The growing demand for quiet unmanned aerial vehicles (UAVs) calls for noise prediction tools capable of capturing the complex aerodynamic interactions occurring in rotor-airframe integrations at low computational cost. This paper presents an analytical framework to predict the potential-interaction tonal noise generated by a propeller operating upstream of its supporting strut, a dominant contributor to the acoustic signature of small UAVs. Aerodynamic sources of loading noise from the propeller and strut are modelled using potential-flow solutions, while a hypotrochoidal conformal mapping is employed to represent the inflow distortion induced by struts with arbitrary non-circular cross-sections. The method requires as input the spanwise distribution of steady loads from an isolated propeller, estimated through an unsteady panel solver that offers a favourable trade-off between computational efficiency and simulation fidelity. Analytical predictions are validated against experimental data for struts of varying cross-section diameters and shapes, different blade numbers, and multiple far-field observer locations. The results confirm that the models accurately capture the dominant physics of propeller-strut potential interaction, predicting sound pressure levels within 3 dB for most observer angles and blade-passing-frequency harmonics. The potential of the proposed methodology to support UAV noise optimisation is demonstrated by addressing the sound radiated by propellers with struts featuring spanwise-varying cross-sections.

With the rapid proliferation of unmanned aerial vehicles, understanding the aeroacoustics of drones in operating conditions is essential to mitigate perceived annoyance. Performing these measurements in a large indoor test hall is particularly attractive, as it allows the execution of complex drone maneuvers under controlled atmospheric conditions, with high-precision trajectory tracking provided by motion capture systems. Yet, not being acoustically treated, these facilities present challenging reverberant conditions for acoustic measurements. This research work focuses on investigating a maneuvering quadcopter drone inside an indoor test hall and proposes a methodology based on phased-array techniques to decontaminate the recorded noise from the reverberation effects using a tailored Green's function. The results indicate that the tonal contributions of the noise spectrum are significantly influenced by drone operation and orientation, with distinct changes in the blade pass frequencies linked to the varying speeds of the front and back rotors during different flight phases. By filtering out spurious broadband noise due to sound reflections, the proposed dereverberation methodology facilitates the tracking of these tonal components, which can be more clearly visualized in the noise spectrum. The study eventually highlights the importance of analyzing the drone trajectory when interpreting the corresponding noise radiation.

The Urban Air Mobility market is currently experiencing rapid growth with significanti nvestments directed toward the development of novel aircraft designs aimed at enhancing performance efficiency and reducing climate impact. Nevertheless, the emergence of noise pollution as a result of aircraft flying closer to previously undisturbed populations is now a pressing concern. Tonal noise, in particular, is widely recognized as the most prevalent and disruptive form of noise pollution. This study proposes a hybrid computational methodology that prioritizes the evaluation of installation effects on a single tractor propeller and wing case, with a specific focus on tonal noise. The hybrid methodology consists of simulating the near-field aerodynamics over the different geometries using an unsteady Reynolds-Averaged Navier-Stokes commercial solver to determine the equivalent sources. Then, the calculation of the acoustic scattering and propagation is handled by a commercial numerical acoustic solver based on the Finite Element Method. This low-order methodology allows discriminating between aerodynamic and acoustic installation effects with moderate computational times. The directivity results indicate that both aerodynamic and acoustic installation effects cause moderate changes in sound level. However, the aerodynamic installation has a greater impacton the directivity, particularly above and below the wing. The noise generated by the unsteady flow over the wing in the propeller-wing geometry is comparable to the levels of propeller noise. However, it radiates in directivity additional to that of the blades, thereby changing the overall directivity and sound level significantly.

Sound emissions of an automotive engine cooling system are studied using both single-microphone directivity measurements and a rotating beamforming technique. These measurements provide reference acoustic data on such a system and some new understanding of the effect that the radiator induces on the distribution of sound sources. Indeed, the beamforming results indicate that, above the frequency limit allowed by the Rayleigh criterion, it is possible to localize and quantify the noise sources even through the heat-exchanger core. Moreover, for the investigated operating points along the fan performance curve, the sources are always distributed at the tip of the blades and, in particular, at the leading edge. The present evidence, confirmed by the similar trends of the frequency spectra with and without the heat exchanger, leads to the conclusion that the dominant sound mechanism is the turbulence-interaction noise. Nevertheless, this turbulence is produced within the gap between the fan ring and its casing rather than generated by the radiator core. The latter appears to induce negligible acoustic transmission losses but, more significantly, is found to have a minimal influence on the aerodynamic modification of sound sources for all the analyzed operating conditions.

Aerodynamic noise emitted by low-speed axial fans has been receiving increasing attention in various sectors of high societal impact, such as automotive and HVAC systems. In this framework, turbulence interaction, flow non-uniformities, trailing-edge boundary layer fluctuations and blade-tip leakages are different mechanisms generating aeroacoustic sources on the rotating blades and contributing to the overall emitted sound. An accurate localization of the sound sources on the surface of the blade is instrumental in separating and isolating these contributions and, therefore, in designing novel sound mitigation concepts. The main objective of this paper is to present an inexpensive and efficient way to isolate and quantify the noise generating mechanisms on rotating blades by means of a irregularly shaped microphone array. The technique is based on ROtating Source Identifier (ROSI) and has been implemented and validated at the von Karman Institute for Fluid Dynamics (VKI). Simulated benchmark datasets that refer to rotating point sources emitting white noise have been considered for the validation of the method. The accuracy in the source localization and in the source strength reconstruction has been evaluated for a fixed and a variable angular rate. Moreover, the algorithm implementation has been parallelized with the purpose of reducing its computational time, which represents the main drawback of ROSI. Finally, the developed technique has been applied to measure the noise sources generated by a forward-skewed subsonic axial fan operated at maximum efficiency. In this case, it has been possible to successfully localize and characterize the major noise sources on the blades. Although further investigation will be necessary to gain better insight into the topic, the present work constitutes an important step for a better understanding of the physical phenomena occurring in the noise generation of an axial fan.

An automotive cooling module working at three different operating conditions is investigated using a microphone array method. Experiments on an open rotor and on a full module configurations were conducted in the ALCOVES anechoic chamber of the von Karman Institute for Fluid Dynamics, Belgium. The acoustic data are post-processed using the ROtating Source Identifier algorithm. As first step, the method was applied for the fan-alone configuration at the nominal operating point. The sound maps of one-third octave band are discussed with respect to the minimum resolving frequency emerging by the Rayleigh criterion. Integrated spectra of the whole beamforming maps are used to validate the method whereas acoustic characteristics of the fan are identified by evaluating integrated spectra of each blade separately. Subsequently, the ROSI method was applied to both configurations, demonstrating the acoustic transparency of the automotive heat exchanger. Depending on the adjusted mass flow rate, sound sources are located on the leading or trailing edges of the fan blades. To complement the analysis, sound directivity measurements have been carried out in the anechoic wind tunnel at the University of Sherbrooke, Canada, on an another sample of the same engine cooling module.