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This study presents a numerical investigation of the noise mitigation effect provided by several fairings placed upstream of a simplified two-wheel landing gear. The chosen configuration is equipped with detachable elements that mimic realistic components, e.g. brakes and torque link, to include representative landing gear noise sources. Several numerical simulations of the flow developing around this landing gear have been carried out, with or without an additional upstream fairing to control the noise generation processes. The chosen configurations match those from related experiments performed at the Delft University of Technology. Both the numerical and the experimental studies are conducted in the framework of the European Union Horizon 2020 research project INVENTOR (INnoVative dEsign of iNstalled airframe componenTs for aircraft nOise Reduction). The numerical method is based on the Zonal Detached Eddy Simulation approach, applied on a set of Cartesian octree grids with a specific immersed boundary wall treatment. Both solid and wire mesh fairings are considered, the latter being accounted for thanks to a specific wire mesh numerical model. Overall, the simulations show a nice agreement with the measurements and allow a thorough analysis of the flow modifications responsible for noise mitigation when the fairings are introduced., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Wind Energy

Noise reduction is one of the major challenges to the societal acceptance of wind turbines. Aerodynamic noise sources are the dominant ones, assuming that mechanical noise can be reduced by correct insulation and periodic maintenance of the gearbox and drive train and by adding insulating material in the wind-turbine nacelle. The dominant aerodynamic noise source is turbulent boundary layer trailing edge noise, caused by the scattering of the pressure fluctuations beneath the turbulent boundary layer at the trailing edge of an airfoil. In order to reduce this source of noise, several strategies have been proposed such as: optimized airfoil design for noise performances; installation of add-ons such as brushes or trailing-edge serrations; and recently, permeable materials at the trailing edge. In this chapter, after a brief introduction to the aerodynamic noise sources, the noise reduction physical mechanisms of trailing-edge serrations, as an industrial state-of-the-art solution, and of porous materials, as an innovative noise reduction strategy, will be discussed., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Wind Energy

This work proposes a semi-empirical framework to predict the noise of wind turbines with serrated trailing edge blades. The framework is employed for studying the reduction of the noise of the SWT 2.3-93 benchmark wind turbine. The framework is verified against field acoustic measurements of the real wind-turbine model and of noise reduction measured for airfoil geometries with serrated trailing edges. Two different serration design strategies are proposed, respectively one with the same serration geometry along the blade and one with serrations scaled with the local boundary-layer properties along the radius. Results show the predicted noise reduction obtained with each of the add-ons and explore the benefits of tailoring the design of the serrations according to the varying flow conditions along the blade span., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Wind Energy

The authors would like to provide the following corrections and clarifications to the article titled “Aeroacoustic Benchmarking of Trailing-edge Noise from a NACA 633–018 Airfoil with Trailing-edge Serrations” which has been published in the AIAA Journal Vol. 61, No. 1, and can be accessed online via https://doi.org/10.2514/1.J061630. The first correction provides clarity in the abstract. Although the main text and Appendices A and B of the original paper provide a thorough analysis of the varying signal-to-noise levels and clearly state that some data points with inherently high noise levels should be excluded in further analysis, the statement in the abstract could lead to misunderstanding that all data points will directly be included in the benchmark activities. It indeed is up to a broader benchmarking team, after considering results among different institutions, to decide which parts of the present dataset will eventually be included. Therefore, for clarity, the text “. . . The present data are to be included in the framework of the Benchmark Problems for Airframe Noise Computation . . . ” should be replaced by “. . . The present data are to be considered among participating institutions and may partially be included in the framework of the Benchmark Problems for Airframe Noise Computation . . . ”. The second correction pertains to the manufacturer of the so-called High-Reynolds Model (HRM) airfoil and a reference mentioned in the second paragraph of Section II.A. The text “. . . manufactured by Deharde . . . [23]" should be “. . . manufactured by RIVAL . . . [23]”. The part of the model considered in this paper was manufactured by RIVAL and Deharde later produced the spanwise extensions for this model to fit in other larger wind tunnels. The authors apologize for this miscommunication. Besides, Ref. [23] in the original paper should be replaced by Ref. [1] of this correction. During the publication process of our paper, this new reference was published and th, Control & Simulation, Reflection & Lifestyle, Wind Energy, Control & Operations

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., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Wind Energy

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., Wind Energy, Group Garcia Espallargas

Aeroacoustic tests in closed wind tunnels are affected by reflections in the tunnel circuit and background noise. Reflections can be mitigated by lining the tunnel circuit. The present study investigates if lining exclusively the most accessible segment of a closed wind tunnel circuit, in particular the test section, is an approach which improves acoustic measurements. Literature shows that a wind tunnel lining material should have high acoustic absorption, low inertial resistivity and low surface roughness. Therefore, the test section of TU Delft's closed Low Turbulence Tunnel is lined with melamine foam wall liners. A total of 4 test section configurations were tested: baseline; test section with lining on the floor and ceiling; test section with lined side--panels; and test section lined at all surfaces (floor, ceiling and side--panels). An omnidirectional speaker is used for evaluating the wind tunnel's acoustic performance. A geometric modelling algorithm, based on the mirror-source method, is used to predict the effect of lining on primary reflections in the test section. In addition, reflections in the test section and in the tunnel circuit are characterized experimentally. The results show that the closed loop of the tunnel circuit is responsible for a long reverberation time in the test section. However, reflections inside the test section itself are the dominant source of acoustic interference at the microphone array location. The low fidelity geometric modelling algorithm is shown to be a valuable approach for an initial estimation of the acoustic benefit of lining, for both flow--off and --on conditions. Lining of the test section walls significantly reduces reflections from the reference source, as well as the aerodynamic background noise that reaches the array., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Wind Energy, Aircraft Noise and Climate Effects, Control & Operations

Blade Element Momentum Theory (BEMT) -based approaches provide 15%–20% inaccurate load predictions under uniform steady inflow. Inaccuracies were related to unclear aerodynamic mechanisms causing the Prandtl Tip-Root Loss (TRL) correction to fail at high Tip-Speed Ratio (TSR) and the ambiguous drag coefficient treatment under Stall Delay (SD). With this study, we provide an in-depth flow analysis to explain what physical mechanisms are poorly captured in TRL and SD corrections and offer a potential solution for the improvement of the Prandtl model. Corrections are assessed by comparison with RANS CFD simulations of the NREL 5MW rotor at its design TSR, TSR 4, and TSR 10. The Prandtl tip-loss correction provides 5%–15% higher loading than CFD at TSR 10. The reason is a lower (0.5°) downwash angle of attack, providing a 5%–10% lift coefficient overestimation. We recommend employing the tip-loss correction of Zhong, as it considers the lift coefficient reduction due to the tip-vortex downwash. The investigated Eggers SD correction predicts an incorrect drag coefficient at TSR 4, while a better agreement is found for the lift coefficient. We conclude that only the latter requires a correction for SD in the inboard blade., Flow Physics and Technology, Wind Energy

The unsteady flow behaviour of two side-by-side rotors in ground proximity is experimentally investigated. The rotors induce a velocity distribution interacting with the ground causing the radial expansion of the rotor wakes. In between the rotors, an interaction of the two wakes takes place, resulting in an upward flow similar to a fountain. Two types of flow topologies are examined and correspond to two different stand-off heights between the rotors and the ground: the first one where the height of the fountain remains below the rotor disks, and a second one where it emerges above, being re-ingested. The fountain unsteadiness is shown to increase when re-ingestion takes place, determining a location switch from one rotor disk to the other, multiple times during acquisition. Consequently, variable inflow conditions are imposed on each of the two rotors. The fountain dynamics is observed at a frequency that is about two orders of magnitude lower than the blade passing frequency. The dominant characteristic time scale is linked to the flow recirculation path, relating this to system parameters of thrust and ground stand-off height. The flow field is analysed using proper orthogonal decomposition, in which coupled modes are identified. Results from the modal analysis are used to formulate a simple dynamic flow model of the re-ingestion switching cycle., Wind Energy, Aerodynamics

This paper presents a noise propagation approach based on the Gaussian beam tracing (GBT) method that accounts for multiple reflections over three-dimensional terrain topology and atmospheric refraction due to horizontal and vertical variability in wind velocity. A semi-empirical formulation is derived to reduce truncation error in the beam summation for receivers on the terrain surfaces. The reliability of the present GBT approach is assessed with an acoustic solver based on the finite element method (FEM) solutions of the convected wave equation. The predicted wavefields with the two methods are compared for different source-receiver geometries, urban settings, and wind conditions. When the beam summation is performed without the empirical formulation, the maximum difference is more than 40 dB; it drops below 8 dB with the empirical formulation. In the presence of wind, the direct and reflected waves can have different ray paths than those in a quiescent atmosphere, which results in less apparent diffraction patterns. A 17-fold reduction in computation time is achieved compared to the FEM solver. The results suggest that the present GBT acoustic propagation model can be applied to high-frequency noise propagation in urban environments with acceptable accuracy and better computational efficiency than full-wave solutions., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Wind Energy

A semi-emperical model is developed, able to capture the aeropropulsive performance characteristics of Over-The-Wing propellers at incidence. The model is based on an hypothesis on the interactions of the propeller- and wing-induced flow fields. Effects of these interactions on the both the thrust and lift are written in a form in which the dominant design parameters appear explicitly. Both the flow hypothesis and model results are validated using experimental data of a single Over-The-Wing propeller. It is shown that for moderate angles of attack, the propulsive thrust is reduced by the wing’s circulation. For angles of attack greater than the stall angle of the isolated wing, thrust is increased by the ingestion of low momentum flow. The propeller is not able to delay stall but induces flow over the wing, which is returned as reduced pressure over the suction side. The model predictions closely match the experimental results for thrust, but integral loading measurements of the wing are required to validate the lift predictions., Aerodynamics, Wind Energy

The present experimental investigation focuses on a flow-field and noise characterization of a CD airfoil experiencing large flow separation and stall. Measurements are performed to investigate the effect of Reynolds number stalling noise signature of the CD airfoil. This study includes investigation of the potential interaction of wind tunnel shear layers with the separating shear layer of the airfoil, in an effort to validate previous experimental studies performed on a similar jet width using Planar-PIV. While a mean flow separation is observed near the leading-edge of the CD airfoil at angles of attack of 15 degrees, the mean reattaches before the trailing-edge region for the case of 15 degrees. In contrast for 22 degrees case the mean flow becomes completely separated and airfoil experiences a deep stall. For the latter, the Sound Pressure Levels are reduced and it is possibly linked to a decrease in overall velocity disturbances and attenuation of modulations in SPL linked to diffraction. More importantly, the velocity disturbances do not scale with overall extent of the separated shear layer or the boundary layer. As such, a one to one correspondence does not exist between SPL and boundary layer thickness., Wind Energy

Rotor noise comprises harmonic features, related to the blade passing frequency, as well as broadband noise. Even though acoustic spectra yield frequency-distributions of acoustic energy within pressure time series, they do not reveal phase-relations between different frequency components. The latter are of critical importance for the development of prediction- and auralization-algorithms, because these phase-relations can result in low-frequency intensity modulation of higher-frequency rotor noise. Baars et al. (AIAA Paper 2021-0713) outlined a methodology to quantify inter-frequency modulation, which in the current work is applied to a comprehensive acoustic dataset of a laboratory-scale rotor at advance ratios ranging from J = 0 to 0.61. PIV measurements of the blade-induced flow disturbances complement the acoustic data to elucidate how the vortical flow structures of one blade impact the inflow of the consecutive blade. The findings strengthen earlier observations for the case of a hovering rotor (J = 0), in which the modulation of the high-frequency noise is strongest at angles of -20 degrees below the rotor plane. For the non-zero advance ratios, the modulation becomes dominant in the sector spanning -45 degrees to 0 degrees, and is maximum in strength for the highest advance ratio tested (J = 0.61). It is hypothesized that the intensity-modulation of high-frequency noise relates to the appearance of different separated-flow features over the suction side of the low Reynolds-number rotor-blades. As recently detailed in the articles by Grande et al. (AIAA J. 60:2 & AIAA J. 60:9, 2022), with increasing J, the separation goes from a fully laminar separation, to one that reattaches and forms a laminar separation bubble, to one that fully separates in a turbulent state. With an increase of modulation strength with J we conjecture that trailing-edge/shedding noise, associated with the broadband features of the separated flow, causes the modulation due to a far, Wind Energy, Aerodynamics

A low-order numerical tool for estimating noise emissions from distributed propeller configurations is presented. The paper describes the tool's computational framework, which uses Hanson's near-field theory to calculate the thickness and loading noise components. The formulation assumes steady blade loading, but an unsteady case can be handled numerically by redefining the pressure distribution over the blade at each new time step. Two representative cases are analyzed to validate the tool: an isolated propeller operating in uniform flow and an array of three propellers in a side-by-side configuration under aerodynamic interference caused by adjacent propellers. The results obtained from the low-fidelity tool are compared to high-fidelity data to evaluate the accuracy and differences in predicting the noise of a distributed propeller system. The low-fidelity tool provides accurate results for both cases, with less than a 1.5 dB difference up to the fifth blade-passage frequency (BPF) when comparing tonal noise predictions at an observer located 10 diameters away and at the propeller plane. When analyzing the source directivity at the first BPF, there is a difference of approximately 0.5 dB at the propeller plane. However, this difference increases to 6 dB as the observer moves toward the inflow direction. This difference is due to the dominance of broadband noise near the propeller axis. The paper concludes with a noise analysis of the distributed propeller system, examining the relative importance of aerodynamic interference in the noise emitted by a propeller. In this case, the unsteady blade loading generated a tonal component of 40 dB at the first BPF in the propeller axis, while it had an insignificant effect at the rotor plane., Wind Energy, Flight Performance and Propulsion

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., Wind Energy

Distributed electric propulsion systems are an emerging technology with the potential of revolutionizing the design and performance of aircraft. When propellers are located in close proximity, they can be subjected to aerodynamic interactions, which affect the far-field noise. In this paper, we study an array of three co-rotating and adjacent propellers to describe both the aerodynamic and acoustic installation effects. A scale-resolving CFD simulation based on the Lattice-Boltzmann/Very-Large-Eddy-Simulation method is used to solve the flow field around the propellers. An acoustic analogy integral approach calculates the far-field noise. Findings show that the helical vortical structures, generated at the tip of each blade undergo a flow deformation at the location of interaction. This causes the loading of each blade to vary during the rotation. Consequently, the unsteady loading noise becomes a dominant noise generation mechanism, driving the noise levels and directivity. It is also shown that introducing a non-zero relative phase angle between the propellers results in a reduction of the unsteady thrust, leading to a mitigation of the unsteady-loading tonal components along the rotation axis. Additionally, the relative phase angle causes constructive/destructive acoustic interference, as demonstrated by analyzing the noise emitted simultaneously by the three propellers., Wind Energy

Experimental results on trailing-edge (TE) noise from a NACA 633 –018 airfoil are presented for a chord-based Reynolds number Rec range between 2 × 105 and 3 × 106. Far-field TE noise from the baseline airfoil with a straight TE and TE serrations is measured with varying Rec, angle of attack, and serration shape and flap angle. Additionally, aerodynamic coefficients and boundary-layer parameters at the TE are also reported. To cover such a broad Rec range, two NACA 633 –018 airfoil models were tested in two different wind tunnels. The measurements include the emitted noise with natural and forced transition locations. For the straight TE, the forced transition location results in up to 5 dB increase of the far-field TE noise level, compared to the natural one. Scaling of the far-field noise spectra from the baseline TE shows that the Strouhal numbers St at which the peak noise level is measured reduce as Rec increases. TE noise spectra for the cases with the TE serrations are found to be dependent on the airfoil lift and Rec. The present data are to be included in the framework of the Benchmark Problems for Airframe Noise Computations category I and are publicly available in a repository with the following digital object identifier (DOI): https://doi.org/10.4121/20940646., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Control & Simulation, Aircraft Noise and Climate Effects, Wind Energy, Control & Operations

Efficient calculation of urban air mobility noise footprint in a vertiport environment, considering acoustic effects of various designs, operational conditions, and environmental factors, is essential to limit the noise impact on the community at an early stage. To this purpose, the computationally efficient low-fidelity approach presented by the authors in Fuerkaiti et al. (2022) [11] is extended to calculate the noise footprint of an aircraft in a generic 3D environment. The straight-ray propagator is replaced with a Gaussian beam tracer that accounts for complex source directivity, 3D varying terrain topology, and wind profiles. The reliability of the Gaussian beam tracer has been verified in previous studies by the authors. In this work, it is further extended to include complex source directivity in the presence of a moving medium. Noise sources, obtained using a low-fidelity toolchain, are stored on a sphere surrounding the aircraft and are propagated through an inhomogeneous anisotropic atmosphere. Noise footprints, predicted for different terrain topologies, source directivities, and wind flow conditions, are compared. It is shown that, compared to flat terrain, for the case under investigation, the building blocks increase on-ground noise levels by 5 dB in the illuminated zone due to multiple reflections; they also shield the incoming sound field by creating shadow zones behind the building. The shielding increases with increasing frequency in a quiescent atmosphere. The change between the source directivities, corresponding to the first and second harmonics of the blade passing frequency, results in a difference of up to 40 dB in the noise footprint. The presence of the wind flow can contribute a significant variation in the acoustic footprint by changing the lobes of the footprint pattern and intensifying the noise levels; the variation increases with increasing frequency. Compared to the straight-ray propagator, the present approach reduces the predic, Wind Energy

Wind Energy, Aerodynamics

Vortex shedding in the wake of a cylinder in uniform flow can be suppressed via the application of a porous coating; however, the suppression mechanism is not fully understood. The internal flow field of a porous coated cylinder (PCC) can provide a deeper understanding of how the flow within the porous medium affects the wake development. A structured PCC (SPCC) was three-dimensionally printed using a transparent material and tested in water tunnel facilities using flow visualisation and tomographic particle image velocimetry at outer-diameter Reynolds numbers of and, respectively. The internal and near-wall flow fields are analysed at the windward and mid-circumference regions. Flow stagnation is observed in the porous layer on the windward side and its boundary is shown to fluctuate with time in the outermost porous layer. This stagnation region generates a quasi-aerodynamic body that influences boundary layer development on the SPCC inner diameter, that separates into a shear layer within the porous medium. For the first time via experiment, spectral content within the separated shear layer reveals vortex shedding processes emanating through single pores at the outer diameter, providing strong evidence that SPCC vortex shedding originates from the inner diameter. Velocity fluctuations linked to this vortex shedding propagate through the porous layers into the external flow field at a velocity less than that of the free stream. The Strouhal number linked to this velocity accurately predicts the SPCC vortex shedding frequency., Wind Energy

Propeller noise generally exhibits a rich mixture of tonal and broadband components related to different physical mechanisms. Specifically, the tones are characterized by having deterministic and persistent characteristics, while the broadband counterpart has random behaviour. The separation is essential for the experimenters as they provide information on the different noise sources. In this framework, the study presents a novel wavelet-based method able to separate the noise emitted by a low Reynolds number propeller into its tonal and broadband components. The technique is applied to an isolated rotor operating under different loading configurations, including hover and cruise conditions. The acoustic pressure data are obtained in the anechoic tunnel (A-tunnel) of the TU Delft low-speed laboratory with a near-field polar and azimuthal distribution of microphones. The method is based upon a threshold varying procedure that separates the tonal and broadband components through the computation of two-point statistics. Advantages and drawbacks with respect to other methodologies already known from the literature are discussed. The application of the method provides the spectral content of the tonal and broadband components as well as the different polar and azimuthal directivity. Specifically, the observed dipole-like shape directivity for the tonal part and flatter broadband OASPL, confirm that the method can provide quite a good separation. Furthermore, the overall flow behaviour is inferred from the decomposition and validated through benchmarked flow visualizations., Wind Energy

The design of efficient rotor blades is affected by the accuracy of aerodynamic prediction methods for load distributions and power computations. Research showed that the accuracy of BEMT-based industrial codes decreases at high inflow-speed under uniform aligned steady conditions. The identified reasons are inaccuracies in the semi-empirical corrections for 3D effects such as stall delay and tip-losses. This study scrutinizes such corrections by comparison with URANS CFD simulations. Results confirm that the accuracy of the rotor thrust and power coefficients reduces up to 30% for a tip speed ratio of 4. The identified causes in the inboard blade are: (1) a more than twice as large drag coefficient given by the Eggers stall delay correction, (2) a 20% loading overestimation due to the unaccounted root-vortex downwash. Furthermore, the linear interpolation between the cylinder and the DU40 airfoil polars near the root as well as the modeling of 2D separation affect the accuracy at least as much as the stall delay correction at a tip speed ratio of 4. Next, the inadequacy of the Prandtl tip-loss factor at a tip speed ratio of 10 provides 5 to 15% higher loads in the outboard blade. It is recommended to extend stall delay corrections or tune the Prandtl root-loss correction to the location of the maximum chord to capture the root-vortex downwash effect, as the phenomenon is observed on the CFD-extracted lift polar and blade flow streamlines. Finally, 2D RANS simulations of the inboard blade profiles should be compared with the 3D ones from the rotating blade to isolate the effect of stall delay on the pressure and skin friction coefficient distributions to further address the modeling of the drag coefficient., Wind Energy

This work presents the first experimental characterization of the flow field in the vicinity of periodically spaced discrete roughness elements (DRE) in a swept wing boundary layer. The time-averaged velocity fields are acquired in a volumetric domain by high-resolution dual-pulse tomographic particle tracking velocimetry. Investigation of the stationary flow topology indicates that the near-element flow region is dominated by high- and low-speed streaks. The boundary layer spectral content is inferred by spatial fast Fourier transform (FFT) analysis of the spanwise velocity signal, characterizing the chordwise behaviour of individual disturbance modes. The two signature features of transient growth, namely algebraic growth and exponential decay, are identified in the chordwise evolution of the disturbance energy associated with higher harmonics of the primary stationary mode. A transient decay process is instead identified in the near-wake region just aft of each DRE, similar to the wake relaxation effect previously observed in two-dimensional boundary layer flows. The transient decay regime is found to condition the onset and initial amplitude of modal crossflow instabilities. Within the critical DRE amplitude range (i.e. affecting boundary layer transition without causing flow tripping) the transient disturbances are strongly receptive to the spanwise spacing and diameter of the elements, which drive the modal energy distribution within the spatial spectra. In the super-critical amplitude forcing (i.e. causing flow tripping) the near-element stationary flow topology is dominated by the development of a high-speed and strongly fluctuating region closely aligned with the DRE wake. Therefore, elevated shears and unsteady disturbances affect the near-element flow development. Combined with the harmonic modes transient growth these instabilities initiate a laminar streak structure breakdown and a bypass transition process., Aerodynamics, Wind Energy

The present work details the steady and unsteady flow topology in the vicinity of an array of periodically spaced super-critical (i.e. causing flow tripping) discrete roughness elements (DRE) applied in a swept wing boundary layer. The stationary flow field is acquired by means of high-magnification dual-pulse tomographic particle tracking velocimetry (3D-PTV), while the unsteady instabilities are investigated through high-resolution hot wire anemometry (HWA). The 3D-PTV time-averaged velocity fields, indicate that the near-element flow region is dominated by the alternation of high- and low-speed streaks. A high-speed region substitutes the wake development shortly downstream of the DRE location, due to the high-speed streaks merging. This initiates a region of strong unsteady fluctuations that expands in the spanwise and wall-normal directions, ultimately leading to the boundary layer transition to turbulence. The spectral content of the stationary flow structures is investigated through a spanwise spatial Fourier transform. The extracted spectra and instability amplitudes, indicate the presence of non-modal mechanisms in the near-element stationary wake region. Nonetheless, the temporal spectral analysis of the HWA velocity signal, identifies the presence of strongly tonal shedding mechanisms initiating and the unsteady instabilities the element vicinity. Their rapid downstream growth and evolution retains a fundamental role in the transitional process., Aerodynamics, Wind Energy

Abstract: This work combines the latest advancements in time marching of 3D vector fields from tomographic particle image velocimetry, with an adapted version of Lighthill’s formulation, for the prediction of far-field jet noise. Three-dimensional velocity vector fields of the jet flow are first reconstructed from a tomographic volume of 4× 3× 9.5 Dj3, with Dj = 5 cm being the jet-exit diameter. (The jet-exit Mach number Mj ranges from 0.10 to 0.20.) The obtained vector fields are then used as input to a recently developed procedure for the time marching of the vorticity field, which relies upon the vortex-in-cell methodology. This yields time series of each three-dimensional velocity field, from which the far-field pressure is computed via Lilley’s acoustic analogy (through evaluation of the Lighthill’s stress tensor). It is shown that the estimate of the far-field noise spectrum compares well with the spectrum measured directly from a far-field microphone in the anechoic A-tunnel facility of TU Delft, in the Strouhal number range from approximately 1 to 12. Graphical Abstract: [Figure not available: see fulltext.], Wind Energy, Aerodynamics

The effect of discrete roughness elements on the development and breakdown of stationary crossflow instability on a swept wing is explored. Receptivity to various element heights and chordwise locations is explored using a combination of experimental and theoretical tools. Forcing configurations, determined based on linear stability predictions, are manufactured and applied on the wing in a low turbulence facility. Measurements are performed using infrared thermography, quantifying the transition front location, and planar particle image velocimetry, providing a reconstruction of stationary crossflow instabilities and their associated growth. Measurements are corroborated with simulations based on nonlinear parabolised stability equations. Results confirm the efficacy of discrete roughness elements in introducing and conditioning stationary crossflow instabilities. Primary instability amplitudes and resulting laminar-turbulent transition location are found to strongly depend on both roughness amplitude and chordwise location. The Reynolds number based on element height is found to satisfactorily approximate the initial forcing amplitude, revealing the importance of local velocity effects in non-zero-pressure gradient flows. Direct estimation of initial perturbation amplitudes from nonlinear simulations suggests the existence of pertinent flow mechanisms in the element vicinity, active in conditioning the onset of modal instabilities. Dedicated velocimetry planes, elucidate the development of a momentum deficit wake which rapidly decays downstream of the element followed by mild growth, representing the first experimental evidence of transient behaviour in swept wing boundary layers. The outcome of this work identifies a strong scalability of the transition dynamics to roughness amplitude and location, warranting the upscaling of roughness elements to more accessible, measurable and spatially resolved configurations in future experiments., Aerodynamics, Wind Energy

A physical description of the flow mechanisms that govern the distribution of the wall-pressure fluctuations over the surface of a serrated trailing edge is proposed. Three main mechanisms that define the variation of turbulent pressure fluctuations across the serrated edge are discussed and semi-empirical models are formulated accordingly. It is shown that the intensity of the wall-pressure fluctuations increases at the tips under the effect of an increased convective velocity as a result of sidewise momentum diffusion. Furthermore, the change of impedance across the edge causes a local reduction of the pressure fluctuations in the vicinity of the trailing edge. Finally, aerodynamic loading over the serrations due to the non-symmetric flow created at different angles of attack establishes secondary flow patterns that induce higher wall-pressure fluctuations over the serration edges. The latter effect is present only for serrations under high aerodynamic loading, while the former ones are observed under any conditions. Semi-empirical models are formulated for predicting the variation of the wall-pressure fluctuations over the serration surface based on the three physical mechanisms described. These models are calibrated and compared against experiments conducted on a symmetric airfoil model at high Reynolds numbers., Wind Energy, Aerodynamics

Cavities placed along wind tunnel walls can attenuate the turbulent boundary layer (TBL) fluctuations as they propagate into the cavity. Placing microphones within the cavities can thus improve the signal-to-noise ratio of acoustic data. However, standing waves form within these cavities distorting the acoustic measurements. This work uses a finite element (FE) solver to evaluate how cavity geometry (depth, diameter, and wall angle) and wall material (hard-walled and melamine foam) affect the amplitude and eigenfrequency of standing waves when excited by an incident acoustic plane wave. Good agreement between predicted and measured acoustic transfer functions is shown. Compared to cylindrical cavities, countersunk and conical cavities improve the overall response, i.e., reducing the quality factor quantifying the resonance and damping characteristics. Stainless steel coverings also reduce the quality factor. A finding is that the shape of the external foam holder rather than the cavity shape drives the standing wave characteristics for the melamine foam cavities. The optimization problem of minimizing the acoustic response while also attenuating the TBL is thus decoupled by using the melamine foam. Consequently, these considerations can be addressed independently by optimizing the outer cavity shape for acoustics and the melamine foam insert for TBL attenuation., Aircraft Noise and Climate Effects, Wind Energy

This manuscript presents the application of a recently developed noise reduction technology, constituted by poro-serrated stator blades on a full-scale aircraft model, in order to reduce rotor-stator interaction noise in the fan stage. This study was carried out using the commercial lattice Boltzmann solver 3DS-SIMULIA PowerFLOW. The simulation combines the airframe of the NASA High-Lift Common Research Model with an upscaled fan stage of the source diagnostic test rig. The poro-serrations on the stator blades have been modeled based on a metal foam with two different porosity values. The results evidence that the poro-serrations induce flow separation on the stator blades, particularly near the fan-stage hub. Consequently, the thrust generated by the modified fan stage is lower and the broadband noise emission at low frequencies is enhanced. Nevertheless, the tonal noise components at the blade-passage frequency and its harmonics are mitigated by up to 9 dB. The poro-serrations with lower porosity achieve a better trade-off between noise emission and thrust penalty. An optimization attempt was carried out by limiting the application of porosity near the tip of the stator blades. The improved leading-edge treatment achieves a total of 1.5 dB in sound power level reduction while the thrust penalty is below 1.5%. This demonstrates that the aerodynamic effects of a leading-edge treatment should be taken into account during the design phase to fully benefit from its noise reduction capability, Wind Energy

This paper investigates the mechanisms by which flow-permeable materials provide noise reduction in an installed jet configuration. Numerical simulations are carried out with a flat plate placed in the near field of a single-stream subsonic jet (Ma=0.3 and Ma=0.5). The trailing-edge region of the plate is replaced by three different permeable structures, with different properties: a metal foam, a perforated plate and a diamond-shaped structure. Due to its complex geometry, the metal foam is modeled with an equivalent region governed by Darcy's law. The perforated plate has the highest resistivity among all investigated configurations, whereas the metal foam and diamond inserts have similar properties. Far-field spectra show significant noise reduction when the solid trailing edge is replaced with the permeable materials, with a maximum decrease of 12 dB at St=0.12, for Ma=0.5. Beamforming results show that the dominant acoustic source is located at the solid–permeable junction for the metal foam and diamond structures, whereas the perforated one has the source positioned near the trailing edge, similarly to the solid case. A breakdown of the far-field noise generated by the plate is also performed, where the contributions from the solid and permeable sections are computed separately. The former has distinct regions in the noise spectrum, which are dominated either by surface pressure fluctuations or trailing-edge scattering. However, for the permeable region, the results point to a significant mitigation of the noise due to scattering, which is no longer the dominant mechanism in any frequency range. This is confirmed by lower values of spanwise coherence, computed from the surface pressure, for the permeable trailing edge, compared to the solid case. Therefore, the clear dominant mechanism in the permeable region of the plate is the unsteady loading to pressure wave impingement. This is verified for all investigated configuration, Wind Energy

The application of an isolated roughness element in the laminar boundary layer developing on the surface of a wing, introduces flow instabilities that eventually lead to the breakdown of the laminar flow structures and the formation of a turbulent wedge. The present work, investigates the instabilities and transition process initiated by an isolated roughness element applied in a swept wing boundary layer. Specifically, the perturbations induced by a cylindrical element are analysed, providing relevant insights regarding the nature of the instabilities developing in the flow field. The global flow features are measured through infrared thermography, while local information on the stationary and unsteady disturbances are provided by hot-wire anemometry. The collected results, prove that the main instabilities responsible for the wedge origin and evolution are related to the shedding process initiated in the wake of the roughness element. Additionally, the dominant flow features identified in the present work, show significant similarities with those pertaining to 2D boundary layer transition initiated by isolated roughness elements., Aerodynamics, Wind Energy

A noise footprint prediction framework for propeller-driven aircraft which couples an aerodynamic model and several aeroacoustic models is presented in this study. The aerodynamic model is based on the blade element momentum theory, while the aeroacoustic models are based on a time-domain compact dipole/monopole Ffowcs-Williams and Hawking's acoustic analogy, a trailing edge noise model, and a noise hemisphere database approach including a straight-ray propagation model, respectively. In order to reduce the runtime, the frequency-domain acoustic formulation, derived by Hanson (1980), is implemented and validated against the compact dipole/monopole Ffowcs-Williams and Hawking's acoustic analogy. The framework evaluates the acoustic effects of variations in the design and operating conditions of a propeller in forward flight. Noise footprints, obtained with different propeller configurations having varying advance ratio and number of blades are compared. It is found that, for a given thrust, a drop in advance ratio alters the source directivity dramatically, which resulting in a variation of up to 30 dBA on the acoustic footprint. When the advance ratio is kept the same and the number of blades increases from 5 to 7, the variation becomes 16 dBA due to the change in the source directivity, but the maximum noise level remains the same. The latter condition reduces the loading for each blade, and consequently the associated noise. However, the total noise remains unchanged as a consequence of increasing thickness noise due to the lower advance ratio, high blade tip Mach number, and addition of extra blades., Aerodynamics, Wind Energy & Propulsion, Wind Energy

This paper presents a computational study of flow incidence effects on the aeroacoustics of a propeller operating at low blade-tip Mach numbers. The numerical flow solution is obtained by using the Lattice-Boltzmann/Very Large Eddy Simulation method, while far-field noise is computed through the Ffowcs-Williams & Hawkings' acoustic analogy applied on the propeller surface. The presence of an angular inflow leads to: (i) the radiation of tonal loading noise along the propeller axis; (ii) the increment/reduction of the sound pressure level in the region from/to which the propeller is tilted away/towards. However, contrarily to propellers operating at high blade-tip Mach numbers, the noise directivity change is found to be governed only by the rise of periodic unsteady loadings, with the modulation of the strength of the noise sources on the blade, associated to the periodic variation of the observer-source relative Mach number (in the blade reference frame), being negligible. Finally, thickness noise and turbulent boundary-layer trailing-edge noise did not show a significant directivity variation due to the propeller yaw angle change., Wind Energy

This paper presents an experimental investigation of a propeller operating at low Reynolds numbers and provides insights into the role of aerodynamic flow features on both propeller performances and noise generation. A propeller operating at a tip Reynolds number regime of 4.3 × 104 − 4.38 × 104 is tested in an anechoic wind tunnel at an advance ratio ranging from 0 to 0.6. Noise is measured by means of a microphone array, while aerodynamic forces are measured with load and torque cells. Oil-flow visualizations are used to show the flow patterns on the blade surface, whereas phase-locked stereoscopic particle image velocimetry (PIV) measurements are carried out to analyze the flow at 60% of the blade radius. The pressure field around the blade section has been computed from the PIV velocity data. Results reveal a complex flowfield with the appearance of a laminar separation bubble at the suction side of the blade. The separation bubble moves toward the leading edge and reduces in size as the advance ratio decreases. At an advance ratio equal to 0.6, the flowfield is characterized by a laminar separation without reattachment. This causes vortex shedding responsible for a high-frequency hump in the far-field noise spectra., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Wind Energy

The mid-span region of wind turbine blades can be thickened to fulfil the structural requirements of the blade. Hence, thick airfoils, that were designed to operate at the root region of the blade, are moved to the mid-span region. This could not imply remarkable variations of the blade performance once its surface is smooth. However, the sensitivity of thick airfoils to roughness could cause significant aerodynamic impacts such as flow separation. This research aims to quantify the impact of the blade thickness, under smooth and rough conditions, in the annual energy production and the fatigue loads of the blade. Ten blade designs, linearly interpolated in thickness, are studied employing aero-elastic computations. The results reveal that the thickest blade increases the annual energy production by 5% with respect to the thinnest blade under rough conditions. Whereas this increase is less than 1% under smooth conditions. The loss of annual energy production varies with the blade thickness linearly for thin blades while it varies exponentially for thick blades up to 22%. Fatigue loads assessment confirmed a reduction of the damage equivalent load under smooth conditions, whereas the thickest blade increased it 28% under rough conditions., Wind Energy, Aerodynamics, Wind Energy & Propulsion

The present study reports a novel wavelet-based method aimed at separating the noise emitted by a single propeller into two contributions, tonal and broadband. An assessment using two different experimental investigations of propellers operating in diverse configurations is presented. The first experiment focuses upon near-field polar microphone array data of a benchmarked low-Reynolds number propeller, in hover and cruise conditions. Measurements were performed in the anechoic tunnel (A-Tunnel) at the low-speed laboratory of TU Delft. The second set of data consists of a test campaign carried out at the Pininfarina Aerodynamic and Aeroacoustic Research Center in Turin (Italy) under the EU funded project ERaCLE. The model comprises a five-bladed propeller installed close to a wing. Pressure signals were acquired using a top-mounted linear microphone array that spans different polar locations. The wavelet-based algorithm able to separate the tonal and broadband contributions through the computation of two-point statistics. The assessment of the decomposition procedure on two very different databases is presented to validate the technique with the aim to extend its range of applications., Wind Energy

The application of a porous coating to a cylinder can significantly reduce the vortex shedding tone when placed in a uniform flow. The mechanism of how this porous media attenuates vortex shedding has been studied more in recent years yet it is still not fully understood. Typical porous materials within a certain range of porosity and airflow resistivity, such as metal foam and porous polyurethane, have been studied extensively; however, the fundamental flow mechanisms responsible for vortex shedding attenuation are very difficult to determine. For example, it is nearly impossible to visualize the internal flow field of porous media with a randomized open-cell internal structure. A Structured Porous Coated Cylinder (SPCC) was designed in recent years to alleviate this internal flow field problem, as the SPCC has clear line of sight along the span and radial direction. SPCC variations have been previously studied and shown to reduce the vortex shedding tone of a bare cylinder in a very similar manner as a randomized porous coated cylinder. In this paper, we present a Tomographic Particle Image Velocimetry study of an SPCC tested in a water tunnel, revealing the previously unseen internal and near-wall flow fields of an SPCC. The flow is visualized in the porous layers, revealing complex interaction between the freestream flow field and the porous structure. Using cross-correlation methods within the flow field, we reveal the entrainment of the flow within the porous layers. Furthermore, implementation of Proper Orthogonal Decomposition shows that vortex shedding occurs within the porous layers., Control & Simulation, Reflection & Lifestyle, Wind Energy

An experimental investigation is conducted to study the aerodynamic behavior of a two-rotor system in ground proximity. The counter-rotating rotors are placed side-by-side in the hovering condition. The time-averaged and unsteady flow behavior is studied when the rotor-to-rotor lateral distance and the distance between the rotors and the ground are varied. The experiments are performed using three-dimensional large-scale volumetric velocimetry with helium-filled soap bubbles as tracers, tracked by the particle motion analysis technique “Shake-The-Box.” The mean velocity field reveals the wake deflection due to the ground plane and the formation of toroidal-shape regions of separated flow below each rotor. The interaction of the wall jets formed by slipstream deflection results in a separation line with the flow emerging from the wall in a fountain-like pattern. Regimes of flow re-ingestion occur when the rotors are sufficiently far apart. The flowfield exhibits the tendency toward asymmetric states, during which the fountain flow column and the domain of re-ingestion shift closer to one of the rotors. A generic classification of flow regimes is proposed in relation to the behavior of two rotors in ground effect., Wind Energy, Aerodynamics

This paper examines the use of both flow control and passive low noise technologies to reduce the aerodynamic noise radiated from a modified LAGOON landing gear, as tested in the EU funded H2020 collaborative research project: INVENTOR, InnoVative dEsign of iNstalled airframe componenTs for aircraft nOise Reduction. At approach to landing, landing gear noise is still a significant contributor to environmental noise in the vicinity of airports. Progress is being made with ambitious projects which aim to develop significantly reconfigured aircraft architectures to reduce airframe noise. The current project examines noise abatement measures which could be retrofit to existing landing gear configurations. Flow control in the form of low TRL "air curtains" which form a fluidic shield or virtual fairing are examined. Amongst the most interesting passive solutions are a selection of higher TRL porous materials in the form of wire mesh, perforated plates and 3D materials. In order to provide a simplified baseline landing gear mock-up on which to test the low noise technologies, the LAGOON NLG is modified with the addition of a torque-link and brakes and is called the "LAGOON-SLG". The porous materials are assessed experimentally in the A-Tunnel aeroacoustic facility in TU Delft, the Netherlands and the air curtains are examined in DLRs AWB aeroacoustic facility in Braunschweig, Germany., Wind Energy

The application of a porous coating to a cylinder can significantly reduce the vortex shedding tone when placed in a uniform flow. The mechanism of how this porous media attenuates vortex shedding has been studied more in recent years yet it is still not fully understood. Typical porous materials within a certain range of porosity and airflow resistivity, such as metal foam and porous polyurethane, have been studied extensively; however, the fundamental flow mechanisms responsible for vortex shedding attenuation are very difficult to determine. For example, it is nearly impossible to visualize the internal flow field of porous media with a randomized open-cell internal structure. A Structured Porous Coated Cylinder (SPCC) was designed in recent years to alleviate this internal flow field problem, as the SPCC has clear line of sight along the span and radial direction. SPCC variations have been previously studied and shown to reduce the vortex shedding tone of a bare cylinder in a very similar manner as a randomized porous coated cylinder. In this paper, we present a Tomographic Particle Image Velocimetry study of an SPCC tested in a water tunnel, revealing the previously unseen internal and near-wall flow fields of an SPCC. The flow is visualized in the porous layers, revealing complex interaction between the freestream flow field and the porous structure. Using cross-correlation methods within the flow field, we reveal the entrainment of the flow within the porous layers. Furthermore, implementation of Proper Orthogonal Decomposition shows that vortex shedding occurs within the porous layers., Wind Energy

The time resolved flow and acoustic fields around a simplified two-wheel nose landing gear configuration featuring brakes, torque link, and a detachable fairing are investigated by numerical and experimental means. The flow field is computed by a lattice Boltzmann method with a collision step based on countable cumulants. Numerical near field flow predictions are validated by experimental results from Delft University of Technology. The comparison shows the quality of the applied computational setup and methodology. The favorable noise mitigation properties of the installed fairing are discussed., Wind Energy

This work presents the first reported experimental characterization of the flow field in the direct vicinity of discrete roughness elements (DRE), in a swept wing boundary layer. High magnification tomographic Particle Tracking Velocimetry (3D-PTV) measurements are used to acquire time-averaged velocity and standard deviation fields in a 3D volume directly aft of the DRE elements. The collected data detail the near-element flow topology, providing information on the developing wake and emerging flow structures, their organization and amplitude evolution. A transient growth behaviour is identified in the element wake, while onset and growth of crossflow instabilities is observed further downstream. As such, the near element flow is confirmed to be a fundamental part of the receptivity process, contributing in setting the initial amplitudes for the crossflow instability evolution., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Aerodynamics, Wind Energy

Microphone measurements in a closed test section wind tunnel are affected by turbulent boundary layer (TBL) pressure fluctuations. These fluctuations are mitigated by placing the microphones at the bottom of cavities, usually covered with a thin, acoustically transparent material. Prior experiments showed that the cavity geometry affects the propagation of TBL pressure fluctuations toward the bottom. However, the relationship between the cavity geometry and the flowfield within the cavity is not well understood. Therefore, a very large-eddy simulation was performed using the lattice Boltzmann method. A cylindrical, a countersunk and a conical cavity are simulated with and without a fine wire-cloth cover, which is modeled as a porous medium governed by Darcy's law. Adding a countersink to an uncovered cylindrical cavity is found to mitigate the transport of turbulent structures across the bottom by shifting the recirculation pattern away from the cavity bottom. Covering the cavities nearly eliminates this source of hydrodynamic pressure fluctuations. The eddies within the boundary layer, which convect over the cover, generate a primarily acoustic pressure field inside the cavities and thus suggesting that the pressure fluctuations within covered cavities can be modeled acoustically. As the cavity diameter increases compared to the eddies' integral length scale, the amount of energy in the cut-off modes increases with respect to the cut-on modes. Since cut-off modes decay as they propagate into the cavity, more attenuation is seen. The results are in agreement with experimental evidence., Wind Energy, Aircraft Noise and Climate Effects, Control & Operations

A Helmholtz resonator with a curved cavity is studied for reducing jet-installation noise in a configuration comprised by a subsonic jet and a nearby flat plate. The face-sheet and cavity are designed with the Guess method and the impedance is verified through an experiment and a simulation of an impedance tube. Both single and double degree-of-freedom liners, the latter with a perforated septum inside the cavity, are studied. A good agreement is obtained for the impedance curves from the design method and the experimental and numerical impedance tube, particularly at frequencies close to the absorption peak. Numerical simulations of the installed jet are performed with an array of resonators placed inside the plate with the face-sheet on the lower side, targeting noise reduction at a ground observer. Far-field spectra show that noise reduction in the order of 7 dB is obtained with respect to the baseline solid plate, at the resonance frequency of the single degree-of-freedom liner. Moreover, there is a broad frequency range around the resonance in which the sound pressure levels of the lined plate are lower. For the configuration with the double degree-of-freedom resonator, further noise reduction (approximately 3 dB) is obtained at higher frequencies, around the second peak in the absorption coefficient curve. The results also show that a slight noise reduction (2 - 3 dB) occurs for an observer on the shielded side of the plate, but in a significantly narrower band around the resonance frequency. This is attributed to the absorption of acoustic waves from the jet itself prior to their scattering at the trailing edge, coupled with a less abrupt impedance discontinuity at the trailing edge. Consequently, the resonators also act by reducing the strength of the acoustic source at the designed resonance frequency., Wind Energy

High-fidelity numerical simulations have been performed to investigate the noise reduction capability of porous leading-edge treatments for mitigating rotor-stator interaction noise in a full-scale aircraft model. The aircraft model consists of the NASA High-Lift Common Research Model (HL-CRM) airframe combined with an up-scaled Source Diagnostic Test (CRM) fan stage. The leading edge of the fan-stage stator blades is made of porous serrations, modelled using the properties of a metal foam. It is found that the porous serrations induce flow separation at the suction side of the stator blades, especially in the inner span regions. As a result, the modified fan stage produces smaller thrust, and the broadband noise emission in the low-frequency range is enhanced. Nevertheless, the tonal noise components at the blade-passage frequency and its harmonics are mitigated by up to 6dB. By limiting the usage of porosity near the tip of the stator blade, the adverse aerodynamic effects can be mitigated, improving the overall benefit of the porous serrations. Compared to the baseline case, such treatment leads to a total sound power level reduction 1.5dB and a thrust penalty that is below 1.5%. Hence, this study suggests that the aerodynamic effects of a porous leading-edge treatment should not be neglected in the design phase in order to maximise the noise reduction benefits., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Wind Energy

This paper explains the presence and relevance of noise caused by a laminar separation bubble (LSB) on a propeller operating at low-Reynolds number. Microphone measurements of a propeller with both clean and forced boundary layer transition blades are carried out in an anechoic wind tunnel by varying the propeller advance ratio J from 0 to 0.6, corresponding to a tip Reynolds number ranging from 4.3 · 10 4 to 10 5 . The flow behaviour on the blade surface and around the propeller is investigated with oil-flow visualizations and particle image velocimetry. At J = 0.4 and 0.6, vortex shedding from the LSB causes high-frequency noise which appears as a hump in the far-field noise spectra. Forcing the location of the boundary layer transition suppresses the LSB and, consequently, the hump, reducing the noise emission of about 5 and 10 dB at J = 0.4 and 0.6, respectively. The fact that the hump is caused by LSB vortex shedding noise is further assessed by using a semi-empirical noise model; it shows that the hump is constituted by tones of different amplitudes and frequencies, emitted at different spanwise sections along the blade., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Wind Energy

The presented work introduces a cancellation technique, based on the linear superposition of stationary crossflow instabilities (CFIs) through the application of a streamwise series of optimally positioned discrete roughness element (DRE) arrays on a swept wing surface. The DRE arrays are designed and arranged with suitable amplitude and phase shift to induce velocity disturbance systems that destructively interact, ultimately damping the developing CFIs. The robustness of this technique is investigated for a smooth wing surface as well as in the presence of enhanced distributed surface roughness. The resulting flow fields are measured with infrared thermography and particle tracking velocimetry, allowing for the extraction of the laminar-to-turbulent transition front location and for the characterization of the local boundary layer development. The acquired data show that the superposition of suitably arranged DRE arrays can successfully suppress monochromatic CFIs, reducing their amplitude and growth and delaying the boundary layer transition to turbulence when applied on a smooth wing surface. However, the presence of elevated background roughness significantly reduces the effectiveness of the proposed method., Aerodynamics, Wind Energy

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., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Wind Energy

This work presents a novel noise propagation approach based on the Gaussian Beam Tracing (GBT) method that accounts for complex source directivity, weather conditions, and irregular ground topology for the evaluation of the noise footprint. The approach takes a precomputed noise sphere as input and propagates the acoustic pressure fluctuations through a moving inhomogeneous atmosphere over realistic three-dimensional (3D) terrain. Noise footprints, obtained with di erent source noise spheres and wind flow conditions, are compared. It is found that, in a quiescent atmosphere, a change in the source directivity results in a variation up to 15 dB on the acoustic footprint. In the presence of the mean flow, the variation in the noise footprint can reach up to 35 dB. The results suggest that any variation in the source directivity and wind flow can cause a significant change in the acoustic footprint predicted in 3D environments with varying terrain topology and wind flow., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Wind Energy

This paper explains the presence and relevance of noise caused by a laminar separation bubble (LSB) on a propeller operating at a low Reynolds number. Microphone measurements of a propeller with both clean and forced boundary-layer transition blades are carried out in an anechoic wind tunnel by varying the propeller advance ratio J from 0 to 0.6, corresponding to a tip Reynolds number ranging from 4.3⋅104 to 105. The flow behavior on the blade surface and around the propeller is investigated with oil-flow visualizations and particle image velocimetry. At J=0.4 and 0.6, vortex shedding from the LSB causes high-frequency noise that appears as a hump in the far-field noise spectra. Forcing the location of the boundary-layer transition suppresses the LSB and, consequently, the hump, reducing the noise emission of about 5 and 10 dB at J=0.4 and 0.6, respectively. The fact that the hump is caused by LSB vortex shedding noise is further assessed by using a semi-empirical noise model; it shows that the hump is constituted by tones of different amplitudes and frequencies, emitted at different spanwise sections along the blade., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Wind Energy