scholarly journals Mechanism of Secondary Currents in Open Channel Flows

2019 ◽  
Author(s):  
Shu-Qing Yang
Keyword(s):  
Open Channel ◽  
Channel Flows ◽  
Open Channel Flows ◽  
Secondary Currents

Numerical study on secondary currents of the second kind in wide shallow open channel flows

2014 ◽  
pp. 179-187 ◽  
Author(s):  
R Suzuki ◽  
I Kimura ◽  
Y Shimizu ◽  
T Iwasaki ◽  
T Hosoda
Keyword(s):  
Open Channel ◽  
Numerical Study ◽  
Channel Flows ◽  
Open Channel Flows ◽  
Secondary Currents

scholarly journals Friction factor decomposition for rough-wall flows: theoretical background and application to open-channel flows

2019 ◽  
Vol 872 ◽  
pp. 626-664 ◽  
Author(s):  
V. I. Nikora ◽  
T. Stoesser ◽  
S. M. Cameron ◽  
M. Stewart ◽  
K. Papadopoulos ◽  
...  
Keyword(s):  
Friction Factor ◽  
Open Channel ◽  
Reynolds Numbers ◽  
Channel Flows ◽  
Turbulent Stress ◽  
Open Channel Flows ◽  
Navier Stokes Equation ◽  
Data Set ◽  
Secondary Currents ◽  
Wide Range

A theoretically based relationship for the Darcy–Weisbach friction factor $f$ for rough-bed open-channel flows is derived and discussed. The derivation procedure is based on the double averaging (in time and space) of the Navier–Stokes equation followed by repeated integration across the flow. The obtained relationship explicitly shows that the friction factor can be split into at least five additive components, due to: (i) viscous stress; (ii) turbulent stress; (iii) dispersive stress (which in turn can be subdivided into two parts, due to bed roughness and secondary currents); (iv) flow unsteadiness and non-uniformity; and (v) spatial heterogeneity of fluid stresses in a bed-parallel plane. These constitutive components account for the roughness geometry effect and highlight the significance of the turbulent and dispersive stresses in the near-bed region where their values are largest. To explore the potential of the proposed relationship, an extensive data set has been assembled by employing specially designed large-eddy simulations and laboratory experiments for a wide range of Reynolds numbers. Flows over self-affine rough boundaries, which are representative of natural and man-made surfaces, are considered. The data analysis focuses on the effects of roughness geometry (i.e. spectral slope in the bed elevation spectra), relative submergence of roughness elements and flow and roughness Reynolds numbers, all of which are found to be substantial. It is revealed that at sufficiently high Reynolds numbers the roughness-induced and secondary-currents-induced dispersive stresses may play significant roles in generating bed friction, complementing the dominant turbulent stress contribution.

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.

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