Subsonic jet noise from nonaxisymmetric and tabbed nozzles

Subsonic jet noise from nonaxisymmetric and tabbed nozzles are investigated experimentally and theoretically It is shown that the noise spectra of the se jets are in good agreement with the similarity spectra found empirically earlier by Tam et al. through a detailed analysis of supersonic jet noise data (Tam, C. K. W., Golebiowski, M., and Seiner, J. M., "On the Two Compon ents of Turbulent Mixing Noise from Supersonic Jets," AIAA Paper 96-1716, 1 996). Furthermore, the radiated noise fields of the jets under study, inclu ding elliptic and large-aspect-ratio rectangular jets, are found to be quit e axisymmetric and are practically the same as that of a circular jet with the same exit area. These experimental results strongly suggest that nozzle geometry modification into elliptic or rectangular shapes is not an effect ive method for jet noise suppression. A lobed nozzle, on the other hand, is found to impart significantly the noise held. Noise from large-scale turbu lent structures, radiating principally in the downstream direction, is effe ctively suppressed, Tabs also impact the noise field, primarily by shifting die spectral peak to a higher frequency. A jetlets model is developed to p rovide a basic understanding of the noise from tabbed jets. The model predi cts that the noise spectrum from a jet with N tabs (N greater than or equal to 2) can be obtained from that of the original jet (no tab) by a simple f requency shift. The shifted frequency is obtained by multiplying the origin al frequency by N-1/2. This result is in fairly good agreement with experim ental data.