10:30
Experimental Aeroacoustic Measurements – Laboratory
10:30
20 mins
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Understanding and Controlling Laminar Separation Bubbles for Quieter Drone Propellers
SIMON WATKINS
Abstract: Small multi rotor drones and emerging Advanced Air Mobility vehicles rely on propellers that operate at low Reynolds numbers, where laminar boundary layer separation and the formation of laminar separation bubbles are common. These flow phenomena reduce aerodynamic efficiency and contribute significantly to increased broadband and tonal noise, limiting endurance, payload capability, and community acceptance in noise sensitive environments. Despite the importance of this regime, experimental data on low Reynolds number propellers and effective flow control strategies remains scarce.
This work presents an experimental investigation into the aerodynamic and acoustic performance of a propeller operating at low Reynolds numbers, with a focus on boundary layer augmentation through surface tripping. The primary research questions address how trip type and placement influence blade surface flow behaviour, propeller efficiency, and noise generation, and whether spanwise specific tripping can offer further performance benefits. A 9×6 propeller operating at a tip Reynolds number range of 4.3×10⁴ to 4.38×10⁴ was tested in a wind tunnel over wind tunnel speeds from 0 to 40 km/h. Aerodynamic forces and moments were measured using a calibrated force and moment balance, while acoustic measurements were obtained using a Brüel and Kjær sound level meter. Surface oil flow visualisation was employed to directly observe boundary layer behaviour, separation, reattachment, and vortex shedding associated with laminar separation bubbles. Tests were conducted in both static conditions and in a controlled inflow corresponding to a max of 40 km/h.
The results show that the chordwise location of boundary layer tripping plays a critical role in performance enhancement. When trips are positioned upstream of the natural laminar separation location, propulsive efficiency improves due to increased thrust generation with nominally similar torque levels. Oil flow visualisations confirm that appropriate tripping suppresses or weakens laminar separation bubbles and associated unsteady vortex shedding. Spanwise variations in Reynolds number were found to produce distinct flow responses along the blade, motivating the investigation of spanwise specific tripping strategies. This approach demonstrates potential efficiency gains beyond those achieved using uniform tripping methods (trips all the way to the tip of the propeller).
Overall, this study provides new experimental insight into low Reynolds number propeller aerodynamics and acoustics, introduces the concept of spanwise specific boundary layer tripping, and contributes data that can inform quieter and more efficient propeller designs for drones and future AAM applications.
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10:50
20 mins
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Potential annoyance from air-taxi manoeuvres in vertiport residents
Julia V. Lippold
Abstract: As urban transportation evolves to bypass traffic, reduce environmental impact, and improve connectivity, the rise of air-based mobility options like electric air-taxis offers a promising alternative to traditional ground transportation. However, the introduction of these air-taxis poses challenges in addressing potential noise-induced annoyance and disturbance in both urban and rural areas, underscoring the importance of balancing the benefits of UAM with residents’ needs and noise pollution limits.
To investigate the potential impact of air-taxi noise, we conducted a laboratory study with 48 participants (gender balanced, aged 19-70 years), testing a total of 144 different scenarios, compiled of the following factors: 4 numbers of movements during the scenario (between 1 and 7 air-taxis), 9 different observer points, consisting of 3 under the trajectory along the center line, 5 on sidelines and 1 in opposite direction to the flight path, urban versus rural background noise context, and 2 flight maneuvers (departure and approach). The scenarios were lasting 23.5 min and are based on auralized air-taxi sounds of a multi-rotor-driven air-taxi with four fixed plus two tilting rotors and real recorded background noises.
The subjects were exposed to six different scenarios, each experiencing at least one scenario with all flyover events, 6 different observer positions, and an equal number of approaches and departures. Background noises were tested as a between-subject factor. Scenario sounds were played through headphones in a laboratory designed to resemble a cozy living room. Subjects provided ratings on a numerical 11-point scale to assess the level of annoyance and disturbance caused by the presented target and background noises, while imaging themselves sitting in a living room, similar to the laboratory setting.
Multiple linear mixed-effects models using random intercepts to account for interindividual baseline differences, revealed that annoyance significantly increased with the number of flyovers. Likewise, annoyance increased closer to the vertiport. Annoyance was, furthermore, rated higher for take-offs compared to approaches whereas there was no significant difference between rural background noise scenarios compared to urban scenarios.
Our results emphasize the need of establishing data-driven regulations regarding flight numbers and flight paths, especially in residential areas, already before the launch of air-taxis.
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11:10
20 mins
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Acoustic characterization of drone response under various atmospheric conditions using an onboard microphone cage
Peter Hartford, Riccardo Zamponi, Christophe Schram
Abstract: As the use of unmanned aircraft systems becomes more frequent in urban areas, perceived noise will play a decisive role in public acceptance of new designs . Drone emission noise is primarily characterized by tonal components generated by the propellers, while electric motors and atmospheric turbulence mainly contribute to broadband noise . Rapid fluctuations in propeller rotational speed, caused by flight controller corrections in response to atmospheric conditions, can introduce additional sources of annoyance. These effects motivate the need for improved methods to localize noise emission sources under real-flight conditions.
The conventional approach for field characterization relies on fly-over measurements using ground-based microphone arrays . However, this method presents several limitations, including propagation effects related to source directivity, intrinsic source modulation, and the need for corrections associated with the drone’s position and orientation.
To address these limitations, we propose a novel measurement approach based on an onboard microphone cage attached to the drone to directly capture and localize noise generation sources. In this measurement campaign, a baseline characterization will first be conducted in an indoor environment, at the same location as in Zamponi et al. , to identify and localize nominal noise sources. Subsequently, a field characterization will be performed under varying atmospheric conditions, with a synchronized meteorological mast to characterize the inflow conditions. This approach will enable a direct comparison between the acoustic fields generated in quiescent and turbulent environments, fueling further discussion regarding strategies and design modifications aimed at reducing drone noise emissions.
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11:30
20 mins
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Mitigation of UAV Airframe Interaction Noise Using Porous Coatings
Craig Gillespie
Abstract: Rapid growth of unmanned aerial vehicles (UAVs) in the last-mile delivery sector has raised concerns over the emission of noise into urban and suburban settings. This is due to the distinct character of the noise which features both tonal components and high frequency broadband noise. In the Irish context commercial drone operations have completed over 200,000 deliveries to date. This scale of commercial operations of commercial deliveries has led to noise complaints despite levels significantly below urban traffic noise. Commercial operators require low-cost, off the shelf solutions to mitigate the noise of operations and increase societal acceptance of drone operations. This project investigated the viability of using a porous mesh applied to the rotor arm/boom supporting the rotor system (motor + propeller) with the intention of reducing airframe interaction noise.
The coatings are additively manufactured with low-cost MSLA printers. The coatings are designed as a custom sleeve that slides onto the boom. The coating consists of a lattice structure of Kelvin cells selected for printability using MSLA processes giving repeatable pore geometry. The porosity of the coating is controlled through a combination of cell size and cell strut thickness. The parameters of the coating are empirically optimised through a cycle of rapid manufacture and testing. The noise reduction is evaluated on both single and coaxial contra-rotating rotor setups with 406 mm diameter rotors in compliance with ISO 3744 sound power measurements. Comparison is made to a control with no coating and several coating types varying coating depth, fairing shape, porosity and unit cell dimensions are tested. The impact of the coatings on thrust performance and aerodynamics is also evaluated. The flow field around the rotor is measured with a hot-wire anemometry probe mounted on a robotic traversing arm to measure the flow field upstream, downstream and inter-rotor. Flow field tests are also conducted with a crossflow of up to 16m/s, a common speed for a commercial delivery drone. It is planned that the optimised coating will be utilised for a flight test study on a commercial drone to assess the noise reduction achieved in the field.
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11:50
20 mins
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Characterisation of Drone Noise in Complex Airflow
Philip McCarthy, Sean McTavish, Hali Barber
Abstract: Operational concepts for Advanced Air Mobility (AAM) vehicles typically consists of extended flights within or in close proximity to urban environments. The airflow present in these environments is often complex, exhibiting localised spatial and temporal variations in wind speed, wind direction, and turbulence levels. Within the broader AAM class of vehicles, the smaller size of drones makes them particularly vulnerable to these complex flow structures; altering the overall levels and sound characteristics, as well influencing the broader noise-propagation from the vehicles into the urban landscape.
When modelling populations noise exposure associated with these operations, the effects of the complex urban airflow are often neglected and instead the noise emissions associated with steady or benign flight conditions are assumed. While in many cases this is adequate, when highly accurate levels are required or human perception is being considered, time-variant sound characteristics must be properly modelled. This study provides an initial attempt to experimentally characterise the noise from a drone flying within this complex airflow, specifically flying in a wind tunnel under controlled turbulence representative of that measured in urban environments.
Wind tunnel testing has been performed using a DJI Matrice 300 drone at the Honda Aerodynamic Laboratories of Ohio (HALO) wind tunnel (See Figure 1). The drone was flown in wind speeds up to 15m/s and turbulence intensities up to approximately 20% of the freestream velocity. Acoustics measurements were conducted using a sideline microphone array, a forward microphone array, and flush mounted floor microphones. Initial spectral analysis from select individual microphones show that turbulent airflow causes a spread in the tonal peaks associated with the fundamental Blade Passage Frequency and its lower order harmonics. This primarily results from the RPM oscillations required to maintain stability within the unsteady flow. Further analysis, including acoustic beamforming and calculation of the psychoacoustic metrics, is currently underway to identify the significant impacts of turbulence on the drone acoustic characteristics that are relevant for noise exposure modelling.
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12:10
20 mins
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Effect of Large scale Turbulence on Propeller Aerodynamic and Aeroacoustic performance
Gokhul Venkataraman Swaminathan, Lourenco Tercio Lima Pereira, Yannick G.A. Chance
Abstract: Recent advancements in the UAM (urban air mobility) sector pave the way for making urban eVTOL (electric vertical take-off and landing) flight a reality. Operating in urban environments, the excessive noise generated by aerial vehicles plays an imperative role in their certification. During urban flight, aerial vehicles will be subjected to a broad spectrum of turbulent structures generated from sources such as flow around high-rise buildings, terrain-level obstacles, incoming ABL (atmospheric boundary layer) interactions and local windspeed variations. Unlike the well-documented effects of aerial vehicles flying under steady freestream or smallscale turbulence impingement, there is limited research on their performance when subjected to large-scale turbulence. This experimental work aims to generate representative large-scale
turbulence in the order of the rotor diameter and analyze its influence on propeller loading and noise emission.
Bluff body shedding from a circular cylinder placed upstream of the propeller is utilized to generate rotor-scale turbulence in the inflow. The presence of an upstream obstruction results in severe penalties to the aerodynamic performance and thrust generation, and the ingestion of rotor-scale turbulence causes an increase in the intensity of low-frequency loading fluctuations. The highly turbulent inflow has a significant impact on noise emissions, mainly causing an increase in broadband content over the entire frequency range and discrete tonal emissions at BPF (blade passing frequency) harmonics. This influence is observed across all the tested advance ratios, which contrasts drastically with the trends of an isolated propeller. With increased turbulence and larger coherent scales ingested near the propeller tip, “Haystacking”
patterns are observed to strongly influence the noise emission, and the resulting spectrum is dominated by broadband content in the high-frequency range above the 2nd BPF. This research shows that rotor-scale turbulence has a strong influence on propeller performance and noise emission, and emphasizes the need for characterizing UAM noise under realistic inflow conditions.
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