Sediment transport by coherent structures in a horizontal open channel flow experiment

1
Sediment transport by coherent structures in a
horizontal open channel flow experiment

• Lorentz Workshop 2006
• Leiden

W.A. Breugem and W.S.J. Uijttewaal
Environmental Fluid Mechanics
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Overview

• Introduction
• Sediment transport in open channel flow
• Experimental setup
• Results
• Concentration profile development
• Drift velocity histogram
• Spatial drift velocity structure
• Conclusions

Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Sediment transport in open channel flow

• Focus
• Suspended sediment, hence outer region
• Dilute flow (for now)

g
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Flume setup
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Particle requirements

• Comparison with DNS
• Be suspended at Re 10,000 (i.e. u/ws 5)
• dp/lk lt 1 (assumption in Maxey Riley eq.)
• Rep lt 1 (assumption in Maxey Riley eq.)
• Experimental procedure
• Phase discrimination dp gt3 dtracer
• Materials
• Tracer hollow glass (dtracer 15 mm)
• Sediment polystyrene (dp 375 mm, rp/rf1.035)

Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Instantaneous result

• Re 500
• Polystyrene particles
• Flow from left to right
• 0.8 ucl subtracted

Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Concentration profiles
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Drift velocity

• Drift from average Stokes drag force (Simonin et
  al., 1993)
• I.e. The drift velocity is the deviation of the
  average fluid velocity seen by a particle.
• Vertical momentum equation

Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Drift velocity profiles
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Drift velocity PDF (Predominantly settling, x16h)
y/h 0.55
vT0.2u
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Drift velocity, deviation from random sampling
(predominantly settling, x 16 h)

• More particles in Q4, less in Q2
• Downgoing particles in Q4
• Upgoing particles in Q1 and Q2

vT0.2u
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Drift velocity histogram (fully developed, x 75
h)
y/h 0.55
vT0.2u
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Drift velocity, deviation from random sampling
(fully developed, x 75h)
vT0.2u
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Conceptual picture of particle transport
Fully developed situation
Settling situation
Velocity Quadrants
u
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Spatial structure

• The spatial structure is studied with conditional
  averages
• Conditional averages are determined with Linear
  Stochastic Estimation (LSE).
• With LSE, conditional averages can be calculated
  from two-point correlations.
• A vortex head (rotating with the mean shear) at
  three different reference heights is used as
  condition.
• Vortex identification is done with swirling
  strength.

Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Fluid flow structure (vortex near the wall)
Q3
Q2
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Drift velocity structure (vortex near the wall)
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Fluid flow structure (vortex at 0.5 h)
Q2
Q3
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Drift velocity structure (vortex at 0.5 h)
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Conceptual model (hairpin vortex)
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Settling situation
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Conceptual picture fully developed situation
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Spatial relation between Q2 Q3
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Spatial relation between Q2 Q4
Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion
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Conclusion

• The increased concentration in either fast
  (settling situation) or slow (fully developed
  situation) flow structures, cause the mean
  particle velocity to differ from the mean fluid
  velocity.
• Downward transport occurs in sweeps (Q4) and
  inward interactions (Q3), upward transport in
  ejections.
• In predominantly settling, particles are found
  less in ejections (Q2) and more often in sweeps
  (Q4).
• In fully developed situation, particles are found
  less sweeps, and more often in both ejections and
  inward interactions. This is due to the alignment
  of several hairpin vortices.

Introduction - Experimental setup Drift
velocity - Flow structures - Conclusion