Serration manufacturing effects on propeller trailing edge noise mechanisms

Trailing edge noise is an important broadband noise source of hovering UAV rotors, which can be reduced by trailing edge serrations. Ayton’s theoretical model provides predictions for serrated trailing edge noise generated by a fully turbulent flow over an infinitesimally thin plane. This study assesses the validity of these assumptions by considering the effects of serration installation and manufacturing. Several propellers of the same design were 3D printed and tested in an anechoic room, where far-field noise and aerodynamic performances were collected. The baseline configuration exhibits clear evidence of laminar boundary-layer instability noise. Cut-in and add-on serrations alleviate this noise mechanism. Similarly, to overcome the influence of laminar to turbulent transition over the blade surface, some propellers also include additional tripping elements to trigger the turbulence. Cut-in serrations experience additional vortex-shedding noise characterized by a Strouhal number based on the serration root thickness. The results show that serrations are a viable method to control trailing edge noise at low RPM, where broadband noise dominates over tonal noise. Finally, the analytical predictions successfully capture the noise reduction trends for the add-on serrations. In particular, the model can capture their frequency range of effectiveness and the relative variation of the mitigation with rotational speed. There is better agreement between the model and the experiments for the design that includes tripping. This outcome highlights the importance of considering the manufacturing method during the design phase.