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Published Articles

The Volume 2, No 1, March 1997

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Progress in Computational Aeroacoustics in Predicting the Noise Radiated from Turbulent Flows

Geoffrey M. Lilley, Xin Zhang and Aldo Rona


In recent years a number of simple unsteady flows involving the interaction between vortices have been studied using computational fluid dynamics. These have been extended to include the sound radiated to the far field either by Direct Numerical Simulation, by the use of acoustic analogies, or by the use of Kirchoff methods. For more complex flows results have been obtained using methods based on solving the time dependent large scale flow structures using the unsteady Reynolds Averaged Navier-Stokes equations and then using acoustic analogies to derive the noise in the radiation field. Some success has been made with the latter methods in the predictions of the noise radiated from the flow over cavities at supersonic speeds, where the noise characteristics are dominated by large scale events associated with self-excited flow oscillations. Similar methods are being applied to other self-excited flows, and ultimately to turbulent flows such as jets. The paper describes these methods and results together with some limited preliminary comparisons with experimental data. In an Appendix an extension of Lighthill's equation for aerodynamic noise is presented covering the effects of flow-acoustic interaction.

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Inverse and Reciprocity Methods for Machinery Noise Source Characterization and Sound Path Quantification Part 1: Sources

Jan W. Verheij


In this article and in a forthcoming companion article some recently explored experimental approaches to the characterization of the noise source strength of machinery and to the ranking of transmission paths are reviewed. They form an addition to the more conventional approaches of the analysis of noise control problems in terms of source-transmission path-receiver schemes. In this first article source strength descriptors are defined both for airborne and for structure-borne sound. Their common basis is that the physical sources are modelled in terms of fictitious elementary sources, such as acoustical monopoles or mechanical point forces. In three of the four methods discussed, the strength of these equivalent substitution sources is determined indirectly. Advantages as well as limitations are considered. One practical advantage concerns the feature that the proposed descriptors are much less affected in strongly different installation environments than the more conventional source strength descriptors. Another practical advantage is that the use of elementary substitution sources as source models facilitates the very convenient application of experimental reciprocity techniques for transmission path ranking. Validation experiments related to applications in ships, road vehicles and office machines are briefly discussed.

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Wavelet Analysis of Vibration Signals Part 2: Wavelet Applications

David E. Newland


This is the second of a pair of papers on the application of wavelet theory to vibration analysis. For transient data analysis, the wavelet transform and the short-time Fourier transform (STFT) produce similar results, but wavelet transforms have computational and other advantages which may be important in particular applications. This part of the paper discusses the construction of time-frequency maps. It is shown how the harmonic wavelet transform, HWT, may be modified to improve the clarity of a time-frequency map and an example is given in which a map computed by the lapped HWT is compared with a corresponding map computed by the STFT. It is found that the windowed HWT is good for time-frequency mapping because of its flexibility and ease of use and the computational efficiency of its algorithm.

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