Study of Sound Generated by Large-Scale Structures in Low Speed Coaxial Jets

Abstract

Sound generation by low speed circular and elliptic coaxial jets is investigated computationally. The secondary to primary jet velocity ratio is limited up to 0.4 and the elliptic jet profile has a 2:1 aspect ratio. Large Eddy Simulations are coupled with the Lighthill's acoustic analogy. Flow incompressibility is assumed limiting the study to the compact quadrupole structures generated during the jet mixing process. As expected, adding a secondary stream is found to extend the mixing region, while changing the jet geometry from circular to elliptic leads to vice versa. Good qualitative agreement in the flow behaviour is achieved with known results. Lighthill's longitudinal streamwise quadrupole is found to dominate the sound generation leading to a mildly high streamwise emission. Increasing the velocity ratio of the secondary stream to the primary stream increases the dominance of the streamwise sound emission. Good qualitative agreement with a previously suggested sound directivity pattern is achieved. Adding a secondary stream to the jet is found to have a mixed effect on the emitted acoustic power. A plausible explanation based on a large scale instability wave model is presented. Dominance of the longitudinal streamwise quadrupole Qxx was observed in all jets and the lateral quadrupoles Qxy and Qxz were of about the same magnitude and only secondary to Qxx. This resulted in high sound emission in the streamwise direction particularly as U1 /U0 was increased to 0.4. Good qualitative agreement was found with a previously suggested expression for the sound directivity pattern [18]. Adding a secondary stream did not cause a significant reduction in the overall acoustic power output as changing the jet geometry from circular to elliptic which was also limited to about 2 dB reduction. Further evidence for the existence of a large scale wave packet structure in the mixing region was produced. The wave packet source model was used to point to the opposing effects on the acoustic power output when adding a secondary stream, which could result in its mild and mixed effect.