Effect of aspect ratio on developing and developed narrow open channel flow with rough bed

This study attempts to unravel the effect of aspect ratio on the turbulence characteristics in developing and fully developed narrow open channel flows. In this regard, experiments were conducted in a rough bed open channel flow. Instantaneous 3D velocities were acquired using an acoustic Doppler velocimeter at various locations along the centerline of the flume. The variables of interest include the mean components of the flow velocity, turbulence intensity, wall normal Reynolds shear stress, correlation coefficient, turbulence kinetic energy, and anisotropy. A new correlation between the equivalent roughness and velocity shift from the smooth wall logarithmic equation as a function of aspect ratio is proposed. Aspect ratio was found to influence the velocity characteristics throughout the depth in the developing flow region, while the effects are confined to the outer layer for fully developed flows. New equations to describe the variation of turbulence intensities and turbulent kinetic energy are proposed for narrow open channel flows. Reynolds stress anisotropy analysis reveals that level of anisotropy in narrow open channel flow is less than in wide open channel flows. Finally, a linear regression model is proposed to predict flow development length in narrow open channel flows with a rough bed.

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Flow Partitioning in Rectangular Open Channel Flow

Mathematical Problems in Engineering ◽

10.1155/2018/6491501 ◽

2018 ◽

Vol 2018 ◽

pp. 1-7

Author(s):

Yu Han ◽

Shu-Qing Yang ◽

Muttucumaru Sivakumar ◽

Liu-Chao Qiu ◽

Jian Chen

Keyword(s):

Shear Stress ◽

Channel Flow ◽

Open Channel ◽

Laser Doppler Anemometry ◽

Reynolds Shear Stress ◽

Three Dimensional ◽

Open Channel Flow ◽

Flow Region ◽

Mathematical Tool ◽

Measured Velocity

Hydraulic engineers often divide a flow region into subregions to simplify calculations. However, the implementation of flow divisibility remains an open issue and has not yet been implemented as a fully developed mathematical tool for modeling complex channel flows independently of experimental verification. This paper addresses whether a three-dimensional flow is physically divisible, meaning that division lines with zero Reynolds shear stress exist. An intensive laboratory investigation was conducted to carefully measure the time-averaged velocity in a rectangular open channel flow using a laser Doppler anemometry system. Two innovative methods are employed to determine the locations of division lines based on the measured velocity profile. The results clearly reveal that lines with zero total shear stress are discernible, indicating that the flow is physically divisible. Moreover, the experimental data were employed to test previously proposed methods of calculating division lines, and the results show that Yang and Lim’s method is the most reasonable predictor.

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A new analytical model for dip modified velocity distribution in fully developed turbulent open channel flow

Canadian Journal of Civil Engineering ◽

10.1139/cjce-2018-0615 ◽

2019 ◽

Vol 46 (8) ◽

pp. 657-668 ◽

Cited By ~ 1

Author(s):

Minakshee Mahananda ◽

Prashanth Reddy Hanmaiahgari ◽

Chandra Shekhar Prasad Ojha ◽

Ram Balachandar

Keyword(s):

Experimental Data ◽

Analytical Model ◽

Channel Flow ◽

Correction Factor ◽

Open Channel ◽

Open Channel Flow ◽

Channel Flows ◽

Superior Performance ◽

Open Channel Flows ◽

Proposed Model

This paper presents a new analytical model to predict the streamwise time-averaged velocity profile affected by the dip phenomenon in open channel flows. The novel approach of the present study is that the Finley wake law has been used instead of Coles’ wake law for the outer layer. To validate the new analytical model, six high quality experiments were conducted in a hydraulically rough bed open channel flow by considering variations of aspect ratio, defined as the ratio of the width of the channel to the depth of flow, from 2 to 4. In these controlled experiments, the time-averaged velocities were measured using a Nortek Vectrino-plus acoustic Doppler velocimeter. In addition, 14 sets of available experimental data, including five field experiments conducted across the globe were also used to test the performance of the proposed model. The proposed model, the Finley-dip-modified-log-wake law (FDMLWL), was used to develop a semiempirical equation to compute the dip position as a function of the dip correction factor and the wake parameter. In addition, using the experimental data and FDMLWL, an empirical equation was developed to compute the dip correction factor for hydraulically smooth open channel flows. The comparison of the FDMLWL model with the experimental data belonging to hydraulically smooth, transition, and rough regimes has consistently indicated better representation of the velocity dip phenomenon. The FDMLWL model has also been compared with other analytical models available in the literature and the superior performance of the proposed model is further observed. Finally, based on the satisfactory validation between experimental data and FDMLWL, it is inferred that the proposed model is better suited for modeling zero velocity gradient at the boundary layer edge, as in open channel flows with dip phenomenon.

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

River Flow 2006 ◽

10.1201/9781439833865.ch7 ◽

2006 ◽

Author(s):

C Manes ◽

D Pokrajac ◽

I McEwan ◽

J Finnigan ◽

V Nikora

Keyword(s):

Open Channel ◽

Shear Velocity ◽

Channel Flows ◽

Open Channel Flows ◽

Rough Bed ◽

Definition Of

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Turbulence structure of non-uniform rough open channel flow

E3S Web of Conferences ◽

10.1051/e3sconf/20184005039 ◽

2018 ◽

Vol 40 ◽

pp. 05039

Author(s):

Priscilla Williams ◽

Vesselina Roussinova ◽

Ram Balachandar

Keyword(s):

Pressure Gradient ◽

Channel Flow ◽

Friction Velocity ◽

Open Channel ◽

Open Channel Flow ◽

Shear Stresses ◽

Turbulence Structure ◽

Mean Flow ◽

Rough Bed ◽

The Mean

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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