Aeroacoustic Testing of Open Rotors at Very Small Scale

This study examines the potential of very-small-scale experiments to simulate the acoustics of large scale counter-rotating open rotors in isolation and with a rectangular-plate shield. The experimental rotor model was based on the historical F7/A7 design developed by General Electric. The 80-mm diameter rotors were fabricated using advanced stereolithography and were powered by two high-performance DC brushless motors. Realistic tip Mach numbers were achieved with the rotors spinning near 50000 rpm. Far-field acoustic surveys were conducted inside an anechoic chamber using a 12-microphone array. The isolated tests captured the tonal and broadband contents of open rotor noise. Rotor-alone tones dominated the sound emission in the vicinity of the rotor plane. In the aft direction, interaction tones were very strong. Addition of the rectangular plate shield created complex trends in the tonal content. Tones radiating in the direction of the rotor plane were well attenuated, while tones emitted in the aft direction were poorly shielded. Some tones were amplified with insertion of the shield. Even though the shield reduced the noise level in an overall sense, the complicated tonal pattern presents challenges for the effective reduction of aircraft noise.