1,721,033 research outputs found
Transient growth mechanisms of low Reynolds number flow over a low-pressure turbine blade
No full textA direct transient growth analysis for three-dimensional perturbations to flow past a periodic array of T-106/300 low-pressure turbine fan blades is presented. The methodology is based on a singular value decomposition of the flow evolution operator, linearised about a steady or periodic base flow. This analysis yields the optimal growth modes. Previous work on global mode stability analysis of this flow geometry showed the flow is asymptotically stable, indicating a non-modal explanation of transition may be more appropriate. The present work extends previous investigations into the transient growth around a steady base flow, to higher Reynolds numbers and periodic base flows. It is found that the notable transient growth of the optimal modes suggests a plausible route to transition in comparison to modal growth for this configuration. The spatial extent and localisation of the optimal modes is examined and possible physical triggering mechanisms are discussed. It is found that for longer times and longer spanwise wavelengths, a separation in the shear layer excites the wake mode. For shorter times and spanwise wavelengths, smaller growth associated with excitation of the near wake are observed
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
Transient growth analysis of the flow past a circular cylinder
Full text linkWe apply direct transient growth analysis in complex geometries to investigate its role in the primary and secondary bifurcation/transition process of the flow past a circular cylinder. The methodology is based on the singular value decomposition of the Navier-Stokes evolution operator linearized about a two-dimensional steady or periodic state which leads to the optimal growth modes. Linearly stable and unstable steady flow at Re=45 and 50 is considered first, where the analysis demonstrates that strong two-dimensional transient growth is observed with energy amplifications of order of 10(3) at U-infinity tau/D approximate to 30. Transient growth at Re=50 promotes the linear instability which ultimately saturates into the well known von-Kaacutermaacuten street. Subsequently we consider the transient growth upon the time-periodic base state corresponding to the von-Kaacutermaacuten street at Re=200 and 300. Depending upon the spanwise wavenumber the flow at these Reynolds numbers are linearly unstable due to the so-called mode A and B instabilities. Once again energy amplifications of order of 10(3) are observed over a time interval of tau/T=2, where T is the time period of the base flow shedding. In all cases the maximum energy of the optimal initial conditions are located within a diameter of the cylinder in contrast to the spatial distribution of the unstable eigenmodes which extend far into the downstream wake. It is therefore reasonable to consider the analysis as presenting an accelerator to the existing modal mechanism. The rapid amplification of the optimal growth modes highlights their importance in the transition process for flow past circular cylinder, particularly when comparing with experimental results where these types of convective instability mechanisms are likely to be activated. The spatial localization, close to the cylinder, of the optimal initial condition may be significant when considering strategies to promote or control shedding
On the change in flow dynamics of a transonic open cavity with trailing edge modifications
No full textThe transonic flow past a cavity is encountered in various mechanical and aerospace problems. A sharp trailing edge from a rectangular cavity produces pressure pulses when impinged by a shear layer. Blunting the trailing edge sharpness reduces the discrete tone emanation. Here, two geometrical configurations are investigated. The base cavity dimension is [L/D] = 3.58 (length-To-depth ratio), and the freestream Mach number is M∞ = 0.8. Attenuation of the pressure pulses in the cavity with elliptic trailing edge is probed through direct numerical simulations coupled with BiGlobal stability analysis at a low Reynolds number (ReD = 1000) and limited experiments at a high Reynolds number (ReD = 3 × 105)
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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