An experimental study of noise generated by tandem blades

To facilitate the low-noise design of tandem lift bodies as applied in aeroengines and aircraft, the acoustic features of tandem blades are investigated by wind-tunnel experiments. This is further specialized for the rotating blades applied in contra-rotating open rotors under the concept of frozen-rotor. A 70-channel phased microphone array and nine high-precision free-field microphones are employed. The beamforming method, enhanced by a source filtering technique, is employed to locate noise sources, providing insights into the source patterns of blade-blade interaction noise concerning flow speed, blade spacing, and aft blade clipping. The results show the following: (A) Sources of tandem-blade noise exist in the form of concentrated source clusters, resulting in two major clusters: the mid-span interaction noise and the tip-induced noise. (B) These source clusters tend to separate as flow speed or blade spacing increases. (C) By increasing blade spacing, the band-pass filtered overall sound pressure level is reduced by 2.9 dB. (D) A two-phase noise suppression pattern is observed with blade clipping, resulting in a total reduction of 3.0 dB for the interaction noise through the removal of tip-induced noise sources and the replacement of mid-span noise sources. Based on these findings, suggestions concerning blade spacing and clipping are discussed.