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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 University of Nebraska jguo2@unl.edu
CONTENTS • Overview • Background • Near wall turbulence • Bedload Initiation • 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? CE, UNL
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
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: IDEA CE, UNL
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
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
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
NEAR WALL TURBULENCE: ROUGHNESS • The above log law can be rewritten as • The roughness effect in the near wall: CE, UNL
NEAR WALL TURBULENCE: TEST WITH DATA Smooth bed Rough bed CE, UNL
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
BEDLOAD INITIATION: ANALYSIS • The hydrodynamic force depends on the flow velocity, u, acting on the particle. Guo CE, UNL 14
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: CRITERION where CE, UNL
BEDLOAD INITIATION: TEST WITH DATA Guo CE, UNL 17
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 • 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 • 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