MULTI-REGION REACTIVE TRANSPORT DUE TO STRONG ANISOTROPY IN UNSATURATED SOILS WITH EVOLVING SCALES OF HETEROGENEITY Page: 3 of 8
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These objectives are being met through an integrated experimental and modeling study that
encompasses three scale- pore-scale (Lattice Boltzman, LB; Smooth particle hydrodynamics,
SPH); centrifuge measurements using the unsaturated flow apparatus (UFA); core scale; and
macroscopic scale.
At the pore scale, experimental and modeling efforts have focused on characterization of pore
morphology and quantifying the effects of pore-scale heterogeneities on flow using pore-scale
models (Lattice Boltzmann and Smooth Particle Hydrodynamics). Small column (5 cm cross
section) measurements have made in the unsaturated flow apparatus (UFA lab centrifuge system)
using repacked and undisturbed cubed-shaped sediment samples to obtain flow and transport data
to validate the pore-scale models. Mesoscale experiments (15 cm cross section) have also been
used in a three-axis permeameter to measure effective hydraulic properties and their directional
dependence. All of the tests have been repeated in a larger three-axis permeameter (30 -50 cm
cross section) on geocentrifuge to quantify parameters at a larger scale using the same sediments
and packing configurations.
3.0 Research Progress and Implications
3.1 Pore-scale Anisotropy
Owing to the difficulty in sampling and interrogating the unconsolidated sediments typical
of Hanford, a physically based sedimentation model was developed to simulate different packing
arrangements of particles ranging from spheres to disks. To gain a quantitative understanding of
the combined effects of three dimensional particle arrangement and particle shape on the
permeability tensor, flow through the different packs was predicted using a lattice-Boltzmann
(LB) flow simulator (Stewart et al, 2004; 2005). Figure 2 shows a random pack of oblate
ellipsoids with an aspect ratio of 10:1. Results show that the
intrinsic properties of the porous medium and the degree of
anisotropy in depends not only upon particle shape and
alignment, but also on the three dimensional structure of the
pack (Stewart et al, 2005; Ward, 2005). With respect to
permeability, trends for random packs indicate that more oblate
particles and higher degrees of particle alignment lead to
decreased pore connectivity and increased tortuosity
perpendicular to the direction of maximum alignment, which
results in greater anisotropy.
Figure 2: Random pack of Figure 3 shows the corresponding anisotropy ratios, Axz=kx/kz
oblate ellipsoids with a 10:1 for body centered 14
aspect ratio. cubic (bc) and 12
face centered 10
cubic (fcc) configurations. As expected, Axz=1 for 8
spherical particles (rX/rz =1), a result consistent with 6
field measurements and continuum scale numerical 4
simulations in coarse sand. A, ranged from 1 to 14 2
in the fcc packs and were significantly higher than in 0
the bec packs increasing as aspect ratio increased. 1 2 3 4 s
In contrast, Axz in the bec packs decreased from a Figure 3: Axzfor the dimensionless k in
maximum of 1 to 0.12 as aspect ratio increased packs of oblate ellipsoids with varying
from 1 to 5. The dependence of permeability on aspect ratios.
aspect ratio is consistent with our underlying hypothesis that particle shape and arrangement
would impact pore scale anisotropy, but the different trends were somewhat counter-intuitive.2
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Ward, Andy. MULTI-REGION REACTIVE TRANSPORT DUE TO STRONG ANISOTROPY IN UNSATURATED SOILS WITH EVOLVING SCALES OF HETEROGENEITY, report, June 1, 2005; Richland, Washington. (https://digital.library.unt.edu/ark:/67531/metadc883189/m1/3/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.