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D. Casalino
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This work investigates the robustness of a transonic FW–H formulation for rotating permeable surfaces, developed to enable stable acoustic integration when the permeable surface moves at sonic conditions relative to the observer. The method, based on the desingularized Formulations 1-DS and 1A-DS by Casalino, is assessed through comparison with classical FW–H approaches. Results show that the de-singularized formulation provides consistent far-field noise predictions, preserving high-frequency content by enabling the use of integration surfaces tightly fitted to the blade geometry. By comparison, the solid formulation underestimates acoustic levels due to the absence of quadrupole contributions, while the classical permeable formulation attenuates high-frequency content as a result of numerical dissipation when the integration surface is located away from the source region. A practical guideline is also provided for selecting the time-step ratio σ between the FW–H and CFD time steps, whose value controls the balance between signal smoothing and maximum resolved frequency. Overall, the proposed formulation offers a robust and efficient approach for aeroacoustic predictions in transonic propeller applications.
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This work investigates the robustness of a transonic FW–H formulation for rotating permeable surfaces, developed to enable stable acoustic integration when the permeable surface moves at sonic conditions relative to the observer. The method, based on the desingularized Formulations 1-DS and 1A-DS by Casalino, is assessed through comparison with classical FW–H approaches. Results show that the de-singularized formulation provides consistent far-field noise predictions, preserving high-frequency content by enabling the use of integration surfaces tightly fitted to the blade geometry. By comparison, the solid formulation underestimates acoustic levels due to the absence of quadrupole contributions, while the classical permeable formulation attenuates high-frequency content as a result of numerical dissipation when the integration surface is located away from the source region. A practical guideline is also provided for selecting the time-step ratio σ between the FW–H and CFD time steps, whose value controls the balance between signal smoothing and maximum resolved frequency. Overall, the proposed formulation offers a robust and efficient approach for aeroacoustic predictions in transonic propeller applications.
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Conference paper
(2026)
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A. Zarri, D. van Wijk, Joren J. Van Cauwenberge, Frits de Prenter, D. Casalino, L. Hirschberg
This work presents a low-order framework to predict aerodynamic interaction and the associated tonal loading noise in closely spaced co-rotating propellers under forward-flight conditions. The modeling approach moves beyond acoustic-interference-based methods by explicitly accounting for rotor–rotor aerodynamic coupling. The wake of each propeller is modeled as a system of de-singularized rigid helical tip vortices, which induce velocity via the Biot–Savart law, both on the generating rotor and on neighboring disks. The formulation relies solely on isolated-propeller forces, computed using blade-element momentum theory. The induced velocity field is used to reconstruct the unsteady inflow and blade loading, with unsteady effects incorporated through a Sears-function-based correction. Comparisons against LB-VLES simulations of three side-by-side propellers show that the model accurately predicts the spatial distribution and phase of the unsteady thrust, with peak locations within approximately 10 deg and amplitude errors of about 30%. The resulting loading is coupled to a rotating-dipole acoustic formulation. At the blade-passing frequency (BPF), the predicted far-field directivity agrees within 1-4 dB in most directions. The model captures both the aerodynamic source modulation and the resulting constructive and destructive interference patterns in the tonal acoustic field. Owing to its low computational cost, the proposed model enables rapid assessment of installation effects in early design stages, including variations in propeller spacing and relative phase angle.
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This work presents a low-order framework to predict aerodynamic interaction and the associated tonal loading noise in closely spaced co-rotating propellers under forward-flight conditions. The modeling approach moves beyond acoustic-interference-based methods by explicitly accounting for rotor–rotor aerodynamic coupling. The wake of each propeller is modeled as a system of de-singularized rigid helical tip vortices, which induce velocity via the Biot–Savart law, both on the generating rotor and on neighboring disks. The formulation relies solely on isolated-propeller forces, computed using blade-element momentum theory. The induced velocity field is used to reconstruct the unsteady inflow and blade loading, with unsteady effects incorporated through a Sears-function-based correction. Comparisons against LB-VLES simulations of three side-by-side propellers show that the model accurately predicts the spatial distribution and phase of the unsteady thrust, with peak locations within approximately 10 deg and amplitude errors of about 30%. The resulting loading is coupled to a rotating-dipole acoustic formulation. At the blade-passing frequency (BPF), the predicted far-field directivity agrees within 1-4 dB in most directions. The model captures both the aerodynamic source modulation and the resulting constructive and destructive interference patterns in the tonal acoustic field. Owing to its low computational cost, the proposed model enables rapid assessment of installation effects in early design stages, including variations in propeller spacing and relative phase angle.
Time-domain impedance boundary conditions for liners that are not based on parameter fitting continue to pose a significant numerical challenge in computational aeroacoustics. This work presents a superposition-based time-domain impedance boundary condition for acoustic liners, including multiple-degree-of-freedom designs and acoustic metamaterials. Three discrete convolution formulations are examined: one using an admittance transfer function, and two using a reflection coefficient transfer function, applied explicitly and implicitly. The study is carried out in one dimension, and the accuracy of each method is assessed by simulating the interaction between an acoustic wave and a liner. Results for a single-degree-of-freedom analytical liner show agreement with frequency-domain benchmark solutions, demonstrating the accuracy of the method. The approach is then extended to two non-analytical realistic liner configurations by reconstructing the required transfer-function data from numerical impedance tube simulations. The framework is applied to both configurations and exhibits consistent behaviour across all three formulations. The findings support the use of this approach for broadband, non-linear, and experimentally derived impedance models.
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Time-domain impedance boundary conditions for liners that are not based on parameter fitting continue to pose a significant numerical challenge in computational aeroacoustics. This work presents a superposition-based time-domain impedance boundary condition for acoustic liners, including multiple-degree-of-freedom designs and acoustic metamaterials. Three discrete convolution formulations are examined: one using an admittance transfer function, and two using a reflection coefficient transfer function, applied explicitly and implicitly. The study is carried out in one dimension, and the accuracy of each method is assessed by simulating the interaction between an acoustic wave and a liner. Results for a single-degree-of-freedom analytical liner show agreement with frequency-domain benchmark solutions, demonstrating the accuracy of the method. The approach is then extended to two non-analytical realistic liner configurations by reconstructing the required transfer-function data from numerical impedance tube simulations. The framework is applied to both configurations and exhibits consistent behaviour across all three formulations. The findings support the use of this approach for broadband, non-linear, and experimentally derived impedance models.
Building-generated turbulence can significantly influence the propagation of noise from advanced air mobility (AAM) vehicles operating in urban environments, yet its impact on acoustic variability remains poorly quantified. In this study, the effect of an isolated building wake on sound propagation is investigated using time-resolved Lattice-Boltzmann very-large-eddy simulations. A simplified tonal acoustic source representative of an AAM vehicle is placed downstream of the building, and the resulting unsteady sound field is analyzed within and downstream of the turbulent wake. The results show that wake-induced turbulence produces pronounced temporal fluctuations in the received sound pressure level, with variability exceeding 3 dB in localized regions. These fluctuations extend beyond the physical extent of the wake due to interference effects and reflected propagation paths from the building and ground. Analysis along selected propagation directions indicates a strong correlation between turbulence-induced velocity fluctuations and acoustic variability along direct propagation paths, while this correlation weakens in regions dominated by multiple reflections. The findings emphasize the importance of accounting for unsteady, building-induced flow effects when evaluating AAM noise in urban environments.
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Building-generated turbulence can significantly influence the propagation of noise from advanced air mobility (AAM) vehicles operating in urban environments, yet its impact on acoustic variability remains poorly quantified. In this study, the effect of an isolated building wake on sound propagation is investigated using time-resolved Lattice-Boltzmann very-large-eddy simulations. A simplified tonal acoustic source representative of an AAM vehicle is placed downstream of the building, and the resulting unsteady sound field is analyzed within and downstream of the turbulent wake. The results show that wake-induced turbulence produces pronounced temporal fluctuations in the received sound pressure level, with variability exceeding 3 dB in localized regions. These fluctuations extend beyond the physical extent of the wake due to interference effects and reflected propagation paths from the building and ground. Analysis along selected propagation directions indicates a strong correlation between turbulence-induced velocity fluctuations and acoustic variability along direct propagation paths, while this correlation weakens in regions dominated by multiple reflections. The findings emphasize the importance of accounting for unsteady, building-induced flow effects when evaluating AAM noise in urban environments.
Journal article
(2026)
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Angelo Paduano, Francesco Scarano, Julio Cordioli, Damiano Casalino, Francesco Avallone
The interaction between acoustic waves and turbulent grazing flow over an acoustic liner is investigated using lattice-Boltzmann very-large-eddy simulations. A single-degree-offreedom liner with 11 streamwise-aligned cavities is studied in a grazing flow impedance tube. The conditions replicate reference experiments from the Federal University of Santa Catarina. The influence of grazing flow (with a centreline Mach number of 0.32), acoustic wave amplitude, frequency and propagation direction relative to the mean flow is analysed. Impedance is computed using both direct (i.e. the in situ method) and modelfitting inference (i.e. the mode-matching) methods. The former reveals strong spatial variations; however, averaged values throughout the sample show minimal differences between upstream- and downstream-propagating waves, in contrast to what is obtained with the latter method. Flow analyses reveal that the orifices displace the flow away from the face sheet, with this effect amplified by acoustic waves and dependent on the wave propagation direction. Consequently, the boundary layer displacement thickness (δ∗) increases along the streamwise direction compared with a smooth wall and exhibits localised humps downstream of each orifice. The growth of δ∗ alters the flow dynamics within the orifices by weakening the shear layer at downstream positions. This influences the acoustic-induced mass flow rate through the orifices at equal sound pressure level, suggesting that acoustic energy is dissipated differently along the liner. The asymmetry of the flow field experienced by the acoustic wave, depending on its propagation direction,highlights the need to consider a spatially evolving turbulent flow when studying the acoustic–flow interaction and measuring impedance.
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The interaction between acoustic waves and turbulent grazing flow over an acoustic liner is investigated using lattice-Boltzmann very-large-eddy simulations. A single-degree-offreedom liner with 11 streamwise-aligned cavities is studied in a grazing flow impedance tube. The conditions replicate reference experiments from the Federal University of Santa Catarina. The influence of grazing flow (with a centreline Mach number of 0.32), acoustic wave amplitude, frequency and propagation direction relative to the mean flow is analysed. Impedance is computed using both direct (i.e. the in situ method) and modelfitting inference (i.e. the mode-matching) methods. The former reveals strong spatial variations; however, averaged values throughout the sample show minimal differences between upstream- and downstream-propagating waves, in contrast to what is obtained with the latter method. Flow analyses reveal that the orifices displace the flow away from the face sheet, with this effect amplified by acoustic waves and dependent on the wave propagation direction. Consequently, the boundary layer displacement thickness (δ∗) increases along the streamwise direction compared with a smooth wall and exhibits localised humps downstream of each orifice. The growth of δ∗ alters the flow dynamics within the orifices by weakening the shear layer at downstream positions. This influences the acoustic-induced mass flow rate through the orifices at equal sound pressure level, suggesting that acoustic energy is dissipated differently along the liner. The asymmetry of the flow field experienced by the acoustic wave, depending on its propagation direction,highlights the need to consider a spatially evolving turbulent flow when studying the acoustic–flow interaction and measuring impedance.
Conference paper
(2026)
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Angelo Paduano, Francesco Scarano, D. Casalino, Júlio A. Cordioli, E.F. Avallone
This work investigates the influence of orifice-edge geometry on the
aeroacoustic performance of perforated acoustic liners exposed to
turbulent grazing flow. Two liner geometries are compared. They differ
only for the shape of the facesheet orifices. One has sharp edge
orifices, while the other chamfered edge orifices. The internal
diameter, the facesheet thickness and the cavity depth is the same.
High-fidelity lattice-Boltzmann very-large-eddy simulations are
performed and compared with experimental measurements to assess both the
acoustic response and the underlying flow physics. Impedance eduction
reveals that the sharp-edged liner exhibits up to 50\% higher acoustic
resistance over the investigated frequency range, whereas the reactance
remains broadly similar, apart from a shift in resonance frequency from
approximately 1.7 to 1.9 kHz. Flow-field analysis indicates that the
chamfered geometry promotes stronger momentum exchange and weaker shear
layers strength above the orifices, effectively behaving as a more
permeable surface. These findings show that small manufacturing-scale
variations in orifice-edge shape can significantly alter both the
aerodynamic development and the acoustic attenuation of liners under
grazing flow, highlighting the need to account for edge geometry in
liner design and predictive modeling.
...
This work investigates the influence of orifice-edge geometry on the
aeroacoustic performance of perforated acoustic liners exposed to
turbulent grazing flow. Two liner geometries are compared. They differ
only for the shape of the facesheet orifices. One has sharp edge
orifices, while the other chamfered edge orifices. The internal
diameter, the facesheet thickness and the cavity depth is the same.
High-fidelity lattice-Boltzmann very-large-eddy simulations are
performed and compared with experimental measurements to assess both the
acoustic response and the underlying flow physics. Impedance eduction
reveals that the sharp-edged liner exhibits up to 50\% higher acoustic
resistance over the investigated frequency range, whereas the reactance
remains broadly similar, apart from a shift in resonance frequency from
approximately 1.7 to 1.9 kHz. Flow-field analysis indicates that the
chamfered geometry promotes stronger momentum exchange and weaker shear
layers strength above the orifices, effectively behaving as a more
permeable surface. These findings show that small manufacturing-scale
variations in orifice-edge shape can significantly alter both the
aerodynamic development and the acoustic attenuation of liners under
grazing flow, highlighting the need to account for edge geometry in
liner design and predictive modeling.
This work presents a high-fidelity aeroacoustic investigation of a distributed-propulsion eVTOL aircraft during representative urban air mobility departure operations. Three flight conditions are analyzed, namely vertical take-off, transition, and cruise in climb. The aerodynamic analysis highlights substantial differences in wake development and rotor–rotor interactions among the investigated configurations. For the particular vehicle configuration, cruise conditions are characterized by the ingestion of coherent wake structures by the tail-mounted propellers, resulting in significant unsteady loading fluctuations. The aeroacoustic analysis, performed using the permeable and solid formulations of the Ffowcs Williams–Hawkings integral equation, shows that the radiated noise is dominated by tonal components, with cruise conditions exhibiting blade passing frequency levels approximately 10 dB higher than the other configurations. Source identification analyses indicate that the dominant acoustic contribution is associated with the unsteady loading noise generated by the tail-mounted propellers due to wake ingestion. Finally, an on-ground footprint analysis based on a complete reference departure trajectory demonstrates compliance with current EASA certification limits, while also highlighting the strong sensitivity of the acoustic footprint to the transition maneuver. The results highlight the importance of rotor wake interactions in cruise conditions for short-aspect- ratio distributed-propulsion eVTOL configurations, where wake ingestion by downstream propellers significantly increases tonal noise levels and directly affects the resulting certification footprint. These findings emphasize the need to account for rotor interaction effects during both preliminary design and acoustic certification assessment.
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This work presents a high-fidelity aeroacoustic investigation of a distributed-propulsion eVTOL aircraft during representative urban air mobility departure operations. Three flight conditions are analyzed, namely vertical take-off, transition, and cruise in climb. The aerodynamic analysis highlights substantial differences in wake development and rotor–rotor interactions among the investigated configurations. For the particular vehicle configuration, cruise conditions are characterized by the ingestion of coherent wake structures by the tail-mounted propellers, resulting in significant unsteady loading fluctuations. The aeroacoustic analysis, performed using the permeable and solid formulations of the Ffowcs Williams–Hawkings integral equation, shows that the radiated noise is dominated by tonal components, with cruise conditions exhibiting blade passing frequency levels approximately 10 dB higher than the other configurations. Source identification analyses indicate that the dominant acoustic contribution is associated with the unsteady loading noise generated by the tail-mounted propellers due to wake ingestion. Finally, an on-ground footprint analysis based on a complete reference departure trajectory demonstrates compliance with current EASA certification limits, while also highlighting the strong sensitivity of the acoustic footprint to the transition maneuver. The results highlight the importance of rotor wake interactions in cruise conditions for short-aspect- ratio distributed-propulsion eVTOL configurations, where wake ingestion by downstream propellers significantly increases tonal noise levels and directly affects the resulting certification footprint. These findings emphasize the need to account for rotor interaction effects during both preliminary design and acoustic certification assessment.
Turbulence from densely built urban structures alters the acoustic signature of advanced air mobility (AAM) vehicles, complicating prediction of noise impact. Ray tracing, using instantaneous frozen velocity-field snapshots from time-resolved simulations, provides an efficient approach for estimating turbulence-induced acoustic variability. Comparisons of the equivalent sound level ( ), variability metric ( ), and transient sound-level fluctuations show good agreement with a time-resolved reference solution, where discrepancies are mainly near the source and ground. Correlation analysis confirms that dominant temporal variability trends are reproduced at most observer locations, demonstrating that the method provides a reliable and computationally efficient framework for assessing AAM noise in urban environments.
...
Turbulence from densely built urban structures alters the acoustic signature of advanced air mobility (AAM) vehicles, complicating prediction of noise impact. Ray tracing, using instantaneous frozen velocity-field snapshots from time-resolved simulations, provides an efficient approach for estimating turbulence-induced acoustic variability. Comparisons of the equivalent sound level ( ), variability metric ( ), and transient sound-level fluctuations show good agreement with a time-resolved reference solution, where discrepancies are mainly near the source and ground. Correlation analysis confirms that dominant temporal variability trends are reproduced at most observer locations, demonstrating that the method provides a reliable and computationally efficient framework for assessing AAM noise in urban environments.
This paper investigates the capability of an industrial multi-purpose flow solver, SIMULIA PowerFLOW®, based on the Lattice-Boltzmann method, to predict the sonic boom characteristics of the JAXA Wing-Body JWB configuration, presented at the Second AIAA Sonic Boom Prediction Workshop, at a free-stream Mach number of 1.6. The near field is initially computed using a three-dimensional supersonic Lattice–Boltzmann scheme. The pressure signature is extracted on the vehicle symmetry plane at two near-field stations, namely (H/L)_1 and (H/L)_3, located at distances equal to 0.85 and 2.55 times the body length from the configuration, respectively. Successively, the computed (H/L)_1 near-field signature is propagated through a stratified atmosphere using the NASA PCBoom code, based on a previously validated propagation setup. At ground level, key parameters, including peak overpressure, pressure rise time, and duration, are analyzed alongside psychoacoustic metrics such as ASEL and PL to assess the accuracy of the propagated signatures. The results provide a consistent near-to-far field assessment for predicting sonic booms using a Lattice–Boltzmann flow solver.
...
This paper investigates the capability of an industrial multi-purpose flow solver, SIMULIA PowerFLOW®, based on the Lattice-Boltzmann method, to predict the sonic boom characteristics of the JAXA Wing-Body JWB configuration, presented at the Second AIAA Sonic Boom Prediction Workshop, at a free-stream Mach number of 1.6. The near field is initially computed using a three-dimensional supersonic Lattice–Boltzmann scheme. The pressure signature is extracted on the vehicle symmetry plane at two near-field stations, namely (H/L)_1 and (H/L)_3, located at distances equal to 0.85 and 2.55 times the body length from the configuration, respectively. Successively, the computed (H/L)_1 near-field signature is propagated through a stratified atmosphere using the NASA PCBoom code, based on a previously validated propagation setup. At ground level, key parameters, including peak overpressure, pressure rise time, and duration, are analyzed alongside psychoacoustic metrics such as ASEL and PL to assess the accuracy of the propagated signatures. The results provide a consistent near-to-far field assessment for predicting sonic booms using a Lattice–Boltzmann flow solver.
Cyclorotors are a unique propulsion type offering rapid, 360° thrust vectoring, which is especially attractive for urban air mobility (UAM) applications. For UAM, noise is a key consideration. However, there is currently little research into cyclorotor noise. This study presents the first high-fidelity aeroacoustic simulations of cyclorotors, using the lattice Boltzmann method with very large eddy simulation (LBM-VLES). A detailed investigation of the noise-generating mechanisms is conducted. Furthermore, a comparison is made with a conventional propeller of equivalent dimensions that provides the same thrust. The results show that cyclorotor noise is dominated by unsteady loading associated with blade-vortex interactions, which offsets the acoustic benefit of the lower blade velocity. In our results, the cyclorotor is not inherently quieter than a conventional propeller operating at similar thrust under isolated conditions.
...
Cyclorotors are a unique propulsion type offering rapid, 360° thrust vectoring, which is especially attractive for urban air mobility (UAM) applications. For UAM, noise is a key consideration. However, there is currently little research into cyclorotor noise. This study presents the first high-fidelity aeroacoustic simulations of cyclorotors, using the lattice Boltzmann method with very large eddy simulation (LBM-VLES). A detailed investigation of the noise-generating mechanisms is conducted. Furthermore, a comparison is made with a conventional propeller of equivalent dimensions that provides the same thrust. The results show that cyclorotor noise is dominated by unsteady loading associated with blade-vortex interactions, which offsets the acoustic benefit of the lower blade velocity. In our results, the cyclorotor is not inherently quieter than a conventional propeller operating at similar thrust under isolated conditions.
Recent studies on distributed electric propulsion systems suggest phase synchronization between rotors as a noise reduction strategy. However, the aerodynamic interactions between propellers' near fields and their influence on far-field tonal noise remain poorly understood, partly due to experimental limitations in microphone placement. This paper addresses this gap through lattice Boltzmann very large eddy simulations of three adjacent, co-rotating rotors, spaced radially at 2% of their diameter, to investigate how relative phase angle affects tonal noise directivity. Results reveal that proximity-induced aerodynamic interactions generate dominant tonal noise in most spatial directions, driven by two mechanisms: time-averaged inflow distortion from nearby propellers and impulsive local effects at blade tips, with the latter influenced by phase angle. While the directivity pattern of the blade-passing frequency harmonic tone remains consistent across phase angles, comparing cases with zero relative phase (blades aligned) and opposite-phase conditions shows sound pressure level shifts of up to 4.5 dB along the primary noise axis, namely, along the inflow direction. Conversely, acoustic interference significantly alters noise directivity, especially in opposite-phase conditions where sound is nearly canceled in specific directions. These findings highlight rotor synchronization as a promising strategy for reducing noise emissions toward sensitive areas.
...
Recent studies on distributed electric propulsion systems suggest phase synchronization between rotors as a noise reduction strategy. However, the aerodynamic interactions between propellers' near fields and their influence on far-field tonal noise remain poorly understood, partly due to experimental limitations in microphone placement. This paper addresses this gap through lattice Boltzmann very large eddy simulations of three adjacent, co-rotating rotors, spaced radially at 2% of their diameter, to investigate how relative phase angle affects tonal noise directivity. Results reveal that proximity-induced aerodynamic interactions generate dominant tonal noise in most spatial directions, driven by two mechanisms: time-averaged inflow distortion from nearby propellers and impulsive local effects at blade tips, with the latter influenced by phase angle. While the directivity pattern of the blade-passing frequency harmonic tone remains consistent across phase angles, comparing cases with zero relative phase (blades aligned) and opposite-phase conditions shows sound pressure level shifts of up to 4.5 dB along the primary noise axis, namely, along the inflow direction. Conversely, acoustic interference significantly alters noise directivity, especially in opposite-phase conditions where sound is nearly canceled in specific directions. These findings highlight rotor synchronization as a promising strategy for reducing noise emissions toward sensitive areas.
Acoustic liners, passive devices to mitigate engine noise, operate under high-speed grazing flow and grazing acoustic waves. To investigate the complex physics governing this interaction, high-fidelity numerical simulations of a spatially evolving turbulent boundary layer grazing a multi-orifice acoustic liner at a bulk Mach number of 0.32 are performed. The simulations replicate conditions from a reference experiment. Grazing tonal plane acoustic waves with amplitudes of 130 dB and 145 dB and propagating in the same direction and the direction opposite to the mean flow are analyzed. The results show that the boundary layer displacement thickness doubles in the presence of the liner and its growth rate is affected by the amplitude and propagation direction of the acoustic wave. The acoustic liner also promotes the formation of an outer hump in both the logarithmic region of the streamwise and wall-normal velocity variance, with these effects becoming more pronounced under acoustic forcing. Furthermore, impedance estimation, using Dean’s method, reveals that near-wall flow modifications, quantified through the displacement thickness, influence the local value of the computed impedance.
...
Acoustic liners, passive devices to mitigate engine noise, operate under high-speed grazing flow and grazing acoustic waves. To investigate the complex physics governing this interaction, high-fidelity numerical simulations of a spatially evolving turbulent boundary layer grazing a multi-orifice acoustic liner at a bulk Mach number of 0.32 are performed. The simulations replicate conditions from a reference experiment. Grazing tonal plane acoustic waves with amplitudes of 130 dB and 145 dB and propagating in the same direction and the direction opposite to the mean flow are analyzed. The results show that the boundary layer displacement thickness doubles in the presence of the liner and its growth rate is affected by the amplitude and propagation direction of the acoustic wave. The acoustic liner also promotes the formation of an outer hump in both the logarithmic region of the streamwise and wall-normal velocity variance, with these effects becoming more pronounced under acoustic forcing. Furthermore, impedance estimation, using Dean’s method, reveals that near-wall flow modifications, quantified through the displacement thickness, influence the local value of the computed impedance.
Aeroacoustics research in Europe
The CEAS-ASC report on 2023 highlights
The Council of European Aerospace Societies (CEAS) Aeroacoustics Specialists Committee (ASC) supports and promotes the interests of the scientific and industrial aeroacoustics community on a European scale and European aeronautics activities internationally. In this context, “aeroacoustics” encompasses all aerospace acoustics and related areas. Each year the committee highlights some of the research and development projects in Europe. This paper is a report on highlights of aeroacoustics research in Europe in 2023, compiled from information provided to the ASC of the CEAS. In addition, during 2023, a number of research programmes involving aeroacoustics were funded by the European Commission. Some of the highlights from these programmes are also summarized in this article, as well as highlights from other projects funded by national governments and industry. Contributions are gathered in sections by topic, and a section covering relevant European scientific events in 2023 is also included. Enquiries concerning all contributions should be addressed to the authors who are given at the end of each subsection.
...
The Council of European Aerospace Societies (CEAS) Aeroacoustics Specialists Committee (ASC) supports and promotes the interests of the scientific and industrial aeroacoustics community on a European scale and European aeronautics activities internationally. In this context, “aeroacoustics” encompasses all aerospace acoustics and related areas. Each year the committee highlights some of the research and development projects in Europe. This paper is a report on highlights of aeroacoustics research in Europe in 2023, compiled from information provided to the ASC of the CEAS. In addition, during 2023, a number of research programmes involving aeroacoustics were funded by the European Commission. Some of the highlights from these programmes are also summarized in this article, as well as highlights from other projects funded by national governments and industry. Contributions are gathered in sections by topic, and a section covering relevant European scientific events in 2023 is also included. Enquiries concerning all contributions should be addressed to the authors who are given at the end of each subsection.
This paper investigates the prediction accuracy and time efficiency of two distinct low-fidelity methods for predicting the tonal and broadband noise of a drone rotor in axial and non-axial inflow conditions. These are both derived from an aerodynamic rotor model based on the blade element momentum theory, respectively coupled with a time- and a frequency-domain solution of the Ffowcs Williams-Hawkings integral equation applied to a radial distribution of acoustically compact and non-compact sources. Experimental data and scale resolving
lattice-Boltzmann/very-large eddy simulation results for a two-bladed small unmanned aerial system in transitional boundary layer conditions are used to validate the low-fidelity approaches. Comparison between low-fidelity, high-fidelity and experimental results reveal that the underlying sound generation mechanisms are accurately modeled by the low fidelity methods, which therefore constitute a valid tool for the preliminary design of quiet drone rotors and for the estimation of the community noise impact of drone operations. ...
lattice-Boltzmann/very-large eddy simulation results for a two-bladed small unmanned aerial system in transitional boundary layer conditions are used to validate the low-fidelity approaches. Comparison between low-fidelity, high-fidelity and experimental results reveal that the underlying sound generation mechanisms are accurately modeled by the low fidelity methods, which therefore constitute a valid tool for the preliminary design of quiet drone rotors and for the estimation of the community noise impact of drone operations. ...
This paper investigates the prediction accuracy and time efficiency of two distinct low-fidelity methods for predicting the tonal and broadband noise of a drone rotor in axial and non-axial inflow conditions. These are both derived from an aerodynamic rotor model based on the blade element momentum theory, respectively coupled with a time- and a frequency-domain solution of the Ffowcs Williams-Hawkings integral equation applied to a radial distribution of acoustically compact and non-compact sources. Experimental data and scale resolving
lattice-Boltzmann/very-large eddy simulation results for a two-bladed small unmanned aerial system in transitional boundary layer conditions are used to validate the low-fidelity approaches. Comparison between low-fidelity, high-fidelity and experimental results reveal that the underlying sound generation mechanisms are accurately modeled by the low fidelity methods, which therefore constitute a valid tool for the preliminary design of quiet drone rotors and for the estimation of the community noise impact of drone operations.
lattice-Boltzmann/very-large eddy simulation results for a two-bladed small unmanned aerial system in transitional boundary layer conditions are used to validate the low-fidelity approaches. Comparison between low-fidelity, high-fidelity and experimental results reveal that the underlying sound generation mechanisms are accurately modeled by the low fidelity methods, which therefore constitute a valid tool for the preliminary design of quiet drone rotors and for the estimation of the community noise impact of drone operations.
Laminar to turbulent transition induced by spanwise periodic arrays of cylindrical roughness elements over a NACA 0012 airfoil is investigated by hotwire anemometry and infrared thermography. The roughness elements are placed in the flow under adverse pressure gradient. Three configurations are investigated, namely an isolated roughness element, a spanwise array of roughness elements, and a pair of arrays in stagger. The streamwise and spanwise interactions between roughness wakes are addressed, focusing on the evolution of mean flow features and mechanisms for the subsequent process of laminar-turbulent transition. The spanwise interaction between roughness elements involves the connections and merging of neighboring low-speed regions (MLS) in the wake, which affects the spanwise distribution and amplitude of the velocity streaks. The maximum effect on promoting transition is observed when two neighboring low-speed regions overlap with each other in the near wake (within 6 times roughness height). The addition of a second roughness array promotes transition when the spanwise spacing is larger than two times the roughness diameter. Spectral analysis of the streamwise velocity fluctuations reveals that the number of roughness elements within the spanwise array affects the number of MLSs and the dominant instability mechanism. For an odd number of MLSs, the Kelvin–Helmholtz instability dominates the growth of velocity fluctuations around the three-dimensional shear layers. For an even number of MLSs, both Kelvin–Helmholtz and asymmetric instabilities appear in the wake. In this case, the dominant mode that leads to transition depends on the spanwise spacing between roughness elements.
...
Laminar to turbulent transition induced by spanwise periodic arrays of cylindrical roughness elements over a NACA 0012 airfoil is investigated by hotwire anemometry and infrared thermography. The roughness elements are placed in the flow under adverse pressure gradient. Three configurations are investigated, namely an isolated roughness element, a spanwise array of roughness elements, and a pair of arrays in stagger. The streamwise and spanwise interactions between roughness wakes are addressed, focusing on the evolution of mean flow features and mechanisms for the subsequent process of laminar-turbulent transition. The spanwise interaction between roughness elements involves the connections and merging of neighboring low-speed regions (MLS) in the wake, which affects the spanwise distribution and amplitude of the velocity streaks. The maximum effect on promoting transition is observed when two neighboring low-speed regions overlap with each other in the near wake (within 6 times roughness height). The addition of a second roughness array promotes transition when the spanwise spacing is larger than two times the roughness diameter. Spectral analysis of the streamwise velocity fluctuations reveals that the number of roughness elements within the spanwise array affects the number of MLSs and the dominant instability mechanism. For an odd number of MLSs, the Kelvin–Helmholtz instability dominates the growth of velocity fluctuations around the three-dimensional shear layers. For an even number of MLSs, both Kelvin–Helmholtz and asymmetric instabilities appear in the wake. In this case, the dominant mode that leads to transition depends on the spanwise spacing between roughness elements.
Journal article
(2024)
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Edoardo Grande, Shubham Shubham, Francesco Avallone, Daniele Ragni, Damiano Casalino
This paper aims to investigate, by means of Lattice-Boltzmann simulations, the flow-field and far-field noise of two co-axial co-rotating rotors operating at 3000 rpm in hover conditions. The two co-rotating configurations are made by 2×2-bladed rotors with a fixed axial separation and two different azimuthal separations Δϕ equal to 84∘ and 12∘. Isolated 2- and 4-bladed rotors, are also simulated at the same operating conditions and used as aerodynamic and aeroacoustic reference. For both Δϕ=84∘ and 12∘, the upper rotor tip vortices are accelerated downstream due to the induction from the lower rotor, avoiding blade vortex interaction (BVI). The lower rotor tip vortices convect into the wake with a lower velocity, causing BVI for Δϕ=12∘. The lower rotor shows a reduction of thrust, relative to the upper rotor, of 36% and 66% for Δϕ=84∘ and 12∘, respectively. For Δϕ=12∘, the lower blades act as a wing flap for the upper ones, increasing their thrust. The tonal noise emission for the co-rotating rotors is driven by the interference between the acoustic waves from upper and lower rotors. Because of destructive interference, the configuration Δϕ=84∘ shows a first harmonic up to 15 dB lower than Δϕ=12∘, but still 4.5 dB higher than the isolated 4-bladed rotor.
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This paper aims to investigate, by means of Lattice-Boltzmann simulations, the flow-field and far-field noise of two co-axial co-rotating rotors operating at 3000 rpm in hover conditions. The two co-rotating configurations are made by 2×2-bladed rotors with a fixed axial separation and two different azimuthal separations Δϕ equal to 84∘ and 12∘. Isolated 2- and 4-bladed rotors, are also simulated at the same operating conditions and used as aerodynamic and aeroacoustic reference. For both Δϕ=84∘ and 12∘, the upper rotor tip vortices are accelerated downstream due to the induction from the lower rotor, avoiding blade vortex interaction (BVI). The lower rotor tip vortices convect into the wake with a lower velocity, causing BVI for Δϕ=12∘. The lower rotor shows a reduction of thrust, relative to the upper rotor, of 36% and 66% for Δϕ=84∘ and 12∘, respectively. For Δϕ=12∘, the lower blades act as a wing flap for the upper ones, increasing their thrust. The tonal noise emission for the co-rotating rotors is driven by the interference between the acoustic waves from upper and lower rotors. Because of destructive interference, the configuration Δϕ=84∘ shows a first harmonic up to 15 dB lower than Δϕ=12∘, but still 4.5 dB higher than the isolated 4-bladed rotor.
Vertical axis wind turbines (VAWTs) have been identified as a technology that, in association with wake steering, can increase power density of wind farms. In this study, we validate a free wake method for VAWT wake prediction, which leads to satisfactory results. We then use this method to simulate wake steering by means of fixed pitched blades and struts. We demonstrate that combining pitched wakes and struts can lead to very advantageous wake behavior, but only when the interactions between the tip vortices are taken into account. The possibility to inject more high momentum flow into the wake while moving the vortex system away from the next turbine could make pitched blades and struts a powerful tool for future wind farms.
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Vertical axis wind turbines (VAWTs) have been identified as a technology that, in association with wake steering, can increase power density of wind farms. In this study, we validate a free wake method for VAWT wake prediction, which leads to satisfactory results. We then use this method to simulate wake steering by means of fixed pitched blades and struts. We demonstrate that combining pitched wakes and struts can lead to very advantageous wake behavior, but only when the interactions between the tip vortices are taken into account. The possibility to inject more high momentum flow into the wake while moving the vortex system away from the next turbine could make pitched blades and struts a powerful tool for future wind farms.
A time-domain inverse aeroacoustic method based on the convective Ffowcs Williams–Hawkings equation is presented. The method allows to determine, in real-time, the unsteady forces exerted on rotating blades in the presence of a moving medium. The inversion procedure is based on a space-time regularization with a mixed l1,2-norm, which guarantees accuracy and smoothness of the solution. The method is initially verified through synthetic acoustic signals emitted by rotating sources in a constant flow, up to a convective Mach number of about 0.88. Then the method is validated through signals generated by a propeller immersed in a wind-tunnel jet flow, up to a Mach number of 0.06. Due to the reduced convective Mach number, the leading aeroacoustic effect is derived from a variation of the blade loading. It is argued that the onset of flow separation at high values of the rotor advance ratio is responsible for the onset of force fluctuations that the inverse method is able to retrieve both qualitatively and quantitatively.
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A time-domain inverse aeroacoustic method based on the convective Ffowcs Williams–Hawkings equation is presented. The method allows to determine, in real-time, the unsteady forces exerted on rotating blades in the presence of a moving medium. The inversion procedure is based on a space-time regularization with a mixed l1,2-norm, which guarantees accuracy and smoothness of the solution. The method is initially verified through synthetic acoustic signals emitted by rotating sources in a constant flow, up to a convective Mach number of about 0.88. Then the method is validated through signals generated by a propeller immersed in a wind-tunnel jet flow, up to a Mach number of 0.06. Due to the reduced convective Mach number, the leading aeroacoustic effect is derived from a variation of the blade loading. It is argued that the onset of flow separation at high values of the rotor advance ratio is responsible for the onset of force fluctuations that the inverse method is able to retrieve both qualitatively and quantitatively.
This work focuses on the assessment of the accuracy of numerical prediction and experimental campaigns on providing the noise emissions of an isolated benchmark propeller. An experimental campaign is carried out with a model low-Reynolds propeller of 0.3 m diameter operating at high RPM, equivalent of a tip-Mach number (M
1) of 0.37 and an advance ratio (J) of 0.4.Measurements are conducted on an open-test section wind tunnel, surrounded by an anechoic chamber. Simulations are carried out with the commercial software PowerFLOW and aim at reproducing the propeller geometry and conditions. BEMT-based noise estimations are also used to demonstrate the expected results. The discussion is focused on the uncertainties of the experimental campaign, and the current accuracy of numerical and analytical predictions, creating a complete picture of the discrepancies expected when predicting propeller noise levels and potential sources of errors. Results point to an accurate ability of the three methodologies to assess the overall noise emissions. Nevertheless, precise description and measurements of the higher harmonics of the tonal emissions and of the broadband noise levels is still lacking and require improvements in experimental conditions and a detailed assessment of the flow over the propeller.
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This work focuses on the assessment of the accuracy of numerical prediction and experimental campaigns on providing the noise emissions of an isolated benchmark propeller. An experimental campaign is carried out with a model low-Reynolds propeller of 0.3 m diameter operating at high RPM, equivalent of a tip-Mach number (M 1) of 0.37 and an advance ratio (J) of 0.4.Measurements are conducted on an open-test section wind tunnel, surrounded by an anechoic chamber. Simulations are carried out with the commercial software PowerFLOW and aim at reproducing the propeller geometry and conditions. BEMT-based noise estimations are also used to demonstrate the expected results. The discussion is focused on the uncertainties of the experimental campaign, and the current accuracy of numerical and analytical predictions, creating a complete picture of the discrepancies expected when predicting propeller noise levels and potential sources of errors. Results point to an accurate ability of the three methodologies to assess the overall noise emissions. Nevertheless, precise description and measurements of the higher harmonics of the tonal emissions and of the broadband noise levels is still lacking and require improvements in experimental conditions and a detailed assessment of the flow over the propeller.
With distributed propulsion and electric vertical take-off and landing aircraft on the rise, fast and accurate methods to simulate propeller slipstreams and their interaction with aircraft components are needed. In this work, we compare results obtained with a filament-based free wake panel method to experimental and previously validated numerical data. In particular, we study a propeller-wing configuration at zero angle of attack and the aerodynamics of the blade-resolved slipstream interaction with the wing. We use a prescribed wake on the wing and a free wake on the propeller, which greatly accelerate the computations. Results indicate that, while forces are overpredicted due to the inviscid nature of the panel method, the free wake is able to capture the slipstream deformation and shearing with remarkable success. We find that a filament-based free wake panel method can be a useful tool for propeller-wing interaction in preliminary aircraft design.
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With distributed propulsion and electric vertical take-off and landing aircraft on the rise, fast and accurate methods to simulate propeller slipstreams and their interaction with aircraft components are needed. In this work, we compare results obtained with a filament-based free wake panel method to experimental and previously validated numerical data. In particular, we study a propeller-wing configuration at zero angle of attack and the aerodynamics of the blade-resolved slipstream interaction with the wing. We use a prescribed wake on the wing and a free wake on the propeller, which greatly accelerate the computations. Results indicate that, while forces are overpredicted due to the inviscid nature of the panel method, the free wake is able to capture the slipstream deformation and shearing with remarkable success. We find that a filament-based free wake panel method can be a useful tool for propeller-wing interaction in preliminary aircraft design.