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L.T. Lima Pereira
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25 records found
1
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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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.
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.
Unified Framework for Scalable Vertiport Allocation with Heterogeneous Fleet Sizing
Case Study on the Republic of Ireland
Advanced Air Mobility (AAM) is expected to transform urban and regional transportation. However, its successful deployment hinges on robust infrastructure planning that balances operational efficiency with scalable growth. Current vertiport allocation models typically deliver single, static network solutions without considering phased growth or aircraft performance limitations. This gap is addressed by presenting a unified framework that generates a scalable vertiport allocation plan in conjunction with system-performance-based heterogeneous fleet sizing. First, potential vertiport networks are generated through a distance-based agglomerative clustering algorithm applied to the census-based synthetically generated demand. Second, the mean vertiport network is established, and undergoes an elimination procedure aimed at maximizing passenger km travelled, to establish phased growth of the vertiport network. In order to determine the optimal development of the fleet as the network grows, the framework employs an agent-based simulation and conducts a parameter sweep across all combinations of vertiport network size, fleet size and fleet composition. This framework is validated on a use case on the Republic of Ireland. The results demonstrate the ability of the framework to deliver a demand-driven phased network introduction and expansion strategy, while identifying the optimal fleet size and composition at each growth stage. This approach provides stakeholders with a replicable foundation for capital allocation and regulatory planning in emerging AAM markets.
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Advanced Air Mobility (AAM) is expected to transform urban and regional transportation. However, its successful deployment hinges on robust infrastructure planning that balances operational efficiency with scalable growth. Current vertiport allocation models typically deliver single, static network solutions without considering phased growth or aircraft performance limitations. This gap is addressed by presenting a unified framework that generates a scalable vertiport allocation plan in conjunction with system-performance-based heterogeneous fleet sizing. First, potential vertiport networks are generated through a distance-based agglomerative clustering algorithm applied to the census-based synthetically generated demand. Second, the mean vertiport network is established, and undergoes an elimination procedure aimed at maximizing passenger km travelled, to establish phased growth of the vertiport network. In order to determine the optimal development of the fleet as the network grows, the framework employs an agent-based simulation and conducts a parameter sweep across all combinations of vertiport network size, fleet size and fleet composition. This framework is validated on a use case on the Republic of Ireland. The results demonstrate the ability of the framework to deliver a demand-driven phased network introduction and expansion strategy, while identifying the optimal fleet size and composition at each growth stage. This approach provides stakeholders with a replicable foundation for capital allocation and regulatory planning in emerging AAM markets.
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.
Current eVTOL certification regulations rely on measurements of the vehicle under controlled, and mostly steady conditions, whereas actual flight operations often involve transient maneuvers and continuously varying rotor operating states. To assess differences in real unsteady operations, this work focuses on eVTOL rotors experiencing unsteady rotational speeds, which are common in electric motors controlled by automatic controllers. The aeroacoustic behavior of a scaled eVTOL rotor undergoing harmonic rotational velocity changes is experimentally investigated in the anechoic wind tunnel of the Delft University of Technology. This was achieved by imposing a harmonic variation in rotational speed of different frequencies ( fℎ) and amplitudes. It is observed that the differences between steady state and unsteady conditions are governed by the reduced frequency parameter, kℎ, which depends on the harmonic frequency and the dimensionless amplitude, λ. The analysis reveals that harmonic variations in rotational speed result in a significant decrease in the rotor’s mean thrust relative to the equivalent static rotational speed, reaching up to 18%. Tonal noise is found to spread around the BPF harmonics, forming a hump containing multiple peaks with a frequency range proportional to the harmonic amplitude of the rotational speed. At higher harmonics, the broadened humps around adjacent BPF tones overlap, resulting in stronger spectral tonal content. In terms of sound pressure levels, a maximum decrease of 2 dB in the tonal noise occurs when the harmonic frequency varies from 0.5 Hz to 2 Hz. At the highest tested frequency, 5 Hz, the tonal noise increases again, and the thrust no longer follows the rotational speed variation as closely as at lower frequencies. This suggests the onset of divergent unsteady behavior, causing deviations from the quasi-steady trends observed at lower frequencies.
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Current eVTOL certification regulations rely on measurements of the vehicle under controlled, and mostly steady conditions, whereas actual flight operations often involve transient maneuvers and continuously varying rotor operating states. To assess differences in real unsteady operations, this work focuses on eVTOL rotors experiencing unsteady rotational speeds, which are common in electric motors controlled by automatic controllers. The aeroacoustic behavior of a scaled eVTOL rotor undergoing harmonic rotational velocity changes is experimentally investigated in the anechoic wind tunnel of the Delft University of Technology. This was achieved by imposing a harmonic variation in rotational speed of different frequencies ( fℎ) and amplitudes. It is observed that the differences between steady state and unsteady conditions are governed by the reduced frequency parameter, kℎ, which depends on the harmonic frequency and the dimensionless amplitude, λ. The analysis reveals that harmonic variations in rotational speed result in a significant decrease in the rotor’s mean thrust relative to the equivalent static rotational speed, reaching up to 18%. Tonal noise is found to spread around the BPF harmonics, forming a hump containing multiple peaks with a frequency range proportional to the harmonic amplitude of the rotational speed. At higher harmonics, the broadened humps around adjacent BPF tones overlap, resulting in stronger spectral tonal content. In terms of sound pressure levels, a maximum decrease of 2 dB in the tonal noise occurs when the harmonic frequency varies from 0.5 Hz to 2 Hz. At the highest tested frequency, 5 Hz, the tonal noise increases again, and the thrust no longer follows the rotational speed variation as closely as at lower frequencies. This suggests the onset of divergent unsteady behavior, causing deviations from the quasi-steady trends observed at lower frequencies.
This study describes a design exploration of Urban Air Mobility (UAM) vehicles, based on top-level aircraft requirements. The exploration focuses on a fully electric vertical takeoff and landing (eVTOL) vehicle that employs an architecture of conventional airframe coupled with tilted rotors, aimed to carry four passengers. Using a low-fidelity design framework, a variety of configurations are investigated by altering design variables such as wing area, number of propellers, operating speed, and range. The influence of these variables on the design is explored from the environmental, societal and, economical perspectives for a 2050 time horizon. The findings suggest that configurations with a small wing area and a large number of small propellers emerges as preferable for minimizing energy consumption (per pax-km) and operating expenses (per pax-km). However, in terms of noise emissions, configurations with fewer but larger propellers are favoured, marking a departure from the design choices that prioritize energy efficiency and cost. Additionally, the study underscores that operations prioritizing commercial viability require high-speed cruising and reduced flight hours, diverging from those that prioritize energy efficiency, thereby emphasizing the necessity of multidisciplinary optimization. Finally, a noise-estimation model is developed to enable quick assessment of the vehicle's sound power level. The model necessitates only fundamental powertrain information as inputs and provides insights into the impact of design choices on noise emission, which is beneficial at preliminary design stage.
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This study describes a design exploration of Urban Air Mobility (UAM) vehicles, based on top-level aircraft requirements. The exploration focuses on a fully electric vertical takeoff and landing (eVTOL) vehicle that employs an architecture of conventional airframe coupled with tilted rotors, aimed to carry four passengers. Using a low-fidelity design framework, a variety of configurations are investigated by altering design variables such as wing area, number of propellers, operating speed, and range. The influence of these variables on the design is explored from the environmental, societal and, economical perspectives for a 2050 time horizon. The findings suggest that configurations with a small wing area and a large number of small propellers emerges as preferable for minimizing energy consumption (per pax-km) and operating expenses (per pax-km). However, in terms of noise emissions, configurations with fewer but larger propellers are favoured, marking a departure from the design choices that prioritize energy efficiency and cost. Additionally, the study underscores that operations prioritizing commercial viability require high-speed cruising and reduced flight hours, diverging from those that prioritize energy efficiency, thereby emphasizing the necessity of multidisciplinary optimization. Finally, a noise-estimation model is developed to enable quick assessment of the vehicle's sound power level. The model necessitates only fundamental powertrain information as inputs and provides insights into the impact of design choices on noise emission, which is beneficial at preliminary design stage.
Conference paper
(2025)
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Yannick G.A. Chance, Lourenco Tercio Lima Pereira, Daniele Ragni, Riccardo Zamponi
Acoustic annoyance is a keen factor in the social acceptance of novel urban air mobility concepts. Although regulations and certification requirements exist for such operations, they rely on measurements of the vehicle under controlled, and mostly steady, conditions. These conditions differ significantly from real envisioned operations, where turbulence from the urban environment, rapid maneuvers, system automatic control, and gusts can affect the vehicle’s noise emissions. To assess such differences, this work focuses on the study of rotors, commonly applied to urban air mobility and transport vehicles, under varying rotational speeds. An experimental campaign is carried out in the anechoic wind tunnel of the Delft University of Technology, where an unsteady rotational speed of the rotor is prescribed. Acoustic measurements are carried out along with the integral loads of the rotor. The work explores both the aerodynamic effects of such an operation and its impact on noise emissions. The final goal is to create a global picture of the relevance and physics of rotor noise under non-steady rotational speeds.
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Acoustic annoyance is a keen factor in the social acceptance of novel urban air mobility concepts. Although regulations and certification requirements exist for such operations, they rely on measurements of the vehicle under controlled, and mostly steady, conditions. These conditions differ significantly from real envisioned operations, where turbulence from the urban environment, rapid maneuvers, system automatic control, and gusts can affect the vehicle’s noise emissions. To assess such differences, this work focuses on the study of rotors, commonly applied to urban air mobility and transport vehicles, under varying rotational speeds. An experimental campaign is carried out in the anechoic wind tunnel of the Delft University of Technology, where an unsteady rotational speed of the rotor is prescribed. Acoustic measurements are carried out along with the integral loads of the rotor. The work explores both the aerodynamic effects of such an operation and its impact on noise emissions. The final goal is to create a global picture of the relevance and physics of rotor noise under non-steady rotational speeds.
TUC-TUC
Development of a test bench for the study of rotor aeroacoustics
This work describes the development of a test bench that allows for a complete assessment of the aerodynamic characteristics and the acoustic emissions of a rotor in flight-like operating conditions. The rotor is named TUC-TUC, after the TU delft Characterization model for roTor aeroacoUstiCs. Its design is driven by the ability of precisely control its configuration and load distribution while facilitating a holistic set of aeroacoustic measurement techniques to take place. The design rationale, technical developments, experimental plans, estimated performance and noise emissions are shown in this study.
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This work describes the development of a test bench that allows for a complete assessment of the aerodynamic characteristics and the acoustic emissions of a rotor in flight-like operating conditions. The rotor is named TUC-TUC, after the TU delft Characterization model for roTor aeroacoUstiCs. Its design is driven by the ability of precisely control its configuration and load distribution while facilitating a holistic set of aeroacoustic measurement techniques to take place. The design rationale, technical developments, experimental plans, estimated performance and noise emissions are shown in this study.
This work describes a multidisciplinary framework proposed for the preliminary design assessment of urban air mobility (UAM) vehicles, which includes estimations of vehicle per- formance and acoustic emissions. Established analytical and semi-empirical methods for aircraft design and performance are combined with estimations of tonal and broadband noise components from the multi-propeller system. The estimation of the tonal components is based on steady blade loading and thickness noise, while the estimation of the broadband compo- nent considers trailing-edge scattering of each isolated propeller. Two design architectures, specifically multicopters and tilted open rotors, are assessed in this investigation. The estimated performance and noise emissions are demonstrated and compared against the available data of existing vehicles. Subsequently, a design exploration is conducted, where key performance parameters are varied and their impact on the vehicle performance, estimated cost, power consumption, and noise emissions are evaluated. The results stress the suitability of each configuration for urban air mobility, along with the operating conditions under which each architecture performs optimally. Multicopter vehicles are lighter for small ranges and speeds and, therefore, are the optimal choice for short distances. The lower MTOM also yields lower noise emissions in takeoff and landing, facilitating integration in populated urban environments. Tiltrotors have greater efficiency at high cruise speeds and, at larger ranges, demonstrate more efficient operations and, consequently, lower noise emissions, more suitable for inter-city and large commuting distances.
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This work describes a multidisciplinary framework proposed for the preliminary design assessment of urban air mobility (UAM) vehicles, which includes estimations of vehicle per- formance and acoustic emissions. Established analytical and semi-empirical methods for aircraft design and performance are combined with estimations of tonal and broadband noise components from the multi-propeller system. The estimation of the tonal components is based on steady blade loading and thickness noise, while the estimation of the broadband compo- nent considers trailing-edge scattering of each isolated propeller. Two design architectures, specifically multicopters and tilted open rotors, are assessed in this investigation. The estimated performance and noise emissions are demonstrated and compared against the available data of existing vehicles. Subsequently, a design exploration is conducted, where key performance parameters are varied and their impact on the vehicle performance, estimated cost, power consumption, and noise emissions are evaluated. The results stress the suitability of each configuration for urban air mobility, along with the operating conditions under which each architecture performs optimally. Multicopter vehicles are lighter for small ranges and speeds and, therefore, are the optimal choice for short distances. The lower MTOM also yields lower noise emissions in takeoff and landing, facilitating integration in populated urban environments. Tiltrotors have greater efficiency at high cruise speeds and, at larger ranges, demonstrate more efficient operations and, consequently, lower noise emissions, more suitable for inter-city and large commuting distances.
The present study focuses on the application of finlet rails as a passive technique of flow control to mitigate trailing-edge noise. Finlet rails are small cylinders whose axes are aligned along the streamwise direction, transversally positioned with respect to the trailing edge. In the first part of this study, the effects of finlet geometry on the aeroacoustic emission of a NACA 633−018 airfoil are investigated using an array of microphones. It is observed that reducing the transversal spacing of finlet rails leads to increasing the maximum noise reduction, found to be of 4 decibels at relatively low frequencies. An optimum for the height of the finlets was determined, equivalent to 1.6δ∗, where δ∗ is the displacement thickness of the boundary layer. With the aim of unveiling the underlying physical mechanism for finlet rails, PIV at high spatial resolution is applied around the surface treatment. It is found that the turbulence energy is lifted-up and moved away from the scattering edge, which attenuates the wall-pressure fluctuations. The observed attenuation of the wall-pressure fluctuations occurs at the energy-containing scales, which is an important difference with finlet fences. In the region underneath the finlet rails, the transversal size of the energetic structures diminishes when the surface treatment is applied. The combination of the lift-up of the turbulence structures, that reduces the wall-pressure fluctuations, with the smaller turbulence scales is responsible for the noise reduction observed for finlet rails.
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The present study focuses on the application of finlet rails as a passive technique of flow control to mitigate trailing-edge noise. Finlet rails are small cylinders whose axes are aligned along the streamwise direction, transversally positioned with respect to the trailing edge. In the first part of this study, the effects of finlet geometry on the aeroacoustic emission of a NACA 633−018 airfoil are investigated using an array of microphones. It is observed that reducing the transversal spacing of finlet rails leads to increasing the maximum noise reduction, found to be of 4 decibels at relatively low frequencies. An optimum for the height of the finlets was determined, equivalent to 1.6δ∗, where δ∗ is the displacement thickness of the boundary layer. With the aim of unveiling the underlying physical mechanism for finlet rails, PIV at high spatial resolution is applied around the surface treatment. It is found that the turbulence energy is lifted-up and moved away from the scattering edge, which attenuates the wall-pressure fluctuations. The observed attenuation of the wall-pressure fluctuations occurs at the energy-containing scales, which is an important difference with finlet fences. In the region underneath the finlet rails, the transversal size of the energetic structures diminishes when the surface treatment is applied. The combination of the lift-up of the turbulence structures, that reduces the wall-pressure fluctuations, with the smaller turbulence scales is responsible for the noise reduction observed for finlet rails.
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.
This preliminary study evaluates the expected psychoacoustics annoyance of an array of distributed propellers operating at different synchrophasing angles between the propeller blades. The study, conducted in a hybrid test section wind tunnel, focused on a three-propeller array in a tractor configuration. The noise emissions were measured using a phased microphone array. Both conventional noise metrics and state-of-the-art psychoacoustic metrics were calculated, enabling a thorough analysis of the noise characteristics produced by these configurations and their expected noise perception. The study found that the relative blade-phase angle significantly influences the levels of the conventional noise metrics and psychoacoustic metrics. These results highlight the potential of controlling relative blade-phase angles to reduce noise annoyance. They also emphasize the importance of conducting psychoacoustic analysis in design configurations like the one examined here.
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This preliminary study evaluates the expected psychoacoustics annoyance of an array of distributed propellers operating at different synchrophasing angles between the propeller blades. The study, conducted in a hybrid test section wind tunnel, focused on a three-propeller array in a tractor configuration. The noise emissions were measured using a phased microphone array. Both conventional noise metrics and state-of-the-art psychoacoustic metrics were calculated, enabling a thorough analysis of the noise characteristics produced by these configurations and their expected noise perception. The study found that the relative blade-phase angle significantly influences the levels of the conventional noise metrics and psychoacoustic metrics. These results highlight the potential of controlling relative blade-phase angles to reduce noise annoyance. They also emphasize the importance of conducting psychoacoustic analysis in design configurations like the one examined here.
This work proposes a semi-empirical framework to predict the noise of wind turbines with serrated trailing edge blades. The framework is employed for studying the reduction of the noise of the SWT 2.3-93 benchmark wind turbine. The framework is verified against field acoustic measurements of the real wind-turbine model and of noise reduction measured for airfoil geometries with serrated trailing edges. Two different serration design strategies are proposed, respectively one with the same serration geometry along the blade and one with serrations scaled with the local boundary-layer properties along the radius. Results show the predicted noise reduction obtained with each of the add-ons and explore the benefits of tailoring the design of the serrations according to the varying flow conditions along the blade span.
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This work proposes a semi-empirical framework to predict the noise of wind turbines with serrated trailing edge blades. The framework is employed for studying the reduction of the noise of the SWT 2.3-93 benchmark wind turbine. The framework is verified against field acoustic measurements of the real wind-turbine model and of noise reduction measured for airfoil geometries with serrated trailing edges. Two different serration design strategies are proposed, respectively one with the same serration geometry along the blade and one with serrations scaled with the local boundary-layer properties along the radius. Results show the predicted noise reduction obtained with each of the add-ons and explore the benefits of tailoring the design of the serrations according to the varying flow conditions along the blade span.
This work addresses the importance of considering the coherent organization of turbulence structures when predicting the noise from serrated trailing edges. Turbulent flow is the source of broadband trailing-edge noise. The existence of coherence in the turbulent flow departs from the state-of-the-art assumption for noise prediction based on the sum of the different wavenumbers excitations in an incoherent manner. This study addresses whether the latter hypothesis is the underlying cause of the noise underestimation observed from theoretical models for serrated trailing edges at high frequencies. A simplified hairpin model in the form of a bounded vortex filament is used as input to the scattering solution. The vortex filament is used to compute the turbulent velocity and wall-pressure fluctuations induced by the modelled coherent structure. This coherent wall-pressure structure is given as input to a numerical acoustic solver of the diffraction problem, yielding the scattered acoustic field. Results focus on the differences between coherent and incoherent assumptions. It is demonstrated that the acoustic scattering of coherent structures differs from the incoherent sum of the wavenumber spectrum of such structures, showing more consistent results with experimental evidences. The results indicates that modelling of the noise from serrated trailing edges can be improved with a detailed description of the turbulent flow.
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This work addresses the importance of considering the coherent organization of turbulence structures when predicting the noise from serrated trailing edges. Turbulent flow is the source of broadband trailing-edge noise. The existence of coherence in the turbulent flow departs from the state-of-the-art assumption for noise prediction based on the sum of the different wavenumbers excitations in an incoherent manner. This study addresses whether the latter hypothesis is the underlying cause of the noise underestimation observed from theoretical models for serrated trailing edges at high frequencies. A simplified hairpin model in the form of a bounded vortex filament is used as input to the scattering solution. The vortex filament is used to compute the turbulent velocity and wall-pressure fluctuations induced by the modelled coherent structure. This coherent wall-pressure structure is given as input to a numerical acoustic solver of the diffraction problem, yielding the scattered acoustic field. Results focus on the differences between coherent and incoherent assumptions. It is demonstrated that the acoustic scattering of coherent structures differs from the incoherent sum of the wavenumber spectrum of such structures, showing more consistent results with experimental evidences. The results indicates that modelling of the noise from serrated trailing edges can be improved with a detailed description of the turbulent flow.
Wind-turbine noise can restrict the growing implementation of renewable energy sources and their application close to urban environments. The largest contributor to the noise of modern turbines is the scattering of the turbulent fluctuations at the blade trailing edge. This source of noise is directly correlated with the turbine’s extracted power. Therefore, operating in noise-restricted environments and at night times entails lower energy production. An extensively applied solution for reducing the noise of wind turbines is the use of trailing-edge serrations, i.e. imposing periodic variations in the geometry of the blade trailing edge. Serrations reduce the effectiveness of the scattering at the trailing edge as the turbulent fluctuations reach the trailing edge at different times along the blade span, consequently reducing the wind-turbine noise. Although extensive literature and knowledge exist on serrations, their measured performance does not compare with the predicted one. Even more problematic, the trends predicted for the geometric alterations of the serrations are not observed in reality. Notably, two things are worth mentioning: first, geometries shown as optimal by theory perform worse than other concepts, and second, the noise from serrations is affected by the angle between the insert and the flow. As a result, the design of trailing-edge serrations still requires dedicated experiments and numerical simulations, hampering the assessment of several geometries necessary for complete optimization of the serration design. This work seeks a physical interpretation of the noise generation mechanisms of trailing edges with serrated add-ons. This interpretation is focused on understanding the underlying physical principles of the flow surrounding a serrated trailing edge. This is carried out in this work with three studies, respectively on the observation, modelling, and control of the flow and acoustic properties of serrated trailing edges...
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Wind-turbine noise can restrict the growing implementation of renewable energy sources and their application close to urban environments. The largest contributor to the noise of modern turbines is the scattering of the turbulent fluctuations at the blade trailing edge. This source of noise is directly correlated with the turbine’s extracted power. Therefore, operating in noise-restricted environments and at night times entails lower energy production. An extensively applied solution for reducing the noise of wind turbines is the use of trailing-edge serrations, i.e. imposing periodic variations in the geometry of the blade trailing edge. Serrations reduce the effectiveness of the scattering at the trailing edge as the turbulent fluctuations reach the trailing edge at different times along the blade span, consequently reducing the wind-turbine noise. Although extensive literature and knowledge exist on serrations, their measured performance does not compare with the predicted one. Even more problematic, the trends predicted for the geometric alterations of the serrations are not observed in reality. Notably, two things are worth mentioning: first, geometries shown as optimal by theory perform worse than other concepts, and second, the noise from serrations is affected by the angle between the insert and the flow. As a result, the design of trailing-edge serrations still requires dedicated experiments and numerical simulations, hampering the assessment of several geometries necessary for complete optimization of the serration design. This work seeks a physical interpretation of the noise generation mechanisms of trailing edges with serrated add-ons. This interpretation is focused on understanding the underlying physical principles of the flow surrounding a serrated trailing edge. This is carried out in this work with three studies, respectively on the observation, modelling, and control of the flow and acoustic properties of serrated trailing edges...
Journal article
(2023)
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Salil Luesutthiviboon, Lourenço Tércio Lima Pereira, Daniele Ragni, Francesco Avallone, Mirjam Snellen
Experimental results on trailing-edge (TE) noise from a NACA 633 –018 airfoil are presented for a chord-based Reynolds number Rec range between 2 × 105 and 3 × 106. Far-field TE noise from the baseline airfoil with a straight TE and TE serrations is measured with varying Rec, angle of attack, and serration shape and flap angle. Additionally, aerodynamic coefficients and boundary-layer parameters at the TE are also reported. To cover such a broad Rec range, two NACA 633 –018 airfoil models were tested in two different wind tunnels. The measurements include the emitted noise with natural and forced transition locations. For the straight TE, the forced transition location results in up to 5 dB increase of the far-field TE noise level, compared to the natural one. Scaling of the far-field noise spectra from the baseline TE shows that the Strouhal numbers St at which the peak noise level is measured reduce as Rec increases. TE noise spectra for the cases with the TE serrations are found to be dependent on the airfoil lift and Rec. The present data are to be included in the framework of the Benchmark Problems for Airframe Noise Computations category I and are publicly available in a repository with the following digital object identifier (DOI): https://doi.org/10.4121/20940646.
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Experimental results on trailing-edge (TE) noise from a NACA 633 –018 airfoil are presented for a chord-based Reynolds number Rec range between 2 × 105 and 3 × 106. Far-field TE noise from the baseline airfoil with a straight TE and TE serrations is measured with varying Rec, angle of attack, and serration shape and flap angle. Additionally, aerodynamic coefficients and boundary-layer parameters at the TE are also reported. To cover such a broad Rec range, two NACA 633 –018 airfoil models were tested in two different wind tunnels. The measurements include the emitted noise with natural and forced transition locations. For the straight TE, the forced transition location results in up to 5 dB increase of the far-field TE noise level, compared to the natural one. Scaling of the far-field noise spectra from the baseline TE shows that the Strouhal numbers St at which the peak noise level is measured reduce as Rec increases. TE noise spectra for the cases with the TE serrations are found to be dependent on the airfoil lift and Rec. The present data are to be included in the framework of the Benchmark Problems for Airframe Noise Computations category I and are publicly available in a repository with the following digital object identifier (DOI): https://doi.org/10.4121/20940646.
Journal article
(2023)
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Lourenço Tércio Lima Pereira, Francesco Avallone, Daniele Ragni, Fulvio Scarano
This work discusses the physics of noise reduction achieved from serrated trailing–edges and its impact by the serration design. An experimental campaign is carried out with a benchmark 2D model based on a NACA 633–018 airfoil. Different trailing-edge serrations are tested under several flow speeds and angles of attack conditions to build a complete dataset of acoustic measurements. Systematic modifications of a reference sawtooth serration design are made to its scale and geometry. Scale modifications are based on sawtooth serrations and comprehend carefully considered variations of the serration height (2h), wavenumber (λ), and aspect ratio (2h/λ). Geometric shape modifications are represented by concave–shaped and combed–sawtooth serrations. This study represents a unique sensitivity–based parametric analysis on the scaling and geometric properties of trailing–edge serrations where the impacts of each modification are studied separately. The results obtained are used to provide guidelines for serration design choices and their impact on broadband noise reduction.
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This work discusses the physics of noise reduction achieved from serrated trailing–edges and its impact by the serration design. An experimental campaign is carried out with a benchmark 2D model based on a NACA 633–018 airfoil. Different trailing-edge serrations are tested under several flow speeds and angles of attack conditions to build a complete dataset of acoustic measurements. Systematic modifications of a reference sawtooth serration design are made to its scale and geometry. Scale modifications are based on sawtooth serrations and comprehend carefully considered variations of the serration height (2h), wavenumber (λ), and aspect ratio (2h/λ). Geometric shape modifications are represented by concave–shaped and combed–sawtooth serrations. This study represents a unique sensitivity–based parametric analysis on the scaling and geometric properties of trailing–edge serrations where the impacts of each modification are studied separately. The results obtained are used to provide guidelines for serration design choices and their impact on broadband noise reduction.
Conference paper
(2022)
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G. Vergés i Plaza, Andreas Fischer, Oliver Lylloff, Christian Bak, Anders S. Olsen, S. Luesutthiviboon, L.T. Lima Pereira, D. Ragni, F. Avallone, More authors...
An experimental aero-acoustic characterisation of the NACA 63
3-018 airfoil is presented in this study, featuring trailing-edge noise emissions with and without serrations. Measurements have been carried out for a chord-based Reynolds number range between 0.18 × 10
6 and 4.8 × 10
6 . Two airfoil models with different chord lengths have been tested in five different wind tunnels. The goal is to compare the measurements in different facilities, quantify the uncertainties, and establish a validation database that can serve as a benchmark for computational studies. The tests have been performed with clean and forced-transition boundary layers for a variety of angles of attack. The effect on the spectral slope and peak levels is evaluated. Scaling laws have been applied to compare different test conditions. The quality and nature of the collapse, as well as the applicability limits of the scaling, are examined. Different serration geometries have been tested at different flap angles. The noise reduction dependence on the aerodynamic loading is discussed. This work is based on an initiative of Task 39 "Quiet Wind Turbine Technology" of the Technology Collaboration Programme (TCP) of the International Energy Agency (IEA).
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An experimental aero-acoustic characterisation of the NACA 63 3-018 airfoil is presented in this study, featuring trailing-edge noise emissions with and without serrations. Measurements have been carried out for a chord-based Reynolds number range between 0.18 × 10 6 and 4.8 × 10 6 . Two airfoil models with different chord lengths have been tested in five different wind tunnels. The goal is to compare the measurements in different facilities, quantify the uncertainties, and establish a validation database that can serve as a benchmark for computational studies. The tests have been performed with clean and forced-transition boundary layers for a variety of angles of attack. The effect on the spectral slope and peak levels is evaluated. Scaling laws have been applied to compare different test conditions. The quality and nature of the collapse, as well as the applicability limits of the scaling, are examined. Different serration geometries have been tested at different flap angles. The noise reduction dependence on the aerodynamic loading is discussed. This work is based on an initiative of Task 39 "Quiet Wind Turbine Technology" of the Technology Collaboration Programme (TCP) of the International Energy Agency (IEA).
Journal article
(2022)
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Lourenço Tércio Lima Pereira, Francesco Avallone, Daniele Ragni, Fulvio Scarano
A physical description of the flow mechanisms that govern the distribution of the wall-pressure fluctuations over the surface of a serrated trailing edge is proposed. Three main mechanisms that define the variation of turbulent pressure fluctuations across the serrated edge are discussed and semi-empirical models are formulated accordingly. It is shown that the intensity of the wall-pressure fluctuations increases at the tips under the effect of an increased convective velocity as a result of sidewise momentum diffusion. Furthermore, the change of impedance across the edge causes a local reduction of the pressure fluctuations in the vicinity of the trailing edge. Finally, aerodynamic loading over the serrations due to the non-symmetric flow created at different angles of attack establishes secondary flow patterns that induce higher wall-pressure fluctuations over the serration edges. The latter effect is present only for serrations under high aerodynamic loading, while the former ones are observed under any conditions. Semi-empirical models are formulated for predicting the variation of the wall-pressure fluctuations over the serration surface based on the three physical mechanisms described. These models are calibrated and compared against experiments conducted on a symmetric airfoil model at high Reynolds numbers.
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A physical description of the flow mechanisms that govern the distribution of the wall-pressure fluctuations over the surface of a serrated trailing edge is proposed. Three main mechanisms that define the variation of turbulent pressure fluctuations across the serrated edge are discussed and semi-empirical models are formulated accordingly. It is shown that the intensity of the wall-pressure fluctuations increases at the tips under the effect of an increased convective velocity as a result of sidewise momentum diffusion. Furthermore, the change of impedance across the edge causes a local reduction of the pressure fluctuations in the vicinity of the trailing edge. Finally, aerodynamic loading over the serrations due to the non-symmetric flow created at different angles of attack establishes secondary flow patterns that induce higher wall-pressure fluctuations over the serration edges. The latter effect is present only for serrations under high aerodynamic loading, while the former ones are observed under any conditions. Semi-empirical models are formulated for predicting the variation of the wall-pressure fluctuations over the serration surface based on the three physical mechanisms described. These models are calibrated and compared against experiments conducted on a symmetric airfoil model at high Reynolds numbers.