Rotor Inflow Noise Caused by a Boundary Layer: Inflow Measurements and Noise Predictions

Detailed measurements of the four-dimensional space time correlation function of a thick turbulent boundary layer have been made and used as source terms for the prediction of sound radiated from a 10-bladed rotor partially immersed in this flow. This simple configuration recreates in a canonical setting the important physics produced when a rotor ingests inhomogeneous anisotropic turbulence, without many of the complications present in most practical applications. The correlation measurements show substantial turbulence anisotropy in the body of the boundary layer which is characterized by streamwise elongated structures inclined relative to the wall. Translated to the rotor frame this anisotropy leads substantial asymmetry in blade-to-blade upwash coherence. Predictions of radiated sound spectra and directivity are performed for a range of conditions. Sound predictions reveal spectra dominated by ‘haystacking’ - distinct peaks at multiples of the blade passing frequency produced by multiple cutting of the same turbulent structures.