Acoustic Performance of Novel Fan Noise Reduction Technologies for a High Bypass Model Turbofan at Simulated Flight Conditions

15th AIAA/CEAS Aeroacoustics Conference (30th AIAA Aeroacoustics Conference)

Parrott

Sutliff

et al. 2009

Two innovative fan-noise reduction concepts developed by NASA are presented -soft vanes and over-the-rotor metal foam liners. Design methodologies are described for each concept. Soft vanes are outlet guide vanes with internal, resonant chambers that communicate with the exterior aeroacoustic environment via a porous surface. They provide acoustic absorption via viscous losses generated by interaction of unsteady flows with the internal solid structure. Over-the-rotor metal foam liners installed at or near the fan rotor axial plane provide rotor noise absorption. Both concepts also provide pressure-release surfaces that potentially inhibit noise generation. Several configurations for both concepts are evaluated with a normal incidence tube, and the results are used to guide designs for implementation in two NASA fan rigs. For soft vanes, approximately 1 to 2 dB of broadband inlet and aft-radiated fan noise reduction is achieved. For over-the-rotor metal foam liners, up to 3 dB of fan noise reduction is measured in the low-speed fan rig, but minimal reduction is measured in the high-speed fan rig. These metal foam liner results are compared with a static engine test, in which inlet sound power level reductions up to 5 dB were measured. Brief plans for further development are also provided.

Section: Fan Rig Evaluationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Assessment of Soft Vane and Metal Foam Engine Noise Reduction Concepts

15th AIAA/CEAS Aeroacoustics Conference (30th AIAA Aeroacoustics Conference)

Parrott

Sutliff

et al. 2009

“…Over-the-rotor (OTR) liners were also evaluated in the same ANCF and LSWT tests. 2,3,14,19,20 Figure 8 provides photographs of one version of OTR liners, which employs metallic foam to achieve a desired impedance spectrum at the surface of the liner. Due to their location, OTR liners are expected to have the most significant effect on noise generated at the tips of the rotors.…”

Section: Reviewmentioning

confidence: 99%

Evaluation of Novel Liner Concepts for Fan and Airframe Noise Reduction

22nd AIAA/CEAS Aeroacoustics Conference

Howerton

2016

This paper presents a review of four novel liner concepts: soft vanes, over-the-rotor liners, external liners, and flap side-edge liners. A number of similarities in the design and evaluation of these concepts emerged during these investigations. Since these were the first attempts to study these particular liner concepts, there was limited information to guide the design process. In all cases, the target frequencies (or frequency range) were known, but the optimum acoustic impedance and optimum liner placement were typically not known. For these cases, the maximum available surface was used and a ρc-impedance was targeted based on the assumption the sound field impinges on the surface at normal incidence. This choice proved fruitful for every application. An impedance prediction model was used to design variable-depth liner configurations, and a graphical design code (ILIAD) was developed to aid in this process. The ability to build increasingly complex liner configurations via additive manufacturing was key, such that multiple designs could quickly be tested in a normal incidence impedance tube. The Two-Thickness Method was used to evaluate available bulk materials, such that bulk liners could also be considered for each application. These novel liner concepts provide sufficient noise reduction to warrant further investigations.

“…NASA has been developing over-the-rotor acoustic treatments for a number of years. The latest iterations of over-the-rotor acoustic treatments have incorporated circumferentially grooved fan cases [2,4,5] to mitigate the issues [6] found when subjecting an acoustic treatment to large dynamic pressure fluctuations. The combination of a fan casing treatment with an acoustic liner led to the desire to isolate the circumferential groove effects from the acoustic liner effects.…”

Section: Introductionmentioning

confidence: 99%

Effect of Grazing Flow on Grooved Over-the-Rotor Acoustic Casing Treatments

Bozak

25th AIAA/CEAS Aeroacoustics Conference

Howerton

et al. 2019

After testing grooved over-the-rotor acoustic casing treatments on a turbofan rotor, a follow-on study was performed to investigate the effect of flow on grooved acoustic liners. The experiment was performed to understand the scaling of acoustic liner absorption with grazing flow and investigate a potential noise source from grooved acoustic liners. Acoustic liner absorption and reflection characteristics were quantified by examining the reduction in amplitude of a plane wave traveling over 2 inch liners with grazing flow. For all liners tested, as the grazing flow Mach number is increased, the absorption curves broadened and the frequency of peak absorption decreased. Grazing flow over a series of grooves was found to generate resonances up to 152 dB sound pressure level. Adding acoustic treatment to the bottom of these grooves was found to reduce the magnitude of this resonance by up to 10 dB sound pressure level and increase its frequency by up to 10%. The quantification of the grazing flow effect and identification of a mechanism behind the noise penalty from the prior turbofan rotor experiment will aid in the design of future over-the-rotor treatments.