Jeff M. Mendoza

Combustion Noise of Auxiliary Power Units

Noise from auxiliary power units (APU) is an important contributor to the overall level of ramp noise. Currently, ramp noise is regulated by international governing bodies as well as by individual airport. A significant component of APU noise is combustion noise. In this study, the unique spectral shape of APU combustion noise is identified. It is found that the spectral shape is the same regardless of engine size, power setting and directivity. Also, it is practically the same as that of open flame combustion noise. The frequency at the peak of the combustion noise spectrum is found to lie in the narrow range between 250 to 350 Hz. The peak sound pressure level of a given APU varies as the square of the fuel consumption rate. In the literature, suggestions have been made concerning a second combustion noise mechanism arising from the passage of hot entropy spots through the exhaust nozzle or constriction. In this investigation, no evidence has been found to indicate the existence of a second APU combustion noise component.

Nacelle In-duct Beamforming using Modal Steering Vectors

A mode measurement array of 119 Kulite transducers was installed in an inlet duct section and tested with a Honeywell Tech977 turbofan engine. 2D mode analysis was performed using conventional beamforming and CLEAN-SC. The modes were determined in terms of sound power level. In-duct spatial source imaging was also performed using free space steering vectors and an annular duct Green’s function. CLEAN-SC is found to be a powerful enhancement for mode measurements, completely removing several types of array artifacts and facilitating reliable sound power estimates. An interesting feature of CLEANSC is that the resulting mode map potentially combines the powers of several modes into a single spot. Applying CLEAN-SC to the spatial imaging results in a confusing distribution of apparent point sources. A need to improve the way CLEAN-SC represents extended sources has been identified.

Baseline Noise Measurements from the Engine Validation of Noise and Emissions Reduction Technology Program

The Engine Validation of Noise and Emissions Reduction Technologies is a test program in which NASA funded engine validations of integrated technologies that reduce aircraft engine noise. These technologies address the reduction of engine fan and jet noise, and noise associated with propulsion/airframe integration. As part of this program, an extensive set of far field noise data was taken to characterize the noise sources of Honeywell’s TECH977 engine. Baseline characterization of this engine included the use of inlet and exhaust barriers for noise shielding. These results showed that the transition from inlet-radiated to aft-radiated noise occurs at a polar directivity angle of 70° to 80° from the inlet and provide a peak noise attenuation approaching 10 dB for the inlet and 20 dB for the exhaust. Treatment sensitivity testing showed that the incremental benefit of additional treatment reduces as the liner length increases. The axial extent of acoustic treatment is a key design parameter for the acoustic attenuation. Comparisons were made between separate flow and mixed flow nozzle exhaust configurations. At the sideline condition, the separate flow nozzle has higher noise levels than the mixed flow nozzle for directivity angles greater than 120 degrees. In addition, the separate flow nozzle has higher noise levels for directivity angles from 60 to 90 degrees at the sideline condition due to an increase in the inlet radiation of the blade passage tone. Turbine noise is better attenuated with mixed flow nozzle exhaust. Smaller reductions in jet noise with the mixed flow nozzle were seen at the cutback and approach operating conditions. Comparison of the pretest predictions with the measured far field data from the baseline testing identified the need for improvements in all source prediction models; however, the prediction methods were a useful tool in performing noise source separations.

Phased Array Beamforming with 100 -foot Polar Arc Microphones in a Static Engine Noise Test

Ground microphones from static engine testing of a Honeywell Tech 977 turbofan engine were used for phased array beamforming. Two arrays were included: the polar array and the “h igh frequency ” array. The polar array consisted of the standard 32 5° microphones covering inlet angles from 5 ° through 160°, and the “h igh frequency ” array comprised 23 microphones nonuniformly spaced over 60° through 90°. All of the microphones were spaced 100 feet from a point below the engine inlet highlight on the tarmac. The primary objectives were to separa te source components and to determine the feasibility of beamforming with the standard microphones. An unexpected case -radiation source was identified and characterized. Conventional beamforming as well as DAMAS and CLEAN -SC were applied. It was found that CLEAN -SC is a powerful tool for reducing sidelobes, dealing w ith coherent sources, and mitigating the effects of turbulent decorrelation, all of which are important in this application.

Static and Flight Aeroacoustic Evaluations of a Scarf inlet

This paper discusses the results pertaining to the design, analysis, and acoustic testing of a scarf inlet as conducted under the Engine Validation of Noise Reduction Concepts (EVNRC) program. Honeywell Engines, Systems & Services (Honeywell), along with team members Boeing, Nordam (previously Calcor Aero Systems), and Allied Aerospace (formally Micro Craft), has performed the EVNRC program. By working in close coordination with NASA, the data obtained from this test was used to validate the advanced technology to meet the NASA Advanced Subsonic Technology (AST) noise reduction goals and expedite the transition of these new technologies into US-powered aircraft. The AST Noise Reduction program goal was to validate technology that reduces engine noise 6 EPNdB and improves nacelle suppression effectiveness by 50 percent relative to 1992 technology. The scarf inlet technology was expected to contribute to these noise reduction goals. Several aspects of the design and evaluation process of this technology are described illustrating key characteristics and acoustic benefits.