1,721,077 research outputs found
A simplified aerodynamic model for floating VAWTs
No full textTo ensure successful modeling of a floating wind turbine, its aerodynamic behaviour has to be investigated. At the time of writing, the only relevant studies on the topic are about VAWT in skewed flow. There is lack of experimental, numerical or theorical studies about floating turbines.
In the following paper, the aerodynamic performance of a periodically oscillating VAWT is investigated through theorical and computational means. The complex dynamics of a floating turbine was simplified to a sinusoidal pitch motion, assuring simplicity without losing meaningfullness.
A theory is given to predict the aerodynamic torque of an oscillating VAWT, obtaining it from the one of the same fixed axis turbine. A blade-element model was developed to achieve this result, taking into account the effect of oscillation on key parameters affecting the torque, that is angles of attack and relative wind speed. The core idea of the method, is to use blade element theory not as a prediction itself, but as a mean to correct the aerodynamic torque of the fixed axis turbine. The latter may be the result of both experiments, or numerical simulations. The simplest though most effective correction developed is
To=Tf (1−k cos(ωot))2 , (1)
where ωo is the frequency of oscillation in rad/s, and k is the ratio of the maximum oscillation speed for a certain
section of the turbine over the freestream wind speed. Three different corrective functions were evaluated, that is one for the effect of the angles of attack, one for the relative wind speed and one considering both at the same time (Eq. (1)). Moreover, the correction may depend on just one representative section, the middle one in this case, or the entire rotor.
Theorical predictions were compared against data from CFD simulations, for two different oscillation frequencies. These were chosen in the typical range of wave energy spectrum, in order to test representative conditions for floating applications. CFD simulations were also performed to obtain the torque of the fixed axis turbine, which was validated against experimental data from the 17m Darrieus-type rotor studied by SANDIA laboratories.
CFD simulations showed aerodynamic forces are deeply affected by oscillation. As intuition suggests, torque increases when the turbine pitches in the opposite direction of the wind, and decreases when it pitches in the same direction. This periodic oscillation causes significative ripple and maximum torque increase. For the higher oscillation frequency, which represents the most extreme condition, maximum torque was about 2 times the one of the fixed turbine. Mean torque was found to be almost unaffected or slightly increased. For the higher oscillation frequency a 4.4% increase was observed.
The theorical model was able to reproduce the behaviour of the oscillating turbine with satisfactory accuracy. To quantify the matching, absolute error was divided by the peak torque of the fixed axis turbine. For the lower oscillation frequency worst accuracy is 13.8%, while mean accuracy is 5.3%. As the frequency grows the hypothesis which the model is based on become less valid, so for the higher oscillation frequency precision decreases, in fact worst accuracy is 29.0% while mean is
9.1%. The theory proved to attain reasonably accurate results notwithstanding its simplicity, making it a cost-effective tool for quick analysis or optimization. Moreover, the theory could insight the way in which oscillation affects the torque.
Considering relative wind speed separately showed that it has little influence, so it can be concluded that torque is affected by oscillation mainly by the change in the angles of attack. Finally, letting the correction depend on the entire rotor or just the middle section yielded almost identical results, proving the simpler model may be used without loss of precision
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
A review and analysis of optimisation techniques applied to floating offshore wind platforms
Full text linkThe deployment of offshore wind in the UK has seen a rapid increase in the past decade and will continue to increase with the securement of the recent Scotwind sites. Floating platforms will be utilised for 60% of these new sites, creating opportunities to try new platform typologies and further solidify the validity of existing concepts. Since there is no consensus on the platform typology, the cost will vary; however, it is predicted to be double the price of traditional fixed platforms. Finding the most optimal solution in terms of cost and performance is key to keeping cost low, allowing the technology to be more competitive. A technique which has been used in other industries is multi-objective optimisation, searching a large design space much more quickly than traditional methods. By carrying out a multi-objective approach, the optimal platform geometry can be identified over the Pareto Frontier, considering conflicting objectives such as cost and performance. The aim of this work is to review the existing literature on multi-objective optimisation of floating offshore wind (FOW) platforms, highlighting the gaps and shortfalls in the current literature. This review highlights the majority of work has been carried out for the 5 MW NREL turbine on a SPAR platform, utilising a genetic algorithm. Cost reduction has been noted as the main objective, however, the models found within the literature are simplistic, with a number of assumptions. The overall findings of this work highlight future work that could be improved: cost models, the inclusion of an energy production model linked to the platform motion, the requirement for analysis of larger turbines and the potential for a concept selection tool to reduce computational time
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
Rigid body dynamic response of a floating offshore wind turbine to waves : identification of the instantaneous centre of rotation through analytical and numerical analyses
Full text linkFloating Offshore Wind Turbines (FOWT) can harness the abundant offshore wind resource at reduced installation requirements. However, a further decrease in the development risks through higher confidence in the design and analysis methods is needed. The dynamic behaviour of FOWT systems is complex due to the strong interactions between the large translational and rotational motions and the diverse loads, which poses a challenge. While the methods to study the FOWT’s general responses are well established, there are no methods to describe the highly complex time-dependent rotational motion patterns of FOWT. For a rigid body in general plane motion, an Instantaneous Centre of Rotation (ICR) can be identified as a point at which, at a given moment, the velocity is zero. However, it is common to assume a centre of rotation fixed in space and time, arbitrarily set at the centre of floatation or gravity. Identification of the ICR is crucial as it may lead to better motion reduction methods and can be leveraged to improve the designs. This includes better-informed fairlead placement and the reduction of aerodynamic load variability. In this paper, we propose a two-fold approach for the identification of the ICR: an analytical solution in the initial static equilibrium position, and a time-domain numerical approach for dynamic analysis in regular and irregular waves to understand the motion patterns and ICR sensitivity to environmental conditions. Results show that the ICR of FOWT depends on wave frequency and, at low frequencies, on wave height, due to the nonlinear viscous drag and mooring loads. An unexpected but interesting result is that the surge-heave-pitch coupling introduced by the mooring system leads to a dynamic phenomenon of signal distortion known as ”clipping” in the nonlinear audio signal processing area, which, through the introduction of higher harmonics, is responsible for the ICR sensitivity to motion amplitude
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