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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

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

CE, UNL

overview
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
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?

CE, UNL

background cont
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

5

slide6
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.

CE, UNL

near wall turbulence buffer layer
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.

CE, UNL

slide9

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)

CE, UNL

slide10

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

10

slide11

NEAR WALL TURBULENCE: ROUGHNESS

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

CE, UNL

slide12

NEAR WALL TURBULENCE: TEST WITH DATA

 Smooth bed

Rough bed

CE, UNL

slide13

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.

CE, UNL

slide14

BEDLOAD INITIATION: ANALYSIS

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

Guo

CE, UNL

14

bedload initiation assumptions
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.

CE, UNL

bedload initiation summary
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

18

potential applications bridge scour
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

19

summary and conclusions
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