Decomposizione ortogonale corretta e analisi aeroacustica wavelet di un'elica a bordo d'uscita dentato per la propulsione di droni

The aim of the present study is to investigate one of the most effective and most studied passive control technique in the literature in order to mitigate the noise emission of a small drone: the Serrated Trailing Edge (STE). Several quiet propellers have been designed and manufactured in order to identify the most silent configuration and to understand the serrations geometry ef fect on the noise generation mechanism. An aeroacoustic pre-qualification has been performed by means of microphone measurements within an ane choic chamber aimed at characterizing the noise signature of all the designed propellers in the near field and at identifying the most interesting configura tion (i.e.ò propellers characterized by the best and worst behavior in terms of noise emission). Then, having identified the most interesting configurations a second experimental campaign has been carried out by means of load cells, microphones and PIV measurements enclosed by an anechoic wind tunnel facility in order to investigate the mechanism that stands behind the noise mitigation. With this purpose, the aerodynamic and aeroacoustic properties and even the velocity and vorticity field along the blade of STE propellers have been characterized. Particular attention is devoted to the fluid-dynamic aspects related to the low Reynolds number flow regime. Results shows that a sensible noise reduction can be achieved by using STE propeller with the main drawback of a lowering of the thrust coefficient in respect to the base line propeller. The spectral analysis reveals that the noise mitigation involve both the tonal and the broadband noise component. In order to give a physi cal interpretation to the results a statistical analysis has been performed and vi the main results is that the serration induce a damping of the tails of the Prob ability Density Function. Such effect is commonly related to intermittency. Furthermore, with the aim of keeping a deeper knowledge of the physical phenomena advanced analysis technique have been performed. A wavelet based analysis has been performed on the pressure time series. Results show the presence of coherent structure embedded in the pressure time series and serrations seems to induce a sensible mitigation of them. Such effect may be related to the noise mitigation. Finally, a Proper Orthogonal Decomposi tion (POD) on the velocity fields has been carried out to identify modes in space devoted at highlighting the fluid-dynamic serration effect in the wake region. The POD modes show that serration seems to modify the coherence of the wake and the tip vortex intensity, the effect on wake coherency is con firmed by means of a bi-dimensional Fourier transform. Furthermore, the modification of the trailing edge geometry produce a re-organization of the POD modes as if the energy associated with lower order modes has moved to higher order ones. Finally, the spanwise correlation length, which is di rectly related with the broadband noise, was influenced by STE leading to a reduction of the propellers noise signature