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3-D Nonstationary Simulation of Hall Thrusters
No full textThe thesis summarizes the work carried out by the author during the PhD course in Aerospace Engineering, XXI Cycle, at Universita' di Pisa, Dipartimento di Ingegneria Aerospaziale ”Lucio Lazzarino” (DIA). The goal of this work is to make a macroscopically exhaustive analysis of the physical phenomena involved in the study of Hall Effect Thrusters (HETs).
The numerical model utilizes and extrapolates the results of several works on HETs and other electric thrusters. Moreover, we draw knowledge from rudiments and solution techniques by other fields of Computational Physics and include them in our simulation model. Among the different kinds of models for the plasma dynamics description, we have chosen to utilize a multifluid-quasineutral nonstationary model with a 3-D structured discretization of the simulation domain. The 3-D description of the problem is fundamentally focused to consider and accurately explore magnetic field configurations that are inherently non-axysimmetric due to the magnetic circuit construction. Moreover, the model aims to take into account a plasma dynamics inherently 3-D despite an axysimmetric construction. The analysis of the unsteady thruster’s operation aims to verify if the nonstationary phenomena, which were highlighted in several theoretical and
experimental works, can be accurately modeled by the fluid framework.
The magnetic model was described using the "magnetostatic approximation", thus it can be considered externally applied such an input parameter to the simulation of the plasma dynamics. Moreover, we split up the total magnetic field into two components, the former externally fixed, and the latter depending on the current induced by the motion of the plasma in-
side the discharge channel. Taking into account the effects induced by the plasma dynamics on the magnetic circuit properties also permits to analyze configurations of magnetic field in series with the discharge.
We started our model description by writing the plasma simulation model in a dimensional form using the fluid equations for a quasineutral three-components plasma. Subsequently, the dimensional fluid equations were expressed in a dimensionless form. The dimensionless form of the model permits to numerically solve it without consider scale effects depending on the value of discharge parameters and extrapolate some typical dimensionless parameters of the problem. Moreover, the dimensionless formulation permits to share the simulation results between different thrusters and effectively present them in scientific publications. Dealing with dimensionless parameters, a scale analysis of physical phenomena can be made. Indeed, using experimental data in the literature
expressed in terms of macroscopic quantities for existing HETs, we can write scaling laws able to keep constant some dimensionless parameters for a reference thruster. The dimensionless analysis is considered of worth because it constitutes the basis of the conceptual design of HETs.
Finally, a frequency analysis of 1-D discharge properties along the axial direction has been performed. This analysis was mainly focused to highlight the presence of low-frequency longitudinal oscillations, thus to validate the pattern of relevant fields with respect to existing experimental data or to numerical models presented in the literature
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
OSCILLAZIONI E TRASPORTO ELETTRONICO NEI PROPULSORI AD EFFETTO HALL
No full textIn questo lavoro vengono trattati due fenomeni che condizionano notevolmente il funzionamento stazionario di un propulsore ad effetto Hall: le oscillazioni dei parametri di scarica ed il trasporto elettronico nella zona di forte campo magnetico.
Intense oscillazioni dei parametri di scarica possono essere presenti anche durante un funzionamento ottimale del propulsore (funzionamento in condizioni di minima corrente elettronica assiale). In regime oscillatorio si presentano fondamentalmente tre problemi: la perdita di efficienza del propulsore poiché viene spesa una parte di potenza di alimentazione per il sostentamento delle oscillazioni, la modifica delle proprietà di trasporto del plasma ed infine un funzionamento irregolare, pericoloso soprattutto dal punto di vista dell'estinzione della scarica. Il parametro che maggiormente caratterizza il funzionamento di ottimo del propulsore risulta l’intensità del campo magnetico.
Nella zona di forte intensità del campo magnetico il trasporto elettronico in direzione assiale differisce notevolmente dal trasporto di tipo classico. Nella letteratura esistente si attribuisce la presenza di questo trasporto anomalo alla presenza delle oscillazioni dei parametri di scarica, della turbolenza e della conduttività di parete. In questo lavoro viene introdotto un ulteriore effetto, dovuto alla topologia dell'induzione magnetica, che puo' causare una modifica notevole del coefficiente di trasporto elettronico assiale, anche di alcuni ordini di grandezza rispetto al valore classico. La turbolenza viene considerata sviluppando un modello di trasporto quasi-lineare per un plasma di Lorentz parzialmente ionizzato, nel caso di turbolenza quasi-elettrostatica. Tale modello permette la determinazione del tensore di trasporto turbolento in funzione delle fluttuazioni temporali di densità, del parametro di Hall e della topologia magnetica.
Si illustrano infine possibili procedure per la determinazione delle grandezze di tipo sperimentale contenute all'interno dei modelli teorici ed un'idea di sviluppo di un software di monitoraggio per la verifica delle previsioni effettuate
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
koamabayili/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
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