Roughness Effects on the Mixing Properties in Open Channel Turbulent Boundary Layers

Tachie, Mark F.; Bergstrom, Donald J.; Balachandar, Ram

doi:10.1115/1.1792265

Cited by 21 publications

(14 citation statements)

References 24 publications

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“…This peak value (i.e., σ u ,max / u * ) has been called a “universal constant” [ Nezu and Nakagawa , 1993] and typical values range from 2.2 to 2.8. Most studies, however, have focused on small relative roughness, i.e., h ≫ k s [ Kironoto and Graf , 1994; Song et al , 1994; Wang and Dong , 1996; Nikora and Goring , 2000; Tachie et al , 2000, 2004; Wu and Yang , 2004] or hydraulically smooth beds [e.g., Nezu and Rodi , 1986]. We are aware of only four studies that have measured σ u ,max / u * for a wide range of relative roughness [ Bayazit , 1975; Wang et al , 1993; Dittrich and Koll , 1997; Carollo et al , 2005].…”

Section: Potential Slope Dependent Effectsmentioning

confidence: 99%

Is the critical Shields stress for incipient sediment motion dependent on channel‐bed slope?

Lamb¹,

Dietrich²,

Venditti³

2008

J. Geophys. Res.

443

728

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[1] Data from laboratory flumes and natural streams show that the critical Shields stress for initial sediment motion increases with channel slope, which indicates that particles of the same size are more stable on steeper slopes. This observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force. Processes that might explain this discrepancy are explored using a simple force-balance model, including increased drag from channel walls and bed morphology, variable friction angles, grain emergence, flow aeration, and changes to the local flow velocity and turbulent fluctuations. Surprisingly, increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical Shields stress are similar for flume experiments and natural streams, and significant variations in bed morphology in flumes is unlikely. Instead, grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed-roughness scale to flow depth (i.e., relative roughness). A model for the local velocity within the grain-roughness layer is proposed based on a 1-D eddy viscosity with wake mixing. In addition, the magnitude of near-bed turbulent fluctuations is shown to depend on the depth-averaged flow velocity and the relative roughness. Extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes.

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Section: Potential Slope Dependent Effectsmentioning

confidence: 99%

Is the critical Shields stress for incipient sediment motion dependent on channel‐bed slope?

Lamb¹,

Dietrich²,

Venditti³

2008

J. Geophys. Res.

443

728

View full text Add to dashboard Cite

show abstract

“…One of the approaches relating the flows and the geometrical factor is the roughness density [25] but the changes in turbulence structure depend on the geometry of individual roughness elements [26]. Although the flows over rough and smooth surfaces share the same similarity [27,28], the conventional smooth-surface scaling is inapplicable to its roughsurface counterpart [29,30].…”

Section: Introductionmentioning

confidence: 99%

A theory of ventilation estimate over hypothetical urban areas

Liu

Wong

2015

Journal of Hazardous Materials

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“…Balachandar and Patel (2002) demonstrated that the velocity profiles measured in a rough open channel flow conform to the classical log-law with an appropriate velocity shift. Tachie et al (2004) showed that surface roughness increases the skin friction coefficient and thickness of the roughness sublayer in an open channel flow. Open channel flows with mobile beds, with and without bedforms, add more complexity to the overlying turbulent flow.…”

Section: Introductionmentioning

confidence: 99%

Turbulence characteristics of flow in an open channel with temporally varying mobile bedforms

Hanmaiahgari

Roussinova

Balachandar

2016

Journal of Hydrology and Hydromechanics

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Turbulence of flow over mobile bedforms in natural open channels is not yet clearly understood. An attempt is made in this paper to determine the effect of naturally formed mobile bedforms on velocities, turbulent intensities and turbulent stresses. Instantaneous velocities are measured using a two-dimensional particle image velocimetry (PIV) to evaluate the turbulence structure of free surface flow over a fixed (immobile) bed, a weakly mobile bed and a temporally varying mobile bed with different stages of bedform development. This paper documents the vertical distribution of velocity, turbulence intensities, Reynolds shear stress and higher-order moments including skewness and turbulent diffusion factors. Analysis of the velocity distributions shows a substantial decrease of velocity near the bed with increasing bedform mobility due to increased friction. A modified logarithmic law with a reduced von Kármán constant and increased velocity shift is proposed for the case of the mobile bedforms. A significant increase in the Reynolds shear stress is observed in the mobile bedforms experiments accompanied by changes over the entire flow depth compared to an immobile bed. The skewness factor distribution was found to be different in the case of the flow over the mobile bedforms. All higher-order turbulence descriptors are found to be significantly affected by the formation of temporally varying and non-equilibrium mobile bedforms. Quadrant analysis indicates that sweep and outward events are found to be dominant in strongly mobile bedforms and govern the bedform mobility.

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Roughness Effects on the Mixing Properties in Open Channel Turbulent Boundary Layers

Cited by 21 publications

References 24 publications

Is the critical Shields stress for incipient sediment motion dependent on channel‐bed slope?

Is the critical Shields stress for incipient sediment motion dependent on channel‐bed slope?

A theory of ventilation estimate over hypothetical urban areas

Turbulence characteristics of flow in an open channel with temporally varying mobile bedforms

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