scholarly journals Assessment of Critical Shear Stress and Threshold Velocity in Shallow Flow with Sand Particles

Water ◽  
2021 ◽  
Vol 13 (7) ◽  
pp. 994
Author(s):  
Reza Shahmohammadi ◽  
Hossein Afzalimehr ◽  
Jueyi Sui

In this study, the incipient motion of four groups of sand, ranging from medium to very coarse particles, was experimentally examined using an acoustic Doppler velocimeter (ADV) in different water depths under the hydraulically transitional flow condition. The transport criterion of the Kramer visual observation method was used to determine threshold conditions. Some equations for calculating threshold average and near-bed velocities were derived. Results showed that the threshold velocity was directly proportional to both sediment particle size and water depth. The vertical distributions of the Reynolds shear stress showed an increase from the bed to about 0.1 of the water’s depth, after performing a damping area, then a decrease toward the water surface. By extending the linear portion of the Reynolds shear stress in the upper zone of the damping area to the bed, the critical shear stress, particle shear Reynolds number, and critical Shields parameter were calculated. Results showed that the critical Shields parameter was located under the Shields curve, showing no sediment motion. This indicates that the incipient motion of sediment particles occurred with smaller bed shear stress than that estimated using the Shields diagram in the hydraulically transitional flow region. The reason could be related to differences between the features of the present experiment and those of the experiments used in the development of the Shields diagram, including the approaches to determine and define threshold conditions, the accuracy of experimental tools to estimate critical shear stress, and sediment particle characteristics. Therefore, the change in the specifications of experiments from those on which the Shields diagram has been based led to the deviation between the estimation using the Shields diagram and that of real threshold conditions, at least in the hydraulically transitional flow region with sand particles.

1981 ◽  
Vol 110 ◽  
pp. 171-194 ◽  
Author(s):  
C. Chandrsuda ◽  
P. Bradshaw

Hot-wire measurements of second- and third-order mean products of velocity fluctuations have been made in the flow behind a backward-facing step with a thin, laminar boundary layer at the top of the step. Measurements extend to a distance of about 12 step heights downstream of the step, and include parts of the recirculating-flow region: approximate limits of validity of hot-wire results are given. The Reynolds number based on step height is about 105, the mixing layer being fully turbulent (fully three-dimensional eddies) well before reattachment, and fairly close to self-preservation in contrast to the results of some previous workers. Rapid changes in turbulence quantities occur in the reattachment region: Reynolds shear stress and triple products decrease spectacularly, mainly because of the confinement of the large eddies by the solid surface. The terms in the turbulent energy and shear stress balances also change rapidly but are still far from the self-preserving boundary-layer state even at the end of the measurement region.


2012 ◽  
Vol 204-208 ◽  
pp. 354-358
Author(s):  
Jun Wang ◽  
Wei Guo ◽  
Hai Tao Xu ◽  
Zhong Wu Jin ◽  
Yin Jun Zhou

The incipient motion mechanism of cohesive fine sediment is different to the one of non-cohesive sediment. It is related to the consolidation while being influenced by the dry unit weight and particle size. By means of the rectangle piping flume, the influence mechanism of dry unit weight and particle size to critical shear stress of cohesive fine sediment is studied. Experimental results show that on the condition of consolidation, the influence of dry unit weight to incipient motion is divided into two different stages, one is that when dry unit weight increase quickly, but the influence to incipient motion is not greatly, another is that when dry unit weight increase slowly, but the influence to incipient motion is very greatly, the critical dry unit weight to two stages decline as mean particle size decrease. So the mean particle size is finer, the degree of dry unit weight influence to critical shear stress is stronger, and the incipient motion is more difficult when consolidation last longer; it is also shown consolidation is more disadvantageous to incipient motion.


2018 ◽  
Vol 2018 ◽  
pp. 1-7
Author(s):  
Yu Han ◽  
Shu-Qing Yang ◽  
Muttucumaru Sivakumar ◽  
Liu-Chao Qiu ◽  
Jian Chen

Hydraulic engineers often divide a flow region into subregions to simplify calculations. However, the implementation of flow divisibility remains an open issue and has not yet been implemented as a fully developed mathematical tool for modeling complex channel flows independently of experimental verification. This paper addresses whether a three-dimensional flow is physically divisible, meaning that division lines with zero Reynolds shear stress exist. An intensive laboratory investigation was conducted to carefully measure the time-averaged velocity in a rectangular open channel flow using a laser Doppler anemometry system. Two innovative methods are employed to determine the locations of division lines based on the measured velocity profile. The results clearly reveal that lines with zero total shear stress are discernible, indicating that the flow is physically divisible. Moreover, the experimental data were employed to test previously proposed methods of calculating division lines, and the results show that Yang and Lim’s method is the most reasonable predictor.


2005 ◽  
Vol 127 (5) ◽  
pp. 929-935 ◽  
Author(s):  
Bo Yu ◽  
Yasuo Kawaguchi

In the present study, we numerically investigated drag-reducing turbulent channel flows by surfactant additives. Surfactant additives were assumed to be uniformly distributed in the entire flow region by turbulent convection and diffusion, etc., but it was assumed that the shear-induced structure (SIS) (network of rod-like micelles) could form either in the region next to the walls or in the center region of the channel, making the fluid viscoelastic. In other regions surfactant additives were assumed to be incapable of building a network structure, and to exist in the form of molecules or micelles that do not affect the Newtonian properties of the fluid. With these assumptions, we studied the drag-reducing phenomenon with coexisting Newtonian and non-Newtonian fluids. From the study we identified the effectiveness of the network structures at different flow regions, and showed that the phenomenon of drag-reduction (DR) by surfactant additives is not only closely associated with the reduction of Reynolds shear stress but also related to the induced viscoelastic shear stress.


2001 ◽  
Vol 37 (8) ◽  
pp. 2273-2283 ◽  
Author(s):  
Audrey B. Shvidchenko ◽  
Gareth Pender ◽  
Trevor B. Hoey

2012 ◽  
Vol 1 (33) ◽  
pp. 32
Author(s):  
Hela Vidura Vithana ◽  
Richard Robert Simons ◽  
Martin Hyde

Flow visualization using Volumetric Three-component Velocimetry (V3V) was carried out during laboratory tests to determine threshold shear stress and forces on idealized spherical stones. Incipient motion tests consisted of light weight spherical particles of specific gravity=1.19-2.65 and diameter=9.6mm-31.8mm. In-line and uplift forces on a 50mm sphere sitting on a rough bed of similar spheres were measured using pressure transducers linked to tappings on the sphere surface. It was found that the Shields critical shear stress and stone protrusion has an exponential relationship. At an exposure of 0.2d and less, bed protection is remarkably stable. Incipient motion data for currents are applicable for waves when the flow is fully developed.


2016 ◽  
Vol 78 (9-4) ◽  
Author(s):  
Charles Hin Joo Bong ◽  
Frederik Josep Putuhena ◽  
Tze Liang Lau ◽  
Aminuddin Ab. Ghani

There are evidences in existing literatures suggesting the incipient motion values for any particle size is substantially lower for rigid boundary condition as compared to loose boundary condition.  The objective of the current study is to determine the effect of sediment deposition thickness on the critical shear stress for incipient motion. Experimental works for incipient motion were carried out in a rectangular flume with varying sediment deposits thickness. Results showed that the sediment deposits thickness has effect on the critical shear stress at low sediment deposits thickness and the effect will slowly diminish as the sediment deposits thickness increases. Multiple linear regression analysis was performed on the experimental data to develop a new critical shear stress equation.  The best regression model has   value of 0.69;    value of 0.60;  value of 0.009 and Mallow’s  value of 3.00. The new equation appears to be more consistent as compared to existing incipient motion equations for rigid boundary condition by having 80% of the predicted data falls within the acceptable discrepancy ratio when tested with data from other authors. The new equation can be used to determine critical shear stress values for self-cleansing sewerage design and other related engineering applications


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