1,720,960 research outputs found
The evolution of spilling breakers: experiments and analyses
No full textL'obiettivo di questa tesi è stato quello di studiare sperimentalmente un particolare tipo di frangimento d’onda (spiller) e di analizzare in dettaglio le caratteristiche geometriche e cinematiche di un sottile strato monofase turbolento. Per la generazione l’onda frangente, è stato utilizzato un tank, realizzato in plexiglass, e messo in movimento attraverso un sistema di esapode. Il moto del tank è stato progettato tramite il solutore numerico HPC e l’intera evoluzione del fenomeno è stata invece valutata riproponendo tale storia temporale in un solutore di tipo N-S. Grazie a quest’ultimo, siamo stati in grado di definire l'area di interesse (1,80 m) per l'analisi e scegliere il setup sperimentale. Per una buona risoluzione temporale e spaziale di ogni telecamera, l'area di studio è stata divisa in due zone diverse e solo quella di monte è stata analizzata perché è lì che il flusso è non stazionario e la curvatura e la rotazione sono più evidenti. Per le misure delle grandezze cinematiche, è stato utilizzato il metodo PIV facendo 2048 realizzazioni al fine di ottenere una buona analisi statistica. Il campo di velocità istantaneo è stato decomposto in uno medio e in uno turbolento. Il flusso medio è caratterizzato da una struttura vorticosa quadrupolare collegato da un punto di sella al disotto della cresta. L'analisi del campo vorticità ha mostrato come essa sia influenzata dalla curvatura della superficie libera, mentre come l'evoluzione dell’energia cinetica turbolenta sia dipendente dalla curvatura e rotazione dell'interfaccia tra strato sottile turbolento monofase e il flusso irrotazionali sottostante. L'analisi nel tempo dello spessore dello strato monofase turbolento evidenzia la non stazionarietà del fenomeno. Infine, il set di dati disponibili ha permesso di verificare una parte del modello analitico di Brocchini e altri collaboratori, riguardante la soluzione analitica del profilo di velocità media.The objective of this thesis is to experimentally study an unsteady spilling breaking wave and to
analyse in details the geometric and kinematic characteristics of the thin single-phase turbulent layer.
A sloshing wave was used for the generation of the breaker. A 3m long, 0.60m depth and 0.10m wide
tank was built in Plexiglass and forced through an hexapode system. The tank motion was designed
by using two different numerical solvers: HPC and Navier-Stokes. The former provides the optimal
tank motion, the latter defines the area of interest (1.80m) for our analysis and choice the equipment
for measurements. For a good temporal and spatial resolution of each camera, the area of interest was
divided into two different zones and only the upstream zone was analysed because it is there that flow
unsteadiness, curvature and rotation are more evident.
For the measurements of the kinematic quantities, the Particle Image Velocimetry (PIV) method was
used. A number of 512 realizations were done in order to obtain a good statistical analysis. The
instantaneous velocity field was decomposed in a mean and turbulent field. The mean flow is
characterized by a quadrupolar vortical structure connected by a saddle point, which at breaking onset
is near the free surface at the lee of the crest. Later on the saddle point is seen to move upstream and
downward of the wave crest. The analysis of the vorticity field shows how it is influenced by the
steepness of the wave and curvature of the free surface. The TKE evolution instead, highlight its
dependence from the curvature and rotation of the interface between the thin single-phase turbulent
layer and irrotational flow underlying. Furthermore, the analysis of the thickness of the layer in time
shows the unsteadiness of the phenomenon and a local instability. Lastly, the data set available has
made it possible to check the analytical model by Brocchini and co-workers, for the analytical
solution of the mean velocity profile
Sloshing in a rotating liquid inside a closed sea cage for fish farming
Get PDFSloshing in a sea cage with a slowly rotating liquid is investigated. The cage is axisymmetric, and the liquid is subjected to a nearly uniform angular velocity about the vertical axis of the cage. Both experimental and theoretical investigations are presented. It is shown that rotation modifies the sloshing regimes of a non-rotating liquid by splitting the natural frequencies. Therefore, resonant sloshing regimes can be manipulated by varying the rotation rate of the liquid
Extra Strain Rates in an unsteady spilling breaking wave
Get PDFAbstract We analyze the extra strain rates that characterize a curved, thin mixing layer induced at an unsteady spilling breaker. We focus on the flow curvature, which induces some extra rates of strain that should be accounted for in algebraic-type turbulence closures. The comparison between the analytical formulation proposed by Brocchini and co-workers for a single-phase turbulent thin layer of fluid and the data, obtained from a Particle Image Velocimetry (PIV) dedicated experimental program, reveals that the order of magnitude of the extra rates of strain induced by the streamline curvature, is comparable with that of the simple shear. This differs from what observed for the geometric curvature terms and from what occurs at hydraulic jumps, typically used to model steady breakers
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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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