Very-large-scale motions in rough-bed open-channel flow

Cameron, Stuart; Nikora, Vladimir; Stewart, Mark T.

doi:10.1017/jfm.2017.24

Cited by 73 publications

(96 citation statements)

References 22 publications

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“…This timescale considers the outer flow variables (Cantwell, ), U bulk and flow depth, as well as the role of the aspect ratio since large‐scale flow structures tend to scale with the channel width (Dinehart, ). T LSE was calculated as

T_{hyd} = T_{LSE} = 5.7 \frac{H}{U_{italicbulk}} {(\frac{B}{H})}^{0.6},

by using the parameterization proposed by Cameron et al () and considering an eddy convective velocity equal to U bulk . The compiled bedload timescales with the predescribed T * normalization are presented as a function of the reach‐averaged, excess normalized grain shear stress, < τ gr > * / τ cr * − 1, in Figure .…”

Section: Resultsmentioning

confidence: 99%

“…by using the parameterization proposed by Cameron et al (2017) and considering an eddy convective velocity equal to U bulk . The compiled bedload timescales with the predescribed T * normalization are presented as a function of the reach-averaged, excess normalized grain shear stress, <τ gr > * /τ cr * À 1, in Figure 12.…”

Section: Journal Of Geophysical Research: Earth Surfacementioning

confidence: 99%

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Boulder Array Effects on Bedload Pulses and Depositional Patches

et al. 2018

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The influence of boulders in high gradient gravel‐bed rivers can be significant. Yet few studies have isolated their role in regulating bedload. It is hypothesized that boulder relative submergence and the corresponding eddies developing around boulders affect the frequency of bedload pulsations and the location of sediment deposits. Results show the occurrence of two distinct timescales in bedload exiting a boulder array, which were identified via time series analysis. These are the small‐scale periodicity, PS, and the large‐scale periodicity, PL. PS was identified in the range of 4–6 min and is believed to result from congested bedload movement due to reduced conveyance area within the array. PL was identified in the range of 8–46 min. It is suggested that PL corresponds to large bedload releases around boulders, which the authors consider to be caused by feedbacks between boulder eddies and bedload deposits. It is also found that boulder submergence and Froude number, Fr, influence the location of predominant deposition. At High Relative Submergence, deposition occurs in the boulder wake regions. At Low Relative Submergence, deposition instead occurs upstream of boulders in locations that depend on Fr. For Fr < 1, material deposits in the stoss of boulders due to the necklace structure of the horseshoe vortex. However, for Fr > 1, material deposits at the flanks of boulders due to the influence of local wave crests around boulders. The trapping efficiency of boulders reduces the dimensionless mean bedload transport rate by three orders of magnitude compared to conditions without boulders.

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T_{hyd} = T_{LSE} = 5.7 \frac{H}{U_{italicbulk}} {(\frac{B}{H})}^{0.6},

Section: Resultsmentioning

confidence: 99%

Section: Journal Of Geophysical Research: Earth Surfacementioning

confidence: 99%

Boulder Array Effects on Bedload Pulses and Depositional Patches

et al. 2018

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“…The main focus is on the identification of the key features of drag spectra and establishing their interrelations with the turbulence structure of the overlying flow. We use spherical roughness elements as an idealised gravel bed 496 S. M. Cameron, V. I. Nikora and I. Marusic particle and adopt the same flow conditions as Cameron, Nikora & Stewart (2017) so that potential links between drag force and the very-large-scale motions (VLSMs or superstructures; e.g. Kim & Adrian 1999;Hutchins & Marusic 2007) identified in that study can be examined.…”

Section: Introductionmentioning

confidence: 99%

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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“…These have received considerable attention in canonical wall flows [11,13,16]. In a river, they were studied by Franca and Lemmin [10] and in a roughbed laboratory open-channel by Cameron et al [1]. These structures consist of streaks of higher and lower streamwise velocities and associated vortices.…”

Section: Introductionmentioning

confidence: 99%

Lateral bed-roughness variation in shallow open-channel flow with very low submergence

et al. 2019

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Quantifying turbulent fluxes and secondary structures in shallow channel flows is important for predicting momentum and mass transfer in rivers as well as channel capacity and associated water levels. Here, we focus on the flow over a lateral bed-roughness variation with very low relative submergence of the roughness elements, h∕k = {3, 2, 1.5} , where h is the flow depth and k is the roughness height. Measurements were performed in a 1.1 m wide and 26 m long glass flume whose bed was fitted with cubes arranged in two regular side-by-side patterns with frontal densities f = 0.2 and 0.4 to create a rough-to-rougher variation. Measurements were performed using stereoscopic PIV in two orthogonal planes, in a vertical transverse plane spanning the two roughness types, and in a longitudinal one at the interface between the roughness types. The results show that the bulk velocity difference between the two sides of the channel increases with decreasing h/k. Also, contrary to what is observed at high relative submergence with smooth-to-rough transitions, higher bulk velocities occur on the side with higher roughness. This difference is increasing as the flow becomes shallower and is shown to be due to increasing effective depths ratios, leading to increasingly lower friction factor ratios with lower friction factors on the high-velocity but rougher side. Although increasing streamwise momentum transfer at the interface is needed as h/k decreases, the turbulent and secondary circulation transfer of momentum is increasingly inhibited. A globally-driven secondary-circulation at h∕k = 3 ceases for lower h/k and roughness-scale circulation becomes dominant. Also, even the increased global shear does not lead to large-scale Kelvin Helmholtz instabilities structures. However, the relative importance of the roughness difference on the flow is augmented as the flow becomes shallower and momentum transfer due to lateral dispersive stresses increases.

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Very-large-scale motions in rough-bed open-channel flow

Cited by 73 publications

References 22 publications

Boulder Array Effects on Bedload Pulses and Depositional Patches

Boulder Array Effects on Bedload Pulses and Depositional Patches

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

Lateral bed-roughness variation in shallow open-channel flow with very low submergence

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