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A turbofan engine includes core and fan nacelles that provide a bypass flow path having a nozzle exit area. The bypass flow path carries a bypass flow to be expelled from the nozzle exit area. A turbofan is arranged within the fan nacelle and upstream from the core nacelle for generating the bypass

A turbofan engine includes core and fan nacelles that provide a bypass flow path having a nozzle exit area. The bypass flow path carries a bypass flow to be expelled from the nozzle exit area. A turbofan is arranged within the fan nacelle and upstream from the core nacelle for generating the bypass flow. A flow control device includes a surface in the bypass flow path including an aperture. The flow device is adapted to introduce a fluid into the bypass flow path for altering a boundary layer of the bypass flow that effectively changes the nozzle exit area. In one example, bleed air is introduced through the aperture. In another example, pulses of fluid from a Helmholz resonator flow through the aperture. By decreasing the boundary layer, the nozzle exit area is effectively increased. By increasing the boundary layer, the nozzle exit area is effectively decreased.

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1. A turbofan engine comprising: core and fan nacelles providing a bypass flow path having a nozzle exit area, the bypass flow path for carrying a bypass flow to be expelled from the nozzle exit area;a turbofan arranged within the fan nacelle and upstream from the core nacelle for generating the byp

1. A turbofan engine comprising: core and fan nacelles providing a bypass flow path having a nozzle exit area, the bypass flow path for carrying a bypass flow to be expelled from the nozzle exit area;a turbofan arranged within the fan nacelle and upstream from the core nacelle for generating the bypass flow;a spool having a turbine mounted thereon and housed within the core nacelle;a gear train interconnecting the turbofan and the spool, the turbofan coupled to the spool through the gear train;a flow control device including a surface in the bypass flow path including an aperture, the flow device adapted to introduce a fluid into the bypass flow path through the aperture for altering a boundary layer of the bypass flow that effectively changes the nozzle exit area; anda controller in communication with the flow control device, the controller configured to determine when chan es in the effective nozzle exit area are desired and command the flow control device to obtain a desired boundary layer. 2. The turbofan engine according to claim 1, comprising a flow source for providing the fluid. 3. The turbofan engine according to claim 2, comprising a compressor arranged within the core nacelle, the compressor providing bleed air as the fluid. 4. The turbofan engine according to claim 3, comprising a low spool and a high spool rotatable relative to one another and housed within the core nacelle, the compressor mounted on the high spool, and the turbofan coupled to the low spool through the gear train. 5. The turbofan engine according to claim 1, wherein the flow control device includes a controller programmed to command a valve for regulating a flow of the fluid through the aperture, wherein the valve is closed during a cruise condition. 6. The turbofan engine according to claim 1, wherein the aperture is arranged to introduce the fluid generally perpendicularly to the bypass flow for increasing the boundary layer. 7. The turbofan engine according to claim 1, wherein the bypass flow path extends axially along a radial space arranged between the core and fan nacelles. 8. A turbofan engine comprising: core and fan nacelles providing a bypass flow path having a nozzle exit area, the bypass flow path for carrying a bypass flow to be expelled from the nozzle exit area;a turbofan arranged within the fan nacelle and upstream from the core nacelle for generating the bypass flow;a flow control device including a surface in the bypass flow path including an aperture, the flow device adapted to introduce a fluid into the bypass flow path through the aperture for altering a boundary layer of the bypass flow that effectively changes the nozzle exit area, wherein the aperture is arranged to introduce the fluid generally in the same direction as the bypass flow for decreasing the boundary layer;a controller in communication with the flow control device, the controller configured to determine when changes in the effective nozzle exit area are desired and command the flow control device to obtain a desired boundary layer; anda compressor arranged within the core nacelle, the compressor providing bleed air as the fluid. 9. The turbofan engine according to claim 8, wherein the bypass flow path extends axially along a radial space arranged between the core and fan nacelles. 10. A method of controlling a turbofan engine comprising the steps of: determining when changes in the effective nozzle exit area of a turbofan bypass flow path are desired;commanding a flow control device to obtain a desired boundary layer by introducing a compressor bleed air into a turbofan bypass flow path to alter a bypass flow through the bypass flow path; andeffectively changing a nozzle exit area of the bypass flow path with the altered bypass flow, including decreasing a boundary layer along a surface within the bypass flow path thereby obtaining the desired boundary layer. 11. The method according to claim 10, wherein the step of effectively changing the nozzle exit area includes increasing a boundary layer along a surface within the bypass flow path. 12. The method according to claim 10, wherein the bypass flow path extends axially along a radial space arranged between a core nacelle and a fan nacelle.