1,720,954 research outputs found
Flutter analysis of piezoelectric beams in MEMS
Full text linkMicro-electro-mechanical systems (MEMS) represent a huge class of devices characterized by the smart coupling between electronics and mechanics, in order to obtain microscopic sensors and actuators. The use of piezoelectric materials in MEMS is steadily increasing, considering both the “direct effect”, e.g. in energy harvesters, and the “indirect effect”, for the cases of resonators, micropumps and other actuators. This paper is devoted to the study of piezoelectric laminate beams in the presence of aeroelastic effects due to the interaction of the structure with a fluid flow, as it may happen if the MEMS is embedded in a fluidic system. More specifically, the analytical conditions for the onset of flutter instability are studied, with the purpose of providing a sound basis for further studies focused on energy harvesting from fluid flows
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
Fluid-structure interactions for energy harvesting
No full textLAUREA MAGISTRALEIl presente lavoro di tesi riguarda lo studio dei più comuni fenomeni aeroelastici applicati a micro-strutture composite in materiale piezoelettrico, con lo scopo di valutare la possibile applicazione nel campo dell'\textit{Energy Harvesting}. I seguenti fenomeni d'interazione fluido-struttura sono stati considerati: Vibrazioni indotte da vortici (VIV) e instabilità da Flutter (FI).
Lo studio teorico si è basato su ipotesi semplificate, adottando il metodo di Rayleigh-Ritz. Modelli a 2 e 3 gradi di libertà sono stati utilizzati rispettivamente per VIV(spostamento trasversale e potenziale) e per FI(spostamento trasversale, rotazione torsionale e potenziale). La teoria dei laminati, opportunamente modificata per integrare l'effetto piezoelettrico, è stata impiegata per studiare il comportamento sezionale delle micro-travi composite. Per gestire ed estrarre energia elettrica dal sistema, è necessario connettere lo strato di piezoelettrico ad un circuito esterno. Due tipologie di circuito sono state analizzate: il primo puramente resistivo(RC) ed un secondo più complesso aggiungendo un induttore in parallelo (RLC).
L'influenza dei parametrici elettrici sulla risposta del sistema è stata ampiamente valutata.
Nel caso di vibrazioni indotte da vortici, un comportamento più efficiente in termini di potenza generata si è ottenuto per fluidi ad alta densità come l'acqua. Questo aspetto è legato innanzitutto all'ampiezza della forza di lift, linearmente proporzionale alla densità del mezzo fluido, e inoltre al fenomeno del lock-in. Infatti, la risposta viene amplificata quando la frequenza di distacco dei vortici si avvicina a quella della struttura. I risultati mostrano che per fluidi a bassa densità, come l'aria, questo fenomeno si ha per bassi valori di numero di Reynolds, al contrario di quanto accade per l'acqua. Bassi numeri di Reynolds comportano basse velocità del fluido e quindi ampiezza limitata della forzante. Le analisi sono state effettuate con i due tipi di circuiti sopra citati, calcolando i valori ottimali di resistenza e induttanza. Il circuito RLC a parità di azione fluidodinamica si è dimostrato più efficiente in termini di potenza elettrica ottenuta.
Il flutter rappresenta un meccanismo di instabilità aeroelastico. In questo contesto è stata valutata l'influenza dell'accoppiamento elettro-meccanico sul valore della velocità critica. Trattandosi di microstrutture è stato necessario opportune derivate di flutter in grado di descrivere il fenomeno per numeri di Reynolds abbastanza limitati. I risultati dimostrano come l'effetto piezoelettrico e il circuito esterno applicato incidano notevolmente sullo smorzamento meccanico del laminato, aumentandone la velocità critica. Successivamente la possibilità di applicare questo meccanismo ad un dispositivo \textit{Energy Harvesting} è stata analizzata.
Gli studi preliminari effettuati nel presente lavoro hanno permesso di capire quale sia la reale efficienza di micro-dispositivi \textit{Flow Energy Harvesting}. I risultati ottenuti sono incoraggianti e possono essere un punto di partenza per soluzioni più complesse in termini geometria, che richiederanno analisi più raffinati(simulazioni CFD, modellazione 3D dell'accoppiamento elettro-meccanico).This work of thesis deals with the study of some of the classical aeroelastic phenomena in the fluid-structure interaction of piezolaminated beams at the microscale, with the aim of evaluating a possible application in the field of \textit{Energy Harvesting}. The performances of this kind of structure are critically assessed by considering the theoretical estimate of electric power generation on the basis of realistic microfluidic flows. The following aeroelastic phenomena are considered: Vortex Induced Vibrations (VIV) and Flutter Instability (FI).
The theoretical study has been based on reduced order models, obtained by applying the Rayleigh-Ritz method. A model with two and three degrees of freedom (dofs) has been used respectively for VIV(dofs are transverse displacement and electric potential) and for FI (dofs are transverse displacement, torsional rotation and electric potential). The sectional behaviour of the beams has been represented by the theory of laminates, account taken of the piezoelectric coupling. Special consideration has been devoted to the electrical circuit, to be connected to the piezoelectric beams in order to extract electrical energy. Two types of circuit have been studied: only resistive electrical circuit (RC) and resistive-inductive electrical circuit (RLC). Preliminary analyses have been performed on cantilever structures, in such a way to understand the real efficiency of a \textit{flow energy harvesting} microsystem.
In the case of VIV, a better behaviour is obtained for fluids like water (with a high density) rather than others, such as air. The reason is twofold: first, the amplitude of the aerodynamic load is linearly influenced by the value of the fluid density; the second reason is linked to the lock-in phenomena. In fact, the syncrhonization occurs when the vortex shedding frequency is close to the frequency of the micro-structure: the results show that, in the case of fluid like air, this phenomena occurs at very low Reynolds numbers, owing to a large value of kinematic viscosity. Low Reynolds numbers involve low fluid speed and low amplitude of aerodynamic loads. Conversely, the kinematic viscosity of water is an order of magnitude less than that of the air, thus achieving a more efficient \textit{energy harvesting} structure. The analyses also show that the solution with RLC coupled circuit exhibits a more efficient response in terms of electric power generation: this fact depends on the low value of the electro-mechanical coupling.
Flutter represents instability phenomenon, so the aim of the study is to evaluate how the piezoelectric coupling influences the critical flow speed. To this purpose, the Author have used flutter derivatives reported in the literature for this kind of structure and for low Reynolds numbers. The results confirm that the electro-mechanical coupling increases the stiffness and the damping of the structure, and consequently increases the critical flow speed.
The preliminary simulations, carried out in the present study, allow for a deeper understanding of energy harvesting from fluid flow at the micro-scale. The achieved results will be used in order to obtain optimized devices, possibly endowed with different and more complex shapes with respect to the simple cantilever considered herein
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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