A Lattice Boltzmann Based Model for Predicting Unsaturated Flow through Soil Macropores and Capillary Pores

该项目的主要目标是将基于Richards方程的土壤毛管孔隙中的非饱和渗流与大孔隙中的渗流耦合起来，采用格子-Boltzmann(LB)方法求解液气两相流动的Navier-Stokes方程。由于计算效率高、基于力学的模型不存在，地下水补给或深层排水估计，或垃圾填埋场的气体排放，往往通过忽略优先流或通过经验建模来进行。双重渗透率模型通过为微观和宏观孔隙分别指定一组不同的非饱和属性来模拟大孔隙流动。然而，通过大孔隙的流动并不遵循达西亚流，而是假设所有的大孔隙是作为一个连续体连接在一起的，而大孔隙往往是离散的和不相连的。该模型的核心思想是，大孔道中的流动类似于不规则形状的管道中的流动，因此使用格子-玻尔兹曼方法求解时，Navier-Stokes方程更合适，数值也更稳定。随着数字X射线CT成像技术的进步，现在可以相对快速和经济地表征土壤的孔隙结构。因此，一个可以利用如此高分辨率的孔结构数据的模型可以彻底改变我们估计长期液体渗入垃圾填埋场或从垃圾填埋场排放气体的方式。土壤系统的宏观和微观孔隙结构将被数字化地输入到模型中，使用从仪器化的野外尺度粘土盖帽采集的土壤样品的X射线计算机层析成像(CT)图像数据来验证建模方法。在三年的时间里，从底特律垃圾填埋场的两个现场规模的粘土盖子测试部分收集了高分辨率的水平衡数据。传统的基于物理的数值模型能够较准确地捕捉毛细或微孔内的非饱和渗流。然而，它们无法模拟由于压实不足、干燥开裂、冻结/融化、根部渗透和啮齿动物活动而形成并持续演化的大孔隙的流动。即将开发的模型将克服模拟液体或气体在大孔隙中流动的挑战。利用X射线成像土壤结构的最新进展来模拟土壤流动的数值模型将为从业者和监管机构提供一种工具，以准确预测长期深层排水到具有环境意义的地下水系统或从垃圾填埋场排放温室气体。将为本科生的垃圾填埋场设计课程准备一个关于废墟中液体和气体迁移的课程模块，并将准备一个包含理论和实验室实验的课程模块，以演示优先通过土壤的流动，并在研究生水平的课程中引入。在夏季训练营期间，将在密歇根州立大学进行动手示威，向高中生伸出援手。

The key objective of this project is to couple the Richards Equation based unsaturated flow in capillary pores of soils with flow through macropores using the Navier-Stokes Equations solved using the Lattice-Boltzmann (LB) Method for two phase (liquid and gas) flow. Because computationally efficient mechanistically-based models do not exist, groundwater recharge, or deep drainage estimates, or gas emissions from landfills are often carried out by ignoring preferential flow or by modeling it empirically. Dual permeability models simulate macropore flow by specifying separate sets of unsaturated properties to the micro and macro pores. However, the flow through macropores does not follow Darcian flow and assumes all macropores are connected as a continuum while often macropores are discrete and disconnected. The central idea behind the model to be developed in this project is that flow through macropores is similar to flow in conduits having irregular shapes and hence Navier Stokes equations are more appropriate and are numerically stable when solved using the Lattice-Boltzmann method. With the advances in digital X-ray CT imaging techniques, it is now possible to have pore structure of soils characterized relatively quickly and economically. Hence, a model that can take advantage of such high resolution pore structure data can revolutionize the way we estimate long-term liquid percolation into landfills or gas emissions from landfills. The macro pore and micro pore structure in the soil system will be digitally input to the model using X-ray Computed Tomography (CT) image data for soil samples collected from instrumented field-scale clay caps to validate the modeling approach. High-resolution water balance data has been collected over a period of three years from two field-scale clay cap test sections located at a landfill in Detroit. The conventional physically-based numerical models can capture the unsaturated flow through capillary or micropores relatively accurately. However, they cannot model flow through macropores which are formed and continuously evolve due to inadequate compaction, desiccation cracking, freeze/thaw, root penetration, and rodent activity. The model that will be developed will overcome the challenge of modeling liquid or gas flow through macropores. A numerical model that takes advantage of recent advances in X-ray imagining of soil structure for modeling flow through soils will provide practitioners and regulators a tool to accurately predict long-term deep drainage into ground water systems of environmental significance or green-house gas emissions from landfills. A course module on migration of liquids and gases from waste sites will be prepared for an undergraduate landfill design class and a course module containing theory and lab experimentation to demonstrate preferential flow through soils will be prepared and introduced in a graduate level course. Outreach to high school students during summer training camps will carried out with hands-on demonstrations at Michigan State University.