Rotor Inflow Noise Caused by a Boundary Layer: Theory and Examples

Abstract

A theory for the noise generated by unsteady loading of an open rotor operating in a turbulent flow is presented. The theory makes no assumptions about the isotropy or homogeneity of the turbulence, and thus can be applied to complex problems such as ingestion of a plane boundary layer into an fan face. The results are illustrated with sample noise calculations for rotors completely and partially immersed in turbulence. When completely immersed the sound spectrum is highly dependent on the advance ratio. For high advance ratios the spectrum has a single broad peak centered slightly above the blade passage frequency. For low advance ratios several peaks (or “haystacks”) appear centered on multiples of the blade rate. When partially immersed this multiple-peaked form is greatly enhanced by the modulation of the mean turbulence flow seen by the blades and dominates the spectrum even at low advance ratios.