Brenda S. Henderson

Fifty Years of Fluidic Injection for Jet Noise Reduction

The paper reviews 50 years of research investigating jet noise reduction through fluidic injection. Both aqueous and gaseous injection concepts for supersonic and subsonic jet exhausts are discussed. Aqueous injection reduces jet noise by reducing main jet temperature through evaporation and main jet velocity through momentum transfer between water droplets and the main jet. In the launch vehicle environment where large quantities of fluid do not have to be carried with the vehicle, water injection is very effective at reducing excess overpressures. For in-flight use, aqueous injection is problematic as most studies show that either large quantities of water or high injection pressures are required to achieve noise reduction. The most effective noise reduction injection systems require water pressures above 2000 kPa (290 psi) and water-to-main-jet mass flow rates above 10% to achieve overall sound pressure level reductions of roughly 6 dB in the peak jet noise direction. Injection at lower pressure (roughly 1034 kPa or 150 psi) has resulted in a 1.6 EPNdb reduction in effective perceived noise level. Gaseous injection reduces noise through jet plume modifications resulting from the introduction of streamwise vorticity in the main jet. In subsonic single-stream jets, air injection usually produces the largest overall sound pressure level reductions (roughly 2 dB) in the peak jet noise direction. In dual-stream jets, properly designed injection systems can reduce overall sound pressure levels and effective perceived noise levels but care must be taken to choose injector designs that limit sound pressure level increases at high frequencies. A reduction of 1.0 EPNdB has been achieved with injection into the fan and core streams. However, air injection into dual-stream subsonic jets has received little attention and the potential for noise reduction is uncertain at this time. For dual-stream supersonic jets, additional research needs to be conducted to determine if reductions can be achieved with injection pressures available from current aircraft engines.

Phased Array Noise Source Localization Measurements of an F404 Nozzle Plume at Both Full and Model Scale

A 48-microphone planar phased array system was used to acquire jet noise source localization data on both a full-scale F404-GE-F400 engine and on a 1/4th scale model of a F400 series nozzle. The full-scale engine test data show the location of the dominant noise sources in the jet plume as a function of frequency for the engine in both baseline (no chevron) and chevron configurations. Data are presented for the engine operating both with and without afterburners. Based on lessons learned during this test, a set of recommendations are provided regarding how the phased array measurement system could be modified in order to obtain more useful acoustic source localization data on high-performance military engines in the future. The data obtained on the 1/4th scale F400 series nozzle provide useful insights regarding the full-scale engine jet noise source mechanisms, and document some of the differences associated with testing at model-scale versus full-scale.

Data Quality Assurance for Supersonic Jet Noise Measurements

The noise created by a supersonic aircraft is a primary concern in the design of future high-speed planes. The jet noise reduction technologies required on these aircraft will be developed using scale-models mounted to experimental jet rigs designed to simulate the exhaust gases from a full-scale jet engine. The jet noise data collected in these experiments must accurately predict the noise levels produced by the full-scale hardware in order to be a useful development tool. A methodology has been adopted at the NASA Glenn Research Center’s Aero-Acoustic Propulsion Laboratory to insure the quality of the supersonic jet noise data acquired from the facility’s High Flow Jet Exit Rig so that it can be used to develop future nozzle technologies that reduce supersonic jet noise. The methodology relies on mitigating extraneous noise sources, examining the impact of measurement location on the acoustic results, and investigating the facility independence of the measurements. The methodology is documented here as a basis for validating future improvements and its limitations are noted so that they do not affect the data analysis. Maintaining a high quality jet noise laboratory is an ongoing process. By carefully examining the data produced and continually following this methodology, data quality can be maintained and improved over time.

Jet noise simulations for complex nozzle geometries

The jet flow from a complex engine nozzle system with multiple jet streams is computed using large eddy simulations. The effects of the fan flow, the impact of installation effects created by the addition of a pylon, and the influence of the core-fluidic injection on the resulting flow field and the acoustic radiation are studied. The potential core length reduces slightly with the introduction of the fan flow and further reduces with the introduction of the fluidic injection nozzle geometry. Computations of fluidic-injection nozzle configurations are validated with experimental data. The agreement in the farfield spectra along the sideline and in the peak propagation directions is good for both the baseline nozzle and the fluidic-injection nozzle configurations. The centerline velocity and the turbulent kinetic energy distribution along the nozzle symmetry plane are in good agreement with the experiments. The parametric study varying the pressure ratio shows that as the injection pressure ratio is increased the jet core moves towards the pylon. For a fluidic injection pressure ratio of 4.0, a reduction of 2.0dB - 2.5dB is observed with respect to the baseline nozzle with a pylon. Fluidic injection is found to produce two sets of counter rotating vortices, one along the nozzle lip line and the second penetrating the nozzle core flow. The potential reason for the noise reduction is investigated from the changes in the turbulence intensity and the convective velocity in the shear layer. It is shown that the turbulence intensity is reduced and the convective velocity at the end of the potential core remains nearly constant for all injection pressure ratios studied.

Acoustics and flow field of slotted air-injection nozzles

Experiments investigating the noise and flow-field characteristics of dual-stream jets with fluidic injectors were performed. Air was delivered to the core stream of a bypass-ratio-five nozzle system via slots in the core-nozzle trailing edge. For dual-subsonic-stream jets, up to 3 dB noise reduction was achieved in the peak-jet-noise direction with an injection total pressure roughly equal to 1.5 times that of the core stream and an injection-to-core mass-flow ratio equal to 2.5%. Particle Image Velocimetry (PIV) studies showed fluidic injection reduced turbulent-kinetic-energy levels downstream of the pylon. Flow asymmetries introduced by the pylon were shown to impact streamwise development of vorticity generated by the injectors which limited the ability of the injectors to enhance jet mixing. For dual-transonic-stream jets, fluidic injection significantly reduced broadband-shock-associated noise but had limited impact on turbulent mixing noise. For single-supersonic-stream jets, fluidic injection reduced broadband-shock-associated noise and turbulent mixing noise over a range of frequencies in the peak-jet-noise direction.