On the role of wave packets in the noise of subsonic jets

This paper discusses experimental data of subsonic jet noise associated with wave-packets. A simple model combining characteristics of point quadrupoles embedded in a plug flow and of a coherent line source with exponential directivity is used to examine the data. The analysis suggests that some results presented in the literature as evidence of wave-packet directivity are due, in fact, to effects of the mean flow on the quadrupolar radiation, and not to super-directional sources. It also suggests that, except for very low Mach number jets, inclusion of these effects is essential for adequate prediction of the resulting far field when the coherent emission of wave-packets is relevant. Discrepancies with respect to the model occur for very low Mach number, when mean flow effects are negligible, showing that a more refined model is needed in this case, in order to adequately capture the details of the sound generation process.