Abstract
Helicopter rotor noise theories are used to determine the noise of subsonic fans produced solely by the rotor, which results from aerodynamic intake flow distortions. Extensions to the theories allow for both stationary, phase-related distortions and distortions which vary randomly with time and may therefore be considered to rotate with random speeds. It is shown that the aerodynamic interaction of intake distortions with the blades will result in both rotor order tones and broad-band noise. The dominance of the one over the other depends on the space-time correlation of the distortions and the magnitude of typical fan length and time scales. Modulation effects are considered. The use of in-duct measurements is discussed and results presented to show the relative importance of steady and unsteady distortions in a simple fan rig. The amplitudes of the resultant blade force distortions have been determined from on-axis acoustic data and used to predict the full forward radiation field.
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