1,720,963 research outputs found
Control of transonic buffet through deployable flaps
No full textWe carry out a series of numerical simulations of transonic flow over a
supercritical airfoil at M=0.7, incidence angle alpha=7°, and chord Reynolds
number Rec=300000, with the aim of suppressing the large-scale shock oscillations (buffet) which are expected to occur under these conditions. For that purpose we use a deployable flap, which is placed in the aft part of the airfoil suction side. The main rationale is to mimic the behavior of shock holders, which are frequently used in experiments to stabilize normal shock waves. For preliminary simulations, the flap has been shaped as a bi-convex airfoil. Two sample configurations have been considered, namely a HIGH configuration, with flap chord cf=0.2c, relative thickness of 6%, and distance of about 0.15c from the airfoil, and a LOW configuration, with flap chord cf=0.1c, relative thickness of 12%, and distance of about 0.075c from the airfoil. The main result is that both the HIGH and the LOW configuration are capable of effectively suppressing buffet, after an initial transient. Both types of control have the main effect of pushing the main shock upstream than its time-average uncontrolled location. However, whereas LOW actuation has the effect of slightly extending the region of flow reversal past the airfoil trailing edge, HIGH actuation is effective in reducing its size. Accompanied with HIGH actuation also comes some small flow separation past the flap, whereas in the LOW case the flap is embedded in a low-speed region, and the flow does not separate. The practical implications of control are observed in the time history of the aerodynamic efficiency. While the efficiency levels off in both cases, LOW actuation retains the (time-average) efficiency of the uncontrolled case, whereas HIGH actuation is responsible for additional losses. A series of numerical simulations covering a wide range of control parameters is currently ongoing, a full account of which will be given in the final paper
Numerical study of transonic buffet on supercritical airfoil with different boundary layer states
Full text linkAccurate numerical simulations of flow over airfoils play an increasingly
important role in the design of aircraft major components such as wings and turbo-
machinery blades. These lifting devices often operate in demanding aerodynamic
conditions for optimum performances, and may experience the presence of shock
waves in operating conditions. Shocks may become unsteady under specific
conditions, undergoing a large-scale, low-frequency periodic motion, which affects
the entire flow-field. This unsteady phenomenon, named transonic buffet, is the
subject of the present numerical investigation, with an oscillating shock over the
suction side of the airfoil.
In this study, a range of transonic Mach numbers and angles of incidence are
considered, but the bulk of the analysis is carried out for flow conditions at free-
stream Mach number M∞ = 0.7 and angle of incidence α = 7°, which show
well established buffet. Large-eddy simulations (LES) with natural and forced
transition carried out at chord Reynolds number Re = 3000000 clearly highlight
the effects of the incoming boundary-layer state on the shock oscillations. While a
laminar upstream boundary layer yields weak oscillations of the shock, a turbulent
incoming boundary layer yields significant buffet. The LES database has been used
to establish veracity (or not) of suggested buffet pathways, mainly based on the
alleged existence of an acoustic feedback loop. This mechanism is actually found
to consist of two separate patterns: coherent pressure disturbances convected from
the shock to the trailing edge, and acoustic waves scattered at the trailing edge,
feeding the shock motion. Additional exploration of the pressure side role in the
unsteadiness reveals that is has but marginal effect on the phenomenon.
Direct numerical simulations (DNS) at lower Reynolds number (Re = 300000)
suggest a reversal in the previously observed trend. In this case, a laminar incoming
boundary layer yields stronger buffet as compared to its turbulent counterpart,
highlighting strong dependence of the buffet phenomenon on the Reynolds number
when natural transition is considered. In order to passively control buffet, we
consider devices whose design is similar to large-eddy break-up devices (LEBU),
consisting of a thin circular-arc airfoil placed between shock and trailing edge, with
the main goal of: i) breaking the eddies originating at the shock, responsible for
the acoustic scattering at the trailing edge; ii) manipulating the acoustic field in the
aft part of the airfoil. RANS simulations show potential for this kind of device for
complete stabilization of buffet. On the other hand, DNS shows that the device is
able to curtail the buffet, but not to eliminate it. Additional tests are needed in
order to assess the effectiveness of the control device, whose practical impact might
be very larg
Reynolds-averaged numerical simulations of conical shock-wave/boundary-layer Interactions
No full textWe carry out a parametric study of conical shock-wave/turbulent boundary-layer interactions by means of
numerical simulation of the Reynolds-averaged Navier–Stokes (RANS) equations, with the eventual goal of
establishing the predictive capabilities of standard turbulence models. Preliminary assessment of several linear-
eddy-viscosity models for the case of planar interactions shows that the k − ε model and its variants as well as the
Spalart–Allmaras model yield accurate prediction of the typical interaction length scale. Numerical simulations of
conical interactions at high Reynolds numbers under attached and separated flow conditions generally support good
capability of RANS to predict the gross flow features, including conditions of incipiently and fully separated flow.
Comparison with direct numerical simulation data at low Reynolds numbers suggests that caution should be made
because certain turbulence models may yield unrealistic incoming velocity profiles. With this caveat, we again find
that RANS models yield satisfactory prediction of the three-dimensional flow organization and associated length
scales. The N-wave mean wall pressure signature is quantitatively predicted, and accurate information about the
fluctuating pressure variance can be obtained from RANS data in a postprocessing stage by extension of correlations
developed for low-speed separation bubbles
Scrutiny of buffet mechanisms in transonic airfoils
No full textWe carry out numerical simulations of transonic flow around a supercritical airfoil at M = 0.7, incidence angle 7 deg, and Reynolds number Re = 3000000, with the aim of elucidating the mechanisms responsible for large-scale shock oscillations (buffet) which are expected to occur under these conditions. An implicit large-eddy simulation (ILES) is carried out with an upstream laminar boundary layer. Companion Reynolds-Averaged Navier-Stokes (RANS) and detached-eddy simulations (DDES) are also carried out to ascertain the feasibility of the various buffet mechanisms. Estimations of the propagation velocities of coherent disturbances and of acoustic waves are obtained, to be inserted in standard predictive models. It appears that the dynamics underlying buffet is highly localized around the mean shock location
Numerical experiments on transonic buffet
No full textWith the aim of investigate the effects of acoustic waves on the transonic buffet phenomenon, unsteady Reynolds-averaged Navier-Stokes simulations (URANS) have been carried out for the flow over a supercritical airfoil by means of an energy-consistent finite-volume solver, at chord Reynolds number Re = 3000000, free-stream Mach number M = 0.7, and angle of incidence of 7 deg. In our previous work [3AF International Conference on Applied Aerodynamics, (2016)] we have shown that URANS contains the essential physical ingredients to make up for transonic buffet. The propagation velocities of coherent disturbances are estimated by cross-correlating the pressure signal at different points, to quantify the actual occurrence of alleged influence of acoustic waves on turbulence and vice-versa. To ascertain whether (as subtended by standard models) buffet involves an acoustic feedback loop, we perform numerical experiments by artificially imposing numerical barriers to upstream propagating acoustic waves, either on the suction or on the pressure side of the airfoil. The results of the numerical experiments are compared with URANS results in the absence of acoustic barriers, in terms of mean and root-mean-square wall-pressure
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
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