Simulating Characteristics Of Open Channel Flow Through Submerged Vegetation By The Immersed Boundary-lattice Boltzmann Method

The open channel flow over vegetation is common in reality.It is an important subject investigated in the water environment and water ecology.For a long time,researchers have done much research on it and obtained many achievements.However,the research on the open channel flow over vegetation remains developable due to the complexity of the turbulence and the fluid-structure interaction.In numerical simulations,the approaching method in the existing literature can be divided into two types,the empirical methods and the direct methods.The empirical methods are usually simple and convenient and have small amount of calculation.But in general,it has bad applicability for rigid bodies with complex geometries and flexible bodies.On the contrary,to directly simulate solids is a general method.It can be used for a solid with any shape.But it usually has large amount of calculation and needs fine grids.The immersed boundary-lattice Boltzmann method(IB-LBM)is a method to simulate fluid-structure interaction.It is simple and easy to implement.The Cartesian grid is used,which makes the grid generation easy and there's no need to adjust the grid with the movement of solids.The lattice Boltzmann method has simple evolution equations and is esay to parallelize.The immersed boundary method can be used to directly describe the shapes and dimensions of the solids.In this paper,the immersed boundary-lattice Boltzmann method is used to simulate the fluid-structure interaction,with the no-slip condition on the surfaces of the solids considered directly.The MRT(multi relaxtion time)model is used to improve the stability of the algorithm,in combination with the large eddy simulation.The local grid refinement method is used.The simulation of the flow over the group of rigid plants with high Reynolds number is done.The effects of the plants' parameters on velocity distribution are analyzed.The main works in this paper are as follows:Part1: using IB-LBM combined with the large eddy simulation and the local grid refinement to simulate the open channel flow over vegetation.The large eddy simulation and considering the shapes and dimensions of the solids directly requires a fine-enough grid to describe the geometry of the solids approximately.Therefore,it's needed to make the space step small enough and solve the problem of high calculating cost at the same time.To solve the problem,in this paper the MRT model with local grid refinement method for it is used.For areas that need refinement,one only need to simply use a finer Cartesian grid to cover them.It is simpler in operation than some grid refinement methods for classic CFD methods.The information tranfer between the coarse grid and the fine grid is carried out with moment variables.Only the values of part of the moment variables related to the stress tensor need to be changed.The accuracy of the method is verified by comparing it with the results of the experiment.Part2: the anlysis of the effects of the double-layer rigid vegetation's density and density ratio on the vertical velocity distribution.With IB-LBM,the open channel flow over double-layer rigid vegetation is simulated under the conditions of different plants' densities and density ratios.I mainly investigate the vertical distribution of the time-averaged longitudinal velocity.I consider two cases.The first case is to keep the density ratio or number ratio of the high plants and the short plants constant and change the plants' densities,analyzing the effects of the plants' densities on the velocity distribution.The second case is is to keep the density of a type of plants constant and change the density of the other to change the density ratio of the two types of plants,analyzing the effects of the density ratio of the high plants and the short plants on velocity distribution.According to the results of the simulation,when the density ratio of the two types of plants is constant and the densities of them are changed,the curvature of the velocity distribution curve near the top of the high plants gets larger as the densities get larger.If I continue to increase the densities,the turning point may move to the top of the high plants.When the number of the high plants is constant and the number of the short plants are changed,so as to change the density ratio of the two types of plants,there are three possible situations.The fist one is only near the top of the high plants there is a turning point.The second is near the top of each of the high plants and the short plants there is an obvious concave arc.The third one is only near the top of the short plants there is a turning point.The density ratios corresponding to the three situations decrease in sequence.Part3: using IB-LBM to simulate the fluid-structure interaction of flexible filaments.Use IB-LBM to simulate the three-dimensional open channel flow through flexible filaments.The simulation is done with a single filament,a row of filaments parallel to the flow direction,a row of filaments vertical to the flow direction and a square array of filaments,respectively.The research on the distribution of velocity and other physical quantity,the deformation and motion of a filament and the interaction between filaments and fluid is done.The results show that the filaments ahead have an effect of decreasing velocity on the filaments behind,which makes the velocities and vorticities near the filaments behind are smaller than those near the filaments ahead,correspondingly.The vorticity has a maximum near the top of the filaments.When a filament with the same stiffness throughout it bends,the curvature at the lower point on it has a larger value.The vertical velocity near the top of the filaments are large.In this paper the IB-LBM combined with the large eddy simulation and the local grid refinement is used to simulate the open channel flow over the rigid vegetation and that over the elastic vegetation respectively.By using the MRT model and the proper values of the relaxation parameters,the stability of the method is improved.With the local grid refinement,the large calculating cost caused by using fine grids in the whole area or areas that take a large proportion is avoided.The effects of the two types of plants' densities and density ratio on the turning point of the velocity distribution curve are analyzed.The fluid-structure interaction of the plant containing an elastic leaf is simulated.Simulating flow involving plant models closer to actual plants is an important developing direction in the field of the fluid-structure interaction of vegetation in the future.IB-LBM is hopeful to get more applications to the problem.