1,720,964 research outputs found
Focussing elastic waves in wing leading edge structures
No full textWings and other aircraft lifting surfaces must be kept free from ice during flight to maintain aerodynamic performance and avoid loss of control. In recent years, as an alternative to traditional thermal methods, several low-energy approaches that use high amplitude shock or vibration have been explored. Amongst them, ultrasonic vibration methods have been successfully validated, but the need to drive the piezoelectric actuators at resonance can cause them to debond from the structure, heat up or crack. To circumvent this problem, a method that achieves amplification through wave focussing instead of resonance has been recently proposed. This approach has been successfully validated for a beam and shown capable of delaminating an ice substitute. However, extensionof this method to realistic wing structures requires prior knowledge of dispersion characteristics. This thesis originates from research questions regarding the assessment of the feasibility of such a method and other wave focussing techniques present in the literature to wing structures, concentrating in particular on the wing slat leading edge where ice accumulates.The thesis starts by introducing an analytical model of an infinite Euler-Bernoulli beam characterised by a single wave mode. Three wave focussing techniques (i.e. the Precompensation, Inverse filter, and Time reversal techniques) are then presented and successfully applied to the same beam model, highlighting the important aspects from the viewpoint of the application. Successively, the formulation of the Semi-Analytical Finite Element (SAFE) method is briefly introduced, followed by a pre-existing SAFE implementation for free wave propagation in the commercial Comsol Multiphysics software. Thanks to a powerful tool of this software that allows one to set up and solve Comsol models directly from the Matlab environment, a procedure to extract the SAFE global mass and stiffness matrices is proposed. A pre-existing SAFE formulation to compute the forced response is also implemented, leading to a customised software that allows conducting the SAFE modelling of an arbitrary waveguide, solving both free and forced wave propagation entirely in the Matlab environment. Through the use of the proposed SAFE implementation, a simply supported plate strip is then modelled and the obtained results are validated with analytical solutions.A SAFE model of an idealised wing slat leading edge is developed to predict the dispersion characteristics and structural response to a concentrated load for the application of wave focussing techniques. To account for energy loss within the material, some structural damping in a small percentage is then introduced through the hysteretic model. Three experimental techniques of wavenumber estimation are presented and applied to an Euler-Bernoulli beam with emulated anechoic terminations. For each technique, the estimated and predicted results are compared, evidencing their capabilities and limitations. Amongst them, the correlation method has been revealed to be the most accurate and is then used to validate the SAFE model of the idealised wing slat leading edge through experimental measurements on a laboratory structure.In the last part of the thesis, the three wave focussing techniques implemented to the Euler-Bernoulli beam model are extended to the SAFE model of the idealised wing slat leading edge, highlighting their strengths and limitations. From the obtained results, the Pre-compensation technique has shown not to be able to provide wave focussing due to the high interference amongst waves. Conversely, the time reversal technique was able to generate a high shock response at the target position. The inverse filter technique, instead, showed ability to reconstruct the original excitation waveform and focusing waves at the target position; however, the enhanced quality of the focal signal came at the expense of lower focal amplitude. The one-bit method, a technique used in time-reversal experiments to increase the amplitude of the focal signal, is also implemented. The results showed that with the one-bit time reversal, the response at the focal point was three times bigger than the conventional time reversal technique, suggesting its use for de-icing applications.The SAFE model is also used to conduct a parametric study on the time reversal technique to investigate the effects of the excitation bandwidth, propagation distance and structural damping on wave focussing. To evaluate how much greater the response obtained using time reversal focussing is compared to another excitation signal used as a reference, an amplification factor in terms of power and peak amplitude is also defined. The results showed that, although different in values, both amplification factors follow a similar trend. In particular, the amplification factor related to peak amplitude increases as the excitation bandwidth and focal distance increase. However, at higher focal distancesand levels of damping, attenuation is responsible for reducing the amplification.Finally, through measured FRFs from a laboratory structure consistent with the SAFE model of the idealised wing slat leading edge, a simpler approach to the conventional time reversal technique is successfully implemented
Wave propagation in an aircraft wing slat for de-icing purposes
No full textIce accretions on aircraft lifting surfaces must be removed in flight to avoid catastrophic accidents. Conventionally, this is done effectively but inefficiently by bleeding hot air from the engines prompting a number of low energy approaches to be explored that use high amplitude shock or vibration. One recently proposed method, which has so far been successfully implemented for a beam, generates a shock response at a target position by focusing elastic waves in time and space from a single actuator. However, prior knowledge of the dispersion characteristics of such waves is required. In order to extend the applicability of the technique, this paper aims to predict the dispersion curves of waves propagating along the leading edge of a Boeing 737 wing slat. A semi-analytical finite element (SAFE) model is implemented in Comsol Multiphysics software. By assuming a spatially harmonic displacement field in the direction of propagation only the 2-D cross section of the waveguide needs to be meshed.Dispersion curves are presented for the leading edge portion of the erosion shield under several assumed sets of boundary conditions. The dispersion curves are compared with measured results from laboratory tests conducted on the real wing slat and found to be in good qualitative agreement. The model will be further developed to predict interfacial shear stresses between the wing skin and accreted ice under transient loading
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Aircraft de-icing using time-reversed guided waves
No full textThe passage of elastic waves in layered structures gives rise to interfacial stresses that can cause delamination. Whilst usually undesirable, this can be exploited to remove unwanted accretions from a substrate, such as ice from an aircraft wing. High amplitude transient waves, such as those induced by electro-expulsive de-icing systems, have proved effective, although their range is limited in part due to pulses spreading out with propagation distance from the actuator. In this paper, the phenomenon of wave dispersion which causes this effect is exploited through time reversality to focus waves both spatially and temporally and hence amplify the peak response at a chosen position. Harmonic finite element analysis is first undertaken to determine the wave bearing characteristics of a simplified representation of a wing leading edge. Transient analysis is then performed on the structure with an attached ice layer to focus disturbances of bandwidths up to 50 kHz at different chosen points. The amplitude of the peak shear stress between the ice and substrate is bandwidth dependent and predicted to be up to 30 times higher than due to a comparable pulse input. Predicted peak accelerations are successfully validated experimentally for a limited bandwidth of 10 kHz
Variations on the Author
Get PDF“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
Get PDFWe 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
Semi-analytical finite-element analysis for free and forced wave propagation using COMSOL and LiveLink for Matlab
No full textThe Semi-Analytical Finite-Element (SAFE) method represents one of the most established numerical approaches for predicting the propagation of elastic waves in one-dimensional structures of arbitrary cross-sections. Its implementation in the commercial finite-element software COMSOL Multiphysics has been proposed in recent years; however, it is limited to only the free wave propagation for computing dispersion curves. To overcome this limitation, this paper proposes an extension of this approach that combines COMSOL and its Livelink for Matlab tool. This enables the extraction from COMSOL of the assembled mass and stiffness SAFE matrices to solve problems of both free and forced wave propagation in the Matlab environment. The resulting customised software takesadvantage of both the potential of commercial FE software and the power of Matlab without worrying about compatibility issues. A model of a simply supported plate strip and that of a more complex geometry are implemented to validate, respectively, the SAFE matrix extraction procedure and the implemented forced response formulation. The results agree well with corresponding analytical and numerical results validating the proposed implementation of the SAFE method
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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