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D. Ragni
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Journal article
(2026)
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Lourenço Tercio Lima Pereira, Daniele Ragni, Gianluca Romani, Damiano Casalino
This work focuses on the assessment of the accuracy of numerical predictions and experimental campaigns on providing aerodynamic characteristics and acoustic emissions of an isolated benchmark propeller. An experimental campaign is carried out with a benchmark model low-Reynolds propeller (89,000 tip Reynolds number) of 0.3 m diameter operating at high RPM, equivalent of a tip-Mach number ( Mt ) of 0.37 and an advance ratio ( J ) of 0.4. Measurements are conducted in the anechoic A-Tunnel, an open test-section wind tunnel, confined by an anechoic chamber. Simulations are carried out with the commercial software PowerFLOW and aimed at reproducing the propeller geometry and conditions. BEMT-based noise estimations are also carried out and used as reference results for the discussion. The study focuses on the assessment of uncertainties of the experimental campaign, and the current accuracy of numerical and analytical predictions, creating a complete picture of the discrepancies expected in similar setups when predicting and experimenting with propeller noise along with the potential sources of errors. Results point to the accuracy of the three methodologies in assessing the overall noise emissions. This follows the precise description of the integral aerodynamic loads on the propeller and the dominant first tonal components of the noise signature. Similarly, the velocity field obtained from the methodologies compares well. Nevertheless, precise descriptions and measurements of the higher harmonics of the tonal emissions and of the broadband noise levels are still lacking and demonstrate the need for improved experimental and numerical methodologies to allow accurate cross-validation between the methodologies.
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This work focuses on the assessment of the accuracy of numerical predictions and experimental campaigns on providing aerodynamic characteristics and acoustic emissions of an isolated benchmark propeller. An experimental campaign is carried out with a benchmark model low-Reynolds propeller (89,000 tip Reynolds number) of 0.3 m diameter operating at high RPM, equivalent of a tip-Mach number ( Mt ) of 0.37 and an advance ratio ( J ) of 0.4. Measurements are conducted in the anechoic A-Tunnel, an open test-section wind tunnel, confined by an anechoic chamber. Simulations are carried out with the commercial software PowerFLOW and aimed at reproducing the propeller geometry and conditions. BEMT-based noise estimations are also carried out and used as reference results for the discussion. The study focuses on the assessment of uncertainties of the experimental campaign, and the current accuracy of numerical and analytical predictions, creating a complete picture of the discrepancies expected in similar setups when predicting and experimenting with propeller noise along with the potential sources of errors. Results point to the accuracy of the three methodologies in assessing the overall noise emissions. This follows the precise description of the integral aerodynamic loads on the propeller and the dominant first tonal components of the noise signature. Similarly, the velocity field obtained from the methodologies compares well. Nevertheless, precise descriptions and measurements of the higher harmonics of the tonal emissions and of the broadband noise levels are still lacking and demonstrate the need for improved experimental and numerical methodologies to allow accurate cross-validation between the methodologies.
Journal article
(2026)
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K. Combey, Omer A. Elsayed, Andrea Magrini , Federico N. Ramirez , S. Wang, Khaoula Qaissi , Hajar Chouiyakh, Lourenço T. Lima Pereira, D. Ragni
The rapid growth of urbanization and increasing road traffic congestion are straining ground transportation infrastructure for both conventional and emergency purposes, driving the need for alternative mobility solutions. urban air mobility (UAM) offers a promising pathway by deploying electric vertical takeoff and landing (eVTOL) aircraft to enable efficient and flexible aerial transport in dense urban environments. However, the successful integration of UAM into city airspace faces critical technical challenges, both at the vehicle and operational levels. In particular, complex aerodynamic and aeroacoustic interactions between closely spaced propellers significantly influence vehicle performance, energy efficiency, and public acceptance. This work presents a review of experimental and computational studies on propeller–propeller and propeller–wing interactions, highlighting the state-of-the-art methodologies and their application to multirotor eVTOL designs. Results indicate that distributed electric propulsion systems arranged in side-by-side configurations exhibit minimal thrust degradation, typically less than 3% compared to an isolated propeller. However, reductions in propeller spacing can induce unsteady blade loading and increase tonal noise levels by up to 10 dB. In contrast, one-after-another configurations may suffer thrust losses of up to 80%, due to slipstream ingestion by the rear propeller, with a lateral separation of at least twice the propeller radius required to recover performance within 4% of the isolated case. The review also addresses propeller–wing interactions that modify local pressure distributions and spanwise lift, particularly in wing-mounted distributed propulsion configurations. The insights provided establish a foundation for developing efficient, low-noise multirotor architectures for future UAM integration.
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The rapid growth of urbanization and increasing road traffic congestion are straining ground transportation infrastructure for both conventional and emergency purposes, driving the need for alternative mobility solutions. urban air mobility (UAM) offers a promising pathway by deploying electric vertical takeoff and landing (eVTOL) aircraft to enable efficient and flexible aerial transport in dense urban environments. However, the successful integration of UAM into city airspace faces critical technical challenges, both at the vehicle and operational levels. In particular, complex aerodynamic and aeroacoustic interactions between closely spaced propellers significantly influence vehicle performance, energy efficiency, and public acceptance. This work presents a review of experimental and computational studies on propeller–propeller and propeller–wing interactions, highlighting the state-of-the-art methodologies and their application to multirotor eVTOL designs. Results indicate that distributed electric propulsion systems arranged in side-by-side configurations exhibit minimal thrust degradation, typically less than 3% compared to an isolated propeller. However, reductions in propeller spacing can induce unsteady blade loading and increase tonal noise levels by up to 10 dB. In contrast, one-after-another configurations may suffer thrust losses of up to 80%, due to slipstream ingestion by the rear propeller, with a lateral separation of at least twice the propeller radius required to recover performance within 4% of the isolated case. The review also addresses propeller–wing interactions that modify local pressure distributions and spanwise lift, particularly in wing-mounted distributed propulsion configurations. The insights provided establish a foundation for developing efficient, low-noise multirotor architectures for future UAM integration.
Porous trailing edges attenuate hydrodynamic pressure fluctuations that scatter as trailing-edge noise, with their effectiveness governed by their material parameters. Conventional measurements of permeability rely on steady-flow rigs, which cannot capture the dynamic response of this parameter, which is relevant for predicting balancing pressure fluctuations under grazing-flow conditions. In this study, we introduce a method for directly determining the dynamic permeability of porous trailing edges from time-resolved particle image velocimetry (PIV) data. The approach employs a lumped-system circuit analogy that links unsteady pressure gradients to through-material velocities, enabling in situ characterisation without specialised porous rigs, thereby further extending its applicability to thin trailing-edge geometries. Two materials with similar porosity but distinct internal architectures are compared against a solid baseline: a structured porous trailing edge (SPTE) and a random foam trailing edge (RFTE). The extracted permeability curves show close agreement with the analytical model of Johnson et al. (J. Fluid Mech., 1987, vol. 176, pp. 379–402), validating the method for both structured and randomised porous materials. The procedure also allows for the estimation of the equivalent viscous characteristic length and tortuosity. A detailed comparison reveals that the SPTE exhibits a lower viscous length scale and tortuosity than the RFTE, with a relatively higher dynamic permeability response at high frequencies.
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Porous trailing edges attenuate hydrodynamic pressure fluctuations that scatter as trailing-edge noise, with their effectiveness governed by their material parameters. Conventional measurements of permeability rely on steady-flow rigs, which cannot capture the dynamic response of this parameter, which is relevant for predicting balancing pressure fluctuations under grazing-flow conditions. In this study, we introduce a method for directly determining the dynamic permeability of porous trailing edges from time-resolved particle image velocimetry (PIV) data. The approach employs a lumped-system circuit analogy that links unsteady pressure gradients to through-material velocities, enabling in situ characterisation without specialised porous rigs, thereby further extending its applicability to thin trailing-edge geometries. Two materials with similar porosity but distinct internal architectures are compared against a solid baseline: a structured porous trailing edge (SPTE) and a random foam trailing edge (RFTE). The extracted permeability curves show close agreement with the analytical model of Johnson et al. (J. Fluid Mech., 1987, vol. 176, pp. 379–402), validating the method for both structured and randomised porous materials. The procedure also allows for the estimation of the equivalent viscous characteristic length and tortuosity. A detailed comparison reveals that the SPTE exhibits a lower viscous length scale and tortuosity than the RFTE, with a relatively higher dynamic permeability response at high frequencies.
The TU Delft Characterization model for roTor aeroacoUstiCs (TUC-TUC) is a modular rotor test bench developed for controlled aerodynamic and acoustic characterization under simplified and well-defined conditions. The platform permits systematic variation of blade pitch and airfoil type and is intended to support the study of rotor aeroacoustic behavior in a more interpretable manner than is typically possible with conventional rotor geometries. This paper presents the commissioning and initial hover characterization of TUC-TUC in the A-Tunnel facility at TU Delft using integral load measurements, acoustic directivity measurements, and particle image velocimetry of the flow field. The measurements are complemented by performance estimates from a blade element momentum theory model coupled with a harmonic source formulation. The results show that the measured aerodynamic response is captured well in trend and magnitude by the performance model, despite an apparent pitch-angle offset between predictions and experiments. The acoustic measurements are likewise found to be broadly consistent with the intended design criteria, including expected velocity scaling and clear source separation between the outer test section and inner structure. Taken together, the results establish TUC-TUC as a suitable basis for continued aerodynamic and aeroacoustic characterization and for future study of rotor-noise mechanisms under controlled conditions.
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The TU Delft Characterization model for roTor aeroacoUstiCs (TUC-TUC) is a modular rotor test bench developed for controlled aerodynamic and acoustic characterization under simplified and well-defined conditions. The platform permits systematic variation of blade pitch and airfoil type and is intended to support the study of rotor aeroacoustic behavior in a more interpretable manner than is typically possible with conventional rotor geometries. This paper presents the commissioning and initial hover characterization of TUC-TUC in the A-Tunnel facility at TU Delft using integral load measurements, acoustic directivity measurements, and particle image velocimetry of the flow field. The measurements are complemented by performance estimates from a blade element momentum theory model coupled with a harmonic source formulation. The results show that the measured aerodynamic response is captured well in trend and magnitude by the performance model, despite an apparent pitch-angle offset between predictions and experiments. The acoustic measurements are likewise found to be broadly consistent with the intended design criteria, including expected velocity scaling and clear source separation between the outer test section and inner structure. Taken together, the results establish TUC-TUC as a suitable basis for continued aerodynamic and aeroacoustic characterization and for future study of rotor-noise mechanisms under controlled conditions.
This study experimentally investigates the performance of vortex generators (VGs) designed for steady stall control in preventing unsteady trailing-edge flow separation and dynamic stall during pitch oscillations occurring on inboard and midboard wind turbine blade sections. Surface pressure measurements are conducted in the TU Delft low-speed wind tunnel on a DU-97-W-300 airfoil undergoing pitch oscillations while equipped with VGs of various vane sizes and shapes. In steady conditions, vanes with heights smaller than the local boundary layer thickness optimally balance delaying stall following trailing-edge separation with achieving maximum lift-to-drag ratio among the tested triangular vane VGs. However, these same VGs with vane heights smaller than or equal to the steady local boundary layer thickness are insufficient to suppress the onset and upstream progression of a trailing-edge separation front in all pitching cycles. VGs whose vane height exceeds the local boundary layer thickness for a larger part of the pitch cycle prevent the onset and upstream progression of the trailing-edge separation front for a larger percentage of cycles. Contrary to past literature, rectangular vanes yield a higher steady aerodynamic efficiency than triangular vanes. Rectangular vanes also suppress trailing-edge flow separation in all pitching cycles at all tested reduced frequencies, indicating more effective boundary layer energization than triangular vanes, thus proving to be a better VG shape for steady and unsteady stall suppression on thick airfoils.
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This study experimentally investigates the performance of vortex generators (VGs) designed for steady stall control in preventing unsteady trailing-edge flow separation and dynamic stall during pitch oscillations occurring on inboard and midboard wind turbine blade sections. Surface pressure measurements are conducted in the TU Delft low-speed wind tunnel on a DU-97-W-300 airfoil undergoing pitch oscillations while equipped with VGs of various vane sizes and shapes. In steady conditions, vanes with heights smaller than the local boundary layer thickness optimally balance delaying stall following trailing-edge separation with achieving maximum lift-to-drag ratio among the tested triangular vane VGs. However, these same VGs with vane heights smaller than or equal to the steady local boundary layer thickness are insufficient to suppress the onset and upstream progression of a trailing-edge separation front in all pitching cycles. VGs whose vane height exceeds the local boundary layer thickness for a larger part of the pitch cycle prevent the onset and upstream progression of the trailing-edge separation front for a larger percentage of cycles. Contrary to past literature, rectangular vanes yield a higher steady aerodynamic efficiency than triangular vanes. Rectangular vanes also suppress trailing-edge flow separation in all pitching cycles at all tested reduced frequencies, indicating more effective boundary layer energization than triangular vanes, thus proving to be a better VG shape for steady and unsteady stall suppression on thick airfoils.
Journal article
(2026)
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Mehdi Doosttalab, Carlos Simão Ferreira, Daniele Ragni, Wei Yu, Christof Rautmann
The accuracy of the Beddoes–Leishman and Risø dynamic stall models is evaluated against experiments on thick wind turbine airfoils with a relative thickness of 35% and trailing edge thicknesses of 10% and 2%, both with and without vortex generators. The dynamic lift, drag, and pitching moment coefficients simulation results are compared with the measurements, obtained in the TU Delft LTT wind tunnel at a Reynolds number of Re=1×106 and dynamic reduced frequency of 0.064. The study revealed that while the aforementioned models successfully predicted the direction of the dynamic cycles, they inaccurately captured the dynamic stall behavior of thick flatback and non-flatback airfoils in all configurations, particularly in separated flows. There was no significant difference observed in the performance of the two models. The reasons for modeling failure are thoroughly examined from both fundamental and mathematical perspectives, and suggestions for improvements are provided. The findings raise concerns regarding the accuracy and reliability of the dynamic load assessment and aeroelasticity analysis for modern large wind turbines, using current dynamic stall models and underscore the necessity for enhancing the existing models.
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The accuracy of the Beddoes–Leishman and Risø dynamic stall models is evaluated against experiments on thick wind turbine airfoils with a relative thickness of 35% and trailing edge thicknesses of 10% and 2%, both with and without vortex generators. The dynamic lift, drag, and pitching moment coefficients simulation results are compared with the measurements, obtained in the TU Delft LTT wind tunnel at a Reynolds number of Re=1×106 and dynamic reduced frequency of 0.064. The study revealed that while the aforementioned models successfully predicted the direction of the dynamic cycles, they inaccurately captured the dynamic stall behavior of thick flatback and non-flatback airfoils in all configurations, particularly in separated flows. There was no significant difference observed in the performance of the two models. The reasons for modeling failure are thoroughly examined from both fundamental and mathematical perspectives, and suggestions for improvements are provided. The findings raise concerns regarding the accuracy and reliability of the dynamic load assessment and aeroelasticity analysis for modern large wind turbines, using current dynamic stall models and underscore the necessity for enhancing the existing models.
Contra-rotating propellers (CRP) are widely employed in multi-rotor vehicles due to their aerodynamic efficiency and compact design. In practical operations, the rotational speed of the propellers can fluctuate about its mean value. These fluctuations lead to azimuthal misalignment between the two propellers, commonly referred to as phase offset. This paper investigates the effect of this phase offset on the tonal noise characteristics of a CRP configuration. A semi-analytical framework is presented to predict tonal noise generated due to unsteady loading arising from potential field interactions. The method requires as input the spanwise distribution of steady aerodynamic loads. The proposed framework is validated by comparing noise predictions with experimental data and is subsequently used to investigate the effects of phase offset. The results show that the odd harmonics of the blade passing frequency are more sensitive to variations in phase offset, whereas the even harmonics remain largely unaffected. Furthermore, the overall sound pressure level shows maximum variation with phase offset in the propeller rotational plane. The study also highlights the potential of the phase offset parameter as a means of incorporating uncertainty due to rotational speed fluctuations into the semi-analytical method proposed for tonal noise prediction.
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Contra-rotating propellers (CRP) are widely employed in multi-rotor vehicles due to their aerodynamic efficiency and compact design. In practical operations, the rotational speed of the propellers can fluctuate about its mean value. These fluctuations lead to azimuthal misalignment between the two propellers, commonly referred to as phase offset. This paper investigates the effect of this phase offset on the tonal noise characteristics of a CRP configuration. A semi-analytical framework is presented to predict tonal noise generated due to unsteady loading arising from potential field interactions. The method requires as input the spanwise distribution of steady aerodynamic loads. The proposed framework is validated by comparing noise predictions with experimental data and is subsequently used to investigate the effects of phase offset. The results show that the odd harmonics of the blade passing frequency are more sensitive to variations in phase offset, whereas the even harmonics remain largely unaffected. Furthermore, the overall sound pressure level shows maximum variation with phase offset in the propeller rotational plane. The study also highlights the potential of the phase offset parameter as a means of incorporating uncertainty due to rotational speed fluctuations into the semi-analytical method proposed for tonal noise prediction.
Conference paper
(2026)
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K. Combey, Omer A. Elsayed, Lourenço T. Lima Pereira, D. Ragni, Khaoula Qaissi , Hajar Chouiyakh, Mustapha Faqir
This study presents an experimental aeroacoustic evaluation of a tip-joined blade (TJB) propeller and compares its performance with conventional two- and four-bladed configurations under similar operating conditions. Acoustic measurements are conducted in the anechoic wind tunnel at Delft University of Technology using an eight-microphone far-field directivity arc. All propellers are designed to deliver equivalent thrust while maintaining identical diameter and chord distributions, enabling a consistent comparison at an advance ratio of J = 0.4 and a rotational speed of 4,000 rpm. The TJB propeller satisfies the thrust requirement but exhibits a propulsive efficiency approximately 4% lower than the four-bladed configuration, primarily due to increased torque associated with the closed-loop geometry. The acoustic results show that the TJB does not provide a uniform reduction across the entire noise spectrum. Broadband noise levels are reduced relative to the four-bladed propeller and fall below those of the two-bladed configuration above approximately 9 kHz, yielding broadband overall sound pressure levels comparable to the two-bladed baseline. In contrast, tonal levels are increased, with the blade-passing frequency peak exceeding those of the two- and four-bladed propellers by approximately 3 dB and up to 17 dB, respectively, at the θ = 90° observer position. Consequently, the total overall sound pressure level of the TJB propeller lies between those of the two-bladed and four-bladed propellers. These findings indicate that the TJB geometry provides effective broadband noise mitigation while exhibiting increased tonal components, highlighting both the potential and limitations of tip-joined blade concepts for propeller noise reduction.
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This study presents an experimental aeroacoustic evaluation of a tip-joined blade (TJB) propeller and compares its performance with conventional two- and four-bladed configurations under similar operating conditions. Acoustic measurements are conducted in the anechoic wind tunnel at Delft University of Technology using an eight-microphone far-field directivity arc. All propellers are designed to deliver equivalent thrust while maintaining identical diameter and chord distributions, enabling a consistent comparison at an advance ratio of J = 0.4 and a rotational speed of 4,000 rpm. The TJB propeller satisfies the thrust requirement but exhibits a propulsive efficiency approximately 4% lower than the four-bladed configuration, primarily due to increased torque associated with the closed-loop geometry. The acoustic results show that the TJB does not provide a uniform reduction across the entire noise spectrum. Broadband noise levels are reduced relative to the four-bladed propeller and fall below those of the two-bladed configuration above approximately 9 kHz, yielding broadband overall sound pressure levels comparable to the two-bladed baseline. In contrast, tonal levels are increased, with the blade-passing frequency peak exceeding those of the two- and four-bladed propellers by approximately 3 dB and up to 17 dB, respectively, at the θ = 90° observer position. Consequently, the total overall sound pressure level of the TJB propeller lies between those of the two-bladed and four-bladed propellers. These findings indicate that the TJB geometry provides effective broadband noise mitigation while exhibiting increased tonal components, highlighting both the potential and limitations of tip-joined blade concepts for propeller noise reduction.
Journal article
(2026)
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Kangni Combey, Omer A. Elsayed, Lourenço Tercio Lima Pereira, Daniele Ragni, Khaoula Qaissi, Hajar Chouiyakh, Mustapha Faqir
This study presents an experimental aeroacoustic investigation of a tip-joined blade (TJB) propeller, compared with 2- and 4-bladed configurations under identical operating conditions and matched thrust. The influence of blade azimuthal angles (30∘, 60∘, and 90∘) was also analyzed to precisely quantify the acoustic benefits of the TJB propeller. Experiments were conducted in an anechoic wind tunnel to quantify and separate tonal and broadband contributions in the acoustic spectra at various radiation angles. At 4,000 rpm and an advance ratio of J=0.4, the TJB produced equivalent thrust to the baseline 2-bladed propeller, with only a 2.8% difference. Under these conditions, it exhibited the strongest first tonal peak at 133 Hz, 2 dB higher than the baseline propeller, while broadband noise above 9 kHz was the lowest among all configurations. The TJB, with slightly reduced propulsive efficiency (56.7% for the 4-bladed 90∘ to 54.5% for the TJB), showed the highest tonal overall sound pressure levels (OASPL) across all directivity angles. This behavior is attributed to aerodynamic interactions between the forward and rearward blade segments, similar to the scissor 4-bladed 30∘ configuration, which also displayed elevated tonal levels. However, the broadband levels of the TJB remained lower than those of the 4-bladed configurations and comparable to the 2-bladed propeller. These trends are associated with reduced trailing-edge noise resulting from the sweeping of the blade elements necessary to form the joined-blade structure. Overall, the results indicate that the TJB propeller provides broadband noise reduction benefits, despite no observed advantage in tonal noise.
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This study presents an experimental aeroacoustic investigation of a tip-joined blade (TJB) propeller, compared with 2- and 4-bladed configurations under identical operating conditions and matched thrust. The influence of blade azimuthal angles (30∘, 60∘, and 90∘) was also analyzed to precisely quantify the acoustic benefits of the TJB propeller. Experiments were conducted in an anechoic wind tunnel to quantify and separate tonal and broadband contributions in the acoustic spectra at various radiation angles. At 4,000 rpm and an advance ratio of J=0.4, the TJB produced equivalent thrust to the baseline 2-bladed propeller, with only a 2.8% difference. Under these conditions, it exhibited the strongest first tonal peak at 133 Hz, 2 dB higher than the baseline propeller, while broadband noise above 9 kHz was the lowest among all configurations. The TJB, with slightly reduced propulsive efficiency (56.7% for the 4-bladed 90∘ to 54.5% for the TJB), showed the highest tonal overall sound pressure levels (OASPL) across all directivity angles. This behavior is attributed to aerodynamic interactions between the forward and rearward blade segments, similar to the scissor 4-bladed 30∘ configuration, which also displayed elevated tonal levels. However, the broadband levels of the TJB remained lower than those of the 4-bladed configurations and comparable to the 2-bladed propeller. These trends are associated with reduced trailing-edge noise resulting from the sweeping of the blade elements necessary to form the joined-blade structure. Overall, the results indicate that the TJB propeller provides broadband noise reduction benefits, despite no observed advantage in tonal noise.
Journal article
(2026)
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A. Sahoo, Akshay Koodly Ravishankara, W. Yu, D. Ragni, Carlos Simao Ferreira
Vortex generators (VGs) are known to delay separation and stall, allowing the design of airfoils with larger stall margins, particularly for thick airfoil sections in the inboard and midboard regions of modern slender wind turbine blades. Including VG effects in blade design studies requires accurate VG models for fast lower-order techniques, like integral boundary layer (IBL) methods. Previous VG models for IBL methods have used engineering approaches tuned on airfoil aerodynamic data. The accuracy of these models depends on the availability of wind tunnel aerodynamic polar datasets for tuning, which are limited and time-consuming to expand for the relevant wind conditions, airfoil sections, and VG configurations being used in continuously growing wind turbine blades. This work proposes a VG model using IBL equations derived from flat-plate boundary layers under the influence of VGs. The new VG model empirically models the shape factor of the boundary layer and the viscous dissipation coefficient in the IBL framework to account for the additional momentum and dissipation in the boundary layer mean flow due to VGs. The model is developed from a wide range of flat-plate boundary layers and VGs to account for variations in VG vane size and placement on the turbulent boundary layer development influencing the airfoil aerodynamic characteristics. The new VG model, called RFOILVogue, is implemented in an in-house code RFOIL, an improvement over XFOIL, and validated with computational fluid dynamics (CFD) data and wind tunnel measurements of flat plates and airfoil sections equipped with VGs. Since it is derived from vortex dynamics in turbulent boundary layers, RFOILVogue better predicts both airfoil performance characteristics, such as positive stall angle, maximum lift, and drag, and boundary layer flow parameters, such as the separation location, compared to the existing tuned VG models. The VG model still suffers from some inherent drawbacks of reduced-order models like RFOIL, and future research directions for thick airfoils are proposed to overcome these drawbacks in VG modelling.
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Vortex generators (VGs) are known to delay separation and stall, allowing the design of airfoils with larger stall margins, particularly for thick airfoil sections in the inboard and midboard regions of modern slender wind turbine blades. Including VG effects in blade design studies requires accurate VG models for fast lower-order techniques, like integral boundary layer (IBL) methods. Previous VG models for IBL methods have used engineering approaches tuned on airfoil aerodynamic data. The accuracy of these models depends on the availability of wind tunnel aerodynamic polar datasets for tuning, which are limited and time-consuming to expand for the relevant wind conditions, airfoil sections, and VG configurations being used in continuously growing wind turbine blades. This work proposes a VG model using IBL equations derived from flat-plate boundary layers under the influence of VGs. The new VG model empirically models the shape factor of the boundary layer and the viscous dissipation coefficient in the IBL framework to account for the additional momentum and dissipation in the boundary layer mean flow due to VGs. The model is developed from a wide range of flat-plate boundary layers and VGs to account for variations in VG vane size and placement on the turbulent boundary layer development influencing the airfoil aerodynamic characteristics. The new VG model, called RFOILVogue, is implemented in an in-house code RFOIL, an improvement over XFOIL, and validated with computational fluid dynamics (CFD) data and wind tunnel measurements of flat plates and airfoil sections equipped with VGs. Since it is derived from vortex dynamics in turbulent boundary layers, RFOILVogue better predicts both airfoil performance characteristics, such as positive stall angle, maximum lift, and drag, and boundary layer flow parameters, such as the separation location, compared to the existing tuned VG models. The VG model still suffers from some inherent drawbacks of reduced-order models like RFOIL, and future research directions for thick airfoils are proposed to overcome these drawbacks in VG modelling.
This study investigates the aeroacoustic behavior of a low-Reynolds-number propeller in forward flight subjected to large-scale inflow disturbances. The incoming flow is modeled as single-frequency sinusoidal vortical gusts, enabling a systematic assessment of the effects of gust frequency, initial phase, and direction on aerodynamic performance and noise generation. The numerical setup is first validated against experimental data under steady inflow conditions. The results show that the loading fluctuations caused by the incoming gust result in discrete tonal components in the acoustic spectrum at frequencies determined by the combination of the gust frequency and multiples of the rotational frequency. These components arise from a double modulation mechanism, and their amplitude is further shaped by inter-blade interference effects. The phase of the gust with respect to the rotor primarily affects the phase of the blade response, thereby modifying the noise directivity, particularly at low frequencies. When the gust is inclined relative to the mean flow, the interaction becomes more complex, leading to a richer tonal spectrum with high intensity tones extending up to the 10th harmonic of the blade passing frequency. Overall, the results provide a physical interpretation of the coupling between rotating blades and large-scale inflow disturbances, supporting the development of improved models for unsteady tonal noise prediction.
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This study investigates the aeroacoustic behavior of a low-Reynolds-number propeller in forward flight subjected to large-scale inflow disturbances. The incoming flow is modeled as single-frequency sinusoidal vortical gusts, enabling a systematic assessment of the effects of gust frequency, initial phase, and direction on aerodynamic performance and noise generation. The numerical setup is first validated against experimental data under steady inflow conditions. The results show that the loading fluctuations caused by the incoming gust result in discrete tonal components in the acoustic spectrum at frequencies determined by the combination of the gust frequency and multiples of the rotational frequency. These components arise from a double modulation mechanism, and their amplitude is further shaped by inter-blade interference effects. The phase of the gust with respect to the rotor primarily affects the phase of the blade response, thereby modifying the noise directivity, particularly at low frequencies. When the gust is inclined relative to the mean flow, the interaction becomes more complex, leading to a richer tonal spectrum with high intensity tones extending up to the 10th harmonic of the blade passing frequency. Overall, the results provide a physical interpretation of the coupling between rotating blades and large-scale inflow disturbances, supporting the development of improved models for unsteady tonal noise prediction.
This 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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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.
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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.
Conference paper
(2026)
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M. De Rosa, A. Glorioso, F. Petrosino, D. Casalino, D. Ragni, A. Aprovitola, L. Iuspa, G. Pezzella, A. Viviani
This paper investigates the capability of the relatively low-dispersion Lattice–Boltzmann methodology to predict the sonic-boom characteristics of an unconventional waverider aircraft. The study focuses on the near-field pressure-signature generated by the CS3 hypersonic scramjet propelled aircraft operating at a free-stream flow speed of Mach 1.6 under stratospheric flight and motor-off conditions. Pressure signatures are evaluated in the aircraft near-field domain and then propagated to the ground to assess the aircraft sonic boom signal. The near-field flow is computed using the commercial multi-purpose solver SIMULIA PowerFLOW®, based on the Lattice–Boltzmann approach; the same flow is solved for comparison with the SU2 Finite Volume solver. Signatures are extracted from the symmetry plane below the aircraft at distances ( 𝐻 𝐿)0.5 and ( 𝐻 𝐿)1, corresponding to half and one body length, respectively. Subsequently, the most representative near-field is propagated using the NASA PCBoom code, and the resulting ground signature is assessed by analysing peak overpressure and rise time. The results demonstrate a consistent near-to-far-field assessment of a sonic boom using a Lattice-Boltzmann flow solver. This method is particularly advantageous for such applications, as it avoids the strict requirement for mesh-aligned grid generation.
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This paper investigates the capability of the relatively low-dispersion Lattice–Boltzmann methodology to predict the sonic-boom characteristics of an unconventional waverider aircraft. The study focuses on the near-field pressure-signature generated by the CS3 hypersonic scramjet propelled aircraft operating at a free-stream flow speed of Mach 1.6 under stratospheric flight and motor-off conditions. Pressure signatures are evaluated in the aircraft near-field domain and then propagated to the ground to assess the aircraft sonic boom signal. The near-field flow is computed using the commercial multi-purpose solver SIMULIA PowerFLOW®, based on the Lattice–Boltzmann approach; the same flow is solved for comparison with the SU2 Finite Volume solver. Signatures are extracted from the symmetry plane below the aircraft at distances ( 𝐻 𝐿)0.5 and ( 𝐻 𝐿)1, corresponding to half and one body length, respectively. Subsequently, the most representative near-field is propagated using the NASA PCBoom code, and the resulting ground signature is assessed by analysing peak overpressure and rise time. The results demonstrate a consistent near-to-far-field assessment of a sonic boom using a Lattice-Boltzmann flow solver. This method is particularly advantageous for such applications, as it avoids the strict requirement for mesh-aligned grid generation.
This work experimentally investigated the feasibility and complementarity of aeroacoustic and infrared thermography (IRT) techniques for detecting damage in rotating wind turbine blades under controlled wind tunnel conditions. Two representative types of damage were considered: trailing edge cracks and internal shear web delamination, created in the scaled blades manufactured in-house. Experiments were conducted in the open jet facility at Delft University of Technology. Acoustic measurements using a two-dimensional microphone array revealed that trailing edge cracks induce distinct tonal noise modifications, which depend on the effective trailing edge thickness and are captured through spectral analysis and acoustic beamforming. The crack-induced tonal noise peaks at a trailing-edge-thickness-based Strouhal number, (Formula presented), between 0.15 and 0.25. IRT, by contrast, are highly sensitive to internal structural features; delaminated regions exhibited localized temperature variations due to changes in thermal properties. Principal component thermography was applied to further enhance the visualization of the internal shear webs and internal delamination. The results demonstrate that the use of aeroacoustic and IRT methods provides a complementary strategy for detecting both edge and internal damage in wind turbine blades.
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This work experimentally investigated the feasibility and complementarity of aeroacoustic and infrared thermography (IRT) techniques for detecting damage in rotating wind turbine blades under controlled wind tunnel conditions. Two representative types of damage were considered: trailing edge cracks and internal shear web delamination, created in the scaled blades manufactured in-house. Experiments were conducted in the open jet facility at Delft University of Technology. Acoustic measurements using a two-dimensional microphone array revealed that trailing edge cracks induce distinct tonal noise modifications, which depend on the effective trailing edge thickness and are captured through spectral analysis and acoustic beamforming. The crack-induced tonal noise peaks at a trailing-edge-thickness-based Strouhal number, (Formula presented), between 0.15 and 0.25. IRT, by contrast, are highly sensitive to internal structural features; delaminated regions exhibited localized temperature variations due to changes in thermal properties. Principal component thermography was applied to further enhance the visualization of the internal shear webs and internal delamination. The results demonstrate that the use of aeroacoustic and IRT methods provides a complementary strategy for detecting both edge and internal damage in wind turbine blades.
When applied to aerofoils with non-negligible thickness, Amiet’s theory for turbulence-interaction noise prediction does not account for the alterations in the velocity field and acoustic response induced by the surface, resulting in an overestimation of the radiated noise. This study proposes a semi-analytical method that models turbulence distortion in the immediate vicinity of the surface starting from upstream flow conditions and considers the resulting effects on the acoustic response of the aerofoil. The distorted spectrum of the upwash velocity component is calculated using the asymptotic results of the rapid distortion theory (RDT) for very large- and small-scale turbulence, overcoming the need to define a representative location where turbulence characteristics are sampled. This distorted spectrum is characterised by an increased energy content that is encompassed in the model by scaling the analytical flat-plate formulation of the aeroacoustic transfer function. The proposed approach relies on defining the aerofoil geometrical feature that affects distortion mechanisms, required to extend the RDT results to such geometries. This parameter is identified as the path travelled by the turbulent eddies from the stagnation point to the position of maximum surface-pressure fluctuations, which is, in turn, related to flow acceleration and leading-edge sharpness. The accuracy of this methodology in enhancing noise prediction is demonstrated using numerical and experimental data of grid-generated turbulence interacting with different aerofoils.
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When applied to aerofoils with non-negligible thickness, Amiet’s theory for turbulence-interaction noise prediction does not account for the alterations in the velocity field and acoustic response induced by the surface, resulting in an overestimation of the radiated noise. This study proposes a semi-analytical method that models turbulence distortion in the immediate vicinity of the surface starting from upstream flow conditions and considers the resulting effects on the acoustic response of the aerofoil. The distorted spectrum of the upwash velocity component is calculated using the asymptotic results of the rapid distortion theory (RDT) for very large- and small-scale turbulence, overcoming the need to define a representative location where turbulence characteristics are sampled. This distorted spectrum is characterised by an increased energy content that is encompassed in the model by scaling the analytical flat-plate formulation of the aeroacoustic transfer function. The proposed approach relies on defining the aerofoil geometrical feature that affects distortion mechanisms, required to extend the RDT results to such geometries. This parameter is identified as the path travelled by the turbulent eddies from the stagnation point to the position of maximum surface-pressure fluctuations, which is, in turn, related to flow acceleration and leading-edge sharpness. The accuracy of this methodology in enhancing noise prediction is demonstrated using numerical and experimental data of grid-generated turbulence interacting with different aerofoils.
Journal article
(2026)
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Ambar Garofano-Soldado, Daniele Ragni, Lourenço T.Lima Pereira, Riccardo Zamponi, Anibal Ollero, Guillermo Heredia
This study focuses on the analysis of the ground effect in counter-rotating coaxial rotors. To investigate the aerodynamic performance of a coaxial rotor system, the aerodynamic loading is measured for different rotor vertical spacing, rotational speed, and height above the ground. To link aerodynamic loading with flow topology, velocity fields in the rotor slipstreams are measured with particle image velocimetry (PIV). A semi-empirical model is additionally proposed to complement existing ground-effect theories from the literature by accounting for the effects of rotor spacing, ground proximity, and rotor-to-rotor aerodynamic interactions. The results of the performance analysis show that the ground effect is more pronounced in coaxial configurations than in single rotors, especially at minimum spacing and height, where the thrust increases about twice the corresponding value of single rotors. The analysis of the PIV velocity fields reveals how the inflow to the bottom rotor accelerates the downstream flow, increasing the flow rate and further reducing the induced velocity near the ground. As the rotor spacing increases, these interactions weaken, causing the aerodynamic loading to converge to that of a single rotor at a spacing around 90 % of the rotor radius. The proposed model inspired by experimental data provides a robust framework for predicting coaxial rotor performance near the ground. It also allows integration of the ground effect model into UAV control strategies for improved flight stability and safety.
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This study focuses on the analysis of the ground effect in counter-rotating coaxial rotors. To investigate the aerodynamic performance of a coaxial rotor system, the aerodynamic loading is measured for different rotor vertical spacing, rotational speed, and height above the ground. To link aerodynamic loading with flow topology, velocity fields in the rotor slipstreams are measured with particle image velocimetry (PIV). A semi-empirical model is additionally proposed to complement existing ground-effect theories from the literature by accounting for the effects of rotor spacing, ground proximity, and rotor-to-rotor aerodynamic interactions. The results of the performance analysis show that the ground effect is more pronounced in coaxial configurations than in single rotors, especially at minimum spacing and height, where the thrust increases about twice the corresponding value of single rotors. The analysis of the PIV velocity fields reveals how the inflow to the bottom rotor accelerates the downstream flow, increasing the flow rate and further reducing the induced velocity near the ground. As the rotor spacing increases, these interactions weaken, causing the aerodynamic loading to converge to that of a single rotor at a spacing around 90 % of the rotor radius. The proposed model inspired by experimental data provides a robust framework for predicting coaxial rotor performance near the ground. It also allows integration of the ground effect model into UAV control strategies for improved flight stability and safety.
This study investigated the noise emission and thrust performance of a heavy-lift unmanned air vehicle (UAV) with a coaxial propulsion system that operates under differential rotor speeds. The UAV adopted an octo-quad architecture, where each rotor pair consists of two propellers with different blades, allowing independent operation of fore and aft rotors in corotating (CR) and contra-rotating (CTR) configurations. Acoustic emissions and thrust were measured under steady conditions. The study compared the performances of CR and CTR configurations and examined the influence of differential rotor speed on the noise emission of the vehicle under different loads for both configurations. The results indicate that the CTR configuration achieves a maximum load factor 0.28 higher than that of the CR configuration and features lower noise at the same thrust when employing differential rotor speed. For both configurations, the drone's noise was influenced by the aerodynamic characteristics of propellers. Specifically, increasing the fore rotor speed relative to the aft rotor amplifies the noise, whereas increasing the aft rotor speed reduces noise without compromising thrust. Corresponding noise spectra were analyzed across different load factors. The results provide insights that can inform about the optimization of noise emission and performance of UAVs with coaxial propulsion systems.
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This study investigated the noise emission and thrust performance of a heavy-lift unmanned air vehicle (UAV) with a coaxial propulsion system that operates under differential rotor speeds. The UAV adopted an octo-quad architecture, where each rotor pair consists of two propellers with different blades, allowing independent operation of fore and aft rotors in corotating (CR) and contra-rotating (CTR) configurations. Acoustic emissions and thrust were measured under steady conditions. The study compared the performances of CR and CTR configurations and examined the influence of differential rotor speed on the noise emission of the vehicle under different loads for both configurations. The results indicate that the CTR configuration achieves a maximum load factor 0.28 higher than that of the CR configuration and features lower noise at the same thrust when employing differential rotor speed. For both configurations, the drone's noise was influenced by the aerodynamic characteristics of propellers. Specifically, increasing the fore rotor speed relative to the aft rotor amplifies the noise, whereas increasing the aft rotor speed reduces noise without compromising thrust. Corresponding noise spectra were analyzed across different load factors. The results provide insights that can inform about the optimization of noise emission and performance of UAVs with coaxial propulsion systems.
Journal article
(2026)
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Mario Alì, Andrea Piccolo, Riccardo Zamponi, Daniele Ragni, Francesco Avallone
This work investigates the effect of grid-generated turbulence ingestion on noise generation in a propeller operating at a low Reynolds number using high-fidelity, scale-resolved simulations. The numerical setup reproduces experiments carried out at Delft University of Technology, where inflow turbulence is generated by a grid placed within a duct. It is found that, upstream of the propeller, the longitudinal correlation length of the streamwise velocity component increases with respect to the case without the propeller. The opposite happens for the transversal one. The turbulent inflow impinging on the propeller blades does not alter the mean flow characteristics over the propeller blades, e.g., the mean static pressure coefficient. However, it increases the root mean square of the pressure fluctuations up to the turbulent reattachment point of the laminar separation bubble, while leaving the downstream region mostly unaffected. This causes a broadband increase in the radiated noise in the low-to-mid frequency range, as confirmed by applying Amiet’s noise-prediction model with input data sampled near the propeller blades’ leading edge. The far-field noise spectra are characterized not only by an increase in the broadband noise with respect to the clean inflow case, but also by tonal components at multiples of the blade-passing frequency. It is found that these tones are caused by the footprint of the turbulence grid that introduces flow inhomogeneities at the propeller location for this specific configuration. It is recommended, when performing experiments and simulations, to verify if any footprint of the turbulence grid is present, not only by performing single-point measurements but also by measuring the time-averaged flow field before installing the propeller.
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This work investigates the effect of grid-generated turbulence ingestion on noise generation in a propeller operating at a low Reynolds number using high-fidelity, scale-resolved simulations. The numerical setup reproduces experiments carried out at Delft University of Technology, where inflow turbulence is generated by a grid placed within a duct. It is found that, upstream of the propeller, the longitudinal correlation length of the streamwise velocity component increases with respect to the case without the propeller. The opposite happens for the transversal one. The turbulent inflow impinging on the propeller blades does not alter the mean flow characteristics over the propeller blades, e.g., the mean static pressure coefficient. However, it increases the root mean square of the pressure fluctuations up to the turbulent reattachment point of the laminar separation bubble, while leaving the downstream region mostly unaffected. This causes a broadband increase in the radiated noise in the low-to-mid frequency range, as confirmed by applying Amiet’s noise-prediction model with input data sampled near the propeller blades’ leading edge. The far-field noise spectra are characterized not only by an increase in the broadband noise with respect to the clean inflow case, but also by tonal components at multiples of the blade-passing frequency. It is found that these tones are caused by the footprint of the turbulence grid that introduces flow inhomogeneities at the propeller location for this specific configuration. It is recommended, when performing experiments and simulations, to verify if any footprint of the turbulence grid is present, not only by performing single-point measurements but also by measuring the time-averaged flow field before installing the propeller.
This study examines the role of turbulence distortion in predicting inflow turbulence (IT) noise generation from large wind turbines via Amiet's theory. Two subsequent distortion mechanisms are investigated: (i) the streamtube expansion in the rotor induction zone and (ii) the interaction with the surface of thick-blade profiles. Large-eddy simulations reveal that the turbulence spectra, which reflect distortion effects, remain largely unaffected by rotor induction within the frequency range relevant for noise generation. As for the other mechanism, the distortion of the turbulence approaching a blade leading edge is modeled with a simplified closed-form solution of Goldstein's rapid distortion theory. This model, based on vorticity deflection, is extended here beyond the high-frequency approximation and integrated into an analytical Amiet-based IT noise tool. Applications to representative test cases show that while distortion effects are minimal for current turbine sizes, they become relevant for future configurations featuring larger rotor sizes and thicker airfoils. The developed model reveals that IT noise levels do not necessarily scale with rotor size but are shaped by spectral changes induced by the blade geometry, operational parameters, and inflow conditions. This model offers a physically consistent, computationally efficient framework for the aeroacoustic assessment of next-generation wind turbine design.
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This study examines the role of turbulence distortion in predicting inflow turbulence (IT) noise generation from large wind turbines via Amiet's theory. Two subsequent distortion mechanisms are investigated: (i) the streamtube expansion in the rotor induction zone and (ii) the interaction with the surface of thick-blade profiles. Large-eddy simulations reveal that the turbulence spectra, which reflect distortion effects, remain largely unaffected by rotor induction within the frequency range relevant for noise generation. As for the other mechanism, the distortion of the turbulence approaching a blade leading edge is modeled with a simplified closed-form solution of Goldstein's rapid distortion theory. This model, based on vorticity deflection, is extended here beyond the high-frequency approximation and integrated into an analytical Amiet-based IT noise tool. Applications to representative test cases show that while distortion effects are minimal for current turbine sizes, they become relevant for future configurations featuring larger rotor sizes and thicker airfoils. The developed model reveals that IT noise levels do not necessarily scale with rotor size but are shaped by spectral changes induced by the blade geometry, operational parameters, and inflow conditions. This model offers a physically consistent, computationally efficient framework for the aeroacoustic assessment of next-generation wind turbine design.