1,720,978 research outputs found
Inversion technique for determining the strength of rotating broadband sources in ducts
No full textAeroengine broadband fan noise is a major contributor to the community noise exposure from aircraft. 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. The technique presented here makes use of a new Green function that takes 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. Simulations demonstrate that for a 26-bladed fan, rotating at Mt = 0.5, the aerodynamic source strengths can be estimated with acceptable robustness and approximately 1dB accuracy, when measurements are made 0.1 acoustic wavelengths from the rotor
Multi-mode sound transmission in ducts with flow
No full textExhaust mufflers, large exhaust stacks, and turbofan engines are common examples of ducted noise. The most useful measure of the sound produced by these noise sources is the sound power transmitted along the duct. When airflow is present, sound power flow can no longer be uniquely determined from the usual measurements of acoustic pressure and particle velocity.One approach to sound power determination from in-duct pressure measurement, and the one discussed in this paper, is to predict the relationship between the sound power and pressure based upon an assumed mode amplitude distribution. This paper investigates the relationship between acoustic pressure and power for a family of idealized source distributions of arbitrary temporal and spatial order. Incoherent monopole and dipole sources uniformly distributed over a duct cross-section can be obtained as special cases. This paper covers the sensitivity of the pressure–power relationship to source multipole order, frequency and, in particular, flow speed. It is shown that the introduction of flow in a hard-walled duct can have a substantial effect on the behavior of the pressure–power relationship for certain source distributions. Preliminary experimental results in a no-flow facility are presented in order to verify some of the main results
An in-duct beamformer for the estimation of far-field directivity
No full textThis paper presents a measurement technique for estimating the far-field directivity of the sound radiated from a duct using measurements of 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. The transfer function is shown to be insensitive to the mode-amplitude distribution inside the duct, and hence a predicted transfer function can be used to predict the directivity in practice where the noise source distribution is unknown. It is suggested that the proposed technique will be especially useful for fan rig experiements, where direct measurement of directivity, for example by use of an anechoic chamber, is impossible. The technique is validated using a no-flow experiment
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
- …
