1,720,973 research outputs found
Effetti di non stazionarietà sulla dinamica delle biforcazioni negli alvei intrecciati
No full textIn due recenti lavori Bolla Pittaluga et al. (2003a) e Miori et al. (2005) hanno studiato, facendo uso di un modello monodimensionale, le configurazioni di equilibrio e la stabilità di una biforcazione fluviale. Gli autori hanno utilizzato condizioni nodali alternative rispetto a quelle presenti in letteratura per definire la distribuzione delle portate solide in corrispondenza della biforcazione. Entrambi i lavori considerano condizioni forzanti del sistema con carattere stazionario. Recenti osservazioni sperimentali di Bertoldi et
al. (2005) hanno tuttavia evidenziato l’importanza di fenomeni non stazionari sull’evoluzione morfologica di una biforcazione, mettendo in particolare in luce il ruolo della presenza di barre migranti nel canale di monte. Nel presente contributo si propone un’estensione del modello proposto da Bolla Pittaluga et al. (2003a) che mette in conto gli effetti di non stazionarietà indotti dalla variabilità della portata liquida che alimenta la biforcazione e dalla presenza di barre migranti nel canale di monte. L’inclusione di tali effetti porta a scenari della dinamica del sistema significativamente più ricchi rispetto al caso di condizioni forzanti stazionarie. L’evoluzione morfologica della biforcazione prevista dal modello riproduce
correttamente, dal punto di vista qualitativo, molti dei comportamenti effettivamente osservati in laboratorio e descritti in letteratura
Morphological evolution of bifurcations in gravel-bed rivers with erodible banks
No full textIn a recent paper Bolla Pittaluga et al. (2003a) proposed a physically based formulation for the nodal point conditions to be adopted, in the context of a one-dimensional approach, to describe channel bifurcations in gravel-bed rivers. The authors employed the nodal conditions to study the equilibrium configurations of a simple bifurcation and their stability, assuming erodible bed and fixed banks. With the present contribution we extend the model proposed by Bolla Pittaluga et al. (2003a) to the case of channels with erodible banks, i. e. channels which may adapt their width to the actual flow
conditions. Such an extension is of a particular interest for river bifurcations in gravel-bed braided streams, in which bed evolution and bank erosion processes may occur over comparable time scales. Moreover, we investigate how the mechanism of generation of the bifurcation may affect its equilibrium configurations, showing that significant differences may arise if the bifurcation is formed through the incision of a new channel or as a consequence of the formation of a central deposit into a channel. In both circumstances, however, the common feature of the equilibrium configurations reached by the system is a strongly unbalanced partition of water and sediment discharges into the two branches. Such a finding agrees with experimental and field observations according to which stable
symmetrical bifurcations are seldom observed in natural gravel-bed rivers
A one-dimensional model of bifurcations in gravel bed channels with erodible banks
No full textIn a recent paper, Bolla Pittaluga et al. proposed a physically based formulation for the nodal point conditions to be adopted at a channel bifurcation in the context of a one-dimensional approach. They employed such conditions to study the equilibrium configurations of a simple bifurcation and their stability, assuming erodible bed and fixed
banks. With the present work we extend the model proposed by Bolla Pittaluga et al. to the case of channels with erodible banks, i.e., channels which may adapt their width to the actual flow conditions. Such an extension is of a particular interest for river bifurcations
in gravel bed braided streams, in which bed evolution and bank erosion processes occur over comparable timescales. Moreover, to study the morphological evolution of a bifurcation, we employ a different approach with respect to Bolla Pittaluga et al. and
introduce a ‘‘local analysis’’ which does not account for the influence exerted on channel morphology by downstream conditions on longer timescales. For values of the controlling parameters typical of gravel bed braided streams, the model shows that the stable equilibrium solutions of a bifurcation are invariably characterized by a strongly
unbalanced partition of water and sediment discharges in the two branches. Numerical simulations, based on a simplified model of the bifurcation evolution, allow us to investigate the role of the mechanism of generation of a bifurcation on its final equilibrium
configurations. It is shown that bifurcations which form through the incision of a new, initially narrow, channel may lead to significantly different equilibrium configurations with respect to bifurcations which generate from a central deposition in a wide channel. In
spite of its simplicity the model seems to retain the most relevant effects which govern the behavior of gravel bed channel bifurcations
Unsteadiness effects on the morphological behaviour of gravel-bed river bifurcations
No full textIn recent works (Bolla Pittaluga et al. 2003a, Miori et al. 2005) the equilibrium configurations of a simple channel division and their stability have been investigated within the context of a one dimensional approach. Physically based relationships for sediment distribution have been employed by the authors at the channel division point, which allow to model the effect of transverse bed slope at the bifurcation on bed load transport. Both works are based on the assumption of steady forcing conditions. Recent experimental observations on gravel-bed channel divisions (Bertoldi et al., 2005) have shown that bar migration along the upstream channel induce time fluctuations in water and sediment feeding of the downstream branches, which significantly affect the morphological behaviour of the system. In the first part of this paper we extend the model proposed
by Bolla Pittaluga et al. 2003a to account for such an effect. In the second part of the paper we also investigate the effect of water discharge fluctuations, which are invariably observed in gravel-bed streams in mountain regions, on the time evolution of a channel bifurcation. Results show that both sources of unsteadiness lead to significantly different and richer scenarios of the possible solutions of the system with respect to the steady case conditions
Usefulness of carotid blood flow peak velocity (ΔVpeak) and velocity time integral (ΔVTI) respiratory variation for fluid responsiveness prediction in ICU shocked mechanically ventilated patients. Preliminary results.
No full textInteraction between migrating bars and bifurcations in gravel-bed rivers
No full textIn the present work we investigate the interaction between migrating alternate bars and the dynamics of river bifurcations. Laboratory experiments are carried out to study a
Y-shaped bifurcation with fixed banks and erodible bed composed of well-sorted sand. The problem is also analyzed by developing a theoretical, one-dimensional model. Results show the occurrence of regular fluctuations in the discharge distribution at the bifurcation node, which are strictly related to bar migration. The effectiveness of bars in conditioning the bifurcation behavior increases with bar amplitude and decreases with bar migration
speed. Four qualitatively different behaviors of the system are observed as the controlling parameters of the flow are varied within a range significant for gravel bed rivers. The theoretical predictions are in good qualitative agreement with the experimental observation
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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