ORCID Profile
0000-0003-1241-2371
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: 20-12-2023
Publisher: Informa UK Limited
Date: 04-07-2019
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: Informa UK Limited
Date: 02-01-2018
Publisher: Cambridge University Press (CUP)
Date: 28-01-2020
DOI: 10.1017/JFM.2020.8
Publisher: Cambridge University Press (CUP)
Date: 25-01-2019
Abstract: The fluctuating drag forces acting on spherical roughness elements comprising the bed of an open-channel flow have been recorded along with synchronous measurements of the surrounding velocity field using stereoscopic particle image velocimetry. The protrusion of the target particle, equipped with a force sensor, was systematically varied between zero and one-half diameter relative to the hexagonally packed adjacent spheres. Premultiplied spectra of drag force fluctuations were found to have bimodal shapes with a low-frequency ( ${\\approx}0.5~\\text{Hz}$ ) peak corresponding to the presence of very-large-scale motions (VLSMs) in the turbulent flow. The high-frequency ( $\\gtrapprox 4~\\text{Hz}$ ) region of the drag force spectra cannot be explained by velocity time series extracted from points around the particle, but instead appears to be dominated by the action of pressure gradients in the overlying flow field. For small particle protrusions, this high-frequency region contributes a majority of the drag force variance, while the relative importance of the low-frequency drag force fluctuations increases with increasing protrusion. The litude of high-frequency drag force fluctuations is modulated by the VLSMs irrespective of particle protrusion. These results provide some insight into the mechanics of bed particle stability and indicate that the optimum conditions for particle entrainment may occur when a low-pressure region embedded in the high-velocity portion of a VLSM overlays a particle.
Publisher: Informa UK Limited
Date: 08-11-2019
Publisher: Informa UK Limited
Date: 18-01-2019
Publisher: Informa UK Limited
Date: 03-09-2023
Publisher: Cambridge University Press (CUP)
Date: 12-03-2021
DOI: 10.1017/JFM.2021.44
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Vladimir Nikora.