1,720,987 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Get PDFWe 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
Numerische Berechnung von Stickoxiden in turbulenten Flammen mit Transportgleichungs-PDF-Verfahren
No full textkoamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Numerical computation of nitric oxides in turbulent flames with transported PDF methods
No full textDie Vermeidung und Reduktion von Schadstoffen ist ein zentraler Aspekt bei der Entwicklung technischer Verbrennungssysteme. Insbesondere bei Stickoxiden werden auf Grund gesetzlicher Regulierungen große Anstrengungen unternommen, um die Emissionen zu senken. Dank dieser Bemühungen ist es beispielsweise mit dem Konzept der Magerverbrennung gelungen in stationären Gasturbinen Emissionen im „single digit“ Bereich zu realisieren, d.h. Emissionen unter 10 vppm NO. Bei der Entwicklung derartiger Technologien spielen Berechnungsmethoden wie etwa die numerische Strömungssimulation (Computational Fluid Dynamics, CFD) eine wichtige Rolle und haben dank verbesserter Modellierungsansätze in den letzten Jahrzehnten immer weiter an Bedeutung gewonnen. Trotz dieser Fortschritte in der Modellierung stoßen Berechnungsmodelle zum Teil an ihre Grenzen, wenn es um die Vorhersage von Stickoxidemissionen im „single digit“ Bereich geht.
Aus diesem Grund wird in dieser Arbeit ein Transportgleichungs-PDF-Verfahren (PDF: Probabilty Density Function, Wahrscheinlichkeitsdichtefunktion) als möglicher Ansatz zur Berechnung von Stickoxiden untersucht. Der Transportgleichungs-PDF-Ansatz bietet den Vorteil, dass er unter Verwendung detaillierter Reaktionsmechanismen den Einfluss turbulenter Fluktuationen auf den chemischen Quellterm exakt darzustellen vermag. Damit verspricht dieser Ansatz bei der Berechnung der kinetisch kontrollierten Stickoxide in turbulenten Strömungen eine hohe Genauigkeit. In der vorliegenden Arbeit wird zu diesem Zweck ein hybrides Finite-Volumen/Lagrange'sches Monte-Carlo-Verfahren verwendet, welches die Transportgleichung für die PDF der thermochemischen Variablen numerisch löst.
Entscheidend für die erfolgreiche Anwendung des Transportgleichungs-PDF-Verfahrens ist die geeignete Modellierung ungeschlossener Terme. Dies betrifft insbesondere den ungeschlossenen Mischungsterm, der den Einfluss molekularer Mischung auf die PDF beschreibt. In diesem Punkt leistet die vorliegende Arbeit einen Beitrag durch ein (zum Zeitpunkt dieser Arbeit) neuwertiges Mischungsmodell. Es berücksichtigt in den Erwartungswerten der thermochemischen Variablen einen zusätzlichen Drift, der aus dem Fick'schen Diffusionsansatz und der Fourier'schen Wärmeleitung resultiert. Mit diesem Ansatz ist es möglich, differentielle Diffusion in Transportgleichungs-PDF-Verfahren zu berücksichtigen.
Neben einer guten Modellierung der Turbulenz-Chemie-Wechselwirkung, ist für NO-Berechnungen auch die Verwendung genauer Reaktionsmechanismen notwendig. Zu diesem Zweck werden zunächst Stickoxide in laminaren Gegenstromdiffusionsflammen berechnet und die Rechenergebnisse mit Messdaten verglichen. Dieser Vergleich gibt Aufschluss über die Güte eines Mechanismus in Bezug auf dessen Genauigkeit bei der Schadstoffberechnung. Durchgeführt wird dieser Vergleich sowohl für Wasserstoff als auch für Methan. Es zeigt sich dabei, dass zum Teil noch große Defizite in der kinetischen Modellierung von NO vorliegen.
Mit Hilfe der Untersuchungen an laminaren Flammen werden Mechanismen ausgewählt, die bei der Oxidation von Wasserstoff und Methan gute Berechnungsergebnisse in den Stickoxiden erzielen. Diese werden anschließend zur Berechnungen von Stickoxiden in turbulenten Wasserstoff- und Methanflammen verwendet. Untersucht werden drei verschiedene Strahlflammen, die sich in ihrer Komplexität unterscheiden. Der einfachste Fall ist eine turbulente Wasserstoff-Luft-Flamme („H3“-Flamme), die im Hinblick auf die Brennstoffoxidation sehr gleichgewichtsnah ist. Höhere Komplexität weist die zweite Flamme auf, eine pilotierte Methan-Luft-Flamme („Sandia-D“-Flamme), bei der lokales Verlöschen auftritt. Die höchste Komplexität bietet eine abgehobene Wasserstoffflamme („Cabra“-Flamme), die als dritte turbulente Flamme untersucht wird. Um die Qualität der Rechnungen zu bewerten, werden in allen Testfällen die Rechenergebnisse mit experimentellen Daten der Erwartungswerte sowie der Standardabweichungen verglichen. Insgesamt zeigt sich eine gute Übereinstimmung zwischen den Resultaten des Transportgleichungs-PDF-Verfahrens und den Messungen. Insbesondere belegen Streudiagramme für die Temperatur, dass bei der diffusionskontrollierten „H3“-Flamme das hier eingeführte Diffusionsmodell Effekte differentieller Diffusion zu beschreiben vermag. Vergleiche mit anderen Verbrennungsmodellen, wie etwa einem multivariaten „assumed-PDF“-Ansatz oder der „laminaren Chemie“ zeigen, dass der Transportgleichungs-PDF-Ansatz deutlich bessere Ergebnisse im thermochemischen Feld und somit in den Stickoxiden liefert. Somit bewirkt die Verwendung des Transportgleichungs-PDF-Verfahrens eine signifikante Erhöhung in der Genauigkeit.The suppression and reduction of pollutants is a central aspect in the design of technical combustion systems. Due to strict legal regulations, great efforts are made to reduce in particular the emissions of nitric oxides. Thanks to such efforts it is possible to reduce for example the emissions of stationary gas turbines to the level of “single digit NOx”, i.e. emissions below 10 vppm NO, by employing lean combustion concepts. In the development of such technologies computational methods like CFD (computational fluid dynamics) play a vital role and have gained in importance during the last decades due to improved modelling capabilities. Despite these improvements, the modelling of nitric oxide remains to some extent still a problem, if “single digit NOx” emissions have to be predicted.
For this reason the transported PDF method is explored in this work as a possible approach for NO prediction. This method offers the advantage, that it is capable to employ detailed chemistry and to treat the effect of turbulence chemistry interaction in closed form. Therefore it promises a high accuracy for the prediction of the kinetically controlled NO formation. In the present work a hybrid finite-volume/Lagrangian Monte-Carlo method is implemented which solves numerically the transport equation of the joint thermochemical PDF.
Decisive for a successful application of the transported PDF method is an appropriate modelling of unclosed terms appearing in the PDF transport equation. Of major importance is the unclosed mixing term, which takes the effect of molecular mixing on the PDF into account. To model this term the present work provides a novel approach. This mixing model takes an additional drift into account in the expectations of the thermochemical variables, which stems from Fickian diffusion and Fourier's law of heat conduction. With this approach it is possible, to include the effect of differential diffusion in the transported PDF method.
In addition to models, which provide closure for turbulence chemistry interaction, reaction mechanisms are required for the prediction of NOx, which model accurately the chemical kinetics of NO formation. To this end the formation of nitric oxides is computed in laminar counterflow diffusion flames and compared to experimental data of NO concentrations. These computations help to assess the quality of NO reaction mechanisms. Comparisons are carried out for both hydrogen and methane flames. With the help of these investigations it is found that (to a certain extent) large deficits still exist in some of the kinetic mechanisms.
With the help of these investigations in laminar flames, reaction mechanisms are chosen, which achieve good accuracy in the prediction of nitric oxide concentrations. These mechanisms are used later on to compute nitric oxides in turbulent hydrogen and methane flames. Three flames are investigated which differ in their degree of complexity. The most simple flame considered is a turbulent hydrogen-air jet flame (“H3” flame) which is, with respect to fuel oxidation, close to equilibrium. A higher degree of complexity offers a piloted methane-air jet flame (“Sandia” flame) which shows a certain amount of local extinction. The highest complexity is given by a lifted hydrogen air flame (“Cabra” flame), which is the third flame investigated in this work. In order to assess the quality of the computational results they are compared to experimental data. Both expectations and standard deviations are compared. In general, a good agreement is observed between calculation and measurement for all flames. Scatterplots of temperature prove in the case of the diffusion controlled “H3” flame the capability of the novel diffusion model presented here to account for differential diffusion effects. Comparisons to other modelling approaches like the “assumed PDF” method or “laminar chemistry” show that the transported PDF method yields an improved accuracy for the prediction of the overall thermochemical field as well as for the prediction of the nitric oxides. Therefore the use of the transported PDF method offers significant improvements in pollutant predictions
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