Characteristic scales and consequences of large-scale horizontal coherent structures in shallow open-channel flows

2012 ◽  
pp. 99-120
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Open Channel ◽  
Channel Flows ◽  
Open Channel Flows ◽  
Characteristic Scales

scholarly journals Flow structure in compound open-channel flows in the presence of transverse currents

2018 ◽  
Vol 40 ◽  
pp. 05024 ◽  
Author(s):  
Sébastien Proust ◽  
Vladimir Nikora
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Open Channel ◽  
Mixing Layer ◽  
Free Surface Flows ◽  
Channel Flows ◽  
Two Dimensional ◽  
Open Channel Flows ◽  
Secondary Currents ◽  
Flow Modification

The structure of free-surface flows is experimentally investigated in a laboratory flume with a compound cross-section consisting of a central main channel (MC) and two adjacent floodplains (FPs). The study focuses on the effects of transverse currents on: (i) mixing layers and quasi-two-dimensional coherent structures at the interfaces between MC and FPs; (ii) secondary currents developing across the channel; and (iii) large and very-large-scale motions that were recently observed in non-compound open channel flows. Transverse currents represent spanwise depth- and time-averaged flow from MC to FPs or vice versa. The study is based on one-point and two-point ADV measurements. Streamwise non-uniform flows are generated by imposing an imbalance in the discharge distribution between MC and FPs at the flume entrance, keeping the total flow rate the same for all scenarios. It is shown that even small transverse currents can be very effective in flow modification, as they can significantly displace the mixing layer, shear-layer turbulence, and coherent structures towards MC or FP, depending on the current direction. They can also alter the distribution and strength of the secondary currents. The interactions of quasi-two-dimensional coherent structures, very-large-scale motions, and secondary currents at different conditions are also part of this study.

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.

scholarly journals EFFECTS OF FROUDE NUMBERS ON COHERENT STRUCTURES IN SIDE-CAVITY OPEN-CHANNEL FLOWS

2003 ◽  
pp. 77-88
Author(s):  
Iehisa NEZU ◽  
Kouki ONITSUKA ◽  
Shunsuke TAKAHASHI ◽  
Masashi YANO
Keyword(s):  
Coherent Structures ◽  
Open Channel ◽  
Channel Flows ◽  
Open Channel Flows
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