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

Michigan State University, East Lansing MI

Investigators

Abstract

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.