This study investigates a widely researched passive noise control strategy for reducing propeller trailing edge noise. The research aims to demonstrate how serrated drone blades can mitigate broadband noise components while simultaneously reducing tonal noise components. An experimental study involving the design, manufacture, and testing of 23 propellers was performed to establish a relationship between serration geometry and noise mitigation. Acoustic characterization during hovering was carried out at a constant rotational speed of Ω=4000 RPM using near-field microphone measurements. Subsequently, detailed aerodynamic and acoustic investigations were performed, employing load cells, Particle Image Velocimetry, and microphone array measurements in an anechoic wind tunnel. The tests were carried out at a constant rotational speed of Ω=5000 RPM and different advance ratios. The results indicate that by properly tuning the serration geometry, a significant reduction in both tonal and broadband noise components can be achieved, with reductions of 3 and 4 dB respectively. However, this comes with the drawback of a nearly 20% loss in thrust coefficient during advanced flight, as well as a 20% reduction in energy consumption. Broadband noise reduction is attributed to the cancellation of spanwise correlation length, while tonal noise is influenced by the reduced load on the blade and tip vortices intensity. Average and root mean square velocity fields reveal that serrated trailing edges promote the break up of peak vorticity in the tip-vortex region, potentially reducing interaction noise between the tip vortex and surrounding drone structures. Proper Orthogonal Decomposition (POD) analysis of the velocity field shows that serrations reduce trailing edge vorticity and tip vortices by shifting energy from large-scale to small-scale structures.

The premise of over-the-wing mounted rotors is that a favorable aerodynamic effect is achieved by interaction with the lifting wing, which also acts as noise shield. A physics-based low-order model is proposed that accounts for aerodynamic interactions in the prediction of the aeroacoustic footprint of the installed rotor. The nonuniform inflow of the rotor disk is modeled by an analytical description of the inviscid potential effects of the wing’s circulation, given as a function of the blade sectional coordinates. Furthermore, the ingestion of the separated boundary layer is considered at large angles of attack. The related steady inflow distortion serves as input to an aeroacoustic noise prediction chain that computes the unsteady loading on the blades and the resulting tonal noise emission by helicoidal surface theory. The model is validated by measurements from a single over-the-wing mounted rotor for a wide range of angle of attack, advance ratios, and rotor positions over the wing’s chord. The predictions and experimental data show an equivalent increase in the tonal components relative to the isolated rotor, and a minimization of the tonal noise for a midchord rotor position, for emission directions around the rotor disk plane over the wing’s suction side.

Laminar to turbulent transition induced by spanwise periodic arrays of cylindrical roughness elements over a NACA 0012 airfoil is investigated by hotwire anemometry and infrared thermography. The roughness elements are placed in the flow under adverse pressure gradient. Three configurations are investigated, namely an isolated roughness element, a spanwise array of roughness elements, and a pair of arrays in stagger. The streamwise and spanwise interactions between roughness wakes are addressed, focusing on the evolution of mean flow features and mechanisms for the subsequent process of laminar-turbulent transition. The spanwise interaction between roughness elements involves the connections and merging of neighboring low-speed regions (MLS) in the wake, which affects the spanwise distribution and amplitude of the velocity streaks. The maximum effect on promoting transition is observed when two neighboring low-speed regions overlap with each other in the near wake (within 6 times roughness height). The addition of a second roughness array promotes transition when the spanwise spacing is larger than two times the roughness diameter. Spectral analysis of the streamwise velocity fluctuations reveals that the number of roughness elements within the spanwise array affects the number of MLSs and the dominant instability mechanism. For an odd number of MLSs, the Kelvin–Helmholtz instability dominates the growth of velocity fluctuations around the three-dimensional shear layers. For an even number of MLSs, both Kelvin–Helmholtz and asymmetric instabilities appear in the wake. In this case, the dominant mode that leads to transition depends on the spanwise spacing between roughness elements.

This study examined the effect of vortex generators on the dynamic stall characteristics of thick wind turbine airfoils with a relative thickness of 35% and trailing edge thickness of 10% and 2%. The experiments were conducted in the TU Delft LTT wind tunnel at a Reynolds number of Re=1×10⁶ and dynamic reduced frequency ranging from 0.032 to 0.096. The study investigated the impact of various factors on the dynamic stall characteristics of the airfoils, including the vortex generator's chord position, trailing edge gap, roughness, mean angle of attack, and reduced frequency. The study found that vortex generators delay dynamic stall for thick airfoils by stabilizing the flow during the upstroke phase. However, this can increase the maximum lift overshoot, particularly with flatback airfoils, resulting in a higher drop in lift during dynamic stall. This can potentially increase the dynamic loads on a wind turbine blade due to stall-induced vibrations. The study noted a significant difference in dynamic stall behavior between flatback and non-flatback airfoils. Overall, this research provides valuable insights into the dynamic stall and flow physics characteristics of thick wind turbine airfoils using vortex generators, aiding in more accurate rotor blade design.

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.

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.

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.

Recent studies on distributed electric propulsion systems suggest phase synchronization between rotors as a noise reduction strategy. However, the aerodynamic interactions between propellers' near fields and their influence on far-field tonal noise remain poorly understood, partly due to experimental limitations in microphone placement. This paper addresses this gap through lattice Boltzmann very large eddy simulations of three adjacent, co-rotating rotors, spaced radially at 2% of their diameter, to investigate how relative phase angle affects tonal noise directivity. Results reveal that proximity-induced aerodynamic interactions generate dominant tonal noise in most spatial directions, driven by two mechanisms: time-averaged inflow distortion from nearby propellers and impulsive local effects at blade tips, with the latter influenced by phase angle. While the directivity pattern of the blade-passing frequency harmonic tone remains consistent across phase angles, comparing cases with zero relative phase (blades aligned) and opposite-phase conditions shows sound pressure level shifts of up to 4.5 dB along the primary noise axis, namely, along the inflow direction. Conversely, acoustic interference significantly alters noise directivity, especially in opposite-phase conditions where sound is nearly canceled in specific directions. These findings highlight rotor synchronization as a promising strategy for reducing noise emissions toward sensitive areas.

Amiet's model for turbulence-ingestion noise prediction for rotors is adapted to incorporate pointwise velocity measurements as input. This is accomplished by using an inverse strip theory approach and transforming the three-dimensional turbulence spectrum, which models inflow conditions, into a one-dimensional term. This latter modification enhances the low-fidelity prediction tool in two key ways. First, it enables its application in cases where turbulence modeling is unavailable, or detailed inflow characterization is impractical. In this way, for example, hot-wire anemometry measurements of the incoming turbulence can be used to compute the acoustic prediction. Second, since the conversion of the turbulence term entails introducing two new functions describing spanwise and axial turbulence correlations; this approach establishes a framework for Amiet's theory in which the contributions to turbulence alteration and noise scattering are separated and represented individually. This “modular” structure enables independent analysis and modeling of these contributions, facilitating the application of Amiet's model to complex flow configurations and rotor geometries. The proposed methodology is successfully validated through experimental measurements of a simplified axial-flight turbulence-interaction setup, where a two-bladed propeller interacts with grid-generated turbulence at three different advance ratios.

A semi-analytical model is proposed that incorporates aerodynamic interactions between the rotor-and winginduced flowfields. Predictions are validated through experiments performed with an array of five rotors above an airfoil, where the angle of attack, advance ratio, and chordwise rotor position are varied. At moderate angles of attack, the propulsive thrust is reduced due to the acceleration induced by the wing’s circulation. Around the stall angle of the isolated wing, the rotors re-energize the boundary layer when operated in low-thrust conditions. By increasing the thrust, a pronounced region of reverse flow between the rotors and wing adversely affects the leadingedge separation delay over the wing that occurs for lower thrust settings. However, in this condition, the wing–rotorarray system exhibits increased thrust compared to the attached flow condition due to the rotors ingesting low-momentum flow. In addition, the rotor-induced flow over the wing augments suction, while the pressure side is subjected to a pressure increase, ascribed to flow entrainment from the rotors. After comparison with the experimental observations, it is confirmed that the model predictions accurately describe the lift and thrust performance trends, aside from a discrepancy in the lift force when the rotors are operated in low-thrust conditions.

The electrification of aircraft is strongly coupled with the use of propellers as a propulsion system because of their high efficiency and their convenient integration with electric motors. Due to the operational flexibility of electric motors, the propeller can also be used in alternative operations, such as negative thrust and power mode. By operating the propeller at negative inflow angles at the blade segments, the torque and thrust are in the opposite direction compared to the conventional positive thrust conditions. This can be useful for control purposes or for energy harvesting. An experimental investigation was carried out to explain the physics behind the aerodynamic performance of a propeller at both positive thrust and energy-harvesting operation. Next to the measured integrated forces and moments on the propeller, stereoscopic particle image velocimetry was used to analyze the flowfield around the blades as well as the slipstream behind the propeller disk to identify the dominating flow phenomena that drive the energy-harvesting operation. The highly cambered blade sections for this typical aircraft propeller do not operate efficiently in energy-harvesting mode due to the associated negative angles of attack. The thin tip blade sections experience separated flow in these conditions, reducing the useful output power, compared to wind turbines, which feature opposite camber. To maximize the output power in the energy-harvesting conditions, a low pitch setting is required in combination with a relatively high advance ratio. However, this also comes at a cost of large negative thrust (drag) values.

This paper presents an improved approach for fast numerical modeling of the mutual aerodynamic interactions between a wing and tractor propellers for preliminary design purposes. Vortex methods are used to model the propeller and wing aerodynamic performance. The blade element momentum (BEM) method, which is used to model the propeller performance, is extended to allow for a nonuniform inflow field, such that the upstream effects of the wing can be included in the propeller performance modeling. The circulation distribution over the propeller blades is then used in the slipstream tube model (STM) to determine the time-averaged propeller slipstream velocities. Finally, an improved vortex lattice method (VLM) is used to model the wing’s spanwise lift distribution. The method includes an often overlooked correction for the finite slipstream dimensions experienced by the wing segments, in both vertical extent and spanwise extent. This physics-based correction, based on the image vortex technique, partially offsets the large discrepancies in the lift augmentation found in previous low-order numerical analyses of propeller–wing interactions, while keeping the analysis routine computationally very cheap. The BEM and STM are validated against experimental data, while the VLM and complete propeller–wing system are validated against high-fidelity numerical data, confirming the accuracy of the used models. Discrepancies are found in regions or operating conditions where viscosity becomes more relevant, such as separated flows. The numerical model derived in this paper can be used for a quick and accurate first-order estimation of the aerodynamic performance of new conceptual distributed propeller aircraft.

This study investigates the relationship between sound quality metrics (SQMs) and noise annoyance caused by airborne wind energy systems (AWESs). In a controlled listening experiment, 75 participants rated their annoyance on the International Commission on Biological Effects of Noise (ICBEN) scale in response to recordings from in-field measurements of two fixed-wing and one soft-wing ground-generation AWES. All recordings were normalized to an equivalent A-weighted sound pressure level of 45 dBA. The results revealed that sharpness was the only SQM predicting participants' annoyance. Fixed-wing kites, characterized by sharper and more tonal and narrowband sound profiles, were rated as more annoying than the soft-wing kite, characterized by higher loudness values. In addition, the effect of some SQMs on annoyance depended on participant characteristics, with loudness having a weaker impact on annoyance for participants familiar with AWESs and tonality having a weaker effect on annoyance for older participants. These findings emphasize the importance of considering psychoacoustic factors in the design and operation of AWESs to reduce noise annoyance.

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.

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.

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.

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.

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.

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.

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).