When applied to aerofoils with non-negligible thickness, Amiet’s theory for turbulence-interaction noise prediction does not account for the alterations in the velocity field and acoustic response induced by the surface, resulting in an overestimation of the radiated noise. This study proposes a semi-analytical method that models turbulence distortion in the immediate vicinity of the surface starting from upstream flow conditions and considers the resulting effects on the acoustic response of the aerofoil. The distorted spectrum of the upwash velocity component is calculated using the asymptotic results of the rapid distortion theory (RDT) for very large- and small-scale turbulence, overcoming the need to define a representative location where turbulence characteristics are sampled. This distorted spectrum is characterised by an increased energy content that is encompassed in the model by scaling the analytical flat-plate formulation of the aeroacoustic transfer function. The proposed approach relies on defining the aerofoil geometrical feature that affects distortion mechanisms, required to extend the RDT results to such geometries. This parameter is identified as the path travelled by the turbulent eddies from the stagnation point to the position of maximum surface-pressure fluctuations, which is, in turn, related to flow acceleration and leading-edge sharpness. The accuracy of this methodology in enhancing noise prediction is demonstrated using numerical and experimental data of grid-generated turbulence interacting with different aerofoils.

This study focuses on the analysis of the ground effect in counter-rotating coaxial rotors. To investigate the aerodynamic performance of a coaxial rotor system, the aerodynamic loading is measured for different rotor vertical spacing, rotational speed, and height above the ground. To link aerodynamic loading with flow topology, velocity fields in the rotor slipstreams are measured with particle image velocimetry (PIV). A semi-empirical model is additionally proposed to complement existing ground-effect theories from the literature by accounting for the effects of rotor spacing, ground proximity, and rotor-to-rotor aerodynamic interactions. The results of the performance analysis show that the ground effect is more pronounced in coaxial configurations than in single rotors, especially at minimum spacing and height, where the thrust increases about twice the corresponding value of single rotors. The analysis of the PIV velocity fields reveals how the inflow to the bottom rotor accelerates the downstream flow, increasing the flow rate and further reducing the induced velocity near the ground. As the rotor spacing increases, these interactions weaken, causing the aerodynamic loading to converge to that of a single rotor at a spacing around 90 % of the rotor radius. The proposed model inspired by experimental data provides a robust framework for predicting coaxial rotor performance near the ground. It also allows integration of the ground effect model into UAV control strategies for improved flight stability and safety.

Porous media are a promising technology to reduce turbulent boundary layer trailing edge noise. However, the fact that the porous material is grazed by turbulent flow on both sides makes its characterization not trivial. This paper describes the modifications resulting from the interaction between the grazing flows through the porous medium, defined as communication. To this end, lattice-Boltzmann simulations of two communicating turbulent channel flows separated by a fully resolved porous medium are carried out. The porous medium is realized as a 75% porous triply periodic minimal surface of type Schwarz’ P. Results are compared against the case with porous medium backed by a solid wall and the smooth wall channel flow. When communication between the two channel flows is allowed, spanwise coherent structures appear that are assimilated to a shear instability at a non-dimensional frequency of. Instantaneous flow through the porous medium is observed and is driven by a time-dependent pressure differential between the channels (with a zero mean and 7.8 Pa standard deviation). This leads to a decrease in energy in turbulent scales smaller than 2.5δ and for bulk scaled frequencies greater than. These flow modifications are not observed in the non-communicating case, with the wall preventing flow through, where the topology of the fluctuating statistics is similar to the smooth wall case. Finally, the drag is found to increase by over 200% with respect to the non-communicating case and 650% with respect to a smooth turbulent channel flow. The drag increase is found to be driven by the velocity fluctuations impinging on the porous topology. The communication does not follow the asymptotic drag relation for the same equivalent roughness, thus entering a different drag regime.

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.

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.

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.

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.

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.

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.

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.

An experimental investigation is carried out to characterize the physical mechanisms by which a trailing-edge crack, idealized as a rectangular cavity to represent delamination damage, affects boundary layer development, coherent vortex shedding, and far-field noise of a National Advisory Committee for Aeronautics 0018 airfoil. Both clean and turbulent inflow conditions are considered to isolate the role of inflow disturbance in modifying these mechanisms. The primary objective is to gain insight into how a geometrical discontinuity at the trailing edge alters the coupled aerodynamic and aeroacoustic behavior. Far-field acoustic measurements and near-wake velocity field data are obtained in the anechoic wind tunnel at Delft University of Technology. Acoustic data from a phased microphone array (from prior work) are combined with new velocity field measurements using particle image velocimetry. The results reveal that increasing crack size leads to enhanced near-wall velocity gradients and stronger coherent vortex shedding, resulting in higher tonal noise levels, particularly at higher frequencies. Normalized tonal frequencies agree with the empirical prediction model of Brooks, Pope, and Marcolini for blunt trailing-edge noise, affirming the relevance of this model even in the presence of geometric imperfections. Under turbulent inflow, the coherent structure scale diminishes slightly, and the tonal frequency increases in the trailing-edge noise spectrum, indicating that inflow turbulence modifies the vortex shedding dynamics and should be accounted for in predictive models. This study is a first step toward understanding and modeling trailing-edge noise in the presence of structural damage, under varying flow conditions.

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.

This paper studies the dynamic stall characteristics of thick flatback and nonflatback wind turbine airfoils. Two airfoils with a maximum thickness of (Formula presented.) were studied, with trailing edge thicknesses of (Formula presented.) and (Formula presented.), respectively. The static and dynamic experimental measurements were performed in the wind tunnel using surface pressure measurements for clean and tripped airfoils at the Reynolds number of (Formula presented.) and dynamic reduced frequency ranging from (Formula presented.) to (Formula presented.). The effects of the trailing edge gap, roughness, mean angle of attack, and reduced frequency on the dynamic stall characteristics of the airfoils were investigated. The results show that increasing the trailing edge gap delays the onset of dynamic stall. However, the lift loss after the onset of dynamic stall is for the flatback airfoil higher than the sharp trailing edge airfoil. Moreover, the flatback airfoil show higher lift overshoot compared to the sharp trailing edge airfoil in the dynamic stall condition. Increasing the reduced frequency affects the dynamic behavior both airfoils differently.

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.

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.

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 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.

Acoustic spectra of rotor noise yield frequency distributions of energy within pressure time series. However, they are unable to reveal phase relations between different frequency components while these play a role in the fundamental understanding of low-frequency intensity modulation of higher-frequency rotor noise. A methodology to quantify interfrequency modulation is applied to a comprehensive acoustic dataset of a fixed-pitch benchmark rotor, operating at a low Reynolds number and at advance ratios ranging from J = 0 to 0.61. Our findings strengthen earlier observations in case of a hovering rotor, in which the modulation of the high-frequency noise is strongest around an elevation angle of θ = −20° (below the rotor plane). For the nonzero advance ratios, modulation becomes dominant in the sector −45° ≲ θ ≲ 0° and is most pronounced at the highest advance ratio tested (J = 0.61). Intensity modulation of high-frequency noise is primarily the consequence of a far-field observer experiencing a cyclic sweep through the noise directivity pattern of relatively directive trailing-edge noise. This noise component becomes more intense with increasing J and is associated with broadband features of the partially separated flow over the rotor blades.

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.