Near Field Acoustics of Core and Fan Chevron Nozzles

Abstract

*† ‡ Tests were conducted to assess the effect of core and fan chevrons on near field jet acoustics. A separate flow exhaust system was used to accurately simulate the discharge of a modern, medium bypass, turbo fan engine. Four nozzle configurations were tested; baseline (conical nozzles on both streams), core chevrons (chevrons on core nozzle and conical fan nozzle), fan chevrons (chevrons on fan nozzle and conical core nozzle), core and fan chevrons (chevron nozzles on both nozzles). Far-field measurements using the same configurations and cycle condition were reported previously 1 . It was shown that configurations with either core or fan chevrons can reduce jet noise. It was also shown that a configuration with both core and fan nozzles chevrons reduces jet noise to a greater degree. Additionally, that reduction was shown to be approximately additive when measured in dB. That is, the reduction due to core chevrons plus the reduction due to fan chevrons was approximately equal to the reduction due to both chevrons. It was suggested that chevrons modify the noise associated with the shear layer upon which they act; core nozzle chevrons affect the noise associated with the core/fan (inner) shear layer, fan nozzle chevrons affect the noise associated with the fan/free stream (outer) shear layer. The mixing region, which is created by the merging of the inner and outer shear layer, can be affected by either chevron nozzle. The total far field noise is thus a superposition of the two sources. The current work seeks to explain these observations by examining the modification of the noise sources within the jet plume. Core chevrons significantly reduce the noise generation region that is typically associated with the inner shear layer. Fan chevrons reduce the noise generation region that is typically associated with the outer shear layer. Both chevrons reduce noise from the mixing region. These effects were observed when chevrons were used separately and when chevrons were used together.