ORCID Profile
0000-0001-8185-1517
Current Organisation
University of Aberdeen
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Publisher: Informa UK Limited
Date: 23-05-2019
Publisher: Informa UK Limited
Date: 04-03-2023
Publisher: Informa UK Limited
Date: 04-07-2019
Publisher: Informa UK Limited
Date: 20-12-2023
Publisher: Springer International Publishing
Date: 2016
Publisher: Cambridge University Press (CUP)
Date: 13-06-2019
DOI: 10.1017/JFM.2019.344
Abstract: A theoretically based relationship for the Darcy–Weisbach friction factor $f$ for rough-bed open-channel flows is derived and discussed. The derivation procedure is based on the double averaging (in time and space) of the Navier–Stokes equation followed by repeated integration across the flow. The obtained relationship explicitly shows that the friction factor can be split into at least five additive components, due to: (i) viscous stress (ii) turbulent stress (iii) dispersive stress (which in turn can be sub ided into two parts, due to bed roughness and secondary currents) (iv) flow unsteadiness and non-uniformity and (v) spatial heterogeneity of fluid stresses in a bed-parallel plane. These constitutive components account for the roughness geometry effect and highlight the significance of the turbulent and dispersive stresses in the near-bed region where their values are largest. To explore the potential of the proposed relationship, an extensive data set has been assembled by employing specially designed large-eddy simulations and laboratory experiments for a wide range of Reynolds numbers. Flows over self-affine rough boundaries, which are representative of natural and man-made surfaces, are considered. The data analysis focuses on the effects of roughness geometry (i.e. spectral slope in the bed elevation spectra), relative submergence of roughness elements and flow and roughness Reynolds numbers, all of which are found to be substantial. It is revealed that at sufficiently high Reynolds numbers the roughness-induced and secondary-currents-induced dispersive stresses may play significant roles in generating bed friction, complementing the dominant turbulent stress contribution.
Publisher: EDP Sciences
Date: 2018
DOI: 10.1051/E3SCONF/20184005061
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.
Publisher: American Society of Civil Engineers (ASCE)
Date: 2020
Publisher: Informa UK Limited
Date: 08-11-2019
Publisher: Informa UK Limited
Date: 18-01-2019
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Mark Stewart.