Aero-Acoustic Propulsion Laboratory Research Articles

High-Speed Turbofan Noise Reduction Using Foam-Metal Liner Over-the-Rotor

A Williams International FJ44-3A turbofan engine was used to demonstrate the high-speed fan noise reduction potential of a foam-metal liner installed in close proximity to the fan rotor. The engine was tested in the NASA Glenn Research Center’s Aeroacoustic Propulsion Laboratory. Two foam-metal liner designs were tested and compared to the hardwall baseline. Traditional single degree-of-freedom liner designs were also evaluated to provide a comparison to the state-of-the art design. This report presents the test setup and documents the test conditions. Far-field acoustic levels and limited engine performance results are also presented. The results show that the foam-metal liner achieved up to 5 dB of attenuation in the forward-quadrant radiated-acoustic power levels, which is equivalent to the traditional single degree-of-freedom liner design. Modest changes in engine performance were noted.

Intermittent Nature of Subsonic Jet Noise

Following on previous works showing that far-field jet noise has significant intermittent aspects, the present work assumes that these intermittent events are the dominant feature of jet noise. A definition and method of detection for intermittent noise events is devised and implemented. Using a large experimental database of acoustically subsonic jets with different jet acoustic Mach numbers (), nozzle diameters (, 5.08, and 7.62 cm), and jet exit temperature ratios (0.84–2.70), these events are extracted from the far-field noise signals measured in the anechoic chamber of the NASA Glenn Aero-Acoustic Propulsion Laboratory. It is shown that a signal containing only these intermittent events retains all of the important aspects of the acoustic spectrum for jet noise radiating to aft angles, validating the assumption that intermittent events are the dominant feature of aft angle jet noise. Statistical analysis of the characteristics of these noise events reveals that these events can be described in terms of three parameters (the variance of the original signal, the mean width of the events, and the mean time between events) and two universal statistical-distribution curves. The variation of these parameters with noise-radiation direction, nozzle diameter, and jet velocity and temperature are discussed. There is a strong correlation between the mean width of the intermittent events and the mean time between the events () in all the cases investigated in this work, implying a strong link between dynamics governing the two quantities.

Computational Analyses of Offset-Stream Nozzles for Noise Reduction

The W ind c omputational fluid dyn amics code was used to perform a series of simulations on two offset stream nozzle concepts for jet noise reduction. The first concept used an S -duct to direct the secondary stream to the lower side of the nozzle. The second concept used vanes to turn th e secondary flow downward. The analyses were completed in preparation of tests conducted in the NASA Glenn Research Center Aeroacoustic Propulsion Laboratory. The offset stream nozzles demonstrated good performance and reduced the amount of turbulence on the lower side of the jet plume. The computer analyses proved instrumental in guiding the development of the final test configurations and giving insight into the flow mechanics of offset stream nozzles. The computational predictions were compared with flowfield results from the jet rig testing and showed excellent agreement.

Experimental and analytical determination of the geometric far field for round jets

An investigation was conducted at the NASA Glenn Research Center using a set of three round jets operating under unheated subsonic conditions to address the question: How close is too close? Although sound sources are distributed at various distances throughout a jet plume downstream of the nozzle exit, at great distances from the nozzle the sound will appear to emanate from a point and the inverse-square law can be properly applied. Examination of normalized sound spectra at different distances from a jet, from experiments and from computational tools, established the required minimum distance for valid far-field measurements of the sound from subsonic round jets. Experimental data were acquired in the Aeroacoustic Propulsion Laboratory at the NASA Glenn Research Center. The WIND computer program solved the Reynolds-Averaged Navier-Stokes equations for aerodynamic computations; the MGBK jet-noise prediction computer code was used to predict the sound pressure levels. Results from both the experiments and the analytical exercises indicated that while the shortest measurement arc (with radius approximately 8 nozzle diameters) was already in the geometric far field for high-frequency sound (Strouhal number >5), low-frequency sound (Strouhal number <0.2) reached the geometric far field at a measurement radius of at least 50 nozzle diameters because of its extended source distribution.

A Large Hemi-anechoic Enclosure for Community-Compatible Aeroacoustic Testing of Aircraft Propulsion Systems

A large hemi-anechoic (absorptive walls and acoustically hard floor) noise control enclosure has been erected around a complex of test stands at the NASA Lewis Research Center in Cleveland, Ohio. This new state-of-the-art Aeroacoustic Propulsion Laboratory (APL) provides an all-weather, semisecure test environment while limiting noise to acceptable levels in surrounding residential neighborhoods. The 39.6 m (130 ft) diameter geodesic dome structure houses the new Nozzle Aeroacoustic Test Rig (NATR), an ejector-powered M = 0.3 free jet facility for acoustic testing of supersonic aircraft exhaust nozzles and turbomachinery. A multi-axis, force-measuring Powered Lift Facility (PLF) stand for testing of Short Takeoff Vertical Landing (STOVL) vehicles is also located within the dome. The design of the Aeroacoustic Propulsion Laboratory efficiently accomodates the research functions of two separate test rigs, one of which (NATR) requires a specialized environment for taking acoustic measurements. Absorptive fiberglass wedge treatment on the interior surface of the dome provides a hemi-anechoic interior environment for obtaining the accurate acoustic measurements required to meet research program goals. The APL is the first known geodesic dome structure to incorporate transmission-loss properties as well as interior absorption into a free-standing, community-compatible, hemi-anechoic test facility.