Experimental Investigation into the Suppression of Propeller Noise Using Over-Tip-Rotor Liners

This paper presents an experimental study on suppressing propeller noise in a short duct configuration by installing Over-Tip-Rotor (OTR) liners on the wall casing. Far-field noise measurements are reported alongside in-duct pressure measurements at the wall obtained using an axial and azimuthal microphone array installed upstream of the propeller plane. In-duct measurements reveal that the OTR liner is effective in reducing broadband noise, achieving up to 4 dB of noise reduction over 1-5 BPFs. The axial decay of tonal components was compared with an analytical solution based on the Green’s function of an axial dipole source, requiring a large number of cut-off modes to match the experimental observations. The findings emphasise the importance of evanescent modes in compact duct acoustics, with direct implications for source modification, scattering effects, and far-field radiation.