Abstract
A theory is developed which predicts the acoustic radiation of an axial-flow fan stator due to interaction with rotor viscous wakes. Calculations are compared in detail with experimental data. Both the harmonic and broadband noise-spectrum components are calculated from a unified model using methodology from the theories of random pulse amplitude modulation, PAM, and pulse position modulation, PPM. The stator is modeled as a circular array of pulsed dipoles. The amplitude and arrival time of each pulse are random variables whose means correspond to the values calculated from harmonic rotor-stator interaction theory. The standard deviations of these random variables are measures of the turbulence levels in the blade wakes. For the model proposed the solution is exact: when PAM is imposed on a periodic stator source, new broadband energy is generated whose spectrum shape is similar to the envelope of harmonics at high frequencies and the harmonic radiation is unchanged; however, when PPM occurs, new broadband energy is radiated but at the expense of harmonic energy. At frequencies significantly above duct cutoff it is shown for a fixed stator solidity that the spectrum is essentially independent of the number of stator vanes.
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