1,721,024 research outputs found
Development of modeling techniques for rolling element bearings
No full textMolti sistemi si affidano a cuscinetti volventi per garantire il moto relativo di corpi (e.g. alberi) al fine di minimizzare l'attrito e garantendo comunque il vincolo richiesto.
Una predizione accurata della dinamica della macchina richiede la modellazione ad alta fedeltà dei sotto sistemi ed i relativi componenti, inclusi modelli accurati di contatto lubrificato e non.
A causa del continuo aumento delle performace richieste e del continuo aumento di densità di potenza, il comportamento a livello di sistema ed il comportamento del singolo componente non possono più essere studiati separatamente. Il forte accoppiamento tra le differenti scale di analisi e l'intrinseca non linearità del contatto non conforme, influenza fortemente la dinamica dell'intero sistema.
Questa tesi mira a sviluppare modelli di contatto che garantiscano livelli di fedeltà crescente, sia per contatti di punto (e.g. Cuscinetti a sfere), sia per contatti di linea (e.g. Cuscinetti a rulli).
Grazie ai modelli di contatto sviluppati siamo in grado di avere una predizione del comportamento del cuscinetto molto accurata, la quale è inoltre in grado di tenere in considerazione vari fenomeni come giochi interni, momenti giroscopici, lubrificazione, forze centrifughe etc.
I modelli proposti offrono la possibilità di investigare il comportamento dei più rilevanti tipi di cuscinetti con 5-dof (gradi di libertà) garantendo un basso costo computazionale senza tuttavia compromettere la fedeltà del modello.
Queste tecniche di modellazione sono orientate sia alla descrizione del comportamento del contatto a se stante, ma hanno anche lo scopo di garantire una efficacie analisi multibody di interi sistemi
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
Model Reduction Techniques to Improve the Efficiency of Flexible Multibody Simulations
No full textThe ever-increasing trend to design machines and processes to befaster, more lightweight, more energy-efficient and more reliablecauses virtual prototyping to gain importance over physicalprototyping. Accurate numerical modeling techniques for flexiblemultibody systems are needed in the different stages of thedesign. Furthermore, numerical models are not only used in thedesign phase; certain applications rely on simulation results ofnumerical models computed in real-time. Real-time targets arecurrently only met for highly simplified multibody models.However, in industry a strong desire exists to extend real-timecapabilities to more advanced models.In model reduction, the evolution of the system state is decomposedin dominant and negligible motion patterns. In the currentstate-of-the-use in flexible multibody modeling mainly body-levelmodel reduction is used. Body-level model reduction refers tomodel reduction applied to the flexibility description ofindividual bodies, i.e. mechanism components. These reduced bodyflexibility models are then incorporated in the model equations ofthe overall system. In case of a multitude of possible loadingpoints on a flexible body, its reduced body flexibility model willbe of significant size, such that time integration of the overallmultibody model becomes expensive. A first goal of this researchis to develop and validate efficient modeling techniques for therepresentation of flexibility in multibody dynamics, especiallytargeting this flaw in the state-of-the-use.A first solution to this problem is interface reduction, in which component interaction is approximated by a limited set of interaction patterns.An alternative interface reduction scheme is proposed and the associated computational hurdles are solved. This offers the user an alternative to model intercomponent interaction, without the numerically introduced artificial stiffness of conventional techniques. In a numerical experiment the effect of this artificial stiffness is illustrated.However, accuracy requirements can impose the use of more detailed body flexibility models. Quite often, many DOFs can be loaded during simulation, but few are loaded simultaneously. The multitude of possibly loaded DOFs imposes an expensive body flexibility description, but at any moment in time only a low-dimensional part of this description contributes to the solution. An innovative methodology is proposed, called Static Modes Switching, which at every time step only includes the strictly needed deformation patterns, so that at every time step an accurate body flexibility description of minimal size is obtained. Considerable simulation speed gains are obtained with an acceptable loss of accuracy.A second goal of this research is to develop and validate system-level model reduction techniques which enable real-time simulation of flexible multibody systems.The presence of both differential and algebraic equations in the model equations, and the number of degrees of freedom needed to accurately represent flexibility prohibit real-time simulation of these systems. Most current model reduction techniques for non-linear systems project the model equations on a set of invariable motion patterns. Only by using configuration-dependent motion patterns, one can transform the model equations from a set of differential-algebraic equations into a set of ordinary differential equations, and achieve a maximal dimension reduction. This is done in Global Modal Parameterization (GMP), for which this research proposes a generalization. Global Modal Parameterization divides the computational load over an expensive preparation phase and a cheap simulation phase. This research focuses on applications where fast online simulation capabilities justify an expensive offline preparation, such as real-time applications.In a first step, modal motion patterns are used. The sources of approximation error of a GMP-description are investigated. The effect of the configuration space discretization coarseness on the different approximation error sources is illustrated. The trade-offs to be defined by the user to control these approximation errors are explained. It was observed that the eigenmodes with eigenfrequencies near or within the frequency range of the excitation should be included in the motion patterns for model reduction. Furthermore, additional motion patterns are required to compensate for the quasi-static contribution of the omitted eigenmodes.Mode veering and mode crossing cause abrupt changes in mode shapes. As a modal GMP approach is based on describing the motion by the contributions of these rapidly changing motion patterns, the degrees of freedom of a GMP-description can vary abruptly for moderate system changes. It is theoretically proven that this even results in singularities for mode veering.Although the individual eigenmodes vary abruptly, the vector space spanned by a pair of veering/crossing modes has limited variability.To exploit this, linear combinations of eigenmodes are proposed to generate smoothly varying motion patterns. This requires an automatic detection of mode veering. A numerical experiment shows that this is unfeasible for systems with multiple parameters defining the dynamics.As a second solution, Krylov subspaces are proposed for the motion patterns of the GMP model reduction. A Krylov subspace also spans the dominant dynamics near a chosen frequency, without singularities due to mode veering.The maximal variability with respect to the system configuration of the first Krylov vector is one order of magnitude lower than the first eigenmode. However, due to the recursive definition and the orthogonalization in the Arnoldi computational process, variability is propagated and amplified through the series of Krylov vectors. By omitting this orthogonalization step, this propagation and amplification of variability vanishes. However, a singularity-free GMP-simulation using Krylov seems to be infeasible.Finally, several future research tracks are proposed.status: Publishe
Modelling and Quantification of the Effect of Mineral Additions on the Rheology of Fresh Powder Type Self-Compacting Concrete
No full textPowder type self-compacting concrete (SCC) mixtures are characterised by a higher powder content compared to traditionally vibrated concrete (TC) mixtures in order to increase the viscosity of the concrete mixture and thus achieving a sufficiently high resistance to segregation, while a third generation (polycarboxylate ether based, PCE) superplasticizer is used in order to increase the flowability of the SCC mixture. Besides cement, powder type SCC mixtures mostly incorporate readily available mineral additions like limestone powder, quartz powder, fly ash or silica fume in order to reduce heat generation during cement hydration.The objective of this work is to describe the workability of powder type SCC by means of a rheological approach. In this way, a fundamental description of the concrete flow behaviour during placement is obtained, which is indispensable in case of special concretes (such as SCC) and/or in cases of congested reinforcements.For fresh concrete, two major different rheological approaches can be used, depending on the scale of observation: the suspension approach (local scale) or the continuum/fluid approach (global scale). In this thesis, mainly the global flow behaviour of fresh SCC is treated and a continuum/fluid approach is used in order to describe the concrete flow behaviour.Due to the viscoplastic behaviour of the fresh concrete, it is generally agreed that, as a good first approximation, fresh concrete can be described by a linear flow curve, according to the Bingham model. However, test results on powder type SCC mixtures revealed that a non-linear flow curve (Herschel-Bulkley model) is often needed in order to describe more correctly the shear rate-shear stress relationship.The wide-gap Couette concentric cylinder rheometer used for the experiments, together with the theoretical derivation of the solution of the Couette inverse problem by means of the integration method for both a Bingham and a Herschel-Bulkley fluid are presented in this work.The description of the shear thickening flow behaviour of powder type SCC in terms of shear rate and shear stress can be used as input for the global scale (CFD) simulations of powder type SCC processing operations (e.g. casting, mixing or pumping) and can be considered as a small step into the promising direction of (fresh) concrete flow simulation/prediction tools.To illustrate the importance of predicting the thixotropic behaviour for a given (powder type) SCC mixture, two practical applications, i.e. formwork pressure and distinct-layer casting are discussed.It was found that the Herschel-Bulkley parameters are reliable, absolute rheometry parameters (at least for the shear rate region actually tested). The wide-gap concentric cylinder rheometer provides a reproducible test procedure, from which the parameters and their influencing factors can be derived.Fresh powder type SCC is susceptible to a shear thickening flow behaviour. This is due to its intrinsic mix design philosophy, i.e. the combination of a sufficient amount of small (powder)particles and a low amount of coarse aggregates (where the hydrodynamic forces dominate), which promotes cluster formation.Furthermore, the addition of a PCE superplasticizer will result in a more dispersed state of the smaller (powder)particles, so that a larger amount of these particles is available for shear thickening, while test results also indicated that the molecular design of the PCE superplasticizer has most-likely a non-negligible impact on the shear thickening effect.The intensity of the shear thickening effect can be modified by the nature and fineness of the mineral addition used. It was found that the limestone, quartz and fly ash addition used in this research project respectively increase, unalter and decrease the shear thickening intensity. Increasing the fineness of the limestone addition resulted in a higher shear thickening intensity.Microstructural interpretations of the rheological behaviour are proposed, based on the order-disorder transition theory and the cluster theory, complemented with local mechanical actions caused by the shear effect.status: Publishe
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