scholarly journals Physical Modeling of Vanishing Bores in Open-Channel Flows

2021 ◽  
Author(s):  
Youkai Li ◽  
Hubert Chanson
Keyword(s):  
Physical Modeling ◽  
Water Surface ◽  
Open Channel ◽  
Flat Channel ◽  
Open Channel Flows ◽  
Data Set ◽  
Undular Bore ◽  
Benchmark Data ◽  
Video Recordings ◽  
Quantitative Results

When an undular bore is propagating upstream against a subcritical flow in a flat channel, its leading front may weaken and vanish. The present study physically investigated the whole decaying process of vanishing bores in a relatively large facility. The invert was made of mobile or fixed natural river gravels. Detailed sampling of water surface elevations and velocities was performed in combination with high-resolution photo and video recordings. Despite some increased turbulence characteristics linked to the bore front arrival, no sediment motion was observed underneath the vanishing bores. The quantitative results provide a unique benchmark data set for further study of the later-stage bore propagations.

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.

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