1,721,109 research outputs found
On the merging of turbulent spots in a supersonic boundary-layer flow
No full textThe complex transition flow physics associated with the merging of turbulent spots in a Mach 2 boundary-layer has been studied using direct numerical simulation. Dynamics of an isolated turbulent spot, merging of laterally displaced spots, and merging of two spots in tandem are considered. The coherent structures associated with the wingtip region of the spot are found to play a major role in destabilising the surrounding laminar fluid. In the merging of laterally displaced spots a strong velocity defect, resulting in unstable inflectional velocity profiles, is observed in the interaction zone. These local inflectional instabilities within the interaction region trigger new large scale coherent structures. During the inline merging, the calmed region behind the tail of the downstream spot is found to suppress the growth of the upstream spot. The upstream spot is ultimately engulfed by the downstream spot
Effect of Mach number on the structure of turbulent spots
Full text linkDirect numerical simulations have been performed to study the dynamics of isolated turbulent spots in compressible isothermal-wall boundary layers. Results of a bypass transition scenario at Mach 2, 4 and 6 are presented. At all Mach numbers the evolved spots have a leading-edge overhang, followed by a turbulent core and a calmed region at the rear interface. The spots have an upstream-pointing arrowhead shape when visualized by near-wall slices, but a downstream-pointing arrowhead in slices away front the wall. The lateral spreading of the spot decreases substantially with the Mach number, consistent with a growth mechanism based on the instability of lateral shear layers. Evidence for a supersonic (Mach) mode substructure is found in the Mach 6 case, where coherent spanwise structures are observed under the spot overhang region
Turbulent spots in a compressible boundary layer flow
No full textDirect simulation of an isolated turbulent spot in a compressible isothermal wall boundary-layer flow has been performed. A bypass transition scenario at Mach 2,4 and 6 is considered. The flow field associated with the transitional and turbulent spots is studied in detail, with results in broad agreement with previous experimental work. The evolved spots are found to have an arrowhead shaped front with a leading edge overhang, followed by a turbulent core and a calmed region at the rear interface. The lateral spreading of the spot is found to decrease substantially with the flow. Evidence for a supersonic (Mack) mode is found in the Mach 6 case: spanwise-coherent structures are observed under the spot overhang region
Turbulent spot/separation bubble interactions in a spatially evolving supersonic boundary-layer flow
No full textThe growth and breakdown of localised disturbances into a turbulent spot in a Mach 2 boundary-layer flow have been investigated by direct numerical simulation. In particular, the interaction of a turbulent
spot with a shock-induced separation bubble is studied. The primary vortex structures triggered by the injected low momentum fluid and the metamorphosis of the hairpin structures into a turbulent spot are
clearly identified in the present study. The spot-separation bubble interaction is found to enhance the breakdown of the hairpin structures i.e. is capable of advancing the transition process. A substantial
increase in the lateral spreading of the spot is observed due to the spot/bubble interaction. Locally averaged profiles of the flow quantities within the spot show behaviour similar to developed turbulent flows
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
Shock-wave/boundary-layer interactions in a model scramjet intake
No full textAn air intake for a Mach 8 flight vehicle concept has been studied using large-eddy simulation with flow conditions corresponding to typical wind-tunnel tests. The flow contains several types of shock-wave/boundary-layer interaction for which large-eddy simulation has advantages over conventional Reynolds-averaged Navier-Stokes approaches. Laminar-to-turbulent transition was triggered using localized blowing trips in the first external ramp surface close to the intake leading edge. The trips lead to turbulent spots that propagate within an otherwise laminar flow, with a lateral spreading angle consistent with previous studies. The transitional/turbulent shock interaction near the. first compression corner was found to enhance the transition to turbulence, leading to a fully turbulent boundary layer after the interaction. Further downstream, a large separation zone forms due to the cowl-lip shock wave impinging on the vehicle-side boundary layer. Perturbations from the vehicle-side turbulent boundary layer were found to enter the cowl-side boundary layer. The final transition of this boundary layer occurred at the end of a separation bubble created by the first cowl-side compression ramp. Statistics for the turbulent How entering the combustor inlet show thick nonequilibrium turbulent boundary layers and trapped compression waves
Strong interaction of a turbulent spot with a shock-induced separation bubble
Full text linkDirect numerical simulations have been conducted to study the passage of a turbulent spot through a shock-induced separation bubble. Localized blowing is used to trip the boundary layer well upstream of the shock impingement, leading to mature turbulent spots at impingement, with a length comparable to the length of the separation zone. Interactions are simulated at free stream Mach numbers of two and four, for isothermal (hot) wall boundary conditions. The core of the spot is seen to tunnel through the separation bubble, leading to a transient reattachment of the flow. Recovery times are long due to the influence of the calmed region behind the spot. The propagation speed of the trailing interface of the spot decreases during the interaction and a substantial increase in the lateral spreading of the spot was observed. A conceptual model based on the growth of the lateral shear layer near the wingtips of the spot is used to explain the change in lateral growth rat
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
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