The purpose of this study was to characterize the sound emitted by a subsonic jet and to predict its farfield radiation pattern based on nearfield measurements. Here, cylindrical near-field acoustical holography (NAH) was used in combination with multi-reference, cross-spectral sound pressure measurements. A strategy for reference microphone positioning is described that accounts for the localized, random, and directional nature of the source. A 0.8 cm diameter burner was used to produce a subsonic turbulent jet with a Mach number of 0.26. Six fixed, linear arrays holding eight reference microphones apiece were disposed circumferentially around the jet. A circular array holding sixteen equally-spaced field microphones was scanned in the axial direction over the 30 cm diameter cylindrical hologram surface. The results revealed that the jet could be modeled as a combination of eleven uncorrelated dipole-, quadrupole, and octupole-like sources. This model then allowed the farfield sound pressure level of the jet to be predicted within one or two decibels. The physical significance of the many partial field components and the possible application of this procedure to larger sources will be discussed.
Nearfield acoustical holography, jet noise, aeroacoustic, visualization
Acoustics and Noise Control
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