In this study, a grain-scale modelling technique has been developed to generate the capillary pressure–saturation curves for swelling granular materials. This model employs only basic granular properties such as particles size distribution, porosity, and the amount of absorbed water for swelling materials. Using this model, both drainage and imbibition curves are directly obtained by pore-scale simulations of fluid invasion. This allows us to produce capillary pressure–saturation curves for a large number of different packings of granular materials with varying porosity and/or amount of absorbed water. The algorithm is based on combining the Discrete Element Method for generating different particle packings with a pore-unit assembly approach. The pore space is extracted using a regular triangulation, with the centres of four neighbouring particles forming a tetrahedron. The pore space within each tetrahedron is referred to as a pore unit. Thus, the pore space of a particle packing is represented by an assembly of pore units for which we construct drainage and imbibition capillary pressure–saturation curves. A case study on Hostun sand is conducted to test the model against experimental data from literature and to investigate the required minimum number of particles to have a Representative Elementary Volume. Then, the capillary pressure–saturation curves are constructed for Absorbent Gelling Material particles, for different combinations of porosity values and amounts of absorbed water. Each combination yields a different configuration of pore units, and thus distinctly different capillary pressure–saturation curves. All these curves are shown to collapse into one curve for drainage and one curve for imbibition when we normalize capillary pressure and saturation values. We have developed a formula for the Van Genuchten parameter $$\alpha $$α (which is related to the inverse of the entry pressure) as a function of porosity and the amount of absorbed water.

中文翻译：

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膨胀和孔隙率变化对毛细管的影响：DEM 结合孔隙单元组装方法

在这项研究中，开发了一种颗粒尺度建模技术来生成膨胀颗粒材料的毛细管压力 - 饱和曲线。该模型仅采用基本的颗粒特性，例如粒度分布、孔隙率和溶胀材料的吸水量。使用该模型，可以通过流体侵入的孔隙尺度模拟直接获得排水和渗吸曲线。这使我们能够为具有不同孔隙率和/或吸水量的大量不同填料的颗粒材料生成毛细管压力 - 饱和曲线。该算法基于将生成不同颗粒填充的离散元方法与孔隙单元组装方法相结合。孔隙空间是使用正则三角剖分提取的，四个相邻粒子的中心形成一个四面体。每个四面体内的孔隙空间称为孔隙单元。因此，颗粒堆积的孔隙空间由一组孔隙单元表示，我们为其构建排水和吸入毛细管压力 - 饱和曲线。对 Hostun 砂进行了案例研究，以根据文献中的实验数据测试模型，并研究具有代表性基本体积所需的最小颗粒数。然后，针对孔隙率值和吸水量的不同组合，构建吸收胶凝材料颗粒的毛细管压力-饱和曲线。每种组合都会产生不同的孔隙单元配置，从而产生明显不同的毛细管压力-饱和度曲线。当我们将毛细管压力和饱和度值归一化时，所有这些曲线都显示为一条排水曲线和一条吸收曲线。我们已经为 Van Genuchten 参数 $$\alpha $$α（与入口压力的倒数相关）开发了一个公式，作为孔隙率和吸水量的函数。