Understanding How Chevrons Modify Noise in a Supersonic Jet with Flight Effects

Tests were conducted to determine the effect of chevrons on the sound radiated from the exhaust of a Mach 1.19 core flow with simulated flight effects. Flight effects were studied by using a secondary coaxial flow varied from static to Mach 0.50. Shadowgraphy, static pressure, and turbulence measurements were used to complement near-field pressure and far-field acoustic measurements. The presence of a secondary flow causes the shock-cell length to slightly lengthen and the shock cells to persist farther downstream; this was true for both the baseline and chevron nozzles. Compared with the baseline nozzle, the shadowgraphy and near-field static pressure showed that the chevrons reduced the shock-cell spacing with minimal effect on the shock-cell strength. Higher turbulence levels near the nozzle exit and comparable shock-cell strength led to higher shock-associated noise for the chevron configuration compared with the baseline. The decrease in shock-cell length caused the peak amplitude of the shock-associated noise to shift to higher frequencies. At the same time, the chevrons significantly reduced any screech tones that were initially present for the baseline case. Finally, it was shown that the chevrons reduced turbulence levels near the end of the potential core, which resulted in a reduction of the low-frequency mixing noise that dominates the aft angles.