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Despite the great number of available models for the prediction of noise produced by large-scale propellers, little is known about their capability of predicting the noise of small-sized rotary-wings, such as found in drones. As these systems operate at much lower Reynolds numbers, different mechanisms of sound generation are expected to take place. This work investigates the capabilities of a gold standard theory, originally developed to assess the tonal noise of large propellers, at predicting the sound of small rotary wings. Thus, a model based on the helicoidal surface theory is considered. The first step of the analysis is carried out by conducting an experimental campaign to characterize the sound produced by drone propellers of different sizes and flight conditions. For all propellers considered, the comparisons between experiments and predictions in terms Δ OASPL at different angles and rotational speeds showed deviations below ± 2 dB for radiation angles between 62.5 ∘ and 135 ∘ . A significant disagreement was observed for angles smaller than 62.5 ∘ . A second analysis, considering the contribution of the individual sources encompassed by the theory, has suggested that the deviation at small radiation angles are due to an overestimation of the source term associated with the blade volume displacement, for which the contribution becomes negligible in the case of small propellers and higher harmonics. The results show that the helicoidal surface theory provides an accurate and inexpensive tool for assessing the far-field noise produced by small propellers, provided that the angles of interest are between 62.5 ∘ and 135 ∘ .

The hydrodynamic noise of surface vessels has recently received significant attention. The unclear mechanism of the scale effect makes it challenging to accurately convert the hydrodynamic noise obtained from a scaled model to the actual ship scale. This paper introduces a dynamic towing method for a Wigley catamaran on the lake to eliminate the impact of propeller noise and mechanical noise. An annular hydrophone array was used to test the underwater radiation noise of the Wigley catamaran at various distances and directions at speeds of 4 knots and 6 knots. The experimental results show that the hydrodynamic noise is a broadband sound with a continuous spectral distribution. The hydrodynamic noise has the maximum energy contribution in the range from 100 Hz to 10 kHz, with the peak frequency 1600 Hz.

High resolution aeroacoustic source analysis is a prerequisite to address the noise concerns and release the full benefits of wake-ingesting propellers. In this work, the aeroacoustic sources of a two-bladed propeller ingesting the wake of an aerofoil are investigated using large eddy simulation in conjunction with two different source identifying approaches. The first approach is the numerical beamforming that utilizes both the classical and wavelet-based beamforming techniques, which determine the phase variations of sources at the low to mid frequencies and reveal that the high-frequency sources are phase-independent. To further improve the spatial resolution of source identification, a new near-field aeroacoustic source analysis approach based on the acoustic analogy is developed in this work. In particular, the on-surface source terms emanating the far-field noise are derived based on the Ffowcs Williams and Hawkings equation for low Mach number flows and constant rotating propellers. Through the incorporation of the simulation results into the proposed source analysis approach, various types of aeroacoustic sources are identified and studied by visualizing their distributions on the propeller surfaces, correlating to flow features and examining the noise spectra and directivity. While the leading edge sources are highly correlated with the wake interaction process, the sources at the mid-chord and the trailing edge of the blade can maintain their strength across most revolving angles. Overall, the proposed analysis approaches extend the capability of computational fluid dynamics and enable the detailed study of noise generation mechanisms of wake-ingesting propeller noise.

This paper describes the design and commissioning of an aeroacoustic test rig for the study of single and coaxial propeller propulsive systems. The size of the propellers matches typical drone applications. The experimental setup, designed and commissioned at the ALCOVES anechoic laboratory of von Karman Institute for Fluid Dynamics, is equipped with aerodynamic sensors for performance analysis and is surrounded by a microphone antenna for the characterization of the noise level and directivity. Thefacility permits varying different parameters such as the longitudinal distance between the rotor planes, and the rotational speed/direction of each propeller. Requirements for the qualification of the test room consist of low-level background noise and minimized turbulence ingestion noise. Two experimental databases have been constituted and are joined to the present paper: (i) for the DJI 9450 two-bladed propeller, verified against data from the literature, and (ii) for single and coaxial contra-rotating Mejzlik two-bladed propellers. The proposed benchmark data will support the validation of low- and high-fidelity numerical methods.

To address the limitation that existing studies mostly focus on noise from single components (bare submarine or isolated propeller) under steady-state conditions, the investigation focuses on the radiated noise characteristics of the SUBOFF submarine and E1619 propeller in both independent and coupled motion scenarios. The Large Eddy Simulation (LES) method and the Ffowcs Williams–Hawkings (FW-H) acoustic model are employed to analyse the noise emissions under these conditions. To verify the numerical method, an inflow velocity of 3.05 m/s was set, and a hydrophone was placed at (2.178, -2, 0). The verification results show that the deviation of the overall sound pressure level (OSPL) from other studies is within 5% (maximum deviation: 4.68 dB), confirming the method’s accuracy. Meanwhile, a cylindrical computational domain is properly configured (e.g., the distances from the propeller’s inflow surface, lateral boundaries, and outflow surface to the propeller itself are 5D, 5D, and 10D, respectively, where D is the propeller diameter). Hydrophones are arranged along the X/Y/Z axes (within the range of 5–50 m) and in the XOY, XOZ, and YOZ planes for noise measurement. The results indicate that: Submarine-propeller coupling significantly increases the submarine’s radiated noise (with an increase of nearly 30 dB in the axial direction) and changes its acoustic directivity from dipole to monopole characteristics; coupling increases the propeller’s axial noise by approximately 10 dB (with little impact on radial noise). Propeller noise dominates the total axial coupled noise (The noise levels of the submarine and propeller are equivalent in the Y/Z directions.). The underlying mechanism is that coupling intensifies flow field unsteadiness, leading to high-frequency fluctuating forces on the submarine hull and propeller surfaces, thus increasing noise levels.

This paper describes a theoretical study – in the time domain – of sound from the interaction of the steady component of the viscous wakes of an upstream propeller with a downstream contra-rotating propeller blade. The study incorporates a two-dimensional model of the upstream propeller wakes and a quasi-three-dimensional blade response function that accounts for downstream blade sweep. For a blade with a straight leading edge, the sound at the observer location, radiated from each blade radius, consists of a series of impulses whose peaks are shown to be influenced by micro Doppler effects and to correspond to the impingement of the propeller wake centrelines on the leading edge of the downstream blade. For radiation from the entire blade span, it is shown that constructive interference of the impulses from all radii can produce impulsive sound of very high amplitude, whereas dephasing of these impulses can reduce significantly the total acoustic signal. For a downstream propeller blade with a swept leading edge, it is shown how the sweep can be designed to ensure that these impulses are de-phased, resulting in significantly lower-amplitude sound at selected observer locations. Finally, to guarantee that the radiated sound is reduced at all possible observer locations, it is shown that the blade leading-edge sweep must be large enough that the trace velocity of the wake centreline, across the leading edge of the downstream propeller blade, is subsonic across the entire span. The benefits are demonstrated for representative blade designs.

With the mass application of drones in daily work, more attention has been paid to the noise problem. The noise generation mechanism of low Reynolds number propellers is studied in this paper. It is found that there are laminar separation bubbles on the suction surface, causing an increase in broadband noise. Leading edge boundary layer trips are added to remove the bubbles. Finally, experimental measurements indicate that isolated propeller noise reduction is up to 5 dBA. It still has a 1.5 dBA reduction in the full-loaded hovering drone test, which brings many conveniences for daily work.

The development of a future transportation concept in cities may also include transportation of passenger and cargo at various altitudes in order to reduce the load on the ground infrastructure. This is known as urban air mobility (UAM), facilitated by the application of electrical vertical take-off and landing (eVTOL) vehicles. Public acceptance is required with regard to safety aspects in densely populated areas, but also in terms of noise emissions. As with almost any aircraft, the propulsion systems, in most cases the propeller blades, are the main source of noise generation. In literature, experimental and numerical results for geometric blade modifications for the purpose of noise reduction are provided only for small diameter propellers. This paper investigates the individual features at larger diameter propellers by means of numeric aeroacoustic simulations with OpenFOAM. The modifications include serrations of the trailing edge, leading edge tubercles and blade tip adaptations. Moreover, the combination of those features is investigated in a parameter study for various rotational velocities and several flight modes. The key effect, namely a reduction of broadband noise, can be observed for several cases, but the strength of it varies. An optimization process is necessary to obtain an efficient noise reduction for all operating conditions of a particular aircraft design.

Based on the known methods for calculating the propeller noise, a fast prediction method of the distributed propulsion noise is proposed. The method takes into account only the component of tonal noise from the load. To test the adequacy of the model, the GL-10 model was used, a concept of an aircraft developed by National Aeronautics and Space Administration to test distributed propulsion as well as vertical take-off and landing maneuvers. The comparison showed a good agreement between the results obtained and the data for the GL-10 model.

This study investigates the aerodynamic and aeroacoustic behavior of propellers operating in ground-effect conditions, with an emphasis on the impact of porous ground surface treatments. The investigation explores the potential of porous materials to reduce propeller noise near the ground, a major barrier to the acceptance and integration of Urban Air Mobility (UAM) systems. Experiments were conducted in an anechoic chamber using an APC inch propeller in a pusher configuration. The setup used a rigid flat plate to act as the ground plane at various distances from the propeller. The ground plane was treated with three types of porous foams, each with different pore densities and thicknesses. Noise measurements were taken using a polar array of microphones positioned in both near-field and far-field locations. The results show that porous surface treatments significantly enhance noise suppression. Coherence analysis revealed that porous treatments improve the spatial consistency of acoustic signals, making noise propagation more predictable and controllable. The study also highlights that the interaction between wake flow and porous surfaces leads to greater noise suppression and stability in the hydrodynamic pressure field. These findings have significant implications for designing quieter, more efficient UAM vehicles, aiding their integration into urban environments.