Computational Analysis of Sound from a Rotor Ingesting a Non-Axisymmetric Turbulent Boundary Layer

Abstract

The noise from a five-bladed rotor ingesting a non-axisymmetric turbulent boundary layer at the rear of a body of revolution (BOR) at 5-degree angle of attack is computed using large-eddy simulation and the Ffowcs Williams-Hawkings equation. The rotor advance ratio is 1.44, corresponding to a nominally zero-thrust condition. The Reynolds number based on the BOR length and free-stream velocity is 1.9 million, and the free-stream Mach number is 0.059. The calculated sound pressure spectra are in reasonable agreement with the experimental measurements of Banks et al. (AIAA-2022-3032). Similar to the axisymmetric, zero angle of attack case, the acoustic field is broadband with haystacking peaks near multiples of the blade passing frequency. While the non-axisymmetry of the rotor inflow generates a strong acoustic response from individual blades at the rotor rotational frequency, its effect on the broadband acoustic response at higher frequencies is negligible, and the sound pressure spectra from the entire rotor only show mild changes in directivity. The blade-to-blade coherence of sectional dipole sources exhibits extremely high values at the rotor rotational frequency and moderately high values at the frequencies of the first two haystacking peaks and the valley between them. The resulting destructive and constructive acoustic interferences are responsible for diminishing the sound at the rotational frequency and generating the haystacking spectral peaks and valleys.