1,720,979 research outputs found
Numerical modelling of cross-flow vertical-axis turbines
No full textClose-packed contra-rotating vertical-axis turbines have potential advantages for wind and hydrokinetic power generation. It is believed that such turbines maximise the fraction of flow passage swept, allowing generation well above the Betz limit for rotors in channels (Salter & Taylor, 2007). The design concept of contra-rotation, following Buntine and Pullin (1989), is based on two vortices of opposite-sign cancelling each other out, and thus conditioning the flow through the turbine while lowering turbulent kinetic energy in the wake. To determine the feasibility of large-scale marine hydrokinetic application of such devices, a numerical model is required.
This thesis describes the development of a numerical model of vertical-axis turbines with torque-controlled and contra-rotating systems using an actuator line model (ALM). This parallelised model, coupled with the open-source OpenFOAM computational fluid dynamics (CFD) code, is used to examine the characteristics of turbulent flow behind vertical-axis turbines. The numerical model is first validated against experimental measurements from a two-bladed H-type wind turbine, where the flow field containing the turbine is simulated by solving the unsteady Reynolds-averaged Navier-Stokes (URANS) equations with a k-ω SST turbulence model. Turbine loading is predicted, and the vorticity distribution is investigated in the vicinity of the turbine. Good overall agreement in terms of the amplitude and profile is obtained between numerical predictions and measured data on thrust coefficients. The rotor is demonstrably driven by the blade-generated lift, which is counteracted by the torque that accelerates the blades and turns the generator. The model captures important three-dimensional flow features that contribute to wake recovery behind a vertical-axis turbine, and enables predictions of the dynamic response of practical vertical-axis turbines to unsteady flow. Close-packed contra-rotating vertical-axis turbines are then implemented for a small-scale case involving four two-bladed wind turbines. The downstream wake evolution behind the turbines undergoes fast velocity recovery in the near-wake region behind the group of rotors. The lower level of turbulent kinetic energy in the wake, owing to the contra-rotation system, has a slight effect on the turbine performance of the present limited test cases in this thesis. However, it is still recommended to have further investigation to show the contributions to enhanced energy extraction in future research.
The numerical model is also validated against experimental measurements from a three-bladed vertical-axis tidal turbine. A parameter study of a pitch control system is carried out to solve the problem of the dynamic stall at low tip speed ratios, and to examine the effect of active pitch on wake turbulence. Tip vortices caused by adjacent foils at different angles are assumed to be suppressed using the turbine spoked-ring wheel (Salter & Taylor, 2007)
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
Numerical simulation and characterization of site specific tidal flow turbulence
Get PDFThe main area of study of this thesis was to research the effect of the bathymetry (the topography of the ocean floor) on the turbulent nature of a tidal energy site by way of high-resolution computational fluid dynamics simulations. Wall Modelled Large Eddy Simulations, for both flood and ebb tides, were carried out of a sampled area of the Fall of Warness, Scotland, which is the European Marine Energy Centre tidal energy site. Results from the simulations were validated with velocity and turbulence data from site measurements at the European Marine Energy Centre site. Both the simulation and the test data correspond to flows at 2.1 m/s turbine hub height reference velocity for ebb and flood tides.
Results show the influence of the sea bottom as a modifier of flow velocities and a driver of turbulence production at local areas important for the purposes of tidal converter performance and loading. High order turbulence parameters such as: Reynolds stresses, length scales, frequency spectra and turbulence intensities are analysed at different depths and locations in the domain, to better understand their local dependence on the changing bathymetry.
Turbulence data presents highly energetic coherent structures of a sufficiently large spatial and temporal size, connected to bathymetry variation. Both flood and ebb tidal simulations show that site bathymetry not only influences the mean flow, but also turbulent structure, energy efficiency of turbulent kinetic energy extracted from wall shear stress and three-dimensional distribution of Reynolds Stresses. Turbulence was found to be more isotropic for larger portions of the water column than found in previous simulations of roughened surface flows, with an especially chaotic flow area covering the bottom fifth of the water column filled with energy rich turbulent events.
Quadrant analysis for multiple depth probe locations is used to give a three-dimensional statistical understanding of bursting events, as modified by the seabed. The results show highly energetic coherent structures of a sufficiently large spatial and temporal size, connected to bathymetry variation.
Ebb and flood turbulent features were compared in an effort to further understand the role bathymetry plays on tidal asymmetry. The temporal, spatial and energetic character of coherent turbulent structures were analyzed, within a context of the direction of the flow and its link to the seabed as a driving force for turbulence generation. Ebb tide was found to have a stronger turbulent intensity and production than flood tide for the same reference mean flow velocities.
The thesis adds to the knowledge of tidal energy site flow characterization and the turbulence of offshore environments by showing the strong imprint of bathymetry on tidal flows. The numerical methodology employed shows a guideline for studying the local and unique seabed bathymetry interactions with the tides. The use of high-resolution large eddy simulations is shown to be a tool capable of reproducing energy rich resolved structures vital to continuous advancement of energy extraction devices
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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