Jet mixing noise suppression by means of a claw mixer: Revision of the nail configuration to improve the high-frequency acoustic property

Jet powered aircraft radiate a substantial amount of noise around airports. Among several noise sources of the aircraft, high-speed jet behind the nozzles of jet engines generates strong sound through mixing process of jet with the surrounding flow. One of the effective measures against the jet mixing noise is to control the flow field so as to weaken shear in the jet plume. The authors have proposed a claw mixer as a retractable mixing control device. The claw, composed of half-edged nails, is placed at the nozzle end. In the preliminary noise test using a jet engine, the nails with a configuration of 60° inclination relative to jet and a 30% penetration provided sufficient noise reduction in the downstream of jet. However, this configuration of nails resulted in increase of additional noise at higher frequencies. To improve this acoustic property, this study attempted to redesign the nail configurations with a help of large eddy simulation. The revised nail configurations, e.g., shallower inclination and less penetration of the nails, suggested less turbulent kinetic energy at higher frequency and thus less additional noise increase to the lateral of jet. The subsequent scale-model test on the revised claw mixer confirmed suppression in both broadband peak noise and the higher-frequency noise.