A numerical investigation has been conducted to characterize the reduction of noise generated by the interaction of incoming turbulence with a flat plate featuring a porous region downstream of the leading edge (referred to as downstream porosity). This work builds on previous experimental and analytical studies where promising noise reduction was achieved and several physical mechanisms potentially contributing to it were identified. These include phase inversions of pressure jumps potentially linked to secondary vorticity phenomena, destructive interference between noise sources, and the alteration of coherent structures within the boundary layer. The present work aims to investigate these mechanisms in detail, corroborating and extending novel experimental findings, to quantify their relative contributions to noise reduction and correlate them with the flow behavior within the perforation holes. The analysis of the unsteady surface pressure over the porous region reveals a marked, periodic phase opposition with respect to the primary noise source at the flat-plate leading edge, persisting across the entire porous section. This behavior indicates that destructive interference underlies the noise-reduction peaks observed experimentally, providing the first evidence for this mechanism. The analysis of the vorticity field, together with the velocity and surface-pressure spectra, supports the presence of a coherent mechanism over the porous region that is associated with this phase opposition. However, its origin and nature remain to be clearly established.

Contra-rotating propellers (CRP) are widely employed in multi-rotor vehicles due to their aerodynamic efficiency and compact design. In practical operations, the rotational speed of the propellers can fluctuate about its mean value. These fluctuations lead to azimuthal misalignment between the two propellers, commonly referred to as phase offset. This paper investigates the effect of this phase offset on the tonal noise characteristics of a CRP configuration. A semi-analytical framework is presented to predict tonal noise generated due to unsteady loading arising from potential field interactions. The method requires as input the spanwise distribution of steady aerodynamic loads. The proposed framework is validated by comparing noise predictions with experimental data and is subsequently used to investigate the effects of phase offset. The results show that the odd harmonics of the blade passing frequency are more sensitive to variations in phase offset, whereas the even harmonics remain largely unaffected. Furthermore, the overall sound pressure level shows maximum variation with phase offset in the propeller rotational plane. The study also highlights the potential of the phase offset parameter as a means of incorporating uncertainty due to rotational speed fluctuations into the semi-analytical method proposed for tonal noise prediction.

The growing demand for quiet unmanned aerial vehicles (UAVs) calls for noise prediction tools capable of capturing the complex aerodynamic interactions occurring in rotor-airframe integrations at low computational cost. This paper presents an analytical framework to predict the potential-interaction tonal noise generated by a propeller operating upstream of its supporting strut, a dominant contributor to the acoustic signature of small UAVs. Aerodynamic sources of loading noise from the propeller and strut are modelled using potential-flow solutions, while a hypotrochoidal conformal mapping is employed to represent the inflow distortion induced by struts with arbitrary non-circular cross-sections. The method requires as input the spanwise distribution of steady loads from an isolated propeller, estimated through an unsteady panel solver that offers a favourable trade-off between computational efficiency and simulation fidelity. Analytical predictions are validated against experimental data for struts of varying cross-section diameters and shapes, different blade numbers, and multiple far-field observer locations. The results confirm that the models accurately capture the dominant physics of propeller-strut potential interaction, predicting sound pressure levels within 3 dB for most observer angles and blade-passing-frequency harmonics. The potential of the proposed methodology to support UAV noise optimisation is demonstrated by addressing the sound radiated by propellers with struts featuring spanwise-varying cross-sections.

This work presents a high-fidelity aeroacoustic investigation of a distributed-propulsion eVTOL aircraft during representative urban air mobility departure operations. Three flight conditions are analyzed, namely vertical take-off, transition, and cruise in climb. The aerodynamic analysis highlights substantial differences in wake development and rotor–rotor interactions among the investigated configurations. For the particular vehicle configuration, cruise conditions are characterized by the ingestion of coherent wake structures by the tail-mounted propellers, resulting in significant unsteady loading fluctuations. The aeroacoustic analysis, performed using the permeable and solid formulations of the Ffowcs Williams–Hawkings integral equation, shows that the radiated noise is dominated by tonal components, with cruise conditions exhibiting blade passing frequency levels approximately 10 dB higher than the other configurations. Source identification analyses indicate that the dominant acoustic contribution is associated with the unsteady loading noise generated by the tail-mounted propellers due to wake ingestion. Finally, an on-ground footprint analysis based on a complete reference departure trajectory demonstrates compliance with current EASA certification limits, while also highlighting the strong sensitivity of the acoustic footprint to the transition maneuver. The results highlight the importance of rotor wake interactions in cruise conditions for short-aspect- ratio distributed-propulsion eVTOL configurations, where wake ingestion by downstream propellers significantly increases tonal noise levels and directly affects the resulting certification footprint. These findings emphasize the need to account for rotor interaction effects during both preliminary design and acoustic certification assessment.

This study examines the role of turbulence distortion in predicting inflow turbulence (IT) noise generation from large wind turbines via Amiet's theory. Two subsequent distortion mechanisms are investigated: (i) the streamtube expansion in the rotor induction zone and (ii) the interaction with the surface of thick-blade profiles. Large-eddy simulations reveal that the turbulence spectra, which reflect distortion effects, remain largely unaffected by rotor induction within the frequency range relevant for noise generation. As for the other mechanism, the distortion of the turbulence approaching a blade leading edge is modeled with a simplified closed-form solution of Goldstein's rapid distortion theory. This model, based on vorticity deflection, is extended here beyond the high-frequency approximation and integrated into an analytical Amiet-based IT noise tool. Applications to representative test cases show that while distortion effects are minimal for current turbine sizes, they become relevant for future configurations featuring larger rotor sizes and thicker airfoils. The developed model reveals that IT noise levels do not necessarily scale with rotor size but are shaped by spectral changes induced by the blade geometry, operational parameters, and inflow conditions. This model offers a physically consistent, computationally efficient framework for the aeroacoustic assessment of next-generation wind turbine design.

This study investigated the noise emission and thrust performance of a heavy-lift unmanned air vehicle (UAV) with a coaxial propulsion system that operates under differential rotor speeds. The UAV adopted an octo-quad architecture, where each rotor pair consists of two propellers with different blades, allowing independent operation of fore and aft rotors in corotating (CR) and contra-rotating (CTR) configurations. Acoustic emissions and thrust were measured under steady conditions. The study compared the performances of CR and CTR configurations and examined the influence of differential rotor speed on the noise emission of the vehicle under different loads for both configurations. The results indicate that the CTR configuration achieves a maximum load factor 0.28 higher than that of the CR configuration and features lower noise at the same thrust when employing differential rotor speed. For both configurations, the drone's noise was influenced by the aerodynamic characteristics of propellers. Specifically, increasing the fore rotor speed relative to the aft rotor amplifies the noise, whereas increasing the aft rotor speed reduces noise without compromising thrust. Corresponding noise spectra were analyzed across different load factors. The results provide insights that can inform about the optimization of noise emission and performance of UAVs with coaxial propulsion systems.

The present study applies a recently proposed methodology that improves the accuracy of Amiet’s model for thick airfoil geometries using rapid distortion theory. The approach is tested against an extensive experimental database from the University of Southampton, where grid-generated turbulence at various free-stream velocities interacts with NACA airfoils of different thicknesses and leading-edge radii. The methodology requires as input only the characteristics of the incoming turbulence and a single geometric parameter of the airfoil that governs the distortion mechanism, related to the pressure gradient at the leading edge. The objectives of the study are twofold: (i) to evaluate the applicability and robustness of the methodology across all tested configurations, and (ii) to assess the feasibility of estimating the geometric parameter using the panel method XFOIL.

This work investigates the unsteady aerodynamic interaction that arises from the impingement of a propeller slipstream on a wing. To this end, an innovative measuring device for unsteady pressure is deployed, comprising a flexible printed circuit board sleeve embedded with MEMS pressure sensors and microphones. The device performance is validated against conventional measurement techniques. The wing is a benchmarked NACA 63₃018 airfoil-based model, and the propeller is the TUD-XPROP-S. In addition to pressure measurements, oil flow visualizations are performed to elucidate the flow pattern on the wing when the propeller operates at advance ratios of 0.8 and 1.8, and nominal blade pitch angles of 30° and 45°. The measurements reveal the formation of a laminar separation bubble on the portion of the wing not washed by the propeller slipstream. The flow is seen to remain attached on the advancing blade side, at least for the tested angles of attack. The microphone measurements capture the trace of the propeller’s tip vortices over the wing and the deformation of the slipstream over the wing. This work serves a dual purpose. Firstly, presenting an innovative measuring device for unsteady pressure, as the sensor-embedded sleeve requires minimal installation efforts and allows for a comprehensive measurement of the unsteady surface pressure field. Secondly, discussing the complex spatio temporal interaction that is formed from the impingement of a propeller slipstream onto a wing.

Current eVTOL certification regulations rely on measurements of the vehicle under controlled, and mostly steady conditions, whereas actual flight operations often involve transient maneuvers and continuously varying rotor operating states. To assess differences in real unsteady operations, this work focuses on eVTOL rotors experiencing unsteady rotational speeds, which are common in electric motors controlled by automatic controllers. The aeroacoustic behavior of a scaled eVTOL rotor undergoing harmonic rotational velocity changes is experimentally investigated in the anechoic wind tunnel of the Delft University of Technology. This was achieved by imposing a harmonic variation in rotational speed of different frequencies ( fℎ) and amplitudes. It is observed that the differences between steady state and unsteady conditions are governed by the reduced frequency parameter, kℎ, which depends on the harmonic frequency and the dimensionless amplitude, λ. The analysis reveals that harmonic variations in rotational speed result in a significant decrease in the rotor’s mean thrust relative to the equivalent static rotational speed, reaching up to 18%. Tonal noise is found to spread around the BPF harmonics, forming a hump containing multiple peaks with a frequency range proportional to the harmonic amplitude of the rotational speed. At higher harmonics, the broadened humps around adjacent BPF tones overlap, resulting in stronger spectral tonal content. In terms of sound pressure levels, a maximum decrease of 2 dB in the tonal noise occurs when the harmonic frequency varies from 0.5 Hz to 2 Hz. At the highest tested frequency, 5 Hz, the tonal noise increases again, and the thrust no longer follows the rotational speed variation as closely as at lower frequencies. This suggests the onset of divergent unsteady behavior, causing deviations from the quasi-steady trends observed at lower frequencies.

This work investigates the effect of grid-generated turbulence ingestion on noise generation in a propeller operating at a low Reynolds number using high-fidelity, scale-resolved simulations. The numerical setup reproduces experiments carried out at Delft University of Technology, where inflow turbulence is generated by a grid placed within a duct. It is found that, upstream of the propeller, the longitudinal correlation length of the streamwise velocity component increases with respect to the case without the propeller. The opposite happens for the transversal one. The turbulent inflow impinging on the propeller blades does not alter the mean flow characteristics over the propeller blades, e.g., the mean static pressure coefficient. However, it increases the root mean square of the pressure fluctuations up to the turbulent reattachment point of the laminar separation bubble, while leaving the downstream region mostly unaffected. This causes a broadband increase in the radiated noise in the low-to-mid frequency range, as confirmed by applying Amiet’s noise-prediction model with input data sampled near the propeller blades’ leading edge. The far-field noise spectra are characterized not only by an increase in the broadband noise with respect to the clean inflow case, but also by tonal components at multiples of the blade-passing frequency. It is found that these tones are caused by the footprint of the turbulence grid that introduces flow inhomogeneities at the propeller location for this specific configuration. It is recommended, when performing experiments and simulations, to verify if any footprint of the turbulence grid is present, not only by performing single-point measurements but also by measuring the time-averaged flow field before installing the propeller.

This study investigates the aeroacoustic behavior of a low-Reynolds-number propeller in forward flight subjected to large-scale inflow disturbances. The incoming flow is modeled as single-frequency sinusoidal vortical gusts, enabling a systematic assessment of the effects of gust frequency, initial phase, and direction on aerodynamic performance and noise generation. The numerical setup is first validated against experimental data under steady inflow conditions. The results show that the loading fluctuations caused by the incoming gust result in discrete tonal components in the acoustic spectrum at frequencies determined by the combination of the gust frequency and multiples of the rotational frequency. These components arise from a double modulation mechanism, and their amplitude is further shaped by inter-blade interference effects. The phase of the gust with respect to the rotor primarily affects the phase of the blade response, thereby modifying the noise directivity, particularly at low frequencies. When the gust is inclined relative to the mean flow, the interaction becomes more complex, leading to a richer tonal spectrum with high intensity tones extending up to the 10th harmonic of the blade passing frequency. Overall, the results provide a physical interpretation of the coupling between rotating blades and large-scale inflow disturbances, supporting the development of improved models for unsteady tonal noise prediction.

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.

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.

With the rapid proliferation of unmanned aerial vehicles, understanding the aeroacoustics of drones in operating conditions is essential to mitigate perceived annoyance. Performing these measurements in a large indoor test hall is particularly attractive, as it allows the execution of complex drone maneuvers under controlled atmospheric conditions, with high-precision trajectory tracking provided by motion capture systems. Yet, not being acoustically treated, these facilities present challenging reverberant conditions for acoustic measurements. This research work focuses on investigating a maneuvering quadcopter drone inside an indoor test hall and proposes a methodology based on phased-array techniques to decontaminate the recorded noise from the reverberation effects using a tailored Green's function. The results indicate that the tonal contributions of the noise spectrum are significantly influenced by drone operation and orientation, with distinct changes in the blade pass frequencies linked to the varying speeds of the front and back rotors during different flight phases. By filtering out spurious broadband noise due to sound reflections, the proposed dereverberation methodology facilitates the tracking of these tonal components, which can be more clearly visualized in the noise spectrum. The study eventually highlights the importance of analyzing the drone trajectory when interpreting the corresponding noise radiation.

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.

Turbulence-ingestion noise, caused by the interaction between incoming turbulence and rotors, is currently a key area of research in the rapidly expanding field of Urban Air Mobility. This is due to the highly turbulent flows characterizing urban environments, where acoustic optimization is especially critical, and to the complexity and diversity of the physical mechanisms involved in noise generation. This makes the analytical modeling for low-fidelity prediction – favored over computationally expensive numerical simulations in the optimization phase — particularly challenging. This study proposes two key modifications to Amiet’s model aimed at enhancing the assessment of inflow-conditions effects on noise generation and prediction. The first allows strip theory to be incorporated to account for radially-varying inflow while preserving the modeling of blade-blade correlation. The second enables the replacement of the original three-dimensional turbulence input, particularly challenging to measure both experimentally and numerically, with a one-dimensional one. This allows probe measurements to be directly used as input to assess the effects of inflow conditions. Additionally, it paves the way for the extension of turbulence-distortion models from rectilinear motion to rotating systems, potentially enhancing prediction accuracy. The approach is validated against experimental acoustic data obtained for a two-bladed propeller under various operating conditions.

Acoustic annoyance is a keen factor in the social acceptance of novel urban air mobility concepts. Although regulations and certification requirements exist for such operations, they rely on measurements of the vehicle under controlled, and mostly steady, conditions. These conditions differ significantly from real envisioned operations, where turbulence from the urban environment, rapid maneuvers, system automatic control, and gusts can affect the vehicle’s noise emissions. To assess such differences, this work focuses on the study of rotors, commonly applied to urban air mobility and transport vehicles, under varying rotational speeds. An experimental campaign is carried out in the anechoic wind tunnel of the Delft University of Technology, where an unsteady rotational speed of the rotor is prescribed. Acoustic measurements are carried out along with the integral loads of the rotor. The work explores both the aerodynamic effects of such an operation and its impact on noise emissions. The final goal is to create a global picture of the relevance and physics of rotor noise under non-steady rotational speeds.

This paper presents a methodology to predict the tonal noise radiated by a propeller-strut configuration by modeling the potential inflow distortion induced by the strut. The approach combines a theoretical description of the potential flow around a circular cylinder with the force distribution and induced velocity computed using an unsteady panel method. The analytical results show satisfactory agreement with measurements from previous studies, demonstrating the suitability of the proposed methodology as a fast and effective prediction tool.

Ingested turbulence affects propeller noise at frequencies above the 2nd Blade Passing Frequency. The extension of the Amiet model to rotating structures is a useful tool to predict this phenomenon. However, comparison with the experimental results reveal discrepancies between predicted and measured acoustic spectra. A likely explanation for this mismatch lies in turbulence distortion.This paper investigates the effects of the propeller-induced flow field on incoming turbulence to obtain a comprehensive description of the flow physics and enhance, in future studies, Amiet’s prediction model.Lattice Boltzmann Very Large Eddy Simulations of a reference propeller operating at low-Reynolds number and subjected to turbulent inflow are performed. The spatial and temporal evolution of isotropic grid-generated turbulence approaching the propeller plane is characterized . The analysis shows that the leading edge interacts with anisotropic turbulence. This is due to the rotational flow induced by the propeller, streamtube contraction, and leading edge distortion.In addition, the effect of turbulence on the laminar separation bubble, conventionally present in flow at low Reynolds number, whose dynamics affects the acoustics at high frequencies, is analyzed. Acoustic spectra, obtained through the Ffowcs-Williams and Hawkings analogy applied to the propeller surface, are then linked to the aerodynamic sources.

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.