High-performance Pore-scale Simulation Method For Fluid Flow And Solute Transport With Applications

Studying the processes of soil water and solute transport at pore scale is of great significance for understanding plant nutrient uptake,microbial activities and groundwater contaminant transport in soil.Since the experimental investigation of water and solute transport at microscopic scale is difficult,using micro-CT technology to obtain high-precision soil structural information at pore-scale level,and then directly modeling water and solute transport based on CT images has become a popular method of pore scale research.The water-flow process in the soil pores can be simulated by the computational fluid dynamics method via solving the Navier-Stokes(N-S)equations if the pore structure is given.The traditional numerical methods solving the N-S equations include finite element method,finite volume method,and finite difference method.However,it is difficult to deal with the complex boundary conditions of the porous media,and the numerical simulations are often difficult to converge.The lattice Boltzmann method(LBM),on the other hand,is a mesoscopic numerical simulation method with macroscopic and microscopical dynamic backgrounds,it can effectively address the convergence issue.Its advantages in dealing with complex boundaries making it widely used to study flow in porous media.The objective of this study was to investigate the transport mechanisms at microscale via simulating water and solute behaviors in real soil formation(resolution 3.7 ?m,the total number of grids 64,000,000),and investigate the influence of spatial heterogeneity of porous media on hydraulic and solute transport parameters.The developed simulation method was then used to quantify the effects of biochar on improving hydraulic properties and its effect on solute transport.The specific results are summarized as follows:(1)We developed a method to simulate three-dimensional saturated soil water and solute transport based on the lattice Boltzmann method and GPU(Graphics Processing Unit)parallel computing.The simulations of pore-water flow and solute transport were carried out using a single relaxation model based on BGK(Bhatnagar,Gross,Krook)approximation,which uses the lattice Boltzmann method D3Q19 model to approximate the N-S equations and the Advection-Diffusion equation respectively.Meanwhile,we realized the parallel computation of water flow and solute transport simulation based on GPU architecture.(2)We used the new method to simulate water flow and solute transport processes in artificial porous structures with different structural heterogeneity.Macroscopic parameters such as permeability,tortuosity,and longitudinal dispersion coefficient were obtained to describe the hydraulic properties of porous media.We then employed the three-dimensional lattice Boltzmann method to simulate water flow and solute transport in real soil structures.After multiple optimizations for GPU parallel computing,the computational efficiency was greatly improved,and the simulations of larger scale samples were realized,which provides a promising way for further research in studying pore-scale water flow and solute transport.(3)We used the developed method to evaluate the impact of biochar amendment on the hydraulic properties of soil aggregates.Biochar addition can improve the structure of clayey soil,which further improves the physical and hydraulic properties.In this study,two soils,a Ultisol(locally named red soil)and a Vertisol(locally named Shajiang black soil),were selected and amended with woodchip biochar.High resolution(3.7 ?m)synchrotron-based X-ray micro-computed tomography images for the aggregates with and without biochar amendment were obtained.We implemented the water flow by the three-dimensional lattice Boltzmann method directly on the reconstructed high-resolution images.Based on the simulated flow fields,we quantified the changes of permeability and tortuosity caused by the biochar amendment.The results of LBM simulations showed that the non-oriented and complex pore structure could result in the anisotropy of permeability and tortuosity of the test samples,a phenomenon that cannot be revealed by the traditional experimental approaches.In addition,we evaluated how sample size might affect permeability and tortuosity determination,i.e.the scale issue.(4)We further investigated the impact of biochar addition on solute transport in the clayey soil.Overall,biochar amendment reduced the spatial variability of pore water velocity and increased the dispersion coefficient inside the amended soil aggregates by one order of magnitude.The differences are not only related to the dilution effect of the solute plume but also directly influenced by the flow field change caused by the pore structure alteration.We observed that with the same hydraulic gradient,the anomalous dispersion is more likely to occur in biochar amended soil aggregates,i.e.,the relationship between the dispersion flux and the concentration gradient deviates from the Fick relationship.This phenomenon is dominated by both mechanical dispersion and molecular diffusion.It was investigated in details by determining the dispersion coefficient with different Pe numbers.In summary,our study developed a simulation method for high-performance porescale three-dimensional saturated soil water flow and solute transport.This research adopted a novel approach to evaluate the hydraulic and solute transport parameters of the aggregates with or without biochar amendment by combining non-destructive Synchrotron-based X-ray micro-computed tomography and three-dimensional lattice Boltzmann simulation.The proposed theory and method can help to understand the mechanisms of soil water and solutes(nutrients and contaminants)movement at microscale.