1,721,017 research outputs found
Numerical study of wind turbine wakes using modal-decomposition techniques and stability analysis
No full textThis thesis is focused on numerical analysis of wind turbines wakes. The flow over wind turbines is simulated performing Large Eddy Simulations (LES), where the rotor blades are modeled using the Actuator Line Method, whereas the Immersed Boundary Method is employed for tower and nacelle. The effect of tower and nacelle on wake dynamics is investigated by means of Proper Orthogonal Decomposition (POD) of numerical velocity data produced by two LES of a model wind turbine: one accounts only for the blades effect; the other includes also tower and nacelle. The turbine operates at Reynolds number Re = 6.3 × 10^5 and tip-speed ratio λ = 3. The two simulations are analysed and compared in terms of mean flow fields and POD modes that mainly characterize the wake dynamics. In the rotor-only case, the most energetic modes in the near wake are composed of high-frequency tip and root vortices, whereas in the far wake, low-frequency modes, mostly located in the wake shear layer region, are found. When tower and nacelle are included, low-frequency POD modes are present already in the near wake, linked to the von Karman vortices shed by the tower. These modes interact non linearly with the tip vortices in the far wake, generating new low-frequency POD modes, some of them lying in the frequency range of wake meandering. An analysis of the mean kinetic energy entrainment of each POD mode shows that tip vortices sustain the wake mean shear, whereas low-frequency modes contribute to wake recovery. This explains why tower and nacelle induce a faster wake recovery. The proper orthogonal decomposition, despite being able to isolate energetic flow structures in the wake, does not provide any physical information on their origin.
In an attempt to determine the physical mechanisms responsible for the emergence of these flow structures, the numerical data obtained without tower and nacelle are further analyzed performing two-dimensional modal and non-modal stability analysis of the turbulent mean flow developing downstream of a wind turbine rotor. Linear stability and optimal forcing analyses have been carried out in different cross-sections sufficiently far from the rotor, where nonparallel effects are rather weak. The frequency content and spatial structure of the most amplified perturbations are compared with that of the most energetic coherent structures recovered by POD analysis. Results show that most unstable modes computed relatively close to the rotor resemble large-scale oscillations isolated by the POD. Moving downstream, this matching is no longer verified; however, restricting the stability analysis to waves having streamwise wavenumber consistent with that of the POD analysis, we find three slightly stable eigenmodes bearing a strong resemblance with the most energetic POD modes.
The analyses described above are based on a model wind turbine; however utility-scale wind turbines operate at far larger Reynolds number, of the order of 10^8 and higher
tip-speed ratio. These differences can lead to a different wake dynamics. For this reason a reference utility-scale wind turbine (i.e. the NREL 5-MW) is simulated and analyzed using Proper Orthogonal Decomposition and Dynamic Mode Decomposition (DMD) in its sparsity promoting variant, which selects a limited subset of dynamically relevant modes. In contrast to the model wind turbine, the wake meanflow is, in this case, essentially aligned with the rotor axis and axisymmetric, suggesting a weaker impact of the tower. The coherent structures isolated by the two modal decomposition
techniques are similar to those observed for the model turbine, but a weaker interaction of tower’s wake and tip vortices is confirmed and a faster breakdown of the latter is reported. Furthermore, POD and DMD of the flow field provide rather different results. Large-scale, low-frequency oscillations are not present among the most energetic POD modes. On the contrary, sparsity-promoting dynamic mode
decomposition suggests that large-scale structures, developing far from the rotor, are relevant to the flow dynamics, despite their energetic content is not sufficiently high to overcome that of the tip vortices and their harmonics, which are among the first POD modes. This result demonstrates that while Proper Orthogonal Decomposition is efficient at identifying coherent structures, it may not be suitable for building a low-dimensional model of a wind turbine wake, while sparsity-promoting DMD can be a better choice
Using nonlinear optimization to understand coherent structures in turbulence and transition
Full text linkLo scopo di questa tesi è quello di investigare i principali meccanismi coinvolti in un flusso transizionale e/o turbolento. L'idea principale è quella di usare una tecnica di ottimizzazione non lineare per indagare l'origine e il ruolo delle strutture coerenti osservate in questi flussi. Questo metodo è stato utilizzato in tre diversi contesti.
Per prima cosa, partendo da un flusso laminare linearmente stabile, sono stati calcolati, tra tutti i disturbi in grado di innescare transizione alla turbolenza, quelli ottimali in termini di energia.
Una volta che la turbolenza è sviluppata, un ottimizzazione 3D non lineare basata sulla massimizzazione dell'energia cinetica turbolenta, è stata utilizzata per studiare le strutture coerenti che popolano i flussi turbolenti, e che sono responsabili della crescita (ottimale) e dello scambio dell'energia stessa.
Infine, la teoria dei sistemi dinamici è stata applicata alle equazioni dei fluidi. La crescita transitoria è stata utilizzata per studiare la geometria dello spazio delle fasi e per rivelare l'importanza della varietà stabile e instabile. Nello stesso framework, è stato utilizzato un algoritmo di minimizzazione non lineare per calcolare le connessioni eterocline tra soluzioni invarianti delle equazioni di Navier-Stokes.This thesis aims at unraveling the main mechanisms involved in
transitional and turbulent flows. The central idea is that of using a nonlinear
optimization technique to investigate the origin and role of coherent structures usually
observed in these flows. This method has been used in three different contexts. First, a
linearly stable laminar flow has been considered and the optimization has been used to
compute the most amplified perturbations among all disturbances able to trigger
transition to turbulence. Once turbulence is well established, a fully 3D nonlinear
optimization maximizing the turbulent kinetic energy is used to study coherent
structures populating turbulent shear flow as well as investigate the mechanisms
responsible for the energy (optimally) growth and exchange. Then, a dynamical system
approach is applied to fluid flow equations. The geometry of the state space is
investigated by using transient growth theory to reveal the importance of the stable and
unstable manifold. In the same framework, a nonlinear minimization algorithm is used
to compute heteroclinic connections among invariant solutions of the Navier-Stokes
equations.Cette thèse vise à démêler les principaux mécanismes impliqués dans les
écoulements transitoires et turbulents. L’idée centrale est d'utiliser une technique
d’optimisation non linéaire pour étudier l’origine et le rôle des structures cohérentes
habituellement observées dans ces écoulements. Cette méthode a été utilisée dans trois
contextes différents. Tout d’abord, un écoulement laminaire linéairement stable a été
considéré et l'optimisation a été utilisée pour calculer les perturbations les plus
amplifiées parmi toutes les perturbations capables de déclencher une transition vers la
turbulence. Une fois que la turbulence est bien établie, une optimisation non linéaire
entièrement 3D maximisant l'énergie cinétique turbulente est utilisée pour étudier les
structures cohérentes qui peuplent l’écoulement turbulent et les mécanismes
responsables de la croissance et de l’échange d’énergie (optimale) sont étudiés. Ensuite,
une approche de type système dynamique est appliquée aux équations du mouvement.
La géométrie de l’espace des phases est étudiée en utilisant la théorie de la croissance
transitoire pour évaluer l’importance des variétés stable et instable dans la dynamique.
Dans le même cadre, un algorithme de minimisation non linéaire est utilisé pour
calculer les connexions hétérocliniques parmi les solutions invariantes des équations de
Navier-Stokes
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
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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