1,720,982 research outputs found
Preliminary design of variable-pitch systems for darrieus wind turbine using a genetic algorithm based optimization procedure
No full textIn recent years, the research in the field of Vertical Axis Wind Turbines (VAWT) has increased significantly because of the capability of these wind energy systems to operate with omni-directional wind with no yaw mechanism. However, VAWTs are commonly affected by a dynamic stall condition that is typically induced by a steep variation of the angle of attack of the blades during the turbine motion. This circumstance increases the noise and reduces the fatigue life of mechanical components. A consolidated strategy to delay the dynamic stall consists in a dynamic variation of the blade pitch angle. In the Literature, both active and passive variable-pitch systems have been presented to replicate an assigned variable-pitch law. An active variable-pitch system changes the blade pitch angle continuously during the turbine rotation by means of actuators that modify the blade aerodynamic layout, achieving the desired (optimal) angle of attach. A passive variable-pitch system modifies directly the pitch using a combination of aerodynamic forces and inertial loads acting on the blade. However, the set-up of passive variable-pitch mechanisms results quite complex and erratic in many cases. In the present paper, two preliminary designs of variable-pitch systems, based on an active approach, are proposed. The first system is characterized by the following components: (i) an eccentric point with respect to the main rotational axis, which position varies dynamically during the turbine motion by means of two linear actuators; (ii) an additional linear actuator that induces a variation of the current distance between a specific hinge point on the airfoil chord and the eccentric point. An optimization procedure based on a binary genetic algorithm is performed to determine the optimal stroke variation of actuators during the turbine motion to reply an a priori known, effective variable-pitch law. The other active system is conceived to be mounted externally to the main VAWT structure (ground fixed) to allow the implementation also in small scale systems characterized by a limited airfoil thickness. In this case the blades are hinged at a quarter of the chord to allow a pitch variation; a pin mounted on a rear point along the chord at the bottom end of the blade is left free to slide into a curved (circular) slot cut out from the shield disk. The pin end is then constrained to follow the radial profile of an external loop during the turbine rotation. The conformation of the external loop has been determined using a genetic algorithm to replicate the kinematics of a previously achieved optimal pitch law
Low-frequency vibrational spectrum of mean-field disordered systems
No full textWe study a recently introduced and exactly solvable mean-field model for the density of vibrational states D(omega) of a structurally disordered system. The model is formulated as a collection of disordered anharmonic oscillators, with random stiffness kappa drawn from a distribution p(kappa), subjected to a constant field h and interacting bilinearly with a coupling of strength J. We investigate the vibrational properties of its ground state at zero temperature. When p(kappa) is gapped, the emergent D(omega) is also gapped, for small J. Upon increasing J, the gap vanishes on a critical line in the (h, J) phase diagram, whereupon replica symmetry is broken. At small h, the form of this pseudogap is quadratic, D(omega) similar to omega(2), and its modes are delocalized, as expected from previously investigated mean-field spin glass models. However, we determine that for large enough h, a quartic pseudogap D(omega) similar to omega(4), populated by localized modes, emerges, the two regimes being separated by a special point on the critical line. We thus uncover that mean-field disordered systems can generically display both a quadratic-delocalized and a quartic-localized spectrum at the glass transition
Mean-field model of interacting quasilocalized excitations in glasses
No full textStructural glasses feature quasilocalized excitations whose frequencies ω follow a universal density of states D(ω)∼ω4
. Yet, the underlying physics behind this universality is
not fully understood. Here we study a mean-field model of quasilocalized excitations in
glasses, viewed as groups of particles embedded inside an elastic medium and described
collectively as anharmonic oscillators. The oscillators, whose harmonic stiffness is taken
from a rather featureless probability distribution (of upper cutoff κ0) in the absence of
interactions, interact among themselves through random couplings (characterized by
a strength J) and with the surrounding elastic medium (an interaction characterized
by a constant force h). We first show that the model gives rise to a gapless density
of states D(ω) = Ag ω4
for a broad range of model parameters, expressed in terms of
the strength of the oscillators’ stabilizing anharmonicity, which plays a decisive role in
the model. Then — using scaling theory and numerical simulations — we provide a
complete understanding of the non-universal prefactor Ag(h, J,κ0), of the oscillators’
interaction-induced mean square displacement and of an emerging characteristic frequency, all in terms of properly identified dimensionless quantities. In particular, we
show that Ag(h, J,κ0) is a non-monotonic function of J for a fixed h, varying predominantly exponentially with −(κ0h
2/3/J
2
) in the weak interactions (small J) regime —
reminiscent of recent observations in computer glasses — and predominantly decays as
a power-law for larger J, in a regime where h plays no role. We discuss the physical interpretation of the model and its possible relations to available observations in structural
glasses, along with delineating some future research directions
Following the Evolution of Hard Sphere Glasses in Infinite Dimensions under External Perturbations: Compression and Shear Strain
No full textWe consider the adiabatic evolution of glassy states under external perturbations. The formalism we use is very general. Here we use it for infinite-dimensional hard spheres where an exact analysis is possible. We consider perturbations of the boundary, i.e., compression or (volume preserving) shear strain, and we compute the response of glassy states to such perturbations: pressure and shear stress. We find that both quantities overshoot before the glass state becomes unstable at a spinodal point where it melts into a liquid (or yields). We also estimate the yield stress of the glass. Finally, we study the stability of the glass basins towards breaking into sub-basins, corresponding to a Gardner transition. We find that close to the dynamical transition, glasses undergo a Gardner transition after an infinitesimal perturbation
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
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