1,720,992 research outputs found
Analysis Of Variable Thrust Hybrid Propulsion For Formation Flight Satellites
Full text linkHybrid motors are a promising brand of space propulsion. Many benefits have been demonstrated, like the fact that they can be "green", low cost safe. In this optics, with this thesis we want to characterize throttle-able hybrid motor for collision avoidance in formation flight.
The purpose is to enhance the performance of a "state of the art" motor, in particular mitigating the effect of some drawbacks like low regression rate and combustion efficiency. The propellant choice has been pressurized nitrous oxide as oxidizer and paraffin wax as fuel.
The work started with a preliminary design of the motor with a 0-D transient analytical code. Objective was to define the main dimensions like nozzle, grain, combustion chamber and oxidizer mass flow, and to help in the sensors choice for the full scale level.
A 3D steady state CFD analysis have been performed, to have a full comprehension of the phenomenon involving vortex flow field inside the combustion chamber. It has been discovered that the most important parameter is the temperature gradient produced by the combustion process, that increase the axial velocity straightening the fluid lines.
This numerical analysis has been used also to characterize the motor performance, simulating different chamber configurations: a) different grain port diameter, to study the combustion at different burning time, b) variation in oxidizer mass flow, involving throttling effects and c) different combustion chamber configuration, using mixer-like devices and pre/post-combustion chambers.
An analytical tool has been developed, using steady state Navier-Stokes in cylindrical coordinates: results showed a forced vortex behaviour of the velocity profile of the fluid field. Moreover, a pressure gradient in radial direction has been discovered. This knowledge helped in the selection of the correct pick-up point of the pressure in combustion chamber, in a position (near the nozzle) where this effect is negligible.
Throttling device selection has devised the use of a needle valve. The selected model, prior to its use in burning test, has been characterized with water (in a preliminary phase) and nitrous oxide in cold test.
The experimental campaign has taken the majority of this work: first cold test have been performed to evaluate the oxidizer mass flow with both axial and vortex injection, discovering that they have the same discharge coefficient; measured mass flow was 350g/s.
Burning test involved all previous investigated field: studying axial and vortex injection have been discovered that the second technique gives an increase in combustion efficiency from 73% to 91%, and an increase in regression rate of 41% with the a coefficient going from 0.19mm/s in the axial case to 0.31mm/s in the vortex case. Pressure oscillations in combustion chamber were reduced from 7.5% to 3.8%.
Throttling has been performed in two levels: 75% and 50%. In both cases pressure oscillations remained the same, while combustion efficiency dropped to a value of 85% for the first case and 82% for last. Regression rate has not changed appreciably.
Last part regarded combustion chamber configuration: we found out that the use of mixer-like devices enhances the regression rate of 6% and raise the combustion efficiency up to 96%. Use of a longer pre-combustion chamber (from 10mm to 17mm) has similar but lower effect: combustion efficiency has an increment of 3%. In both cases, pressure chamber oscillation drop to a level lower than 2%.I motori ibridi fanno parte di un promettente ramo della propulsione aerospaziale. Sono già stati dimostrati diversi benefici, quali il fatto che siano "green", a basso costo e con una elevata sicurezza nelle operazioni. In quest'ottica, con questa tesi si vuole caratterizzare un motore ibrido a spinta variabile da utilizzarsi per collision avoidance in formation flight.
Lo scopo è di incrementare un motore già esistente con le tecnologie allo stato dell'arte, ed in particolare cercare di mitigare gli effetti negativi classici degli ibridi come il basso regression rate ed efficienza di combustione. La scelta dei propellenti è ricaduta su N2O pressurizzato come ossidante e paraffina come propellente.
La prima fase del progetto ha visto la creazione di un modello transient 0-D analitico per il design preliminare del motore. In particolare, si sono ricavate le dimensioni di grano, ugello e camera di combustione e la portata di massa obiettivo.
Successivamente, è stato analizzato il campo di moto fluido interno alla camera di combustione tramite un codice stazionario 3-D CFD. Il risultato emerso è che il parametro fondamentale per la fluidodinamica interna è il gradiente di temperatura prodotto dalla combustione, che accelera la componente assiale di velocità raddrizzando le linee fluide.
Questo stesso strumento è stato impiegato anche per caratterizzare in via numerica le configurazioni di motore che sono state poi testate. Tre gruppi di analisi sono state effettuate: a) differente portata di ossidante, per studiare il fenomeno del throttling dal punto di vista fluidodinamico, b) diverso diametro di porta del grano, per studiare il comportamento del motore in diversi istanti temporali e c) differenti configurazioni di camera di combustione, con l'impiego di componenti tipo mixer o pre/post-camere.
Un tool analitico che utilizza le equazioni di Navier-Stokes in coordinate cilindriche è stato sviluppato con lo scopo di aumentare la conoscenza del campo di moto. In particolare, è stato scoperto che il profilo di velocità del flusso segue un vortice forzato, ed è presente un gradiente di pressione in senso radiale. Questa informazione ha permesso un più accurato posizionamento dei sensori, in modo da avere un minore effetto gradiente nella misura del segnale di pressione.
Il throttling ha richiesto una selezione di dispositivi regolatori di flusso: la scelta è caduta su una valvola a spillo. Questa è stata caratterizzata dal punto di vista sperimentale mediante test a freddo sia con acqua che con l'ossidante reale utilizzato nei successivi test a fuoco.
La campagna sperimentale è iniziata con dei test a freddo per caratterizzare i due iniettori, assiale e vortex; il risultato è che hanno la stessa portata (350g/s) a parità di salto di pressione, e quindi lo stesso coefficiente di scarica.
Successivamente sono state studiate le performance con dei burn test: con la metodologia di iniezione vortex si ottengono incrementi sia di regression rate(+41%) che di efficienza di combustione (si passa da 73% a 91%). Le oscillazioni di pressione in camera scendono da un valore sulla media di pressione di 7.5% al 3.8%.
Il throttling è stato studiato in due livelli: 75% e 50% della portata di massa di ossidante. La riduzione di portata è stata accompagnata da una riduzione di efficienza di combustione (85% per il primo caso, 82% per il secondo), mentre le oscillazioni di pressione sono rimaste le stesse.
L'ultima parte ha visto lo studio di diverse configurazioni della camera di combustione. In particolare, si è visto che dispositivi tipo mixer permettono un incremento di efficienza fino al 96%, ed un aumento del regression rate del 6%. Anche l'impiego di pre-camere più lunghe (da 10mm a 17mm) fornisce prestazioni simili ma con incrementi minori: l'efficienza è innalzata del 3%. In entrambi i casi vi è un effetto positivo nella riduzione delle oscillazioni di pressione, che si attestano ad un valore inferiore al 2%
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
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
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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