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Simulating the Gas Diffusion Coefficient in Macropore Network Images: Influence of Soil Pore Morphology

^a Lab. for Plant-Soil Interaction Processes, Ministry of Education, College of Resources and Environment, China Agricultural Univ., No. 2 Yuanmingyuan Xi Lu, Beijing 100094, P.R. China
^b USDA-ARS, George E. Brown, Jr., Salinity Lab., 450 West Big Springs Rd., Riverside, CA 92507, USA

View larger version (44K): [in a new window]   Fig. 1. (a) Comparison of measured and finite element (FEM) simulated value of with the commonly used Milling-Quirk models. Data sources: Karimi et al. (1987); Jin and Jury (1996); Bruckler et al. (1989); Xu et al. (1992); and Moldrup et al. (2003, 2004). FEM results are marked by + (inlet at the left boundary) and x (inlet at the right boundary). (b) Enlarged area within gray lines of (a).

View larger version (72K): [in a new window]   Fig. 2. Binary images of soil thin sections used for the finite element (FEM) solutions (black indicates pores). (a) Top of thin section and (b) bottom of thin section (from Anderson et al., 1996).

View larger version (43K): [in a new window]   Fig. 3. (a) Plot of the shortest paths for soil thin Section 3b. (b) Diffusive fluxes for soil thin Section 3b for diffusion from left to right (bottom) and right to left (top) after 10 s (left) and 150 s (right).

View larger version (53K): [in a new window]   Fig. 4. (a) Schematic of the two-dimensional diffusion simulation system in which diffusion occurs from the dark area on the left of the soil thin section to the right. (b) and (c) represent enlarged parts of the thin section as displayed within the gray lines of (a) and (b). (d) shows the unstructured mesh adopted for the finite element (FEM) simulations, including a typical enlarged 6-node triangular element (indicated by arrow) used in the simulations. (e) shows nodes of the computational mesh. (f) gives a contour plot of the concentration field at time t = 150 s. (g) example of concentration distribution (t = 150 s) along the paths (x = 1, 3, and 5 cm).

View larger version (45K): [in a new window]   Fig. 5. Contour plots of concentrations obtained with the finite element (FEM) simulations for diffusion in soil thin Section 3b.

View larger version (26K): [in a new window]   Fig. 6. Results of the finite element (FEM) simulations; (a) breakthrough curve (BTC) obtained with the FEM simulation (C) and corresponding fitted curve (solid line). (b) FEM simulation with different mesh densities for Soil Sample 3b.

View larger version (35K): [in a new window]   Fig. 7. Relationships between soil structure parameters [(a) Dmp, (b) Dmin, (c)min, (d) lmin/l0, (e) l/l0, (f) < and >, respectively]. (g) values obtained by fitting the finite element (FEM) simulated breakthrough curve (BTC) (results for diffusion from left to right and right to left).