This paper describes engine noise tests conducted in an outdoor environment using a revised notched nozzle. A notch is a small dent formed at the nozzle edge that penetrates into the primary jet. The notched nozzle is expected to improve the acoustic performance with less deterioration in aerodynamic performance relative to that of a conventional nozzle. The slight penetration of the notch causes small disturbances immediately after the nozzle, driving the subsequent mixing process in the shear layer. This mixing process helps suppress both large-scale vortices in the far downstream region and excessive shear stress near the nozzle.

The authors have researched and developed various notched nozzles. Previous engine tests using a 6-notched nozzle showed that the notch itself caused additional noise by increasing the sound pressure level at higher frequencies. To counter this problem, a revised 18-notched nozzle was developed through computational and experimental studies. The authors’ previous paper [Ishii, et al.; ASME Paper GT2012-69507, 2012] showed that this nozzle increased the noise reduction toward the side direction of the nozzle under hot-jet conditions. However, there remain some unsolved issues. One issue is the scale of the nozzle. Another issue is the test conditions, such as the different effective cross-sectional areas.

In this light, a larger-scale nozzle with a diameter five times larger than that in the hot-jet model was prepared so as to adjust the nozzle aerodynamic performance. Noise tests of this nozzle were carried out using a turbojet engine together with far-field and phased array microphones, and the revised notched nozzle was found to show improved noise reduction performance compared to the previous design.

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