格子波尔兹曼法在胶体潜流交换研究中的应用

Application of Lattice Boltzmann methods to the colloidal hypothetic exchange study

采用室内试验与数值模拟的方法,结合X射线差异微断层扫描(XDMT)和格子波尔兹曼法(Lattice Boltzmann Method,LB),从孔隙尺度研究胶体颗粒在河流系统潜流交换(Hyporheic Exchange)中的动力学过程,量化分析了胶体沉积导致的河床介质性质的改变。结果表明,胶体颗粒通过潜流交换作用进入河床介质后,其形式分布表现出高度的非均质性,上层河床介质的孔隙率削减幅度比下层介质的大,且上游河床上下层的渗透性都比下游河床的渗透性下降幅度大。通过拟合河床介质的局部渗透率和局部孔隙率的关系,得到两者的关系式为k∝φ2.9,表明介质的局部渗透率随着局部孔隙率的增大而增加。

This paper is to present our study of the applicability of Lattice Boltzmann methods to the colloidal hyporheic exchange via the flume experiment while trying to use the X-ray Difference Micro-tomography（XDMT） and Lattice Boltzmann Methods in combination with the study of the colloidal dynamic process in the hyporheic exchange taking place in a riverbed,in order to quantify the spatial variation of the riverbed caused by the colloid deposition.For our research purpose,we have conducted a laboratory flume experiment with the deposition of the colloidal ZrO2 particles in a streambed composed of granular glass beads.At the same time,we have done observations via XDMT and LB simulation and quantitative analysis of the effects of pore-scale colloidal deposits on the local porosity and permeability of the sand riverbed.The 3D tomographic reconstructions have shown that the colloid deposition proves to be highly heterogeneous at the pore scale.After entering the upstream surface of the bed form,most colloids were deposited on the top surface of the glass beads,which implies that the process of interception and gravitation tends to dominate the particle deposition because of the approximately downward of the water current in the given river section.In such sections of the river,we can find the accumulation effects of the grain contacts,resulting from the physical straining mechanism.The porosity reduction at the top level of the bed proves generally greater than those in the bottom level.At the same time,we can also find that both at the top level and the bottom level,the permeability reduction at the upstream surface of the riverbed proves to be generally higher than the downstream surface,which implies that a small amount of colloidal deposition can lead to significant permeability reduction by clogging the bed surface.When we investigated the overall evolution of the deposit structure and permeability distributions of the sites in the streambed due to colloid filtration in the streambed by using XDMT and LB simulations,it can be found that by fitting the relationship between the local permeability and the local porosity of the bed media,the deposit structure and permeability distributions of the sites in the streambed turn out to follow a power with k∝φ2.9.This indicates that the local permeability tends to increase with the increase of the local porosity,which suggests that the permeability was a strong function of porosity in this case.The data obtained here before and after deposition of the local porosity and permeability make us gain a better understanding of the complicated coupling between the changing pore structure,the pore water current,the solute migration,and the stream subsurface exchange.