ROTOR NOISE DUE TO ATMOSPHERIC-TURBULENCE INGESTION .2. AERO-ACOUSTIC RESULTS

A previously developed analysis for noise prediction due to turbulent inflow to a propeller or helicopter rotor has been extended to treat nonisotropic turbulence. The turbulence is calculated using a mean-flow contraction model combined with rapid distortion theory as described in the companion paper.1 The noise model is based on an analysis for a flat plate in rectilinear motion through a turbulent field. By assuming that the rotor is in rectilinear motion at each instant of time, the overall spectrum is calculated by averaging the instantaneous spectrum over time. The mean-flow distortion stretches the turbulence, decreasing the velocities along the principal axis of the stretching (strain). When the principal axis of the stretching lies close to the rotor axis, the distortion acts to decrease the upwash velocities on the rotor and thus decreases the noise from that for isotropic turbulence. The correlation area in the plane of the rotor also is decreased tending to decrease the noise further. Conversely, if the principal stretching direction lies close to the rotor plane, an increase in the noise is possible. Acoustic energies at the observer location are calculated and compared to the energy of the turbulence as affected by the contraction. © 1989 by United Technologies Corporation. © The American Institute of Aeronautics and Astronautics, Inc. All rights reserved.