Relative Clocking of Enhanced Mixing Devices for Jet Noise Benefit Vinod G. Mengle * The Boeing Company, Seattle, WA 98124-2207 In dual stream nozzles of modern turbofan engines there may be two sets of enhanced mixing devices, like chevrons or lobe mixers, on the two nozzles for jet noise reduction. The effect of relative azimuthal location or clocking between these two sets of mixers on jet noise is explored. A preferred clocking hypothesis, applicable to both separate flow nozzles and internally mixed nozzles, is presented for inwardly immersed chevrons, based on the alignment of radial flow velocity components induced by axial vortices in the cross-plane. This argument is verified experimentally for internally mixed nozzles using lobe mixers or chevrons on the internal nozzle, and external chevrons on the mixed nozzle. Results show that preferred clocking positions can give additional noise reductions at all angles and, especially, in the annoying frequency spectrum, over a broad range of typical operating conditions. Extensions of this preferred clocking mechanism are presented by developing a simple "clocking index" for different chevron immersion patterns, namely, immersed in, out and alternately in/out. I. Introduction tringent community noise regulations for jet aircraft are being met by implementing enhanced mixing devices, such as, chevrons and lobe mixers in modern dual-stream turbofan engine nozzles. For separate flow nozzles, where the outer nozzle is upstream of the inner nozzle, chevrons on both nozzle lips have sometimes proven to be more beneficial 1 than a single mixer. However, the effect of relative azimuthal location or clocking between these two sets of chevrons has not been explored in the past literature. On the other hand, for internally mixed nozzles, where the outer nozzle is downstream of the inner nozzle, generally lobe mixers have been the mixing device of choice 2 and, to the author’s knowledge, the effect of another set of mixers, like chevrons, on the outer nozzle (or, the mixed nozzle duct) has not been explored at all in the published literature. With more and more stringent aircraft noise regulations in the future we would like to seek additional jet noise benefit with these proven mixing devices even if it is incremental. With this spirit of enquiry we study in this paper if the two given sets of mixers can be clocked preferentially with respect to each other so as to yield additional jet noise reduction. Clocking by itself will not add weight and is not expected to change the nozzle thrust significantly; then it would be a practically acceptable solution if proven beneficial. S Both chevrons and lobe mixers are known to enhance mixing between the two neighboring streams with the aid of streamwise or axial vorticity, although the mechanism to generate it is different in each case. Chevrons, which are special serrations on the nozzle lip (typically, triangular shaped) and slightly immersed in one of the streams, generate a pair of streamwise vortices from its side edges, mainly due to the pressure difference on its two sides – like wing tip vortices, except that chevrons have flows at two different speeds on its top and bottom surfaces, and are thus also operating in a shear layer. Lobe mixers, which are special corrugations of the nozzle lip, produce axial vorticity, on the other hand, by forcing juxtaposition of two annular flows in neighboring radial sectors and imparting a different radial velocity component to each flow stream. Lobe mixers typically generate much stronger axial vorticity than chevrons and, hence, their induced velocity effect lasts over longer downstream distances. In either case, an individual chevron or lobe in a lobe mixer produces a pair of axial vortices of opposite type freely convecting downstream. When a family of such chevrons or corrugated lobes is present in a nozzle, the whole array of axial vortices induces cross-stream velocities in both the radial and the azimuthal directions. Hence, if * Engineer/Scientist, Acoustics & Fluid Mechanics Dept., P.O. Box 3707, MC 67-ML. Senior AIAA member. American Institute of Aeronautics and Astronautics 1 43rd AIAA Aerospace Sciences Meeting and Exhibit 10 - 13 January 2005, Reno, Nevada AIAA 2005-996 Copyright © 2005 by The Boeing Company. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.