1,721,051 research outputs found
Numerical analysis of film cooling in advanced rocket nozzles
No full textThe key demand on future space transportation systems is the concurrent reduction of Earth-to-orbit launch costs and increase of launcher reliability and operational efficiency. A common way of slightly improving performance of gas-generator open-cycle engines is the injection of the turbine exhaust gas into the nozzle divergent section, which is also used for wall film cooling. The present study focuses on a numerical parametric analysis of the film-cooling efficiency in dual-bell nozzles. The secondary gas injection is made in the first bell, and it is found that the expansion fan originating from the inflection helps the film to better protect the wall. The results of fully-attached-flow simulations are also used to study the influence of film cooling on the expected behavior of nozzle side loads during operation with separated flow in the second bell. Copyright © 2009 by E. Martelli, F. Nasuti, and M. Onofri
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
Numerical simulation of hot-gas side heat transfer enhancement in thrust chambers by wall ribs
No full textHigh performance liquid rocket engines environment is characterized by high temperature and high heat fluxes. Heat transfer accurate prediction is one of the key features for the development of these engines, especially in the case of expander cycle feed system. In the present study, hot-gas side heat transfer enhancement due to ribbed walls in an expander cycle engine thrust chamber is analyzed by means of a Reynolds-Averaged Navier-Stokes (RANS) equations solver. As a validation of the solver, a ribbed wall experimental test case is reproduced to assess the capability of the solver to properly capture thermal boundary layer and heat transfer enhancement. In this test case, heat enhancement is quantified by measuring water coolant temperature increase along the duct. Then a simplified coupling procedure is adopted to compare numerical simulation against experimental data. Finally an expander cycle engine thrust chamber is studied focusing on heat transfer enhancement due to ribbed walls, compared to the smooth wall case. In this study the capability of the solver to be employed as a design tool for wall ribbed thrust chambers is shown. © 2011 by the American Institute of Aeronautics and Astronautics, Inc
Numerical parametric analysis of dual-bell nozzle flows
No full textDual-bell nozzles permit two different operating modes, which provide higher performance than conventional bell nozzles when applied to rocket engines operating from sea level. During the low-altitude mode, the flow is separated and the separation line is located near the inflection of the nozzle wall, in a region characterized by a negative value of the wall pressure gradient, as in conventional nozzles, and in which side loads may occur. The characteristics of this region in hot full-scale applications are addressed by means of numerical simulations, and scaling laws are sought for cold subscale side-load experiments. Moreover, the flow behavior during the transition between the two operating modes is analyzed by time-accurate simulations
Flow Separation Response to Unsteady External Disturbances in Dual Bell Nozzles
No full textThe dual bell nozzle belongs to the family of Altitude Compensating Nozzles (ACN), which constitutes an option of great interest to improve launcher first stage performance. The internal flow of this nozzle adapts to the external pressure by separating at the wall inflection at low altitude (first operating mode) and by full flowing at high altitude (second operating mode). Therefore the dual bell nozzle operates with a separated flow at take off and in the initial portion of the flight trajectory. This characteristic is of important concern since the external flow is neither steady nor axisymmetric, and its coupling with the internal flow separation can cause dangerous side loads. This work presents the results of a time accurate numerical analysis of the effect of unsteady ambient pressure on the separation point and on the shock system inside the dual bell nozzle, operating in the first mode. The external computational domain is characterized by an acoustically open downstream boundary. The numerical simulations show that the oscillation of the separation point is very sensitive to frequencies near the acoustic wave characteristic frequencies
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