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@unl.edu. CONTENTS. Overview Background Near wall turbulence 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

jguo2@unl.edu

CONTENTS
• Overview
• Background
• Near wall turbulence
• Potential applications: bridge scour
• Summary and Conclusions

CE, UNL

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

CE, UNL

3

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

Guo

CE, UNL

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• 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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• 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.

CE, UNL