Finite-element and lattice Boltzmann based numerical modeling of environmental mass transfer processes

Abstract

The inherent heterogeneity of subsurface porous media, as well as the occurrence of highly non-linear rate-limiting mass transfer processes, results in significant challenges to accurate and efficient modeling of contaminant flow and transport. This dissertation provides comprehensive Web-based modeling tools and advanced numerical methods for students and researchers to better investigate fluid flow and mass transfer processes in natural and model systems under water-saturated conditions. A Web-based virtual mass transfer processes laboratory (MTVLab) was developed for students and researchers to access and study state of the art understanding of mass transfer mechanisms at the particle scale. Meanwhile, MTVLab system architecture provides a proof-of-principle framework from which to develop more sophisticated Web-based models that can employ computationally efficient, high-level computer programs. MTVLab is available at http://www.vanderbilt.edu/mtvlab. Lattice Boltzmann methods (LBM) were used to study fluid flow in two-dimensional randomly generated porous media. An innovative method was developed to construct permeability cumulative distribution functions through the combination of LBM and first order reliability method (FORM). LBM FORM was found to be approximately 13 to 120 times more efficient than traditional Monte Carlo-based simulations while maintaining similar orders of accuracy. A novel least squares finite element lattice Boltzmann method (LSFE-LBM) was developed, extending LBM to unstructured meshes. LSFE-LBM is able to more efficiently simulate fluid flow and solute transport in domains that contain complex or irregular geometric boundaries. LSFE-LBM provided the foundation for the numerical modeling efforts to elucidate the relative contributions of transport-related and sorption/desorption - related nonequilibrium factors on mass transfer processes in a whole class of porous media exemplified by randomly generated porous media. Applications of LSFE-LBM to simulate phenanthrene transport in porous media represent an initial effort to bridge comprehensive sorption/desorption mechanistic studies with pore–scale modeling, the results of which help advance our understanding of the effects of soil organic matter and soil structure configurations on fate and transport of organic chemicals in subsurface systems.