Flow characteristics in curved open channel with partial suspended vegetation

Suspended vegetation in open channels such as natural river,lake,reservoir usually affect the flow structure,causing the change of the water environment,sediment transport,bed deformation.In order to study the water flow behavior in curved open channels under the influence of suspended vegetation,experiments were conducted in a Ushaped flume with 180°bend where the suspended vegetation substituted by cylindrical glass rods were partially placed.The particle image velocimeter(PIV)system was employed to measure the flow velocities of various cases with different vegetation arrangements and roots length.Comparison and analysis were conducted for measured data,such as water level,velocity,head loss,Reynolds stress,and turbulence kinetic energy(TKE)to obtain certain general rules of water flow in open curved channels with suspended vegetation.It can be found from the measured data that the water level and the head loss in the vegetation area are closely related to the arrangements of the suspended vegetation.Furthermore,the drag force of the vegetation can not only lead to smaller increments of longitudinal TKE above the vegetation tail than that below it,but also lead to the flow velocities in vegetation area much smaller than areas without vegetation.In addition,suspended vegetation weakens the vortex strength near the water surface and increases the vortex strength below the vegetation tail,and the flow velocity in the vegetation area has a close relationship with the suspended vegetation length in the water.Therefore,it can be concluded that the reasonable arrangements of suspended vegetation group in curved open channel can protect the concave bank from being scoured,and protect the convex bank from being deposited.

Study on the flow structure in curved open channels with suspended vegetation using multi relaxation time lattice Boltzmann method

Suspended vegetation in rivers,lakes,reservoirs and canals can change flow structure,which will in turn affect the sediment transport and cause the variation of water ecological environment.In order to study the characteristics of bend flow through suspended vegetation,three-dimensional numerical simulations are carried out by using the multi-relaxation-time lattice Boltzmann method(MRT-LBM).The drag force induced by vegetation is added in the velocity correction in the equilibrium distribution and a hybrid format combined bounce and specular reflection scheme is applied in the solid-fluid boundaries.After the validation of this model,six cases are designed to conduct the numerical simulations according to the root depth and the arrangement of vegetation.The simulated results show that the suspended vegetation can redistribute the flow structure in curved open channels.After the arrangement of suspended vegetation,the main flow moves to the side without vegetation,and the distribution of velocity tends to be balanced when vegetation is arranged on the entire cross section,the range of circulating current is reduced from the whole cross section to the local position without vegetation,however,the circulating current can still exist in the curve where the suspended vegetation enters less than half of the water depth.In addition,it can also be concluded that the suspended vegetation can affect the lateral gradient of flow velocity,and the bed shear stress in the curved channel.

3-D numerical simulation of curved open channel confluence flow with partially non-submerged rigid vegetation

For the study of the effects of partially non-submerged rigid vegetation on the free-surface confluence flow in a curved open channel,a numerical simulation is carried out by using the Volume of Fluid model combined with the porous media model with the software OpenFOAM.The model is first validated by using available experimental measurement data with a good agreement.Then,the characteristics of the separation zone generated by the centrifugal forces and the confluence flow are analyzed.Due to the resistance created by the vegetation,the velocities in the separation zone are more chaotic and the separation zone becomes smaller and more irregular.The reduction of the separation zone area of the vegetated flow in the convex bank is more significant than that in the concave bank.The velocities in the vegetated region become much smaller and remains so in the downstream flow after the vegetation region.Meanwhile,the vegetation compresses and divides the circulations in the flow area,rebuilding a structure with smaller circulations in the main flow and unclear circulations in the vegetation region.In addition,the bed wall shear stresses are significantly smaller in the vegetation region and the separation zone compared to the non-vegetated flow.This implies that the vegetation can have the effect of protecting the river bed from erosion.

Effect of vegetation on flow structure and dispersion in strongly curved channels

The effect of vegetation on the flow structure and the dispersion in a 180 o curved open channel is studied. The Micro ADV is used to measure the flow velocities both in the vegetation cases and the non-vegetation case. It is shown that the velocities in the vegetation area are much smaller than those in the non-vegetation area and a large velocity gradient is generated between the vegetation area and the non-vegetation area. The transverse and longitudinal dispersion coefficients are analyzed based on the experimental data by using the modified N- zone models. It is shown that the effect of the vegetation on the transverse dispersion coefficient is small, involving only changes of a small magnitude, however, since the primary velocities become much more inhomogeneous with the presence of the vegetation, the longitudinal dispersion coefficients are much larger than those in the non-vegetation case.