Subsonic Jet Noise from Nonaxisymmetric and Tabbed Nozzles

Abstract

Subsonic jet noise from nonaxisymmetric and tabbed nozzles are investigated experimentally and theoretically. It is shown that the noise spectra of these 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 Components of Turbulent Mixing Noise from Supersonic Jets, AIAA Paper 96-1716, 1996). Furthermore, the radiated noise fields of the jets under study, including elliptic and large-aspect-ratio rectangular jets, are found to be quite 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 effective method for jet noise suppression. A lobed nozzle, on the other hand, is found to impact significantly the noise field. Noise from large-scale turbulent structures, radiating principally in the downstream direction, is effectively suppressed. Tabs also impact the noise field, primarily by shifting the spectral peak to a higher frequency. A jetlets model is developed to provide a basic understanding of the noise from tabbed jets. The model predicts that the noise spectrum from a jet with N tabs (N ≥ 2) can he obtained from that of the original jet (no tab) by a simple frequency shift. The shifted frequency is obtained by multiplying the original frequency by N 1/2 . This result is in fairly good agreement with experimental data.