1,721,085 research outputs found
Post-flutter analysis of flexible high-aspect-ratio wings
No full textThe nonlinear aeroelastic modeling and post-flutter behavior of HALE wings are discussed. By employing a geometrically exact 3-dimensional structural beam model coupled with the modified Beddoes-Leishman incompressible unsteady aerodynamic model, the study of a reduced-order model obtained by the Galerkin method is carried out. Continuation tools are employed to path follow both the nonlinear static equilibria as well as the limit cycles past the flutter velocity where the Hopf bifurcation occurs. Aeroelastic simulations are concurrently performed by reducing the governing equations to a form amenable to numerical integration in a finite element solution platform. Initial investigations are carried out on the nonlinear HALE wing model coupled with a quasi-steady aerodynamic model. It is highlighted that the number of mode shapes in the reduced-order models for the equilibrium response in the pre-flutter range becomes important for increasing air speeds. The stable post-flutter branches terminate at fold bifurcations, a somewhat expected result which, however, represents an important assessment of the conducted investigations. Further investigations present the effects of the unsteady aerodynamics for moderately large angles of attack described by the modified Beddoes-Leishman incompressible unsteady aerodynamic model that accounts for flow separation and dynamic stall. The ongoing investigations aim to study the aeroelastic behavior of these highly nonlinear wings, for an improved understanding of the nonlinear behavior in the neighborhood of the flutter boundary and in the post-critical regime, and in high angle-of-attack regimes where the unsteady aerodynamic effects due to flow separation and stall are more significant. ©2012 AIAA
UNSTEADY AERODYNAMIC MODELING AND FLUTTER ANALYSIS OF LONG-SPAN SUSPENSION BRIDGES
No full textA parametric one-dimensional model of suspension bridges is employed to investigate their static and dynamic aeroelastic behavior in response to a gust load and at the onset of flutter. The equilibrium equations are obtained via a direct total Lagrangian formulation where the kinematics for the deck, assumed to be linear, feature the vertical and the chord-wise displacements of the deck mean axis and the torsional rotations of the deck cross sections, while preserving their shape during rotation. The cables elasto-geometric stiffness contribution is obtained by condensing the equilibrium in the longitudinal direction assuming small horizontal displacements and neglecting the cable kinematics along the bridge chord-wise direction. The equations of motion are linearized about the prestressed static aeroelastic configuration and are obtained via an updated Lagrangian formulation. The equations of motion governing the structural dynamics of the bridge are coupled with the incompressible unsteady aerodynamic model obtained by a set of reduced-order indicial functions developed for the cross section of a suspension bridge, here represented by a rectangular cross-section. The space dependence of the governing equations is treated using the Galerkin approach borrowing as set of trial functions, the eigenbasis of the modal space. The time integration is subsequently performed by using a numerical scheme that includes the modal reduced dynamic aeroelastic Ordinary Differential Equations (ODEs) and the added aerodynamic states also represented in ODE form, the latter being associated with the lag-state formulation pertinent to the unsteady wind-induced loads. The model is suitable to analyze the effect of a time and space non uniform gust load distributed on the bridge span. The obtained aeroelastic system is also suitable to study the onset of flutter and to investigate the sensitivity of the flutter condition on geometrical and aerodynamic parameters. The flutter instability is evaluated using appropriate frequency and time domain characteristics. The parametric continuum model is exploited to perform dynamic aeroelastic flutter analysis and gust response of the Runyang Suspension Bridge over the Yangtze river in China
Post-flutter bifurcation behavior in long-span suspension bridges
No full textA linearized parametric continuum model of a long-span suspension bridge is coupled with a nonlinear quasi-steady aero-dynamic model giving the aeroelastic partial differential equations of motion reduced to the state-space ordinary differential form by adopting the Galerkin method. Numerical time-domain simulations are performed to investigate the limit cycle oscillations occurring in the range of post-flutter wind speeds. Continuation tools are thus employed to path follow the limit cycles past the flutter speed where the Hopf bifurcation occurs. The stable post-flutter behavior, which can significantly affect the bridge by fatigue, terminate at a fold bifurcation. This result represents an important assessment of the conducted aeroelastic investigations. The stability range of the limit cycle oscillations is evaluated by carrying out sensitivity analyses with respect to the main design parameters, such as the structural damping and the initial wind angle of attack. Copyright © 2013 by ASME
System identification from GVT and taxiing of an unmanned aerial vehicle
No full textThe modal parameters of an aircraft structure are currently estimated from ground vibration tests (GVT). These tests are carried out on ground in order to estimate the frequency response functions first and then the modal parameters. Such estimates require one or more shakers to excite the structure together with simultaneous measurements of both the input and the output signals. Recent developments in operational modal analysis allowed such modal identification from the measurements of the output responses only, provided that the unmeasurable excitation is practically a white noise stochastic input in the frequency range of interest and is randomly exciting the different parts of the structure. In this paper the effects of the different test setup on the modal parameter estimates will be presented. Both input-output based experimental modal analysis and operational modal analysis are performed on an Unmanned Aerial Vehicle, the Clarkson University Golden Eagle. The modal parameters estimated from the standard ground vibration test are compared with the ones achieved by using the engine induced vibrations as means of excitation while the UAV is taxiing
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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