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, drones demand 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 to 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 the design of aerial vehicles for 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 this reason, 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 technologies available in 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.

Acoustic and phase portrait analysis of leading-edge roughness element on laminar separation bubbles at low Reynolds number flow

“…44 On the other hand, the broad-band noise of an airfoil is generally produced by three primary sources: noise related to the turbulence of the incoming flow (LE noise), the noise produced by the interaction of the turbulent boundary layer over the trailing edge (TE noise), and noise generated by the possible separation bubble and flow (separation noise). [45][46][47][48] Figure 14 is a visual representation of the noise sources expressed. This energy conversion often occurs entirely at sharp edges, such as the trailing edge, and can also be affected by the shear layers' interaction and composition.…”

Quantifying modulation in the acoustic field of a small-scale rotor using bispectral analysis

“…These data contain all noise field characteristics of (small-scale) rotors in hover. 27 Descriptions of the experimental acquisition and data are provided in § II and § III, respectively. Subsequently, § IV describes the methodology for capturing the BPF modulation, resulting in a single parameter, for a given acoustic signal.…”

“…A Reynolds number of Re c75 ≡ c 75 2πω0.75R/ν = 8.0 • 10 4 , based on the blade chord of c ≈ mm at a radial position of 0.75R, indicates that the propeller operates with a reduced efficiency, and predicting the noise can be difficult. 27,48 Hence, the acoustic data can support simulation validation-type of studies. In terms of aerodynamic performance, the rotation rate ensured a thrust of F z = 1.014 kgf (close to our nominal set-point of 1 kgf) with a reaction torque of the motor-rotor assembly of τ = 0.169 Nm.…”

Considerações iniciais sobre a regulamentação de ruído para aeronaves eVTOL

“…O número de Reynolds típico para para o escoamento sobre a hélice de um UA é da ordem de Re c (0, 75R) ∼ 10 4 − 10 5 , caracterizando um regime transicional-turbulento, enquanto que para helicópteros é da ordem de Re c (0, 75R) ∼ 10 6 , descrevendo um regime turbulento. Estas diferenças dificultam o uso de ferramentas semi-empíricas de predição de ruído de rotores convencionais [13,14] e fazem com que os trabalhos sobre ruído de aeronaves eV-TOL sejam atualmente bastante experimentais.…”