Near Field Source Investigation in Shrouded Propellers

This paper investigates the near-field pressure of a shrouded propeller with particular emphasis on the physical nature of the in-duct pressure field. While previous studies on shrouded propellers generally either treated the source as an idealised acoustic source, or focused on mean flow effects, blade loading, wake dynamics, and far-field noise generation, limited attention has been paid to the measured or computed in-duct pressure signals. To address this issue, a combined experimental and numerical study is carried out using in-duct microphone measurements and lattice Boltzmann method (LBM) simulations. The analysis focuses on the axial and azimuthal pressure distributions at the blade passing frequencies (BPFs). The in-duct axial decay is shown to be well described by a potential field like exponential scaling, in contrast to the slower acoustic decay observed outside the duct. Modal decomposition is then applied on the in-duct pressures field at three axial planes upstream of the propeller plane to filter out the acoustic pressure components. The results indicate that the total in-duct pressure is dominated by the hydrodynamic contribution, providing new physical insight into the composition of shrouded propeller in-duct pressure and highlighting the importance of distinguishing acoustic and non-acoustic contributions when interpreting near-field measurements. Beyond investigating the in-duct pressure field, a proposed modal filtering procedure provides a practical means of extracting the acoustic modal content generated by the propeller, which can subsequently be used as a representative source input for analytical ducted propeller radiation models.