An MDOE Investigation of Chevrons for Supersonic Jet Noise Reduction

The impact of chevron design on the noise radiated from heated, overexpanded, supersonic jets is presented. The experiments used faceted bi-conic convergent-divergent nozzles with design Mach numbers equal to 1.51 and 1.65. The purpose of the facets was to simulate divergent seals on a military style nozzle . The nozzle throat diameter was equal to 4.5 inches. Modern Design of Experiment (MDOE) techniques were used to investigate the impact of chevron penetration, length, and width on the resulting acoustic radiation. All chevron configurations used 12 chevrons to match the number of facets in the nozzle. Most chevron designs resulted in increased broadband shock noise relative to the baseline nozzle. In the peak jet noise direction, the optimum chevro n design reduced peak sound pressure levels by 4 dB relative to the baseline nozzle. Th e penetration was the parameter having the greatest impact on radiated noise at all observatio n angles. While increasing chevron penetration decreased acoustic radiation in the pea k jet noise direction, broadband shock noise was adversely impacted. Decreasing chevron length increased noise at most observation angles. The impact of chevron width on radiated noise depended on frequency and observation angle. I. Introduction He application of chevrons (serrations applied to a nozzle trailing edge that protrude into the exhaus ting flow) to military aircraft is particularly attractive becaus e existing engines can be retrofitted rather than r edesigned to incorporate these devices. At takeoff, high perfor mance tactical aircraft typically have overexpanded , supersonic jet-exhausts that contain noise sources not present in the subsonic exhausts of commercial aircraft en gines. As a result, chevrons that have been optimized for noise reduction in commercial aircraft may not perform a dequately on tactical aircraft. While a reasonably large number of investigations have studied the impact of chevr ons on subsonic jets, similar studies for supersonic flows are limi ted. The present investigation uses a Modern Desig n of Experiments (MDOE) approach to explore the impact of chevron design on the acoustic radiation of overe xpanded supersonic jets. An overexpanded jet resulting from operating a convergent-divergent nozzle at a stagnation pressu re below that corresponding to the nozzle design Mach number contains a quasi-periodic shock cell structure that can persist for several diameters downstream of the nozzle exit. T he constructive interference of sound waves produce d by the interaction of large-scale jet disturbances with th e shock waves within the shock cell structure resul ts in broadband shock noise 1,2,3 . Shock noise can dominate the acoustic spectra at upstream and broadside observation angles relative to the nozzle exit. Additionally, mixing noise sources are present and are associated with l arge scale jet disturbances (radiating in the downstream direction ) that become very effective noise sources when the ir phase speeds (relative to the ambient speed of sound) bec ome supersonic 4 , and with fine scale turbulence 5 (radiating in the upstream direction). Mixing noise sources are also present in subsonic jets but the large-scale distu rbances typically have subsonic phase speeds. In subsonic jets, properly designed chevron no zzles produce lower overall acoustic radiation leve ls than those of a corresponding round nozzle 6,7,8,9 . Experiments have shown that increasing chevron p enetration decreases low frequency noise and often increases high frequency noise (sometimes referred to as high frequency cros sover). The number of chevrons also impacts the acoustic radiat ion but not as significantly as the penetration. J et shear velocity (the velocity difference between the inner and oute r jet streams) impacts chevron acoustic performance with increases in shear velocity increasing low frequenc y noise reduction but sometimes increasing high fre quency noise * Researcher, Acoustics Branch, MS 54-3, 21000 Brookpark Rd., Cleveland, OH 44135. † Reseacher, Acoustics Branch, MS 54-3, 21000 Brookpark Rd., Cleveland, OH 44135, Associate Fellow.