多孔介质中溶质扩散滞后的试验测定与建模研究

Recent pore-scale studies have shown that liquid phase distribution in porous media is subject to the water saturation history. Soil water hysteresis refers to the unequal water contents during a wetting process and a drying process even at the same matric potential. Since the water present in soil pores is the dominant diffusive pathway, solute diffusion may be subject to the impact of water retention hysteresis as well, but this effect has not been systematically investigated. In this project, the hysteresis of solute diffusion will be studied by both experimental and modeling approaches. Based on the wetting and drying methods, effective solute diffusion coefficients (and thus diffusion tortuosity) at different water content/matric potentials of a siltstone and three soils of different textures will be measured. Then this complete set of data will be used to develop solute diffusion hysteresis models as a function of either water content or matric pressure. Physically based models for solute diffusion hysteresis in variably saturated porous media will also be developed by computational fluid dynamics (CFD) techniques, including the lattice Boltzmann method (LBM) and finite element method (FEM) for Navier-Stokes (N-S) equation. Since diffusion is one of the most important mechanisms of dissolved solute transport, quantifying solute diffusion hysteresis has significant consequences for accurately modeling solute transport in porous media, which is important for developing efficient fertilizer application, water conservation, and soil remediation best management practices, as well as for numerous engineering applications.

近来孔隙尺度研究表明，多孔介质中的液相分布会受到水分饱和历史的影响。土壤水分滞后指在变干和变湿过程中，同样的基质势条件下，土壤的含水量不相等。由于土壤孔隙中的水分是主要的溶质扩散路径，溶质扩散也可能会受到水分滞后效应的影响，但该效应尚未得到系统性的研究。本项目将通过试验和模拟途径研究溶质扩散的滞后效应。在利用干湿方法的基础上，我们将试验测定不同含水量和基质势条件下一种粉砂岩和三种不同质地的土壤的等效溶质扩散系数（也即相应的扩散曲折度）。随后该整批数据将用来建立溶质扩散滞后的统计模型，将其表示为含水量或基质势的函数。我们还将利用格子玻尔兹曼法和有限元求解纳维-斯托克斯方程等计算流体力学方法，为溶质扩散滞后过程建立起孔隙尺度的物理模型。由于扩散是溶质运移最重要的机理之一，量化溶质扩散滞后可以准确模拟多孔介质中的溶质运移，对于高效施肥、节水、土壤修复以及各种工程应用等方面都有重要的指导意义。

研究孔隙尺度上的土壤水流与溶质运移过程对于了解植物根际养分吸收、微生物活动、地下水污染过程有重要意义。由于在微观尺度上进行实验研究具有较大难度，利用微观CT技术获取较高精度的土壤孔隙结构信息，再基于CT图像进行直接建模已成为孔隙尺度研究的热点方法。进行直接建模已成为孔隙尺度研究的热点方法。在孔隙结构给定的情况下，水流过程的模拟可以通过计算流体力学方法求解Navier-Stokes方程实现。传统的数值模拟手段主要通过有限元法、有限体积法、有限差分法等对N-S方程进行离散求解。但是，这些方法在处理多孔介质的复杂边界时常常遇到困难，模拟结果难以收敛。本项目本文基于格子Boltzmann法在同步辐射X射线显微CT获取到的高精度多孔介质结构中进行水流运动和溶质运移的模拟，实现了对真实三维土壤团聚体（分辨率3.7μm，节点数达64,000,000）中水流运动和溶质运移过程的直接模拟探究了多孔介质空间异质性对土壤水力学特性以及溶质运移特性的影响，并利用发展起来的模型方法量化分析了生物质炭在改良土壤团聚体结构中的水力学效应及对溶质运移的影响。本项目最终开发了一套高性能孔隙尺度三维饱和壤水流与溶质运移的模拟方法，首次结合同步辐射X射线显微CT和孔隙尺度物理模型研究了生物质炭改良对土壤团聚体水力学特性和其内部的溶质运移过程的影响。论文中提出的理论和方法对于认识多孔介质中水流运动和养分迁移的微观机制有着重要意义。