Measurement of Noise Reduction from Acoustic Casing Treatments Installed Over a Subscale High Bypass Ratio Turbofan Rotor

Abstract

NASA is continuing to develop over-the-rotor acoustic liners for turbofan applications. A series of low Technology Readiness Level experiments were conducted to better understand the acoustic and aerodynamic effects of these acoustic liners. The final experiment included the evaluation of four acoustic casing treatment concepts and two baseline configurations in an internal flow axial compressor facility with a 1.5 pressure-ratio high-bypass turbofan rotor. An inlet in-duct array was utilized to extract sound power levels propagating forward from the turbofan rotor. The effect of a circumferentially grooved relative to a hardwall fan case was found to reduce the in-duct sound power level by about 1.5dB for frequencies less than 2kHz while increasing noise from 4 to 8kHz by as much as 7.5dB at low fan speeds. The four acoustic treatment concepts were incorporated into the bottoms of the circumferential grooves and found to provide an additional 1 to 2dB sound power level reduction under 2kHz. The sound power level reduction was found to be even greater, 2.5 to 3.5dB, when evaluating the reduction on rotor alone duct modes (co-rotating modes). The acoustic treatments also appeared to reduce multiple pure tone noise at transonic fan speeds. Depending on the acoustic treatment concept, the high-frequency noise created by the circumferential grooves was reduced by 1.5 to 5 dB. The total noise reduction from acoustic treatments embedded into the bottoms of circumferential grooves relative to a hardwall baseline was found to be 2.5 to 3.5dB sound power level. The sound power level reduction for rotor alone (co-rotating) modes was found to be 3.5 to 4.5dB. These results show the potential for significant turbofan noise reduction by incorporating acoustic treatments over-the-rotor.