1,720,957 research outputs found
An experimental analysis of dispersion and diffusion processes in compound channels
No full textThe analysis of mixing processes in natural streams is very important to understand and predict
water contamination and nutrient dispersion. Pollutant behavior is usually modeled by a standard
advection-diffusion equation for the concentration. This equation allows an adequate description
of the evolution of the passive tracers once the diffusivity tensor has been provided, estimation
of which strongly depends on the local character of the flow field.
An experimental campaign based on Particle Image Velocimetry (PIV) measurements of free-
surface velocities forms the basis for an analysis of mixing processes which occur in a compound
channel flow. The flow mixing is analyzed from a Lagrangian point of view, i.e. in terms of single
and multiple particle statistics (absolute and relative dispersion and diffusivity) and of the related
mean flow characteristics. In the present study the asymptotic behavior of mixing characteristics
for larger times, once the ballistic regime has been attained, has been analyzed in terms of the
absolute diffusivity in order to characterize typical values of longitudinal and transversal
dispersion coefficients. Different series of experiments (related to the ratio r h between the main
channel flow depth and the floodplain depth, which classifies the flow in Shallow, Intermediate
and Deep, and Fr the Froude number) have been performed and the asymptotic value of the
absolute diffusivity has been evaluated. The present study highlights a stronger dependence of
coefficients on the main flow characteristics. The results are compared with several dispersion
analyses for both the longitudinal dispersion coefficient and the transverse turbulent mixing
coefficient presented in the literature
Lagrangian mixing in Compound Channels
No full textIn a recent study Stocchino & Brocchini (2010) have investigated the dynamics of two dimensional large-scale vortices with vertical axis evolving in a straight compound channel under quasi-uniform °ow conditions. They have showed that properties of quasi two dimensional turbulence strongly depend on the ratio rh between the main channel flow depth (h_mc) and the floodplain flow depth (h_f p). In the case of shallow flows, when the flow depth in the main channel largely exceeds the depth in the flood-plains, the macrovortices are generated owing to the vorticity production concentrated at the transition region, i.e. between the main channel and the floodplains. The source of vorticity was found to be related to the depth jump at the transition. Another fundamental property of this class of flow is that once the turbulence is fully developed, the typical size of the macrovortices is independent of the streamwise coordinate. In the present contribution, we discuss the results of a new extensive experimental campaign in terms of Lagrangian properties of the °ow. Di®erent Lagrangian measures of
mixing (i.e. single and multiple particle statistics, FSLE) are introduced with the aim to characterize the mixing processes that occur in a compound channel for varying depth ratio (rh) and Froude number (Fr). The results suggest that the presence of the macrovortices at the transition region between the main channel and the °ood plains delays the establishment of fully developed conditions, increasing the decorrelation time beyond which a Brownian regime is recovered. Moreover, the growth in time of the absolute dispersion is strongly influenced by the presence of long-lived macrovortices causing a non monotonic behaviour Moreover, the dependence of the relative diffusivity on the pair separation shows different behaviours as the flow ratio rh is increased characterized by a local dynamics
Transversal and longitudinal mixing in compound channels.
No full textAn experimental campaign, based on particle image velocimetry (PIV) measurements of free-surface velocities, forms the basis for an analysis of the mixing processes which occur in a compound-channel flow. The flow mixing is characterized in terms of Lagrangian statistics (absolute dispersion and diffusivity) and of the related mean flow characteristics. Mixing properties strongly depend on the ratiorh between the main channel flow depth (h*mc) and the floodplain depth (h*fp), and three flow classes can be identified, namely shallow, intermediate, and deep flows. In the present study the large time asymptotic behavior of the mixing characteristics is analyzed in terms of the absolute diffusivity in order to characterize typical values of longitudinal and transversal diffusivity coefficients. Various sets of experiments, which cover a wide range of the governing physical parameters, have been performed and the asymptotic values of the absolute diffusivity have been evaluated. The results are then compared with several studies of flow dispersion for both the longitudinal diffusivity coefficient and the transversal turbulent mixing coefficient. The present results highlight a stronger dependence of such coefficients with the flow-depth ratio than with the flow regime (Froude number)
Lagrangian mixing in straight compound channels
No full textRecently Stocchino & Brocchini (J. Fluid Mech., vol. 643, 2010, p. 425 have studied the dynamics of two-dimensional (2D) large-scale vortices with vertical axis evolving in a straight compound channel under quasi-uniform flow conditions. The mixing processes associated with such vortical structures are here analysed through the
results of a dedicated experimental campaign. Time-resolved Eulerian surface velocity fields, measured using a 2D particle-image velocimetry system, form the basis for
a Lagrangian analysis of the dispersive processes that occur in compound channels when the controlling physical parameters, i.e. the flow depth ratio (rh) and the Froude number (Fr) are changed. Lagrangian mixing is studied by means of various approaches based either on single-particle or multiple-particle statistics (relative and absolute statistics, probability density functions (p.d.f.s) of relative displacements and finite-scale Lyapunov exponents). Absolute statistics reveal that transitional macrovortices, typical of shallow flow conditions, strongly influence the growth in time of the total absolute dispersion, after the initial ballistic regime, leading to a nonmonotonic behaviour. In deep flow conditions, on the contrary, the absolute dispersion displays a monotonic growth because the generation of transitional macrovortices does not take place. In all cases an asymptotic diffusive regime is reached. Multiple-particle dynamics is controlled by rh and Fr. Different growth regimes of the relative diffusivity have been found depending on the flow conditions. This behaviour can be associated with different energy transfer processes and it is further confirmed by the p.d.f.s of relative displacements, which show a different asymptotical
shape depending on the separation scales and the Froude number. Finally, an equilibrium regime is observed for all the experiments by analysing the decay of the finite-scale Lyapunov exponents with the particle separations
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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