Book Description

This experimental research project varied nozzle discharge geometries in an effort to reduce jet noise. Chevrons and other mixing enhancement devices introduced streamwise vortices into the initial shear layer of the jet. Single Flow Noise Reduction: Air was injected into the center of streamwise vortices. They were energized and increased mixing across the shear layer, which reduced low frequency noise. The vortices broke down with lower circumferential velocities, which reduced high frequency noise. More robust streamwise vortices created additional mixing. This reduced low frequency noise but created additional high frequency noise. The size of the streamwise vortices was reduced. This did not effect low frequency noise, but reduced high frequency noise. Coaxial Flow Noise Reduction: The noise reduction when using fan and core chevrons together was shown to be additive. Reductions attributed to core chevrons plus reductions attributed to fan chevrons were roughly equal to reductions when both chevron nozzles were installed. Low frequency noise was predominantly generated in the mixed region, which can be modified by either nozzle. The effect was compounded when both chevron nozzles were installed. High frequency noise was predominantly generated in the shear layers. Under the current test conditions, the shear layers were close enough that acoustic measurements could only resolve one noise source. Either chevron nozzle modified noise in the shear layer(s), the effect was compounded when both were installed. Underexpanded Flow Noise Reduction: Baseline and chevron nozzles were used on an underexpanded core stream. Chevrons increased shock associated noise for lower fan flow velocities but reduced shock associated noise for the highest fan flow velocity. Shock associated noise is created as turbulence interacts with shock cells. Both the values of turbulence and the magnitude of pressure fluctuations are important. For the lower fan flow velocities, chevrons increased turbulence but did not effect the pressure fluctuations. This increased shock noise. For the highest fan flow velocity, chevrons did not change turbulence levels but significantly reduced pressure fluctuations. This reduced shock associated noise.