Large and very large scale motions in roughbed open-channel flow

Stuart Cameron* (Corresponding Author), Vladimir Nikora, Mark Stewart, Andrea Zampiron

*Corresponding author for this work

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Long duration PIV measurements in rough-bed (glass beads) open-channel flow (OCF) reveal that the pre-multiplied spectra of the streamwise velocity has 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. Length scales of the large and very large scale motions 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. Supplementary data for flows over self-affine fractal rough beds support these findings and additionally indicate that the length of VLSMs may grow along the extensive distance from the channel entrance. The origin and nature of LSMs and VLSMs are still to be resolved, but differences in their scaling suggest that VLSMs in rough-bed open-channel flows form independently rather than as a spatial alignment of LSMs.

Original languageEnglish
Article number05061
Number of pages8
JournalE3S Web of Conferences
Publication statusPublished - 5 Sept 2018
Event9th International Conference on Fluvial Hydraulics, River Flow 2018 - Lyon-Villeurbanne, France
Duration: 5 Sept 20188 Sept 2018

Bibliographical note

The study has been supported by two EPSRC/UK grants, “High-resolution numerical and experimental studies of turbulence-induced sediment erosion and near-bed transport” (EP/G056404/1) and “Bed friction in rough-bed free-surface flows: a theoretical framework, roughness regimes, and quantification” (EP/K041088/1).


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