Advanced Air Mobility (AAM) vehicles are usually capable to operate in both forward and vertical flight thanks to their design configurations featuring multiple (tilting) rotors. Such maneuvering agility can be leveraged to minimize their acoustic footprint during operations close to the ground. The present work compares the acoustic footprint of optimal trajectories of AAM aircraft using low-order aero-acoustic models. The methodology is applied to the case of an AAM quad-rotor aircraft, which is modelled as a rotating point-mass with three Degrees of Freedom and two input controls. The trajectories are globally optimal in the sense of standard mission objectives, such as maneuver time or traveled horizontal distance, and are subject to realistic performance constraints. Results show that minimum-time trajectories generate higher and more concentrated noise footprints compared to minimum-distance trajectories, which distribute noise levels more evenly and result in overall lower noise footprints. Departure trajectories exhibit lower noise levels than approach trajectories. Adopting minimum-distance trajectories can significantly reduce noise impacts for both approach and departure maneuvers. @en

This paper investigates the prediction accuracy and time efficiency of two distinct low-order methods, Opty∂B and LOPNOR , for predicting tonal and broadband noise of a drone rotor in axial and non-axial flow conditions. These are both derived from an aero- dynamic rotor model based on the blade element momentum theory, respectively coupled with a time- (Opty∂B) and frequency-domain (LOPNOR) solution of the Ffowcs Williams- Hawkings (FW-H) integral equation applied to a radial distribution of acoustically compact and non-compact sources. Experimental data and scale-resolving lattice-Boltzmann/very- large eddy simulation (LB/VLES) results for a two-bladed small unmanned aerial system (sUAS) in transitional boundary layer conditions are used to validate the low-order ap- proaches. Comparison between low-order, high-fidelity and experimental results reveal that the underlying sound generation mechanisms are accurately modelled by the low-fidelity methods, which therefore constitute a valid tool for preliminary design of quiet drone rotors or to estimate the noise impact of drone operations.@en

This paper introduces a methodology for predicting the noise footprint of urban air mobility (UAM) vehicles in vertiport environments during approach and departure maneuvers. The methodology integrates a flight mechanics model, an aerodynamic model, aeroacoustic models, and a noise propagation model. The flight mechanics model employs a point-mass dynamic model to determine optimal trajectories based on prescribed criteria. The aerodynamic model utilizes blade element momentum theory, while aeroacoustic models include frequency-domain acoustic formulation and a noise propagation model based on Gaussian beam propagation method, which accounts for 3D variations in terrain and atmospheric profiles. Noise footprints are computed for several waypoints, featuring significant variations in vehicle flight speed and pitch angle, and are subsequently compared. It is observed that variations in vehicle pitch angle significantly influence noise radiation directivity. Specifically, when the vehicle pitches up, on-ground noise levels beneath the source increase, while those at receivers farther away decrease. Conversely, when the vehicle pitches down, on-ground noise levels beneath the source decrease, while those at receivers farther away increase. Additionally, as flight speed increases, on-ground noise levels rise accordingly regardless of whether the vehicle pitches up or down. This trend suggests that lower flight speeds during approach and departure maneuvers are desirable to reduce the noise footprint. Furthermore, it is noted that building blocks further shield incoming noise and decrease noise levels at receivers distributed behind them. These findings underscore the necessity of the proposed approach in evaluating the noise footprint of UAM flight trajectories in vertiport environments, providing valuable insights for early design stages.@en

This paper presents a low-order method for assessing tonal noise from full-electric propeller-driven aircraft during outdoor operations. A high-fidelity numerical simulation and several outdoor measurements were performed to validate the approach and identify the dominant noise source. Outdoor measurements involve constant-altitude level flight and three take-off flights. The low-order method focuses exclusively on blade and hub geometry, while the numerical simulation considers propeller blades and the spinner. Comparison of outdoor measurements, numerical simulations, and low-order model predictions reveals the propeller as the primary noise source for the specified aircraft configuration, with negligible interactions with the airframe. The study further demonstrates that during take-off flights noise levels at higher harmonics are more sensitive to variations in propeller disk angles of attack. These findings underscore the importance of addressing propeller noise in full-electric propeller-driven aircraft. Additionally, the paper emphasizes the practical application of the low-order approach for evaluating the aircraft's noise footprint during take-off flights, providing crucial insights for early design stages.@en

Efficient calculation of urban air mobility noise footprint in a vertiport environment, considering acoustic effects of various designs, operational conditions, and environmental factors, is essential to limit the noise impact on the community at an early stage. To this purpose, the computationally efficient low-fidelity approach presented by the authors in Fuerkaiti et al. (2022) [11] is extended to calculate the noise footprint of an aircraft in a generic 3D environment. The straight-ray propagator is replaced with a Gaussian beam tracer that accounts for complex source directivity, 3D varying terrain topology, and wind profiles. The reliability of the Gaussian beam tracer has been verified in previous studies by the authors. In this work, it is further extended to include complex source directivity in the presence of a moving medium. Noise sources, obtained using a low-fidelity toolchain, are stored on a sphere surrounding the aircraft and are propagated through an inhomogeneous anisotropic atmosphere. Noise footprints, predicted for different terrain topologies, source directivities, and wind flow conditions, are compared. It is shown that, compared to flat terrain, for the case under investigation, the building blocks increase on-ground noise levels by 5 dB in the illuminated zone due to multiple reflections; they also shield the incoming sound field by creating shadow zones behind the building. The shielding increases with increasing frequency in a quiescent atmosphere. The change between the source directivities, corresponding to the first and second harmonics of the blade passing frequency, results in a difference of up to 40 dB in the noise footprint. The presence of the wind flow can contribute a significant variation in the acoustic footprint by changing the lobes of the footprint pattern and intensifying the noise levels; the variation increases with increasing frequency. Compared to the straight-ray propagator, the present approach reduces the prediction error by 5 dB in the illuminated zones and 35 dB in the terrain shadow zones.

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This paper presents a noise propagation approach based on the Gaussian beam tracing (GBT) method that accounts for multiple reflections over three-dimensional terrain topology and atmospheric refraction due to horizontal and vertical variability in wind velocity. A semi-empirical formulation is derived to reduce truncation error in the beam summation for receivers on the terrain surfaces. The reliability of the present GBT approach is assessed with an acoustic solver based on the finite element method (FEM) solutions of the convected wave equation. The predicted wavefields with the two methods are compared for different source-receiver geometries, urban settings, and wind conditions. When the beam summation is performed without the empirical formulation, the maximum difference is more than 40 dB; it drops below 8 dB with the empirical formulation. In the presence of wind, the direct and reflected waves can have different ray paths than those in a quiescent atmosphere, which results in less apparent diffraction patterns. A 17-fold reduction in computation time is achieved compared to the FEM solver. The results suggest that the present GBT acoustic propagation model can be applied to high-frequency noise propagation in urban environments with acceptable accuracy and better computational efficiency than full-wave solutions.

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A noise footprint prediction framework for propeller-driven aircraft which couples an aerodynamic model and several aeroacoustic models is presented in this study. The aerodynamic model is based on the blade element momentum theory, while the aeroacoustic models are based on a time-domain compact dipole/monopole Ffowcs-Williams and Hawking's acoustic analogy, a trailing edge noise model, and a noise hemisphere database approach including a straight-ray propagation model, respectively. In order to reduce the runtime, the frequency-domain acoustic formulation, derived by Hanson (1980), is implemented and validated against the compact dipole/monopole Ffowcs-Williams and Hawking's acoustic analogy. The framework evaluates the acoustic effects of variations in the design and operating conditions of a propeller in forward flight. Noise footprints, obtained with different propeller configurations having varying advance ratio and number of blades are compared. It is found that, for a given thrust, a drop in advance ratio alters the source directivity dramatically, which resulting in a variation of up to 30 dBA on the acoustic footprint. When the advance ratio is kept the same and the number of blades increases from 5 to 7, the variation becomes 16 dBA due to the change in the source directivity, but the maximum noise level remains the same. The latter condition reduces the loading for each blade, and consequently the associated noise. However, the total noise remains unchanged as a consequence of increasing thickness noise due to the lower advance ratio, high blade tip Mach number, and addition of extra blades.

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This work presents a novel noise propagation approach based on the Gaussian Beam Tracing (GBT) method that accounts for complex source directivity, weather conditions, and irregular ground topology for the evaluation of the noise footprint. The approach takes a precomputed noise sphere as input and propagates the acoustic pressure fluctuations through a moving inhomogeneous atmosphere over realistic three-dimensional (3D) terrain. Noise footprints, obtained with di erent source noise spheres and wind flow conditions, are compared. It is found that, in a quiescent atmosphere, a change in the source directivity results in a variation up to 15 dB on the acoustic footprint. In the presence of the mean flow, the variation in the noise footprint can reach up to 35 dB. The results suggest that any variation in the source directivity and wind flow can cause a significant change in the acoustic footprint predicted in 3D environments with varying terrain topology and wind flow.

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This paper presents an atmospheric propagation model, based on ray acoustics, that accounts for realistic weather conditions in the evaluation of the noise footprint of an aircraft. Noise sources, obtained using the Ffowcs Williams and Hawkings acoustic analogy applied to scale-resolved flow simulation data, are stored on a hemisphere surrounding the vehicle. These noise sources are propagated using a propagation model that takes into account the vertical variability of air temperature and wind velocity. The electric vertical takeoff and landing aircraft, presented by Casalino, van der Velden, and Romani [(2019). in Proceedings of the AIAA Scitech 2019 Forum, January 7-11, San Diego, CA, pp. 1834-1851], is used as a case study; noise footprints, obtained considering various vertically varying temperature and wind velocity distributions, are compared. It is shown that weather conditions in the acoustic wave propagation can contribute to mismatch up to 4 dBA in the illuminated zone and a significant drop in the refractive shadow zone caused by the vertical air temperature and wind velocity gradients. This work constitutes the first accomplishment in including realistic atmospheric effects in aircraft community noise prediction based on scale-resolved flow simulations.

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