1,720,994 research outputs found
Propagation of viscous gravity currents inside confining boundaries: the effects of fluid rheology and channel geometry
No full textA theoretical and experimental investigation of the propagation of free-surface, channelized viscous gravity currents is conducted to examine the combined effects of fluid rheology, boundary geometry and channel inclination. The fluid is characterized by a power-law constitutive equation with behaviour index n. The channel cross section is limited by a rigid boundary of height parametrized by k and has a longitudinal variation described by the constant b ≥ 0. The cases k 1 are associated with wide, triangular and narrow cross sections. For b>0, the cases k ≷ 1 describe widening channels and squeezing fractures; b=0 implies a constant
cross-sectional channel. For a volume of released fluid varying with time like tα, scalings for current length and thickness are obtained in self-similar
forms for horizontal and inclined channels/fractures. The speed, thickness and aspect ratio of the current jointly depend on the total current volume (α), the
fluid rheological behaviour (n), and the transversal (k) and longitudinal (b) geometry of the channel. The asymptotic validity of the solutions is limited to
certain ranges of parameters. Experimental validation was performed with different fluids and channel cross sections; experimental results for current radius and
profile were found to be in close agreement with the self-similar solutions at intermediate and late times
Non-Newtonian power-law gravity currents propagating in confining boundaries
Full text linkThe propagation of viscous, thin gravity currents of non-Newtonian liquids in horizontal and inclined channels with semicircular and triangular cross-sections is investigated theoretically and experimentally. The liquid rheology is described by a power-law model with flow behaviour index n, and the volume released in the channel is taken to be proportional to ta, where t is time and a is a non-negative constant.
Some results are generalised to power-law cross-sections. These conditions are representative of environmental flows, such as lava or mud discharges, in a variety of conditions. Theoretical solutions are obtained in self-similar form for horizontal channels, and with the method of characteristics for inclined channels. The position of the current front is found to be a function of the current volume, the liquid rheology,
and the channel inclination and geometry. The triangular cross-section is associated with the fastest or slowest propagation rate depending on whether a < ac or a > ac. For horizontal channels, ac = n=(n+1) < 1, whereas for inclined channels, ac = 1, irrespective of the value of n. Experiments were conducted with Newtonian and power-law liquids by independently measuring the rheological parameters and releasing currents with constant volume (a = 0) or constant volume flux (a = 1) in right triangular and semicircular channels. The experimental results validate the model for horizontal channels and inclined channels with a = 0. For tests in inclined channels with a = 1, the propagation rate of the current front tended to lower values than predicted, and different flow regimes were observed, i.e., uniform flow with normal depth or instabilities resembling roll waves at an early stage of development.
The theoretical solution accurately describes current propagation with time before the transition to longer roll waves. An uncertainty analysis reveals that the rheological parameters are the main source of uncertainty in the experiments and that the model is most sensitive to their variation. This behaviour supports the use of carefully designed laboratory experiments as rheometric tests
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
Gravity-driven flow of non-Newtonian fluids in heterogeneous porous media: a theoretical and experimental analysis
Full text linkA theoretical and experimental analysis of non-Newtonian gravity-driven flow in porous media with spatially variable properties is presented. The motivation for our study is the rheological complexity exhibited by several environmental contaminants (wastewater sludge, oil pollutants, waste produced by the minerals and coal industries) and remediation agents (suspensions employed to enhance the efficiency of in-situ remediation).
Natural porous media are inherently heterogeneous, and this heterogeneity influences the extent and shape of the porous domain invaded by the contaminant or remediation agent. To grasp the combined effect of rheology and spatial heterogeneity, we consider: a) the release of a thin current of non-Newtonian power-law fluid into a 2-D, semi-infinite and saturated porous medium above a horizontal bed; b) perfectly stratified media, with permeability and porosity varying along the direction transverse (vertical) or parallel (horizontal) to the flow direction. This continuous variation of spatial properties is described by two additional parameters.
In order to represent several possible spreading scenarios, we consider: i) instantaneous injection with constant mass; ii) continuous injection with time-variable mass; iii) instantaneous release of a mound of fluid, which can drain freely out of the formation at the origin (dipole flow).
Under these assumptions, scalings for current length and thickness are derived in self similar form. An analysis of the conditions on model parameters required to avoid an unphysical or asymptotically invalid result is presented.
Theoretical results are validated against multiple sets of experiments, conducted for different combinations of spreading scenarios and types of stratification. Two basic setups are employed for the experiments: I) direct flow simulation in an artificial porous medium constructed superimposing layers of glass beads of different diameter; II) a Hele-Shaw (HS) analogue made of two parallel plates set at an angle. The HS analogy is extended to power-law fluid flow in porous media with variable properties parallel or transverse to the flow direction.
Comparison with experimental results show that the proposed models capture the propagation of the current front and the current profile at intermediate and late time
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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