Contributions of very large-scale motions to turbulence statistics in open channel flows

2020 ◽  
Vol 892 ◽  
Author(s):  
Yanchong Duan ◽  
Qigang Chen ◽  
Danxun Li ◽  
Qiang Zhong
Keyword(s):  
Large Scale ◽  
Open Channel ◽  
Channel Flows ◽  
Turbulence Statistics ◽  
Open Channel Flows ◽  
Image Position

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.

Stability analysis of open-channel flows with secondary currents

2021 ◽  
Vol 927 ◽  
Author(s):  
Carlo Camporeale ◽  
Fabio Cannamela ◽  
Claudio Canuto ◽  
Costantino Manes
Keyword(s):  
Free Surface ◽  
Stability Analysis ◽  
Large Scale ◽  
Open Channel ◽  
Secondary Flows ◽  
Channel Flows ◽  
Open Channel Flows ◽  
Secondary Currents ◽  
Turbulence Structures ◽  
Wall Flows

This paper presents some results coming from a linear stability analysis of turbulent depth-averaged open-channel flows (OCFs) with secondary currents. The aim was to identify plausible mechanisms underpinning the formation of large-scale turbulence structures, which are commonly referred to as large-scale motions (LSMs) and very-large-scale motions (VLSMs). Results indicate that the investigated flows are subjected to a sinuous instability whose longitudinal wavelength compares very well with that pertaining to LSMs. In contrast, no unstable modes at wavelengths comparable to those associated with VLSMs could be found. This suggests that VLSMs in OCFs are triggered by nonlinear mechanisms to which the present analysis is obviously blind. We demonstrate that the existence of the sinuous instability requires two necessary conditions: (i) the circulation of the secondary currents $\omega$ must be greater than a critical value $\omega _c$ ; (ii) the presence of a dynamically responding free surface (i.e. when the free surface is modelled as a frictionless flat surface, no instabilities are detected). The present paper draws some ideas from the work by Cossu, Hwang and co-workers on other wall flows (i.e. turbulent boundary layers, pipe, channel and Couette flows) and somewhat supports their idea that LSMs and VLSMs might be governed by an outer-layer cycle also in OCFs. However, the presence of steady secondary flows makes the procedure adopted herein much simpler than that used by these authors.

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.

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