Hydraulic resistance in open-channel flows over self-affine rough beds

Stewart, Mark T.; Cameron, Stuart; Nikora, Vladimir; Zampiron, Andrea; Marušić, Ivan

doi:10.1080/00221686.2018.1473296

Cited by 37 publications

(64 citation statements)

References 33 publications

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“…The relative offsets between the velocity profiles when plotted versus z/H reflects the expected trend of increasing friction factor (∼u 2 * /U 2 ) with decreasing relative submergence (H/D; e.g. Stewart et al 2018). Analysis in Cameron et al (2017) indicated that for these flows the measured velocity distributions support a logarithmic range up to z/H ≈ 0.5 with a von Kármán coefficient of κ = 0.38.…”

Section: Resultsmentioning

confidence: 62%

Drag forces on a bed particle in open-channel flow: effects of pressure spatial fluctuations and very-large-scale motions

2019

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The fluctuating drag forces acting on spherical roughness elements comprising the bed of an open-channel flow have been recorded along with synchronous measurements of the surrounding velocity field using stereoscopic particle image velocimetry. The protrusion of the target particle, equipped with a force sensor, was systematically varied between zero and one-half diameter relative to the hexagonally packed adjacent spheres. Premultiplied spectra of drag force fluctuations were found to have bimodal shapes with a low-frequency (${\approx}0.5~\text{Hz}$) peak corresponding to the presence of very-large-scale motions (VLSMs) in the turbulent flow. The high-frequency ($\gtrapprox 4~\text{Hz}$) region of the drag force spectra cannot be explained by velocity time series extracted from points around the particle, but instead appears to be dominated by the action of pressure gradients in the overlying flow field. For small particle protrusions, this high-frequency region contributes a majority of the drag force variance, while the relative importance of the low-frequency drag force fluctuations increases with increasing protrusion. The amplitude of high-frequency drag force fluctuations is modulated by the VLSMs irrespective of particle protrusion. These results provide some insight into the mechanics of bed particle stability and indicate that the optimum conditions for particle entrainment may occur when a low-pressure region embedded in the high-velocity portion of a VLSM overlays a particle.

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Section: Resultsmentioning

confidence: 62%

Drag forces on a bed particle in open-channel flow: effects of pressure spatial fluctuations and very-large-scale motions

2019

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“…This effect was observed to be as great as 49 % between the R1 and R3 beds and it was not dependent on the channel aspect ratio. The present paper is, to a certain degree, a follow up of the work reported in Stewart et al (2019), with a particular focus on the mechanisms contributing to the overall friction factor. 3.2.…”

Section: Roughness Characteristicsmentioning

confidence: 96%

“…Its equivalent presentation k s ≈ (0.88-1.13)∆ provides additional support for choosing ∆ = 4σ z for characterising roughness heights in our study. Besides, Stewart et al (2019) also showed that the spectral slope β appears to be closely related to the roughness 'effective slope' ES, which is equal to the double solidity λ (i.e. the ratio of the projected frontal area of the roughness elements to the wall-parallel projected area; Napoli, Armenio & De Marchis 2008;Flack & Schultz 2010).…”

Section: Roughness Characteristicsmentioning

confidence: 98%

“…Three self-affine fractal patterns, referred to hereafter as R1, R2 and R3, were generated using the inverse discrete Fourier transform of the designed spectra. Full details of the applied spectral synthesis procedure and the algorithms used are provided in Stewart et al (2019).…”

Section: Roughness Characteristicsmentioning

confidence: 99%

“…A detailed assessment and analysis of the manufactured plates, as well as extensive measurements of the friction factor for a wide range of hydraulic conditions, are reported in Stewart et al (2019). Specifically, Stewart et al (2019) found that the exponent β should be considered as a significant factor of the overall hydraulic resistance, in addition to the conventionally used length scales such as characteristic roughness heights and wavelengths. The results demonstrated that with all other parameters being the same, a decrease in β leads to a visible increase in the friction factor.…”

Section: Roughness Characteristicsmentioning

confidence: 99%

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Friction factor decomposition for rough-wall flows: theoretical background and application to open-channel flows

Nikora

Stoesser

Cameron³

et al. 2019

J. Fluid Mech.

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A theoretically based relationship for the Darcy–Weisbach friction factor $f$ for rough-bed open-channel flows is derived and discussed. The derivation procedure is based on the double averaging (in time and space) of the Navier–Stokes equation followed by repeated integration across the flow. The obtained relationship explicitly shows that the friction factor can be split into at least five additive components, due to: (i) viscous stress; (ii) turbulent stress; (iii) dispersive stress (which in turn can be subdivided into two parts, due to bed roughness and secondary currents); (iv) flow unsteadiness and non-uniformity; and (v) spatial heterogeneity of fluid stresses in a bed-parallel plane. These constitutive components account for the roughness geometry effect and highlight the significance of the turbulent and dispersive stresses in the near-bed region where their values are largest. To explore the potential of the proposed relationship, an extensive data set has been assembled by employing specially designed large-eddy simulations and laboratory experiments for a wide range of Reynolds numbers. Flows over self-affine rough boundaries, which are representative of natural and man-made surfaces, are considered. The data analysis focuses on the effects of roughness geometry (i.e. spectral slope in the bed elevation spectra), relative submergence of roughness elements and flow and roughness Reynolds numbers, all of which are found to be substantial. It is revealed that at sufficiently high Reynolds numbers the roughness-induced and secondary-currents-induced dispersive stresses may play significant roles in generating bed friction, complementing the dominant turbulent stress contribution.

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On the small‐scale fractal geometrical structure of a living coral reef barrier

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Summary The topographical complexity of coral reefs is of primary importance for a number of hydrodynamical and ecological processes. The present study is based on a series of high‐resolution seabottom elevation measurements along the Maupiti Barrier Reef, French Polynesia. Several statistical metrics and spectral analysis are used to characterize the spatial evolution of the coral geometrical structure from the reef crest to the backreef. A consistent fractal‐like power law exists in the spectral density of bottom elevation for length scales between 0.1 and 7 m, while at larger scale, the reef structure shows a different pattern. Such a fine characterization of the reef geometrical structure provides key elements to reconstruct the reef history, to improve the representation of reef roughness in hydrodynamical models and to monitor the evolution of coral reef systems in the context of global change. © 2020 John Wiley & Sons, Ltd.

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Hydraulic resistance in open-channel flows over self-affine rough beds

Cited by 37 publications

References 33 publications

Drag forces on a bed particle in open-channel flow: effects of pressure spatial fluctuations and very-large-scale motions

Drag forces on a bed particle in open-channel flow: effects of pressure spatial fluctuations and very-large-scale motions

Friction factor decomposition for rough-wall flows: theoretical background and application to open-channel flows

On the small‐scale fractal geometrical structure of a living coral reef barrier

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