1,721,105 research outputs found
Capturing nonlinear time-dependent aircraft dynamics using a wind tunnel manoeuvre rig
Full text linkThis paper considers a novel multi-degree-of-freedom dynamic manoeuvre rig,with the aim of assessing its potential for capturing aircraft model nonlinear time dependent dynamics in the wind tunnel. The dynamic manoeuvre rig capabilities are demonstrated via a series of experiments involving a model aircraft in a closed section low-speed wind tunnel. A series of open loop experiments show that the aircraft model exhibits nonlinear time dependent dynamics. This nonlinear behaviour manifests itself as limit cycle oscillations that increase in complexity with the number of degrees-of-freedom in which the aircraft is allowed to move. Two real-time closed loop control experiments further illustrate the manoeuvre rig potential: first, using a pitch motion configuration, an experiment is conducted to investigate the limit cycle behaviour in more detail, allowing the stability properties of the pitch oscillations to be assessed; secondly, using a 5-DOF motion configuration, the test motion envelope is extended by using a compensating feedback control law to track the aircraft’s roll motion. Together, these experiments demonstrate the manoeuvre rig potential to reveal aircraft nonlinear and unsteady phenomena.<br/
Intermittent gear rattle due to interactions between forcing and manufacturing errors
No full textThe interaction between eccentricity and an external forcing fluctuation in gear rattle response is investigated experimentally. The experimental rig consists of a 1:1 ratio steel spur gear pair, the input gear being controlled in displacement and the output gear being under no load. Gear transmission errors recorded using high accuracy encoders are presented. Large variations in backlash oscillation amplitude are observed as the relative phase of the input forcing and the sinusoidal static transmission error due to eccentricity is varied. A simplified mathematical model incorporating eccentricity is developed. It is compared with experimental findings for three different gear eccentricity alignments by way of plots relating backlash oscillation amplitude to forcing amplitude and phase relative to eccentricity sinusoid. It is shown that eccentricity does not fully account for the experimentally observed large variations in amplitude. Through analysis of the experimental data, it is suggested that further tooth profiling errors may explain the discrepancie
The effect of nonlinear cross-coupling on reduced-order modelling
Full text linkThe use of reduced-order models (ROMs) for nonlinear systems has received significant attention due to their potential to greatly reduce computational cost, compared to full nonlinear finite-element models. Here, we consider and compare two indirect methods; the applied modal force and enforced modal displacement techniques, paying particular attention to the effect of nonlinear cross-coupling terms. The analysis we present shows that the applied modal force technique is able to account for some effects arising from modes that are not retained in the ROMs, but the resulting accuracy of the ROM depends on the amplitudes selected for the set of forces used to estimate the coefficients of the ROMs. This analysis also shows that the enforced modal displacement technique does not compensate for the effect of modal interactions with modes that are not included in the ROM, but its accuracy is independent of the amplitude of the forces used to estimate the coefficients. The mechanisms that lead to the differences between these techniques is firstly demonstrated using a two conceptually-simple, discrete systems, before a nonlinear beam model is considered
Nonlinear Modal Decomposition Using Normal Form Transformations
No full textIn this paper we discuss a technique for decomposing multi-degree-of-freedom weakly nonlinear systems into a simpler form. This type of decomposition technique is an established cornerstone of linear modal analysis. Extending this type of technique to nonlinear multi-degree-of-freedom systems has been an important area of research in recent years. The key result in this work is that a theoretical transformation process is used to reveal both the linear and nonlinear system resonances. For each resonance, the parameters which characterise the backbone curves and higher harmonic components of the response, can be obtained. The underlying mathematical technique is based on a near identity normal form transformation for systems of equations written in second-order form. This is a natural approach for structural dynamics where the governing equations of motion are written in this form as standard practice. The example is a system with cubic nonlinearities, and shows how the transformed equations can be used to obtain a time independent representation of the system response. It is shown that when the natural frequencies are close to an integer multiple of each other, the backbone curve bifurcates. Examples of the predicted responses are compared to time-stepping simulations to demonstrate the accuracy of the technique
Experimental Investigation of Aerodynamic Hysteresis Using a Five-Degree-of-Freedom Wind-Tunnel Maneuver Rig
Full text linkThe high incidence aerodynamics of a lightweight jet trainer aircraft model has been investigated using a novel five degree-of-freedom (DoF) dynamic manoeuvre rig, recently updated with improved actuation and data acquisition systems, in the 7' x 5' closed-section low-speed wind tunnel at the University of Bristol. The major focus was to identify the nonlinear and unsteady aerodynamic characteristics specific to the stall region and which affect free-to-move aircraft model behaviour. First, the unstable equilibrium states in the limit cycle regions were stabilized, and so observed, over a wide range of angles of attack using a simple elevator feedback control law based on pitch angle and pitch-rate sensor measurements.Tests with two degrees-of-freedom, namely the aircraft model and rig arm pitch angles, revealed the existence of static hysteresis in the normal force acting on the aircraft model in the stall region. Unlocking the aircraft model in roll and yaw accompanied by feedback stabilization of the lateral-directional modes of motion demonstrated onset of asymmetric aerodynamic rolling and yawing moments in this four degree-of-freedom configuration. This observation implicitly indicates a link between the static hysteresis in the normal aerodynamic force with an onset of aerodynamic asymmetry. The experimental results show the efficiency of the updated multi-degree-of-freedom actively controlled manoeuvre rig in providing insight into complicated aerodynamic effects within the stall region
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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