孔隙尺度下三维多孔介质扩散迂曲度的SPH计算
Pore-scale SPH simulations of diffusive tortuosity in 3-D porous media
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摘要: 多孔介质的扩散迂曲度是污染物在多孔介质中扩散速率的重要量度,研究扩散迂曲度对于研究污染物在多孔介质中的运移规律具有重要意义。迂曲度作为反映孔隙结构的复杂程度的量化指标,其值的大小与孔隙结构关系密切;而在常规宏观试验中难以实现对孔隙结构和试验条件的精准控制,因此有必要发展数值仿真试验技术对扩散迂曲度与孔隙结构的关系进行机理性研究。基于光滑粒子流体动力学(SPH)方法对重构三维土柱进行仿真试验,再采用曲线拟合可获取介质的扩散迂曲度值并确定有效扩散系数。分别采用颗粒流法(PFC)和蒙特卡洛方法随机生成三维球型颗粒堆积土柱和不同倾角的三维片状型颗粒重构土柱用于仿真试验,并根据计算结果探讨了颗粒类型和片状颗粒片层倾角对多孔介质扩散迂曲度的影响。计算结果表明:对片层状多孔介质,片层与浓度梯度方向的夹角对介质的扩散迂曲度影响明显;扩散迂曲度随着片层偏角的增大而增大;当片层走向与浓度梯度方向一致时迂曲度甚至可能小于同孔隙率的球状颗粒堆积体的迂曲度值。Abstract: The diffusive tortuosity is an important measurement of the diffusion rate of pollutants in porous media. Determining the diffusive tortuosity of porous media is of great significance for studying the migration of pollutants. Based on the Smooth particle hydrodynamics (SPH) method, the effective diffusion coefficient and the values of diffusive tortuosity can be obtained conveniently and accurately by simulating the diffusive experiment on 3-D reconstructed soil columns. The particle fluid code (PFC) and the Monte Carlo method are used to generate 3-D soil columns by stacking spherical particles and by filling random layered elements, respectively. The calculated results show that for the layered porous media, the angle between the layer and the concentration gradient direction has a significant influence on the diffusive tortuosity. The values of diffusive tortuosity generally grow with the increasing included angle. When the orientation of layers consists with the concentration gradient direction, the diffusive tortuosity is minimized and may even be less than the tortuosity value of the spherical particle packing media with the same porosity.