1,720,955 research outputs found
Performance analysis of vortex based mixers for confined flows
Get PDFThe hybrid rocket is still sparsely employed within major space or defense projects due to their relatively poor combustion efficiency and low fuel grain regression rate. Although hybrid rockets can claim advantages in safety, environmental and performance aspects against established solid and liquid propellant systems, the boundary layer combustion process and the diffusion based mixing within a hybrid rocket grain port leaves the core flow unmixed and limits the system performance. One principle used to enhance the mixing of gaseous flows is to induce streamwise vorticity. The counter-rotating vortex pair (CVP) mixer utilizes this principle and introduces two vortices into a confined flow, generating a stirring motion in order to transport near wall media towards the core and vice versa. Recent studies investigated the velocity field introduced by this type of swirler. The current work is evaluating the mixing performance of the CVP concept, by using an experimental setup to simulate an axial primary pipe flow with a radially entering secondary flow. Hereby the primary flow is altered by the CVP swirler unit. The resulting setup therefore emulates a hybrid rocket motor with a cylindrical single port grain. In order to evaluate the mixing performance the secondary flow concentration at the pipe assembly exit is measured, utilizing a pressure-sensitive paint based procedure
Self-Excited Instability Regimes of Confined Turbulent Premixed Jet Flames
No full textThe demand for more efficient and cleaner terrestrial gas turbine engines for energy generation has accelerated with stringent emission regulations. Most terrestrial gas turbine engine configurations utilize lean premixed jet stabilized flames for energy extraction. These highly turbulent flames provide the necessary thermal power densities and low NOx emissions but are prone to combustion instabilities. This work studies the stability of a canonical premixed turbulent jet flame to changes in operation condition at elevated pressure. The underlying coupling mechanisms leading to the excitation of different modes are of interest. Different fuel injection schemes are considered, to delineate the influence of system coupled excitation and flow field intrinsic mechanisms that lead to self-excitation of longitudinal and transverse modes in an axisymmetric jet flame.The fundamental longitudinal mode sensitivity to operation conditions was investigated in a technically-premixed configuration, for which the fuel injector is exposed to the system dynamics. Global equivalence ratio fluctuations (φ ) paired with flame-vortex interactions were observed to sustain the 1L mode, for which leaner operating conditions lead to an increase of the limit-cycle amplitude.To solely focus on the shear layer dynamics involved in the feedback loop of the 1L instability a “fully-premixed” configuration (FPC) of the combustor was designed. The elimination of potential φ lead to the excitation of longitudinal as well as transverse, and spinning modes, which can be associated with specific burner operation envelope regions. 1L mode coupled flame dynamics indicate axisymmetric emission patterns in OH∗ emission imaging corresponding to axisymmetric instabilities in the shear layer. Transverse modes correlate with an asymmetric shear layer roll-up process and a flapping motion of the flame. Spinning modes are characterized by high levels of limit-cycle amplitude and a single wave is observed that travels in the annulus of the recirculation zone. From the high speed imaging an azimuthal wave speed of up to 90% of the Chapman-Jouguet velocity for the natural gas - air mixture is computed.The transverse mode is found to be sensitive to changes in chamber pressure and injector velocity. For a baseline injector velocity at which 1T mode excitation occurs, an increase in system pressure lead to an increase of the 1T mode amplitude. The 1T mode excitation is found to be sensitive to the injector velocity, where the highest amplitude are observed for a Strouhal number band of 1.6-1.7 based on the injector diameter.A linear stability analysis (LSA) of the underlying base flow field is performed in order to assess if the underlying shear layer instability modes determine the selection of the instability regime for a given flame condition. Two flow perturbation modes are supported by the flow field, Mode 1 is associated with the recirculation zone domain, where preferred mode frequencies favors coupling with longitudinal acoustics chamber modes. Mode 2 resides in the mixture jet, for which preferred mode frequencies match local 1T acoustic eigenfrequencies of the chamber. It is found that for 1T mode dominated operation cases, the recirculation zone associated mode is stabilized leading to to the excitation of the 1T mode in the injector near field. For operation cases showing predominantly longitudinal combustion instability, the recirculation zone mode (Mode 1) shows elevated growth rates in the injector near field paired with preferred frequencies that are compatible with the longitudinal acoustic eigenmodes
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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