Farfield multimodal acoustic radiation directivity

Saule, A. V.; Rice, E. J.

doi:10.1121/1.2016139

Cited by 3 publications

(2 citation statements)

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“…Approximate expressions for inlet radiation have been developed in terms of mode cutoff ratio _ 'fr1/ a mn y 1-MD where the dimensionless frequency Ti= f D/c; with the eigenvalue of the (m,n)th mode, a mn; duct diameter, D; duct Mach number, M D ; and speed of sound, c. The key simplification realized by the cutoff ratio formulation is that modes with the same C and p propagate similarly to the far field. This has been demonstrated for radiation from a flanged duct without flow (63) and is fairly accurate for principal lobe radiation (64). Two important duct mode propagation angles, ¢ x and ¢ x , are defined in (65) Here, ¢ x and q) x are the angles which the vector normal to the wave front and the group velocity vector, respectively, make with the duct axis.…”

Section: Inlet Radiationmentioning

confidence: 97%

Aircraft Turbofan Noise

Groeneweg

Rice

1983

Volume 5: Ceramics; Structures and Dynamics; Controls, Diagnostics and Instrumentation; Education; Process Industries

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Recent advances in the understanding of turbofan noise generation and suppression in aircraft engines are reviewed with particular emphasis on NASA research. The review addresses each link in the chain of physical processes which connect unsteady flow interactions with fan blades to far field noise. Mechanism identification and description, duct propagation, radiation and acoustic suppression are discussed. Recent advances in the experimental technique of fan inflow control assure that in-flight generation mechanisms are not ma.,ked by extraneous sources in static tests. Rotor blade surface pressure and wake velocity measurements aid the determination of the types and strengths of the generation mechanisms. Approaches to predicting or measuring acoustic mode content, optimizing treatment impedance to maximize attenuation, translating impedance into porous wall structure and interpreting far field directivity patterns are illustrated by comparisons of analytical and experimental results. A persistent theme of the review is the interdependence of source and acoustic treatment design to minimize far field noise. Areas requiring further research are discussed and the relevance of aircraft turbofan results to quieting other turbomachinery installations is addressed.

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Section: Inlet Radiationmentioning

confidence: 97%

Aircraft Turbofan Noise

Groeneweg

Rice

1983

Volume 5: Ceramics; Structures and Dynamics; Controls, Diagnostics and Instrumentation; Education; Process Industries

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“…The condition for the calculations were MO w 0.6 and I . 7.111. At these conditions, it was estimated there would be 100 propagating modos.…”

Section: -Mpmentioning

confidence: 99%

Farfield radiation of aft turbofan noise

Rice

Saule

1980

The Journal of the Acoustical Society of America

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The farfield radiation of turbofan noise from the inlet of an engine has recently received considerable study. Relatively simple approximate expressions have been developed for single mode radiation as a function of cutoff ratio and for multimodal radiation. Convective effects upon radiation which are essential for interpreting static engine tests have been developed. This information has been essential for the design of inlet suppressors. This paper represents an initial effort to provide the same type of radiation expressions for the noise radiation from the aft duct. The results of approximate aft radiation equations are compared to more exact Wiener-Hopf radiation results. Refraction as well as convective effects in the multiple flow streams must be considered. The possibility that the acoustic power distribution is altered during transmission is also discussed. This has an impact on the design and testing of aft duct suppressors.

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