1,720,994 research outputs found
Modelling droplet impact in fluid-structure interaction problems
Full text linkThe impact of a liquid droplet onto a deformable substrate is a physical phenomenon that appears ubiquitously in both natural and industrial contexts. The fluid-structure interaction between the droplet and the substrate results in a complex and highly nonlinear system, and is a significant modelling challenge. In this thesis, analytical and numerical modelling techniques for these systems are presented with the aim of providing physical insight and quantitative predictions on the dynamics of the droplet as the properties of the substrate are changed.
In Chapter 1, we motivate the thesis by outlining relevant real-world scenarios involving droplet impact onto deformable substrates, and present a review of the existing literature. In Chapter 2, we setup the canonical droplet impact system that will be used throughout the thesis, including the statement of governing equations, non-dimensionalisation and any assumptions that apply throughout. We present an analytical model for this system in Chapter 3, where we exploit the small timescale of impact phenomena to derive a matched asymptotic solution based on Wagner's theory of impact. To validate the analytical model and investigate regimes which Wagner theory cannot reach, a numerical modelling technique is then considered in Chapter 4, where we use the volume-of-fluid method to conduct direct numerical simulations of the system. We present a novel approach to model the fluid-structure interaction within a two-phase scenario by employing a moving frame coordinate transformation. In Chapters 5 and 6, we apply the previously developed models to study droplet impact onto a spring-supported plate and an elastic membrane, respectively. We find how the deformation of the substrate affects the dynamics of the droplet via slowing down the spreading, decreasing the pressure and reducing the amount of fluid ejected across the substrate. By making use of both modelling techniques, we explore a wide range of substrates by varying the elasticity, tension, mass and dampening properties of the substrate. We then quantify how changing these physical properties of the substrate affects the dynamics of the droplet and in turn the displacement of the substrate during the impact process. Finally, in Chapter 7, we summarise the main results and discuss directions for further work
Electrically induced stabilization of liquid films coating the underside of a surface
No full textThe subject of this talk is the electrostatic stabilization of a viscous thin liquid film wetting the underside of a horizontal surface in the presence of an electric field applied parallel to the undisturbed interface. The formulated asymptotic model includes the effect of bounding solid dielectric regions typically found in experiments above and below the fluids. The competition between gravitational forces, surface tension, and the non-local effect of the applied electric field is captured analytically in the form of a nonlinear evolution equation. State-of-the-art computational tools based on the volume-of-fluid method are also implemented to both assess the range of validity of the derived model and guide these arguments towards practical (and highly nonlinear) contexts involving mixing, pumping and directed polymer assembly. Joint work with T.G. Anderson (Caltech), D.T. Papageorgiou (Imperial College London) and P.G. Petropoulos (NJIT).Non UBCUnreviewedAuthor affiliation: University of OxfordPostdoctora
Modelling, analysis and simulation of incompressible multi-fluid flows
Full text linkMulti-fluid flows are omnipresent in our lives, from the fabrication of integrated circuit components in most electronics to the miniature laboratories inside medical tools, and even as a drop of rain splashes onto the wing of an aeroplane. In this thesis we use theoretical and numerical tools to investigate topics from the fascinating world of interfacial flows.
The first part of this dissertation is dedicated to the study of multi-fluid systems in small scale channel geometries in the presence of electric fields. We develop the theoretical machinery to address the stabilisation (to the point of complete suppression) of the classical Rayleigh-Taylor instability under the action of an electric field acting in the plane of the liquid-liquid interface. In a related context, in many situations electric fields normal to the fluid-fluid interface may be employed in order to accurately drive instabilities towards beneficial goals. In particular, we discuss novel mechanisms to generate pumping and mixing in millimetre-sized geometries without requiring moving parts or an oncoming flow.
In the second part of this thesis we turn our attention to the area of aerodynamics, thus investigating multi-fluid flows in a very different regime, dictated by high speed environments. We initially address one of the canonical problems in fluid mechanics, drop impact onto solid or liquid coated surfaces. This situation arises naturally on an aircraft in either rain or de-icing conditions. A new model for water catch on a surface is proposed, incorporating the violent splashing dynamics occurring in realistic conditions. The impingement of a large number of droplets ultimately leads to the formation of a liquid layer on the surface. We extend the powerful asymptotic framework of triple-deck theory to analyse changes in the flow separation process in the presence of an additional liquid. Flows past surface roughnesses and corners/flaps are discussed as practical examples.Open Acces
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
Droplet impact and the transition to splashing
Full text linkDroplet impact onto a variety of surfaces is a familiar everyday experience: from a dripping tap to spray painting or droplets ejected during a sneeze. An underappreciated question, but often of great importance, is what happens to the droplet after impacting a surface, be it solid or another liquid. For sufficiently high speed impacts the droplet can break up into several smaller droplets in what we define as a splash. In the case of impact onto a liquid pool the pool itself can be sufficiently deformed such that it breaks up into droplets meaning there could be a splash originating from either droplet or pool.
Whether or not a droplet impact results in a splash is often a consequence of the very early time dynamics. What happens in these earliest times is of great importance to understanding and potentially even predicting the outcome of the impact. Whilst some insights have been found for the case of impact onto smooth flat solids the dynamics are much more intricate in the more complex cases considered here.
In this thesis we investigate the early time dynamics of droplet impact onto liquid pools, curved solids, and liquid films floating on pools using a combination of high speed imaging experiments, high resolution direct numerical simulations, and mathematical modelling. First we analyse the early time motion of the common interface between an impacting droplet and a liquid pool. Building upon previous results that only considered the case of identical liquids to a general multi-fluid case we find an explicit equation for the speed of the interface. Furthermore, we extend this to the case of a floating film on top of a deep pool, investigating the motion of both the upper droplet-film interface and the lower film-pool interface.
We then investigate the variation of the threshold to splash in two different cases. The first case is the impact of a droplet onto a deep immiscible viscous pool to examine how the splashing threshold depends on the pool viscosity. We identify two different regimes where the splashing threshold shows a different variation with the pool viscosity. Applying the earlier result (penetration speed) we explain the observations and derive an equation for the splashing threshold in the case of high pool viscosity, as well as the composition of the splashed liquid. Finally we investigate the impact of a droplet onto both concave and convex curved surfaces, finding a consistent change in the splashing threshold for both cases, and explaining these results in the context of recent theoretical models
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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