A Multivariate Statistical Analysis of the Noise Emitted by an Installed Propeller

Novel-aircraft concepts consider the possibility of placing the propulsor very close to the fuselage to ingest the incoming airframe boundary layer. In this configuration, the engine takes the inflow at a reduced velocity, consuming less fuel in the combustion process. However, this induces a series of noise consequences that alter tonal and broadband noise components. The present work reports an experimental investigation to analyse the sound emitted by a propeller ingesting a turbulent boundary layer. Experiments have been performed in the anechoic wind tunnel at the University of Bristol and the set-up consisted of a two-bladed propeller close to a tangential flat plate to simulate the installed effects. A tripping device was placed 1 m upstream of the propeller and was used to generate a turbulent boundary layer at the propeller location. The wind tunnel velocity was fixed at 33 m/s keeping the advance ratio at J = 0.65. Far-field noise has been acquired using a microphone array positioned parallel to the plate, directly overhead of the propeller. The data were analysed in the frequency domain, providing a characterisation of spectral quantities. Furthermore, wavelet analysis was performed to investigate the time evolution of the identified pressure features. Results show evident haystacking humps close to higher BPF harmonics, due to the ingestion of the boundary layer. Moreover, the wavelet analysis revealed the intermittent nature of the haystacking humps, clearly visible at higher BPF harmonics with low-frequency energy content.