The Council of European Aerospace Societies (CEAS) Aeroacoustics Specialists Committee (ASC) supports and promotes the interests of the scientific and industrial aeroacoustics community on a European scale and European aeronautics activities internationally. In this context, “aeroacoustics” encompasses all aerospace acoustics and related areas. Each year the committee highlights some of the research and development projects in Europe. This paper is a report on highlights of aeroacoustics research in Europe in 2023, compiled from information provided to the ASC of the CEAS. In addition, during 2023, a number of research programmes involving aeroacoustics were funded by the European Commission. Some of the highlights from these programmes are also summarized in this article, as well as highlights from other projects funded by national governments and industry. Contributions are gathered in sections by topic, and a section covering relevant European scientific events in 2023 is also included. Enquiries concerning all contributions should be addressed to the authors who are given at the end of each subsection.

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Eduction methods are adopted to characterize acoustic liners. In this paper, several impedance eduction techniques are compared using a numerical database obtained with scale resolved lattice-Boltzmann simulations of a reference acoustic liner in the presence or not of a grazing turbulent flow. Three impedance eduction techniques are compared: an inverse approach based on the Mode-Matching (MM) method, the straightforward method based on the Prony-like Kumaresan-Tufts (KT) algorithm, and one approach based on a minimization problem between reference measurements and the solution of the Pierce’s equation. Furthermore, the educed impedance is compared with the one obtained using local impedance measurements with the Dean’s method with virtual probes located on the entire face-sheet. Results show that impedance values obtained with the Deans’ method are highly dependent on the sampling location and that they vary largely over each cavity. Results from the eduction methods are similar amongst them with few discrepancies found for the method based on the Pierce’s equation. In particular, the highest value of resistance obtained using the Deans’ method is similar to the one obtained using the KT and MM eduction methods.@en

Distributed Electric Propulsion systems are an emerging technology. Aerodynamic interactions between propellers in close proximity can, however, cause periodic variations in the blade loading. Together with acoustic interference, these installation effects can form a dominant noise source in such systems. In this contribution, we investigate a low-cost computational modeling approach to predict the unsteady loading of the propeller blades, and thereby the interaction noise of an array of side-by-side propellers. To inform this low-cost model, a numerical campaign of scale-resolving Lattice Boltzmann/Very Large Eddy Simulations (LBM/VLES) has been performed on the Dutch National Supercomputer Snellius. The goal of this model development is to gain a better understanding of the blade-to-blade interaction mechanisms and to determine to which extent the model can be applied for purposes like preliminary design, uncertainty quantification, or control, for which the computational cost of high-fidelity simulations is prohibitive. As a practical example, the optimal relative phase angle in an array of propellers is determined and validated.@en

Motivated by the potential benefit of Strut-Braced Wing (SBW) configurations in reducing fuel burn, this manuscript investigates the noise generated by the interaction between the propeller slipstream and the SBW. The flow field on a regional transport aircraft is computed in high-lift condition by means of a high-fidelity Lattice Boltzmann solver with very large eddy simulation approach. The acoustic far field is obtained using the Ffowcs-Williams and Hawkings acoustic analogy. The analysis shows that the main aeroacoustic effect due to the interaction between the SBW and the propeller slipstream is the emergency of a tonal noise scattering from the region bounded by the pressure side of the main wing and the suction side of the strut at twice the blade passing frequency.@en

A time-domain inverse aeroacoustic method based on the convective Ffowcs Williams–Hawkings equation is presented. The method allows to determine, in real-time, the unsteady forces exerted on rotating blades in the presence of a moving medium. The inversion procedure is based on a space-time regularization with a mixed l1,2-norm, which guarantees accuracy and smoothness of the solution. The method is initially verified through synthetic acoustic signals emitted by rotating sources in a constant flow, up to a convective Mach number of about 0.88. Then the method is validated through signals generated by a propeller immersed in a wind-tunnel jet flow, up to a Mach number of 0.06. Due to the reduced convective Mach number, the leading aeroacoustic effect is derived from a variation of the blade loading. It is argued that the onset of flow separation at high values of the rotor advance ratio is responsible for the onset of force fluctuations that the inverse method is able to retrieve both qualitatively and quantitatively.@en

This study presents a numerical investigation of the aerodynamic and aeroacoustics behavior of side-by-side rotors in proximity to the ground. The configuration developed in the European H2020 project ENODISE is used as a reference. It features two counter-rotating APC 6x4 propellers, positioned at a height H/R = 2 above the ground, where R is the propeller radius, with a tip-to-tip distance S/R = 2, operating at a rotational speed of 167 Hz. Scale-resolved simulation is carried out using the Lattice-Boltzmann/Very-Large-Eddy-Simulation CFD solver SIMULIA PowerFLOW®. Analysis of the data describes the spatio-temporal of the flow field and describes in detail the bi-stable behavior of wake in the presence of the ground, which is then re-ingested by the rotor disks. This motion, occurring at a frequency two orders of magnitude lower than the rotor blade passing frequency, is similar to what was found experimentally. In addition, the present work shows the impact of this phenomenon on far-field noise by comparing sound pressure levels (SPL) pre- and post-re-ingestion in a semi-spherical manner. The findings reveal a significant increase in SPL across a broad frequency range as the occurrence of the re-ingestion.@en

Correction Notice 1. The vertical axis ticks in the subfigures of Fig 10 should be reverted to align with the global coordinate system of the simulation domain. The updated figure is presented below. Not the one presented in the original manuscript. (Figure Presented).

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This paper aims to investigate, by means of Lattice-Boltzmann simulations, the flow-field and far-field noise of two co-axial co-rotating rotors operating at 3000 rpm in hover conditions. The two co-rotating configurations are made by 2×2-bladed rotors with a fixed axial separation and two different azimuthal separations Δϕ equal to 84∘ and 12∘. Isolated 2- and 4-bladed rotors, are also simulated at the same operating conditions and used as aerodynamic and aeroacoustic reference. For both Δϕ=84∘ and 12∘, the upper rotor tip vortices are accelerated downstream due to the induction from the lower rotor, avoiding blade vortex interaction (BVI). The lower rotor tip vortices convect into the wake with a lower velocity, causing BVI for Δϕ=12∘. The lower rotor shows a reduction of thrust, relative to the upper rotor, of 36% and 66% for Δϕ=84∘ and 12∘, respectively. For Δϕ=12∘, the lower blades act as a wing flap for the upper ones, increasing their thrust. The tonal noise emission for the co-rotating rotors is driven by the interference between the acoustic waves from upper and lower rotors. Because of destructive interference, the configuration Δϕ=84∘ shows a first harmonic up to 15 dB lower than Δϕ=12∘, but still 4.5 dB higher than the isolated 4-bladed rotor.@en

Vertical axis wind turbines (VAWTs) have been identified as a technology that, in association with wake steering, can increase power density of wind farms. In this study, we validate a free wake method for VAWT wake prediction, which leads to satisfactory results. We then use this method to simulate wake steering by means of fixed pitched blades and struts. We demonstrate that combining pitched wakes and struts can lead to very advantageous wake behavior, but only when the interactions between the tip vortices are taken into account. The possibility to inject more high momentum flow into the wake while moving the vortex system away from the next turbine could make pitched blades and struts a powerful tool for future wind farms.

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Commercial aircraft electrification and novel urban air mobility vehicles under development have brought renewed interest in propeller noise to the research community. The complex noise generation mechanisms and broad range of possible configurations lead to the development of a large number of propeller noise test rigs. However, the effects of test rig configuration, installation, and instrumentation on the experimental results are still unclear. In this work, the comparison of the results obtained by two different institutes, the Federal University of Santa Catarina (Brazil) and TU Delft (The Netherlands), is presented.A pair of identical propellers, used for acoustic benchmark studies, were tested under the same conditions to compare the effects of the test rigs on the noise results. A good agreement is observed for thrust and torque. Some divergences on the acoustic data were observed. However, good agreement was noted at the first harmonic and overall sound pressure level, with a maximum difference of 1.8 and 2.8 dB, respectively.@en

This study investigates the aerodynamic and acoustic response of a multi-orifice acoustic liner grazed by a planar acoustic wave and turbulent flow, at centerline Mach number equal to 0.32. High-fidelity flow simulations are carried out using a Lattice-Boltzmann Very-LargeEddy-Simulation solver and the in-situ technique is used to calculate impedance. The triple decomposition technique is adopted to separate the mean-flow effects from those due to grazing tonal acoustic waves with different frequencies and amplitudes. This study highlights the sensitivity of in-situ measurements on the position of the face-sheet probe used to sample the unsteady pressure fluctuations. It is found that the resistance changes up to a factor of three along each cavity. The acoustic-induced velocity field reveals the intricate interaction between the acoustic waves and the turbulent flow. It is shown that the wake shed by the upstream cavity impacts the downstream one, affecting the spatial distribution and the amplitude of the acoustic-induced velocity within the orifice. Furthermore, a vortex within the hole is observed; it is found that its impact on resistance depends on the acoustic wave propagation with respect to the mean flow.

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With distributed propulsion and electric vertical take-off and landing aircraft on the rise, fast and accurate methods to simulate propeller slipstreams and their interaction with aircraft components are needed. In this work, we compare results obtained with a filament-based free wake panel method to experimental and previously validated numerical data. In particular, we study a propeller-wing configuration at zero angle of attack and the aerodynamics of the blade-resolved slipstream interaction with the wing. We use a prescribed wake on the wing and a free wake on the propeller, which greatly accelerate the computations. Results indicate that, while forces are overpredicted due to the inviscid nature of the panel method, the free wake is able to capture the slipstream deformation and shearing with remarkable success. We find that a filament-based free wake panel method can be a useful tool for propeller-wing interaction in preliminary aircraft design.

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Numerical simulations of a wind turbine blade with and without trailing-edge serrations are validated with full-scale field test of a 130 m diameter onshore wind turbine. Simulations focus on trailing-edge noise and are conducted on extruded airfoil sections of the blade using the lattice-Boltzmann method and very large eddy simulations, which are then propagated to the far-field using the Ffowcs Williams-Hawkings approach, simulating the rotation of the sections and the noise of the entire rotor. Far-field noise spectra at two mean wind speeds are used for validation, with the sound power level of the simulations being within 2.5 dB of field test and the total noise reductions attributed to the serrations being captured within 0.6 dB.

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This work focuses on the assessment of the accuracy of numerical prediction and experimental campaigns on providing the noise emissions of an isolated benchmark propeller. An experimental campaign is carried out with a model low-Reynolds propeller of 0.3 m diameter operating at high RPM, equivalent of a tip-Mach number (M ₁) of 0.37 and an advance ratio (J) of 0.4.Measurements are conducted on an open-test section wind tunnel, surrounded by an anechoic chamber. Simulations are carried out with the commercial software PowerFLOW and aim at reproducing the propeller geometry and conditions. BEMT-based noise estimations are also used to demonstrate the expected results. The discussion is focused on the uncertainties of the experimental campaign, and the current accuracy of numerical and analytical predictions, creating a complete picture of the discrepancies expected when predicting propeller noise levels and potential sources of errors. Results point to an accurate ability of the three methodologies to assess the overall noise emissions. Nevertheless, precise description and measurements of the higher harmonics of the tonal emissions and of the broadband noise levels is still lacking and require improvements in experimental conditions and a detailed assessment of the flow over the propeller.

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An inverse acoustic method is presented in this work, which allows to determine the spatial and temporal distribution of unsteady rotating forces from microphone array measurements. The method is based on the usage of a space–time regularization with a mixed norm. The proposed method can take advantage of a prior knowledge of the space–time characteristics of the unsteady rotating forces to ensure an accurate force reconstruction in real-time, using a smaller number of input signals compared to more conventional inverse methods. Different properties of the proposed method are initially investigated by using synthetic acoustic signals radiated from rotating point sources and computed via an acoustic analogy formulation. Finally, the method is validated by using experimental acoustic signals radiated from the rotor of an unmanned aerial vehicle.

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Distributed electric propulsion systems are an emerging technology with the potential of revolutionizing the design and performance of aircraft. When propellers are located in close proximity, they can be subjected to aerodynamic interactions, which affect the far-field noise. In this paper, we study an array of three co-rotating and adjacent propellers to describe both the aerodynamic and acoustic installation effects. A scale-resolving CFD simulation based on the Lattice-Boltzmann/Very-Large-Eddy-Simulation method is used to solve the flow field around the propellers. An acoustic analogy integral approach calculates the far-field noise. Findings show that the helical vortical structures, generated at the tip of each blade undergo a flow deformation at the location of interaction. This causes the loading of each blade to vary during the rotation. Consequently, the unsteady loading noise becomes a dominant noise generation mechanism, driving the noise levels and directivity. It is also shown that introducing a non-zero relative phase angle between the propellers results in a reduction of the unsteady thrust, leading to a mitigation of the unsteady-loading tonal components along the rotation axis. Additionally, the relative phase angle causes constructive/destructive acoustic interference, as demonstrated by analyzing the noise emitted simultaneously by the three propellers.@en

This paper presents low speed fluid structure interaction simulations of a highly flexible wing at various flow conditions, including flutter and excitation from sinusoidal gusts. Such wings are becoming more relevant in recent years, due to their potential for improving aerodynamics and reducing weight, while their flutter characteristics are particularly challenging to address, as the modal properties of the wings change as deflections increase. Calculations are based on time domain coupling of a geometrically exact beam structural model and a 3D free wake panel method, modeling the outer surface of the wing, which allow for nonlinear effects in terms of geometrical deformations and the flow at low computational cost. Static and aeroelastic wing deflections are in line with experimental data of the Pazy wing, which is a benchmark for highly flexible wings from Technion. Two flutter mechanisms are predicted within 1 to 3 m/s of the experimental range. An analysis of the flutter modes is performed, showing that the second torsion mode plays a role in flutter, something that had not been published before. Limit cycle oscillations are achieved and are shown to compare well with reference data, with the frequency being within 1% of the experimental value. Finally, results of gust simulations of the Pazy wing are compared to data from experiments and corrections for the wind tunnel measurements are proposed, which should facilitate future validation efforts. This work serves as a contribution to the Pazy wing dataset and is a step towards mid-fidelity simulations for more complex configurations.

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We investigate the aerodynamics of a surging, heaving, and yawing wind turbine with numerical simulations based on a free-wake panel method. We focus on the UNAFLOW (UNsteady Aerodynamics of FLOating Wind turbines) case: a surging wind turbine which was modeled experimentally and with various numerical methods. Good agreement with experimental data is observed for amplitude and phase of the thrust with surge motion. We achieve numerical results of a wind turbine wake that accurately reproduce experimentally verified effects of surging motion. We then extend our simulations beyond the frequency range of the UNAFLOW experiments and reach results that do not follow a quasi-steady response for surge. Finally, simulations are done with the turbine in yaw and heave motion, and the impact of the wake motion on the blade thrust is examined. Our work seeks to contribute a different method to the pool of results for the UNAFLOW case while extending the analysis to conditions that have not been simulated before and providing insights into nonlinear aerodynamic effects of wind turbine motion.@en

This paper presents a noise propagation approach based on the Gaussian beam tracing (GBT) method that accounts for multiple reflections over three-dimensional terrain topology and atmospheric refraction due to horizontal and vertical variability in wind velocity. A semi-empirical formulation is derived to reduce truncation error in the beam summation for receivers on the terrain surfaces. The reliability of the present GBT approach is assessed with an acoustic solver based on the finite element method (FEM) solutions of the convected wave equation. The predicted wavefields with the two methods are compared for different source-receiver geometries, urban settings, and wind conditions. When the beam summation is performed without the empirical formulation, the maximum difference is more than 40 dB; it drops below 8 dB with the empirical formulation. In the presence of wind, the direct and reflected waves can have different ray paths than those in a quiescent atmosphere, which results in less apparent diffraction patterns. A 17-fold reduction in computation time is achieved compared to the FEM solver. The results suggest that the present GBT acoustic propagation model can be applied to high-frequency noise propagation in urban environments with acceptable accuracy and better computational efficiency than full-wave solutions.

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Efficient calculation of urban air mobility noise footprint in a vertiport environment, considering acoustic effects of various designs, operational conditions, and environmental factors, is essential to limit the noise impact on the community at an early stage. To this purpose, the computationally efficient low-fidelity approach presented by the authors in Fuerkaiti et al. (2022) [11] is extended to calculate the noise footprint of an aircraft in a generic 3D environment. The straight-ray propagator is replaced with a Gaussian beam tracer that accounts for complex source directivity, 3D varying terrain topology, and wind profiles. The reliability of the Gaussian beam tracer has been verified in previous studies by the authors. In this work, it is further extended to include complex source directivity in the presence of a moving medium. Noise sources, obtained using a low-fidelity toolchain, are stored on a sphere surrounding the aircraft and are propagated through an inhomogeneous anisotropic atmosphere. Noise footprints, predicted for different terrain topologies, source directivities, and wind flow conditions, are compared. It is shown that, compared to flat terrain, for the case under investigation, the building blocks increase on-ground noise levels by 5 dB in the illuminated zone due to multiple reflections; they also shield the incoming sound field by creating shadow zones behind the building. The shielding increases with increasing frequency in a quiescent atmosphere. The change between the source directivities, corresponding to the first and second harmonics of the blade passing frequency, results in a difference of up to 40 dB in the noise footprint. The presence of the wind flow can contribute a significant variation in the acoustic footprint by changing the lobes of the footprint pattern and intensifying the noise levels; the variation increases with increasing frequency. Compared to the straight-ray propagator, the present approach reduces the prediction error by 5 dB in the illuminated zones and 35 dB in the terrain shadow zones.

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