Assessment of Soft Vane and Metal Foam Engine Noise Reduction Concepts

Abstract

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.

Experimental study of vortex–structure interaction noise radiated from rod–airfoil configurations

“…Compared with the solid leading edge, the slotted leading edge gives a noise reduction of about 4 dB, whereas an improved noise reduction of 6 dB can be obtained with the soft vane by covering the slotted leading edge with 0.2 mm-thick cloth. A similar behaviour using the soft vane concept was also reported by Jones et al (2009) for the experimental study of the engine fan noise reduction. The corresponding sound pressure spectra are shown in Fig.…”

“…The leading edge slot is 20 mm (depth) Â 2 mm (width). This soft vane concept was investigated by Jones et al (2009) on engine noise reduction. As indicated by Jones et al the slotted leading edge may allow communication of pressure fluctuations on both sides of the surface and the chamber formed within the cloth should provide pressure release (relative to the solid surface it replaced) on the leading edge surface.…”

Analysis of Turbofan Design Options for an Advanced Single-Aisle Transport Aircraft

“…24 In addition to conventional liners, two advanced technologies were applied for fan noise reduction; soft vane stators and over-the-rotor foam metal treatment . 25 Both of these technologies are applications of acoustic treatment in areas of the engine which currently do not have treatment: the fan vanes and above the fan rotor tips. Acoustic tests of both of these technologies were conducted at NASA Glenn in 2008.…”

Noise Assessment of NASA Environmentally Responsible Aviation (ERA) Technologies and Concepts

“…By using a more resilient and structurally sound metal mesh, this technology addresses this drawback and reduces adverse aerodynamic effects of typical materials. In an assessment of the technology by NASA, a 3dB fan inlet noise reduction was obtained with low tip speed fan applications such as the GTF, but little reduction was seen for fans with high tip speeds [27]. …”

“…In addition, the soft surface provides a pressure release for sources produced at the surface of the vane. [27] These mitigated sources include those generated by the interaction of wakes, turbulence, and vortices impinging on the vane surface.…”