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.

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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.@en

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.

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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.

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

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.

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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.

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Modern wind turbines employ thick airfoils in the outer region of the blade with strong adverse pressure gradients and high sensitivity to flow separation, which can be anticipated by leading-edge roughness. However, Reynolds average Navier-Stokes simulations currently overpredict the Reynolds shear stresses near the surface, and the flow separation is not correctly predicted. Hence, these methods are not representative enough to optimize the blade design to avoid flow separation, which becomes relevant for rough blades. While several eddy-viscosity corrections in the (Figure presented.) turbulence model have been previously studied to predict flow separation over smooth airfoils, the present study aims to extend their applicability to airfoils with leading-edge roughness. Two corrections, whose effect on flow physics has not been empirically quantified, are addressed. Particle image velocimetry measurements have been performed on a 30% thick airfoil to quantify the impact of these corrections. The reduction of the eddy viscosity introduced by the corrections leads to a shift of the peak location of the Reynolds shear stresses away from the surface, which, in turn, promotes flow separation and improves the prediction of the mean velocity and the pressure-coefficient distribution. Besides, the ratio between the main turbulent shear stress and turbulent kinetic energy is demonstrated to be lower than the standard value used in the (Figure presented.) turbulence model at the boundary-layer outer edge. Adjusting this ratio for an angle of attack of 0° decreases the error on the predicted lift and drag coefficients from 75% to 3% and from 58% to 39%, respectively.

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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.@en

Integrating a porous material into the structure of an aerofoil constitutes a promising passive strategy for mitigating the noise from turbulence–body interactions that has been extensively explored in the past few decades. When a compact permeable body is considered in the aeroacoustic analogy derived by Curle to predict this noise source, a dipole associated with the non-zero unsteady Reynolds stresses appears on the surface in addition to the dipole linked to the pressure fluctuations. Nevertheless, the relative contribution of this source to the far-field noise radiated by a porous wing profile has not been clarified yet. The purpose of the current research work is twofold. On the one hand, it investigates the impact of porosity on the surface-pressure fluctuations of a thick aerofoil immersed in the wake of an upstream circular rod at a Mach number of 0.09. On the other hand, it quantifies the relevance of the Reynolds-stresses term on the surface as a sound-generation mechanism. The results from large-eddy simulations show that the porous treatment of the wing profile yields an attenuation of the unsteady-pressure peak, which is localised in the low-frequency range of the spectrum and is induced by the milder distortion of the incoming vortices. However, porosity is ineffective in breaking the spanwise coherence or in-phase behaviour of the surface-pressure fluctuations at the vortex-shedding frequency. The Reynolds-stresses term is found to be considerable in the stagnation region of the aerofoil, where the transpiration velocity is larger, and partly correlated with the unsteady surface pressure, suggesting constructive interference between the two terms. This results in a non-negligible contribution of this term to the far-field acoustic pressure emitted by the porous wing profile for observation angles near the stagnation streamline. The conclusions drawn in the present study eventually provide valuable insight into the design of innovative and efficient passive strategies to mitigate surface–turbulence interaction noise in industrial applications.

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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.@en

The aerodynamic noise radiated by the flow past a cylinder in the subcritical regime can be modeled by a quadrupolar sound source placed at the onset position of the vortex-shedding instability that is scattered by the surface with a dipolar directivity. When the cylinder is coated with a porous material, the intensity of the shed vortices is greatly reduced, determining a downstream shift of the instability-outbreak location. Consequently, sound diffraction is less efficient, and noise is mitigated. In this paper, an innovative design approach for a flow-permeable coating based on a further enhancement of such an effect is proposed. The results of phased-microphone-array measurements show that, once the leeward part of the cover is integrated with components that make the flow within the porous medium more streamlined, the quadrupolar source associated with the vortex-shedding onset is displaced more downstream, yielding additional noise attenuation of up to 10 dB with respect to a uniform coating. Furthermore, the same noise-control mechanism based on the weakening of the sound scattering can be exploited when these components are connected to the bare cylinder without the porous cover. In this case, the mitigation of overall sound pressure levels is comparable to that induced by the coated configurations due to the lack of noise increase produced by the inner flow interacting within the pores of the material. Remarkable sound reductions of up to 10 dB and a potential drag-force decrease are achieved with this approach, which paves the way for disruptive and more optimized noise-attenuation solutions.

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Integrating porous materials into the structure of an airfoil constitutes a promising passive strategy for mitigating the noise from turbulence-body interactions that has been extensively explored in the past few decades. When a compact permeable body is considered in the aeroacoustic analogy derived by Curle to predict this noise source, a dipole associated with the nonzero unsteady Reynolds stresses appears on the surface in addition to the dipole linked to the pressure fluctuations. Nevertheless, the relative contribution of this source on the far-field noise radiated by a porous wing profile has not been clarified yet. The purpose of the current research work is twofold. On the one hand, it investigates the impact of porosity on the surface-pressure fluctuations of a thick airfoil immersed in the wake of an upstream circular rod at a Mach number of 0.09. On the other hand, it quantifies the relevance of the Reynolds-stresses term on the surface as a sound-generation mechanism. Results from large-eddy simulations show that the porous treatment of the wing profile yields an attenuation of the unsteady-pressure peak, which is localized in the low-frequency range of the spectrum and is induced by the milder distortion of the incoming vortices. However, porosity is ineffective in breaking the spanwise coherence or in-phase behavior of the surface-pressure fluctuations at the vortex-shedding frequency. The Reynolds-stresses term is found to be considerable in the stagnation region of the airfoil, where the transpiration velocity is larger, and partly correlated with the unsteady surface pressure. This results in a nonnegligible contribution of this term to the far-field acoustic pressure emitted by the porous wing profile for observation angles near the stagnation streamline. The conclusions drawn in the present study eventually provide valuable insight into the design of innovative and efficient passive strategies to mitigate surface-turbulence interaction noise in industrial applications.@en

Sound emissions of an automotive engine cooling system are studied using both single-microphone directivity measurements and a rotating beamforming technique. These measurements provide reference acoustic data on such a system and some new understanding of the effect that the radiator induces on the distribution of sound sources. Indeed, the beamforming results indicate that, above the frequency limit allowed by the Rayleigh criterion, it is possible to localize and quantify the noise sources even through the heat-exchanger core. Moreover, for the investigated operating points along the fan performance curve, the sources are always distributed at the tip of the blades and, in particular, at the leading edge. The present evidence, confirmed by the similar trends of the frequency spectra with and without the heat exchanger, leads to the conclusion that the dominant sound mechanism is the turbulence-interaction noise. Nevertheless, this turbulence is produced within the gap between the fan ring and its casing rather than generated by the radiator core. The latter appears to induce negligible acoustic transmission losses but, more significantly, is found to have a minimal influence on the aerodynamic modification of sound sources for all the analyzed operating conditions.

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A small didactic wind tunnel demonstrator has been designed and manufactured at the von Karman Institute for Fluid Dynamics to illustrate the physical principles at stake in flow-induced noise generation, offer an audible perception of the effectiveness of noise-mitigation strategies, and serve as a practical test bench for aeroacoustic education and research. Seven mitigation technologies are embedded in a single facility, which addresses the noise generation by an airfoil, noise propagation in a duct, and noise transmission through a flexible panel. A challenging objective of this facility was to offer a perceptible impression of various aeroacoustic noise mechanisms at low flow speeds and a live assessment of the effectiveness of noise-reduction technologies. Different approaches combining multiple microphones, advanced signal-processing techniques, and real-time audio feedback have been implemented to this end. A digital twin has been developed to assist the design of the facility and test the concepts implemented in it. The results establish that the demonstrator enables a clear perception of the effectiveness of the noise-mitigation technologies. The facility is also suitable for fast and inexpensive preliminary investigations of future noise-reduction concepts, taking advantage of rapid prototyping techniques.

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A small didactic wind tunnel demonstrator has been designed and manufactured at the von Karman Institute for Fluid Dynamics to illustrate the physical principles at stake in flow-induced noise generation, offer an audible perception of the effectiveness of noise-mitigation strategies, and serve as a practical test bench for aeroacoustic education and research. Seven mitigation technologies are embedded in a single facility, which addresses the noise generation by an airfoil, noise propagation in a duct, and noise transmission through a flexible panel. A challenging objective of this facility was to offer a perceptible impression of various aeroacoustic noise mechanisms at low flow speeds and a live assessment of the effectiveness of the noise-reduction technologies. Different approaches combining multiple microphones, advanced signal-processing techniques, and real-time audio feedback have been implemented to this end. The results establish that the demonstrator enables a clear perception of the effectiveness of the noise-mitigation technologies. The facility is also suitable for fast and inexpensive preliminary investigations of future noise-reduction concepts, taking advantage of rapid prototyping techniques.

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Coating a circular cylinder with porous materials constitutes an effective passive strategy for reducing the flow-induced noise linked to the vortex shedding. Despite the number of investigations in the last decade, the noise-mitigation mechanisms associated with this technique remain unclear. The present research aims to clarify the role played by the alterations in the flow field due to porosity in the aerodynamic-sound attenuation of a cylinder coated with metal foam. Far-field acoustic and particle-image-velocimetry (PIV) measurements were performed at the Delft University of Technology for Reynolds numbers ranging in the subcritical regime. The aeroacoustic results show that a significant tonal and broadband suppression could be achieved with the porous treatment of the body. For the coated cylinder, the dominant sources do not appear to be distributed over the surface but rather are situated several diameters downstream of it, with a lower amplitude. The PIV data reveal that the main effect of the coating is to stabilize the cylinder wake, which results in an elongation of the vortex-formation length and a decrease in the turbulence kinetic energy. In particular, the position where the vortex shedding starts corresponds to the region of the dominant noise sources. The conclusions drawn in this study potentially provide an insightful indication for the design of more effective sound-control solutions.@en

An experimental investigation on jet-installation noise reduction with permeable flaps on an aircraft half-model is performed, focusing on the effects on the noise levels as well as on the aerodynamic properties of the model (lift and drag forces). A nozzle with an exit diameter = 113 mm is included for generating a single-stream jet flow in the vicinity of the airframe. Two perforated flaps with different hole distributions are investigated. The first one has a uniform hole distribution with equal hole spacing in both streamwise and spanwise directions, whereas the second one has a gradient permeability with the hole spacing progressively decreasing towards the trailing edge. Aerodynamic force measurements, carried out with a balance, show that the permeable flaps are responsible for a lift reduction in the order of 7%, and a slight drag increase, in the order of 0.5%, with the gradient permeability flap outperforming the uniform one. Flow-field maps indicate that the effect on lift is linked to the formation of side-edge vortices at the spanwise positions corresponding to the discontinuity between solid and porous regions. Acoustic results obtained from phased array measurements show that the source at the flap trailing edge, generated by the interaction with the jet, reduces by 3 dB in amplitude with both permeable flaps, which perform similarly. Therefore, the permeable flaps are able to significantly reduce jet-installation noise for an aircraft configuration in flight conditions, however care must be taken in the design of the permeable structure in order to avoid aerodynamic degradation.

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The distortion of homogeneous isotropic turbulence interacting with a porous cylinder is calculated by means of the Rapid Distortion Theory (RDT). The porous treatment, characterised by a constant static permeability, is modelled as an impedance boundary condition accounting for the Darcy’s flow within the body. The RDT algorithm is first validated through comparisons with published velocity measurements in the stagnation region of an impermeable cylinder placed downstream of a turbulence grid. Subsequently, the impact of porosity on the velocity field is investigated through the analysis of the one-dimensional spectra at different locations near the body and the velocity variance along the stagnation streamline. The porous surface affects the incoming turbulence distortion near the cylinder by reducing the blocking effect of the body and by altering the vorticity deformation caused by the mean flow. The former leads to an attenuation of the one-dimensional velocity spectrum in the low-frequency range, whereas the latter results in an amplification of the high-frequency components. This trend is found to be strongly dependent on the turbulence scale and influences the evolution of the velocity fluctuations in the stagnation region. The porous RDT is finally adapted to model the turbulence distortion in the vicinity of the leading edge of a NACA-0024 profile fitted with melamine foam. The good agreement between the calculations and the experimental results demonstrates that the present methodology can improve the understanding of the physical mechanisms involved in the aerofoil-turbulence interaction noise reduction through porosity and be instrumental in designing such passive noise-mitigation treatments.@en

Aerodynamic noise emitted by low-speed axial fans has been receiving increasing attention in various sectors of high societal impact, such as automotive and HVAC systems. In this framework, turbulence interaction, flow non-uniformities, trailing-edge boundary layer fluctuations and blade-tip leakages are different mechanisms generating aeroacoustic sources on the rotating blades and contributing to the overall emitted sound. An accurate localization of the sound sources on the surface of the blade is instrumental in separating and isolating these contributions and, therefore, in designing novel sound mitigation concepts. The main objective of this paper is to present an inexpensive and efficient way to isolate and quantify the noise generating mechanisms on rotating blades by means of a irregularly shaped microphone array. The technique is based on ROtating Source Identifier (ROSI) and has been implemented and validated at the von Karman Institute for Fluid Dynamics (VKI). Simulated benchmark datasets that refer to rotating point sources emitting white noise have been considered for the validation of the method. The accuracy in the source localization and in the source strength reconstruction has been evaluated for a fixed and a variable angular rate. Moreover, the algorithm implementation has been parallelized with the purpose of reducing its computational time, which represents the main drawback of ROSI. Finally, the developed technique has been applied to measure the noise sources generated by a forward-skewed subsonic axial fan operated at maximum efficiency. In this case, it has been possible to successfully localize and characterize the major noise sources on the blades. Although further investigation will be necessary to gain better insight into the topic, the present work constitutes an important step for a better understanding of the physical phenomena occurring in the noise generation of an axial fan.

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