1,720,968 research outputs found
Closure mechanism of the A1 and A2 modes in jet screech
No full textThis paper explores the screech closure mechanism for different axisymmetric modes in shock-containing jets. While many of the discontinuities in tonal frequency exhibited by screeching jets can be associated with a change in the azimuthal mode, there has to date been no satisfactory explanation for the existence of multiple axisymmetric modes at different frequencies. This paper provides just such an explanation. As shown in previous works, specific wavenumbers arise from the interaction of waves in the flow with the shocks. This provides new paths for driving upstream-travelling waves that can potentially close the resonance loop. Predictions using locally parallel and spatially periodic linear stability analyses and the wavenumber spectrum of the shock-cell structure suggest that the A1 mode resonance is closed by a wave generated when the Kelvin-Helmholtz mode interacts with the leading wavenumber of the shock-cell structure. The A2 mode is closed by a wave that arises owing to the interaction between the Kelvin-Helmholtz wave and a secondary wavenumber peak, which arises from the spatial variation of the shock-cell wavelength. The predictions are shown to closely match experimental data, and possible justifications for the dominance of each mode are provided based on the growth rates of the absolute instability
The axisymmetric screech tones of round twin jets examined via linear stability theory
No full textSpatial linear stability analysis is used to study the axisymmetric screech tones generated by twin converging round nozzles at low supersonic Mach numbers. Vortex-sheet and finite-thickness models allow for identification of the different waves supported by the flow at different conditions. Regions of the frequency-wavenumber domain for which the upstream-propagating guided jet modes are observed to be neutrally stable are observed to vary as a function of solution symmetry, jet separation, S, and the velocity profile used. Screech-frequency predictions performed using wavenumbers obtained from both models agree well with experimental data. Predictions obtained from the finite-thickness model better align with the screech tones measured experimentally and so are seen to be an improvement on predictions made with the vortex sheet. Additionally, results from the finite-thickness model predict both symmetric and antisymmetric screech tones for low S that are found in the vortex-sheet model only at greater S. The present results indicate that the feedback loop generating these screech tones is similar to that observed for single-jet resonance, with equivalent upstream and downstream modes
Analysis of axisymmetric screech tones in round twin-jets using linear stability theory
No full textThe axisymmetric screech modes, which arise as part of a resonance cycle, are considered here for purely converging round twin-jets. Linear stability models, vortex-sheet and finite thickness, are employed to consider the underlying physics of the resonance involved. Strong agreement is found between both the models and experimental acoustic data, in particular forming a sharp bound to the cut-on screech frequency. When considering the symmetry of the screech modes, symmetric or antisymmetric, the experimental measurements indicated regions of antisymmetry that were not supported by the vortex-sheet model. Whereas the finite thickness model could support this across part of the nozzle pressure range and provided distinct predictions for the two symmetries
Dynamics of round jet impingement
No full textThe impingement of a round jet issued from a convergent nozzle is known to produce intense tonal noise. These tones are generated by a feedback process involving a Kelvin-Helmholtz instability wave and an upstream-travelling guided jet mode. The frequency structure of these tones is experimentally explored by varying the jet Mach number from 0.7 to 1.5 and we observe evidence that the upstream-travelling guided modes of the jet are active in the feedback process. Tone frequency predictions obtained using the intrinsic jet modes are compared to the standard model involving free-stream acoustic waves: the upstream-travelling guided waves model provides a convincing agreement with the experimental data.this make
Understanding twin-jet screech using a vortex-sheet model
No full textA twin-jet vortex-sheet model is applied to study the axisymmetric screech modes, generated by simply converging nozzles, for low supersonic Mach numbers. This allows for the identification of the different waves supported by the flow for different conditions. Propagative regions for the upstream-propagating neutral guided jet mode are observed to vary both as a function of solution symmetry, SS and SA, and the jet separation, whilst bounding the screech tones. The A1 screech mode is observed to be cut-off sharply by the SS lower bound. Screech frequency predictions are performed using the wavenumbers obtained from the vortex-sheet model, which provided a good qualitative agreement with the experimental data. Additional A1 and A2 screech tones were observed in the acoustic spectrum as the jet separation was increased, pointing to the existence of new resonance cycles associated with the different symmetries of the flow. These additional tones exist simultaneously and are seen to merge together as increases leaving only a single A1 and A2 tone
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
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