Studio fluidodinamico e aeroacustico degli effetti di interazione reciproca rotore-rotore nei droni

Small scale unmanned aerial vehicles (UAVs) using multirotor propulsion systems have received considerable attention for a wide range of military and commercial applications in recent years. In the multirotor configuration, the rotor interaction phenomenon occurs severely because the rotors are located in close proximity to each other. Therefore, the separation distance between the adjacent rotor tips has a strong effect on the wake structures and flow fields, which consequently play an important role in determining the aerodynamic performance and noise level of the multirotor vehicle. The aim of the present study is just to investigate the Rotor-to-Rotor Interaction. Investigation of twin co-rotating rotors interaction effects was performed by load, flow field, and aeroacoustics measurements. Wake interaction effects were evaluated in terms of rotor performance by thrust and torque measurements, flow field dynamic by time-resolved particle image velocimetry (TR-PIV), and acoustic emission using a microphone array. Two co-rotating rotors, each with a diameter 𝐷 = 393.7 𝑚𝑚 and equipped with three blades, were investigated in side-by-side configuration at two mutual distances: 𝑑1 = 1.02𝐷 and 𝑑2 = 1.04𝐷. Four different rotating speeds, i.e., 2620, 3500, 4360 and 5200 RPM, were explored. Using a custom control system, it has been possible to study the effect of phase between the two rotors in the aeroacoustic field. In a side-by-side configuration, thrust and torque undergo a reduction compared to that found for a single propeller configuration. The level of aerodynamic load fluctuations increases as well. Intricate flow patterns characterized by periodic vortical structures are formed in the wake of the rotors in twin configuration at the rotor-to-rotor distance of 1.02D. The interaction of the wakes produces a recirculation region at the external periphery of the shear layers. The prominence of the wake contractions occurs near the rotor disks; however, further downstream, the slipstream promotes the wake interaction. The modal analysis of the radial velocity component using Proper Orthogonal Decomposition (POD) highlights that the interacting wakes are dominated by a wave-like motion pulsating at Harmonics of the Blade Passing Frequency (HBPF) of 1/3. Higher orders of POD modes capture coherent vortical structures including tip vortices pulsating at HBPF = 1. A wavelet analysis applied to the POD time coefficients confirmed the persistency of the wave-like motion pulsating at a frequency equal to that of the rotating speed of the propellers. A variety of modulations across the frequency highlights the dynamical behavior of the large-scale coherent structures over the acquisition time. The aeroacoustic investigation shows that the noise level, in terms of OverAll Sound Pressure Level (OASPL), increases up to 10 dB with respect to that generated by the single propeller. Increasing the distance d between the two rotors, no remarkable reductions in noise, in terms of OASPL, were observed, especially for the two high-speed regimes. Finally, having control over the phase between the two rotors has advantages from the point of view of acoustic emission