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Characterization of the behavior of cryogenic propellants in tanks
Get PDFWith the introduction of cryogenic propellant combinations for space engines, there has been an
increasing interest in the study of the behavior of the propellant in the tanks in which it is stored.
This study can be carried out in two ways. The first approach involves both the dynamic behavior
of the propellant, i.e. the sloshing analysis, and the thermo-fluid-dynamics behavior, which are
analyzed as coupled phenomena. Conversely, in the second approach, the two phenomena are
studied separately. In this thesis, the second approach has been used, and only the thermo-fluiddynamics
characterization of the behavior of cryogenic propellant in tanks has been carried out, since
a consolidated reduced order activity on the sloshing analysis is already present in the literature.
Cryogenic propellants, having a very low boiling point, are subject to phase transitions even
when exposed to very low heat leaks. As a result, the pressure raises inside a closed non-venting
tank. Moreover, the tanks, especially those belonging to the upper stages of space launchers, are
subjected to time-varying heat leaks and gravity levels during their operating life. The thermo-fluiddynamics
characterization mainly aims at estimating the evolutions of tank pressure, of the local
fluid temperatures, of the phase change rate, and of the propellant location (in the case of reduced
gravity). All these analyses, are necessary in order to guarantee the reliability and safety of a tank
operation (feed turbopumps with propellant at appropriate temperature and pressure conditions,
guarantee that only liquid drains from the tank outlet, ensure that only vapor comes out of the
venting valve, etc.).
Due to the cost and complexity associated with the experiments, numerical tools of various
levels of complexity have been implemented over the years to carry out the thermo-fluid-dynamics
characterization. The first approach is characterized by the use of reduced order models. The
latter allow quick estimates of the main observables, and, therefore, represent an engineering tool
necessary in the case when a large number of parametric analyses has to be carried out, as in an
industrial design process. The second approach involves Computational Fluid Dynamics (CFD)
simulations. These last allow to perform more accurate analyses than the previous approach, and
to study the complex physical phenomena at play. However, the drawback of CFD simulations is
the elevated computational cost with respect to the reduced order models.
The aim of this study is twofold. On the one hand, a suitable zero-dimensional model has been
developed, implemented in Matlab, and successfully verified through the comparison with the results
of a one-dimensional code developed by NASA (see Chap. 2). Later, it has been used to simulate
two tank characteristic operating conditions, for which experimental data were also available. A
ground-based self-pressurization experiment in a small-scale liquid N2 tank (see Chap. 4) and a
ground-based active-pressurization experiment in a different small-scale liquid N2 tank (see Chap.
5). On the other hand, the aim of the study has been the development of a state-of-the-art CFD
methodology (see Chap. 3), capable of estimating the main thermo-fluid-dynamics observables,
and to study the main physical phenomena at play inside tanks of various species of cryogenic
propellant, and characterized by different operating conditions. The proposed methodology has
been successfully validated through the comparison with the experimental data of three benchmark
test cases, characterized by different propellants, geometries, and operating conditions. This has
been done in order to develop a computational tool which guarantees reliability during most of the
operating life of the tank. Moreover, for the various validation test cases, different physical and
numerical models and parameters have been tested, in order to find the ones which allow the best
reproduction of a particular operating condition, as well as the optimal parameters in terms of the
trade-off between accuracy and computational time. The three validation test cases which have
been simulated are:
• A ground-based liquid N2 self-pressurization test case, inside a small-scale tank (see Chap.
4).
• A ground-based liquid N2 active-pressurization test case, inside a small-scale tank (see Chap.
5).
• A self-pressurization test case carried out in the liquid H2 tank of the second stage of the
Saturn IB AS-203 vehicle, while it was into circular low Earth orbit (see Chap. 6).
Then, the validated CFD methodology has been used to carry out a parametric study on the gravity
level. In particular, four gravity levels, from normal gravity (gE = 9.81 m/s2) until the reduced
gravity of 10−3gE, have been considered. This study has been performed using the same geometry,
fluid, and operating conditions of the first validation test case. The parametric study has allowed
to see the effect of the gravity level on the pressurization behavior, phase change, and liquid-ullage
interface shape.
Finally, the developed, and repeatedly validated zero-dimensional model and CFD methodology
have demonstrated to allow to perform, with a different level of detail, the thermo-fluid-dynamics
characterization of the behavior of cryogenic propellant, under most of the operating conditions of
interest for tank design. Nevertheless, the operating conditions considered in the present thesis do
not involve the case of fluid with sloshing. In order to carry out a more comprehensive study of the
actual behaviour of cryogenic propellant in tanks, the sloshing phenomenon should be included in
future work. Indeed, sloshing happens very often in cryogenic propellant tanks and, although being
a mainly dynamic in nature process, it has a strong influence on the propellant thermal physical
phenomena
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
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
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