Sound propagation in closed test section wind tunnels suffers from reflections and diffraction, which compromise acoustic measurements. In this article, it is proved possible to improve the post-processing of phased-array microphone measurements by using an approach based on the combination of numerical acoustic simulations and beamforming. A Finite Element Method solver for the Helmholtz equation is used to model the acoustic response of the experimental facility. The simulations are compared with acoustic experiments performed at TU Delft's Low Turbulence Tunnel, using both fully reflective (baseline) and lined test sections. The solver accurately predicts the acoustic propagation from a monopole sound source at the centre of the test section to the microphones in the phased-array, for frequencies in the range 500Hz<f<2000Hz. It is shown that a (lower fidelity) geometric modelling method is unable to precisely predict the acoustic response of the Low Turbulence Tunnel at these frequencies, due to strong acoustic diffraction. The numerical results are used to implement corrections to the post-processing of experimental data. A corrected version of the Source Power Integration method is able to increase the accuracy of the source's noise levels calculation, based on a single numerical simulation with the source at the same location as in the experiment. A Green's function correction increases the beamforming resolution and the source's noise levels estimation accuracy from the beamforming maps, without a priori knowledge of the source's location. Both corrections perform well at processing flow-on acoustic measurements, and the Green's function correction shows an additional benefit. The improvement in beamforming spatial resolution leads to an increase of the signal to noise ratio.

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This paper presents an experimental investigation into the aeroacoustic and aerodynamic impact of various flow-permeable fairings having different levels of airflow resistivity, including wire meshes, perforated plates, and 3D-printed materials based on the repetition of diamond-lattice unit cells. The fairings are installed upstream of a scaled LAGOON landing gear model, which incorporates a torque link and brake-like protuberances to replicate realistic noise sources. Acoustic-imaging measurements carried out on the baseline model reveal that these additional components contribute significantly to far-field acoustic radiation, altering both the location and strength of dominant noise sources. The flow-permeable fairings decrease the model loading and turbulence kinetic energy in its wake compared to a fully solid configuration due to less abrupt flow deflection, with a positive impact on undesired noise possibly arising from interactions with downstream, uncovered gear components. Furthermore, fairings characterized by high airflow resistivity offer comparable or superior sound reductions to the solid fairing within a frequency range where the self-noise produced by the airflow through material pores does not dominate. Beyond generating an extensive dataset to support the validation of numerical simulations, this study provides valuable insight into the development of innovative and more efficient passive sound-control solutions for landing gear systems.

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This study investigates the acoustic characteristics of a distributed propeller-wing system in a tractor configuration, focusing on the potential for noise reduction using the synchrophasing technique. The experimental setup features three propellers with nacelles mounted side-by-side on the leading edge of a wing, resulting in aerodynamic interference between adjacent propellers due to a tip clearance of 5% of the radius. The study investigates the acoustic impact of varying relative blade-phase angles (synchrophasing) and further explores the implications of changing the angle of attack. The findings show that synchrophasing influences noise emissions at the locations investigated, potentially amplifying or reducing noise relative to a random configuration. Moreover, these effects persist when investigating higher angles of attack. @en

The present study focuses on the application of finlet rails as a passive technique of flow control to mitigate trailing-edge noise. Finlet rails are small cylinders whose axes are aligned along the streamwise direction, transversally positioned with respect to the trailing edge. In the first part of this study, the effects of finlet geometry on the aeroacoustic emission of a NACA 63₃−018 airfoil are investigated using an array of microphones. It is observed that reducing the transversal spacing of finlet rails leads to increasing the maximum noise reduction, found to be of 4 decibels at relatively low frequencies. An optimum for the height of the finlets was determined, equivalent to 1.6δ^∗, where δ^∗ is the displacement thickness of the boundary layer. With the aim of unveiling the underlying physical mechanism for finlet rails, PIV at high spatial resolution is applied around the surface treatment. It is found that the turbulence energy is lifted-up and moved away from the scattering edge, which attenuates the wall-pressure fluctuations. The observed attenuation of the wall-pressure fluctuations occurs at the energy-containing scales, which is an important difference with finlet fences. In the region underneath the finlet rails, the transversal size of the energetic structures diminishes when the surface treatment is applied. The combination of the lift-up of the turbulence structures, that reduces the wall-pressure fluctuations, with the smaller turbulence scales is responsible for the noise reduction observed for finlet rails.

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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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Two passive add-on porous fairings, comprised of diamond grids varying in size, are numerically and experimentally investigated for their effectiveness in mitigating landing gear noise. The baseline landing gear, a modified version of the LAGOON landing gear with its inner rim cavities closed, along with two configurations equipped with diamond-lattice fairings, are numerically simulated using the Improved Delayed Detached-Eddy Simulation (IDDES) in combination with the Ffowcs Williams and Hawkings (FW-H) analogy. Instead of resolving the detailed flow features through the diamond-lattice fairings, a numerical model is employed to represent the effect of fairing. Prior to integrating the numerical model into the simulations of landing gears, rigorous validation of the model against experimental data in a channel flow is performed. Subsequently, the predicted flow fields and far-field noise spectra of the baseline and controlled landing gears are validated against the experiments conducted in the anechoic A-Tunnel at Delft University of Technology. The results indicate that implementing a diamond-lattice fairing upstream of the landing gear can effectively diminish far-field noise in the frequency range exceeding 200 Hz. For the baseline landing gear, the torque link and brakes are potent noise sources. For the controlled landing gears, both diamond-lattice fairings mitigate the pressure fluctuations on the torque link and brakes, leading to a reduction of surface noise sources. The noise directivity shows that the DL 4.5mm fairing produces a noise reduction of 2-6 dB whereas the DL 2.5mm fairing generates a noise reduction of 3-7 dB across all radial directions. These findings pave the way for the low-noise design of aircraft landing gears.@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 work describes a multidisciplinary framework proposed for the preliminary design assessment of urban air mobility (UAM) vehicles, which includes estimations of vehicle per- formance and acoustic emissions. Established analytical and semi-empirical methods for aircraft design and performance are combined with estimations of tonal and broadband noise components from the multi-propeller system. The estimation of the tonal components is based on steady blade loading and thickness noise, while the estimation of the broadband compo- nent considers trailing-edge scattering of each isolated propeller. Two design architectures, specifically multicopters and tilted open rotors, are assessed in this investigation. The estimated performance and noise emissions are demonstrated and compared against the available data of existing vehicles. Subsequently, a design exploration is conducted, where key performance parameters are varied and their impact on the vehicle performance, estimated cost, power consumption, and noise emissions are evaluated. The results stress the suitability of each configuration for urban air mobility, along with the operating conditions under which each architecture performs optimally. Multicopter vehicles are lighter for small ranges and speeds and, therefore, are the optimal choice for short distances. The lower MTOM also yields lower noise emissions in takeoff and landing, facilitating integration in populated urban environments. Tiltrotors have greater efficiency at high cruise speeds and, at larger ranges, demonstrate more efficient operations and, consequently, lower noise emissions, more suitable for inter-city and large commuting distances.@en

The distortion of turbulence interacting with thick airfoils is analyzed with scale-resolved numerical simulations to elucidate its impact on leading-edge-noise generation and prediction. The effect of the leading-edge geometry is investigated by considering two airfoils with different leading-edge radii subjected to grid-generated turbulence. The velocity field is shown to be altered near the stagnation point, in a region whose extension does not depend on the leading-edge radius. Here, the deformation of large-scale turbulence causes the amplitude of the upwash velocity fluctuations to increase in the low-frequency range of the spectrum because of the blockage exerted by the surface. Conversely, the distortion of small-scale structures leads to an exponential decay of the spectrum at high frequencies due to the alteration of the vorticity field. The prevalence of a distortion mechanism over the other is found to depend on the size of the turbulent structures with respect to the curvilinear length from the stagnation point to the location where surface-pressure fluctuations and pressure gradient peak. This occurs at the curvilinear abscissa where the curvature changes the most. The same high-frequency exponential-decay slope observed for the upwash velocity is retrieved for surface-pressure spectra in the leading-edge region, suggesting that the airfoil unsteady response is induced by the distorted velocity field. This physical mechanism can be accounted for in Amiet's model by using a distorted turbulence spectrum as input and accounting for the increased amplitude of the distorted gust in the aeroacoustic transfer function, retrieving an accurate noise prediction for both airfoils.

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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 paper elucidates the link between the near-wake development of a circular cylinder coated with a porous material in a low Mach-number flow and the related aerodynamic sound attenuation. It accomplishes this by formulating the cylinder flow-induced noise as a diffraction problem. The necessity for such an approach is driven by experimental evidence obtained through acoustic beamforming and particle-image-velocimetry measurements, which reveal that the dominant noise sources for a coated cylinder are not localised on the body surface but rather in the wake, specifically at the outbreak position of the shedding instability. The acoustic field at the vortex-shedding frequency can be hence modelled by considering a compact lateral quadrupole at this location and employing an exact Green's function tailored to a cylindrical geometry. Because of the diffraction of the sound waves radiated by the quadrupolar source on the cylinder surface, the resulting far-field directivity pattern resembles that of a dipole. The study demonstrates that the porous coating has the two-fold effect of decreasing the strength of the point quadrupole in the wake and moving its origin further downstream, reducing, in turn, the efficiency of the sound scattering. Consequently, the diffracted part of the acoustic field, which dominates the far-field noise for a bare cylinder in accordance with classical theory, provides a contribution that is comparable to the direct part. The results eventually indicate that quadrupolar sources must be considered to accurately predict the noise radiated from a porous-coated cylinder, even at low Mach numbers.

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The noise emission of a simplified two-wheel nose landing gear configuration featuring a detachable porous fairing is investigated by a hybrid CFD/CAA approach. The noise mitigation properties of the porous fairing are discussed and compared against two reference configurations, i.e., baseline configurations with and without a solid fairing. The time resolved flow and acoustic near field is computed by a lattice Boltzmann (LB) method with a collision step based on countable cumulants, and the noise radiation into the far field is predicted by solving a permeable surface Ffowcs Williams and Hawkings formulation. The porous material is represented by an equivalent forcing term in the LB equation based on a Forchheimer-extended Darcy model. The porous material has a deterministic geometric structure such that the modeling parameters describing the porous material properties are determined by results from a flow simulation through the resolved micro-structures in a periodic pressure drop setup. The effect of the different fairings on the flow field and the resulting acoustic far field pressure are discussed.@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

A solid fairing and a wire-mesh fairing consisting of very fine wires and pores are numerically and experimentally investigated for the mitigation of landing gear noise. A slightly modified LAGOON landing gear and two configurations, one equipped with a solid fairing and the other with a wire-mesh fairing, are numerically simulated using the Improved Delayed Detached-Eddy Simulation (IDDES) in combination with the Ffowcs Williams and Hawkings (FW-H) analogy. Instead of resolving the detailed flow features through the wire mesh, a recently proposed numerical model is used to represent the effect of the wire-mesh fairing. The simulated flow fields and the far-field noise spectra are validated against the experiments conducted in an anechoic wind tunnel. The superiority of the recently proposed wire-mesh model over a classical wire-mesh model in modelling both the aerodynamic and aeroacoustic effects of the wire mesh is demonstrated. Results also show that the dense wire-mesh fairing functions very similarly to the solid fairing and that significant noise can be reduced through the installation of a solid fairing or a wire-mesh fairing upstream of the landing gears. For the baseline landing gear, the torque link and the brakes are identified noise sources. With the aerodynamic penalty of a 50% increase in drag, both fairings mitigate the pressure fluctuation on the torque link and brakes, resulting in the reduction of surface noise sources. The noise directivity shows that a solid fairing or a dense wire-mesh fairing contributes to a noise reduction of 4-6 dB in all radial directions. The findings in this study pave the way for the low-noise design of aircraft landing gears.

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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

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

This article presents an experimental investigation into the use of serrations to reduce propeller trailing edge noise. The study aims to demonstrate the potential of serrated drone blades in reducing tonal noise components while concurrently influencing the broadband noise components in the acoustic spectrum. A series of propellers was designed, manufactured, and tested to establish a relationship between serration geometry and noise mitigation. Subsequently, an aerodynamic and aeroacoustic characterization was performed using load cells, microphone arrays, and Particle Image Velocimetry measurements in an anechoic wind tunnel facility. The results suggest that a proper design of serration geometry can lead to a significant reduction in both tonal and broadband noise components, with the main drawback being a loss in thrust coefficient. Additionally, serrated trailing edge propellers promote enhanced diffusion of peak vorticity in the tip-vortex region, potentially reducing the noise produced by the interaction of the propeller tip-vortex and the surrounding drone structures. Finally, the breakdown of the fluid dynamic correlation length is observed, leading to the reduction of broadband noise.@en

The attachment of porous media to a blunt trailing edge (TE) can significantly suppress vortex shedding processes and the related tonal noise, yet the near-wall and internal flow fields of porous media are difficult to analyze experimentally and rely on numerical simulations to elucidate the internal flow features. A structured porous trailing edge (SPTE) has been recently designed that follows a methodology of a structured porous coated cylinder. The SPTE acoustic response was compared against randomized porous media with 10 and 30 pores/in. in an anechoic wind tunnel over a range of flow velocities. Acoustic beamforming revealed that the dominant acoustic sources were at the end of the solid plate, even when a porous TE was attached. A region of integration was used to extract acoustic spectra without additional noise sources, revealing that the SPTE possesses superior noise reduction capability. Dipolar directivity patterns were observed at the vortex shedding frequency for each TE, and the coherence between microphones revealed the complex acoustic propagation of the high-frequency content. A wavelet analysis revealed how the SPTE breaks periodic vortex shedding cycles into smaller cycles over a wider frequency range, leading to an overall noise reduction relative to the other TEs.

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This study analyzes the alteration undergone by turbulent eddies as they approach a propeller operating at low Reynolds number, with the purpose of investigating the resulting effects on the noise emitted by the propeller. The two mechanisms affecting turbulence distortion, the streamtube contraction and the interaction between the turbulent structures and the blade, have been investigated experimentally. The set-up consists of a propeller with a diameter of 30 cm operating at a 75% chord-based Reynolds number of 10.8 × 104 interacting with the turbulence produced by a rectangular grid. The flow behavior has been studied by particle image velocimetry (PIV) and hot-wire anemometry (HWA), while a microphone arc was installed for the acoustic analysis. The results reveal that the interaction between incoming turbulence and the propeller plays a dominant role in the alteration of turbulence with respect to the streamtube contraction. This is due to the relatively low contraction ratio of the propeller at this regime, equal, in this case, to C.R.=1.3. Turbulence characteristics are used as input for two different analytical noise-prediction models, both based on Amiet’s theory for turbulence-impingement noise. The first implements the original formulation of Amiet for propeller noise, which requires a position along the blade to be specified to define all the inputs. The second has been developed in the present work to account for the variations of the blade geometry and turbulence conditions in the radial direction. The comparison between the noise predictions and the experimental measurements shows that a better agreement can be obtained with the second model. This reveals that noise generation is strongly dependent on the variation of the flow conditions and propeller geometry along the radial direction, confirming that the description of these characteristics can enhance the accuracy of low-fidelity noise-prediction methods.

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