Broadband Shock-Cell Noise from Dual Stream Jets

Abstract

Broadband shock-cell noise from dual stream jets is investigated. The present work is confined to the cases of supersonic secondary jet and subsonic primary jet. Previous studies on single stream supersonic jets reveal that broadband shock-cell noise from these jets is generated by the interaction of the large turbulence structures and the shock cells in the jet plume as the former propagate downstream through the latter. A prominent characteristic of the radiated noise is that it is most intense in the upstream direction and drops off with an increase in inlet angle. An important result of the present investigation is the discovery that there are two sets of broadband shock-cell noise. One set is the classical shock-cell noise, similar to that of single stream jets. The second set radiates sound primarily in the downstream direction. Its intensity is low at 90°, but the intensity increases with inlet angle until a critical angle is reached beyond which it starts to decrease rapidly. It is believed that the first sound field is generated by the interaction of the large turbulence structures and the shock cells in the outer shear layer of the dual stream jet. The second sound field is generated in a similar manner involving the large turbulence structures and the shock cells in the inner shear layer of the jet. The inner shear layer separates the secondary and the primary jet. A simplified mathematical model capable of elucidating the generation, transmission and radiation of broadband shock-cell noise from dual stream jets is developed. The model provides formulas relating the direction of radiation and the frequencies of broadband shock-cell noise at the peaks of the noise spectra for both sets of sound fields. Good agreement is found between the predictions from these formulas and experimental measurements, thereby providing support for the validity of the model.