Impacts of turbulent flow over a channel bed with a vegetation patch on the incipient motion of sediment

2018 ◽  
Vol 45 (9) ◽  
pp. 803-816
Author(s):  
Reza Shahmohammadi ◽  
Hossein Afzalimehr ◽  
Jueyi Sui
Keyword(s):  
Experimental Study ◽  
Reynolds Stress ◽  
Streamwise Velocity ◽  
Stress Pattern ◽  
Submerged Vegetation ◽  
Incipient Motion ◽  
Vegetation Patch ◽  
Motion Process ◽  
Velocity Turbulence ◽  
Vegetation Patches

In this experimental study, the effect of a submerged vegetation patch with individual plants on the incipient motion of sediment has been examined. Results showed that a vegetation patch in the channel bed affected the incipient motion process of sediment and resulted in significant impacts on flow velocity, turbulence intensities, turbulent kinetic energy, and Reynolds stress distribution. The presence of vegetation patches completely change vertical distribution of velocity and Reynolds stress pattern and leads to negative Reynolds stress values in zones with negative velocity gradient. In the presence of a vegetation patch, threshold cross-averaged streamwise velocity is 20% smaller than that without vegetation, but because of the occurrence of the preferential path around the sheath section, the near-bed streamwise velocity is the same as without vegetation. Also, vegetation increases turbulence intensity, thus encouraging sediment motion. In the presence of a vegetation patch, the Shields parameter is, on average, one and half times that of without vegetation over the bed.

scholarly journals Formation of Coherent Flow Structures beyond Vegetation Patches in Channel

Water ◽  
2021 ◽  
Vol 13 (20) ◽  
pp. 2812
Author(s):  
Masoud Kazem ◽  
Hossein Afzalimehr ◽  
Jueyi Sui
Keyword(s):  
Experimental Study ◽  
Considerable Effect ◽  
Middle Part ◽  
Vortex Structures ◽  
Flow Structures ◽  
Finite Width ◽  
Vegetation Patch ◽  
Weak Part ◽  
Side Flow ◽  
Vegetation Patches

By using model vegetation (e.g., synthetic bars), vortex structures in a channel with vegetation patches have been studied. It has been reported that vortex structures, including both the vertical and horizontal vortexes, may be produced in the wake in the channel bed with a finite-width vegetation patch. In the present experimental study, both velocity and TKE have been measured (via Acoustic Doppler Velocimeter—ADV) to study the formation of vortexes behind four vegetation patches in the channel bed. These vegetation patches have different dimensions, from the channel-bed fully covered patch to small-sized patches. Model vegetation used in this research is closely similar to vegetation in natural rivers with a gravel bed. The results show that, for a channel with a small patch (Lv/Dc = 0.44 and Dv/Dc = 0.33; where Lv and Dv are the length and width of patch and Dc is the channel width, respectively), both the flow passing through the patch and side flow around the patch have a considerable effect on the formation of flow structures beyond the patch. The results of further analysis via 3D classes of the bursting events show that the von Karman vortex street splits into two parts beyond the vegetation patch as the strong part near the surface and the weak part near the bed; while the middle part of the flow is completely occupied by the vertical vortex formed at a distance of 0.8–1 Hv beyond the vegetation patch, and thus, the horizontal vortexes cannot be detected in this region. The octant analysis is conducted for the coherent shear stress analysis that confirms the results of this experimental study.

scholarly journals Riparian Vegetation Density Mapping of an Extremely Densely Vegetated Confined Floodplain

Hydrology ◽  
2021 ◽  
Vol 8 (4) ◽  
pp. 176
Author(s):  
István Fehérváry ◽  
Tímea Kiss
Keyword(s):  
Riparian Vegetation ◽  
Woody Species ◽  
Submerged Vegetation ◽  
Vegetation Types ◽  
Vegetation Density ◽  
Vegetation Zone ◽  
Density Mapping ◽  
Vegetation Zones ◽  
Branch Density ◽  
Vegetation Patches

The most crucial function of lowland-confined floodplains with low slopes is to support flood conveyance and fasten floods; however, obstacles can hinder it. The management of riparian vegetation is often neglected, though woody species increase the vegetation roughness of floodplains and increase flood levels. The aims are (1) to determine the branch density of various riparian vegetation types in the flood conveyance zone up to the level of artificial levees (up to 5 m), and (2) to assess the spatial distribution of densely vegetated patches. Applying a decision tree and machine learning, six vegetation types were identified with an accuracy of 83%. The vegetation density was determined within each type by applying the normalized relative point density (NRD) method. Besides, vegetation density was calculated in each submerged vegetation zone (1–2 m, 2–3 m, etc.). Thus, the obstacles for floods with various frequencies were mapped. In the study area, young poplar plantations offer the most favorable flood conveyance conditions, whereas invasive Amorpha thickets and the dense stands of native willow forests provide the worst conditions for flood conveyance. Dense and very dense vegetation patches are common in all submerged vegetation zones; thus, vegetation could heavily influence floods.

The limiting behaviour of turbulence near a wall

1986 ◽  
Vol 170 ◽  
pp. 265-292 ◽  
Author(s):  
Dean R. Chapman ◽  
Gary D. Kuhn
Keyword(s):  
Correlation Coefficient ◽  
Reynolds Stress ◽  
Computational Models ◽  
Streamwise Velocity ◽  
Streamwise Vorticity ◽  
Courant Number ◽  
Fine Mesh ◽  
Wall Boundary Conditions ◽  
Limiting Behaviour ◽  
Good For

Three different Navier-Stokes computational models of incompressible viscoussublayer turbulence have been developed. Comparison of computed turbulence quantities with experiment is made for the mean streamwise velocity, Reynolds stress, correlation coefficient and dissipation; for the r.m.s. fluctuation intensities of streamwise vorticity, Reynolds stress and three velocity components; and for the skewness and flatness of fluctuating streamwise velocity and Reynolds stress. The comparison is good for the first three of these quantities, and reasonably good for most of the remainder.Special computer runs with a very fine mesh and small Courant number were made to define the limiting power-law behaviour of turbulence near a wall. Such behaviour was found to be confined to about 0.3 wall units from the wall, and to be: linear for streamwise turbulence, spanwise turbulence, vorticity normal to the wall, and for the departures from their respective wall values of dissipation, streamwise vorticity and spanwise vorticity; second power for turbulence normal to the wall; third power for Reynolds stress; and a constant value of the correlation coefficient for Reynolds stress. A simple physical explanation is given for the third-power variation of Reynolds stress and for the broad generality of this limiting variation.Applications are made to Reynolds-average turbulence modelling: damping functions for Reynolds stress in eddy-viscosity models are derived that are compatible with the near-wall limiting behaviour; and new wall boundary conditions for dissipation in k-ε models are developed that are similarly compatible.

scholarly journals Experimental study of incipient motion in mixed-size sediment

1988 ◽  
Vol 24 (7) ◽  
pp. 1137-1151 ◽  
Author(s):  
Peter R. Wilcock ◽  
John B. Southard
Keyword(s):  
Experimental Study ◽  
Incipient Motion

Numerical study on the drag characteristics of rigid submerged vegetation patches

2021 ◽  
Vol 33 (8) ◽  
pp. 085123
Author(s):  
Mengyang Liu ◽  
Wenxin Huai ◽  
Bin Ji ◽  
Peng Han
Keyword(s):  
Numerical Study ◽  
Submerged Vegetation ◽  
Vegetation Patches
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