Clocking Effect of Chevrons with Azimuthally-Varying Immersions on Shockcell/Cabin Noise

Abstract

*† ‡ § Prior scale model and flight tests have shown that azimuthally varying fan chevrons, especially, those with more enhanced mixing near the pylon, are quieter than conventional azimuthally uniform chevrons for both jet mixing noise, as well as shockcell noise, and can be designed to have insignificant thrust loss at cruise conditions. In this paper we study the effect of clocking of chevrons with azimuthally varying immersions on shockcell noise at cruise conditions using a Boeing 777-300ER aircraft with GE90-115B engines. Three types of clockings are studied: (i) highest immersion near the pylon and decreasing monotonically away from it, (ii) highest immersion on the side away from the fuselage and decreasing monotonically as the fuselage is approached, and (iii) uniformly immersed chevrons except for those near the pylon which have higher immersions. 14 variable geometry fan chevrons made of shape memory alloy, heating elements and feedback control were used to change their immersions during flight, with and without conventional core chevrons. The unsteady pressure on the fuselage side is analyzed for shockcell noise during cruise conditions at 0.84 Mach no. and 35,000 ft altitude from start to finish. For each cruise segment one of the above chevron immersion clockings was found to be more beneficial than others. At midcruise conditions the second clocking gave slightly better results than the first one for low frequencies, but the last clocking gave the highest noise benefit above 5 dB. Addition of core chevrons increased the high frequency noise for all three clockings and are not deemed useful for shockcell noise reduction. The interior cabin noise spectra mostly corroborate the low frequency benefits and high frequency penalties found on the fuselage exterior skin.