Effect of Reynolds number, near-wall perturbation and turbulence on smooth open-channel flows

Bushra, A.; Faruque, Mdabdullah Al; Balachandar, Ram

doi:10.3826/jhr.2009.3113

Cited by 26 publications

(13 citation statements)

References 39 publications

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“…Velocity measurements were done using A commercial two-component fiberoptic LDA system (Dantec Inc.) which is powered by a 300-mW Argon-Ion laser. Details of this is avoided for brevity because using the same system in several previous studies [20,23,9]. A Bragg cell and a focusing lens of 500 mm with beam spacing of 38 mm are the optical elements of the LDA system.…”

Section: The Laser Doppler Anemometrymentioning

confidence: 99%

“…The research in open channel turbulent flow is much less comparing the vast amount of research done on turbulent boundary layer and pipe flow. Although there are significance in engineering application for the flow over rough surfaces but research study on turbulent flow over smooth surfaces [3][4][5][6][7][8][9] in both form of experimental and numerical since 1970 superseded the research on flow over rough surfaces. As research grows on the flows over rough surfaces but remains to be the Achilles heel of turbulent research [10].…”

Section: Introduction 11 Open Channel Flow: Generalmentioning

confidence: 99%

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Roughness effects on turbulence characteristics in an open channel flow

Faruque

Balachandar

2010

Can. J. Civ. Eng.

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A comprehensive study was carried out to understand the effect of roughness on the turbulence characteristics of flow in an open channel. To this end, tests were conducted with four different types of bed surface conditions at two different Reynolds number (Re = 47 500 and 31 000). This includes the use of an impermeable smooth bed, impermeable rough bed, permeable sand bed, and an impermeable bed with distributed roughness. The roughness is generated using sand grains of median diameter 2.46 mm. The effect of bed roughness is seen to have penetrated through most of the flow depth, disputing the conventional "wall similarity" hypothesis. The results show that the distributed roughness generates the largest roughness effect. The differences in the characteristics as noted by the velocity triple products exceed 200% between the flow over the smooth and rough beds. Although the same sand grain is used to create the different rough bed conditions, there are differences in turbulence characteristics, which is an indication that specific geometry of the roughness has an influence. A quadrant analysis indicates that roughness increases the contribution of the extreme turbulent events that produce very large instantaneous Reynolds shear stress and consequently influence the flow.

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Section: The Laser Doppler Anemometrymentioning

confidence: 99%

Section: Introduction 11 Open Channel Flow: Generalmentioning

confidence: 99%

Roughness effects on turbulence characteristics in an open channel flow

Faruque

Balachandar

2010

Can. J. Civ. Eng.

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“…Prior to each experiment, the water was seeded with appropriate silver coated spherical glass particles with mean diameter of 10 μm and specific gravity of 1.1 to facilitate velocity measurements. Details of the LDV configuration are avoided for brevity, and are available in Afzal et al, (2009) [16] and Roussinova et al (2008) [17]. Due to the limitation of the transmitting optics, simultaneous two-component velocity measurements were available only for 85% of the flow depth.…”

Section: Methodsmentioning

confidence: 99%

Turbulence structure of non-uniform rough open channel flow

2018

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This paper focuses on the turbulence structure in a non-uniform, gradually varied, sub-critical open channel flow (OCF) on a rough bed. The flow field is analysed under accelerating, near-uniform and decelerating conditions. Information for the flow and turbulence parameters was obtained at multiple sections and planes using two different techniques: two-component laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Different outer region velocity scaling methods were explored for evaluation of the local friction velocity. Analysis of the mean velocity profiles showed that the overlap layer exists for all flow cases. The outer layer of the decelerated velocity profile was strongly affected by the pressure gradient, where a large wake was noted. Due to the prevailing nature of the experimental setup it was found that the time-averaged flow quantities do not attained equilibrium conditions and the flow is spatially heterogeneous. The roughness generally increases the friction velocity and its effect was stronger than the effect of the pressure gradient. It was found that for the decelerated flow section over a rough bed, the mean flow and turbulence intensities were affected throughout the flow depth. The flow features presented in this study can be used to develop a model for simulating flow over a block ramp. The effect of the non-uniformity and roughness on turbulence intensities and Reynolds shear stresses was further investigated.

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“…This provides additional motivation for experimentalists to continue to design high-quality experiments in an effort to refine the near-bed data in turbulent boundary layer flows. Also, as a conceptual study, flow over a smooth bed is still of interest to many researchers in both open channel and turbulent boundary layer flow (Roussinova et al 2008;Afzal et al 2009;Gaudio and Dey 2013;Segalini et al 2013) .…”

Section: R a F T D R A F T 1 Introductionmentioning

confidence: 99%

Reconsideration of the overlap region in smooth shallow open channel flows

et al. 2017

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In this paper, data sets for mean velocity distributions in smooth shallow open channel flow are reconsidered to evaluate the characteristics of the overlap region and estimate the friction velocity (u∗). Both new and existing velocity measurements are used in the analysis. The velocity profiles are obtained using laser Doppler velocimetry and particle image velocimetry at typical Reynolds numbers (20 000–60 000) achieved in laboratory flumes. Validation of the estimated u∗ values using different forms of power law is established by comparing these values with the ones available in literature. Also, the Reynolds shear stress distribution based on two-dimensional measurements validate the estimated u∗. The availability of new data sets allows one to verify the usefulness of the log-law and evaluate the log-law constants. Different fitting methods; least squares, derivative, and scattered methods are used to evaluate the value of von Kármán coefficient. It is found that the value of κ obtained from the least squares method varies between 0.35 and 0.51 and depends on the Reynolds number. This refutes the conventional constant value assumption for the von Kármán coefficient (κ = 0.41). By considering the Prandtl’s mixing-length theory, the present values of the von Kármán coefficient are used to evaluate the mixing length distributions. The mixing length distributions in smooth open channel flow are found to depend on Reθ.

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Effect of Reynolds number, near-wall perturbation and turbulence on smooth open-channel flows

Cited by 26 publications

References 39 publications

Roughness effects on turbulence characteristics in an open channel flow

Roughness effects on turbulence characteristics in an open channel flow

Turbulence structure of non-uniform rough open channel flow

Reconsideration of the overlap region in smooth shallow open channel flows

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