Experimental investigation on the application of serrated trailing edge propellers for drone noise reduction

This study investigates a widely researched passive noise control strategy for reducing propeller trailing edge noise. The research aims to demonstrate how serrated drone blades can mitigate broadband noise components while simultaneously reducing tonal noise components. An experimental study involving the design, manufacture, and testing of 23 propellers was performed to establish a relationship between serration geometry and noise mitigation. Acoustic characterization during hovering was carried out at a constant rotational speed of $\Omega =4000$ RPM using near-field microphone measurements. Subsequently, detailed aerodynamic and acoustic investigations were performed, employing load cells, Particle Image Velocimetry, and microphone array measurements in an anechoic wind tunnel. The tests were carried out at a constant rotational speed of $\Omega =5000$ RPM and different advance ratios. The results indicate that by properly tuning the serration geometry, a significant reduction in both tonal and broadband noise components can be achieved, with reductions of 3 and 4 dB respectively. However, this comes with the drawback of a nearly 20% loss in thrust coefficient during advanced flight, as well as a 20% reduction in energy consumption. Broadband noise reduction is attributed to the cancellation of spanwise correlation length, while tonal noise is influenced by the reduced load on the blade and tip vortices intensity. Average and root mean square velocity fields reveal that serrated trailing edges promote the break up of peak vorticity in the tip-vortex region, potentially reducing interaction noise between the tip vortex and surrounding drone structures. Proper Orthogonal Decomposition (POD) analysis of the velocity field shows that serrations reduce trailing edge vorticity and tip vortices by shifting energy from large-scale to small-scale structures.