Abstract
The present work deals with the modeling of the aerodynamic sound generated by the propellers of small-size drones, taking into account the effects of horizontal forward flight with negative pitch and of installation on supporting struts. Analytical aeroacoustic formulations are used, dedicated to the loading noise. The fluctuating lift forces on the blades are expanded as circular distributions of acoustic dipoles, the radiated field of which is calculated by using the free-space Green’s function. This provides descriptions of the sound field, valid in the entire space. The stationary mean-flow distortions responsible for the lift fluctuations and at the origin of the sound are estimated from existing numerical flow simulations and from ad hoc models. Installation and forward-flight effects are found to generate much more sound than the steady loading on the blades associated with thrust. Therefore, the models are believed reliable fast-running tools that could be used for preliminary low-noise design through repeated parametric calculations, or for noise-impact estimates corresponding to prescribed urban traffic.
Highlights
Small-to-medium size classes of flying objects or vehicles are likely to become part of everyday life in cities in the near future, relying on distributed electric propulsion
The simple results obtained in this work with a set of realistic parameters indicate that the tonal noise from small-size quadrotors in operation is essentially generated by periodic blade forces due to forward flight and/or installation effects
Steady-loading noise associated with thrust contributes in horizontal directions for normal operating conditions, only at the lowest frequencies
Summary
Small-to-medium size classes of flying objects or vehicles are likely to become part of everyday life in cities in the near future, relying on distributed electric propulsion. Tonal noise is attributed to the steady-state aerodynamics of the blades, on the one hand, and to periodic variations of the aerodynamics of the blades with the period of rotation, on the other hand The latter find their origin in all deviations from pure axisymmetry of the flow through a rotor disk, referred to as stationary distortions. Apart from its generation, rotating-blade noise is scattered by the main body and struts of a drone, which a priori modifies the radiating properties in the environment This is called the acoustic installation effect. In a purely axisymmetric flow, steady-loading noise and thickness noise are the only expected contributions They are intrinsic to the design of the blades and are directly related to thrust. Concluding remarks are made about the use of the models for noise-impact assessment
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