1,721,007 research outputs found
Turbulence characterization in the far wake of a wind turbine
No full textThe power output and aerodynamic loading of a wind farm depend strongly on the turbulence characteristics in the far-wake region of the constituent wind turbines. Predicting such turbulence is vital to wind-farm optimization but is challenging for existing wind-turbine wake models. To accurately model the far-wake turbulence, it is first necessary to understand how turbulence itself is generated in the wake region as well as in the atmospheric flow. In this thesis, this challenge is addressed in three successive stages. Firstly, an atmospheric boundary layer is studied for its ability to represent the lower region of the atmosphere in which wind farms operate. A large-eddy simulation (LES) solver is used to examine the effects of (i) the subgrid-scale model, (ii) the wall model, (iii) the von-Kármán constant and (iv) the grid-cell aspect ratio. It is found that although all of these factors influence the accuracy of the solution, it is the grid-cell aspect ratio that has the greatest effect on the variances of the spanwise and vertical velocity components. In particular, applying nearly isotropic grid cells causes all three components of the velocity variance to match the turbulence scaling laws prescribed by the attached eddy hypothesis. Based on these findings, the LES solver is improved such that it captures all components of the attached eddy hypothesis-based scaling laws, providing a reliable turbulent inflow condition for wind-turbine simulations. Secondly, the improved LES solver is coupled with an actuator disk model to simulate wind-turbine wakes under various atmospheric boundary layer inflow and tip-speed ratio conditions. It is found that different components of the velocity fluctuations contribute differently towards the far-wake evolution: (i) at low frequencies, e.g. St < 0.3 (St = ƒD/U is the Strouhal number, where ƒ is the frequency, D is the turbine diameter and U is the mean incident velocity), the spanwise (v′) and vertical (w′) velocity fluctuations are responsible for deflecting the wake via the passive wake meandering mechanism, (ii) at 0.1 < St < 1.0 all three components of the velocity fluctuations contribute to turbulent kinetic energy (TKE) generation via convective instabilities but v′ and w′ cause greater TKE generation than streamwise fluctuations u′ of the same magnitude, and (iii) at low frequencies, e.g. St < 0.3, u′ generates thrust fluctuations via fluctuations in the angle-of-attack, which also introduces near-wake corrections in the mean-wake deficit profiles. Simulation- and resolvent-based componentwise input-output analyses are used to explain why v′ and w′ play dominant roles in generating TKE: the convective instabilities in turbine wakes are more receptive to transverse forcing than to streamwise forcing. These results highlight the importance of accounting for the velocity fluctuation components individually when modeling the wake evolution. Another key conclusion is that the wake deflection from the passive wake meandering mechanism and the TKE generation from the convective instability mechanism can be modeled separately, because they mostly occur in different frequency ranges and thus interact mainly via the mean flow. Thirdly, resolvent analysis is performed on the simulated mean wake flow to exploit the convective instability mechanism for predicting the far-wake turbulence. In the formulation of the resolvent operator, the mean wake flow is assumed to be axisymmetric, with the effects of small-scale flow structures modeled via an eddy-viscosity term. Singular value decomposition of the resolvent operator is then used to identify a set of forcing and response modes ranked by their energy gain. The resolvent gain is found to peak at a Strouhal number of around 0.2 and at an absolute azimuthal wavenumber of 1. The forcing modes are concentrated upstream of the response modes. These findings reveal the role of convective instabilities in generating far-wake turbulence. The eddy-viscosity models enable the response modes to agree reasonably well with modes educed using spectral orthogonal decomposition on the instantaneous wake flow. To summarize, eddy-viscosity- based resolvent analysis can not only capture the role of convective instabilities, but can also predict the energetic structures in the far wake region. Finally, a comprehensive wake model is developed from the above findings. The scale dependence of the wake-center deflection observed in the LES is incorporated for predicting wake meandering caused by the passive wake meandering mechanism. The dominant role of convective instabilities in generating turbulence is accounted for to predict the TKE generation in the far wake. Compared with its original version, the present wake model significantly improves the predictions of the mean wake evolution, the wake meandering spectra and the TKE spectra. Moreover, the present wake model is efficient in capturing both the static and dynamic wake evolution, making it suitable for real-time calculations of wind farm performance. In this thesis, the turbulence characteristics in the far wake of wind turbines are predicted with resolvent analysis and dynamic modeling, based on the far-wake dynamics elucidated by LES. This modeling framework can be used to optimize the design and operation of wind farms, even under realistic atmospheric conditions, accelerating the transition towards a zero-carbon world.</p
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
DEVELOPMENT OF IMMERSED BOUNDARY METHOD FOR FLUID-STRUCTURE INTERACTION AND ITS APPLICATION
No full textPh.DDOCTOR OF PHILOSOPHY (CDE-ENG
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
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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