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The Tower Shadow Effect in Downwind Wind Turbines
Get PDFScarcity of land sites applicable for wind turbines is pushing the technology offshore. By going offshore the expenses of nearly all components increases, which has triggered extensive research with the aim of decreasing component costs, and increasing reliability, as also maintenance costs increase as going offshore.
The focus of this thesis is to look at ways to reduce aerodynamic loads which can contribute in lowering structural fatigue loads and thereby component costs for offshore wind turbines. The loading is mainly reported as blade root flapwise bending moment on wind turbines intentionally made for bottom fixed offshore sites. The load reduction is investigated using a different wind turbine configuration with a downwind mounted rotor and further compared with the conventional upwind mounted rotor on a monopile tower. Blades on downwind mounted rotors are exposed to the fluctuating wake behind the towers, known as the tower shadow. The influence from the tower shadow on blade fatigue loads is investigated using three different types of towers; a full height truss type tower, a faring (airfoil shaped) tower and a monopile tower.
For reliable wind turbine simulations with downwind mounted rotors, an accurate tower shadow model is essential. Thorough investigations of the tower shadow is presented, including the detailed flow picture of the mean velocity deficit, unsteady and turbulent motions, as well as velocity spectra. The tower shadow is investigated in three different ways; using three dimensional physical model scale experiments, steady tower shadow models and two dimensional computational fluid dynamic (CFD) simulations.
By use of the tower shadow from the CFD simulations, an artificial increase in blade fatigue loading is seen as the transversal grid is made coarser. Although this is a ’safe-fail’ design (for the coarser grid), it should be kept in mind as this ’simulated’ safety factor from the coarse grid simulations could increase wind turbine costs.
A method for improving the accuracy of the steady tower shadow models (currently the most frequently used model in commercial software) through a preprocessing step, where the results are directly applicable in commercial software for full wind turbine simulations, is presented. This method will improve the reliability of the simulated results. The parameters of the steady tower shadow model and the turbulence intensity are fitted and calibrated with short CFD simulations of the relevant tower geometries. This method accounts for any deviation between the mean velocity deficit obtained from the steady tower shadow model and the CFD simulations, as well as the unsteady motions and turbulence due to the presence of the tower through the calibration of the turbulence intensity (maximum deviation of ±3 percent with respect to the tower shadow based on the CFD simulations, measured as blade fatigue loading).
The response measured as blade fatigue load show an increased loading for the blades on the downwind mounted rotors using the original blades, compared to the conventional upwind mounted rotor on a monopile tower. Introducing softer and lighter blades changed this result, with reductions in blade fatigue loading (compared to the upwind mounted rotor with the original blades) of three, four and five percent for the monopile, truss and fairing towers, respectively.PhD i bygg, anlegg og transportPhD in Civil and Transport Engineerin
Resolution of tower shadow models for downwind mounted rotors and its effects on the blade fatigue
Get PDFA simulation study on the wind field resolution in computer load simulations has been conducted, both in transversal/vertical and longitudinal direction, to determine the effect on blade fatigue loading. Increasing the transversal/vertical resolution decreased the loading significantly, while only small changes to the load, at very low frequencies were found for increased longitudinal resolution. Next the influence of the tower shadow for a downwind mounted rotor was investigated, with respect to blade fatigue loading. The influence of different components to the total tower shadow effect was studied, both for a monopile and a truss tower, latter at inclination 0 and 22.5 degrees with respect to the incoming wind direction. Four components were considered, both individually and in combinations: mean wind speed, mean velocity deficit, unsteady motions from vortex shedding, and turbulence. The mean velocity deficit and turbulence were the main contributors to blade fatigue loading, and the unsteady motions can be neglected for the truss tower. For the monopile, neglecting the unsteady motions resulted in an underestimation of fatigue loading in the order of 3 percent.publishedVersionContent from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd
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
Resolution of tower shadow models for downwind mounted rotors and its effects on the blade fatigue
No full textA simulation study on the wind field resolution in computer load simulations has been conducted, both in transversal/vertical and longitudinal direction, to determine the effect on blade fatigue loading. Increasing the transversal/vertical resolution decreased the loading significantly, while only small changes to the load, at very low frequencies were found for increased longitudinal resolution. Next the influence of the tower shadow for a downwind mounted rotor was investigated, with respect to blade fatigue loading. The influence of different components to the total tower shadow effect was studied, both for a monopile and a truss tower, latter at inclination 0 and 22.5 degrees with respect to the incoming wind direction. Four components were considered, both individually and in combinations: mean wind speed, mean velocity deficit, unsteady motions from vortex shedding, and turbulence. The mean velocity deficit and turbulence were the main contributors to blade fatigue loading, and the unsteady motions can be neglected for the truss tower. For the monopile, neglecting the unsteady motions resulted in an underestimation of fatigue loading in the order of 3 percent
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
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