Aeroacoustic Investigation of Shrouded Propeller Systems

This paper presents an experimental, numerical, and analytical investigation into the influence of shroud length on the acoustic performance of ducted propeller systems, with relevance to Urban Air Mobility (UAM) and Unmanned Aerial Vehicle (UAV) applications. Building on previous work that examined intake geometry and acoustic liners, this study isolates the effect of the shroud length by comparing two configurations of different lengths under identical intake and thrust conditions. Far-field and in-duct noise measurements are presented and complemented by hot-wire flow diagnostics and numerical simulations using the Lattice-Boltzmann (LB) method. Additionally, a simplified analytical model is presented to assess the role of acoustic near-field effects in noise radiation. The results show that the longer duct configuration increases broadband noise at lower frequencies, while the shorter duct exhibits higher tonal and broadband noise at higher frequencies, particularly in the rear arc. Analytical predictions confirm that radiation effects due to source proximity to the intake are only significant when the source lies within half a wavelength of the duct inlet. For the configurations tested, these effects contribute less than 1 dB to the overall noise difference, indicating that source-related flow effects are dominant. The LB simulations show good agreement with experimental data and provide a foundation for future analysis of the noise generation mechanisms.