1,721,176 research outputs found
Drop collisions with simple and complex surfaces
No full textDrop impact onto surfaces has long been a popular and important subject of experimental, numerical and theoretical studies to explain phenomena observed both in nature and in many engineering applications. Progress in understanding and describing the hydrodynamics involved in drop impacts has been rapid in recent years, due partly to the availability of high-speed cameras, but also because of accompanying advances in theoretical and numerical approaches. Thus, for simple surfaces, i.e. smooth surfaces of uniform chemistry, the outcome of a drop impact can be well predicted over a large range of impact parameters, including quantitative values of spread dynamics and splash characteristics. This article comprehensively reviews the present level of understanding for such impact situations.However many practical applications involve impacts onto surfaces of higher complexity, either morphologically or chemically, involving textured or porous surfaces or surfaces with non-uniform wettability characteristics. This expands greatly the parameter space for which descriptions of the impact must be found and the present understanding is significantly more rudimentary compared to drop impacts onto simple surfaces. In this review such impacts are discussed by considering effects introduced by morphological changes to the surface and by changes of the wettability. Comparisons to corresponding impacts onto simple surfaces are drawn to underline the additional physical mechanisms that must be considered.</p
Über den Aufprall von Hochgeschwindigkeitstropfen auf trockene und benetzte Oberflächen
Full text linkThis thesis focuses on the high-speed impact of liquid drops on dry smooth surfaces and thin liquid films in the presence of a strong gas flow using experimental and numerical methods. A flywheel experiment was designed to study such challenging conditions by analyzing the impact with different drop diameters, velocities, liquids and gas mixtures. A sophisticated algorithm was also developed for image analysis and to handle a large number of experimental data. Selected experimental cases were complemented with numerical simulations, which provided insights on the flow around and inside the impacting drop. The results of the drop impact on dry surfaces indicate that the splashing scenario depends strongly on the physical properties of the liquids and not on the surrounding gas or the kinematic impact conditions. It was also demonstrated that the mechanism of gas entrapment at the early stage of the impact is not responsible for the splashing of drops; however, the physical properties of the surrounding gas influenced the spreading lamella and the ejection of secondary droplets. The results demonstrated that the prompt splash is indeed well described by the Rayleigh-Taylor instability in the spreading lamella. The drop impact on wetted surfaces always generates a chaotic and thin corona, which bends and deforms itself during the entire splashing process. Contrary to the splashing on dry surfaces, the break-up of the corona can be attributed to at least three different instabilities: rim-bending, Rayleigh-Taylor, and Rayleigh-Plateau instabilities. Those instabilities generate a host of small secondary droplets in a wide range of sizes and velocities. It was found that the thin ejected corona leads to hole formation and the eventual detachment from its base at the last stage of impact. The thickness of the corona was also estimated using two different theoretical methods. The outcome of high-speed impact on dry and wetted surfaces were quantified in this thesis. The results of the theoretical, experimental, and numerical analyses were combined to describe completely the formation of the secondary droplets. The presented methods allow the prediction of the splashing phenomenon for a drop impacting dry or wetted surfaces at low and high speed.In dieser Studie wurde der Aufprall von Flüssigkeitstropfen bei hohen Geschwindigkeiten auf glatte trockene und benetzte Oberflächen mit experimentellen und numerischen Methoden untersucht. Dafür wurde ein Schwungrad-Experiment entwickelt, mit dem der Tropfenaufprall mit verschiedenen Durchmessern, Aufprallgeschwindigkeiten, Flüssigkeiten und Gasgemischen analysiert werden konnte. Ausgewählte experimentelle Fälle wurden durch numerische Simulationen ergänzt, welche einen tieferen Einblick in die Strömung um und innerhalb des aufprallenden Tropfens ermöglichten. Die Ergebnisse des Tropfenaufpralls auf trockenen Oberflächen deuten darauf hin, dass das Zerspritzen-Szenario stark von den physikalischen Eigenschaften der Flüssigkeiten und nicht von dem umgebenden Gas oder den kinematischen Aufprallbedingungen abhängt. Außerdem konnte nachgewiesen werden, dass der Mechanismus des Gaseinschlusses in der frühen Phase des Aufpralls nicht für das Zerspritzen der Tropfen verantwortlich ist. Jedoch haben die physikalischen Eigenschaften des umgebenden Gases einen Einfluss auf die Entstehung von Sekundärtropfen. Mittels einer theoretischen Analyse konnte bewiesen werden, dass das Prompt-Zerspritzen durch die Rayleigh-Taylor-Instabilität in der Flüssigkeitslamelle beschrieben werden kann. Der Tropfenaufprall auf benetzte Oberflächen erzeugt immer eine Korona, die sich während des gesamten Zerspritzenvorgangs verbiegt und verformt. Im Gegensatz zum Zerspritzen auf trockene Oberflächen kann der Zerfall der Korona auf mindestens drei verschiedene Instabilitäten zurückgeführt werden: Biege-, Rayleigh-Taylor- und Rayleigh-Plateau-Instabilitäten. Diese Instabilitäten erzeugen viele Sekundärtropfen in einem weiten Bereich von Größen und Geschwindigkeiten. Es konnte festgestellt werden, dass die ausbreitende Korona in der letzten Phase des Aufpralls zur Lochbildung und Ablösung führt. Anhang zweier theoretischer Methoden konnte die Dicke der Korona abgeschätzt werden. Im Rahmen dieser Arbeit wurden die Sekundärtropfen beim Tropfenaufprall auf trockene und benetzte Oberflächen quantifiziert. Die Ergebnisse aus den theoretischen, experimentellen und numerischen Analysen konnten vereint werden, um die Entstehung der Sekundärtropfen vollständig zu beschreiben. Die vorgestellten Methoden ermöglichen die Vorhersage des Zerspritzen-Phänomens für einen Tropfen, der mit niedriger und hoher Geschwindigkeit auf trockene oder benetzte Oberfläche aufprallt
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
Crater evolution after the impact of a drop onto a semi-infinite liquid target
No full textThis paper is devoted to an experimental and theoretical investigation of the crater formed by the impact of a single drop onto a semi-infinite target of the same liquid. The shape of the crater at various time instances after impact has been observed using a high-speed video system and then accurately characterized. A theoretical model for the crater expansion has been developed, which is able to predict the temporal variation of the crater depth for sufficiently high Weber, Froude, and Reynolds numbers. The flow around the crater is approximated by an irrotational velocity field past a moving and expanding sphere. The equations describing the propagation of the surface of the crater have been obtained from the balance of stresses at the crater interface, accounting for inertia, gravity, and surface tension. The temporal evolution of the crater depth has been calculated by numerical solution of the equations of motion. The agreement between the theoretical predictions and experimental data are rather good
Single drop impact onto a deep pool: experimental observations and theoretical model for the crater evolution
No full textDispelling 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
- …
