1,721,175 research outputs found
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
Computational resources and numerical accuracy in density driven flow problems
No full textLAUREA MAGISTRALELa simulazione numerica di problemi di flusso e trasporto a densità variabile implica un alto costo computazionale sia in termini di hardware che di tempo di calcolo; ciò è dovuto all’accoppiamento non lineare tra l’equazione di flusso e quella di trasporto. Il problema è complicato dalla mancanza di una conoscenza deterministica dei parametri che caratterizzano il sottosuolo e dunque dalla necessità di applicare metodi stocastici. In letteratura sono stati presentati metodi numerici efficienti per la risoluzione di problemi a densità variabile e linee guida per una discretizzazione spazio-temporale che assicuri soluzioni accurate. Una metodologia per valutare la convergenza dei momenti dei campioni delle variabili di interesse ottenuti tramite il metodo Monte Carlo è stata sviluppata da Ballio e Guadagnini (2004) e testata in scenari relativamente semplici (densità costante). L’allocazione ottimale delle risorse computazionali, considerando discretizzazione spaziale e numerosità del campione, non è tuttora definita in problemi di flusso e trasporto a densità variabile.
L’ obbiettivo fondamentale della tesi è quello di selezionare, tra diverse soluzioni ottenute variando l’algoritmo numerico, la discretizzazione spazio-temporale e l’ampiezza del campione, la soluzione più accurata e la metodologia più efficiente. Sono stati risolti due casi studio che corrispondono al problema di Henry modificato in acquiferi omogenei caratterizzati da diverse dispersività. Un terzo caso studio, che include l’eterogeneità dell’acquifero, è stato preso in considerazione. Il risultato dell’analisi è stato l’identificazione di una soluzione trade off in termini di discretizzazione spazio-temporale e numerosità del campione.
Sono stati utilizzati due codici (SUTRA e DDFLOW) che differiscono per algoritmo numerico implementato per la soluzione delle equazioni di flusso e trasporto, tecniche di discretizzazione dello spazio e del tempo e metodo iterativo per la risoluzione del sistema non lineare di equazioni. Il risultato principale dello studio è che DDFLOW fornisce soluzioni accurate in minor tempo di SUTRA. L’efficienza di DDFLOW è particolarmente evidente in condizioni di bassa dispersivitàNumerical simulation of density driven flow problems requires large computational cost both in terms of computer hardware requirements and CPU time due to the non-linear coupling between flow and transport equations. The problem is further exacerbated by the lack of deterministic knowledge of subsurface characteristic parameters, and therefore the need to tackle the problem within a stochastic framework.
Efficient numerical methods to solve density driven flow have been developed in the past and guidelines about the space-time discretization techniques needed to acquire accurate solutions have been proposed. A methodology to assess the convergence of the Monte Carlo-based ensemble moments of quantities of interest has been developed by Ballio and Guadagnini (2004) and tested within relatively simple (constant density) scenarios. The optimal allocation of computational resources, considering both space-time discretization and ensemble size, has never been investigated for any density driven flow problem.
A key goal of this thesis is to determine the “most accurate” solution to be implemented within seawater intrusion problems by varying the numerical algorithm, the space-time discretization and the ensemble size. Two test cases, simulating a modified Henry problem within homogeneous aquifers characterized by diverse dispersivities, have been investigated. Additionally a third test case, including aquifer’ heterogeneity, has been analyzed. As a result, a trade off solution is identified in terms of space-time and ensemble discretization.
Two codes (SUTRA and DDFLOW) have been tested. The two codes differ in the numerical algorithm implemented for the solution of flow and transport equations, space-time discretization techniques, and iterative scheme for the solution of non-linearities. A key result of our study is that DDFLOW provides accurate solutions involving less CPU time than SUTRA. DDFLOW efficiency is particularly evident within low dispersivity conditions
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
Flow parameter estimation in fractured porous media : inversion and adaptive multi-scale parameterization
No full textCe travail a pour objectif de développer et d’appliquer une méthode originale permettant de simuler l’écoulement dans un milieu poreux fracturé. Cette méthode repose sur une approche multicouches double continuum permettant de séparer le comportement des différents aquifères présents sur un site. La résolution des écoulements, basée sur la méthode des Eléments Finis de Crouzeix-Raviart, est associée à une méthode inverse (minimisation de type Quasi-Newton combinée à la méthode de l’état adjoint) et à une paramétrisation multi-échelle.La méthode est appliquée dans un premier temps sur l’aquifère fracturé du site expérimental de Poitiers. Les résultats montrent une bonne restitution du comportement de l’aquifère et aboutissent à des champs de transmissivité plus réguliers par rapport à ceux de l’approche simple continuum. L’application finale est réalisée sur le site de Cadarache (taille plus importante et données d’entrée moins denses). Le calage des deux aquifères présents sur le site est satisfaisant et montre que ceux-ci se comportent globalement de façon indépendante. Ce calage pourra être amélioré localement grâce à données de recharge plus fines.The aim of this study is to develop and validate a new method for the simulation of flow in fractured porous media. This method is based on a multi-layered and dual continuum approach allowing to discriminate the behavior of different aquifers present on a site. The flow equations are solved using a Crouzeix-Raviart Finite Element method, in association with an inverse method (Quasi-Newton minimization combined with the adjoint state method) and a multi-scale parameterization.The method is first applied and validated on the fractured aquifer of the Hydrogeological Experimental Site of Poitiers. The results closely reproduce the flow behavior of the aquifer and lead to a transmissivity field much more homogeneous than the one obtained with a simple continuum approach. The final application is performed on the site of Cadarache (large scale problem with heterogeneously distributed input data). The model calibration of both aquifers is rather satisfactory and shows that their behavior is globally independent. It could locally be improved if more accurate groundwater recharge data is made available
Development of an experimental approach to better understand the evolution of soil fertility in mid-mountain areas
No full textLes forêts sont soumises à de nombreuses perturbations naturelles et/ou anthropiques, à court et à long terme. Une meilleure compréhension des interactions sol/eau/plante et des réponses des écosystèmes aux modifications devient de plus en plus cruciale pour une meilleure gestion des ressources naturelles et renouvelables. Dans un environnement granitique, comme dans les Vosges (NE de la France), le déclin des forêts peut être lié au lessivage des nutriments du sol par les pluies acides et par les processus naturels d’acidification des sols qui peuvent être plus importants avec des résineux qu’avec des feuillus. L’acidification peut modifier la disponibilité des nutriments dans le sol et l'équilibre entre les différents réservoirs de nutriments.Avec cette étude, nous développons une approche expérimentale afin de reproduire et d'analyser en laboratoire le lessivage des sols à plus petite échelle. Les résultats montrent un comportement chimique très variable selon l'élément, le type de sol, les horizons et le type d'acide. De plus, nous avons observé des résultats inattendus pour le calcium : la quantité totale de Ca extraite après 150 jours correspond à une très faible proportion du calcium échangeable obtenu avec la cobaltihexamine, soulignant une biodisponibilité complexe de ce nutriment dans le sol.Forests are subject to several natural and/or anthropogenic perturbations, over short and long terms time. A better understanding of how soil/water/plant interactions can react and response to modifications become more and more crucial for a better management of natural and renewable resources. In granitic environment, as in the Vosges Mountains (NE France) forest decline can be related to soil nutrient leaching by acid rain. Acidity from atmospheric pollution can modify the availability of nutrients in soil, and the equilibrium between different nutrient pools.With this study we develop an experimental approach to reproduce and analyze in laboratory soil leaching at smaller scale. The results show highly variable chemical behavior versus element, type of soil, horizons and type of acid. In addition, we observed unexpected results for calcium: the total quantity of Ca extracted after 150 days correspond to a very low proportion of the calcium exchangeable obtained with cobaltihexamine, underlining a complex bioavailability of this nutrient in soil
2D benchmark experiments and simulations of density coupled flow problems
Full text linkVariations of fluid densities can alter flow patterns and transport processes, if solute concentration differences are high enough to cause relevant density contrasts. Since numerous environmental problems are related to these phenomena, the need for accurate process description and modeling continues to increase. The numerical simulation of such processes is challenging due to the strong non-linear coupling of flow and transport processes. Therefore, experimental studies are required to elaborate the basic principles and to test numerical codes in order to provide reliable tools for water resources management and planning. In this thesis, density-coupled flow processes under the influence of geometrical boundary conditions are studied and numerical codes are tested against high resolution experimental data.
Photometric methods were further developed to increase the accuracy of measurements in flow tank experiments. They directly related digitally measured intensities of a tracer dye to solute concentrations. This enabled an effective processing of a large number of images in order to compute concentration time series at various points of the flow tank and concentration contour lines. Perturbations of the measurements were lens flare effects and the image resolution. Transmissive and reflective intensity measurements were compared. The reflection images were more homogeneous in spatial illumination than the transmission images. Major perturbations of the transmissive images were lens flare effects and light dispersion within the bead-water-Plexiglas system which smeared the front of the plume. Based on the conducted evaluation of transmissive and reflective intensity measurements, the reflection data delivered more reliable intensity values to derive solute concentrations in intermediate scale flow tank experiments. The newly developed resistivity measurement system used two different input voltages at gilded electrode sticks to enable the measurement of salt concentrations from 0 to 300 g/l. The method was highly precise and the major perturbations were caused by temperature changes, which can be controlled in the laboratory. The two measurement approaches, photometric and resistivity methods, were compared with regard to their usefulness in providing data for benchmark experiments. Due to the unknown measurement volume of the electrodes, the photometric method was better to determine experiments in a series of laboratory-scale 2D porous medium tank experiments.
Various density-driven flow problems were investigated using well-defined experimental parameters and boundary conditions. The experiments were carried out both in a rectangular flow tank (158×100×4 cm3) and in a more complex geometrical setup aiming to study variable density flow in geological formations of aquifers and aquicludes connected via fault zones. An impermeable layer within the porous medium tank forced the solutes to pass through a channel to reach the outlet of the tank. The porous medium was homogeneous in both cases. The image analysis technique deliverd 2, 10, 50 and 80% salt concentration isolines at distinct times and breakthrough curves of the dyed saltwater. The experimental data were presented as benchmark problems to evaluate numerical codes. A numerical model based on Mixed Finite Elements for the fluid flow problem and a combination of Discontinuous Galerkin Finite Element and Multi-Point Flux Approximation methods for the transport turned out to be adequate for the simulation of the physical experiments. The high data availability made the proposed benchmark experiments a valuable tool for assessing the performance of density-coupled flow models. Heterogeneous porous medium experiments were conducted with a low permeability zone in the centre of the tank. Three different boundary conditions, corresponding to different localizations of the inflow and the outflow openings at the opposite edges of the tank, were applied and different flow scenarios are observed in the heterogeneous tank. The numerical model used for the simulations was based on efficient advanced approximations for both spatial and temporal discretizations. The Method Of Lines (MOL) was used to allow higher-order temporal discretization and the model adapted in both the order of approximation and time step to provide the necessary accuracy. The model was able to reproduce the experiments. The numerical results were improved by assuming a non-Fickian dispersivity for high density experiments
Dissolution kinetics : from a mechanistic approach at atomic scale to macroscopic description at centimeter scale
No full textLes cinétiques de dissolution représentent un composant majeur des flux de matière sur Terre. Lors de cette thèse, plusieurs facteurs pouvant impacter la dissolution ont été étudiés afin de lier différentes échelles spatiales. Cette thèse reporte l’étude de la dissolution de minéraux modèles (enstatite et calcite) de l’échelle de l’atome à celle de la face pour le premier, et de celle de la face à celle du milieu poreux pour le deuxième. L’anisotropie de dissolution observée expérimentalement sur l’enstatite a pu être reproduite à l’aide d’un modèle probabiliste des ruptures de liaisons. L’intérêt de ce type de modèle est démontré et des propositions quant à l’extension de son utilisation ont été faites. Un modèle de substitution du taux de dissolution de l’enstatite liant l’échelle atomique à celle de la face a également été proposé. L’étude de la calcite a, quant à elle, démontré l’importance du choix de la loi de vitesse dans les études de mise à l’échelle des taux de dissolution. Une expérience en colonne et sa simulation à l’aide d’un modèle de transport-réactif ont démontré que l’hétérogénéité de l’écoulement du fluide autour d’un minéral joue un rôle de premier ordre dans la dissolution des minéraux.Dissolution kinetics represent a major challenge in the understanding of geological phenomena. During this thesis, several factors that could impact dissolution were studied in order to link different spatial scales. The dissolution of the model minerals of this thesis, enstatite and calcite, was studied from the atomic scale to the face scale for the first and from the face to the porous medium scale for the second. The dissolution anisotropy observed experimentally on enstatite could be reproduced using a probabilistic model. The interest of this type of model has been demonstrated and proposals for extending their use have been made. A surrogate model for the dissolution rate of enstatite linking the atomic scale to the face scale was also established. The study of calcite has demonstrated the importance of the selection of the rate law in dissolution rates upscaling studies. A column experiment and its simulation using a reactive-transport model demonstrated that the heterogeneity of the fluid flow around a mineral plays a major role in the dissolution of minerals
Application of artificial neural networks in hydrogeology : identification of unknown pollution sources in contaminated aquifers
No full text[...]Certaines caractéristiques concernant la qualité et l’hydrogéologie des eaux souterraines, dans de nombreux cas, ne sont pas directement mesurables et doivent être évaluées en fonction d'autres paramètres directement mesurables. Le problème de la détermination des paramètres inconnus du modèle est généralement dénommé "problème inverse". La résolution du problème inverse pour la modélisation de l'écoulement et le transport des contaminants dans les eaux souterraines est l'objectif principal de ce travail de recherche.
Quant à la résolution du problème inverse, dans le présent document, nous avons pour objectif la définition d’une méthodologie qui permette l’identification des caractéristiques dans l’espace et le temps des sources inconnues de contaminations. Dans ce travail de recherche, le problème inverse est résolu sur la base de mesures de concentrations du contaminant dans les puits de surveillance situés dans un domaine d'intérêt. Une fois connu l’effet d’un certain phénomène, nous cherchons à reconstruire la cause qui l’a généré.
Ainsi, la recherche a-t-elle été élaborée selon les points suivants : - Modélisation de la contamination des eaux souterraines par l'utilisation d'un logiciel non-commercial pour la modélisation des flux et le transport des contaminants dans les milieux poreux. -Modélisation des relations cause-effet de la contamination des eaux souterraines par les Réseaux de Neurones Artificiels (RNA). - Application des RNA pour la résolution du problème inverse dans deux cas de contamination des eaux souterraines étudiés. [...][...]In many cases, some hydrogeological and groundwater quality characteristics, are not directly measurable and must be physically assessed in function of directly measurable parameters. The problem of determining the unknown model parameters is usually identified as "inverse problem". Solving the inverse problem is the main goal of modeling groundwater flow and contaminant transport. The validity of an aquifer forecasting model is closely related to the reliability and accuracy of the parameters assessment. With respect to the resolution of the inverse problem, this work aims at defining a methodology that allows to identify the features in space and time of unknown contamination sources. In our case, the inverse problem is solved on the basis of measurements of contaminant concentrations in monitoring wells located in the studied areas.
In the framework of this thesis, the research is developed under the following themes: - groundwater contamination modeling using a non-commercial software for the flux and transport model in porous media. - modeling of the cause and effect relationships in groundwater contamination with Artificial Neural Networks (ANN) technology. - application of ANN to solve the inverse problem in two cases of groundwater contamination.
Over the past decades, Artificial Neural Networks (ANN) have become increasingly popular as a problem solving tool and have been extensively used as a forecasting tool in many disciplines.[…
- …
