1,720,959 research outputs found
Investigating hydrodynamics and turbulent effects in rivers for different flow conditions using spatial complexity metrics
No full textIn non-uniform river flows, hydrodynamic features, such as gradients of velocity in flow directions, are of particular importance for explaining the abundance of aquatic habitats. Hydraulic complexity metrics referred to as M1, M2, and M3 play an important role when it comes to the analysis of habitat metrics on a 3D spatial level. Parameter M1 is proportional to the drag force experienced by an organism, parameter M2 represents how much more energy an organism must expend if it moves from the lower velocity to the higher velocity location, and parameters M3 illustrates the circulation in flow. The specific aim of the present study is to apply those parameters to characterize, under different flow conditions, the cross-sectional distribution of kinetic energy and coherent structures which are both relevant for many aquatic organisms. To this aim, laboratory data as well as field observations along Tiber River, in central Italy, were considered and the hydraulic complexity metrics were investigated in dimensionless form. On the laboratory-scale, the dimensionless parameters M1*, M2* and M3* identify the velocity gradient related to the high/low cross-sectional velocity and the high/low vorticity areas in selected cross-sections along the flume. Then, the evaluation of the parameter M2* in the horizontal plane allows to verify the relation between the localization of the high/low kinetic energy areas in the longitudinal direction. For the field-scale, the parameters were investigated, under high, moderate and low flow conditions, in a gauged site in the Tiber River. The results indicate that an aquatic organism should spend more energy where M3*, which is related to flow circulation, assumes high values. Furthermore, significant values of M1* and M2* are observed, which are linked to gradients in the cross-sectional distribution of velocity. These values are predominantly found at the river centerline for M1* and at the banks for M2*. In terms of echo-hydraulics, the results based on both laboratory and field data indicate that they are complementary, showing that for the larger magnitudes of M1 and M3, which are related to the kinetic energy and flow circulation, respectively, an aquatic organism should spend more energy in these zones. Overall, the results based on both laboratory and field studies suggest that parameters M1 and M2 are inversely linked, i.e., M1 decreased with increasing M2, while, there is no relationship between parameters M3 and M1. The findings of the present research would be of particular interest in quantifying biologically important flow patterns occurring at different spatial scales within different streams and under different flow conditions
Dam-break flow dynamics over a stepped channel with vegetation
No full textIn spite of the insistence of a variety of studies on floods triggered by a dam failure, the effects of channel unevenness and vegetation have not been fully explored. Some hydrological aspects, such as the sudden change in the topography of a river section, the density of vegetation, and its influence on flood development, need to be further addressed. Therefore, the present work investigates the complex effects of vegetation and channel's step on wave propagation during a dam break across a dry downstream channel. The Flow-3d Computational Fluid Dynamics package was used to simulate the flow phenomena during a dam break, adopting different geometric conditions and a densely vegetated area of the downstream channel in the far-field. Three-dimensional flow characteristics were reproduced by solutions of Navier-Stokes equations coupled with the standard volume of fluid to track the evolution of the free surface. The turbulent flow characteristics were represented by different approaches frequently used in the scientific literature. The computational model was optimized and validated using experimental data published in the literature. The results showed that the model had high accuracy in predicting the evolution of the free surface, moving hydraulic jump, flow velocity, and flow regime. In addition, the model was able to predict the formation and development of transitional, rotational, and transverse flows, jet flow, nappe flow, wave breaking, the bore evolution in different directions, and the change of the flow regime under the influence of the channel step and vegetation. Accordingly, the flow fluctuations during dam break, wave attenuation, flow separation, and turbulence structure evolution in the vegetated area were predicted
Bed compaction effect on dam break flow over erodible bed; experimental and numerical modeling
No full textDisastrous floods result in severe loss of human lives and intense destruction to the infrastructure and economic activities. Furthermore, it leads to environmental and ecological damages located at the downstream area of the dam. The present study aims at investigating experimentally and numerically the sediment transport and morphological evolution of the erodible bed induced by dam-break flows. Experimental runs were conducted in a flume, specially designed for dam-break flow process, equipped with a thin vertical gate at its middle. Based upon an initial plane bed, the effect of three different compaction rates of the bed at both up-and downstream of the dam was investigated. The experimental data consistently suggested that increasing the bed compaction rate resulted in decreasing the scouring and sedimentation depth, as well as the sediment transport rate. Further, increasing the bed compaction rate led to increasing the wave-front celerity, reducing the scouring rate and the erosion depth. Both the experimental and numerical outputs highlighted the process of air entrainment at the leading edge of the wave-front. The numerical results showed that the maximum void fraction was associated with the maximum flow velocity at the zone of wave-front. Comparisons were made between the experimental results and those are provided from a numerical study using standard volume of fluid VOF method, where three different turbulence closure schemes; RNG, k-ε and k-ω, and three bed load sediment transport approaches, Van Rijn, Meyer-Peter-Muller and Nielson, were applied. Accordingly, by considering the error values in predicting the free surface height and the bed deformation, the k-ω closure model showed the highest accuracy in capturing the turbulence features while the Mayer Peter-Muller formula had the highest performance in predicting the bed deformation in non-compacted (NC) bed. Besides, the RNG closure model and Nielsen bed load formula revealed the highest accuracy in predicting the aforementioned features in a semi-compacted (SC) bed. The k-ɛ closure model and Mayer Peter-Muller formula showed the highest accuracy in reproducing the turbulence characteristics and bed profile in a fully-compacted (FC) bed
Prediction of river discharges at confluences based on Entropy theory and surface-velocity measurements
No full textHydrodynamic features of the confluence zone of large rivers are complicated because of their three-dimensional flow structure. The confluence between the Rio Negro and the Rio Solimões, characterised by black and white waters, respectively, ranks among the largest river junctions on Earth. An Entropy-based investigation was carried out to assess the discharge and analyse the 2D structure of velocity distribution for large river flows relying on monitoring of near-surface velocity only. The estimated flow data where compared with in-situ ADCP data gathered across some transects of the Negro and Solimões rivers during both low and relatively high flow conditions. Results are illustrated through some transects at the confluence zone, upstream and downstream of the zone and in the Careiro Channel. Comparisons highlight that the Entropy-based flow velocity in terms of depth-averaged velocity, cross-sectional mean flow velocity and vertical velocity distributions, starting from the measured surface velocity, is in agreement with those determined by the ADCP measurements, with an error in mean flow velocity and discharge lower than 15%. The research highlights the potential of the Entropy method to estimate the discharge and velocity in very large rivers just relying on near-surface velocities monitoring
Prediction of river discharges at confluences based on Entropy theory and surface-velocity measurements
No full textHydrodynamic features of the confluence zone of large rivers are complicated because of their three-dimensional flow structure. The confluence between the Rio Negro and the Rio Solimões, characterised by black and white waters, respectively, ranks among the largest river junctions on Earth. An Entropy-based investigation was carried out to assess the discharge and analyse the 2D structure of velocity distribution for large river flows relying on monitoring of near-surface velocity only. The estimated flow data where compared with in-situ ADCP data gathered across some transects of the Negro and Solimões rivers during both low and relatively high flow conditions. Results are illustrated through some transects at the confluence zone, upstream and downstream of the zone and in the Careiro Channel. Comparisons highlight that the Entropy-based flow velocity in terms of depth-averaged velocity, cross-sectional mean flow velocity and vertical velocity distributions, starting from the measured surface velocity, is in agreement with those determined by the ADCP measurements, with an error in mean flow velocity and discharge lower than 15%. The research highlights the potential of the Entropy method to estimate the discharge and velocity in very large rivers just relying on near-surface velocities monitoring
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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