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  • English  (89)
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  • 1
    Language: English
    In: Computational Geosciences, 2013, Vol.17(1), pp.139-149
    Description: Carbon capture and storage is a recently discussed new technology, aimed at allowing an ongoing use of fossil fuels while preventing the produced CO 2 to be released to the atmosphere. CCS can be modeled with two components (water and CO 2 ) in two phases (liquid and CO 2 ). To simulate the process, a multiphase flow equation with equilibrium phase exchange is used. One of the big problems arising in two-phase two-component flow simulations is the disappearance of the nonwetting phase, which leads to a degeneration of the equations satisfied by the saturation. A standard choice of primary variables, which is the pressure of one phase and the saturation of the other phase, cannot be applied here. We developed a new approach using the pressure of the nonwetting phase and the capillary pressure as primary variables. One important advantage of this approach is the fact that we have only one set of primary variables that can be used for the biphasic as well as the monophasic case. We implemented this new choice of primary variables in the DUNE simulation framework and present numerical results for some test cases.
    Keywords: Two-phase flow ; Multicomponent flow ; Porous medium ; CO storage
    ISSN: 1420-0597
    E-ISSN: 1573-1499
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  • 2
    Language: English
    In: Advances in Water Resources, April 2017, Vol.102, pp.161-177
    Description: In the geostatistical inverse problem of subsurface hydrology, continuous hydraulic parameter fields, in most cases hydraulic conductivity, are estimated from measurements of dependent variables, such as hydraulic heads, under the assumption that the parameter fields are autocorrelated random space functions. Upon discretization, the continuous fields become large parameter vectors with elements. While cokriging-like inversion methods have been shown to be efficient for highly resolved parameter fields when the number of measurements is small, they require the calculation of the sensitivity of each measurement with respect to all parameters, which may become prohibitive with large sets of measured data such as those arising from transient groundwater flow. We present a Preconditioned Conjugate Gradient method for the geostatistical inverse problem, in which a single adjoint equation needs to be solved to obtain the gradient of the objective function. Using the autocovariance matrix of the parameters as preconditioning matrix, expensive multiplications with its inverse can be avoided, and the number of iterations is significantly reduced. We use a randomized spectral decomposition of the posterior covariance matrix of the parameters to perform a linearized uncertainty quantification of the parameter estimate. The feasibility of the method is tested by virtual examples of head observations in steady-state and transient groundwater flow. These synthetic tests demonstrate that transient data can reduce both parameter uncertainty and time spent conducting experiments, while the presented methods are able to handle the resulting large number of measurements.
    Keywords: Inverse Modeling ; Geostatistical Inversion ; Uncertainty Quantification ; Nonlinear Conjugate Gradients ; Preconditioning ; Engineering
    ISSN: 0309-1708
    E-ISSN: 1872-9657
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  • 3
    In: Computational Geosciences, 17(1), 139-149 (2013)
    Description: Carbon Capture and Storage (CCS) is a recently discussed new technology, aimed at allowing an ongoing use of fossil fuels while preventing the produced CO2 to be released to the atmosphere. CSS can be modeled with two components (water and CO2) in two phases (liquid and CO2). To simulate the process, a multiphase flow equation with equilibrium phase exchange is used. One of the big problems arising in two-phase two-component flow simulations is the disappearance of the nonwetting phase, which leads to a degeneration of the equations satisfied by the saturation. A standard choice of primary variables, which is the pressure of one phase and the saturation of the other phase, cannot be applied here. We developed a new approach using the pressure of the nonwetting phase and the capillary pressure as primary variables. One important advantage of this approach is the fact that we have only one set of primary variables that can be used for the biphasic as well as the monophasic case. We implemented this new choice of primary variables in the DUNE simulation framework and present numerical results for some test cases. Comment: 11 pages, 11 figures, accepted for publication in Computational Geosciences
    Keywords: Physics - Computational Physics ; 76s05, 35q86
    Source: Cornell University
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  • 4
    Language: English
    In: Vadose Zone Journal, 2015, Vol.14(5), p.0
    Description: The guest editors introduce the seven contributions to the special issue on processes in capillary fringes, with a focus on the complex interaction of biological, chemical, and physical processes in this environemnt. Processes in capillary fringes (CFs) have a complex nature due to the interactions between the solid, liquid, and gaseous environments. Despite a considerable body of literature on CFs coming from different disciplines, the ongoing processes and their complex interactions are yet only partially understood.
    Keywords: Soils ; Solids ; Vadose Water ; Methods and Instruments ; General;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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  • 5
    Language: English
    In: Mathematics and Computers in Simulation, 2011, Vol.81(10), pp.2051-2061
    Description: For the simulation of transport processes in porous media effective parameters for the physical processes on the target scale are required. Numerical upscaling, as well as multiscale approaches can help where experiments are not possible, or hard to conduct. In 2009, Bastian and Engwer proposed an Unfitted Discontinuous Galerkin (UDG) method for solving PDEs in complex domains, e.g. on the pore scale. We apply this method to a parabolic test problem. Convergence studies show the expected second order convergence. As an application example solute transport in a porous medium at the pore scale is simulated. Macroscopic breakthrough curves are computed using direct simulations. The method allows finite element meshes which are significantly coarser then those required by standard conforming finite element approaches. Thus it is possible to obtain reliable numerical results for macroscopic parameter already for a relatively coarse grid.
    Keywords: Discontinuous Galerkin Method ; Higher Order ; Unfitted Finite Elements ; Numerical Upscaling ; Flows in Porous Media ; Convection–Diffusion-Problems ; Parabolic Equations ; Computer Science
    ISSN: 0378-4754
    E-ISSN: 1872-7166
    Source: ScienceDirect Journals (Elsevier)
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  • 6
    Language: English
    In: Computers and Geosciences, July, 2012, Vol.44, p.78(8)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cageo.2012.03.014 Byline: Ronnie L. Schwede (a), Adrian Ngo (b), Peter Bastian (b), Olaf Ippisch (b), Wei Li (a), Olaf A. Cirpka (a) Abstract: Hydraulic conductivity is a key parameter for the simulation of groundwater flow and transport. Typically, it is highly variable in space and difficult to determine by direct methods. The most common approach is to infer hydraulic-conductivity values from measurements of dependent quantities, such as hydraulic head and concentration. In geostatistical inversion, the parameters are estimated as continuous, spatially auto-correlated fields, the most likely values of which are obtained by conditioning on the indirect data. In order to identify small-scaled features, a fine three-dimensional discretization of the domain is needed. This leads to high computational demands in the solution of the forward problem and the calculation of sensitivities. In realistic three-dimensional settings with many measurements parallel computing becomes mandatory. In the present study, we investigate how parallelization of the quasi-linear geostatistical approach of inversion can be made most efficient. We suggest a two-level approach of parallelization, in which the computational domain is subdivided and the evaluation of sensitivities is also parallelized. We analyze how these two levels of parallelization should be balanced to optimally exploit a given number of computing nodes. Author Affiliation: (a) University of Tubingen, Center for Applied Geoscience, Holderlinstr. 12, 72074 Tubingen, Germany (b) University of Heidelberg, Interdisciplinary Center of Scientific Computing, Im Neuenheimer Feld 368, 69120 Heidelberg, Germany Article History: Received 20 December 2011; Revised 24 February 2012; Accepted 19 March 2012
    Keywords: Hydrogeology -- Analysis ; Groundwater Flow -- Analysis ; Hydraulic Flow -- Analysis ; Groundwater -- Analysis ; Geostatistics -- Analysis
    ISSN: 0098-3004
    Source: Cengage Learning, Inc.
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  • 7
    In: Journal of Geophysical Research: Atmospheres, 27 January 2003, Vol.108(D2), pp.ALT 4-1-ALT 4-11
    Description: We apply an energy balance model to the snow cover for snowpack accumulation and ablation at a continuous permafrost site on Spitsbergen for the snow‐covered periods from fall 1998 to winter 2000. The model includes net radiative, turbulent, ground, snow, and rain heat flux. The balance yields two distinct types of snow ablation: winter and spring ablation. Energy transferred by sensible heat and rain input reduces the snow cover during the winter, creating internal ice lenses and basal ice. The snowpack ablates during spring in two stages in both years. During the first stage, surface melt and subsequent internal freezing compact and reduce the snow cover, but no runoff is produced. This phase lasts more than twice as long as the second stage. During the second stage, which takes 14 days in both years, melt rates from the snowpack are represented well using the energy balance model. Ground heat fluxes are comparable during spring in both years, but the long persistence of the snow cover in 2000 delays the thawing of the ground. Due to the duration of the snow cover during spring snow melt of both years, the total energy supplied to the ground is significant, between 30 and 50% of the total energy supplied by net radiation.
    Keywords: Snow Cover ; Snow Melt ; Frozen Ground ; Energy Balance ; Spitsbergen
    ISSN: 0148-0227
    E-ISSN: 2156-2202
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  • 8
    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;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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  • 9
    Language: English
    In: Vadose Zone Journal, 2013, Vol.12(4), p.0
    Description: The hydraulic behavior of soil is determined by its hydraulic properties and their variability in space. In agricultural soils, this heterogeneity may stem from tillage or may have natural origin. The root distribution of plants will adapt to some extent to this soil heterogeneity. However, the combined impact of soil heterogeneity and root water uptake (RWU) on long-term soil water budgets has not received much attention. Numerical experiments helped identify how soil heterogeneity affects plant transpiration, soil evaporation, and groundwater recharge. Two-dimensional virtual soils with hierarchical heterogeneity, both natural and tillage induced, served as a basis for modeling soil water dynamics for a 10-yr climate record from two weather stations in Germany that vastly differ in annual precipitation. The complex interactions between soil and vegetation were explored by (i) comparing different RWU strategies (depth-, structure-, and time-dependent root profiles), (ii) land use types (perennial grass and annual winter crops), (iii) a combination of textures (silt above sand and sand above loam), and (iv) RWU with or without a compensation mechanism. The simulations were repeated with one-dimensional, effective representations of these virtual soils. In the framework of hydropedology, this study shed some light on the interaction between plants and pedological features and its impact on the macroscopic soil water budget. We demonstrated that land use has a major impact on the annual water balance through the partitioning of evapotranspiration into bare soil evaporation and plant transpiration. Compensational RWU becomes important for the annual water balance when the root zone comprises contrasting materials with respect to water holding capacity. Soil heterogeneity has in fact a minor impact on long-term soil water budgets. As a consequence, the relative contribution of plant transpiration, soil evaporation, and groundwater recharge to the total soil water loss was well reproduced by simulations in one-dimensional effective soil profiles. This advocates the application of one-dimensional soil-atmosphere-vegetation transfer (SVAT) models at larger scales. These findings only hold for assumptions made in our numerical simulations including flat area without lateral flow and no macropore flow.
    Keywords: Environmental Geology ; Soils ; Atmosphere ; Boundary Conditions ; Central Europe ; Eastern Germany ; Europe ; Field Studies ; Germany ; Grain Size ; Heterogeneity ; Hydrodynamics ; Hydrology ; Hydropedology ; Julicher Borde Germany ; Land Use ; Magdeburg Germany ; Mapping ; North Rhine-Westphalia Germany ; Numerical Models ; One-Dimensional Models ; Rhizosphere ; Saxony-Anhalt Germany ; Scale Factor ; Size Distribution ; Soil-Atmosphere-Vegetation Transfer ; Soils ; Topography ; Two-Dimensional Models ; Unsaturated Zone ; Vegetation ; Water Balance ; Western Germany;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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  • 10
    Language: English
    In: Computers and Geosciences, July 2012, Vol.44, pp.78-85
    Description: Hydraulic conductivity is a key parameter for the simulation of groundwater flow and transport. Typically, it is highly variable in space and difficult to determine by direct methods. The most common approach is to infer hydraulic-conductivity values from measurements of dependent quantities, such as hydraulic head and concentration. In geostatistical inversion, the parameters are estimated as continuous, spatially auto-correlated fields, the most likely values of which are obtained by conditioning on the indirect data. In order to identify small-scaled features, a fine three-dimensional discretization of the domain is needed. This leads to high computational demands in the solution of the forward problem and the calculation of sensitivities. In realistic three-dimensional settings with many measurements parallel computing becomes mandatory. In the present study, we investigate how parallelization of the quasi-linear geostatistical approach of inversion can be made most efficient. We suggest a two-level approach of parallelization, in which the computational domain is subdivided and the evaluation of sensitivities is also parallelized. We analyze how these two levels of parallelization should be balanced to optimally exploit a given number of computing nodes. ► Parallel geostatistical inversion. ► Two level parallelization. ► Efficient C++ programming.
    Keywords: Parallel Computing ; Geostatistical Inversion ; Adjoint ; Parallelization ; Geology
    ISSN: 0098-3004
    E-ISSN: 1873-7803
    Source: ScienceDirect Journals (Elsevier)
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