This study describes a design exploration of Urban Air Mobility (UAM) vehicles, based on top-level aircraft requirements. The exploration focuses on a fully electric vertical takeoff and landing (eVTOL) vehicle that employs an architecture of conventional airframe coupled with tilted rotors, aimed to carry four passengers. Using a low-fidelity design framework, a variety of configurations are investigated by altering design variables such as wing area, number of propellers, operating speed, and range. The influence of these variables on the design is explored from the environmental, societal and, economical perspectives for a 2050 time horizon. The findings suggest that configurations with a small wing area and a large number of small propellers emerges as preferable for minimizing energy consumption (per pax-km) and operating expenses (per pax-km). However, in terms of noise emissions, configurations with fewer but larger propellers are favoured, marking a departure from the design choices that prioritize energy efficiency and cost. Additionally, the study underscores that operations prioritizing commercial viability require high-speed cruising and reduced flight hours, diverging from those that prioritize energy efficiency, thereby emphasizing the necessity of multidisciplinary optimization. Finally, a noise-estimation model is developed to enable quick assessment of the vehicle's sound power level. The model necessitates only fundamental powertrain information as inputs and provides insights into the impact of design choices on noise emission, which is beneficial at preliminary design stage.@en

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.

@en

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.

@en

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

Ducted rotors are configurations known to outperform their unducted reference baselines when aerodynamic performance is concerned. Aside from aerodynamic benefits in hover, a duct also affects acoustic emissions. One of the most contended design parameters of a duct-rotor assembly is the radial distance between the blade tip and the duct wall, referred to as the “tip gap”. The present study explains how the aerodynamic performance of a ducted-rotor system is affected by the tip-gap distance, taking into account the performance of the rotor and those of the duct's inlet lip and diffuser sections. Separate thrust measurements of the rotor and duct establish that the latter can generate up to half of the total thrust of the assembly. Static wall-pressure measurements along the inner wall of the duct reveal a low pressure suction zone over the duct's inlet lip area. This allows the assembly to generate more thrust than the rotor alone, even though the duct's diffuser section generates a drag component (negative thrust). From the velocity fields it is further shown that the performance-deterioration with an increasing tip gap distance is associated with a contraction of the rotor slipstream in the duct diffuser.

@en

A physical analysis has been conducted to assess the effects of turbulence distortion on leading-edge noise generation and low-fidelity modeling in the framework of Amiet's theory. This model retrieves the power spectral density (PSD) of far-field noise using as input the upwash velocity spectrum of incoming turbulent flow and an aeroacoustic transfer function modeling the aerodynamic and acoustic response of the airfoil to the perturbation. The study has been carried out by investigating the interaction of grid-generated turbulence with a NACA 0012 and a NACA 0012-103, featuring the same thickness but different leading-edge shape. The alteration of the velocity field has been shown to occur in agreement with the analytical findings of the rapid distortion theory (RDT), identifying in particular an exponential decay for the upwash-velocity component spectrum at high frequencies. The same decay is observed for the surface-pressure spectra close to the leading edge, suggesting that sound is generated by a pressure distribution induced by the altered velocity field and hence proving that turbulence-distortion effects should be included to enhance the noise-prediction accuracy of low-fidelity models. This has been further demonstrated by using as input in Amiet's model the altered upwash velocity spectrum sampled in the immediate vicinity of the leading edge: an improved prediction of the high-frequency decay is yielded, but the evident overestimation of the noise levels with respect to the results provided by the Ffowcs-Williams and Hawkings' (FWH) analogy indicates the necessity of correcting the aeroacoustic transfer function as well.@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).

@en

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

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 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

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.

@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

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

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

Permeable materials are a promising trailing edge noise reduction technique. The noise reduction is a result of the unsteady interaction between the two communicating boundary layers, in a process referred to as the pressure release mechanism. However, in practice the aeroacoustic performance of permeable trailing edges degrades under lifting conditions, i.e. with a pressure and velocity differential. This study aims at investigating such flow physics using the Lattice Boltzmann Method through 3DS Power FLOW. A numerical setup was created to explore the impact of velocity and pressure differentials between two communicating boundary layers and relate them to the aeroacoustic performance of porous media. The proposed numerical setup consists of two vertically stacked temporally developing channel flows separated by a porous medium (6δ × (4δ + t) × 2 δ), where δ and t are the half-channel height and the porous medium thickness respectively. The two channel flows communicate through fully resolved porous media, here, 75% porous triply periodic minimal surfaces. A large drag increase is observed for all geometries. An increase in anti-correlation between the pressure fluctuations between the channels is found to be related to a drag increase. It was concluded that the spanwise coherent turbulent structures drive the increase in drag. These structures are also affected by the geometry of the porous medium at the surface of the grazing flows. The presence of large coherent turbulent structures leads to a shift in turbulent energy scales. This is related to the modification of the wall pressure spectrum, where it was observed that less energy is present at low frequencies, whereas a peak was observed at a higher frequency. The crossover frequency is between 150Hz and 600Hz.

@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.

@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.

@en

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

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.

@en