Abstract: Noise perceived below a turbofan is reduced by providing a thicker fan flow below the turbofan primary exhaust flow than is present in conventional turbofans having generally circular and concentric fan and exhaust flows. To thicken the fan flow, the exhaust nozzle is offset upward in the fan nozzle, or the exhaust nozzle discharge end is canted to offset the exhaust flow upward, or the exhaust nozzle is flattened to a generally rectangular or an elliptical shape having a major horizontal axis. In any of these cases, the thickness of the fan flow below the primary exhaust flow is increased and less noise is perceived below the turbofan than if circular concentric fan and exhaust flows were produced. In one embodiment a noise-suppressing thickened fan flow is used in combination with a conventional noise-suppressing multilobe or multitube mixing nozzle to further reduce noise perceived below the turbofan.

Abstract: A discharge nozzle of a jet engine has lobes, tubes or deflectors for promoting mixing of the engine discharge flow with ambient air to reduce jet noise. By providing the lobes, tubes or deflectors at only the upper portion of the discharge nozzle, jet noise perceived below the engine is reduced substantially without shifting the noise spectrum to include a higher proportion of piercing and irritating high frequency noise components. Additionally, in a turbofan, the fan or secondary flow is discharged below the primary exhaust flow to further reduce noise perceived below the turbofan.