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Berlin Brandenburg

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  • 1
    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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  • 2
    In: Water Resources Research, November 2015, Vol.51(11), pp.9094-9111
    Description: We study the impact of pore structure and surface roughness on capillary trapping of nonwetting gas phase during imbibition with water for capillary numbers between 10 and 5 × 10, within glass beads, natural sands, glass beads monolayers, and 2‐D micromodels. The materials exhibit different roughness of the pore‐solid interface. We found that glass beads and natural sands, which exhibit nearly the same grain size distribution, pore size distribution, and connectivity, showed a significant difference of the trapped gas phase of about 15%. This difference can be explained by the microstructure of the pore‐solid interface. Based on the visualization of the trapping dynamics within glass beads monolayers and 2‐D micromodels, we could show that bypass trapping controls the trapping process in glass beads monolayers, while snap‐off trapping controls the trapping process in 2‐D micromodels. We conclude that these different trapping processes are the reason for the different trapping efficiency, when comparing glass beads packs with natural sand packs. Moreover, for small capillary numbers of 10, we found that the cluster size distribution of trapped gas clusters of all 2‐D and 3‐D porous media can be described by a universal power law behavior predicted from percolation theory. This cannot be expected a priori for 2‐D porous media, because bicontinuity of the two bulk phases is violated. Obviously, bicontinuity holds for the thin‐film water phase and the bulk gas phase. The snap‐off trapping process leads to ordinary bond percolation in front of the advancing bulk water phase and is the reason for the observed universal power law behavior in 2‐D micromodels with rough surfaces. Surface roughness controls capillary trapping efficiency The transition‐zone model can be applied to 2‐D micromodels with rough surfaces The 2‐D and 3‐D porous media belong all to the same universality class
    Keywords: Surface Roughness ; Precursor Thin‐Film Flow ; Snap‐Off Trapping ; Universal Power Law ; Ordinary Bond Percolation
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 3
    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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  • 4
    In: Water Resources Research, November 2018, Vol.54(11), pp.9033-9044
    Description: Structural hierarchy is a fundamental characteristic of natural porous media. Yet it provokes one of the grand challenges for the modeling of fluid flow and transport since pore‐scale structures and continuum‐scale domains often coincide independent of the observation scale. Common approaches to represent structural hierarchy build, for example, on a multidomain continuum for transport or on the coupling of the Stokes equations with Darcy's law for fluid flow. These approaches, however, are computationally expensive or introduce empirical parameters that are difficult to derive with independent observations. We present an efficient model for fluid flow based on Darcy's law and the law of Hagen‐Poiseuille that is parameterized based on the explicit pore space morphology obtained, for example, by X‐ray μ‐CT and inherently permits the coupling of pore‐scale and continuum‐scale domain. We used the resulting flow field to predict the transport of solutes via particle tracking across the different domains. Compared to experimental breakthrough data from laboratory‐scale columns with hierarchically structured porosity built from solid glass beads and microporous glass pellets, an excellent agreement was achieved without any calibration. Furthermore, we present different test scenarios to compare the flow fields resulting from the Stokes‐Brinkman equations and our approach to comprehensively illustrate its advantages and limitations. In this way, we could show a striking efficiency and accuracy of our approach that qualifies as general alternative for the modeling of fluid flow and transport in hierarchical porous media, for example, fractured rock or karstic aquifers. A model for the simulation of pore‐scale and continuum‐scale flow in hierarchically structured porous media is developed Explicit pore space morphology obtained by image analysis of X‐ray micro‐CT images is used for parameterization Predictions of solute breakthrough obtained by particle tracking perfectly match observations
    Keywords: Darcy'S Law ; Particle Tracking ; Column Experiments ; X‐Ray Μ‐Ct ; Pore Space Morphology ; Image Analysis
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 5
    In: Water Resources Research, March 2007, Vol.43(3), pp.n/a-n/a
    Description: Large‐scale models of transient flow processes in the unsaturated zone require, in general, upscaling of the flow problem in order to capture the impact of heterogeneities on a small scale, which cannot be resolved by the model. Effective parameters for the upscaled models are often derived from second‐order stochastic properties of the parameter fields. Such properties are good quantifications for parameter fields, which are multi‐Gaussian. However, the structure of soil does rarely resemble these kinds of fields. The non‐multi‐Gaussian field properties can lead to strong discrepancies between predictions of upscaled models and the averaged real flow process. In particular, the connected paths of parameter ranges of the medium are important features, which are usually not taken into account in stochastic approaches. They are determined here by the Euler number of one‐cut indicator fields. Methods to predict effective parameters are needed that incorporate this type of information. We discuss different simple and fast approaches for estimating the effective parameter for upscaled models of slow transient flow processes in the unsaturated zone, where connected paths of the material may be taken into account. Upscaled models are derived with the assumption of capillary equilibrium. The effective parameters are calculated using effective media approaches. We also discuss the limits of the applicability of these methods.
    Keywords: Richards Equation ; Unsaturated Flow ; Upscaling
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 6
    In: Water Resources Research, April 2009, Vol.45(4), pp.n/a-n/a
    Description: High‐resolution optical bench‐scale experiments were conducted in order to investigate local gas flow pattern and integral flow properties caused by point‐like gas injection into water‐saturated glass beads. The main goal of this study was to test the validity of the continuum approach for two‐fluid flow in macroscopic homogeneous media. Analyzing the steady state experimental gas flow pattern that satisfies the necessary coherence condition by image processing and calibrating the optical gas distribution by the gravimetrical gas saturation, it was found that a pulse‐like function yields the best fit for the lateral gas saturation profile. This strange behavior of a relatively sharp saturation transition is in contradiction to the widely anticipated picture of a smooth Gaussian‐like transition, which is obtained by the continuum approach. This transition is caused by the channelized flow structure, and it turns out that only a narrow range of capillary pressure is realized by the system, whereas the continuum approach assumes that within the representative elementary volume the whole spectrum of capillary pressures can be realized. It was found that the stochastical hypothesis proposed by Selker et al. (2007) that bridges pore scale and continuum scale is supported by the experiments. In order to study channelized gas flow on the pore scale, a variational treatment, which minimizes the free energy of an undulating capillary, was carried out. On the basis of thermodynamical arguments the geometric form of a microcapillary, macrochannel formation and a length‐scale‐dependent transition in gas flow pattern from coherent to incoherent flow are discussed.
    Keywords: Air Sparging ; Continuum Modeling ; Pore‐Scale Modeling ; Gas Flow Pattern ; Instability Analysis ; Image Processing
    ISSN: 0043-1397
    E-ISSN: 1944-7973
    Source: John Wiley & Sons, Inc.
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  • 7
    In: Water Resources Research, May 2006, Vol.42(5), pp.n/a-n/a
    Description: This paper presents a vision that advocates hydropedology as an advantageous integration of pedology and hydrology for studying the intimate relationships between soil, landscape, and hydrology. Landscape water flux is suggested as a unifying precept for hydropedology, through which pedologic and hydrologic expertise can be better integrated. Landscape water flux here encompasses the source, storage, flux, pathway, residence time, availability, and spatiotemporal distribution of water in the root and deep vadose zones within the landscape. After illustrating multiple knowledge gaps that can be addressed by the synergistic integration of pedology and hydrology, we suggest five scientific hypotheses that are critical to advancing hydropedology and enhancing the prediction of landscape water flux. We then present interlinked strategies for achieving the stated vision. It is our hope that by working together, hydrologists and pedologists, along with scientists in related disciplines, can better guide data acquisition, knowledge integration, and model‐based prediction so as to advance the hydrologic sciences in the next decade and beyond.
    Keywords: Catchment Hydrology ; Landscape Processes ; Scale ; Soil Hydrology ; Soil Physics ; Vadose Zone
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 8
    In: Water Resources Research, August 2019, Vol.55(8), pp.6653-6672
    Description: —a key process for water exchange between soil and atmosphere—is controlled by internal water fluxes and surface vapor fluxes. Recent studies demonstrated that the dynamics of the water flow in corners determine the time behavior of the evaporation rate. The internal water flux of the porous media is often described by capillary flow assuming . Particularly, the crucial influence of , that is, the nonlinear dependency of the capillary flow has been neglected so far. The focus of the paper is to demonstrate that SiO‐surfaces can exhibit contact angles of about 40°. This reduces the internal capillary flow by 1 order of magnitude compared to complete wetting. First, we derived the contact angle by inverse modeling. We conducted a series of evaporation experiments in a 2‐D square lattice microstructure connected by lognormal distributed throats. We used an explicit analytical power series solution of the single square capillary model. A contact angle of 38° ± 1° was derived. Second, we directly measured the contact angle of the Si‐SiO wafer using the Drop Shape Analyzer Krüss 100 and obtained an averaged contact angle of 42° ± 2°. The results support the single square capillary model as an appropriate model for the description of the evaporation process in an ideal square capillary. Evaporation rate dependence on contact angle and temperature: Influence of capillary, viscous, and gravitational forces Visualization micromodel experiments of corner flow: Micromodels produced by a new interval‐based ICP‐DRIE technology Analytical solution for 1‐D corner flow and analysis of the fluid‐fluid patterns and geometric characteristics of the evaporation process
    Keywords: Water Flow ; Silicon Dioxide ; Water Exchange ; Surfaces ; Internal Water ; Power Series ; Water Exchange ; Soil Water ; Evaporation ; Contact Angle ; Dependence ; Evaporation Rate ; Water Flow ; Water Exchange ; Microstructure ; Evaporation ; Porous Media ; Porous Media ; Silica ; Evaporation ; Water Flow ; Capillary Flow ; Atmospheric Models ; Wetting ; Silicon Dioxide ; Fluxes ; Soils ; Evaporation ; Evaporation Rate ; Water Exchange ; Contact Angle;
    ISSN: 0043-1397
    E-ISSN: 1944-7973
    Source: John Wiley & Sons, Inc.
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  • 9
    Language: English
    In: Water Resources Research, 05/1998, Vol.34(5), pp.1241-1249
    Description: Observed data sets containing values above or below the analytical threshold of measuring equipment are referred to as censored. Such data are frequently encountered in quality and quantity monitoring applications of water, soil, and air samples. Most of the previous literature on the statistical analysis of censored data relates to the problems of moment, parameter, and quantile estimation methods. Such estimation methods usually assume an underlying probability distribution. Few goodness-of-fit methods exist for censored data. We introduce L moment diagrams for the evaluation of the goodness of fit of alternative distributional hypotheses for left-censored data. Experiments with artificial censored data sets document the conditions under which L moment diagrams should be useful. Our approach, like Hosking's [1995] approach for right censoring, derived L moment diagrams for left-censored observations from partial probability-weighted moments. Copyright 1998 by the American Geophysical Union.
    Keywords: Hydrogeology ; Environmental Geology ; Data Management ; Environmental Analysis ; Information Management ; Monitoring ; Pollution ; Probability ; Soils ; Statistical Analysis ; Surface Water ; Water Quality;
    ISSN: Water Resources Research
    E-ISSN: 00431397
    E-ISSN: 19447973
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