ORCID Profile
0000-0001-8093-9015
Current Organisation
University of Aberdeen
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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: Springer Science and Business Media LLC
Date: 08-2023
DOI: 10.1007/S10652-023-09934-0
Abstract: The results of large-eddy simulations of open-channel flows over spanwise heterogeneous surface ridges at two representative spanwise spacings are presented. Flows at moderate Froude and Reynolds numbers over smooth channel beds with streamwise-orientated rectangular ridges are considered. The ridge spacing has a profound effect on the flow: at small spacing relatively small secondary cells occur, whilst at large ridge spacing secondary cells occupy the entire flow depth. The instantaneous flow features secondary flow instabilities and the meandering of alternating low- and high-momentum regions. The quasi-periodical nature of the meandering of the instantaneous large-scale motion is visualised and quantified for both ridge spacings. Although time-averaged clockwise and counter-clockwise secondary current cells are symmetrical about the ridge-axis, they exhibit quasi-periodical increase and decrease in size as well as lateral and vertical movement in space over the meandering period.
Publisher: Copernicus GmbH
Date: 09-12-2020
DOI: 10.5194/ESURF-8-1039-2020
Abstract: Abstract. In natural open-channel flows over complex surfaces, a wide range of superimposed roughness elements may contribute to flow resistance. Gravel-bed rivers present a particularly interesting ex le of this kind of multiscalar flow resistance problem, as both in idual grains and bedforms may contribute to the roughness length. In this paper, we propose a novel method of estimating the relative contribution of different physical scales of in-channel topography to the total roughness length, using a transform-roughness correlation (TRC) approach. The technique, which uses a longitudinal profile, consists of (1) a wavelet transform which decomposes the surface into roughness elements occurring at different wavelengths and (2) a “roughness correlation” that estimates the roughness length (ks) associated with each wavelength based on its geometry alone. When applied to original and published laboratory experiments with a range of channel morphologies, the roughness correlation estimates the total ks to approximately a factor of 2 of measured values but may perform poorly in very steep channels with low relative submergence. The TRC approach provides novel and detailed information regarding the interaction between surface topography and fluid dynamics that may contribute to advances in hydraulics, bedload transport, and channel morphodynamics.
Publisher: Cambridge University Press (CUP)
Date: 28-07-2023
DOI: 10.1017/JFM.2023.498
Abstract: We present measurements of turbulent drag reduction (DR) in boundary layers at high friction Reynolds numbers in the range of $4500 \\le Re_\\tau \\le 15\\ 000$ . The efficacy of the approach, using streamwise travelling waves of spanwise wall oscillations, is studied for two actuation regimes: (i) inner-scaled actuation (ISA), as investigated in Part 1 of this study, which targets the relatively high-frequency structures of the near-wall cycle, and (ii) outer-scaled actuation (OSA), which was recently presented by Marusic et al. ( Nat. Commun. , vol. 12, 2021) for high- $Re_\\tau$ flows, targeting the lower-frequency, outer-scale motions. Multiple experimental techniques were used, including a floating-element balance to directly measure the skin-friction drag force, hot-wire anemometry to acquire long-time fluctuating velocity and wall-shear stress, and stereoscopic particle image velocimetry to measure the turbulence statistics of all three velocity components across the boundary layer. Under the ISA pathway, DR of up to 25 % was achieved, but mostly with net power saving (NPS) losses due to the high-input power cost associated with the high-frequency actuation. The low-frequency OSA pathway, however, with its lower input power requirements, was found to consistently result in positive NPS of 5–10 % for moderate DRs of 5–15 %. The results suggest that OSA is an attractive pathway for energy-efficient DR in high-Reynolds-number applications.
Publisher: Copernicus GmbH
Date: 20-07-2020
Abstract: Abstract. In natural open-channel flows over complex surfaces, a wide range of superimposed roughness elements may contribute to flow resistance. Gravel-bed rivers present a particularly interesting ex le of this kind of multiscalar flow resistance problem, as both in idual grains and bedforms can potentially be important roughness elements. In this paper, we propose a novel method of estimating the relative contribution of different physical scales of river bed topography to the total drag, using a transform-roughness correlation (TRC) approach. The technique, which requires only a single longitudinal profile, consists of (1) a wavelet transform which decomposes the surface into roughness elements occurring at different wavelengths, and (2) a `roughness correlation' that estimates the drag associated with each wavelength based on its geometry alone, expressed as ks. We apply the TRC approach to original and published laboratory experiments and show that the multiscalar drag decomposition yields estimates of grain- and form-drag that are consistent with estimates in channels with similar morphologies. Also, we demonstrate that the roughness correlation may be used to estimate total flow resistance via a conventional equation, suggesting that it could replace representative roughness values such as median grain size or the standard deviation of elevations. An improved understanding of how various scales contribute to total flow resistance may lead to advances in hydraulics as well as channel morphodynamics.
Publisher: Cambridge University Press (CUP)
Date: 28-07-2023
DOI: 10.1017/JFM.2023.499
Abstract: Turbulent drag reduction (DR) through streamwise travelling waves of the spanwise wall oscillation is investigated over a wide range of Reynolds numbers. Here, in Part 1, wall-resolved large-eddy simulations in a channel flow are conducted to examine how the frequency and wavenumber of the travelling wave influence the DR at friction Reynolds numbers $Re_\\tau = 951$ and $4000$ . The actuation parameter space is restricted to the inner-scaled actuation (ISA) pathway, where DR is achieved through direct attenuation of the near-wall scales. The level of turbulence attenuation, hence DR, is found to change with the near-wall Stokes layer protrusion height $\\ell _{0.01}$ . A range of frequencies is identified where the Stokes layer attenuates turbulence, lifting up the cycle of turbulence generation and thickening the viscous sublayer in this range, the DR increases as $\\ell _{0.01}$ increases up to $30$ viscous units. Outside this range, the strong Stokes shear strain enhances near-wall turbulence generation leading to a drop in DR with increasing $\\ell _{0.01}$ . We further find that, within our parameter and Reynolds number space, the ISA pathway has a power cost that always exceeds any DR savings. This motivates the study of the outer-scaled actuation pathway in Part 2, where DR is achieved through actuating the outer-scaled motions.
Publisher: The International Association for Hydro-Environment Engineering and Research (IAHR)
Date: 09-2019
Publisher: Cambridge University Press (CUP)
Date: 12-03-2021
DOI: 10.1017/JFM.2021.44
Publisher: Elsevier BV
Date: 05-2021
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: Cambridge University Press (CUP)
Date: 28-01-2020
DOI: 10.1017/JFM.2020.8
Publisher: American Society of Civil Engineers (ASCE)
Date: 2020
Publisher: Informa UK Limited
Date: 03-09-2023
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Andrea Zampiron.