The radiation of transient sound from a duct termination exhausting subsonic flow

Abstract

A tunable arc transient technique was developed to measure the duct exit response to sound in subsonic flow. An assymptotic formulation based upon the theory of sound radiation from a duct exit in flow by Munt, was developed. Experimental and theoretical results were compared to enhance the current understanding of core radiation from a turbofan aeroengine. An intense, arc discharge acoustic source tunable in both level (145-170dB induct) and frequency bandwidth, was developed. The radiated sound was predictable from arc current alone and was the basis of a transient method. The transient method developed in flow was shown to be useful in measuring duct propagation attenuation, sound pressure reflection coefficients, transmission coefficients and radiation directivities of the duct exit. These results compared well to other experimental methods. The assymptotic theory developed, used large argument expansions for the decomposition of the Wiener-Hopf kernel in the approximations by Cargill to the Fourier integral formulation by Munt. The theory, valid for plane wave sound radiation in the flight and static cases for Ka> 05, shows excellent agreement to Levine and Schwinger's results. The theory and transient test, identified nine flow effects to the radiation of sound from a duct exit. Flow convection scales as four powers of doppler factor. Enhancement of diffraction of sound into forward arc. Radiation interference pattern familiar to classic piston directivity. Radiation interference pattern shifts with cone of silence. Exponential mean flow refraction valley about the jet axis. Transmission coefficient across the jet/external interface. In the flight case, additional effects relative to the static case are: For equal external and jet mean flow velocities, flow convection scales as two powers of doppler factor. The cone of silence angle decreases. Radiation interference pattern becomes more constructive in forward arc.

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