Jet exhaust noise reduction

1. A method for reducing jet noise, the method including weakening Mach cones in a jet exhaust gas streamtube by modifying exhaust gas flow in a longitudinal axial core of the exhaust gas streamtube, the step of modifying exhaust gas flow including passively weakening exhaust plume Mach cones by reducing pressure differentials across Mach cone shock waves, the step of reducing pressure differentials including deploying a transpiration element along the longitudinal axial core of the exhaust gas streamtube.

2. The method of claim 1 in which the step of modifying exhaust gas flow includes generating a low energy wake along the longitudinal axial core of the exhaust gas streamtube in a diverging section of the jet exhaust nozzle such that the low energy wake displaces the axial core of the exhaust gas streamtube, thereby forming the streamtube into an annular configuration.

3. The method of claim 2 in which the step of generating a low energy wake includes forming the exhaust gas streamtube into an annular configuration having, at an exit plane of the nozzle, an effective cross-sectional area that approaches an ideal cross-sectional area for a perfectly expanded state.

4. The method of claim 3 in which the step of generating a low energy wake includes deploying a transpiration element in the exhaust nozzle along a central longitudinal axis of the nozzle in a position passing through at least portions of both converging and diverging sections of the nozzle such that exhaust gas passing through the converging section of the nozzle is absorbed into the transpiration element and is expelled from the transpiration element into the diverging section of the nozzle.

5. The method of claim 4 in which the step of deploying a transpiration element includes deploying a porous tube.

6. The method of claim 5 in which the step of deploying a porous tube includes deploying a flexible porous tube.

7. The method of claim 1 in which the step of deploying a transpiration element includes deploying a porous tube along the longitudinal axial core of the exhaust gas streamtube.

8. The method of claim 7 in which the step of deploying the porous tube includes deploying the tube to extend approximately three nozzle diameters aft of the nozzle exit plane.

9. The method of claim 7 in which the step of deploying a transpiration element includes deploying the transpiration element while the jet exhaust gas streamtube is present.

10. A jet noise reduction device comprising a gas flow modifier including a transpiration element, at least a portion of the transpiration element being deployable along a central longitudinal axis of a jet exhaust nozzle and configured to passively weaken jet exhaust streamtube Mach cones by modifying gas flow along a longitudinal axial core of the jet exhaust gas streamtube such that pressure differentials are reduced across Mach cone shock waves.

11. A jet noise reduction apparatus as defined in claim 10 in which the exhaust gas flow modifier includes a flexible porous tube deployable along the central longitudinal axis of the jet exhaust nozzle through at least portions of both converging and diverging sections of the nozzle, the porous tube being configured to absorb exhaust gas in the converging section of the nozzle and to expel the absorbed exhaust gas into the diverging section of the nozzle.

12. A jet noise reduction apparatus as defined in claim 11 in which the porous tube includes an array of pores through which the tube expels exhaust gas to form a wake that forms the exhaust gas streamtube into an annular configuration having, at the nozzle exit plane, an effective cross-sectional exit area that approaches an ideal expansion state.

13. A jet noise reduction apparatus as defined in claim 10 in which the transpiration element comprises a flexible porous tube.

14. A jet noise reduction apparatus as defined in claim 13 in which the exhaust gas flow modifier is configured to deploy the porous tube to extend approximately three nozzle diameters aft of the nozzle exit plane.

15. A jet noise reduction apparatus comprising:

a gas flow modifier including a transpiration element, at least a portion of which is deployable along a longitudinal axial core of a jet exhaust gas streamtube and configured to passively weaken jet exhaust streamtube Mach cones by modifying gas flow along a longitudinal axial core of the jet exhaust gas streamtube such that pressure differentials are reduced across Mach cone shock waves;

the transpiration element comprises a flexible porous tube; and

the exhaust gas flow modifier includes a reel supported in an exhaust cone of a jet and configured to alternately deploy and retract the flexible porous tube along the longitudinal axial core of the exhaust gas streamtube produced by the jet.

16. The method of claim 1 in which the step of deploying a transpiration element includes deploying a porous tube.

17. The method of claim 16 in which the step of deploying a porous tube includes deploying a flexible porous tube.

18. The method of claim 1 in which the step of deploying a transpiration element includes deploying a porous tube configured to absorb exhaust gas into the tube through pores disposed adjacent high pressure portions of Mach cone shock waves and to allow the absorbed exhaust gases to escape the tube through pores disposed adjacent relatively low pressure portions of the Mach cone shock waves.

19. A jet noise reduction apparatus as defined in claim 10 in which the exhaust gas flow modifier includes a flexible porous tube deployable along the central longitudinal axis of the jet exhaust nozzle, the porous tube being configured to absorb exhaust gas through pores disposed adjacent high pressure portions of Mach cone shock waves and to allow the absorbed exhaust gases to escape through pores disposed adjacent relatively low pressure portions of the Mach cone shock waves.