1,721,037 research outputs found
Numerical simulation of fully-developed compressible flows over wavy surfaces
No full textRough surfaces are common on high-speed vehicles, for example on heat shields, but compressibility isnot usually taken into account in the flow modelling other than through the mean density. In the presentstudy, supersonic fully-developed turbulent rough wall channel flows are simulated using direct numericalsimulation to investigate whether strong compressibility effects significantly alter the mean flow andturbulence properties across the channel. The simulations were run for three different Mach numbersM = 0.3, 1.5 and 3.0 over a range of wall amplitude-to-wavelength ratios from 0.01 to 0.08, correspondingto transitionally and fully rough cases respectively. The velocity deficit values are found to decrease withincreasing Mach number. It is also found that at Mach 3.0 significant differences occur in the mean flowand turbulence statistics throughout the channel and not just in a roughness sublayer. These differencesare found to be due to the presence of strong shock waves created by the peaks of the roughnesselements
On the Mach number and temperature dependence of jet noise: Results from a simplified numerical model
No full textNumerical simulations of sound radiation from perturbed round jets are used, firstly toexplore the structure of the sound sources and then to carry out a parametric study ofthe effect of jet Mach number and jet temperature. The simplified model problemincludes a steady base jet flow, maintained in the absence of disturbances, superimposedwith instability waves that are free to interact nonlinearly. Simulations over arange of subsonic jet Mach numbers show that a nonlinear mechanism dominates overa linear mechanism for low-frequency sound radiation, while for supersonic Machnumbers the linear mechanism is dominant. Additional insight is gained from afrequency-wavenumber analysis, including a transformation in the radial direction.With this decomposition, the acoustic field is located by the arc of a circle in plots ofradial against streamwise wavenumber for discrete frequencies. The transformation isapplied to both the pressure field, showing the sound directivity, and to selected sourceterms, showing characteristic directivity patterns for the streamwise and radialquadrupole terms. Decreasing the Mach number leads to a reduction in amplitude ofthe sources and of the sound radiation. Simulations with broadband forcing show thatthe qualitative effects of Mach number and jet heating are captured by this approach,which requires less resolution than a direct numerical simulation. A significant increasein the strength of the acoustic radiation for cold jets is observed, which is worthy offurther investigation
Laminar-turbulent transition induced by a discrete roughness element in a supersonic boundary layer
Full text linkThe linear instability and breakdown to turbulence induced by an isolated roughnesselement in a boundary layer at Mach 2:5, over an isothermal flat plate withlaminar adiabatic wall temperature, have been analysed by means of direct numericalsimulations, aided by spatial BiGlobal and three-dimensional parabolized (PSE-3D)stability analyses. It is important to understand transition in this flow regime sincethe process can be slower than in incompressible flow and is crucial to predictionof local heat loads on next-generation flight vehicles. The results show that theroughness element, with a height of the order of the boundary layer displacementthickness, generates a highly unstable wake, which is composed of a low-velocitystreak surrounded by a three-dimensional high-shear layer and is able to sustain therapid growth of a number of instability modes. The most unstable of these modes areassociated with varicose or sinuous deformations of the low-velocity streak; they area consequence of the instability developing in the three-dimensional shear layer as awhole (the varicose mode) or in the lateral shear layers (the sinuous mode). The mostunstable wake mode is of the varicose type and grows on average ?17% faster thanthe most unstable sinuous mode and ?30 times faster than the most unstable boundarylayer mode occurring in the absence of a roughness element. Due to the high growthratesregistered in the presence of the roughness element, an amplification factor ofN D 9 is reached within ?50 roughness heights from the roughness trailing edge. Theindependently performed Navier–Stokes, spatial BiGlobal and PSE-3D stability resultsare in excellent agreement with each other, validating the use of simplified theories forroughness-induced transition involving wake instabilities. Following the linear stagesof the laminar–turbulent transition process, the roll-up of the three-dimensional shearlayer leads to the formation of a wedge of turbulence, which spreads laterally at a ratesimilar to that observed in the case of compressible turbulent spots for the same Machnumber
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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