Noise Reduction and Flow Characteristics in Asymmetric Dual-Stream Jets

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

Effect of Wedge-Shaped Deflectors on Flow Fields of Dual-Stream Jets

The effect of wedge-shaped fan flow deflectors on the mean and turbulent flow-fields of dual-stream jets is investigated. Several wedge-shaped deflector concepts were used to create asymmetry in the plume of a dual-stream jet issuing from a scaled down version of the NASA Glenn ‘5BB’ bypass-ratio 8 turbofan nozzle. The deflector configurations comprised internal and external wedges with and without a pylon. Some external wedges incorporated local extensions of the fan nacelle. All the deflectors reduced radial velocity gradients, magnitudes of peak Reynolds stresses, and peak turbulent kinetic energy beneath the jet centerplane, with an increase above the jet centerplane. A correlation was obtained between the maximum radial velocity gradient and the peak turbulent kinetic energy in the dominant noise source region

Flow Field of a Dual-Stream Jet with External Wedge- Shaped Deflector

The research effort is a Reynolds-Averaged Navier-Stokes (RANS) investigation that looks at the flow-field and performance of a bypass ratio 8 nozzle with an external wedgeshaped noise suppressor, simulating the exhaust of a turbofan engine at takeoff conditions. Peak turbulence was reduced by the wedge on the side opposite the deflector, and it was increased in the initial region of the jet behind the deflector. Flow-field trends agreed with the expectations based on static jet experiments. The calculated thrust loss was 1.1% at takeoff conditions