ORCID Profile
0000-0002-2333-5271
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: The Company of Biologists
Date: 15-11-2010
DOI: 10.1242/JEB.040790
Abstract: Blind Mexican cave fish (Astyanax fasciatus) are able to sense detailed information about objects by gliding alongside them and sensing changes in the flow field around their body using their lateral line sensory system. Hence the fish are able to build hydrodynamic images of their surroundings. This study measured the flow fields around blind cave fish using particle image velocimetry (PIV) as they swam parallel to a wall. Computational fluid dynamics models were also used to calculate the flow fields and the stimuli to the lateral line sensory system. Our results showed that characteristic changes in the form of the flow field occurred when the fish were within approximately 0.20 body lengths (BL) of a wall. The magnitude of these changes increased steadily as the distance between the fish and the wall was reduced. When the fish were within 0.02 BL of the wall there was a change in the form of the flow field owing to the merging of the boundary layers on the body of the fish and the wall. The stimuli to the lateral line appears to be sufficient for fish to detect walls when they are 0.10 BL away (the mean distance at which they normally swim from a wall), but insufficient for the fish to detect a wall when 0.25 BL away. This suggests that the nature of the flow fields surrounding the fish are such that hydrodynamic imaging can only be used by fish to detect surfaces at short range.
Publisher: Informa UK Limited
Date: 08-11-2019
Publisher: Informa UK Limited
Date: 18-01-2019
Publisher: Cambridge University Press (CUP)
Date: 12-03-2021
DOI: 10.1017/JFM.2021.44
Publisher: Informa UK Limited
Date: 23-05-2019
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: Oxford University Press (OUP)
Date: 22-02-2021
DOI: 10.1093/ICB/ICAB001
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: 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: The Company of Biologists
Date: 15-11-2010
DOI: 10.1242/JEB.040741
Abstract: Blind Mexican cave fish (Astyanax fasciatus) sense the presence of nearby objects by sensing changes in the water flow around their body. The information available to the fish using this hydrodynamic imaging ability depends on the properties of the flow field it generates while gliding and how this flow field is altered by the presence of objects. Here, we used particle image velocimetry to measure the flow fields around gliding blind cave fish as they moved through open water and when heading towards a wall. These measurements, combined with computational fluid dynamics models, were used to estimate the stimulus to the lateral line system of the fish. Our results showed that there was a high-pressure region around the nose of the fish, low-pressure regions corresponding to accelerated flow around the widest part of the body and a thick laminar boundary layer down the body. When approaching a wall head-on, the changes in the stimulus to the lateral line were confined to approximately the first 20% of the body. Assuming that the fish are sensitive to a certain relative change in lateral line stimuli, it was found that swimming at higher Reynolds numbers slightly decreased the distance at which the fish could detect a wall when approaching head-on, which is the opposite to what has previously been expected. However, when the effects of environmental noise are considered, swimming at higher speed may improve the signal to noise ratio of the stimulus to the lateral line.
Publisher: Informa UK Limited
Date: 03-09-2023
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Stuart Cameron.