1,720,975 research outputs found
Linear Stability Models for Reacting Mixing Layers
Full text linkWe develop a physics-based reduced-order model of the aero-acoustic sound sources in reacting mixing layers as a method for fast and accurate predictions of the radiated sound. Instabilities in low-speed mixing layers are known to be dominated by the traditional Kelvin–Helmholtz (K–H)-type “central” mode, which is expected to be superseded by the “outer” modes as the chemical-reaction-based heat-release modifies the mean density, yielding new peaks in the density-weighted vorticity profiles. Although, these outer modes are known to be of lesser importance in the near-field mixing, how these radiate to the far-field is uncertain, on which we focus primarily, when the mixing layer is supersonic, but also report subsonic cases. On keeping the flow compressibility fixed, the outer modes are realized via biasing the respective mean density of the fast (oxidizer) or slow (fuel) side. In the linearized model that we use, the mean flow are laminar solutions of two-dimensional compressible boundary layers with an imposed composite turbulent spread rate, which we show to correctly predict the growth of instability waves by saturating them earlier, similar to in non-linear calculations, but obtained here via solving the linear parabolized stability equations (PSE). The chemical reaction is modeled via a single-step, single-product overall process which introduces a heat release term in the mean temperature equation. As the flow parameters are varied, modes that are unstable on the slow side are shown to be more sensitive to heat release, potentially exceeding equivalent central modes, as these modes yield relatively compact sound sources with lesser spreading of the mixing layer, when compared to the corresponding fast modes. In contrast, the radiated sound, obtained directly from the PSE solutions, seems to be relatively unaffected by a variation of mixture equivalence ratio, except for a lean mixture which is shown to yield a pronounced effect on the slow mode radiation by reducing its modal growth. For subsonic mixing layers, the sensitivity of central mode is explored, which in addition requires an acoustic analogy based method (e.g. the Lilley–Goldstein equations) to predict the sound from the linearized PSE sources, as used here, unlike in supersonic cases
Structure of Turbulent, Swirling Round Jets
No full textThe present study deals with the numerical analysis of the effect of the swirl in the self-preservation region of the turbulent round jet. However, a large number of literature exists for the analysis of near-exit regions—very few deals with the self-preservation region of the jets far downstream. The present study attempts to provide insights into the effect of swirl on the turbulent mixing and jet spread rate by examining the self-similar solution in the far-field region of the jet. The study is divided into two main portions: a comparison of the turbulent swirling and non- swirling jets and the comparison between the turbulent jets having low to moderate values of swirls.
A standard computation for a non-swirling jet is used to validate the flow solver. Simulations are carried out at a Reynolds number of 2,400 for the top-hat velocity profile at the inlet. All flow characteristics are computed in detail and compare the results with existing DNS data. Velocity profiles at different streamwise locations collapse on a single curve and closely match the available data. The jet decay and spread rates also align with the standard computed data.
Large eddy simulation has been performed for non-swirl (S = 0), weak swirls (S = 0.3, 0.5) and moderate swirl (S = 0.7) at a Reynolds number of 11,000. In both the non-swirling and swirling cases, special care is taken to ensure that the computational domain is large enough to study the jet’s behaviour in a self-similar region. The research presents the effects of the swirl on a turbulent flow and compares the simulation results with available experimental data. Comparing the swirling and non-swirling cases indicates a changed turbulence structure to the effect that the swirling jet spreads and mixes faster than the non-swirling. With increasing degrees of swirl, the angle of spread of the jets is increased, and correspondingly, the decay of the maximum values of velocity components along the lengths of the jets is faster. Flow entrainment shows that the entrainment increases with swirls. The numerical simulations showed that the flow quickly achieved a self-similarity for the mean axial velocity. In contrast, the radial and azimuthal mean velocities reached a self-similar state after a longer period of jet development. Results of the decay of velocity and jet spread rate in the self-similar region of the swirling jet without vortex breakdown were found to vary linearly with the streamwise direction of the jet irrespective of the magnitude of swirl number, which is in line with the findings from experiments of Rose (1962), Chigier & Chervinsky (1967) & Pratte & Keffer (1972). In contrast, Craya & Darrigol (1967) has theoretically shown that axial velocity decay varies as three halves along the length of the jet. Additionally, mass flux shows higher mixing in swirling jets compared with non-swirling. The integrated axial fluxes of linear and angular momentums were conserved along the jet’s axis in the self-preserving region
On late stages of transition in round jets
No full textA round jet is one of the simplest canonical axisymmetric free shear flows that occur both
in engineering applications and natural phenomena. A laminar-turbulent transition process
is inherent to its development involving multiple stages of instabilities – a topic of intense
research due to its importance in mixing and noise generation. This study focuses on the
last two stages of this transition process, .i.e. instability of vortex rings and its breakdown
to turbulence. Vortex rings are subjected to two types of short wavelength azimuthal insta-
bilities: elliptic and curvature, with the former most commonly seen during the transition of
round jets. A global linear stability analysis was performed to examine elliptic instability,
and the results were compared with asymptotic theories and Direct Numerical Simulations
(DNS). The stability analysis explored two types of base flows: Gaussian and equilibrated
– a skewed Gaussian, which differ in their core vorticity distribution. The growth rates of
the stationary modes are found to be very sensitive to the vorticity distribution, in a way
not accounted for in the asymptotic theories. This is demonstrated by the equilibrated ring
results where a 9% difference in inviscid growth rates is observed for rings with the same
slenderness ratio but evolved with different Reynolds numbers, leading to slight variations
in their vorticity distribution. However, inviscid growth rates are very close to asymptotic
theory predictions if corrections for the ring radius evolution and equilibrated distribution
are considered, while for Gaussian rings, these are smaller by 19 − 33%. This difference in
Gaussian rings is attributed to the absence of a local contribution in the vorticity distribution
that comes from the deformation of the ring core due to the strain field of induced velocity,
which gets naturally included for equilibrated rings. The inclusion of viscosity reduces the
growth rates due to damping, which agree very well with those extracted from the DNS for
equilibrated rings, but there are differences for the Gaussian rings. Additionally, for viscous
cases non-zero frequency rotating modes were isolated for both the base flow types which
appeared in multiple branches, physically corresponding to both curvature and elliptic insta-
bilities.
The large-scale structures from the nonlinear evolution of elliptic instability modes were
next explored using DNS and linear stability analysis. The evolution of the most unstable
mode leads to the formation of an inner core, wrapped around by halo vorticity, which even-
tually breaks down due to nonlinear interactions, yielding a turbulent vortex ring with the
ejection of multiple hairpin vortices in the wake of the ring. The evolution of an isolated hairpin vortex was studied with other vortex structures during the transition modelled as a
background flow: a uniformly convecting stream and a uniform shear. In all the cases inves-
tigated in this work, the entire hairpin vortex moved upward due to its self-induced velocity,
and due to large curvature, its tip rose above its plane, resulting in the two legs approaching
each other upstream of the tip. Further evolution leads to the viscous vortex reconnection
process at the point of closest approach, which splits the hairpin into a vortex ring and a
second hairpin. In this study, with the addition of a background flow the reconnection plane
gets convected downstream, while three stages of the reconnection process: inviscid advec-
tion, bridging and threading, observed in other configurations, are identified. Reconnection
occurs early and at increasingly smaller timescales with an increase in Reynolds number,
while the addition of a positive and negative shear accelerates and decelerates the onset of
the reconnection process, unlike a uniformly convecting stream. The present work identifies
the vortex reconnection process as an important mechanism for the formation of small-scales
during the last stages of breakdown of a laminar vortex ring into turbulence
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
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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