Browse Topic: Noise
This SAE Recommended Practice establishes the procedure for measuring the sound level of recreational motorboats in the vicinity of a shore bordering any recreational boating area during which time a boat is operating under conditions other than stationary mode operation. It is intended as a guide toward standard practice and is subject to change to keep pace with experience and technical advances.
In the context of developing new rotorcrafts dedicated to Advanced Air Mobility, aeroacoustic simulations of co-axial rotor systems have been conducted using the lattice Boltzmann method with the ProLB code. Eight configurations, spanning from co-rotating rotors to contra-rotating shrouded rotors, were analyzed in stationary conditions through comparisons with experimental data and flow field analysis. This investigation validates our numerical methodology, based on direct noise simulation, and enhances our understanding of noise generation and propagation of such propulsive systems. Our simulations successfully replicate all measured trends in global aerodynamic performance, average noise levels, and noise directivities. Maximum discrepancies were 1.5 N (9%) and 2.1 dB (averaged noise level on the considered microphones). Based on our analysis, the following observations have been made. The open co-rotating rotors is the quietest configuration due to reduced Blade-Vortex Interaction
Carbon/epoxy stiffened panels are being increasingly used in transport rotorcraft. The reduced mass density and high stiffness of carbon/epoxy composites can lead to higher levels of vibration relative to comparable metallic structures, which themselves can have vibrations and interior noise high enough to damage the hearing of crew and passengers. The current investigation explores a method to reduce the vibration of carbon/epoxy stiffened panels by introducing thickness tapers known as acoustic black holes (ABHs). The ABH feature is integrated into either the stiffeners or plate of a representative stiffened panel configuration. A finite element (FE) parametric study was used to guide designs that reduce the vibration of the panel without compromising the compressive buckling capability or mass of the panel. FE studies showed that a 30 ply to 12 ply thickness taper longitudinally oriented in the blade stiffener can reduce vibrations and increase compressive buckling capability
The performance and acoustics of a scaled propeller designed for an eVTOL vehicle were investigated in axial and edgewise flight. The measured performance compared well with BEMT predictions in axial flight conditions. The noise produced by the propeller is dominated by broadband noise sources, where there is evidence of contributions from blade wake interaction noise, turbulent boundary layer trailing edge noise, and laminar boundary layer vortex shedding noise. The directivity of the noise was found to be dependent on the advance ratio. Beamform maps also identified changes in the dominant noise source at different observer locations as a function of advance ratio.
This paper explores a significant step forward, regarding the further detailed understanding of the Fenestron®. Since its patent in 1968 – for the Gazelle helicopter –, the shrouded tail rotor has been resized, inclined, modulated, etc. and has thus been continuously enhanced on different rotorcraft. Half a century after its invention, Airbus is once again exploring in more detail the magic of the Fenestron®, with the objective of optimizing it even further, for future helicopter applications. To grasp and observe properly some specific phenomena, a model (scaled to one third) capable of both unprecedented functions and modularities, was developed. The present paper will describe in detail the novel model and the related challenges and solutions. This model is capable of high rotor speed and dynamic pitch inputs, delivering power levels high enough to reach stall effects, while allowing the measurement of propulsive efficiency and to differentiate rotor vs fairing thrust. Furthermore
Acoustic flight testing of rotorcraft often involves generating noise source hemispheres to gain an understanding about the aircraft's acoustic emissions. However, aerodynamically complex Urban Air Mobility and Future Vertical Lift vehicles may not maintain a steady aerodynamic state during flight, making source hemispheres measured using traditional linear arrays unreliable or difficult to interpret. To address this challenge, all emission angles need to be measured simultaneously. This has lead to the concept of the two dimensional 'snapshot' array layout. A mathematically defined microphone distribution was utilized to achieve uniform coverage on the source hemisphere. Within the chosen distribution, two lower microphone count distributions are embedded, allowing for a comparison of the effects of number of microphones. The array was deployed as part of a joint Army/NASA acoustic research flight test in July of 2024. Data were collected using an MD530F helicopter as the test vehicle
This paper investigates the relationship between broadband noise behavior and helical wake structure in coaxial corotating rotors. Experimental measurements were conducted across variations in collective pitch (9.4°, 12.5°, and 15.0°) and rotor speeds (1500–4500 RPM). The inflow ratio (λ) was shown to govern the slope of broadband noise trends mapped in phase offset versus separation distance space, with experimental and theoretical λ values agreeing within 1%. Tip vortex core growth was estimated using the Ramasamy-Leishman model and normalized by the blade tip chord, reflecting the location of tip vortex formation. Across collective pitch variations, initial vortex core radii ranged between 7.5% and 9.1% and across rotor speeds, it ranged between 7.5% to 8.5% of the blade tip chord. When broadband noise trends became less coherent across phase offset angles, the corresponding vortex core radii were observed to approach or exceed 10% of the tip chord. At 4500 and 3500 RPM, vortex
Survivability in the future operating environment is becoming more challenging as threat systems evolve and become more sophisticated. The ability to tailor and manage signatures will be one of the key methods to improve survivability, allowing operators to minimise detection and maximise the effectiveness of countermeasures. This paper presents the findings of an investigation into the application of classical Signal Detection Theory (SDT) to the aural detectability of helicopter noise signatures, considering human auditory capabilities. The paper has thus developed a novel methodology, applied it to both the experimental and numerical helicopter acoustics signatures of an LH platform, and used these results to infer the detectability characteristics of the aircraft, as well as how they are affected by the presence of background noise in different environments.
A hybrid RANS/LES simulation of the Ideally Twisted Rotor (ITR) in hover was interrogated to identify bluntness vortex shedding (BVS) and determine the contribution to the predicted rotor broadband self-noise. Three rotor blade stations were extracted to study spanwise variations in the BVS shedding frequency and amplitude. Corresponding 2-D airfoil simulations were performed to evaluate a simplified modeling approach that effectively isolates BVS. The BVS shedding frequencies predicted by the 2-D airfoil simulations differed by less than 2% from the corresponding rotor stations in the 3-D simulation. The increased computational cost incurred by performing 3-D airfoil simulations did not lead to a worthwhile increase in simulation fidelity. Farfield noise was predicted for the three rotor stations and the 2-D airfoil simulations, and trends in frequency agreed well. The 2-D approach overpredicted the 3-D peak amplitudes by 5 - 10 dB. This work demonstrates that 2-D hybrid RANS/LES
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