1,721,094 research outputs found
Design and testing of transonic linear cascade tunnel with optimized slotted walls
No full textIn linear cascade wind tunnel tests, a high level of pitchwise periodicity is desirable to reproduce the azimuthal periodicity in the stage of an axial compressor or turbine. Transonic tests in a cascade wind tunnel with open jet boundaries have been shown to suffer from spurious waves, reflected at the jet boundary, that compromise the flow periodicity in pitch. This problem can be tackled by placing at this boundary a slotted tailboard with a specific wall void ratio s and pitch angle a. The optimal value of the s-a pair depends on the test section geometry and on the tunnel running conditions. An inviscid two-dimensional numerical method has been developed to predict transonic linear cascade flows, with and without a tailboard, and quantify the nonperiodicity in the discharge. This method includes a new computational boundary condition to model the effects of the tailboard slots on the cascade interior flow. This method has been applied to a six-blade turbine nozzle cascade, transonically tested at the University of Leicester. The numerical results identified a specific slotted tailboard geometry, able to minimize the spurious reflected waves and regain some pitchwise flow periodicity. The wind tunnel open jet test section was redesigned accordingly. Pressure measurements at the cascade outlet and synchronous spark schlieren visualization of the test section, with and without the optimized slotted tailboard, have confirmed the gain in pitchwise periodicity predicted by the numerical model
WAVE REFLECTION ON POROUS WALLS: NUMERICAL MODELLING AND APPLICATION TO TRANSONIC WIND TUNNELS
No full textTransonic wind tunnel tests are often affected by wave reflections
stemming from the edge of the test section that may be attenuated by
using perforated end walls. In this work, a numerical model is
presented that includes a novel boundary condition for thick walls,
that can be used to enhance perforated wind tunnel test section
designs.
An in-house finite volume scheme for compressible inviscid flows is
enhanced by the addition of a new perforated wall boundary condition.
This wall condition is based on the assumption that the wall
thickness is larger than the perforation size or diameter, which is
common of transonic wind tunnels with single leaf walls. The model
was validated against a simple oblique shock reflection test case.
The model predictions were shown to be an improvement with respect
to those from a benchmark perforated wall boundary condition that
assumes a small wall thickness in relation to the perforation size.
A numerical and experimental study was conducted of the end wall
effects in a chocked nozzle turbine linear cascade, discharging at
Mach 1.27. Without a perforated wall, the cascade displays strong
end wall trailing edge shock reflections, giving a pitchwise
non-periodic discharge. A numerical model flow
with a 50% void ratio tailboard indicated some regain in flow
periodicity, as verified by experiment. The thick wall model better
reproduced the flow features documented in the flow visualisations
than the benchmark model, indicating that this is a better boundary
condition to enhance perforated tailboard designs for compressible
model flows
Time accurate numerical study of turbulent supersonic jets
No full textA time accurate numerical study is presented of an over-expanded Mach 2 circular turbulent jet in which the flow is assumed axisymmetric. The focus of this investigation is on the jet screech phenomenon resulting from the interaction between the large-scale turbulent mixing region instabilities and the regular spacing of the shock wave-expansion system, (shock cells), in the over-expanded jet. The solution is obtained of the "short" time-dependent Reynolds averaged Navier–Stokes equations (TRANS), using a two-equation, k–?, turbulence model. The time accurate method was first calibrated for the given model geometry when the flow was fully expanded, and the resulting mean flow characteristics were compared with experimental data. The results were in broad agreement for the first 10 diameters of the jet downstream of the exit. Further downstream the time-averaged axial velocity decayed at a slightly faster rate than in the experiments. In an ideal inviscid fully expanded jet no shock cells would be present but in the turbulent jet calculations weak shock cells appeared which gradually died out beyond about 10 diameters from the nozzle exit. The calculated non-dimensional time-averaged transverse velocity profiles showed self-similarity, when allowance was made for the false origin of the shear layer, in agreement with the measured results.In the calculations for the over-expanded jet it was found, in agreement with experimental data, that the interaction between shock cell modulated instability waves and the shock-expansion system generated jet screech. It was found, as part of the screech phenomenon, that the shocks and the shock cells oscillated over a small distance which increased from the axis to a maximum within the shear layer. This shock unsteadiness resulted in the shocks being smeared when viewed in the equivalent steady flow calculations
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
Slot width augmentation in a slotted-wall transonic linear cascade wind tunnel
No full textTransonic tests in linear cascade wind tunnels can suffer from
significant test section boundary interference effects in pitch.
A slotted tailboard has been designed and optimised with an
in-house Euler numerical method to reduce such effects. Wind
tunnel measurements on an overspeed Mach 1.27 discharge from a
Rolls-Royce T2 cascade, featuring strong end-wall shock-induced
interference, showed a 77% reduction in the flow pitchwise
periodicity error with the optimised tailboard, with respect to
the baseline open-jet cascade flow. Two-dimensional Euler
predictions were also cross-validated against a three-dimensional
Reynolds averaged computation, to explore the three-dimensionality
of the discharge
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