Aeroacoustics of the propeller Boundary Layer Ingestion configuration for varying thrusts

The Boundary Layer Ingestion (BLI) configuration is a promising installed propulsion arrangement for aircraft owing to its potential to boost aerodynamic efficiency. However, it is prone to generate additional noise as the inflow to the propulsion system is non-uniform and turbulent due to the ingested boundary layer. Nevertheless, the acoustic characteristics of the BLI configuration and its sensitivity to different operating parameters (e.g., boundary layer thickness, turbulence contents and propeller rotational speeds) are not well understood. Thus, the present study reports a detailed experimental campaign to investigate the aerodynamic thrust, far-field acoustics and plate surface pressure of a two-bladed propeller ingesting distinct turbulent boundary layers (TBL) for a range of advance ratios. All results were compared against a reference ‘No BLI’ case. It was found that BLI noise comprises mostly broadband noise that is directed predominantly in the downstream direction parallel to the plate, whilst orthogonal to the plate, the noise signature is mostly tonal. When increasing thrust, the effect of BLI on noise is reduced. Scaling of the acoustic spectra with tip Mach number identifies that leading-edge-turbulence interaction was the main source of broadband noise in the BLI configuration and more importantly, showed two distinct operating regimes based on thrust conditions. Lastly, phase-averaged acoustic results showed marked noise variations over a rotation with an increase when the propeller begins to ingest the boundary layer, particularly for the thicker and more turbulent boundary layer, indicating that the broadband components became dominant during TBL interaction and were partly responsible for the modulation of the emitted noise.