A PIV Flow Field Investigation of Medium Bypass Chevron Nozzles

Abstract

Particle image v elocimetry tests were c onducted to assess the effect of core and fan nozzle chevrons on the flow field. 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 stream and conical nozzle on fan stream), fan chevrons (chevrons on fan stream and conical nozzle on core stream), core and fan chevrons (chevron nozzles on both streams ). Far field acousti c measurements using the same configurations and cycle condition have been reported previously 1 . It was shown that configurations with chevrons on either core or fan nozzle reduce jet noise. It was also shown that a configuration with both core and fan n ozzle chevrons reduces jet noise to a greater degree. Additionally, that reduction was shown to be approximately additive when measured in decibel. 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 in general, 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 che vrons 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 noise heard in the far field is thu s a superposition of the sources. In this way, jet noise is additive. Near -field acoustic measurements using the same configurations and cycle condition were also reported previously 2 . These results confirm this thesis. Core chevrons significantly redu ce 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 eff ects were observed when chevrons were used separately and when chevrons were used together. Results of the current tests demonstrate that in general, core chevrons modify the inner shear layer and the mixed region. Fan chevrons modify the outer shear lay er and mixing region. Variations in t urbulent kinetic energy have been correlated with variations in near field jet noise.