1,720,961 research outputs found
Dynamic interaction between OWC system and Wells turbine: a comparison between CFD and lumped parameter model approaches
No full textSystems composed of an oscillating water column and a self-rectifying turbine are among the most well-known devices for wave energy conversion. These systems have been largely studied experimentally and numerically, focusing either on OWC hydrodynamic or power-unit performance. The turbine has been studied in experimental facilities that reproduced the periodic motion of the water column in the chamber by an artificially moved piston, while OWC performance has been generally studied neglecting or crudely simplifying the turbine interaction. The objective of this work is to introduce a lumped parameter model, able to reproduce the interaction of the turbine with the air mass in the OWC chamber. Results are compared with experimental and CFD data, and demonstrate that the often discussed hysteresis is caused by compressibility effects in the air chamber and not, as previously assumed, by turbine aerodynamics. The model is also used to estimate energy loss and phase delay in full-scale OWC systems, proving the importance of a correct sizing of OWC-turbine systems. The lumped parameter model allows to rapidly isolate parameters with the largest influence on system performance, and could be integrated with existing wave-to-wire models to improve the understanding of the hydrodynamic/aerodynamic interaction in the overall system
Evaluation of entropy generation methods in wells turbines
No full textEntropy generation analyses have been applied, in recent years, to a variety of systems, including Wells turbines. This can be a very powerful method, as it can provide important insights into the irreversibilities of the system, as well as a methodology for identifying, and possibly minimizing, the main sources of loss. However, some of the simplifications used in recent studies raise more than a concern on the validity of the approach. This work proposes a method based on RANS equations to evaluate the en-tropy production in Wells turbines. An estimation of the second-law efficiency of different Wells turbine rotors is also presented, under conditions representative of the air flow inside an OWC device. The main sources of entropy generation are highlighted and compared for the different geometries
Wells turbines dynamic simulation using cfd and a lumped parameter model
No full textThe hydrodynamics and aerodynamics of oscillating columns systems have been largely studied both experimentally and numerically, especially through laboratory facilities which focused either on OWC or power unit performance, usually aWells turbine. For the latter, a common simplification is made by substituting the periodic wave motion with the alternative motion of a piston in a chamber that is connected to the duct that hosts the turbine. This setup led to detailed studies on the difference between turbine performance during acceleration and deceleration, usually attributed to a hysteretic behavior of the turbine. This work demonstrates, using both CFD and a lumped parameter model, that the OWC hysteresis is caused by compressibility effects in the air chamber and not, as previously assumed, by an aerodynamic hysteresis of the turbine
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
On the Hysteretic Behaviour of Wells Turbines
No full textThe Wells turbine is a self-rectifying axial flow turbine employed in Oscillating Water Column systems to convert low-pressure airflow into mechanical energy. A number of studies highlighted a variation in turbine performance between acceleration and deceleration phases, generally ascribed to the interaction between blade trailing edge vortices and blade boundary layer. This explaination is in opposition with the large existing literature on rapidly pitching airfoils and wings, where it is generally accepted that a hysteretic behavior can be appreciated only at non-dimensional frequencies significantly larger than the ones typically found in Wells turbine. This work presents a critical re-examination of the phenomenon and a new analysis of some of the test cases originally used to explain its origin. The results demonstrate how the behavior of a Wells turbine is not dissimilar to that of an airfoil pitching at very low reduced frequencies and that the causes of the alleged hysteresis are in a different phenomenon
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