Small-Scale Rotor Aeroacoustics for Drone Propulsion: A Review of Noise Sources and Control Strategies

Abstract

In the last decade, the drone market has grown rapidly for both civil and military purposes. Due to their versatility, the demand for drones is constantly increasing, with several industrial players joining the venture to transfer urban mobility to the air. This has exacerbated the problem of noise pollution, mainly due to the relatively lower altitude of these vehicles and the proximity of their routes to extremely densely populated areas. In particular, both the aerodynamic and aeroacoustic optimization of the propulsive system and of its interaction with the airframe are key aspects of unmanned aerial vehicle design that can signify the success or the failure of their mission. The industrial challenge involves finding the best performance in terms of loading, efficiency and weight, and, at the same time, the most silent configuration. For these reasons, research has focused on an initial localization of the noise sources and, on further analysis, of the noise generation mechanism, focusing particularly on directivity and scattering. The aim of the present study is to review the noise source mechanisms and the state-of-the-art control strategies, available in the literature, for its suppression, focusing especially on the fluid-dynamic aspects of low Reynolds numbers of the propulsive system and on the interaction of the propulsive system flow with the airframe.

Microfiber coating for propeller noise reduction

“…LBLVS can also generate broadband noise at a higher frequency range than the discrete frequency peaks observed in tonal noise 25,[27][28][29] . LBLVS noise is associated with laminar flow that is separating from the surface (See Figure 8 (a)) 7,[21][22][23] . The peak of the broadband noise was shown to be around 6 kHz.…”

“…Propeller noise can be reduced by applying both active and passive flow-control technologies. Passive means, such as the use of engineered surfaces, materials, or coatings, are particularly advantageous as they do not require an external power source or mechanical/electrical installation 7 . Serrated leading or trailing edges have been used for noise reduction [8][9][10][11][12] .…”

“…Based on these findings, it is hypothesized that the microfiber-induced flow of these features can play a role in reducing propeller noise. Here, the sources of the propeller noise are schematically described in Figure 1 7,12,[21][22][23] . These are (A) the interaction of the turbulent boundary layer and the trailing edge known as the turbulent boundary layer trailing-edge noise (TBLTE), (B) vortex shedding associated with the laminar boundary layer known as the laminar boundary layer vortex-shedding noise (LBLVS), (C) the interaction of the propeller blade and the blade-tip vortex known as the blade vortex interaction noise (BVI), and (D) the interaction of the propeller blade and the turbulent wake formed by a preceding blade known as the blade wake interaction noise (BWI).…”

Aeroacoustics Effects Generated by the Interaction Rotor-Airframe in a sUAS

“…To date, research in aeroacoustics has predominantly focused on larger rotary-wing and fixed-wing aircraft (Brocklehurst and Barakos 2013;Molino 1982;Roger 2019), with limited attention directed towards the unique characteristics of sUAS. However, the reduced scale and distinctive design of sUAS introduce novel challenges and opportunities in mitigating and managing aeroacoustic effects (Candeloro et al 2022). The compact size, lightweight structures, and increased reliance on electric propulsion systems further underscore the need for targeted investigations into the acoustic signature of sUAS, particularly concerning rotorairframe interactions.…”

On the iInfluence of Inlet Geometry on Turbocharger Compressor Noise.

“…In addition to the aeronautical industry, turbomachinery is present in many applications, such as vacuum cleaners, dryers, or domestic fans, whose annoying acoustic signature is well known worldwide. Recently, drone technology has posed new noise problems of aeroacoustic nature in cities [20]. It is clear, then, that the investigation methods on aeroacoustic sources in turbomachinery, to which this thesis contributes, are of great interest to industry and society today.…”