Noise Reduction and Flow Characteristics in Asymmetric Dual-Stream Jets

Abstract

This research effort is motivated by the advent of asymmetric nozzle concepts for directional suppression of jet noise from turbofan engines. The specific method addressed is the fan flow deflection (FFD) technique, whereby aerodynamic devices deflect downward the fan stream of the turbofan exhaust and thus create an asymmetry in the plume of the jet exiting an otherwise coaxial nozzle. The asymmetry reduces jet noise emissions in downward and sideward directions affecting airport communities. Flow field and acoustic measurements were conducted to understand what flow quantities are affected by the departure from symmetry and how their changes impact noise emission. The experiments were complemented by computations that included the effect of forward flight. It is found that FFD reduces the radial gradients of mean velocity, the turbulent kinetic energy, and the Reynolds stress on the underside of the jet. A preliminary correlation between downward velocity gradient and downward sound emission indicates that velocity gradient reduction is an important ingredient for noise suppression using FFD. Further, a correlation between the maximum radial gradient of the axial velocity component and peak turbulent kinetic energy was obtained. In an additional related aspect of this work, the effect of baseline nozzle geometry on efficacy of methods that create jet asymmetry was studied. A phenomenological investigation between nozzles with parallel exit flow lines and converging exit flow lines was conducted. Jets with uniformly reduced radial gradients below the centerplane were found to be acoustically superior to jet plumes with focused or narrow gradient reduction.

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