New acoustic and aerodynamic phenomena due to non-uniform rotation of propellers

A study is reported of the influence of non-uniform rotation-which is inher ent to piston engine driven propellers-on the aerodynamics and aeroacoustic s of multi-blade propellers by numerical simulation. The combination of aer odynamic predictions with a 3-D unsteady free wake panel method and aeroaco ustic predictions based on Farassat's Formulation 1A of the Ffowcs Williams and Hawkings equation is used to achieve this goal. The numerical results show that non-uniform rotation has a significant influence on propeller aer odynamics and can lead to an increase in the generated noise. In case of a mismatch between the periodicity of the non-uniformity and the basic blade passage frequency, additional harmonics ("subharmonics") are generated. For a periodicity coincidence, the effects are masked due to an overlapping of the frequencies. The level of such subharmonics may be high enough to incr ease the overall A-weighted noise. The azimuthal directivity of the of the propeller noise remains no longer axisymmetric, and changes to a wave-like harmonic variation. The number of undulations per revolution depends on the order of the non-uniformity and is not related to the number of propeller blades. The polar directivity pattern also changes substantially from that known for uniform rotation. A frequency domain analysis of the unsteady pre ssure distribution shows that the subharmonics perceived at a space-fixed l ocation are not due to an aerodynamic or acoustic interaction but rather th e consequence of a motion geometry or Doppler effect. (C) 1999 Academic Pre ss.