Computational and Experimental Investigations of Jet Noise Reduction Concepts for Low-Bypass Ratio Military Gas Turbine Engines

Abstract

Computational and experimental activities supporting the design and evaluation of jet noise reduction concepts for low bypass-ratio military gas turbine engines are presented. Work has been focused on evaluations of lobed nozzle concepts, utilizing typical Field Carrier Landing Practice (FCLP) engine cycle conditions. At FCLP, the engine operates slightly over-expanded, with the result that jet noise emissions also include shock noise contributions. Reduction of nozzle exit area via introduction of lobes permits the nozzle to operate closer to ideal conditions, while also introducing vortices into the plume that may alter turbulence structure and noise emissions. Structured grid, computational fluid dynamics (CFD) investigations of isolated noise nozzles have been conducted. Determination of the optimal number of lobes for a nozzle has been a key objective of our studies. It was found that a six-lobe configuration is optimal, and that two-, three- and four- and twelve-lobe designs fail to provide performance improvement relative to the six-lobe design. While all lobe designs provided the same amount of area reduction for the baseline nozzle, the twelve-lobe configuration restricted the growth of the vortices introduced into the plume by the lobes. With reduced number of lobes, the far-field mixing of the exhaust plume remained unaffected, although some near-field benefits were observed. However, the turbulence characteristics, and hence noise, were not altered and the six-lobe design provided best performance, as demonstrated during tests at NCPA. The noise attenuation benefits of nozzle beveling were also assessed. These studies demonstrate the strong effect of dual jet interactions for the closely spaced, inward canted exhaust nozzles of a twin-engined aircraft. These dual nozzle plume-plume interactions have been found to have a very large effect on the turbulence structure, and hence noise. The analysis of lobed nozzle concepts with engine-engine interactions and vehicle aerodynamic/plume interactions has required the usage of multi-element unstructured grid numerics. Evaluations of aspirated lobe concepts were also conducted, where mass flux is introduced into the plume at the trailing edge of the lobes. Sensitivities to mass flow rates were examined and laboratory measurements of noise emissions in NCPA’s anechoic chamber are presented. Finally, impact of the lobes on nozzle performance during altitude flight are presented, along with CFD modeling upgrades required for performing simulations of complete aircraft/plume interactions.