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• 1
Article
Language: English
In: Transport in Porous Media, 2016, Vol.112(1), pp.207-227
Description: According to experimental observations, capillary trapping is strongly dependent on the roughness of the pore–solid interface. We performed imbibition experiments in the range of capillary numbers ( Ca ) from $$10^{-6}$$ 10 - 6 to $$5\times 10^{-5}$$ 5 × 10 - 5 using 2D-micromodels, which exhibit a rough surface. The microstructure comprises a double-porosity structure with pronounced macropores. The dynamics of precursor thin-film flow and its importance for capillary trapping are studied. The experimental data for thin-film flow advancement show a square-root time dependence. Based on the experimental data, we conducted inverse modeling to investigate the influence of surface roughness on the dynamic contact angle of precursor thin-film flow. Our experimental results show that trapped gas saturation decreases logarithmically with an increasing capillary number. Cluster analysis shows that the morphology and number of trapped clusters change with capillary number. We demonstrate that capillary trapping shows significant differences for vertical flow and horizontal flow. We found that our experimental results agree with theoretical results of percolation theory for $$Ca =10^{-6}$$ C a = 10 - 6 : (i) a universal power-like cluster size distribution, (ii) the linear surface–volume relationship of trapped clusters, and (iii) the existence of the cutoff correlation length for the maximal cluster height. The good agreement is a strong argument that the experimental cluster size distribution is caused by a percolation-like trapping process (ordinary percolation). For the first time, it is demonstrated experimentally that the transition zone model proposed by Wilkinson (Phys Rev A 30:520–531, 1984) can be applied to 2D-micromodels, if bicontinuity is generalized such that it holds for the thin-film water phase and the bulk gas phase.
Keywords: 2D-micromodel with rough surface ; Precursor thin-film flow ; Snap-off trapping ; Universal power law ; Ordinary bond percolation
ISSN: 0169-3913
E-ISSN: 1573-1634
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• 2
Article
Language: English
In: Advances in Water Resources, 2011, Vol.34(2), pp.314-325
Description: ► Stochastic reconstruction with a combination of multi-point statistics. ► Good rendition of connectivity with Minkowski functions and Chord length distributions. ► Transport behavior compares well between reference media and reconstructed media. ► Pressure field tends to bridge local discontinuities within highly conductive regions. Flow and transport in porous media is determined by its structure. Beside spatial correlation, especially the connectivity of heterogeneous conductivities is acknowledged to be a key factor. This has been demonstrated for well defined random fields having different topological properties. Yet, it remains an open question which morphological measures carry sufficient information to actually predict flow and transport in porous media. We analyze flow and transport in classical, two-dimensional random fields showing different topology and we determine a selection of structural characteristics including classical two-point statistics, chord-length distribution and Minkowski functions (four-point statistics) including the Euler number as a topological measure. Using the approach of simulated annealing for global optimization we generate analog random fields that are forced to reproduce one or several of theses structural characteristics. Finally we evaluate in how far the generated analogons reproduce the original flow and transport behavior as well as some more elaborate structural characteristics including percolation probabilities and the pair connectivity function. The results confirm that two-point statistics is insufficient to capture functional properties since it is not sensitive to connectivity. In contrast, the combination of Minkowski functions and chord length distributions carries sufficient information to reproduce the breakthrough curve of a conservative solute. Hence, global topology provided by the Euler number together with local clustering provided by the chord length distribution seems to be a powerful condensation of structural complexity with respect to functional properties.
Keywords: Simulated Annealing ; Solute Transport ; Minkowski Functionals ; Chord Length Distribution ; Local Percolation Probability ; Pair Connectivity Function ; Engineering
ISSN: 0309-1708
E-ISSN: 1872-9657
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• 3
Article
Language: English
In: Vadose Zone Journal, 2012, Vol.11(3), p.0
Description: Recently, a new approach was introduced to directly measure unsaturated hydraulic conductivity in flux-controlled experiments--the multistep flux experiment. Thereby an overshoot in matric potential h (sub m) across drainage and infiltration fronts was observed. We extended this experimental approach to simultaneously measure the volumetric water content Theta within the sample and applied the method to a sand and a clay loam soil. The detailed trajectories within the h (sub m) -Theta space were obtained during a number of decreasing and increasing steps in infiltration rate. This clearly demonstrates the type and magnitude of hydraulic nonequilibrium under transient conditions where water content and matric potential deviate from a well-defined static relation. We also compared the directly measured hydraulic conductivities with those obtained from classical multistep outflow experiments and found that nonequilibrium dynamics might lead to an underestimation of hydraulic conductivity when obtained from an inverse solution of Richards" equation. We provide a qualitative explanation of nonequilibrium that depends on the structure of the material and the type and magnitude of external forcing. The new experimental setup is considered to be a valuable tool to actually quantify nonequilibrium effects. This will make it possible to represent this relevant phenomenon in future modeling concepts.
Keywords: Hydrogeology ; Aquifers ; Climate Forcing ; Discharge ; Drainage ; Experimental Studies ; Ground Water ; Hydraulic Conductivity ; Hydrodynamics ; Hysteresis ; Infiltration ; Models ; Richards Equation ; Saturation ; Soil Mechanics ; Solute Transport ; Transport ; Unsaturated Zone;
E-ISSN: 1539-1663
Source: CrossRef
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• 4
Article
In: Water Resources Research, May 2014, Vol.50(5), pp.4514-4529
Description: A major difficulty in modeling multiphase flow in porous media is the emergence of trapped phases. Our experiments demonstrate that gas can be trapped in either single‐pores, multipores, or in large connected networks. These large connected clusters can comprise up to eight grain volumes and can contain up to 50% of the whole trapped gas volume. About 85% of the gas volume is trapped by gas clusters. This variety of possible trapped gas clusters of different shape and volume will lead to a better process understanding of bubble‐mediated mass transfer. Since multipore gas bubbles are in contact with the solid surface through ultrathin adsorbed water films the interfacial area between trapped gas clusters and intergranular capillary water is only about 80% of the total gas surface. We could derive a significant (R = 0.98) linear relationship between the gas‐water‐interface and gas saturation. We found no systematic dependency of the front velocity of the invading water phase in the velocity range from 0.1 to 0.6 cm/min corresponding to capillary numbers from 2 × 10 to 10. Our experimental results indicate that the capillary trapping mechanism is controlled by the local pore structure and local connectivity and not by thermodynamics, i.e., by the minimum of the , at least in the considered velocity range. Consistent with this physical picture is our finding that the trapping frequency (= bubble‐size distribution) reflects the pore size distribution for the whole range of pore radii, i.e., the capillary trapping process is determined by statistics and not by thermodynamics. No systematic dependency of trapping efficiency on capillary number Majority of trapped gas bubbles (85%) are multipore trapped Trapping of gas clusters is determined by statistics and not by thermodynamics
Keywords: Gas Clusters ; Capillary Trapping ; Interfacial Area
ISSN: 0043-1397
E-ISSN: 1944-7973
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• 5
Article
Language: English
In: Vadose Zone Journal, 2015, Vol.14(5), p.0
Description: We used X-ray computed microtomography to study gas trapping in a fluctuating water table. Our results show that capillary forces control trapping and phase distribution in dynamic capillary fringes. In porous media, the nonwetting phase is trapped on water saturation due to capillary forces acting in a heterogeneous porous structure. Within the capillary fringe, the gas phase is trapped and released along with the fluctuation of the water table, creating a highly active zone for biological transformations and mass transport. We conducted column experiments to observe and quantify the magnitude and structure of the trapped gas phase at the pore scale using computed microtomography. Different grain size distributions of glass beads were used to study the effect of the pore structure on trapping at various capillary numbers. Viscous forces were found to have negligible impact on phase trapping compared with capillary and buoyancy forces. Residual gas saturations ranged from 0.5 to 10%, while residual saturation increased with decreasing grain size. The gas phase was trapped by snap-off in single pores but also in pore clusters, while this single-pore trapping was dominant for grains larger than 1 mm in diameter. Gas surface area was found to increase linearly with increasing gas volume and with decreasing grain size.
Keywords: Grain Size ; Water Table ; Mass Transport ; Buoyancy ; Pores ; Porous Media ; Particle Size ; Water Table ; Saturation ; Vadose Water ; Fluctuations ; Trapping ; Buoyancy ; Methods and Instruments ; General;
E-ISSN: 1539-1663
Source: CrossRef
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• 6
Article
Language: English
In: Vadose Zone Journal, 2011, Vol.10(2), p.654
Description: The unsaturated hydraulic conductivity function is the dominant material property for modeling soil water dynamics. Because it is difficult to measure directly, it is often derived from the water retention characteristic combined with a geometric model of the pore space. In this study, we developed an automated, simple multistep flux (MSF) experiment to directly measure unsaturated conductivities, K(psi (sub m) ), at a number of water potentials, psi (sub m) , using the experimental setup of classical multistep outflow (MSO) experiments. In contrast to the MSO experiment, the MSF experiment measures the conductivity directly at a spatially constant water potential assuming macroscopically homogeneous materials. Additionally, the proposed method reveals the hysteresis of K(psi (sub m) ) with respect to increasing and decreasing water potentials as well as the temporal dynamics of K(psi (sub m) ) during transient-flow conditions. This temporal behavior is explained by the dynamics of fluid configurations at the pore scale during drainage and imbibition leading to hydraulic nonequilibrium. It may provoke a systematic underestimation of hydraulic conductivity using inverse optimization of K(psi (sub m) ) based on classical MSO experiments. The new approach will improve the determination of K(psi (sub m) ) and it provides an experimental tool to quantify the effects of hydraulic nonequilibrium under transient conditions.
Keywords: Hydrogeology ; Experimental Studies ; Geometry ; Ground Water ; Hydraulic Conductivity ; Hysteresis ; Inverse Problem ; Mathematical Methods ; Measurement ; Models ; Movement ; Optimization ; Phase Equilibria ; Soils ; Unsaturated Zone;
E-ISSN: 1539-1663
Source: CrossRef
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• 7
Article
Language: English
In: Vadose Zone Journal, 2013, Vol.12(3), p.0
Description: Soils are structured on multiple spatial scales, originating from inhomogeneities of the parent material, pedogenesis, soil organisms, plant roots, or tillage. This leads to heterogeneities that cause variability of local measurements of hydraulic state variables and affects the flow behavior of water in soil. Whereas in real-world systems, the true underlying structures can never be absolutely known, it is appealing to employ synthetic or "virtual" experiments for assessing general properties of flow in porous media and grasping the main physical mechanisms. With this aim, three two-dimensional virtual realities with increasing structural complexity, representing cultivated soils with hierarchical spatial heterogeneity on multiple scales were constructed by the interdisciplinary research group Virtual Institute of the Helmholtz Association (INVEST). At these systems, numerical simulations of water dynamics including a heavy rain, a redistribution, and a long-lasting evaporation period were performed. The technical aspects of the construction of the virtual soils and results of the forward simulations have been presented in a paper by Schluter et al. (2012). In this follow-up paper, we use inverse modeling to investigate measurements in virtual vertical soil profiles, mimicking typical field monitoring campaigns with moisture content and matric potential sensors placed at five depths. Contrary to the real situation, we can interpret observed data, their variability, estimated hydraulic properties, and predicted water balance in the light of the known truth. Our results showed that measurements, particularly those of water contents, varied strongly with measuring position. Using data from single profiles in systems similar to our virtual soils thus will lead to very different estimates of the soil hydraulic properties. As a consequence, the correct calculation of the water balance is rather a lucky coincidence than the rule. However, the average of the predicted water balances obtained from the one-dimensional simulations, and the estimated soil hydraulic properties agreed very well with those attained from the two-dimensional systems.
Keywords: Soils ; Hydrogeology ; Boundary Interactions ; Evaporation ; Grain Size ; Heterogeneous Materials ; Hydrodynamics ; Infiltration ; Interpretation ; Inverse Problem ; Irrigation ; Matric Head ; Measurement ; Moisture ; One-Dimensional Models ; Quantitative Analysis ; Simulation ; Size Distribution ; Soils ; Spatial Distribution ; Tdr Data ; Two-Dimensional Models ; Unsaturated Zone ; Van Genuchten-Mualem Parameters ; Water ; Water Balance;
E-ISSN: 1539-1663
Source: CrossRef
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• 8
Article
In: Water Resources Research, June 2017, Vol.53(6), pp.4709-4724
Description: The relaxation dynamics toward a hydrostatic equilibrium after a change in phase saturation in porous media is governed by fluid reconfiguration at the pore scale. Little is known whether a hydrostatic equilibrium in which all interfaces come to rest is ever reached and which microscopic processes govern the time scales of relaxation. Here we apply fast synchrotron‐based X‐ray tomography (X‐ray CT) to measure the slow relaxation dynamics of fluid interfaces in a glass bead pack after fast drainage of the sample. The relaxation of interfaces triggers internal redistribution of fluids, reduces the surface energy stored in the fluid interfaces, and relaxes the contact angle toward the equilibrium value while the fluid topology remains unchanged. The equilibration of capillary pressures occurs in two stages: (i) a quick relaxation within seconds in which most of the pressure drop that built up during drainage is dissipated, a process that is to fast to be captured with fast X‐ray CT, and (ii) a slow relaxation with characteristic time scales of 1–4 h which manifests itself as a spontaneous imbibition process that is well described by the Washburn equation for capillary rise in porous media. The slow relaxation implies that a hydrostatic equilibrium is hardly ever attained in practice when conducting two‐phase experiments in which a flux boundary condition is changed from flow to no‐flow. Implications for experiments with pressure boundary conditions are discussed. What happens to fluids in a porous medium after pumping is stopped? Fast X‐ray tomography shows that even in a sample smaller than a sugar cube fluid interfaces continue to move for hours until an optimal fluid configuration is reached. The pace is limited by slow relaxation of dynamic contact angles. Therefore hydrostatic equilibrium, which is the state at which all fluid interfaces come to rest, is hardly ever attained in practice when conducting two‐phase flow experiments where the flow is stopped in much larger soil or rock samples. Relaxation dynamics through internal redistribution of fluids after fast drainage occurs in two stages A quick dissipation within seconds is followed by slow relaxation within several hours due to relaxation of dynamic contact angles Fluid configurations during relaxation are very different from those during quasi‐static drainage and imbibition
Keywords: Two‐Phase Flow ; Dynamic Effects ; Hydraulic Nonequilibrium ; Dynamic Contact Angle ; Fluid Configuration ; Fluid Topology
ISSN: 0043-1397
E-ISSN: 1944-7973
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• 9
Article
Language: English
In: Vadose Zone Journal, 2012, Vol.11(3), p.0
Description: The rhizosphere has a controlling role in the flow of water and nutrients from soil to plant roots; however, its hydraulic properties are not well understood. As roots grow, they change the pore size distribution of the surrounding soil. Roots release polymeric substances such as mucilage into their rhizosphere. Microorganisms living in the rhizosphere feed on these organic materials and release other polymeric substances into the rhizosphere. The presence of these organic materials might affect the water retention properties and the hydraulic conductivity of the rhizosphere soil during drying and rewetting. We used neutron radiography to monitor the dynamics of water distribution in the rhizosphere of lupin (Lupinus albus L.) plants during a period of drying and rewetting. The rhizosphere was shown to have a higher water content than the bulk soil during the drying period but a lower one during the subsequent rewetting. We evaluated the wettability of the bulk soil and the rhizosphere soil by measuring the contact angle of water in the soil. We found significantly higher contact angles for the rhizosphere soil than the bulk soil after drying, which indicates slight water repellency in the rhizosphere. This explains the lower soil water content in the rhizosphere than the bulk soil after rewetting. Our results suggest that the water holding capacity of the rhizosphere is dynamic and might shift toward higher or lower values than those of the surrounding bulk soil, not affected by roots, depending on the history of drying and rewetting cycles.
Keywords: Soils ; Hydrogeology ; Absorption ; Carbohydrates ; Compactness ; Concentration ; Ecology ; Habitat ; Hydraulic Conductivity ; Hydrologic Cycle ; Hydrology ; Hydrophobic Materials ; Imagery ; Lipids ; Lupinus Albus ; Measurement ; Microorganisms ; Moisture ; Nuclear Magnetic Resonance ; Nutrients ; Organic Compounds ; Physical Properties ; Plantae ; Polymers ; Polysaccharides ; Porosity ; Rhizosphere ; Roots ; Soil Profiles ; Soil-Water Balance ; Soils ; Spectroscopy ; Tomography ; Wettability ; X-Ray Data;
E-ISSN: 1539-1663
Source: CrossRef
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• 10
Article
Language: English
In: Hydrology and Earth System Sciences, 2016, Vol.20(10), pp.4017-4030
Description: Prediction and modeling of localized flow processes in macropores is of crucial importance for sustaining both soil and water quality. However, currently there are no reliable means to predict preferential flow due to its inherently large spatial variability. The aim of this study was to investigate the predictive performance of previously developed empirical models for both water and air flow and to explore the potential applicability of X-ray computed tomography (CT)-derived macropore network characteristics. For this purpose, 65 cylindrical soil columns (6#xE2;#x80;#xAF;cm diameter and 3.5#xE2;#x80;#xAF;cm height) were extracted from the topsoil (5#xE2;#x80;#xAF;cm to 8.5#xE2;#x80;#xAF;cm depth) in a 15#xE2;#x80;#xAF;m#xE2;#x80;#xAF;#xE2;#x80;#x89;#xC3;#x97;#xE2;#x80;#x89;#xE2;#x80;#xAF;15#xE2;#x80;#xAF;m grid from an agricultural field located in Silstrup, Denmark. All soil columns were scanned with an industrial X-ray CT scanner (129#xE2;#x80;#xAF;#xC2;#xB5;m resolution) and later employed for measurement of saturated hydraulic conductivity, air permeability at -30 and -100#xE2;#x80;#xAF;cm matric potential, and gas diffusivity at -30 and -100#xE2;#x80;#xAF;cm matric potential. Distribution maps for saturated hydraulic conductivity, air permeability, and gas diffusivity reflected no autocorrelation irrespective of soil texture and organic matter content. Existing empirical predictive models for saturated hydraulic conductivity and air permeability showed poor performance, as they were not able to realistically capture macropore flow. The tested empirical model for gas diffusivity predicted measurements at -100#xE2;#x80;#xAF;cm matric potential reasonably well, but failed at -30#xE2;#x80;#xAF;cm matric potential, particularly for soil columns with biopore-dominated flow. X-ray CT-derived macroporosity matched the measured air-filled porosity at -30#xE2;#x80;#xAF;cm matric potential well. Many of the CT-derived macropore network characteristics were strongly interrelated. Most of the macropore network characteristics were also significantly correlated with saturated hydraulic conductivity, air permeability, and gas diffusivity. The predictive Ahuja et al.#xC2;#xA0;(1984) model for saturated hydraulic conductivity, air permeability, and gas diffusivity performed reasonably well when parameterized with novel, X-ray CT-derived parameters such as effective percolating macroporosity for biopore-dominated flow and total macroporosity for matrix-dominated flow. The obtained results further indicate that it is crucially important to discern between matrix-dominated and biopore-dominated flow for accurate prediction of macropore flow from X-ray CT-derived macropore network characteristics.
Keywords: Hydrogeology – Analysis ; Permeability – Analysis ; Porosity – Analysis ; Cat Scans – Analysis;
ISSN: Hydrology and Earth System Sciences
ISSN: 10275606
E-ISSN: 1607-7938
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