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Large and Very Large Scale Motions in Rough-Bed Open-Channel Flow

Author(s): Stuart Cameron, Vladimir Nikora, Mark Stewart

Linked Author(s): Vladimir Nikora

Keywords: Open channel flow, turbulence, hydraulic resistance, sediment transport, flow-biota interactions

Abstract: Long duration PIV measurements in rough-bed 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. The data reported in this paper fill this gap. 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. 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


Year: 2017

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