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.

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.

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.

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.

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.

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.

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 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.

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 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.

The empirical calibration of remote microphone probes (RMP), used to acquire wall-pressure fluctuations, can introduce spurious resonance into the sensor transfer function due to the difference in the pressure field inside the calibrator geometry over multiple calibration steps. Such spurious resonance subsequently propagates into the unsteady-pressure data at which the calibration is applied, hindering the accuracy of the measurements. Current post-processing methods for tackling these issues are often manual and strongly dependent on the operator's expertise. In this study, we propose an original semi-empirical calibration method to remove spurious resonance in a less operator-reliant manner. The approach is based on fitting an existing analytical fluid-dynamical model for the propagation of pressure waves in the probe to the empirical calibration data using Bayesian inference. The proposed method is successfully applied to three datasets, from a simple probe recessed behind a pinhole to a more complex branching RMP. For all the configurations, spurious resonance is eliminated from the transfer function with a strongly reduced impact of the operator intervention while retaining the resonant features that are characteristic of the RMP. The affected frequency bands are then replaced using the underlying physical model. In this way, the detrimental impact of spurious resonance is removed from the measured wall-pressure spectra. Furthermore, the RMP parameters retrieved by the fit can also be used as inputs to corrective models, specifically to account for averaging effects due to the probe sensing area or for the impact of grazing flow or temperature variations on the transfer function.

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 RANS inaccuracy in predicting flow separation becomes relevant in thick-modern blades, especially once they get dirt or eroded. The literature has tackled the problem by modifying the coefficient a 1 in the k - ω - SST turbulence model without considering its dependency on the adverse pressure gradient condition. This study aims to assess the mentioned dependency by estimating a 1 per blade side and angle of attack. Pressure-coefficient measurements for 18 %, 25 %, and 30 % thick airfoils in clean-inflow conditions at a Reynolds number based on chord of 3 × 106 are considered in the estimation. Hence, a unique variation of a 1 is obtained per each airfoil, justifying the need to adapt a 1 locally within the OpenFOAM code. This modification considerably improves the flow separation prediction with respect to the standard a 1 value and the fb method. As a result, the CL error is reduced by 8 %, whereas a certain CD error remains for all the RANS solutions. Finally, an aeroelastic model of a 4.5 MW wind turbine reveals that the non-corrected RANS solution causes an overshoot of power and loads near rated power, whereas the corrected RANS replicates the shape of the power and loads curves. Consequently, the suggested methodology can be used to search for the required a 1 relation to local flow conditions.

The application of a porous coating to a bluff body, such as a cylinder, is an efficient passive flow control method and has applications in wind, aerospace, and civil engineering. Despite its effectiveness in reducing vortex-shedding processes and the associated noise-generating mechanisms, there exists a trade-off in terms of the generation of higher frequency noise. Such acoustic emissions can be irritating to the human ear and thereby diminish some of the benefits of porous media as a form of passive flow and noise control. To date, explanations of the mechanisms responsible for high-frequency noise generation of a porous coated cylinder (PCC) are qualitative, and there is no clear consensus. In this paper, it is hypothesized that the high-frequency noise is caused by local vortex shedding around the structural members of the pores that comprise the porous media on the cylinder windward side. To investigate this claim, a mathematical model is derived based on the premise that a porous cylinder potential flow model accurately captures the total velocity at the outer diameter of a PCC. The acoustic shedding frequencies are assumed to be related to the Strouhal number relationship, and the acoustic intensity is assumed to be consistent with Curle’s theory, where acoustic intensity scales with the sixth power of flow velocity and the characteristic length squared. A lattice-Boltzmann method simulation of a simplified structured PCC, following a previously published design, is used to test the analytical model, yielding excellent agreement with the mathematical model in terms of acoustic intensity and frequency range.

The aeroacoustic performance of porous materials for sound-control applications depends on the flow communication through the medium. Hence, flow-permeable noise-reduction technologies should be tailored to the flow they operate within. A large-scale simulation setup has been developed in this work to aid the design of porous materials for airframe-noise mitigation by modeling their aerodynamic and acoustic behavior. However, this aerodynamic modeling setup requires validation on a more fundamental flow case. To this purpose, large-eddy simulations of the turbulent boundary-layer flow over two porous materials and a reference solid wall are compared against wind-tunnel measurements. This analysis includes velocity-derived boundary-layer profiles and unsteady wall-pressure measurements on the upper and lower surfaces of the flow-permeable medium. The generated experimental data are additionally made publicly available as a benchmark for boundary-layer flows over a porous wall-insert. The results of the simulation show a satisfactory agreement with the experimental data in most cases, especially for the solid wall. The mean-velocity and turbulence-intensity profiles and the wall-pressure spectra of the boundary layer over the porous materials show a dependence on the streamwise position along the surface, leading to a decrease in wall-pressure energy below a Strouhal number based on the boundary-layer thickness and the outer-flow velocity of 3 and an increase above it. Future research will be aimed at developing a new model for porous media flow centered on the optimization of the flow communication paths within them. This will potentially allow the development of porous materials with favorable acoustic properties while minimizing their aerodynamic penalty.

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.

The analytical model for leading-edge noise prediction formulated by Amiet, developed for a flat plate, relates the far-field acoustic pressure to the upstream inflow conditions, modeled by canonical turbulence spectra. The inaccurate results provided by this low-fidelity method when applied to thick airfoils has been attributed to the distortion experienced by turbulent structures when approaching the airfoil, not modeled in the original formulation of Amiet. The first attempts to account for the effects of this physical mechanism consisted of modifying the term representing the incoming turbulence by means of the analytical results of the rapid distortion theory, obtaining a promising improvement of the noise-prediction accuracy. This paper aims to set up the physical framework to investigate the relation between turbulence distortion and noise-generation mechanisms with the purpose of enhancing inflow-turbulence noise modeling. A numerical database obtained for a rod-airfoil configuration has been chosen to allow the analysis of the vortex dynamics when interacting with a body. The analysis of the velocity field near the leading edge has highlighted that the extension of the region where turbulence distortion occurs depends on the size of the incoming turbulence structures. Furthermore, surface pressure fluctuations have been observed to peak at the same position along the airfoil where the pressure gradient in the streamwise direction is maximum. A novel approach has been proposed to account for turbulence distortion in Amiet's model by using as input the turbulence spectrum directly sampled in this position. A satisfactory agreement with the prediction provided by the solid formulation of the Ffowcs-Williams and Hawkings analogy has been obtained.

The present study assesses the ability to numerically predict turbulence-interaction noise of a NACA0012 airfoil with grid-generated turbulence by utilizing the Lattice Boltzmann solver PowerFLOW. Both the near-field flow characteristics and far field noise are bench-marked against an existing experimental study. The grid was chosen to match that from the experiment to provide evidence that the present numerical approach in physically placing a grid upstream of the airfoil can reproduce the turbulence characteristics observed from the benchmark experiment and thus accurately capture the turbulence-interaction noise generated. The comparison of the results show that the turbulence statistics, including turbulence intensity, integral length scales and anisotropy are highly consistent with the experiment. Moreover, far field acoustics of the turbulence interaction as well as the near-field flow properties near the leading-edge and the unsteady wall pressure fluctuations of the airfoil are also analyzed and the results agreed well with the experimental measurements. The present study confirms that the grid-generated approach is suitable for numerical investigation of turbulence-interaction noise and its potential mitigation strategies.

The sound emitted by the flow past a circular cylinder can be described by a quad rupole placed at the outbreak location of the shedding instability and diffracted surface into the far field by the body with a dipolar directivity. This mechanism is greatly stabilized for a cylinder coated with a porous material, which features a substantial downstream shift of the onset location of the shed vortices that leads, in turn, to a reduction in the efficiency of the sound scattering and consequent noise mitigation. In this research, a novel design for a porous treatment of the cylinder based on the enhancement of this effect is proposed. Far-field acoustics tests were performed at the Delft University of Technology for Reynolds numbers based on the cylinder diameter ranging in the subcritical regime. The outcomes of the analysis demonstrate that, when the aft part of the flow-permeable coating is modified to make the internal flow more streamlined, an additional sound attenuation of up to 10 dB is achieved in comparison with a uniform porous cover. Moreover, a significant noise decrease of up to 10 dB and potential drag reduction are obtained if these components are connected to the bare cylinder without the use of a porous coating. This result can open up interesting opportunities to design disruptive and more optimized sound-mitigation solutions.