1,720,985 research outputs found
Background-noise effects on modelling transitional shear flows
No full textPresented in this article is an experimental and numerical study aimed at analysing the wake dynamics downstream of a cylinder in an unsteady flow. The experimental study was carried out by measuring the mean velocity field with a laser Doppler anemometer, visualizing the fluid field by means of laser induced fluorescence, and performing a statistical analysis of the instantaneous velocity signals using Fourier and wavelet transforms. The visualized images were processed to determine the mean-statistical characteristics of large-scale structures in the wake. Experimental results provided the essential information for the numerical investigations of the temporal evolution of large-scale structures. A theoretical model based on the theory of dynamical systems with few degrees of freedom was adopted. The effects induced by the unsteady nature of the incoming flow were simulated. Experimental and numerical results were in excellent agreement, underlining the validity of the methods of analysis and simulation proposed
Effects of unsteadiness in transitional shear-flows: Application to a wake flow
No full textIn recent years, results from experimental and numerical investigations suggest that large scale dynamics plays a dominating role in transitional shear-flows. Many authors (Karniadakis et Al., 1992, Williamson et Al., 1993) have clearly shown that large scale structures can result from the instability of the mean velocity profile. These structures undergo deterministic evolution strongly affected by reciprocal non-linear interactions. In this context a dynamical system approach is a suitable tool to model large scale dynamics in transitional shear-flows because it is able to encode these deterministic processes. A dynamical system is presented, describing the local time evolution of large scale motions for a general three-dimensional disturbance superimposed to the basic velocity field. In this paper the model was applied to a two-dimensional cylinder wake in an unsteady mean flow. At this purpose a extension of the mathematical model was performed in order to achieve information on the effects of unsteadiness of the basic flow in transitional wake dynamics. Flow visualizations and velocity measurements were also carried out in a hydraulic tunnel. This produced useful information to select the main features of the large scale motion in the cylinder wake. A low dimensional system, obtained as a truncated form of the proposed model, was used to describe the dynamical behaviour of the wake. A good agreement was found between the experimental and numerical results. The role of a low frequency basic flow noise in the wake dynamics was described, thus confirming other authors findings (Miksad et Al., 1992)
Experimental investigation of 3D separation in a cylinder rough flat plate junction
No full textIn this paper the results of an experimental study concerning the separated flow region which develops upstream from a cylindrical obstacle vertically placed on a rough surface and subjected to a fully developed turbulent boundary layer, will be analyzed. This region is characterized by a strongly time-variant vortex system, whose main features were described by a great number of authors through velocity and pressure measurements and flow visualizations, for a wide range of Reynolds number. The experimental results were interpreted using the topological approach by Hunt et al. (1978), Baker (1980), Dargahai (1989). However for a turbulent inlet boundary layer the number of vortices observed is still a controversial issue. This type of flow occurs in several technical applications, e.g. local scouring at bridge piers. To this avail bed roughness could play an important role in defining the vortex system structure and stability. In this paper, the instantaneous characteristics of the vortical structure were highlighted by the flow visualizations. LDV measurements of the longitudinal and vertical velocity components were carried out for three different Reynolds numbers. In one of these conditions, the crosswise mean velocity component was computed and a statistical analysis of the instantaneous velocity signals was performed. Furthermore by applying the topological concepts of critical point and sectional streamline the main features of the separated flow were identified
On the role of hemodynamics in predicting rupture of the abdominal aortic aneurysm
No full textHemodynamics plays a crucial role in the growth of an abdominal aortic aneurysm (AAA) and its possible rupture. Due to the serious consequences that arise from the aneurysm rupture, the ability to predict its evolution and the need for surgery are of primary importance in the medical field. Furthermore, the presence of intraluminal thrombus (ILT) strongly affects the evolution of the pathology. In this study, we analyzed the influence of hemodynamics on the growth and possible rupture of AAAs. Numerical investigations of pulsatile non-Newtonian blood flow were performed in six patient-specific AAAs reconstructed from diagnostic images, having different sizes and shapes, and with or without ILT. Wall shear stress and vorticity distribution in the bulge and their evolution during the cardiac cycle were analyzed. The results indicate that blood flow dynamics acts synergistically with atherosclerotic degeneration in the development of the disease. The high surface complexity and tortuosity of the aneurysms significantly affect the blood motion, and the presence of inflection in the aneurysm centerline has a noticeable effect on the vortex dynamics. Links between regions of slow recirculating flows, low values of time-averaged wall shear stress, high values of oscillatory shear index, and zones of ILT deposition were found. In the absence of ILT, possible thrombus accumulation areas and consequent aneurysm growth were identified. The findings of this study highlight the importance of hemodynamics in assessing the vulnerability of the aortic wall and underline the crucial role of patient-specific investigations in predicting the rupture of individual aneurysms
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
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