Near Wall Turbulence and Bedload Initiation

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Near Wall Turbulence and Bedload Initiation

• Dr. Junke (Drinker) Guo
• Assistant Professor and Director of Flow
  Simulation Lab
• Department of Civil Engineering
• University of Nebraska
• jguo2_at_unl.edu

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CONTENTS

• Overview
• Background
• Near wall turbulence
• Bedload Initiation
• Potential applications bridge scour
• Summary and Conclusions

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OVERVIEW

• Topic Near wall velocity profile Bedload
  initiation
• What we know The linear law in the viscous
  sublayer the log law in the inertia layer the
  Shields diagram for bedload initiation
• What is the gap (1) buffer layer model between
  the viscous and the inertia layer (2) roughness
  model for transitional (3) foundation for the
  Shields diagram.
• Why important (1) without an accurate buffer
  layer model, CFD modeling is expensive (2)
  bedload transport and bridge scour prediction can
  never be improved.
• What we propose (1) an accurate mean flow model
  for near wall turbulence (2) an accurate bedload
  initiation criterion.

Guo
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BACKGROUND

• Turbulence Irregular, random motion of fluids.
• Why turbulence? (Re inertia / viscous)
• Method (dimensional analysis, asymptotic, mean
  flow fluctuation)
• Near wall turbulence
• Vertical structure Buffer layer law?
• Boundary condition Transitional roughness
  function?

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BACKGROUND (Cont.)

• Bedload Sediment is transported by rolling,
  sliding and saltation.
• Bedload initiation
• Under what condition, bedload starts to move?
• The Shields diagram (1) How to connect near wall
  turbulence to the shields diagram? (2) How about
  small particle initiation?

Guo
CE, UNL
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• OBJECTIVES
• Find a general mean flow model for near wall
  turbulence, which includes the buffer layer law
  and roughness effect.
• Find a theoretical bedload initiation criterion,
  which includes the small particle initiation.

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NEAR WALL TURBULENCE IDEA
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NEAR WALL TURBULENCE BUFFER LAYER

• The arctangent law in the inner region
• Idea The inner region law connects the log law
  through the additive constant B.

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NEAR WALL TURBULENCE LAW OF THE WALL

• Composition of the arctangent law and the log
  law
• Determination of the value of C?
• Comparison (previous figure)

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NEAR WALL TURBULENCE ROUGHNESS

• According to Nikuradse (1933), roughness only
  shifts the velocity profile with a constant. For
  the log law, we have

Guo
CE, UNL
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NEAR WALL TURBULENCE ROUGHNESS

• The above log law can be rewritten as
• The roughness effect in the near wall

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NEAR WALL TURBULENCE TEST WITH DATA
? Smooth bed
Rough bed
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NEAR WALL TURBULENCE SUMMARY

• The proposed mean velocity profile model
  reproduces all asymptotes, fills the gap in the
  buffer layer, and accounts for the effect of
  roughness.
• The proposed model agrees well with laboratory
  data in hydraulically smooth, transitional, and
  rough flow regimes.

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BEDLOAD INITIATION ANALYSIS

• The hydrodynamic force depends on the flow
  velocity, u, acting on the particle.

Guo
CE, UNL
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BEDLOAD INITIATION ASSUMPTIONS

• The drag coefficient, CD, is similar to that
  of sediment settling.
• The acting velocity, u, is estimated by the
  proposed near wall velocity profile model.

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BEDLOAD INITIATION CRITERION

• where

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BEDLOAD INITIATION TEST WITH DATA
Guo
CE, UNL
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BEDLOAD INITIATION SUMMARY

• The proposed criterion rationally connects the
  near wall turbulence and bedload initiation.
• The proposed criterion is valid for all particle
  Reynolds numbers, including small particle
  initiation.

Guo
CE, UNL
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POTENTIAL APPLICATIONS BRIDGE SCOUR

• Bedload initiation criterion is the most
  important parameter in bridge scour predictions.
• The immediate application of the proposed
  criterion is to predict the general scour depth
  due to flow contractions.
• Combining the proposed criterion with CFD
  software, the time rate of local scour depth due
  to floods can be simulated.

Guo
CE, UNL
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SUMMARY AND CONCLUSIONS

• We completed our two objectives by making two
  contributions
• We propose a universal law for near wall
  turbulence, which fills the gaps in the buffer
  layer and the effect of transitional roughness
  and
• We derive a universal criterion for bedload
  initiation, which rationally connect the
  turbulent boundary layer and bedload initiation
  and includes small particle initiation.
• We expect this work will significantly improve
  bedload transport modeling, and bridge scour
  prediction.

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Thanks!!