An investigation of the noise reduction mechanisms of over-the-rotor liners for electric ducted fan applications using a duct acoustics approach

Abstract

This work extends our prior study (Physics of Fluids, 35, 107,144, 2023) on over-the-rotor (OTR) liners, which revealed that liners in the fan near-field exhibit dramatically different orifice flow characteristics compared to conventional liners subject to purely acoustic excitation superimposed on a grazing flow. While previous work primarily focused on orifice flow dynamics, this study systematically investigates the noise reduction mechanisms. A novel method is proposed to separate the acoustic attenuation and source modification effects of the OTR liner, allowing a quantitative analysis of the two effects by examining changes in duct mode amplitudes. Results show that amplitudes of the duct modes excited by rotor blades decrease across a broadband due to the source modification effect, and further noise reduction is observed with increasing fan speed. The acoustic attenuation effect is primarily observed within the frequency range corresponding to the acoustic absorption bandwidth of the OTR liner, but its contribution to overall noise reduction is less significant than that of the source modification effect. Moreover, the OTR liner has a marginal impact on the amplitudes of the duct modes excited by the stator. The proposed method offers a new approach to studying the noise reduction mechanisms of OTR liners.