Jet exhaust noise reduction

1. A method for reducing jet noise, the method including weakening primary Mach cones in a jet exhaust gas streamtube exiting a jet exhaust nozzle by emitting a secondary gas plume from a subjet disposed on a central longitudinal axis of the jet exhaust nozzle, into the jet exhaust nozzle along a longitudinal axial core of the exhaust gas streamtube.

2. The method of claim 1 in which emitting a secondary gas plume includes generating a low energy wake along at least a portion of the longitudinal axial core of the exhaust gas streamtube in the jet exhaust nozzle such that the low energy wake forms the streamtube into an annular configuration by displacing the axial core of the exhaust gas streamtube.

3. The method of claim 2 in which the low energy wake is generated in a diverging section of the jet exhaust nozzle.

4. The method of claim 2 in which emitting a secondary gas plume includes forming the exhaust gas streamtube into an annular configuration having, at an exit plane of the nozzle, an ideal effective cross-sectional area for a perfectly expanded state.

5. The method of claim 3 in which generating a low energy wake includes:

deploying a porous tube along a central longitudinal axis of the nozzle in a position extending at least partially into the diverging section of the nozzle; and

routing air into the tube from an air source such that the routed air is expelled through pores of the tube into the diverging section of the nozzle.

6. The method of claim 1 in which emitting a secondary gas plume comprises emitting a supersonic secondary gas plume from the subjet, within and along the central longitudinal axial core of the exhaust gas streamtube such that secondary Mach cones form in the secondary gas plume and weaken the primary Mach cones.

7. The method of claim 6 in which the step of emitting a secondary supersonic plume includes emitting the secondary supersonic plume from a secondary subjet disposed on a central longitudinal axis of the jet exhasut nozzle.

8. The method of claim 6 in which the step of emitting a secondary supersonic plume includes emitting the secondary supersonic plume from a secondary subjet disposed in a diverging section of the jet nozzle.

9. A jet noise reduction apparatus comprising a porous tube including an array of pores shaped and arranged to expel gas in such a way as to form a low energy wake along a longitudinal axis of an exhaust gas streamtube, forming the exhaust gas streamtube into an annular configuration having, at the nozzle exit plane, an ideal effective cross-sectional exit area for a perfectly expanded state.

10. The jet noise reduction apparatus of claim 9 in which the porous tube is a flexible porous tube.

11. The jet noise reduction apparatus of claim 9 in which the porous tube is supported on a reel supported in an exhaust cone of a jet, the reel being operable to alternately deploy and retract the porous tube along the axial core of the exhaust gas streamtube.

12. A jet noise reduction apparatus comprising a subjet positioned on a central longitudinal axis of a jet exhaust nozzle and shaped to emit gas flow along the longitudinal axial core of an exhaust gas streamtube such that a secondary supersonic plume is formed and positioned to generate secondary Mach cones that weaken Mach cones generated by the exhaust gas streamtube.

13. A jet noise reduction apparatus as defined in claim 12 in which the subjet is positioned to emit the gas flow into a diverging section of a jet exhaust nozzle.