Prevention of buzz-saw noise by acoustic lining

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Prevention of buzz-saw noise by acoustic lining

Sijtsma, P.

In modern high-bypass ratio turbofan engines, the fan of the axial flow compressor is the dominant source of noise. Most of this noise is radiated from the inlet of the surrounding duct. When the rotor operates at subsonic relative (helical) tip speed, sound is mainly generated by the interaction of rotor and stator. The acoustic signal is periodic with the blade-passing frequency (BPF), which is the shaft rotation frequency multiplied by the number of rotor blades. In the sound spectrum, peaks appear at n multiplied by BPF. At supersonic helical tip speed, the noise caused by motion of the rotor itself becomes important. Shock waves arise near the rotor blade leading edges and propagate upstream towards the duct inlet. By weakly non-linear shock propagation theory, it can be shown that the sound power decays towards the inlet, however, not exponentially as is the case with subsonic rotor noise. The smaller the inlet radius of the duct, the more decay is expected. Expressions are derived describing the generation of buzz-saw noise, occuring upstream of a ducted rotor with supersonic relative tip speed. In contrast with earlier work on this matter, a fully three-dimensional, non-linear theory is used, in which the effect of acoustic treatment is included. In the sound spectrum of a hard-walled duct, the theory predict peaks at close-to-resonance frequencies. Moreover, a substantial suppression of buzz-saw noise is predicted to a duct wall with acoustic lining, depending on the liner characteristics. Calculations with estimated input are in good agreement with measurements.