1,720,966 research outputs found
Numerical Modelling of Alluvial Rivers by Shock Capturing Methods
Full text linkThe problem of modelling both the unsteady hydrodynamics and the bed morphological variations in natural channels is generally performed by solving the De Saint Venant balance equations for the liquid phase together with the Exner continuity equation for the sediments carried as bed-load. This thesis focuses on the development of an high-order accurate centred scheme of the finite volume type for the numerical solution of the coupled De Saint Venant-Exner system. A new scheme, called PRICE-C, is proposed. It solves the system of equations in a non-conservative form, however it has the important characteristic of reducing automatically to a conservative scheme if the underlying PDE system is a conservation law. It is applied to the shallow water equations in the presence of either a fix or a movable bed. The scheme is first introduced in a one-dimensional framework, and it is then extended to the two-dimensional case. The extension is not straightforward in the case of an unstructured mesh, since averages over suitable edge-based control volumes have to be performed. The scheme is extended to high order of accuracy in space and time via the ADER-WENO and MUSCL technique respectively for the one- and twodimensional case. The well-balanced property of the scheme is proven, i.e. the ability to reach steady states also in the presence of discontinuous water surface or discontinuous bottom profile. The scheme can deal with subcritical and supercritical flows, as well as transcritical situations. Moreover the proposed approach leads to a correct estimate of the celerity of surface discontinuities as well sediment bores and small bottom perturbations. The main characteristic of the scheme is its simplicity: it is based on a simple centred approach, that means that the knowledge of the eigenvalues of the matrix of the system is not required. This is important since the interaction between sediment transport and water flow not always admits detailed knowledge of the eigenstructure. Hence this scheme can be useful to engineers since they need simple numerical tools that can be easily used without entering in the mathematical detail of the differential hyperbolic system under consideration. Moreover the centred strategy gives generality to the scheme: in fact, it can be applied without modification to any kind of hyperbolic equations with non-conservative terms.La modellazione dell’idrodinamica e delle variazioni orfologiche in canali naturali `e generalmente effettuata risolvendo numericamente le equazioni delle onde lunghe in acque basse, che regolano il moto della fase fluida, assieme all’equazione di Exner, che descrive l’evoluzione del fondo. L’argomento della presente tesi consiste nello sviluppo di un schema ai volumi finiti di tipo ”centrato” per la soluzione accoppiata di tale sistema di equazioni. Un nuovo schema, denominato PRICE-C, `e qui introdotto: esso risolve le equazioni in forma conconservativa, ma ha l’importante propriet`a di degenerare in uno schema conservativo se il sottostante sistema di equazioni ammette una forma conservativa. Lo schema `e applicato alle equazioni delle onde lunghe in acque basse sia nel caso di fondo fisso che di fondo mobile, dapprima in un ambito unidimensionale e successivamente in quello bidimensionale. L’estensione non `e immediata nel caso
in cui il reticolo di calcolo sia non-strutturato, dal momento che le equazioni differenziali devono essere mediate su opportuni volumi di controllo. Lo schema `e poi esteso ad alti ordini di accuratezza nello spazio e nel tempo attraverso le procedure ADER-WENO e MUSCL rispettivamente per il caso unidimensionale e bidimensionale. Inoltre si dimostra come lo schema proposto verifichi la ”well-balanced property”, che consiste nella capacit`a di raggiungere soluzioni stazionarie, anche in presenza di discontinuit`a della superficie libera e del fondo. Condizioni di corrente lenta e rapida, come pure condizioni di tipo transcritico vengono correttamente risolte. Inoltre lo schema in grado di riprodurre le celerit`a di propagazione di discontinuit`a della superficie e fronti di sedimenti al fondo, cos`? come la celerit`a di propagazione di piccoli disturbi del fondo. Caratteristica principale dello schema `e la sua semplicit`a: `e basato su un semplice approccio di tipo centrato, cio`e non necessita la conoscenza degli autovalori
della matrice del sistema. Questa `e un’importante caratteristica dal momento che non sempre autovalori e autovettori sono calcolabili analiticamente, in particolare nel caso di complesse formule di chiusura per il trasporto al fondo. Quindi questo schema pu`o rivelarsi utile per l’ingegnere che spesso necessita di un semplice strumento numerico che possa essere applicato ad un sistema di equazioni differenziali di tipo iperbolico senza dover entrare nel dettaglio delle propriet`a atematiche del sistema stesso. Data la sua generalit`a, infatti, lo schema pu`o essere applicato ad ogni tipo di sistema iperbolico contenente termini non-conservativi
Elementi speciali nella modellazione ai volumi finiti di correnti di densità: applicazione numerica al Fiume Adige.
No full textLong-term evolution and morphodynamic equilibrium of a tidal channel flanked by tidal flats
No full textThis contribution investigates the long term morphodynamic equilibrium of a movable bed
tidal channel flanked by tidal flats. The two dimensional shallow water equations, the bed
evolution equation, and the advection-diffusion equation for suspended-load are solved
numerically.
As far as the channel evolves, starting from a given straight configuration, with spatially
constant depth and width, it turns out to be always ebb dominated near the inlet and flood
dominated in the landward part. As a consequence, the net sediment flux within a tidal cycle
is directed seaward near the inlet and landward in the remaining part of the channel, leading to
sediment scour in the outer part of the channel and deposition in the inner part, where a
sediment front propagates landward. The system evolves asymptotically toward an
equilibrium configuration characterized by an approximately symmetric suspended-load
transport rate during the tidal cycle. Moreover a nearly constant value of the maximum
flood/ebb speed along the channel is observed, that implies a important morphologic
adjustment toward a constant erosion potential in all the sections of the channel. The width of
the channel is found to decrease exponentially toward the land.
The numerical results show that tidal flats are progressively carved by channels, with a
spacing varying between 200 and 1000 m. The relationship between inlet cross section of
these channels and their watershed is approximately linear
Finite volume modelling of a stratified flow with the presence of submerged weirs
No full textThe saltwater intrusion in the estuary of the Adige River has been investigated by a two-dimensional finite volume shock-capturing model. Owing to the relative small tide range characterizing the river mouth, a sharply stratified salt wedge tends to form during low discharge periods (e.g. in summer). Suitable hydraulic relations have been introduced to model the action of a submerged barrage, located close to the estuary mouth and built to hinder seawater intrusion. Field measurements of salinity profiles have been used to calibrate the model. The numerical results suggest that, as a consequence of increased water withdrawal that occurred in the last years, the barrage does not prevent efficiently the intrusion of the salt wedge any more
Tidal hydrodynamics and erosional power in the Fly River delta, Papua New Guinea
No full textA two‐dimensional numerical model is used to study tidal hydrodynamics and
distribution of bed shear stresses in the Fly River delta, Papua New Guinea. The model
describes the propagation of the tidal wave within the delta and along the river. Model
results indicate that tidal discharge at the mouths of the distributary channels is between
10 and 30 times larger than the river discharge, and that the upstream part of the delta
is flood‐dominated, whereas near the mouth, the delta is ebb‐dominated. Numerical
simulations allow us to investigate the sensitivity of fluxes and bottom stresses with
respect to the variations of sea level and the area of delta islands. The results suggest that a
decrease in the total area of the delta occupied by islands increases the tidal prism and,
therefore, the bed shear stresses. Similarly, an increase in sea level reduces the dissipation
of the tidal signal and speeds up the propagation of the tidal wave within the delta, thus
yielding higher discharges and increased bed shear stresses
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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