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Nonlinear vibration isolators with asymmetric stiffness
No full textA vibration source is commonly coupled to a receiving structure by a vibration isolator. A key trade-off in the choice of vibration isolator is the requirement for a wide frequency range of isolation without excessive static deflection. This compromise can, in principle, be circumvented by employing a softening nonlinear isolator that presents a high stiffness to the weight of the isolated mass but a low tangent stiffness in the vicinity of the equilibrium position. The first part of this thesis is concerned with the static response of a number of elements that are expected to exhibit such a nonlinear stiffness characteristic. A mechanism with geometrical nonlinearity is studied first and found to offer some benefits compared with a similar one reported in the literature. Beams are commonly employed as linear springs and their suitability as nonlinear isolators is considered here. It is shown that the stiffness of a simply supported beam loaded transversely at its centre is of a hardening type in contrast to what is reported in the literature. Post-buckled beams are also investigated as candidates for nonlinear springs of vibration isolators although the sudden change in stiffness at the buckling point is unfavourable. Curved beams and beams with eccentric loading are investigated as alternatives to a straight axially loaded post-buckled beam. Static analyses are presented which show that curvature or eccentricity in loading can be incorporated to smooth the force-deflection curves. A commercially available rubber isolation mount is also studied as an example of an axially loaded curved element and its force-deflection characteristic measured. The hypothesis that it can be modelled by a curved beam is found not to hold. The inter-variability observed between samples is evaluated which illustrates the potential for mistune of nonlinear mounts in general.Nonlinear stiffness gives rise to the possibility of asymmetry about the equilibrium position, either as an inherent characteristic of the isolator or as a result of a mistuned added mass or static preload. A nonlinear isolator with asymmetric stiffness is modelled as a Duffing oscillator. The force transmissibility of the oscillator is obtained analytically using the Harmonic Balance Method from which the performance of the isolator is evaluated quantitatively as a function of both static load and mistuned mass. A study is presented for the case of a nonlinear isolator comprised of a curved beam. The high stiffness of the beam in extension causes impulsive behaviour in the transmitted force which is alleviated by the inclusion of a linear spring placed in series. It is shown that this combination significantly outperforms a linear isolator with the same static deflection
Large deflection of a simply supported beam
No full textThe large deflection of a simply-supported beam loaded in the middle is a classic problem in mechanics which has been studied by many people who have implemented different methods to determine the solution, such as analytical exact solutions and the finite element method. The problem is investigated again here but the Galerkin method is used to obtain an approximate force-deflection characteristic of the beam. It is shown that the beam can be modelled with a Duffing-type stiffness with hardening nonlinearity. The exact solution and that from the finite element method are used to validate the results. The accuracy of the results and the suitability of the Sine function to model the deflected shape of the beam in the Galerkin method are investigated.The large deflection of a simply-supported beam due to a pure bending moment is also investigated. The exact solution is obtained and the results are used to describe the behaviour of the beam
Developing and analysing an electromechanical model of a bio-inspired flapping wing mini UAV
Full text linkThe purpose of this paper is to describe and analyse an electromechanical model of a mini flapping wing mechanism for the purpose of system optimisation. The system comprises of a small DC motor connected to a voltage source, a gearbox and a slider crank that drives two wings. The DC gearmotor is modelled considering its both mechanical and electrical components. An equivalent viscous damper is considered to model the mechanical losses of the gearmotor. The crank mechanism is assumed massless and the inertia of the wings only considered in the model. The aerodynamic drag and lift are modelled using an equivalent viscous damping model as an energy sink. The parameters of the system are estimated using published experimental data and manufacturer datasheets for the corresponding DC gearmotor. The energy efficiency as the ratio of aerodynamic power to the input electrical power of the system and also the aerodynamic power are used as two measures to evaluate the system performance
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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