1,721,013 research outputs found
Dynamic wind farm flow control using free-vortex wake models
No full textIn the current state of model-based wind farm flow control, the implementation of yaw-based wake steering based on steady-state models has demonstrated potential for improving wind farm power production. However, for realistic, time-varying wind directions, the dynamics of wake propagation may impact the effectiveness of wake redirection. This dissertation presents the development of an economic model-predictive wind farm flow control strategy and assesses the potential for improved power production from wake steering in wind farms under time-varying conditions.At the core of such a model-based control strategy is a control-oriented model of the wind farm flow. A free-vortex wake model is formulated based on an actuator-disc representation of the wind turbine rotor. A validation study is included for power predictions in the mid to far wake of turbines operating under yaw misalignment using data from wind tunnel experiments. Finally, a distributed strategy for control optimisation is constructed to provide a scalable solution for dynamic wind farm flow control which is tested in a large-eddy simulation environment under realistic conditions. This novel controller yields additional gains in power production during wind direction transients and reduces the increase in yaw actuator usage from wake steering.Team Jan-Willem van Wingerde
Approximate basis and a priori closure models for energy-conserving reduced order modeling of fluid flows
No full textModel order reduction (MOR) has been a field of active research in the past twenty years, more recently also in fluid dynamics. The main advantage of MOR is computational cost reduction, which, along with equally important accuracy, constitute main objective in the MOR community.A main ongoing issue is that data volumes in fluid flow simulations (such as turbulent flows) are usually very large, hence processing is costly. An example of a high-cost operation in MOR is the construction of reduced basis (RB) via singular value decomposition (SVD) of snapshot data of turbulent flows. The present research, in its first objective, aims at tackling this problem by applying and adapting an incremental SVD algorithm (iSVD). The procedure does not require simultaneous access to the entire snapshot matrix, butthe price to pay is that accurate approximations of RB via iSVD are obtained only for low-index. ROMs require exactly those, therefore application of iSVD is plausible. The algorithm is tested on high-fidelity data representing transitional and turbulent flow solutions, obtained with an energy-conserving code (INS3D). Important iSVD paramters are identified and their influence on key properties of RB: orthogonality, zero-divergence and fidelity w.r.t. conventional SVD basis is examined.The second objective concerns closure modeling. MOR by definition neglects a part of information.Hence inaccuracies and/or instabilities often develop in the reduced order model (ROM) solution. The applied ROM framework is energy-conserving (EC-ROM), thereby ensuring non-linear stability. Accuracy is not guaranteed, therefore a correction is desirable. In ROM context several strategies exist. In the present research one such strategy, dissipation via a closure term, is examined in an `a priori' test. Based on the full order model (FOM) data and projection of it onto the reduced space, exact expression for missing information (exact closure term) is derived. Subsequently, an eddy viscosity (EV) ansatz is applied, whereby also high-fidelity data is used to compute EV. The turbulence model is of mixing-length type. The related turbulent diffusive term with variable EV is regressed on the exact closure term.It is concluded that iSVD is a feasible algorithm in MOR applications, particularly in combination with EC-ROM, provided that parameters of iSVD: increment size, maximum dimension of RB and threshold are far from their lower bounds. EV mixing length model is considered inadequate as a means of improving accuracy of EC-ROM in periodic shear-layer.Aerospace Engineerin
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
Quadrature Methods for Wind Turbine Load Calculations
Full text linkTwo sources of uncertainty can be distinguished in models for wind turbine calculations. Firstly, the environment the wind turbine has to withstand is uncertain and has a direct impact on the life time of the turbine. Secondly, the models used to predict the forces acting on the turbine contain an unknown error, which can also be modeled as a random variable. This thesis discusses numerical methods based on polynomial approximation to study these two types of uncertainty. In essence the computationally costly model is replaced by a polynomial, which is cheap to evaluate using a computer. The first part of the thesis is mainly focused on computing the loads acting on a wind turbine. The key uncertainties in this case originate from the variability in the environmental conditions (such as the weather). For load cases, the main interest is on integral quantities of the computationally expensive model. For the purpose of computing integral quantities, polynomial approximation is equivalent to smartly constructing interpolatory quadrature rules. Various algorithms are proposed to construct such quadrature rules. Their efficiency is demonstrated by computing loads acting on a turbine using measurement data obtained at the Dutch North Sea. Modeling the uncertainty arising from model error is significantly less trivial. Two different approaches, either based on interpolation using Leja nodes or integration based on quadrature rules, are discussed. Which approach is best in a certain computational test case depends on the specific quantity of interest. Examples of the applicability of all proposed methods are discussed throughout the thesis. A common theme in all results is that high convergence rates are obtained for models that can be approximated well using polynomials, which is usually the case for models arising in the field of wind energy.Wind Energ
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
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