Studv On Flow Characteristics Of Rivers With Double-layer Rigid Vegetation Based On LES Method

Aquatic vegetation affects the physical,biological and chemical processes of aquatic environment.The existence of vegetation can significantly change the internal structure of water flow.It is important to explore the impact of vegetation on water flow movement,sustainable utilization of water resources,ecological restoration of rivers and lakes,river regulation and management.The structure of flow when passing double vegetation is very complex,so it is necessary and indispensable to study the turbulent structure of the flow.In this paper,Large Eddy Simulation(LES)is used to simulate the flow in rectangular open channel with double vegetation under 8 cases with different vegetation arrangements.The numerical simulation results are calibrated with the physical experiment results under the same conditions and the results showed good agreement.In this paper,the flow characteristics of rectangular open channel with double vegetation are studied.The flow velocity,turbulence characteristics and vorticity field are analyzed.It is found that the distribution of the main flow velocity in the channel is significantly affected by dourble-layer vegetation setup.The main flow velocity in vegetation area increases significantly in the top area of vegetation layer,while in free water layer it conforms to the logarithmic law.Within the low vegetation with double-layer vegetation flow,it can be divided into three areas according to the velocity of flow.The first area is the area between two rows of vegetation,and its velocity is relatively fast;the third area is the vegetation backflow area,whose velocity is relatively slow;the second area is the vegetation upflow area,whose velocity is between one and three areas.Reynolds stress peaks in the top area of low vegetation,and is small and constant in the lower part of low vegetation.Reynolds stress decreases gradually in the area above low vegetation,and increases slightly near the free surface due to the influence of high vegetation.The effect of different vegetation layout and vegetation density on Reynolds stress distribution is obvious.The turbulence intensity of flow reaches its peak below the top of the vegetation layer where the flow is highly sheared,and the flow layer above the vegetation layer tends to be the same and remains relatively stable.Large eddy simulation method can better simulate the eddy current structure caused by turbulent anisotropy of fluid.The presence of vegetation greatly strengthens the intensity of water vorticity,especially the coherent eddy band near the top of vegetation layer.The flow around vegetation produces cylindrical flow and develops downstream to form different wake patterns.Unstable eddies are formed downstream of vegetation,which promoted momentum exchange and vertical diffusion.When the vegetation density is low,a long wake area is formed downstream of the vegetation,while the wake area is short when the vegetation density is high.Velocity distributions at different locations of vegetation-laden flow are characterized by one or more inflection points.The vorticity corresponding to these inflection points is relatively strong.It can be seen that the key factor to form the vorticity of vegetation-laden flow is the gradient of flow velocity.The maximum vorticity is near the top of the vegetation layer,which is due to the large disturbance of the flow through this area,resulting in a large velocity gradient.However,height differences in the two-layer vegetation setup lead to different vorticity distributions and maximum vorticity occurrence heights under different working conditions.