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L.T. Lima Pereira
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1
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.
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.
...
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.
Self-sustained whistling in long-cavity Helmholtz resonators with bias flow
Experimental characterization and analytical modelling
Journal article
(2026)
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Andrey R. da Silva, Niklaus Lima, Julio Cordioli, Tsukasa Yoshinaga, Tercio L. Pereira
This study examines the hydrodynamic-acoustic feedback mechanisms responsible for whistling in double-neck Helmholtz resonators under bias flow, with emphasis on long-cavity configurations representative of human whistling. Combined flow and acoustic experiments were performed using particle image velocimetry and pressure measurements to characterize vortex dynamics, sound generation, and oscillation regimes across a range of Reynolds numbers and geometric parameters. Within the self-sustained oscillation regime, sound pressure levels scale consistently with the Strouhal number, exhibiting a peak near St=0.32. Experimental data collapse is achieved using a modified Reynolds number that incorporates cavity-length effects. Moreover, two-point coherence and cross-spectral analyses show that acoustic generation occurs predominantly inside the resonator chamber, with maximum sound production located approximately 2.3 diameters downstream of the upstream neck. Flow visualization reveals that vortices dissipate before reaching the downstream neck, indicating a feedback mechanism distinct from classical hole-tone and Rossiter-type oscillations. To interpret these findings, an empirical model was developed, consisting of a two-degree-of-freedom lumped acoustic system coupled to a nonlinear oscillator representing vortex shedding. The model reproduces the onset, lock-in, saturation, and abrupt extinction of self-sustained oscillations, capturing the essential coupling that transfers energy from the mean flow to the acoustic field. These results provide new insight into long-cavity whistles and offer a predictive framework for Helmholtz-type resonators under bias flow.
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This study examines the hydrodynamic-acoustic feedback mechanisms responsible for whistling in double-neck Helmholtz resonators under bias flow, with emphasis on long-cavity configurations representative of human whistling. Combined flow and acoustic experiments were performed using particle image velocimetry and pressure measurements to characterize vortex dynamics, sound generation, and oscillation regimes across a range of Reynolds numbers and geometric parameters. Within the self-sustained oscillation regime, sound pressure levels scale consistently with the Strouhal number, exhibiting a peak near St=0.32. Experimental data collapse is achieved using a modified Reynolds number that incorporates cavity-length effects. Moreover, two-point coherence and cross-spectral analyses show that acoustic generation occurs predominantly inside the resonator chamber, with maximum sound production located approximately 2.3 diameters downstream of the upstream neck. Flow visualization reveals that vortices dissipate before reaching the downstream neck, indicating a feedback mechanism distinct from classical hole-tone and Rossiter-type oscillations. To interpret these findings, an empirical model was developed, consisting of a two-degree-of-freedom lumped acoustic system coupled to a nonlinear oscillator representing vortex shedding. The model reproduces the onset, lock-in, saturation, and abrupt extinction of self-sustained oscillations, capturing the essential coupling that transfers energy from the mean flow to the acoustic field. These results provide new insight into long-cavity whistles and offer a predictive framework for Helmholtz-type resonators under bias flow.
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.
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 = 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 = 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 = 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 = 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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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.
...
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.
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.
...
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.
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.
...
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.
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.
...
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.
...
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 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.
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 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.
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.
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.
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...
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.