Research On Characteristics Of The Water Flow With Suspended Vegetation In A Curved Flume By Experiments And Numerical Simulations

Aquatic vegetation in the natural curved river could change the water flow structure,and further affect the sediment transport.According to the vegetation location in the water,the vegetation can be divided into emergent vegetation,submerged vegetation and suspended vegetation,such as Eichhornia crassipes,Hydrocharis dubia,Pistia stratiotes,Salvinia natans,etc.The roots of suspended vegetation hang in the water,so the hydraulic factor of open channel flow cannot simply be used to describe its flow characteristics.The water flow with suspended vegetation is studied combined with physical experiment and numerical calculations.A series of measurements have been conducted on the water flow with suspended vegetation in a laboratory U-shaped flume using particle image velocimeter and other instruments.Multi-relaxation lattice Boltzmann model with vegetation drag force is established and is applied to numerical simulate the bend flow characteristics in a flume with suspended vegetation.The main research contents and conclusions are as follows:Firstly,the basic idea of computational fluid dynamics and the fundamental theory of lattice method are introduced.Through theoretical analysis,boundary treatment and algorithm design,the different lid-driven cavities flow is calculated by MATLAB software.The results show that the semi curved cavity will promote the separation of the main vortices,the fully curved cavity can eliminate the secondary vortices generated in the boundary,and making the bottom boundary of the rectangular cavity into a cylindrical surface can simplify the movement to the greatest extent.Secondly,a series of experiments are carried out in a laboratory flume,and the suspended vegetation is substituted by cylindrical glass rod.Six typical operating conditions were set up.Four positions of vegetation arragements are on concave banks,on convex banks,on both sides and on the whole section,and two lengths of vegetation roots are 7cm and 3.5cm.PIV and other instruments have been employed to measure the water level and velocities.The measured results show that suspended vegetation will raise the water level,and arranging vegetation on one side will increase water surface lateral gradient in the vegetation area.The flow velocity shows a trend of "small at both ends and large in the middle" along the vertical direction.The suspended vegetation will weaken the vorticity at the water surface,forming an obvious vortex near the root tip.Finally,a three-dimensional numerical simulation on the water flow in a curved flume with suspended vegetation is carried out.In order to avoid the divergence in the numerical calculation process,the MRT-LBM introduced the drag force of plants is chosen for simulation,and the mixed method of rebound and specular reflection is used to simplify the calculation of the upper boundary.After processing,a D3Q15multi-relaxation model is established to numerical simulates the flow structure in a curved flume with suspended vegetation under several typical cases.The results show that the bend circulation only exists in the non-vegetation area.The flow velocity first increases and then decreases in the area below the root tip,and is evenly distributed above the root tip.For the four conditions of vegetation on the half side,the vegetation in the middle has the greatest impact on the water flow structure.Under different vegetation root depths,the deeper the vegetation roots,the weaker the circulation intensity and the bedshear stress.The larger the Reynolds number,the smaller the flow velocity,but the greater the turbulent intensity and lateral mixing.