An experimental study of propeller noise due to cyclic flow distortion

Abstract

Large scale non-uniformities in the approach flow to a propeller result in periodic unsteady blade forces that radiate tonal noise at the blade passing frequency and its higher harmonics. The current research experimentally 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-cycle and four-cycle azimuthal distortion of the flow field. As the inflow distortions contain just one dominant Fourier mode, a well-defined axisymmetric free-field directivity exists only for the fundamental tone. Higher harmonics, originating from small scale flow gradients, have a random directivity. An extensive comparison of the measured tonal noise levels with the theoretical predictions shows an excellent agreement, both in magnitude and directivity. © 1995 by Academic Press, Inc.