Drag forces on a bed particle in open-channel flow

Effects of pressure spatial fluctuations and very-large-scale motions

S M Cameron (Corresponding Author), V I Nikora, I Marusic

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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 ( ) peak corresponding to the presence of very-large-scale motions (VLSMs) in the turbulent flow. The high-frequency ( ) 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 amplitude 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.
Original languageEnglish
Pages (from-to)494-512
Number of pages19
JournalJournal of Fluid Mechanics
Volume863
Early online date25 Jan 2019
DOIs
Publication statusPublished - 25 Mar 2019

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open channel flow
Open channel flow
drag
Drag
beds
low frequencies
Pressure gradient
Velocity measurement
entrainment
Turbulent flow
particle image velocimetry
Time series
pressure gradients
Flow fields
Mechanics
turbulent flow
Surface roughness
flow distribution
roughness
low pressure

Keywords

  • sediment transport
  • shear layer turbulence
  • turbulent boundary layers

Cite this

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title = "Drag forces on a bed particle in open-channel flow: Effects of pressure spatial fluctuations and very-large-scale motions",
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 ( ) peak corresponding to the presence of very-large-scale motions (VLSMs) in the turbulent flow. The high-frequency ( ) 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 amplitude 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.",
keywords = "sediment transport, shear layer turbulence, turbulent boundary layers",
author = "Cameron, {S M} and Nikora, {V I} and I Marusic",
note = "The study has been supported by two EPSRC/UK grants, ‘High-resolution numerical and experimental studies of turbulence-induced sediment erosion and near-bed transport’ (EP/G056404/1) and ‘Bed friction in rough-bed free-surface flows: a theoretical framework, roughness regimes, and quantification’ (EP/K041088/1). I.M. also acknowledges the support of the Australian Research Council.",
year = "2019",
month = "3",
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doi = "10.1017/jfm.2018.1003",
language = "English",
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publisher = "Cambridge Univ. Press.",

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T2 - Effects of pressure spatial fluctuations and very-large-scale motions

AU - Cameron, S M

AU - Nikora, V I

AU - Marusic, I

N1 - The study has been supported by two EPSRC/UK grants, ‘High-resolution numerical and experimental studies of turbulence-induced sediment erosion and near-bed transport’ (EP/G056404/1) and ‘Bed friction in rough-bed free-surface flows: a theoretical framework, roughness regimes, and quantification’ (EP/K041088/1). I.M. also acknowledges the support of the Australian Research Council.

PY - 2019/3/25

Y1 - 2019/3/25

N2 - 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 ( ) peak corresponding to the presence of very-large-scale motions (VLSMs) in the turbulent flow. The high-frequency ( ) 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 amplitude 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.

AB - 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 ( ) peak corresponding to the presence of very-large-scale motions (VLSMs) in the turbulent flow. The high-frequency ( ) 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 amplitude 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.

KW - sediment transport

KW - shear layer turbulence

KW - turbulent boundary layers

UR - http://www.mendeley.com/research/drag-forces-bed-particle-openchannel-flow-effects-pressure-spatial-fluctuations-verylargescale-motio

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DO - 10.1017/jfm.2018.1003

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JO - Journal of Fluid Mechanics

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SN - 0022-1120

ER -