Control of flow recirculation for accurate rotor noise measurement

Abstract

As the interest in multirotor UAVs and drones has grown in recent years, numerous studies investigating the performance of these unmanned aerial vehicles have been actively pursued. To improve the aerodynamic performance of drones or reduce the noise emitted from the drone, various studies have been conducted on the rotor, which has the greatest influence on the performance of the drone. To conduct a performance study of the rotor, experiments are typically carried out by installing the rotor in a controlled and constrained environment, aiming to prevent external interference. However, within such a confined setting, the operation of the rotor often leads to the development of rotor wake, resulting in the occurrence of recirculation as the flow is reintroduced back into the rotor plane. This recirculating flow is widely recognized for significantly amplifying the noise emitted from the rotor. With few exceptions involving efforts to validate the onset of recirculation or mitigate its impact, there have been scarce attempts to directly control the influence of recirculation. Therefore, the objective of this investigation is to introduce an innovative mesh grid configuration aimed at mitigating the consequences of recirculation, even post its occurrence. To avert the recirculation of flow while minimizing interference with the rotor's wake, a mesh grid was devised to obstruct all sides except at the precise location traversed by the rotor's wake. Furthermore, a comprehensive case study was conducted, employing variations in both mesh size and the height of the mesh grid as parameters, with the aim of identifying an optimal configuration that most efficiently suppresses recirculation. In order to thoroughly investigate the impact of recirculation on rotor performance, measurements were undertaken to evaluate both aerodynamic and aeroacoustic aspects. To understand how recirculation impacts various aspects of performance, Particle Image Velocimetry (PIV) flow visualization was implemented within the internal space of the chamber. To examine the influence of recirculation on the aeroacoustic performance of the rotor, an initial application of spectrogram identified the onset of recirculation. Subsequently, a noise decomposition technique was applied to quantitatively assess the influence of recirculation on the aeroacoustic performance of the rotor by separating the measured noise data from the microphone into tonal and broadband noise components. It was confirmed that the tonal noise component was amplified by approximately 3.36 dB, while that of the broadband noise component was 0.24 dB and the noise amplified by recirculation was emitted in the direction of 30-40 degrees. The results of PIV flow visualization revealed that the strong downwash was formed in the wake region of the rotor, but up-wash flow was formed near the wall, finally establishing a recirculating flow within the internal space of the chamber. When this flow was re-ingested into the rotor, the vorticity in the rotor inflow region was significantly increased, pointing to the recirculation induces flow instability in the rotor inflow region. To install a mesh grid within the chamber revealed that it can suppress and control the influence of recirculation. The tonal noise component of the case with a fine mesh grid at tip path plane (TPP) of the rotor was amplified by approximately 0.39 dB, while that of a coarse mesh grid at TPP was amplified by approximately 0.65 dB, which were much smaller than that of without mesh case, but the fine mesh grid was more effective to suppress the effect of recirculation. Consequently, it was confirmed that a finer mesh size resulted in less noise amplification compared to the case with a coarser mesh because the finer mesh grid can more effectively impede the upwash flow. Additionally, siting the mesh grid at TPP proves to be the most efficacious method for recirculation suppression regardless of the mesh size. Placing the mesh grid at a lower position permits the blocked flow to redevelop, as it provides spatial room for regeneration. Keywords : Aerodynamics, Aeroacoustics, Recirculation, Recirculation suppression device, Noise decomposition technique, PIV flow visualization