Open Fan Acoustic Tests in Low- and High-Speed Configurations at S1MA Wind Tunnel

Unducted aircraft engines, such as the CFM RISE Open Fan engine, represent a promising alternative to conventional turbofan engines for the next generation of commercial aircraft. Noise reduction in unducted aero-engines is a significant challenge. Wind tunnel testing is required to obtain accurate noise measurements and to assess the accuracy of CFD-CAA predictions. Safran Aircraft Engines has recently completed an extensive test campaign on a 1/5.5th scale model of an Open Fan at ONERA S1MA wind tunnel in Modane, France. The tests included several blade sets to gain a better understanding of the main noise sources of an Open Fan. Validation of multi-disciplinary blade design optimization was performed under different operating conditions. For the low-speed tests at Mach numbers below 0.3, acoustic measurements were performed in an anechoic test section with a diameter of 8 meters, simulating both approach and take-off conditions. To this end, microphone arrays were mounted on the walls of the wind tunnel to measure far-field noise directivity at circumferential locations relevant for community noise assessments, such as fly-over and sideline conditions. For the high-speed tests at Mach ~ 0.8, the microphone arrays were positioned closer to the fan blade tip to measure near-field noise, which is suitable for assessing noise levels on the aircraft fuselage. A parametric study was performed to evaluate the influence of pitch settings, fan rotational speed, in-flight Mach number, and angle of attack on noise emissions. Promising aerodynamic and acoustic results have been obtained, confirming the expectations of the Open Fan and its potential for civil aviation applications. The key design drivers for an Open Fan have been validated, and numerous opportunities have been identified to further reduce noise from cycle and blade design optimizations.