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A system for reducing jet noise emission from an internally mixed gas turbine engine exhaust, comprising a fan/core flow mixer having a plurality of mixer lobes and a common flow nozzle having an equal number of tabs located along a circumferential edge of an aft end of the nozzle. There is a predet

A system for reducing jet noise emission from an internally mixed gas turbine engine exhaust, comprising a fan/core flow mixer having a plurality of mixer lobes and a common flow nozzle having an equal number of tabs located along a circumferential edge of an aft end of the nozzle. There is a predetermined clocking relationship between the plurality of mixer lobes and the plurality of nozzle tabs that results in reduced exhaust noise emission, most evident in the lower frequency range. A method for reducing jet noise emission from an internally mixed gas turbine engine exhaust comprises selectively aligning a circumferential distribution of a mixed flow vorticity field produced by a fan/core mixer with a circumferentially distributed exhaust flow vorticity field produced by a modified common flow nozzle at an exit plane of the engine exhaust.

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We claim: 1. A system for reducing jet noise emission from an internally mixed gas turbine engine exhaust, comprising: a double-lobe mixer used for mixing a core flow and a bypass fan flow, said mixer having a plurality of mixer lobes each having a major peak and a major valley; and an exhaust flow

We claim: 1. A system for reducing jet noise emission from an internally mixed gas turbine engine exhaust, comprising: a double-lobe mixer used for mixing a core flow and a bypass fan flow, said mixer having a plurality of mixer lobes each having a major peak and a major valley; and an exhaust flow nozzle having a plurality of triangular tabs each having an upstream base end and a downstream tip or tip edge center end, located along a circumferential edge of an aft end of the nozzle, wherein there is a predetermined clocking relationship between the plurality of mixer lobes and the plurality of nozzle tabs. 2. The system of claim 1, wherein there is an equal number of mixer major lobes and nozzle tabs, and the predetermined clocking relationship provides that each of the lobe major peaks is axially aligned with a respective tab tip. 3. The system of claim 1, wherein the predetermined clocking relationship provides that each of the mixer lobe major valleys is aligned with a respective tab tip. 4. The system of claim 1, wherein the predetermined clocking relationship provides that the alignment of the plurality of mixer lobes and the plurality of nozzle tabs is between a range of 0째 and 360째, where 0째 and 360째 represent a tab tip to lobe peak alignment and 180째 represents a tab tip to lobe valley alignment. 5. The system of claim 1, wherein the mixer is a conventional lobe mixer. 6. The system of claim 5, wherein the mixer has a lobe count in the range between 6 to 24 lobes. 7. The system of claim 1, wherein each major peak includes a minor peak and a minor valley. 8. The system of claim 7, wherein the tip of each tab is aligned with the minor valley of each respective major peak. 9. The system of claim 7, wherein the tip of each tab is aligned with the major valley of each respective mixing structure. 10. The system of claim 7, wherein the double-lobe mixer has a lobe count in the range between 6 to 24 lobes. 11. The system of claim 1, wherein each of the plurality of lobes radially penetrates within a selected range relative to a radial tab protrusion. 12. The system of claim 1, wherein each of the plurality of tabs has a leading edge integral with the circumferential edge of the aft end of the nozzle, at least two side edges, and a tip edge, which define a shape of the tab, the shape being selected from one of a triangle, a trapezoid, or a shape having smooth, continuous corners, further wherein the tip edge is a point, a flat edge, a curved edge, or a plurality of shaped edges. 13. The system of claim 11, wherein the double lobe mixer produces a mixed flow having a circumferential distribution located in a plane of the exhaust, the nozzle tab orientation being aligned with said circumferential location. 14. The system of claim 13, wherein the nozzle produces streamwise vorticity in an external exhaust stream, and further wherein the internal mixed flow streamwise vorticity and the tab nozzle-induced streamwise vorticity are aligned in circumferential location at a desired exit plane of the exhaust. 15. A method for reducing jet noise emission from an internally mixed gas turbine engine exhaust, comprising: providing an internally mixed gas turbine engine including double-lobe mixer used for mixing a core flow and a bypass fan flow, and an exhaust nozzle having a geometrically modified aft end protrusion which includes a plurality of triangular tabs with each having an upstream base end and a downstream tip end; generating via the flow mixer streamwise vorticity at a given circumferential location in an exit plane of the exhaust; and generating via the geometrically modified aft end protrusion an exhaust flow streamwise vorticity at the given circumferential location in the exit plane of the exhaust and in the external domain extending downstream of the nozzle exit. 16. The method of claim 15, wherein generating the exhaust flow vorticity at the given circumferential location in the exit plane of the exhaust comprises orienting a plurality of the tabs in a radially inward protruding direction by a predetermined amount relative to a predetermined amount of mixer lobe radial penetration. 17. A method for reducing jet noise emission from an internally mixed gas turbine engine exhaust, comprising: selectively aligning a circumferential distribution of a mixed flow vorticity field produced by an internal double-lobe mixer used for mixing a core flow and a bypass fan flow, with a circumferentially distributed exhaust flow vorticity field produced by a tab nozzle at an exit plane of the engine exhaust, with radial and circumferential positioning of the elements of the internally-generated vorticity field relative to those of the externally-generated vorticity field such that the noise-reduction benefit of the two-stage mixing process is optimized. 18. The method of claim 17, comprising providing a lobe mixer as the fan/core mixer, and providing a selective geometric alteration of an aft end of the nozzle to generate the exhaust flow vorticity field. 19. The method of claim 18, wherein the lobe mixer is a double-lobe mixer. 20. The method of claim 18, wherein providing the selective geometric alteration of the aft end of the nozzle comprises forming a plurality of tabs, and orienting at least some of the tabs in a selected range between a radially inward-protruding direction and a radially outward-protruding direction. 21. The method of claim 20, comprising providing a plurality of tabs on the nozzle that is equal to a plurality of major lobes of the lobe mixer. 22. The method of claim 21, comprising clocking each one of the plurality of tabs with a corresponding one of the plurality of lobes between an alignment range of 0째 to 360째, where 0째 and 360째 represent a tab tip to a lobe peak alignment and 180째 represents a tab tip to lobe valley alignment. 23. The method of claim 22, comprising aligning each one of the plurality of tab tips with a corresponding mixer major lobe peak. 24. The method of claim 22, comprising aligning each one of the plurality of tab tips with a corresponding mixer major lobe valley. 25. The system of claim 14 wherein the internal mixed flow streamwise vorticity and the tab nozzle-induced streamwise vorticity are in the same rotational directions. 26. A method of claim 15 wherein the flow mixer streamwise vorticity and the exhaust flow streamwise vorticity are in the same rotational directions.