The analysis of a propeller's low-frequency vibration and sound radiation characteristics: Jellyfish mode

  Objectives  We found a new component of propeller noises which is induced by the excitation of the low-frequency elastic mode of the propeller. In particular, the model where all the blades of a propeller vibrate in the same phase is stimulated will result in strong acoustic radiation. We have named the mode shape in which all the blades of a propeller vibrate in the same phase as "Jellyfish Mode". Also, the low-frequency narrow-band sound radiation of the propulsion system induced by the excitation of "Jellyfish Mode" is named by the "Jellyfish Effect". To reveal its mechanism, a study of the low frequency intrinsic modal characteristics of the propeller is carried out.

  Methods  the refined mesh FEM method is utilized to study the dynamic feature of the propeller. The modal characteristics of the propeller both in air and water are obtained and the influence of the structural detuning of the propeller on the modal characteristics is analyzed. Then, the cyclic symmetric structure vibration theory is cited to validate and further study the low-frequency dynamic property of the propeller.

  Results  The research revealed the grouping modal characteristics of the propeller both in air and water. Furthermore, the single frequency mode and repetition frequency mode characteristics in a mode group are also brought out. The research results showed that the structural detuning of the propeller has a great effect on the repetition frequency mode while it has little effect on the single frequency mode. The refined mesh FEM numerical analysis results are consistent with the theoretical results.

  Conclusions  The study revealed the low frequency natural dynamic characteristics of the propeller and the research above can provide theoretical supports for propeller noise analysis and control, which is of great significance in theoretical and practical engineering.