Understanding How Chevrons Modify Noise in Supersonic Jet with Flight Effects

Abstract

Tests were conducted to determine the effect of chevrons on the sound radiated from the exhaust of a Mach 1.19 core flowwithsimulated flighteffects.Flighteffectswerestudiedbyusingasecondarycoaxial flowvariedfromstatic 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 chevronsreducedtheshock-cellspacingwithminimaleffectontheshock-cellstrength.Higherturbulencelevelsnear thenozzleexitandcomparableshock-cellstrengthledtohighershock-associatednoiseforthechevronconfiguration 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 initiallypresentforthebaselinecase.Finally,itwasshownthatthechevronsreducedturbulencelevelsneartheendof the potential core, which resulted in a reduction of the low-frequency mixing noise that dominates the aft angles.