re Kmentioning

confidence: 99%

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The variation of flow and turbulence across the sediment–water interface

2017

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A basic framework characterising the interaction between aquatic flows and permeable sediment beds is presented here. Through the permeability Reynolds number (Re K = √ Ku * /ν, where K is the sediment permeability, u * is the shear velocity and ν is the fluid viscosity), the framework unifies two classical flow typologies, namely impermeable boundary layer flows (Re K 1) and highly permeable canopy flows (Re K 1). Within this range, the sediment-water interface (SWI) is identified as a transitional region, with Re K in aquatic systems typically O(0.001-10). As the sediments obstruct conventional measurement techniques, experimental observations of interfacial hydrodynamics remain extremely rare. The use of refractive index matching here allows measurement of the mean and turbulent flow across the SWI and thus direct validation of the proposed framework. This study demonstrates a strong relationship between the structure of the mean and turbulent flow at the SWI and Re K . Hydrodynamic characteristics, such as the interfacial turbulent shear stress, velocity, turbulence intensities and turbulence anisotropy tend towards those observed in flows over impermeable boundaries as Re K → 0 and towards those seen in flows over highly permeable boundaries as Re K → ∞. A value of Re K ≈ 1-2 is seen to be an important threshold, above which the turbulent stress starts to dominate the fluid shear stress at the SWI, the penetration depths of turbulence and the mean flow into the sediment bed are comparable and similarity relationships developed for highly permeable boundaries hold. These results are used to provide a new perspective on the development of interfacial transport models at the SWI.

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“…In particular, the permeability along the vertical direction is much higher than the permeability along all directions lying on the x − z plane. As shown by Manes et al (2011aManes et al ( ,b, 2012, enhanced permeability favours momentum transport at the interface between a turbulent ow and a permeable canopy, which weakens the velocity gradient at the canopy top and ultimately the intensity of the associated shear layer. When switching from C S to C 95 , the permeability for the vertical component over the patch top increases much more than the permeability associated with lateral ow movements at the anks.…”

Section: Velocity Measurementsmentioning

confidence: 93%

Characterisation of drag and wake properties of canopy patches immersed in turbulent boundary layers

2016

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The wakes and the drag forces of canopy patches with di erent densities, immersed in turbulent boundary layers, are investigated experimentally. The patches are circular (with outer diameter D) and are made of several identical circular cylinders (height, H and diameter, d). The bulk aspect ratio of all the patches (AR = H D) was xed at 1 and the patch density (φ = Ncd 2 D 2 , also referred to as solidity) is altered by varying the number of cylinders (Nc) in the patch. Drag measurements show that the patch drag coe cient increases with increasing density. However, the drag coe cient of the highest investigated density (φ ≈ 0.25) is greater than the drag coe cient of a solid patch (i.e. φ = 1, which is a solid cylinder with AR = 1). PIV measurements were carried out along streamwise-wall-normal (x − y) plane along the centreline of patch and in the streamwise-spanwise (x − z) plane at its mid height (i.e. y = H 2). Mean velocity elds show that the porosity of the patch promotes bleeding along all directions. It was observed that bleeding along the vertical/horizontal direction increases/decreases with increasing φ. Furthermore, it was also observed that bleeding along the lateral direction dictates the point of ow separation along the sides of the patch and makes it independent of φ. All these aspects make wakes for porous patches markedly di erent from their solid counterpart and provide a rational basis to explain the observed trends in the drag coe cient.

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Turbulent boundary layers over permeable walls: scaling and near-wall structure

Cited by 133 publications

References 44 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

The variation of flow and turbulence across the sediment–water interface

Characterisation of drag and wake properties of canopy patches immersed in turbulent boundary layers

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