1,720,961 research outputs found
Replication Data for: Sub-surface turbulence or non-breaking capillary waves: which dominates air-water gas transfer?
No full textThis dataset contains the data necessary to reproduce the plots in the publication 'Sub-surface turbulence or non-breaking capillary waves: which dominates air-water gas transfer?'. It contains results obtained by particle image velocimetry, planar laser-induced fluorescence, and O2 concentration measurements for flows with different combinations of sub-surface turbulence and surface wave properties
Numerical implementation of the pressure-patch model of ships on flows of arbitrary shear profiles
Full text linkI denne oppgaven er det utviklet en numerisk metode for å finne en ekstern trykkfordeling som modellerer et skip på vannoverflaten gitt skrogets form, fartøyets fart, vanndybden og en dybdeavhengig skjærprofil. Trykkflekker har ofte blitt brukt for å modellere skip i kombinasjon med lineære overflatebølger gjennom årene. Disse trykkflekkene holdes imidlertid konstant for forskjellige fartshastigheter, vanndybde og skjærstrøm, som vil gjøre at det effektive skroget i vannet endres etter forholdene. Derfor er den tradisjonelle trykkflekkmetoden ikke en nøyaktig modell for skipsskrog, og det er behov for en metode som finner trykkflekken som korrekt modellerer skipsskroget, gitt skrogets geometri, hastighet, vanndybde og skjærstrømning.
Den numeriske metoden er brute en force-metoden der en kombinasjon av "mesh"-integrasjon, FFT og "finite difference" brukes til å løse problemet. Integral ligningen som brukes til å finne trykklappen, er en Fredholm integral likning av den første typen, og å løse for likningen for trykket kan betraktes som et inverst problem. Ved å bruke en Green's funksjon og diskretisere ligningen, kan problemet løses ved å løse det lineære ligningssystemet. Problemet løses i både 2D og 3D, og den numeriske metoden kan beregne løsninger for en vilkårlig dybdeavhengig skjærprofil.
Nøyaktigheten av løsningene som er gitt ved den numeriske metoden er funnet å være ganske gode. Metoden kan reprodusere alle punktene til den gitte overflaten med en øvre grense for relativ feil på mindre enn 0,5 \% for alle testene i dette studiet. Det ble funnet at effekten av skipshastighet, vanndybde og skjærstrømprofil hadde signifikante effekter på trykkflekken. Dessuten er den beregnede trykkflekken veldig forskjellig fra den konstante trykkflekken som tradisjonelt brukes, hvor den tradisjonelt brukte flekken bare gir riktig skipsskrog når skipets hastighet er null. Imidlertid er det noen utfordringer med å få løsninger for det inverse problemet. Siden Greens funksjonen er divergerende for punktet i tillegg til å være sakte konvergende for punktene nær null, vil Greens funksjonen ikke være fullt konvergert. I tillegg må strålingsbetingelsesparameteren settes til en relativt høy verdi for å unngå periodisk grense-problemer.
Likevel sikrer de resulterende trykkflekkene ønsket høyde på vannoverflaten. Imidlertid viser de store trykktaggene på baksiden av trykkflekken behovet for en mer fysisk ønsket overflatehøyde i skipets område. I den virkelige verden vil et skip bli forhøyet og rotert bakover ettersom trykk-kraften virker på skroget. Derfor er de beregnede trykkflekkene en rettferdig representasjon av skip i det unaturlige tilfellet av ikke-skiftende effektive skipsskrog.In this thesis, a numerical method is developed for finding an external pressure distribution that models a ship on the water surface given the shape of the hull, the speed of the moving ship, the water depth, and a depth-dependent shear profile. Using a pressure patch to model a ship has commonly been done in the linear surface waves field over the years. These pressures are, however, typically kept constant for different ship velocities, water depth, and shear flow, and the effective ship hull in the water will change with these conditions. The traditional pressure patch method is therefore not an accurate model for a moving ship, and there is a need for a method that finds the pressure source that correctly models the ship hull, given the geometry of the hull, ship velocity, water depth, and shear flow.
The numerical method is a brute force method where a combination of mesh integration, FFT and finite differences is used to solve the problem. The integral equation used for finding the pressure patch is a Fredholm integral equation of the first kind, and solving the equation for the pressure can be regarded as an inverse problem. By using a Green's function method and discretizing the equation, the problem is solved by solving the linear equation system. Furthermore, the problem is solved in both 2D and 3D, and the numerical method can calculate solutions for an arbitrary depth-dependent shear profile.
The accuracy of the solutions provided by the numerical method is found to be reasonably accurate. The method can reproduce all the points in the given surface elevation with an upper bound relative error of less than 0.5\% for all the cases in this study. It was found that the effect of ship velocity, water depth, and shear current profile had significant effects on the pressure patch. Moreover, the calculated pressure patch is dramatically different from the constant pressure patch that is traditionally used, where the traditionally used patch only gives the correct effective ship hull when the ship velocity is zero. However, there are some challenges with obtaining the solution of the inverse problem. As the Green's function is divergent for the point in addition to being slowly convergent in the points close to zero, the Green's function will not be fully converged. Also, the radiation condition parameter needs to be set to a relatively high value in order to avoid periodic boundary issues.
Nevertheless, the resulting pressure patches are ensuring the wanted prescribed surface elevation on the water surface. However, the big pressure spikes occurring at the back of the pressure patches are indicating the need for a more physical prescribed surface elevation in the region of the ship. In the real world, a moving ship would be elevated and tilted as the pressure forces act on the hull. Hence, the calculated pressure sources should give a fair representation of moving the ship in the unnatural case of a non-changing effective ship hull
Numerical implementation of the pressure-patch model of ships on flows of arbitrary shear profiles
No full textI denne oppgaven er det utviklet en numerisk metode for å finne en ekstern trykkfordeling som modellerer et skip på vannoverflaten gitt skrogets form, fartøyets fart, vanndybden og en dybdeavhengig skjærprofil. Trykkflekker har ofte blitt brukt for å modellere skip i kombinasjon med lineære overflatebølger gjennom årene. Disse trykkflekkene holdes imidlertid konstant for forskjellige fartshastigheter, vanndybde og skjærstrøm, som vil gjøre at det effektive skroget i vannet endres etter forholdene. Derfor er den tradisjonelle trykkflekkmetoden ikke en nøyaktig modell for skipsskrog, og det er behov for en metode som finner trykkflekken som korrekt modellerer skipsskroget, gitt skrogets geometri, hastighet, vanndybde og skjærstrømning.
Den numeriske metoden er brute en force-metoden der en kombinasjon av "mesh"-integrasjon, FFT og "finite difference" brukes til å løse problemet. Integral ligningen som brukes til å finne trykklappen, er en Fredholm integral likning av den første typen, og å løse for likningen for trykket kan betraktes som et inverst problem. Ved å bruke en Green's funksjon og diskretisere ligningen, kan problemet løses ved å løse det lineære ligningssystemet. Problemet løses i både 2D og 3D, og den numeriske metoden kan beregne løsninger for en vilkårlig dybdeavhengig skjærprofil.
Nøyaktigheten av løsningene som er gitt ved den numeriske metoden er funnet å være ganske gode. Metoden kan reprodusere alle punktene til den gitte overflaten med en øvre grense for relativ feil på mindre enn 0,5 \% for alle testene i dette studiet. Det ble funnet at effekten av skipshastighet, vanndybde og skjærstrømprofil hadde signifikante effekter på trykkflekken. Dessuten er den beregnede trykkflekken veldig forskjellig fra den konstante trykkflekken som tradisjonelt brukes, hvor den tradisjonelt brukte flekken bare gir riktig skipsskrog når skipets hastighet er null. Imidlertid er det noen utfordringer med å få løsninger for det inverse problemet. Siden Greens funksjonen er divergerende for punktet i tillegg til å være sakte konvergende for punktene nær null, vil Greens funksjonen ikke være fullt konvergert. I tillegg må strålingsbetingelsesparameteren settes til en relativt høy verdi for å unngå periodisk grense-problemer.
Likevel sikrer de resulterende trykkflekkene ønsket høyde på vannoverflaten. Imidlertid viser de store trykktaggene på baksiden av trykkflekken behovet for en mer fysisk ønsket overflatehøyde i skipets område. I den virkelige verden vil et skip bli forhøyet og rotert bakover ettersom trykk-kraften virker på skroget. Derfor er de beregnede trykkflekkene en rettferdig representasjon av skip i det unaturlige tilfellet av ikke-skiftende effektive skipsskrog
Current Mapping from the Wave Spectrum
Full text linkIn this chapter we review methods by which near--surface ocean currents can
be measured remotely using images of the water surface, as obtained by X-band
radar in particular. The presence of a current changes the dispersive behavior
of surface waves, so our challenge is to solve the inverse problem: to infer
the spatially-varying current from measurements of the wavy surface. We examine
how remote sensing of currents is achieved in practice by analyzing the wave
spectrum, as may be measured for example by X-band radar. A set of consecutive
backscatter images recorded as a function of time is Fourier-transformed to
produce the spectrum, which gives information concerning the propagation of
waves whose dispersion is altered by currents. X-band radar images measure the
wave field over multiple square kilometers, and analyzing various spatial
subsets of the images allows a map of the spatial variation of the currents to
be reconstructed. Several algorithms for obtaining empirical dispersion
relations from the measured spectrum and extracting the currents are reviewed:
the least squares and iterative least squares method, the normalized scalar
product method, and the polar current shell method. We go on to describe how
the same methods and algorithms can be extended to also allowing the
depth-dependence of the current to be determined. Reasonable agreement between
radar-derived currents and in situ measurements has been demonstrated in
multiple field measurements. However, more validation is necessary especially
in the context of depth-varying flows. Understanding the extent to which Stokes
drift is measured as part of the radar-derived current is not well-understood
yet potentially important.Comment: Chapter 15 of Huang & Gill (eds) "Ocean Remote Sensing Technologies:
High frequency, marine and GNSS-based radar", chapter 15, pp. 357-378,
Institution of Engineering and Technology (IET), 2021. This is a preprint, 24
pages, 8 figure
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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