Three-Stream Jets Research Articles

Investigation of vortical and near-acoustic fields in three-stream jets.

The connections between the vortical and near-acoustic fields of three-stream, high-speed jets are investigated for the ultimate purpose of developing linear surface-based models for the noise source. Those models would be informed by low-cost, Reynolds-averaged Navier-Stokes (RANS) computations of the flow field. The study uses two triple-stream jets, one is coaxial and the other has eccentric tertiary flow that yields noise suppression in preferred directions. Large eddy simulations (LES) validate the RANS-based models for the convective velocity Uc of the noise-generating turbulent eddies. In addition, the LES results help define a "radiator surface" on which the jet noise source model would be prescribed. The radiator surface is located near the boundary between the rotational and irrotational fields and defined as the surface on which the Uc distribution, obtained from the space-time correlations of the pressure, matches that inferred from the RANS model. The edge of the mean vorticity field is nearly coincident with the radiator surface, which suggests a RANS-based criterion for locating this surface. The two-dimensional space-time correlations show how the asymmetry of the tertiary stream and the resulting thicker low-speed flow weakens the generation of acoustic disturbances from the vortical field.

Synthesis of convection velocity and turbulence measurements in three-stream jets

Key flow regions linked to jet noise sources are investigated through comparison of convection velocity and turbulence measurements in high speed three-stream nozzles. A time-resolved Doppler global velocimetry (DGV) instrument was applied in the Nozzle Acoustic Test Rig (NATR) at NASA's Aero-Acoustic Propulsion Lab to measure 250 kHz repetition laser scattering signals arising from seeding particles at 32 points in high speed flow. Particle image velocimetry (PIV) measurements previously reported by NASA provided mean velocity and turbulence intensity. Results for convection velocity of the particle concentration field were obtained from the DGV data for three-stream nozzle configurations using a cross-correlation approach. The three-stream cases included an axisymmetric and an asymmetric, offset nozzle configuration. For the axisymmetric case, areas of high convection velocity and turbulence intensity were found to occur 4 to 6 area-equivalent nozzle diameters downstream from the nozzle exit. Comparison of convection velocity between the axisymmetric and offset cases show this same region as having the greatest reduction in convection velocity due to the offset, up to 20% reduction for the offset case compared to the axisymmetric case. These findings suggest this region along the centerline near the end of the potential core is an important area for noise generation with jets and contributes to the noise reductions seen from three-stream offset nozzles.

Modelling of noise reduction in complex multistream jets

The paper presents a low-order prediction scheme for the noise change in multistream jets when the nozzle geometry is altered from a known baseline. The essence of the model is to predict the changes in acoustics due to the redistribution of the mean flow as computed by a Reynolds-averaged Navier–Stokes (RANS) solver. A RANS-based acoustic analogy framework is developed that addresses the noise in the polar direction of peak emission and uses the Reynolds stress as a time-averaged representation of the action of the coherent turbulent structures. The framework preserves the simplicity of the Lighthill acoustic analogy, using the free-space Green’s function, while accounting for azimuthal effects via special forms for the space–time correlation combined with source–observer relations based on the Reynolds stress distribution in the jet plume. Results are presented for three-stream jets with offset secondary and tertiary flows that reduce noise in specific azimuthal directions. The model reproduces well the experimental noise reduction trends. Principal mechanisms of noise reduction are elucidated.

Noise sources in a commercial supersonic jet

Stringent noise regulations currently limit commercial aviation. These regulations make supersonic commercial flight impractical. The development of an engine that can meet these strict rules is paramount to making supersonic commercial flight a reality. One method of noise reduction is to add additional streams to an engine. As such, the three-stream jet has potential to help reduce exhaust noise. Understanding the noise sources in the jet plume can help to design nozzles that are quieter. To accomplish this, high-fidelity, high-speed data are required. Data for an axisymmetric and offset three-stream nozzle were generated using the LES code JENRE developed by the Naval Research Laboratory. The simulation data has been shown to match well with experimental data. Advanced analyses methods that are based on Proper Orthogonal Decomposition (POD), wavelet decomposition, and Stochastic estimation have been applied to extract noise sources in the jet plume.

Three-stream jet noise measurements and predictions

An experimental and numerical investigation of the noise produced by high-subsonic three-stream jets was conducted. The exhaust system consisted of externally mixed-convergent nozzles and an external plug. Bypass- and tertiary-to-core area ratios between 1 and 1.75, and 0.4 and 1.0, respectively, were studied. Axisymmetric and offset tertiary nozzles were investigated for heated and unheated conditions. For axisymmetric configurations, the addition of the third stream was found to reduce mid- and high-frequency acoustic levels in the peak-jet-noise direction, with greater reductions at the lower bypass-to-core area ratios. The addition of the third stream also decreased peak acoustic levels in the peak-jet-noise direction for intermediate bypass-to-core area ratios. For the offset configurations, an s-duct was found to increase acoustic levels relative to those of the equivalent axisymmetric-three-stream jet while half-duct configurations produced acoustic levels similar to those for the axisymmetric jet for azimuthal observation locations of interest. Comparisons of noise predictions with acoustic data are presented for selected unheated configurations. The predictions are based on an acoustic analogy approach with mean flow interaction effects accounted for using a Green’s function, computed in terms of its coupled azimuthal modes, and a source model previously used for round and rectangular jets.

Aeroacoustics of Three-Stream High-Speed Jets from Coaxial and Asymmetric Nozzles

The development of three-stream variable-cycle turbofan engines for tactical aircraft provides an opportunity for noise reduction by shaping the exhaust of the secondary and tertiary streams. The paper reviews an experimental study of subscale three-stream jets operating at high specific thrust and issuing from rapid-prototyped nozzles. Exhaust conditions at four set points, with variable tertiary pressure ratio and bypass ratio, were determined using thermodynamic cycle analysis. Nozzles were fabricated for each set point and featured coaxial and non-coaxial exit geometries. The operating conditions were simulated using precisely metered helium–air mixtures, and far-field noise surveys were collected using a 24-microphone array. For a low-bypass ratio, a coaxial configuration offers no significant noise benefit compared to the single-stream primary jet. Configurations with offset secondary or tertiary streams offer significant noise reduction in the direction of the thicker flow. For an overall bypass ratio around 0.5, reductions of 5.1 dB in overall sound pressure level and 4.2 dB in effective perceived noise level were attained.