1,720,958 research outputs found
Phase calibration of in-duct axial microphone arrays
No full textIn-duct microphone arrays are used in experimental test facilities to understand the generation of noise by aeroengine componants such as fan-blades. A typical microphone array used to perform modal analysis, for example, is a linear array of wall-mounted microphones. Accurate results can only be obtained if the microphone array is well calibrated for both magnitude and phase. As in-situ calibration is often impossible the microphones must therefore be calibrated outside of the duct. This is unsatisfactory as changes in the phase response that result from mounting the microphones in the duct wall after calibration, where they are exposed to flow and temperature effects, and the effects of the microphone mounting are not accounted for. This paper presents a method to calibrate the microphones in-situ using properties of the broadband noise generated, for example, by rotating fan blades. The technique allows the relative phase difference between individual microphones to be determined. The technique is validated using experimental data obtained from a laboratory scale no-flow rig
Determining the strength of rotating broadband sources in ducts by inverse methods
No full textAeroengine broadband fan noise is a major contributor to the community noise exposure from aircraft. It is currently believed that the dominant broadband noise mechanisms are due to interaction of the turbulent wake from the rotor with the stator, and interaction of the turbulent boundary layers on the rotor blades with their trailing edges. Currently there are no measurement techniques that allow the localisation and quantification of rotor-based broadband noise sources. This paper presents an inversion technique for estimating the broadband acoustic source strength distribution over a ducted rotor using pressure measurements made at the duct wall. It is shown that the rotation of acoustic sources in a duct prevents the use of standard acoustic inversion techniques. A new technique is presented here for inverting the strength of rotating broadband sources that makes use of a new Green function taking into account the effect of source rotation. The new Green function is used together with a modal decomposition technique to remove the effect of source rotation, thereby allowing an estimation of the rotor-based source strengths in the rotating reference frame. It is shown that the pressure measured at the sensors after application of this technique is identical to that measured by sensors rotating at the same speed as the rotor. Results from numerical simulations are presented to investigate the resolution limits of the inversion technique. The azimuthal resolution limit, namely the ability of the measurement technique to discriminate between sources on adjacent blades, is shown to improve as the speed of rotation increases. To improve the robustness of the inversion technique, a simplifying assumption is made whereby the sources on different blades are assumed to be identical. It is also shown that the accuracy and robustness of the inversion procedure improve as the axial separation between the rotor and sensors decreases. Simulation results demonstrate that for a 26-bladed fan, rotating with a blade tip Mach number of Mt=0.5, the aerodynamic source strengths can be estimated with acceptable robustness and approximately 1 dB accuracy, when measurements are made approximately 0.1 acoustic wavelengths from the rotor
Estimation of the far-field directivity of broadband aeroengine fan noise using an in-duct axial microphone array
No full textThis paper presents a measurement technique for estimating the far-field directivity of the sound radiated from a duct using measurements of acoustic pressure made inside the duct. The technique is restricted to broadband, multi-mode sound fields whose directivity patterns are axi-symmetric, and whose modes are mutually uncorrelated. The technique uses a transfer function to relate the output from an in-duct axial beamformer to measurements of the far-field polar directivity. A transfer function for a hollow cylindrical duct with no flow is derived, and investigated in detail. Transfer functions for practical cases concerning aeroengine exhausts are also presented. The transfer function is shown to be insensitive to the mode-amplitude distribution inside the duct, and hence can be used to predict the directivity in practice where the noise source distribution is unknown. The technique is then validated using a no-flow facility, and is shown to be able to predict variations in the far-field directivity pattern and also estimate the far-field sound pressure levels to within 2 dB. It is suggested that the proposed technique will be especially useful for fan rig experiments, where direct measurement of directivity, for example by use of an anechoic chamber, is impossible
A technique for the in situ phase calibration of in-duct microphone arrays
No full textIn aeroengine noise experiments in-duct microphone arrays are often used to make detailed measurements of the sound field transmitted along the duct. The individual microphones in the array must be calibrated with respect to magnitude, and often more critically with respect to phase. Calibration is difficult to perform in situ due to the presence of the duct. This paper presents a technique to allow in situ phase calibration of axial microphone arrays. It relies on the observation that the measured cross-spectral pressure matrix at the array has a Hermitian Töplitz form in the case where the propagating duct modes are mutually incoherent. Using this property a system of equations can be written which, when solved, allows the phase calibration factors to be obtained. The technique is verified experimentally using a no-flow laboratory rig by comparing the phase calibration factors obtained with those measured in free-field conditions. The accuracy of the phase calibration factors obtained by the technique is limited by the degree of deviation of the measured cross-spectral matrix from Töplitz behaviour. In the experimental results shown this is less than 15° at duct frequencies below ka=25. The technique is a robust and rapid method for calibrating in-duct axial microphone arrays
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
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