Instantaneous Directivity in Jet Noise by Multipole Decomposition

Noise generated by nearly axisymmetric vortex pairing in an excited low Mach number jet was studied experimentally as a prototypical jet noise source using a three-dimensional microphone array. The measured time-dependent sound field was decomposed into its multipole components using general linear regression techniques. Although the vortex interactions were very nearly axisymmetric and thus a nearly axisymmetric lateral quadrupole sound field was expected, the sound field was found to have large departures from axisymmetry. This acoustic sensitivity to symmetry breaking in the vorticity field was also shown in calculations using numerical vortex models and vortex sound theory. The directivity of the far-field sound gets smeared out in the time averaged data due to the tilting of the quadrupoles; this emphasizes the need for studying the jet sound pressure field in four dimensions (x, t) to better understand the mechanisms of noise generation.