On the aeroacoustics of turbulent boundary layer ingesting propellers

The present work experimentally examines the aeroacoustic characteristics of a propeller ingesting various planar turbulent boundary layers. The experimental setup consists of a two-bladed propeller, operating at a constant advance ratio, positioned close to a flat plate in the boundary layer ingestion configuration (BLI) with a fixed propeller-plate clearance. To investigate the sensitivities of the far-field noise signature of the BLI configuration to the inflow conditions, three incoming turbulent boundary layers of varying thicknesses and turbulence energy contents were developed. The resulting far-field acoustics and near-field velocity were captured using microphone arrays and hot-wire anemometry. Far-field acoustic results show that increasing the boundary layer thickness, and turbulence contents, sees a significant rise of the broadband components in the mid-frequencies, with strong directivity, a useful property to manipulate during aircraft design. Upstream flow field results confirm that the presence of the propeller induces a mild elevation in both the velocity and its fluctuations within the boundary layers, with the highest increase associated with the thickest boundary layer. Additionally, to understand the distinct acoustic and flow behavior over a revolution of the propeller, i.e., when the propeller is moving in and out of the boundary layer, phase-averaging of the acoustic and velocity signals was performed. The phase-averaged results showed strong pulsation in the steady and unsteady velocities around the propeller and also identified that the peak of the broadband noise generation occurs when the propeller is close to perpendicular to the plate and during peak local velocity perturbation.