Noise characteristics of two parallel jets with unequal flow

Abstract

Many jet noise suppression devices employ tubes, lobes, and similar devices to break up the flow. To understand the suppression mechanisms of these devices, a two-parallel-jet model experiment was conducted. The three-dimensional noise field was mapped for the case when one jet was at a velocity of 549 m/s and a total temperature of 538°C (jet 1) and the other was at 351 m/s and 93°C (jet 2). Mean flow profiles were also obtained at three axial stations. The experiments showed that noise from a high-velocity jet is reduced by placing a second jet of lower velocity parallel to it on the same side as the listener. The noise reduction is maximum on the lower velocity jet side of the plane containing the common diameter of the two jets. The noise reduction decreases as one moves azimuthally around the jets from this plane. No noise reduction is obtained at approximately 75 deg to this plane. Flow profiles showed insignificant mean flow interaction up to five nozzle diameters of the first jet. From a detailed examination of the noise results and earlier jet noise source location studies, it is concluded that for the present configuration with a spacing of about 1.5 diameters, acoustic shielding is the dominant mechanism responsible for the observed noise reduction as compared to mean flow interaction. Application of the two-parallel-jets results to bypass engine jet noise reduction is demonstrated through the example of a suppressor called the four-tube nonsymmetric nozzle.