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• Lattice Boltzmann Method
• 1
Article
Language: English
In: Transport in Porous Media, 2013, Vol.96(2), pp.271-294
Description: The 3D description of the soil structure at the pore scale level can help to elucidate the biological functioning of soil. The water–air distribution in the 3D-pore space is of particular interest because it determines the diffusion pathways of nutrients and the localisation of active soil microorganisms. We used the Shan–Chen interparticle-potential approach to simulate spontaneous phase separation in complex academic and real 3D-porous media using the advanced TRT lattice Boltzmann scheme. The equation of state and phase diagram were calculated and the model was verified using hydrostatic laws. The 3D pattern of water/air interface in two complex academic pore geometries was accurately computed. Finally, 3D maps of static liquid–gas distribution were simulated in a real 3D X-ray computed tomography image obtained from an undisturbed soil column sampled in a silty clay loam soil. The simulated soil sample of 1.7 cm 3 was described at a voxel-resolution of 60 μm. The range of the simulated saturations (from 0.5 to 0.9) was in a good agreement with the expected saturations calculated from the phase diagram.
Keywords: Lattice Boltzmann method ; Water meniscus ; TRT ; Shan–Chen ; Porous media
ISSN: 0169-3913
E-ISSN: 1573-1634
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• 2
Article
Language: English
In: Transport in Porous Media, 2014, Vol.105(2), pp.391-410
Description: Numerical models that solve transport of pollutants at the macroscopic scale in unsaturated porous media need the effective diffusion dependence on saturation as an input. We conducted numerical computations at the pore scale in order to obtain the effective diffusion curve as a function of saturation for an academic sphere packing porous medium and for a real porous medium where pore structure knowledge was obtained through X-ray tomography. The computations were performed using a combination of lattice Boltzmann models based on two relaxation time (TRT) scheme. The first stage of the calculations consisted in recovering the water spatial distribution into the pore structure for several fixed saturations using a phase separation TRT lattice Boltzmann model. Then, we performed diffusion computation of a non-reactive solute in the connected water structure using a diffusion TRT lattice Boltzmann model. Finally, the effective diffusion for each selected saturation value was estimated through inversion of a macroscopic classical analytical solution.
Keywords: Lattice Boltzmann method ; Effective diffusion ; TRT ; Unsaturated ; Porous media
ISSN: 0169-3913
E-ISSN: 1573-1634
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• 3
Article
Language: English
In: Transport in Porous Media, 2017, Vol.116(3), pp.975-1003
Description: Hydrogen gas migration modeling through water-saturated engineering barriers and the host rock of a deep geological repository for radioactive waste is of concern for safety assessment of such facilities. A two-phase two-relaxation-time lattice Boltzmann model using the Rothman and Keller approach was parallelized on graphic processing units to simulate hydrogen gas migration in a 3D image obtained by X-ray microtomography of Opalinus clay microfractures. A dimensional analysis combined with a grid refinement analysis was carried out to set the model parameters to reproduce the realistic viscous, capillary and inertial forces of the natural system. Relative permeabilities curves were first calculated in a simple regular fracture with different initial two-phase configurations. We observed that segmented gas flow configurations led to a drop in the relative gas permeability by two orders of magnitude as compared to parallel flow configuration. The model was then applied to 4 $$\times$$ × refined 3D images. For lower water saturation values ( $$0.5 \le S_\mathrm{w} 〈 0.7$$ 0.5 ≤ S w 〈 0.7 ), hydrogen gas migrated through continuous gas paths oriented in the flow direction. At high water saturation values ( $$S_\mathrm{w}\ge 0.7$$ S w ≥ 0.7 ), the relative gas permeability dropped to zero because the hydrogen phase segmented into gas pockets that were stuck in local narrow throats of the clay fracture. The study pointed out that the high capillary forces prevented the gas bubbles from distorting themselves to pass through these narrow paths.
Keywords: LBM ; Two-phase flow ; Relative permeability ; RK ; TRT ; GPU
ISSN: 0169-3913
E-ISSN: 1573-1634
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• 4
Article
Language: English
In: Advances in Water Resources, September 2015, Vol.83, pp.123-136
Description: Soil structure and interactions between biotic and abiotic processes are increasingly recognized as important for explaining the large uncertainties in the outputs of macroscopic SOM decomposition models. We present a numerical analysis to assess the role of meso- and macropore topology on the biodegradation of a soluble carbon substrate in variably water saturated and pure diffusion conditions . Our analysis was built as a complete factorial design and used a new 3D pore-scale model, LBioS, that couples a diffusion lattice-Boltzmann model and a compartmental biodegradation model. The scenarios combined contrasted modalities of four factors: meso- and macropore space geometry, water saturation, bacterial distribution and physiology. A global sensitivity analysis of these factors highlighted the role of physical factors in the biodegradation kinetics of our scenarios. Bacteria location explained 28% of the total variance in substrate concentration in all scenarios, while the interactions among location, saturation and geometry explained up to 51% of it.
Keywords: Biodegradation ; Lattice-Boltzmann Method ; Pore-Scale Heterogeneity ; Spatial Distribution ; Substrate Diffusion ; Microbial Habitats ; Engineering
ISSN: 0309-1708
E-ISSN: 1872-9657
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• 5
Article
Language: English
In: Computers and Geosciences, February 2019, Vol.123, pp.20-37
Description: In recent years, technological advances have stimulated researchers to try to unravel the extremely complex microscale processes that control the activity of microorganisms in soils. In particular, significant work has been carried out on the development of models able to accurately predict the microscale distribution of water, and the location of air–water interfaces in pores. A comparison, by Pot et al. (2015), of two different modeling approaches with actual synchrotron-based tomography data, shows that a two-phase lattice Boltzmann model (LBM) is able to predict remarkably well the location of air–water interfaces but is extremely slow, whereas a morphological model (MOSAIC), representing the pore space as a collection of spherical balls, provides a reasonable approximation of the observed air–water interfaces when each ball is allowed to drain independently, but does so blazingly fast. Interfaces predicted by MOSAIC, however, tend to have nonphysical shapes. In that general context, the key objective of the research described in the present article, based on the same tomography data as Pot et al. (2015), was to find out to what extent the use of ellipsoids instead of spherical balls in MOSAIC could not appreciably speed up computations, or at least, at equal computational time, provide a quantitatively better approximation of water-air interfaces. As far as we know, this is the first time an ellipsoids-based approximation of the soil pore space is proposed. A secondary objective was to assess whether ellipsoids might yield smoother, more physical, interfaces. Simulation results indicate that the use of ellipsoids provides a sizeable increase in accuracy in the prediction of air-water interfaces, an approximately 6-fold drop in computation time, and much more realistic-looking interfaces, compared to what is obtained with spherical balls. These observations are encouraging for the use of models based on geometric primitives to describe a range of microscale processes, and to address the still daunting issue of upscaling to the macroscopic scale.
Keywords: Pore Scale ; Synchroton X-Ray Micro Computed Tomography ; Soil Air-Water Interfaces ; Computational Geometry ; 3d Volume Segmentation ; Morphological Modeling ; Geology
ISSN: 0098-3004
E-ISSN: 1873-7803
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