1,720,980 research outputs found
Design approach to predict synthetic jet formation and resonance amplifications
No full textA predictive approach for the Synthetic Jet (SJ) formation, which couples formation criteria already available
from the literature, with outcome to Lumped Element Modeling (LEM), which is able to predict the resonance
velocity amplifications, is presented. In this respect, the formation criteria yield the minimum jet velocity to
obtain an experimentally observable jet, whereas the LEM approach predicts the jet velocity, to be compared
with the minimum one of the formation criterion. A deep experimental investigation is carried out to detect jet
formation and to measure resonance velocity amplifications on various types of actuators manufactured in
house, including, among others, the influence of single and double orifices. Jet velocity is recorded by means of
hot-wire anemometry, at both the device orifice exit and one orifice diameter downstream, i.e. downstream of
the expected stagnation (saddle) point. Jet formation and resonance amplification thresholds are identified in
terms of the dimensionless Stokes and Strouhal parameters. Based on the experimental finding, a description of
the relevant modeling is reported, devoted to the evaluation of the velocity magnification factor with respect to
the so-called incompressible (static) solution. It takes advantage from the physical consideration that the actuator
behaves as a driven system of two-coupled mechanical oscillators, the Helmholtz’s one and the structural
one, for which resonance frequencies are predicted by means of simple relationships, together with appropriate
damping factors. Jet formation thresholds closely agree with the present experimental findings. The predicted
overdamped conditions are compared with classic analytic boundary correlations of the literature and previous
experimental results as well. Practical and simple relationships, that can help the designer to manufacture a
device having desired performances, are presented
PERFORMANCES OF TWO OPEN-SOURCE SOLVERS IN THE NUMERICAL SIMULATION OF SYNTHETIC JETS
No full textWe report direct numerical simulations of a synthetic jet issuing in a quiescent environment performed by two widespread open-source computational fuid dynamics
(CFD) codes: Nek5000 and the OpenFOAM solver pimpleFoam. While the former employs a high-order spectral-element method, the latter is based on finite-volume, lower-
order schemes. The
flow parameters are based on the experimental data of Yao et al.
(2006). The performances of the two codes are compared thoroughly, with regard to accuracy as well as computational cost. Our computations show that Nek5000 is globally
more efficient than pimpleFoam for the problem under study
Development of a physical model for plasma synthetic jet actuators
No full textAmong various devices used in active flow control, plasma synthetic jet actuators seem
to be a promising technology to improve aircraft performances. This paper presents a new physical
model able to predict the temporal evolution of the main thermodynamic variables of the device.
Results for both single pulse mode and repetitive working regime are reported, providing insights
of their characteristics. The work is completed by an analysis of the actuator frequency response,
followed by a comparison with literature results
Experimental and CFD Characterization of a Double-Orifice Synthetic Jet Actuator for Flow Control
Full text linkThe paper presents a joint experimental and numerical characterization of double-orifice synthetic jet actuators for flow control. Hot-wire measurements of the flow field generated by the device into a quiescent air environment were collected. The actuation frequency was systematically varied to obtain the frequency response of the actuator; its coupled resonance frequencies were detected and the velocity amplitude was measured. Direct numerical simulations (DNS) of the flow field generated by the device were subsequently carried out at the actuation frequency maximizing the jet output. The results of a fine-meshed parametric analysis are outlined to discuss the effect of the distance between the orifices: time-averaged flow fields show that an intense jet interaction occurs for small values of the orifice spacing-to-diameter ratio; phase-averaged velocity and turbulent kinetic energy distributions allow to describe the vortex motion and merging. A novel classification of the main regions of dual synthetic jets is proposed, based on the time- and phase-averaged flow behaviour both in the near field, where two distinct jets converge, and in the far field, where an unique jet is detected. The use of three-dimensional DNS also allows to investigate the vortex merging for low values of the jet spacing. The work is intended to provide guidelines for the design of synthetic jet arrays for separation control and impinging configurations
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
On low-frequency unsteadiness in swept shock wave–boundary layer interactions
Full text linkWe derive a scaling law for the characteristic frequencies of wall pressure fluctuations
in swept shock wave/turbulent boundary layer interactions in the presence of cylindrical
symmetry, based on analysis of a direct numerical simulations database. Direct numerical
simulations in large domains show evidence of spanwise rippling of the separation line,
with typical wavelength proportional to separation bubble size. Pressure disturbances
around the separation line are shown to be convected at a phase speed proportional
to the cross-flow velocity. This information is leveraged to derive a simple model for
low-frequency unsteadiness, which extends previous two-dimensional models (Piponniau
et al., J. Fluid Mech., vol. 629, 2009, pp. 87–108), and which correctly predicts growth of
the typical frequency with the sweep angle. Inferences regarding the typical frequencies in
more general swept shock wave/turbulent boundary layer interactions are also discussed
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
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