AN EXPERIMENTAL-STUDY OF PROPELLER NOISE DUE TO CYCLIC FLOW DISTORTION

Large scale non-uniformities in the approach flow to a propeller resul t in periodic unsteady blade forces that radiate tonal noise at the bl ade passing frequency and its higher harmonics. The current research e xperimentally investigates the free-field directivity of a four-bladed propeller interacting with matched and mismatched cyclic distortions. The study employs non-uniform flow generators that provide a three-cy cle and four-cycle azimuthal distortion of the flow field. As the infl ow distortions contain just one dominant Fourier mode, a well-defined axisymmetric free-field directivity exists only for the fundamental to ne. Higher harmonics, originating from small scale flow gradients, hav e a random directivity. An extensive comparison of the measured tonal noise levels with the theoretical predictions shows an excellent agree ment, both in magnitude and directivity.