DIRECT EVALUATION OF AEROACOUSTIC THEORY IN A JET

This paper provides a unique, detailed evaluation of basic aeroacoustic theory applied to low-Mach-number (M = 0.08) cold jets. ln contrast to most prior studies comparing theoretical predictions of jet noise with experimental results, our comparison uses a relatively complete knowledge of the flow field and employs vortex sound theory - an acoustic analogy which is shown to be insensitive to those aspects of the flow field about which our knowledge is incomplete. The primary result is that the measured sound field directivity of vortex ring pairing in circular jets is verv similar to that predicted by theory: a stationary, axisymmetric, lateral quadrupole. This directivity is very unlike the monotonic polar dependence found in time-average measures of jet noise fields and unlike the directivity found in similar excited jet experiments. Although not perfect, the agreement between experiment and theory here is satisfyingly close in comparison to the discrepancies found by Huerre & Crighton (1983). Our result also proves that pairing of purely axisymmetric coherent structures is not the dominant sound source in low-Mach-number jets and that vortex asymmetry must be an essential aspect of the vortex motions which produce noise in such jets.