/ TM — 2007 - 214992 October 2007 AIAA – 2007 – 3664

The concepts studied during Offset Stream Technology (OST) test were all designed to produce directional jet noise reduction in a separate flow engine configuration by redirecting the fan stream relative to the core stream, creating a thicker fan flow on one side of the jet. Previous studies, conducted at bypass ratios of five or less showed that lower noise levels would result on the thickened side. The OST test, conducted using nozzles with bypass ratios 5, 8, and 13, included flow and acoustic measurements over a wide range of engine bypass ratios, jet conditions, and flight speeds for three offset stream concepts: wedges, vanes, and an S-duct. Mean axial velocity data, measured in cross-stream planes showed that all these concepts successfully created the fan stream offset as intended. Additionally, far field acoustic measurements showed that the vanes and S-duct reduced the noise on the side of the jet with the thicker fan stream below the levels measured from the concentric baseline jet. However, adding a flight stream, to simulate Mach 0.2 forward flight, diminished the effectiveness of offset stream devices, reducing or completely eliminating most of the noise benefits. Furthermore, while engine condition (i.e., takeoff and cutback) did not have a large impact on the noise, engine bypass ratio had a significant impact on the effectiveness of the offset stream concepts. The offset stream devices had much less impact on the jet noise at higher bypass ratios than in lower bypass ratio configurations. In fact, only modest reductions were noted from the best bypass ratio 8 configuration. Above bypass ratio 8, the results from the offset jet were very similar to the baseline. Future work should, therefore, be directed at lower bypass ratio applications.