Numerical Study Of Open Channel Flow With Vegetation By The Lattice Boltzmann Method

The water flow can be disturbed by the aquatic vegetation in the natural rivers,reservoirs and lakes.As a result,the sediment transport,the riverbed revolution,the water environment and water ecology in the rivers,reservoirs or lakes can also be influenced.In this study,the three dimensional laser Doppler velometer,LDV)and other instruments are applied to measure and analyse the water flow characteristics in a laboratory flume.The multi-relax time lattice Boltzmann equation(MRT LBE)was developed and its numerical methods are studied.The model is applied to numerically simulate the laborotary flume flow with aquatic vegetation.The research results not only has thoretical significance,but also has value in engineering applications,which can provide reference in the field of river regulation,water ecology environment construction.Specifically speaking,the research results are as followings:At first,the origin and development of computational fluid dynamics are introduced.The comparison is present among traditional numerical methods and the lattice Boltzmann method(LBM).The discrete velocity model of the lattice Boltzmann method(LBM)and the boundary treatment method are studied,and verified by two classical examples.Secondly,a three-dimensional laser Doppler flow meter(LDV)was used to measure the flow velocity in a laboratory flume with different diameters of vegetation's groups.In the laboratory flume,measurements are conducted under four typical cases according the arrangement of the plants in the channel i.e.sparse and parallel,dense and parallel,sparse and staggered,dense and staggered.The measured data were dealed with by some softwares,such as Matlab.The water level fluctuation,flow velocity change,water surface slope,hydraulic slope and the relationship between them and the vegetation density were discussed.After analysis and comparison of the measured data,the flow characteristics in the vegetation area under the four cases were obtained.Finally,the LBM is used for the 2D numerical simulation of the flow structure in the laboratory flume with rigid vegetation.Because the single relaxation time model(SRT)is usually unstabled under high Renolds numbers,multi-relaxation time(MRT-LBE)model is introduced in this study,and the drag force of vegetation is added in the model after comparison the numerical simulation results with measured data.Furthermore,the algorithm of MRT-LBE model is presented.Numerical simulations are conducted under four different vegetation's arrangements.By comparison with the measured results,simulation results and the measured data are in good agreement,indicating that the model can reasonably simulate the flow in open channels or rivers with rigid vegetation.Because the channel is symmetric about it center line,only a half of the channel is used for simulation to save computation time and memory space.At the inlet,the flow velocity is given according to the measured data.At the outlet,full development condition is used.At the solid wall boundary,bounce back is applied.