Abstract
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.
Highlights
Long duration particle image velocimetry (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 VLSM length scale for the self-affine roughness appears to be quite similar to that obtained for the glass beads roughness, the latter data was obtained at fixed x=12.7m but with x/H varying for the different flow depths
Our measurements show that the pre-multiplied spectra in rough-bed open-channel flows exhibit a bimodal shape in the outer flow corresponding to the presence of large and very large scale motions
Summary
Kim & Adrian [1] discovered a bimodal distribution in the streamwise velocity premultiplied spectrum of pipe flow and described the two apparent scales as large and very large scale motions. They proposed that very large scale motions (VLSMs) may result from a streamwise alignment of large scale motions (LSMs) which themselves might represent a collection of smaller hairpin shaped vortices. Most studies of VLSMs and superstructures in pipes, closed channels, and boundary layers have been undertaken with smooth-wall conditions. The main objective is to identify and quantify VLSMs in open-channel flows over hydraulically fully rough beds represented by glass beads and by self-affine fractal surfaces. The main findings are briefly summarised in the Conclusions (Section 4)
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