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Effects of bed form structure on particle-turbulence interaction in unsteady suspended sediment-laden laboratory open-channel flows . Fereshteh Bagherimiyab Ulrich Lemmin. Introduction Experimental set-up Procedure Results Conclusion. Introduction. Turbulence plays an essential role in

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slide1

Effects of bed form structure on particle-turbulence interaction in unsteady suspended sediment-laden laboratory open-channel flows

Fereshteh Bagherimiyab

Ulrich Lemmin

  • Introduction
  • Experimentalset-up
  • Procedure
  • Results
  • Conclusion
slide2

Introduction

  • Turbulence plays an essential role in
  • suspended sediment transport
    • Suspension of fine sediments
    • Generates bed forms
    • Affects Water quality in rivers
    • Impacts on the environment
slide3

Experimental set-up

ADVP withhousing

B = 0.6m

h = 0.8m

L=17m

side walls = transparent glass

gravel layer= 0.1 m thick

gravel size range = 3 to 8 mm

D50 = 5.5 mm

slide4

Experimental procedure

Constant discharge

?

slide5

Results

Range of variations of discharge, water depth, mean velocity and Reynolds number

slide6

Results

  • Velocity measurements without sediment

Longitudinal mean velocity and mean velocity from log fit during the unsteady flow stages against depth changes

slide7

Results

Friction velocity u* distribution for the unsteady ranges of 30s and 60 s

slide8

Flow withparticle motion

Sediment suspension

1 m

D50 = 0.16 mm

Thickness = 6 mm

slide9

Results

Particle velocity with ADVP

Mean particle velocities during the accelerating and the peak flow stages

slide10

Results

Particle Tracing Velocimetry (PTV)

a)

b)

  • suspension is nearly uniform in a shallow layer above the bed
  • Suspension into the water column above occurs in burst-like events in the final phase, strongest near the ripple crest
  • The highest concentration is found near the bottom
  • total suspension, increase in the final phase. This confirms the importance of burst structures with strongest bursts near the ripple crests

PTV results during the (a) initial and (b) final phase of the accelerating stage. Arrows indicate particle velocity vectors

  • Particle velocity profiles extend higher into the water column in the final phase

concentration (gr cm-3)

Particle concentrations in the same image slices

Mean particle velocities in ten image slices for the (a) initial and (b) final phases of the accelerating stage

slide11

Results

Example of PTV results during the final phase of the accelerating flow range. Arrows indicate particle velocity vectors

a)

b)

  • The highest concentration is found in the position where ripples are formed (x = 10.2 cm with a strong gradient towards the trough
  • This confirms the burst structure pattern with strongest bursts near the ripple crests

Particle velocities in six positions of images

  • Particle concentrations in six positions of images

Bed form formation during the final phase of the accelerating flow range

slide13

Results

ripples were observed

L= 0.8 water depth

h=5 mm

fine particles rolled along ripples

  • Ripples formed quickly, within about 3 sec after fine sediment particle saltation started
slide14

Results

  • during the initial saltation of particles, the upper layer of the whole bed began to move (Fig. a)
  • Particles may rise about 1 cm above the bed
  • Saltation above the bed level occurs in bursts
  • The ripple started forming 2.5 s after (Fig. b)
  • vortex is formed in the lee side of the crest
  • the vertical velocity component is strong in the near bottom layer

Close-up view of sediment particle trajectories related to ripple formation

slide15

Results

Remains of the initial reference bed level

  • ripples are formed from some instability on the bed
  • towards the end of the acceleration range, ripples occurred nearly simultaneously along the whole bed and quickly formed a pattern
  • The whole pattern slowly moves in the direction of the flow, but the distribution does not change significantly
  • Over time during the steady peak flow of about 3 minutes, the colour of the dark area changes little

Large scale image of ripple formation, seen from the top. Mean spacing of the ripples is about 6 cm

slide17

Conclusion

  • Sediment suspension stronglycharacterized by burstevents and the presence of ripples on the bed
  • High sediment suspension continued to occur during the decelerating flow even though the flow velocity decreased
  • Sediment suspension in unsteady flow is controlled by the same large scale turbulence processes as in steady flow
  • The wavelength of the ripples depended on the rate of acceleration
  • Faster acceleration produced shorter ripples due to a much stronger friction velocity
  • The shorter ripples produced stronger and higher suspension
  • Ripples did not change in appearance or dimension for the duration of the experiments