Sound Generation by a Rotor Interacting with a Casing Turbulent Boundary Layer

Abstract

A new method for predicting the noise generated by a ducted rotor interacting with inhomogeneous and nonisotropic turbulence has been developed. The analytical formulation used a model of the two-point correlation function of the turbulent velocity in the space–time domain. The study focused on a specific condition where the dominant noise source was the interaction between a rotor and a casing turbulent boundary layer. The axial length scale of this turbulence was found to be large enough to generate unsteady lift that was correlated between multiple rotor blades. This led to tonal sound at the blade passing frequency in the absence of mean velocity variations. The analytical formulation was validated with a set of measurements obtained in a ducted rotor facility. The prediction accurately modeled both the tonal and broadband features of the sound spectra.