1,720,959 research outputs found
AN INVESTIGATION ON ENERGY DYNAMICS OF COMPLEX RESONATORS
Get PDFIn the last decades the design process of engineering structures was characterized by an increasing demanding of the prediction capabilities at the design stage, which would have allowed to asses not only the performance and the integrity of the structure but also the vibro-acoustic performances. The more realistic is the model, the greater the opportunity to produce optimal design; this implies the need of using a model with a very high number of degrees of freedom. Predicting the response of a complex structural-acoustic system presents some difficulties. In fact, direct numerical solution of the governing equations of the system, while possible in principle, can be so computationally demanding as to be impractical. Furthermore, the application of deterministic methods of analysis to the prediction of its vibrational response would be not appropriate. In fact, in a complex system the presence of a high number of heterogeneous subsystems (beams, plates, acoustic cavities, etc.) and, above all, the high number of joints connecting the components are sources of a certain degree of variability of the system. However, geometric and fabrication tolerances, variation in material properties, structural irregularities, non uniform damping distribution are some of the cause of variability that lead to different dynamical behaviours of samples of a set of nominally identical manufactures. Moreover, many important engineering problems involve high frequency vibrations. In fact, especially in recent years, the use of light structures in aircraft and aerospace engineering, broad band excitations due to engines of increasing power and the increasing interest for the high speeds in ship, automotive and train engineering, created a general attention in the analysis of the high frequency vibration and of the acoustic problems. Approaching the high frequency problem both the difficulties above presented meet two serious limitations. In fact, high frequency problems involve vibrations characterized by short wavelengths compared with a typical length of the system. In conventional vibrational analysis based on a discretization of the continuum domains into small elements, being the size of these elements dependent on the minimum wavelength of interest, the computational time demanding could be so high that it would become prohibitive. Besides the problem of high time demanding, which could be in principle overcome with the growth of the computer power, the use of a very fine mesh also implies an accurate modelling of the system details. Obviously, such a modelling is difficult to obtain and it also introduces a certain degree of variability. On the other hand, at high frequency the response of the system becomes increasingly sensitive to small perturbations of its parameters: even small variations in geometrical component dimensions, material properties and assembly tolerances imply large variations in the mid- and high- frequency responses. In fact, while low order eigenvalues of the system are lightly affected by small variation of the system parameters, the more the order of the eigenvalues increases the more their values are modified, such that the behaviour of the structure becomes unpredictable. These considerations led to introduce a different way of tackling dynamic problems, based on a thermodynamic analogy. It is in fact in that frame that the problem related with system characterized by a high number of degrees of freedom (atoms) was studied for the first time. The necessity to reduce the computational cost has led to introduce global parameter as descriptor of the systems, rather than the local characterization, i.e. the punctual displacement. Further, to overcome the second problem, i.e. the inherent uncertainties of the system parameters, a statistical approach was favourite to a deterministic one, so that the attention is focused on an ensemble of similar system which characteristics are defined with a certain degree of variability. The Statistical Energy Analysis (SEA) represents in this scenario the most fruitful method inspired up to now. For the first time, some concepts reminiscent of thermodynamic were applied on the analysis of the dynamical behaviour of a mechanical system. In fact, SEA states an analogy between the energy flow in mechanical systems and the heat flow in thermal problems, such that the energy diffuses from one substructure to another at a rate proportional to the difference in "temperature", i.e. the modal energy, of the substructures and it is dissipated internally in each substructure at a rate proportional to the "temperature" of the substructure. Even if the analogy with thermodynamic problems brought important results in the energy analysis of vibrating structures, this analogy still present limits not yet completely understood. Moreover, the evaluation of the transient response to not steady excitation cannot be obtained by the classical SEA or other thermal-like approach, being their use limited at the steady state. Such an excitation is encountered in a large number of engineering structures: slamming load and breaking waves upon the bow of the ship hull, pyrotechnic shock in space vehicle structures, earthquake in civil engineering are only few examples of this. The impulsive excitation is particularly severe for the system, since can cause structural failure or unwanted noise. This work is concerned with the development of a predictive method for describing in the time domain the energy sharing among complex vibrating systems, affected by inherent uncertainty of their parameters. Starting from a new approach to the problem, based on a suitable combination of statistical thermodynamics and classical structural dynamics, the goal relies in a better understanding of both the initial transient energy sharing among the subcomponents and the long term energy response of the subcomponents. Before concluding this introduction, a brief description of the argument exposed in the following chapters is presented. In the next chapter, a critical analysis of the available method that, up to now, can be used for the analysis of complex systems is given. The attention is mainly focused upon the discernment of the advantages of each method, as well as upon the difficulties and the limitations of their application. In particular, the attention is directed to those methods that are partially inspired by a thermal analogy. In chapter 3 a new probabilistic energetic approach, (the Time Asymptotic Energy ensemble Average -TAEA), which has represented the foundation of the present work, is described. The originality of this methods lies in the development of an asymptotic expansion technique, that permits to evaluate the energy distribution between two subcomponents of a weakly coupled system, in both transient and steady state conditions with a low computational cost. As a particular result, this method also provides the long term energy responses of the two subsystems that, under some assumptions that will be later presented, expresses a condition reminiscent of the Energy Equipartition Principle (EEP) stated in Statistical Mechanics (SM). In the light of this results, it seemed interesting to go deep into the principle of energy equipartition as stated in SM, trying to understand the conditions under which it holds, the definitions introduced in SM and how these last could be translated to structural dynamics. This analysis, described in chapter 4, underlines that the EEP is not an obvious result in the analysis of dynamical systems. This is clear through the consideration that to approach the EEP in SM a number of assumptions are necessary, that are often not satisfied for engineering systems. This chapter is addressed investigating the conditions that can lead to the appearance of EEP and, as far as possible, a classification of the inhibiting or promoting factors for the reaching of energy equipartition conditions is presented. The analysis explores the field of linear and nonlinear vibrations, the effects of non-homogeneity and localization, the system dimension (i.e. its number of degrees of freedom), the weak or the strong coupling as well as the effect of the initial energy distribution among the subsystems. Chapter 5 is focused upon the theoretical improvement of TAEA theory to include an arbitrary strong coupling between the substructures and the interaction among more than two subsystems. These analyses has led to reformulate the theory introducing a new energetic parameter, which is the sum of two terms: the "blocked energy term", which represents the sum of the kinetic and the potential energy terms of the subsystem when the other subsystems are considered blocked and the "mixed energy term", which provide the energy contribution to the subsystem energy due to the motion of the others. This new formulation allows the determination of the energy sharing among two or more subsystems even in presence of strong coupling among them. In chapter 6 a validation of TAEA is presented through a comparison of the energy distribution among subcomponents of a system obtained applying the developed method and those obtained experimentally. Two different two-dimensional systems were analysed: the former is composed by two plates coupled by means of straps, while the second system is made up of three plates connected through straps among the subsystems. Despite a small difference between the theoretical results and the measured energies at the first instants, the agreement between them is very satisfactory in both cases. Chapters 7 and 8 close this thesis summarising the achieved results and showing some possible perspectives of the present study
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
Characteristics of the fluctuating pressure field beneath the turbulent boundary layer attached to the hull of a high speed vessel
Get PDFThe fluctuating pressure field generated by the turbulent boundary layer is one of the main sources of excitation and radiated noise for aircraft as well as for marine vehicles when for instance the induced vibrations in the hull plate of high-speed vessels or in the sonar dome are considered. In this frame, the hydrodynamic load acting on an elastic structure is usually described by the wave-number frequency spectrum that is a quantity difficult to obtain by direct measurements and it is usually provided by mathematical models deduced from the behaviour of the normalised cross spectrum. However also this last analysis is not easy to perform because the finite size of the pressure sensors implies a poor spatial resolution and signal attenuation at high frequency. On the other side, the background noise of the facilities often causes contamination of the data at low frequency. The wall pressure fluctuations spectra, when considering a body completely immersed in a fluid, depend on Reynolds number while, in the case of a surface piercing body such as a ship, a contribution from the Froude number must be taken into account hence introducing a new variable in its description. In view of the evaluation of the noise radiated on board of civil high speed vessels generated by flow exciting vibrations, an experimental campaign aimed to measure the pressure fluctuations beneath the turbulent boundary layer attached to the hull of a ship model was performed in a towing tank. The chance to use this kind of facility, characterised by a very low background noise, allows also the description of the low frequency part of the pressure spectra. An array of pressure transducers, flush mounted on a ship hull, were used to derive general laws for the streamwise cross spectral density behaviour at model scale based on the Corcos model. A detailed discussion of the scaling laws for the power spectral density in the different frequency range is also provide
INSEAN second year report
No full textThe main objective for the second year report was to provide the model scale pressure spectra produced by the turbulent boundary layer measured on the hull of the MISTRAL and of the JUMBO CAT. The acquired data were used to understand the principal characteristics of the TBL (turbulent boundary layer) induced noise. At the same time the final design of the experimental set up relative to SRF (surface random flow) excitation was completed with the help of numerical simulations
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
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