Double-Averaging Concept for Rough-Bed Open-Channel and Overland Flows: Theoretical Background

Nikora, Vladimir; McEwan, Ian; McLean, S. R.; Coleman, Stephen E.; Pokrajac, Dubravka; Walters, Roy A.

doi:10.1061/(asce)0733-9429(2007)133:8(873)

Cited by 346 publications

(313 citation statements)

References 44 publications

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“…All cases under investigation were expected to belong to the class of intermediate submergence (type II) flows as defined by Nikora et al (2007a). The boundary layer was therefore not expected to comprise the logarithmic layer that occurs in smooth bed flows at high Reynolds numbers and in rough bed flows at 836 R. McSherry et al Journal of Hydraulic Research Vol.…”

Section: Resultsmentioning

confidence: 99%

Free surface flow over square bars at intermediate relative submergence

McSherry

Chua

Stoesser

et al. 2018

Journal of Hydraulic Research

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Results from large-eddy simulations and complementary flume experiments of turbulent open channel flows over bed-mounted square bars at intermediate submergence are presented. Scenarios with two bar spacings, corresponding to transitional and k-type roughness, and three flow rates, are investigated. Good agreement is observed between the simulations and the experiments in terms of mean free surface elevations and mean streamwise velocities. Contours of simulated time-averaged streamwise, streamfunction and turbulent kinetic energy are presented and these reveal the effect of the roughness geometry on the water surface response. The analysis of the vertical distribution of the streamwise velocity shows that in the lowest submergence cases no logarithmic layer is present, whereas in the higher submergence cases some evidence of such a layer is observed. For several of the flows moderate to significant water surface deformations are observed, including weak and/or undular hydraulic jumps which affect significantly to the overall streamwise momentum balance. Reynolds shear stress, form-induced stress and form drag are analysed with reference to the momentum balance to assess their contributions to the total hydraulic resistance of these flows. The results show that form-induced stresses are dominant at the water surface and can contribute significantly to the overall drag, but the total resistance in all cases is dominated by form drag due to the presence of the bars.

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Section: Resultsmentioning

confidence: 99%

Free surface flow over square bars at intermediate relative submergence

McSherry

Chua

Stoesser

et al. 2018

Journal of Hydraulic Research

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“…In this case the double-averaged momentum conservation equation (e.g. Nikora et al 2007) in the streamwise direction for the flow region above the roughness tops reduces to…”

Section: Bulk Statisticsmentioning

confidence: 99%

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

2017

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Long-duration particle image velocimetry measurements in rough-bed open-channel flows (OCFs) reveal that the pre-multiplied spectra of the streamwise velocity have a bimodal distribution due to the presence of large-and very-large-scale motions (LSMs and VLSMs, respectively). The existence of VLSMs in boundary layers, pipes and closed channels has been acknowledged for some time, but strong supporting evidence for their presence in OCF has been lacking. The data reported in this paper fill this gap. Length scales of the LSMs and VLSMs in OCF exhibit different scaling properties; whereas the streamwise length of the LSM scales with the flow depth, the VLSM streamwise length does not scale purely with flow depth and may additionally depend on other scales such as the channel width, roughness height or viscous length. The transverse extent of the LSMs was found to increase with increasing elevation, but the VLSM transverse scale is anchored around two flow depths. The origin and nature of LSMs and VLSMs are still to be resolved, but differences in their scaling suggest that VLSMs in rough-bed OCFs form independently rather than as a spatial alignment of LSMs.

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“…Here, the flow is highly 3D and heterogeneous, and accounting for spatial variability effects with the Reynolds equation becomes complex. To smoothen flow irregularities, a spatial averaging operation may be applied to the RANS equations, thus resulting in the so-called Double-Averaged (in both time and space) Navier-Stokes (DANS) equations (Nikora et al 2007a). If averaging is performed at sufficiently large scales to smoothen the flow spatial heterogeneity, then uniformity may be assumed simplifying the hydrodynamic equations.…”

Section: Homogeneous and Heterogeneous Flowsmentioning

confidence: 99%

Turbulence in Rivers

Franca

Brocchini

2015

Rivers – Physical, Fluvial and Environmental Processes

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The study of turbulence has always been a challenge for scientists working on geophysical flows. Turbulent flows are common in nature and have an important role in geophysical disciplines such as river morphology, landscape modeling, atmospheric dynamics and ocean currents. At present, new measurement and observation techniques suitable for fieldwork can be combined with laboratory and theoretical work to advance the understanding of river processes. Nevertheless, despite more than a century of attempts to correctly formalize turbulent flows, much still remains to be done by researchers and engineers working in hydraulics and fluid mechanics. In this contribution we introduce a general framework for the analysis of river turbulence. We revisit some findings and theoretical frameworks and provide a critical analysis of where the study of turbulence is important and how to include detailed information of this in the analysis of fluvial processes. We also provide a perspective of some general aspects that are essential for researchers/ practitioners addressing the subject for the first time. Furthermore, we show some results of interest to scientists and engineers working on river flows.

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Double-Averaging Concept for Rough-Bed Open-Channel and Overland Flows: Theoretical Background

Cited by 346 publications

References 44 publications

Free surface flow over square bars at intermediate relative submergence

Free surface flow over square bars at intermediate relative submergence

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

Turbulence in Rivers

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