Coherent Structures In Open-Channel Flows Over a Fixed Dune

2005 ◽  
Vol 127 (5) ◽  
pp. 858-864 ◽  
Author(s):  
Wusi Yue ◽  
Ching-Long Lin ◽  
Virendra C. Patel
Keyword(s):  
Free Surface ◽  
Coherent Structures ◽  
Open Channel ◽  
Surface Deformation ◽  
Channel Flows ◽  
Mean Flow ◽  
Open Channel Flows ◽  
Near Surface ◽  
Numerical Predictions ◽  
The Mean

Turbulent open-channel flow over a two-dimensional laboratory-scale dune is studied using large eddy simulation. Free surface motion is simulated using level set method. Two subgrid scale models, namely, dynamic Smagorinsky model and dynamic two-parameter model, are employed for assessing model effects on the free surface flow. The present numerical predictions of mean flow field and turbulence statistics are in good agreement with experimental data. The mean flow can be divided into two zones, an inner zone where turbulence strongly depends on the dune bed geometry and an outer layer free from the direct influence of the bed geometry. Streaky structures are observed in the wall layer after flow reattachment. Quadrant two events are found to prevail in near-wall and near-surface motions, indicating the predominance of turbulence ejections in open-channel flows. Large-scale coherent structures are produced behind the dune crest by a strong shear layer riding over the recirculation zone. These quasistreamwise tubelike vortical structures are transported downstream with the mean flow and most are destructed before arriving at the next crest. Free surface deformation is visualized, demonstrating complex patterns of upwelling and downdraft.

Size and effect on the mean flow of large-scale horizontal coherent structures in open-channel flows: an experimental studyThis paper is one of a selection of papers in this Special Issue in honour of Professor M. Selim Yalin (1925–2007).

2009 ◽  
Vol 36 (10) ◽  
pp. 1643-1655 ◽  
Author(s):  
Ana Maria Ferreira da Silva ◽  
Habib Ahmari
Keyword(s):  
Flow Velocity ◽  
Coherent Structures ◽  
Large Scale ◽  
Open Channel ◽  
Channel Flows ◽  
Flow Depth ◽  
Mean Flow ◽  
Velocity Measurements ◽  
Open Channel Flows ◽  
The Mean

The size of the largest horizontal coherent structures (HCSs) of turbulence in open-channel flows is investigated experimentally on the basis of three series of flow velocity measurements. These are further used to explore the dynamics and morphological consequences of HCSs. The flow velocity measurements were carried out in a 21 m long and 1 m wide channel, with a bed formed by sand with average grain size of 2 mm. The bed surface was flat. The turbulent and subcritical flow under investigation was uniform, with a flow depth of 4 cm. The bed slope of 0.0015 was such that, for the present flow depth, the bed shear stress acting on the bed was substantially below the threshold for initiation of motion, thus ensuring that the bed remained flat throughout the measurements. To the knowledge of the writers, this work is a first attempt to systematically investigate HCSs in open-channel flows. It should be viewed as an extension to the case of horizontal structures of work previously carried out by a number of authors on large-scale organized turbulence motion in open-channel flows, so far focusing exclusively on vertical coherent structures (VCSs). The horizontal burst length was found to be between five and seven times the flow width. A slight internal meandering of the flow caused by the superimposition of burst sequences on the mean flow was detectable. Both of these findings lend support to the longstanding belief expressed by many prominent researchers that the formation of large-scale river forms is directly related to the large-scale turbulence. In particular, the present measurements for the first time provide some direct evidence in support of hypotheses previously raised by Yalin and da Silva regarding the formation of alternate bars and meanders through the action of HCSs on the mean flow and the mobile bed and banks.

Coherent Structures in Open-Channel Flows Over a Fixed Dune

2004 ◽  
Author(s):  
Wusi Yue ◽  
Ching-Long Lin ◽  
Virendra C. Patel
Keyword(s):  
Coherent Structures ◽  
Open Channel ◽  
Wall Layer ◽  
Mean Flow ◽  
Near Surface ◽  
Vortical Structures ◽  
Eddy Simulation ◽  
Numerical Predictions ◽  
Scale Models ◽  
Strong Shear

Turbulent open-channel flow over a two-dimensional laboratory-scale dune is studied using large eddy simulation. Free surface motion is simulated using a level set method. Two subgrid scale models, namely dynamic Smagorinsky and dynamic two-parameter models, are employed. The present numerical predictions of mean flow field and turbulence statistics are in good agreement with experimental data. Streaky structures are observed in the wall layer after flow reattachment. Quadrant two events dominate near-wall and near-surface motions. Coherent structures are produced behind the dune crest by strong shear layer riding over the recirculation zone. These tube-like vortical structures are transported downstream with the mean flow and most are destroyed before arriving at the next crest.

Additional spanwise vortices near the free surface in open channel flows

2021 ◽  
Vol 924 ◽  
Author(s):  
Yanchong Duan ◽  
Qiang Zhong ◽  
Guiquan Wang ◽  
Qigang Chen ◽  
Fujun Wang ◽  
...  
Keyword(s):  
Free Surface ◽  
Open Channel ◽  
Channel Flows ◽  
Open Channel Flows

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