1,721,040 research outputs found
Sensitivity and identifiability of hydraulic and geophysical parameters from streaming potential signals in unsaturated porous media
No full textFluid flow in a charged porous medium generates electric potentials called streaming potential (SP). The SP signal is related to both hydraulic and electrical properties of the soil. In this work, global sensitivity analysis (GSA) and parameter estimation procedures are performed to assess the influence of hydraulic and geophysical parameters on the SP signals and to investigate the identifiability of these parameters from SP measurements. Both procedures are applied to a synthetic column experiment involving a falling head infiltration phase followed by a drainage phase. GSA is used through variance-based sensitivity indices, calculated using sparse polynomial chaos expansion (PCE). To allow high PCE orders, we use an efficient sparse PCE algorithm which selects the best sparse PCE from a given data set using the Kashyap information criterion (KIC). Parameter identifiability is performed using two approaches: the Bayesian approach based on the Markov chain Monte Carlo (MCMC) method and the first-order approximation (FOA) approach based on the Levenberg-Marquardt algorithm. The comparison between both approaches allows us to check whether FOA can provide a reliable estimation of parameters and associated uncertainties for the highly non-linear hydrogeophysical problem investigated. GSA results show that in short time periods, the saturated hydraulic conductivity (K-s) and the voltage coupling coefficient at saturation (C-sat) are the most influential parameters, whereas in long time periods, the residual water content (theta(s)), the Mualem-van Genuchten parameter (n) and the Archie saturation exponent (n(a)) become influential, with strong interactions between them. The Mualem-van Genuchten pa-rameter (alpha) has a very weak influence on the SP signals during the whole experiment. Results of parameter estimation show that although the studied problem is highly nonlinear, when several SP data collected at different altitudes inside the column are used to calibrate the model, all hydraulic (K-s, theta(s), alpha, n) and geophysical parameters (n(a),C-sat) can be reasonably estimated from the SP measurements. Further, in this case, the FOA approach provides accurate estimations of both mean parameter values and uncertainty regions. Conversely, when the number of SP measurements used for the calibration is strongly reduced, the FOA approach yields accurate mean parameter values (in agreement with MCMC results) but inaccurate and even unphysical confidence intervals for parameters with large uncertainty regions
Modélisation de l'écoulement de l'eau, du transport réactif et du transfert de chaleur dans les milieux poreux
No full textCe travail traite de la modélisation de l'écoulement, du transport réactif et du transfert de chaleur dans les milieux poreux en se concentrant sur trois applications principales : (i) le transport réactif multicomposant dans le processus d'électrodiffusion, (ii) la convection thermohaline dans les milieux poreux saturés et (iii) l'écoulement, la dissolution et le transport dans les roches fracturées. L'objectif principal de ce travail est d'améliorer l'efficacité et la précision des modèles numériques et de fournir une compréhension approfondie des processus physiques sous-jacents. Les premiers efforts sont consacrés à l'évaluation des hypothèses communes sur la formulation des études de transport réactif multicomposant pour l'électrodiffusion. Une partie importante est consacrée aux études de la convection thermohaline dans les milieux poreux saturés. Une solution semi-analytique est développée pour le cas de la convection thermohaline 3D et un modèle numérique avancé est développé pour la convection thermohaline réactive avec l'application de la séquestration du CO2. La dernière partie de la thèse est consacrée à l'étude de l'écoulement, de la dissolution et du transport dans les roches fracturées, basée sur une technique numérique avancée.This work addresses the modeling of flow, reactive transport and heat transfer in porous media with focusing on three main applications: (i) multicomponent reactive transport in the electro-diffusion process, (ii) Thermohaline convection in saturated porous media and (iii) Flow, dissolution and transport in fractured rocks. The main objective of this work is to improve the efficiency and accuracy of the numerical models and provide a deep understanding of the underlying physical processes. The first efforts are devoted to the evaluation of common assumptions on the formulation of multicomponent reactive transport studies for electro-diffusion. A significant part is dedicated to studies of thermohaline convection in saturated porous media. A semi-analytical solution is developed for the case of the 3D thermohaline convection and an advanced numerical model is developed for the reactive thermohaline convection with the application of CO2 sequestration. The last part of the thesis is devoted to the studies of flow, dissolution and transport in fractured rocks based on an advanced numerical technique
A 3-D Semianalytical Solution for Density-Driven Flow in Porous Media
No full textExisting analytical and semianalytical solutions for density-driven flow (DDF) in porous media are limited to 2-D domains. In this work, we develop a semianalytical solution using the Fourier Galerkin method to describe DDF induced by salinity gradients in a 3-D porous enclosure. The solution is constructed by deriving the vector potential form of the governing equations and changing variables to obtain periodic boundary conditions. Solving the 3-D spectral system of equations can be computationally challenging. To alleviate computations, we develop an efficient approach, based on reducing the number of primary unknowns and simplifying the nonlinear terms, which allows us to simplify and solve the problem using only salt concentration as primary unknown. Test cases dealing with different Rayleigh numbers are solved to analyze the solution and gain physical insight into 3-D DDF processes. In fact, the solution displays a 3-D convective cell (actually a vortex) that resembles the quarter of a torus, which would not be possible in 2-D. Results also show that 3-D effects become more important at high Rayleigh number. We compare the semianalytical solution to research (Transport of RadioACtive Elements in Subsurface) and industrial (COMSOL Multiphysics®) codes. We show cases (high Raleigh number) where the numerical solution suffers from numerical artifacts, which highlight the worthiness of our semianalytical solution for code verification and benchmarking. In this context, we propose quantitative indicators based on several metrics characterizing the fluid flow and mass transfer processes and we provide open access to the source code of the semianalytical solution and to the corresponding numerical models.Q. Shao acknowledges the support of the National Natural Science Foundation of China (Grant 11702199) and the Natural Science Foundation of Hubei Province (Grant 2017CFB147). The Editor, Associate Editor, and the three reviewers are thanked for their comments that helped us to improve the paper. The source code for the semianalytical solution (with all necessary input files) as well as the TRACES model are available at https://lhyges.unistra.fr/FAHS-Marwan or by contacting the author directly ([email protected]). The COMSOL models are available on the COMSOL application exchange website
Hybridizable interior penalty discontinuous galerkin methods for mass transfer in porous media
No full textCette thèse est consacrée à la résolution de l’écoulement et du transport de masse dans les milieux poreux hétérogènes, anisotropes et fracturés par des méthodes Hybrides de Galerkine Discontinues (HDG). Au cours de la dernière décennie, cette méthode connait un succès indéniable auprès de la communauté scientifique. Son formalisme flexible offre de nombreux avantages sur le plan numérique : traitement des discontinuités du milieu ou de la solution du problème, flux localement conservatifs, stratégies de raffinement-h et -k, elligibilité à la condensation statique, et utilisation de maillages non-conformes. L’objectif de ce travail concerne le développement d’une famille de méthodes de Pénalisation Intérieure Hybrides de Galerkine Discontinues (H-IP), pour l’amélioration de la simulation des transferts de masse dans les milieux poreux et fracturés. L’originalité de ce formalisme réside dans la mise en place d’une stratégie de pénalisation adaptative, qui nous permet d’inclure plusieurs situations physiques : (i) les problèmes purement diffusif ou convectif, (ii) les régimes mixtes combinant ces deux processus pour un large éventail du nombre de Péclet, et (iii) les problèmes de diffusion localement évanescente. Enfin, nous présentons l’étude de l’écoulement en milieux poreux fracturés, dans lesquels les fractures sont traitées comme des interfaces de dimension (d−1) immergées dans la roche. Une série d’expériences numériques est réalisée pour évaluer la robustesse, la stabilité et la convergence du schéma pour ces situations physiques délicates. Ces résultats révèlent la souplesse et la pertinence de cette classe pour la modélisation des phénomènes dans les réservoirs naturels.This thesis focuses on the numerical modeling of flow and mass transport in heteroge- neous, anisotropic, and fractured porous media with Hybrid Discontinuous Galerkin (HDG) methods. During the last decade, this method have received tremendous success from scientific community. Its flexibility provides several benefits regarding the numerical aspects : treatment of discontinuities of the medium and the solution, local conservative fluxes, h- and k-refinement strategies, eligibility to static condensation, and use of non-matching meshes. The objective of this work is to develop families of Hybridizable Interior Penalty (H-IP) schemes, to enhance numerical modeling and simulation of mass transfer in fractured porous media. The originality of the proposed methodology lies in the definition of an adaptative penalty strategy, which allow us to encompass different physical situations : (i) purely diffusive or convective problems, (ii) mixed-regimes combining both processes for a wide range of Péclet number, and (iii) locally evanescent diffusion problems. Finally, we present the study of flow in fractured porous media, in which the fractures are treated as an interface of (d − 1)-dimension immersed in the bulk. A series of numerical experiments are performed to evaluate the robustness, stability, and convergence of the method for these delicate physical situations. The results prove the flexibility and the suitability of this class for the modeling phenomena in natural reservoirs
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Modeling of multicomponents reactive transport in saturated porous media
No full textLa modélisation du transport réactif se fait séquentiellement avec l’approche de séparation d’opérateur (OS) ou bien simultanément avec l’approche globale (GA). Les premiers travaux de modélisation de transport réactif ont montré la supériorité de (OS) par rapport à (GA). Des travaux récents ont cependant mis en évidence la faiblesse de OS dans certaines configurations. Dans la première partie de ce travail nous avons étudié les approches (OS) et (GA) dans différentes configurations chimie-transport. Afin d’éviter l’inconvénient majeur de (GA), à savoir son coût élevé en temps de calcul, on a développé un nouveau modèle de calcul robuste et efficace. Les résultats qu’on a obtenus montrent qu’avec ce modèle, (GA) devient plus efficace et plus précise que (OS). Cet avantage devient prononcé dans le cas d’une chimie complexe avec des réactions de sorption et/ou de précipitation/dissolution.
La deuxième partie de ce travail s’intéresse au développement des schémas numériques pour la modélisation du transport réactif. La plupart des modèles existants utilisent des méthodes Eulériennes qui imposent des discrétisations très fines en espace et en temps et par conséquent des temps de calculs prohibitifs. Afin de remédier à ce problème on a utilisé la méthode ELLAM. L’objectif de ce travail est de développer les potentialités des méthodes ELLAM pour résoudre le problème du transport réactif.
Une première étape consiste à combiner les ELLAM avec OS et GA. Les résultats obtenus montrent que pour les réactions cinétiques OS_ELLAM est la plus efficace. Cependant, pour les réactions à l’équilibre, ELLAM_GA devient préférable.
La deuxième étape est l’amélioration des ELLAM afin d’éviter le problème de diffusion numérique. On a donc développé une nouvelle formulation des ELLAM. Cette formulation a été testée pour le transport linéaire et non linéaire. Les résultats montrent que notre nouvelle approche n’est pas sensible au pas de temps et évite toute diffusion numérique.The modeling of reactive transport is done sequentially with the operator splitting approach or simultaneously with the global approach. The first work in reactive transport showed the superiority of (OS) compared to (GA). However, recent work highlighted the OS weakness in certain configurations. In the first part of this work we studied the tow approaches (OS) and (GA) for various configurations of chemistry-transport. In order to avoid the major disadvantage of (GA), who is the high cost in CPU time, we developed a new efficient and robust model. The obtained results show that, with this model, (GA) becomes more efficient and more precise than (OS). This advantage becomes more important in the case of complex chemistry with sorption and precipitation/dissolution reactions.The second part of this work is interested to the development of numerical schemes for modeling reactive transport. In fact, the majority of the existing models use Eulerian methods for the transport equations. These methods require a very fine space and time discretization and consequently a prohibitory CPU times. In order to solve this problem, we use in this work the Eulerian Lagrangian Localized Adjoint Method (ELLAM). The objective of this work is to develop the potentialities of ELLAM to solve the reactive transport problem.The first step of this part consists in combining the ELLAM with OS and GA. The obtained results show that for kinetic reactions, OS_ELLAM is more efficient than GA_ELLAM. However, for the equilibrium reactions, ELLAM_GA becomes preferable.The second step is the improvement of the ELLAM in order to avoid the numerical diffusion problem. A new formulation of ELLAM is developed. This formulation was tested for linear and nonlinear transport. The result shows that our new approach is less sensitive to the time step and avoids any numerical diffusion
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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