On the definition of the shear velocity in rough bed open channel flows.

Dubravka Pokrajac, J. J. Finnigan, C. Manes, Ian Kenneth McEwan, Vladimir Ivanovich Nikora

Research output: Contribution to conferenceOther

20 Citations (Scopus)

Abstract

Shear velocity u(*) is a well-known fundamental velocity scale of the wall turbulence, widely used for scaling various flow quantities such as mean velocity and turbulent intensities. The definition of shear velocity is closely related to the universal Logarithmic law of the wall, which was originally derived for constant shear stress boundary layers. Because the shear stress is constant, the momentum flux rho u(*)(2) is equal to the wall shear stress, so u(*) is often understood as some kind of synonym for the wall shear stress. The universal Log-law is also used for the flows with linear shear stress such as pipe flows and uniform open channel flows. In such flows the turbulent momentum flux delivered to the roughness differs from the wall shear stress, but the difference is negligible for the rough walls where boundary layer thickness (flow depth in case of uniform open channel flow), is much larger than roughness height. In rough wall flows where the flow depth and the roughness height are of the similar order, the difference between the turbulent momentum flux representative of the free outer flow and the wall shear stress may become significant. For such flows it is important to distinguish between the wall shear stress and the shear stress used for evaluation of the shear velocity. This paper highlights the problems associated with the definition of the shear velocity in shallow open channel flows and its use for evaluation of the bed shear stress. The influence of the definition of u(*) on the data analysis is illustrated using experimental results.

Original languageEnglish
Number of pages89
Publication statusPublished - 2006

Keywords

  • VEGETATION
  • TURBULENCE
  • RESISTANCE
  • CANOPIES
  • LAYER

Cite this

On the definition of the shear velocity in rough bed open channel flows. / Pokrajac, Dubravka; Finnigan, J. J.; Manes, C.; McEwan, Ian Kenneth; Nikora, Vladimir Ivanovich.

2006.

Research output: Contribution to conferenceOther

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title = "On the definition of the shear velocity in rough bed open channel flows.",
abstract = "Shear velocity u(*) is a well-known fundamental velocity scale of the wall turbulence, widely used for scaling various flow quantities such as mean velocity and turbulent intensities. The definition of shear velocity is closely related to the universal Logarithmic law of the wall, which was originally derived for constant shear stress boundary layers. Because the shear stress is constant, the momentum flux rho u(*)(2) is equal to the wall shear stress, so u(*) is often understood as some kind of synonym for the wall shear stress. The universal Log-law is also used for the flows with linear shear stress such as pipe flows and uniform open channel flows. In such flows the turbulent momentum flux delivered to the roughness differs from the wall shear stress, but the difference is negligible for the rough walls where boundary layer thickness (flow depth in case of uniform open channel flow), is much larger than roughness height. In rough wall flows where the flow depth and the roughness height are of the similar order, the difference between the turbulent momentum flux representative of the free outer flow and the wall shear stress may become significant. For such flows it is important to distinguish between the wall shear stress and the shear stress used for evaluation of the shear velocity. This paper highlights the problems associated with the definition of the shear velocity in shallow open channel flows and its use for evaluation of the bed shear stress. The influence of the definition of u(*) on the data analysis is illustrated using experimental results.",
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T1 - On the definition of the shear velocity in rough bed open channel flows.

AU - Pokrajac, Dubravka

AU - Finnigan, J. J.

AU - Manes, C.

AU - McEwan, Ian Kenneth

AU - Nikora, Vladimir Ivanovich

PY - 2006

Y1 - 2006

N2 - Shear velocity u(*) is a well-known fundamental velocity scale of the wall turbulence, widely used for scaling various flow quantities such as mean velocity and turbulent intensities. The definition of shear velocity is closely related to the universal Logarithmic law of the wall, which was originally derived for constant shear stress boundary layers. Because the shear stress is constant, the momentum flux rho u(*)(2) is equal to the wall shear stress, so u(*) is often understood as some kind of synonym for the wall shear stress. The universal Log-law is also used for the flows with linear shear stress such as pipe flows and uniform open channel flows. In such flows the turbulent momentum flux delivered to the roughness differs from the wall shear stress, but the difference is negligible for the rough walls where boundary layer thickness (flow depth in case of uniform open channel flow), is much larger than roughness height. In rough wall flows where the flow depth and the roughness height are of the similar order, the difference between the turbulent momentum flux representative of the free outer flow and the wall shear stress may become significant. For such flows it is important to distinguish between the wall shear stress and the shear stress used for evaluation of the shear velocity. This paper highlights the problems associated with the definition of the shear velocity in shallow open channel flows and its use for evaluation of the bed shear stress. The influence of the definition of u(*) on the data analysis is illustrated using experimental results.

AB - Shear velocity u(*) is a well-known fundamental velocity scale of the wall turbulence, widely used for scaling various flow quantities such as mean velocity and turbulent intensities. The definition of shear velocity is closely related to the universal Logarithmic law of the wall, which was originally derived for constant shear stress boundary layers. Because the shear stress is constant, the momentum flux rho u(*)(2) is equal to the wall shear stress, so u(*) is often understood as some kind of synonym for the wall shear stress. The universal Log-law is also used for the flows with linear shear stress such as pipe flows and uniform open channel flows. In such flows the turbulent momentum flux delivered to the roughness differs from the wall shear stress, but the difference is negligible for the rough walls where boundary layer thickness (flow depth in case of uniform open channel flow), is much larger than roughness height. In rough wall flows where the flow depth and the roughness height are of the similar order, the difference between the turbulent momentum flux representative of the free outer flow and the wall shear stress may become significant. For such flows it is important to distinguish between the wall shear stress and the shear stress used for evaluation of the shear velocity. This paper highlights the problems associated with the definition of the shear velocity in shallow open channel flows and its use for evaluation of the bed shear stress. The influence of the definition of u(*) on the data analysis is illustrated using experimental results.

KW - VEGETATION

KW - TURBULENCE

KW - RESISTANCE

KW - CANOPIES

KW - LAYER

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ER -