This research presents the design, numerical optimization, and experimental validation of two bio-inspired membrane-type acoustic metamaterials—one based on a hexagonal honeycomb topology and the other on a spider-web configuration—for targeted low-frequency sound insulation in railway cabins. Field noise measurements from Fadak passenger trains identified two dominant interior frequency bands: a primary peak near 630 Hz and a secondary band around 1.6 kHz. The proposed meta-panels were modeled as full three-dimensional cellular arrays in COMSOL Multiphysics, with sound transmission loss (STL) computed in 1/6-octave bands and analyzed through modal decomposition. The honeycomb design, characterized by a lower first natural frequency, demonstrated enhanced attenuation at very low frequencies, whereas the spider-web structure exhibited a denser modal distribution, enabling broader suppression near 1.6 kHz. A constant volume/mass parametric analysis was followed by a single-objective Nelder–Mead optimization to maximize STL at 630 Hz by tuning two key geometric parameters. The optimized configurations achieved up to ∼10 dB improvement at the target frequency, with the honeycomb variant offering superior performance-to-mass ratio, and the spider-web variant delivering stronger attenuation peaks at the expense of increased mass. Prototypes fabricated via fused deposition modeling (FDM) using flexible thermoplastic polyurethane (TPU) and thin polyethylene membranes were tested in a coupled reverberant–anechoic setup, confirming the numerical predictions. These findings demonstrate that the integration of bio-inspired geometries with lightweight, flexible membrane structures enables precise control of low-frequency acoustic performance. The proposed designs offer a scalable and weight-efficient solution for mitigating interior noise in railway passenger environments, with potential applicability across a range of transportation and architectural contexts.
With the rapid development of electric vertical take-off and landing (eVTOL) aircraft for urban air mobility (UAM), rotor noise has become a major barrier to their large-scale deployment. To reduce the cruise noise of tiltrotor eVTOL aircraft, this study develops a medium-to high-fidelity multi-rotor aeroacoustic prediction framework by coupling the reformulated vortex particle method (rVPM) with the Ffowcs Williams–Hawkings (FW–H) acoustic analogy. Based on this framework, phase-synchronisation analyses are first conducted for a tandem twin-rotor system to investigate the effects of relative rotor phase on far-field noise radiation. The aeroacoustic model is then integrated with a self-adaptive constrained Bayesian optimisation algorithm to minimise the average sound pressure level at the blade passing frequency (BPF) evaluated at seven observer locations. The optimised phase configuration reduces the target BPF sound pressure level by approximately 31.3 dB and decreases the overall sound pressure level at the point directly beneath the aircraft by about 7 dB. The analysis of the noise-reduction mechanism indicates that rotor phase adjustment can mitigate the wake-induced loading fluctuations of the rear rotors while promoting destructive acoustic interference among different rotors at far-field observation points. The proposed method causes no appreciable loss in rotor propulsive efficiency and provides a promising approach for low-noise rotor phase design in multi-rotor eVTOL aircraft.
Exposure to high sound pressure levels (SPL) is a leading cause of noiseinduced hearing loss, and earplugs are a primary means of protection. The effectiveness of earplug protection depends on the seal formed in the ear canal. An ill-fitting earplug leaves air gaps that leak sound and degrade attenuation. The acoustics of these leak paths and their dissipation mechanisms under high-amplitude sound remain poorly characterized. This study quantifies sound transmission through modeled earplug leakage with rigid-wall idealization and identifies the governing dissipation mechanisms by combining impedance-tube experiments and direct numerical simulation (DNS). The methods were first verified against stand-alone slit resonators and orifices, for which extensive published data are available, and then used to measure the transmission loss (TL) and acoustic power absorption coefficient of modeled earplug–canal configurations over 1–5 kHz at overall incident SPLs of 120–150 dB. Leakage from an ill-fitting silicone rubber earplug reduced its TL by approximately 18 dB relative to a sealed configuration at an overall incident sound pressure levels (OISPL) of 120 dB, and the leakage-path TL increased with SPL, indicating a nonlinear attenuation mechanism. DNS resolved the SPL-dependent mechanism, showing that incident acoustic energy is converted into vortical kinetic energy at the leak edges at an OISPL of 150 dB, more strongly at lower frequencies. These findings show that this nonlinear mechanism activates under modeled leakage and identify the leak path as the dominant SPLdependent transmission route in ill-fitting, non-absorbing earplugs, underscoring the importance of seal integrity in high-SPL environments.
Acoustic testing of propellers in enclosed environments presents a fundamental challenge in reproducing static free-flight conditions due to the onset of flow recirculation, as reported in previous studies. Building on the concept of a facility time-scale parameter, this paper proposes and demonstrates a methodology for identifying the onset of flow recirculation without the use of advanced flow-measurement instrumentation. An experimental campaign was conducted in an anechoic chamber across a range of test points, encompassing multiple low-Reynolds-number propeller sizes and rotational speeds. The onset of recirculation was inferred through facility-scale considerations and analysis of the resulting acoustic response, rather than through direct flow diagnostics. The extent and severity of acoustic contamination were quantified using sound pressure level measurements obtained from a far-field microphone array. The results elucidate clear dependencies between the recirculation onset and key parameters governing the facility time scale, including propeller diameter and angular speed. The results confirm that flow recirculation measurably alters the propeller tonal acoustic signature. The strongest effect was observed in the 2nd to 9th BPF harmonics, where differences of up to 7 dB were recorded between time segments, while the 10th–15th harmonics showed smaller variations. The findings contribute toward the development of more robust and standardised methodologies for acoustic testing of propellers and unmanned aerial vehicles.
A duct containing a flat plate closely represents air-conditioning ducts in which air flow generates unwanted acoustic noise. This work studies the flow and acoustic fields in a duct of 50 × 50 ( m m ) rectangular cross-section containing a flat plate of 1 ( m m ) thickness, focusing on the conditions that lead to acoustic resonance. The computational procedure employs one-way coupling between a Large Eddy Simulation (LES) incompressible flow solver and an acoustic solver based on the Perturbed Convective Wave Equation (PCWE) model. The effects of plate length, number of plates, inflow Reynolds number (Re), and plate angle on the acoustic behaviour are investigated. The results show that changing the plate length alters both dominant resonance frequency and sound pressure levels, while varying the inflow velocity changes the sound pressure levels. Additionally, adjusting the plate angle broadens the dominant frequency towards lower values. Moreover, while by increasing number of plates, the flow turbulence intensifies and new modes arise, the sound pressure levels at dominant resonance frequency diminish.
The concept of thin-layer swirl as a means of turbulent jet mixing and noise control is investigated. Swirl is introduced at the exit of a conical nozzle through 12 short vanes which impart angular momentum primarily to the outer shear layer. In addition to the uniform swirl (case m = 0) design, three azimuthally modulated swirl configurations are created by varying the exit vane angle. To investigate the development of the small disturbances, the spatial linear instability analysis is performed on an inviscid parallel jet with thin-layer swirl to identify the unstable modes and estimate their spatial growth rates. The azimuthal modes, m = = ± 1 , ± 2 , a n d ± 3 over a range of swirl number S are investigated. For low–moderate swirl, the negative helical modes (m < 0) are more spatially unstable than their positive counterparts; as swirl increases, the modal ordering reverses, with positive modes (m > 0) becoming dominant. High-order LES is used to assess how thin-layer swirl influences jet mixing and noise. Entrainment is quantified using the radial-velocity method and thin-layer swirl cases show a modest enhancement over a plain jet. The simulations show that the uniform swirl case (m = 0) achieves stronger mixing than the modulated designs (m = 1,2,3). Far-field acoustics is estimated by Ffowcs Williams-Hawkings (FWH) method and it shows a net OASPL reduction of 2–4 dB at θ = 60 ° and 90 ° . The gross thrust penalty for introducing low-aspect ratio swirl vanes at the nozzle exit is about 7-11%. Future work targets the optimization of vane geometry and swirl strength to retain acoustic benefits while reducing thrust loss.
This special issue of the Journal of Aeroacoustics is presented in honor of Prof. Stewart Glegg, and his many professional accomplishments. The enthusiasm of the many contributors to this volume, representing just a small subset of those who have benefitted from his work and collaboration, is just one indication of Stewart’s phenomenal impact. This article is an attempt to summarize Stewart’s personal and professional biography to date. His contributions to the aeroacoustics community include insightful technical advances in a surprisingly broad set of areas, extensive service particularly through his university and AIAA, and the inspiration, mentoring and advancement of many students and colleagues.
The noise emitted by axisymmetric, dual-stream, internally mixed jets was studied. At a jet Mach number of 0.90, the jet howled loudly (i.e., acoustic tones dominated over the broadband jet noise) for multiple mixing-duct lengths and for both unheated and heated core flows. Particle-image velocimetry data revealed that the jet's potential core was substantially shortened when the howling occurred and there were intense velocity fluctuations in the jet's shear layer. Using schlieren images, it was shown the jet's instability waves were excited at the frequency of the fundamental acoustic tone, suggesting a flow/acoustic interaction was responsible for these observations.
Existing aircraft noise models often rely on static databases and fail to comprehensively account for dynamic environmental and operational factors, resulting in relatively significant prediction errors in a single flight noise event. To make up these issues, a noise attenuation model integrating ellipsoid and nonlinear function has been proposed for the first time, this paper will further improve our previously proposed model. The key improvements are two aspects: firstly, geometric constraints derived from the analysis of NPD database to combined to ensure that the noise attenuation curve remains physically reasonable, overcoming the limitations of previously pure data fitting. Secondly, the objective function is redefined, thereby reducing the errors that assuming a constant sound source level in the original model, and improving the model's balance and extrapolation performance. The comparison results show that the ME, MAE and fluctuation range of the calibration and validation errors have all been reduced to some extent. The improved model also obtains a more balanced error distribution in predicting the EPNL. The sound level attenuation curves generated are more concentrated, and closer to the noise attenuation process in real environments. It is more conducive to analyze the noise attenuation characteristics of aircraft noise and establish corresponding noise databases. Furthermore, the model simulates the directivity of noise propagation through a simple geometric parameter adjustment of ellipsoid model, demonstrating a good environmental adaptability. The value of these enhancements not only lies in combining physical laws with data fitting, further enhancing the performance of the model, but also offering a new research direction for developing a more adaptable, interpretable, and reliable aircraft noise model.
This study examines the effect of interface configuration on the aeroacoustic prediction of cross-flow fans using a combined experimental and numerical approach. Unsteady Reynolds Averaged Navier Stokes (URANS) simulations, coupled with the Ffowcs Williams-Hawkings (FW-H) acoustic analogy, were conducted to capture both aerodynamic behavior and noise emissions. Two interface strategies were evaluated: a single-interface and a dual-interface configuration. While steady-state simulations provided basic aerodynamic trends, they were not able to capture the unsteady flow structures that are important for noise prediction. The URANS approach allows resolution of these transient features. The results indicate that the single-interface configuration tends to better preserve coherent vortical structures and provides acoustic predictions that are more consistent with the measured tonal components, including the blade-passing frequency (BPF). In contrast, the dual-interface configuration shows improved aerodynamic performance prediction but reduced acoustic levels. This difference may be associated with the attenuation of unsteady flow structures, which could be influenced by numerical dissipation effects related to the interface treatment. However, this interpretation remains qualitative and has not been quantitatively verified in the present study. The results highlight the trade-off between aerodynamic accuracy and acoustic resolution, emphasizing the importance of interface treatment in fan noise modeling.
This study presents a computational analysis of an NREL phase-VI HAWT blade, incorporating a winglet at the tip of the blade, inspired by its use in aircraft. An aeroacoustic analysis is conducted on baseline and winglet blades across varying wind speeds and receiver locations using the FW-H model, with results validated against reference data. The unsteady flow characteristics reveal that pressure differences at the blade tip form vortices, winglets weakens and confines these vortices closer to the tip, reducing energy-dissipating turbulence and improving aerodynamic performance. Velocity helicity, which measures fluid flow's swirling characteristics, indicates that the base blade generates higher helical flow patterns at the blade tip compared to the winglet blade. The reduced helical patterns with the winglet suggest decreased rotational energy losses, improving power generation. Quantitatively, the winglet blade exhibits higher normal force coefficients across the blade span, with the most significant improvement observed near the tip (i.e. 95% of the radial location of the blade). Increasing wind speed results in higher overall sound pressure levels due to increased turbulence and blade-tip noise, with observed increases ranging from 6.4% to 9.6%. Blade design significantly influences wind turbine noise, with winglet blades reducing aerodynamic noise by decreasing vortex intensity, resulting in 6.9% to 8.1% lower OASPL compared to baseline blades. Noise emission from the HAWT varies with the downstream receiver locations, the results show that the noise-reduction effect of the winglet blade persists across all measured positions, demonstrating its spatial robustness. Overall, the findings highlight the potential of winglets to reduce aerodynamic noise while improving flow characteristics near the blade tip.
This study investigates the effects of nozzle-plate spacing and nozzle pressure ratio (NPR) on the noise characteristics of both free and impinging jets through wind tunnel experiments. For free jets, the results show that when 2.05 < NPR <2.84, the far-field noise overall sound pressure level (OASPL) is stronger in the downstream direction. However, for NPR >2.84, the OASPL in the midstream direction exceeds that in the downstream direction, mainly due to the attenuation of turbulent mixing noise and the increase in broadband shock-associated noise. The free jet noise spectrum also exhibits two main discrete tones and their harmonics, with the transition attributed to the gradual formation of shock cell structures interacting with large-scale vortex structures. For impinging jets, the far-field noise in the upstream direction is similar to that of free jets when 2.89 < NPR <3.1, but midstream and downstream noise levels decrease. This is primarily due to the inclined plate transforming the turbulent mixing region in the fifth and sixth shock cells of the free jet into two distinct regions. One region, located downstream of the plate, generates noise that primarily radiates upstream, while the other is situated in the turbulent mixing zone of the wall jet. Additionally, the primary source region of the screech tone is near the upper edge of the strong vorticity region at the third shock cell, with the second and third harmonics originating from the strong vorticity regions at the fourth and fifth shock cells. The discrete tone at 3737 Hz corresponds to the position where the strong vorticity region on the side of the inclined plate along the flow direction (SVRiP-FD) fully merges with the upper edge of the strong vorticity region at the fifth shock cell.
This study experimentally investigates the impact of finlet integration on the aeroacoustic and aerodynamic characteristics of a T-MOTOR 18*6.1 carbon fiber propeller operating at low Reynolds number regimes. Far-field noise and thrust measurements were conducted under static conditions across rotational speeds ranging from 2000 to 6000 RPM and various sound radiation polar angles. Aerodynamic results indicated that finlet integration led to an average thrust performance reduction of 8% across the entire operating range, primarily attributed to increased profile drag. From an aeroacoustic perspective, findings revealed a strong dependency of finlet performance on both propeller rotational speed and noise propagation angle. Notably, finlets at 2000 RPM resulted in broadband noise reduction at high frequencies across all polar angles. Their optimal aeroacoustic performance was observed at lower rotational speeds (2000 RPM) and suction-side polar angles, particularly at 30 degrees, where the broadband noise reduction averaged approximately 6 dB at low frequencies and 3.5 dB at high frequencies. While a tonal noise increase of approx. 1.5 dB occurred at the first blade passing frequency at this speed, the study indicates an influence on flow patterns and pressure fluctuations, inferred from the acoustic and thrust results. However, further optimization of finlet design and selection is essential to expand their beneficial impact across diverse operating conditions.
In this study the aeroacoustics performance of owl airfoil compared to hawk and National Advisory Committee for Aeronautics (NACA) 0012 airfoils by two chord numbers of C = 150 mm and C = 250 mm, were investigated experimentally by the help of CPV (Coherent Particle Velocity) method. The effects of different parameters such as free-stream velocity, Reynolds number, angle of attack and hotwire position on the six different airfoils were examined in wind tunnel experiments. The CPV method results show that for frequencies below 2 kHz under attached flow conditions, the owl airfoil exhibited the lowest Sound Pressure Levels (SPL), aligning with self-noise cancelation hypothesis. At Re = 74 & times; 103, the SPL reached a minimum, likely due to the elimination of laminar separation bubbles during boundary layer transition. Also, for the hawk airfoil, a distinct behavior was observed below 50 Hz, where SPL increased linearly to a peak at 50 Hz, likely due to vortex shedding, with minimal Reynolds number influence. In addition, SPL variations above 1 kHz were less sensitive to Reynolds number changes, aligning with noise from shear layer instabilities. The owl airfoil exhibited the same behavior, including the elimination of Laminar Separation Bubbles (LSB) at identical Reynolds numbers for both chord lengths. In contrast, the hawk and NACA0012 airfoils showed significant variations in vortex shedding and shear layer instabilities at the same Re.
The Unified Transform Method (UTM) is applied for the first time to acoustic scattering by poroelastic plates in uniform grazing flow. We extend the UTM framework to incorporate low-Mach-number flow over either a finite poroelastic plate or a composite plate composed of alternating impermeable rigid and poroelastic sections. The method captures edge singularities through tailored basis functions and avoids the kernel factorization difficulties of Wiener-Hopf approaches, while remaining computationally efficient. A flow-dependent Rayleigh conductivity is introduced to quantify how porosity effects are modified by grazing flow under both plane-wave and point quadrupole excitation. Results reveal a Strouhal-controlled regime in which flow can modulate the noise-reducing properties of pores, but beyond this limit the effects of pores are nullified completely. For composite plates, the placement and length of poroelastic inserts and their rigid supports relative to the trailing edge are critical: downstream inserts, when sufficiently long, provide the most effective suppression of trailing-edge noise and elastic insert displacement. These findings demonstrate the UTM as a versatile semi-analytical framework for analyzing noise-control strategies with poroelastic surfaces in flow.
The aeroacoustic effects of rounding a forward-facing step in a low-Mach-number turbulent boundary layer are investigated using large-eddy simulation and Lighthill's theory. The step height is 26 % of the thickness of the unperturbed boundary layer at R e theta = 4755 , and the rounding radius relative to the step height ranges from 0 to 100 % . Consistent with previous experimental findings, step rounding is shown to cause reduced flow separation and increased peak pressure fluctuations on the step upper-surface except in the 100 % rounding case. The acoustic radiation is significantly weakened by step rounding, and the noise reduction increases with increasing rounding radius and frequency. The acoustic source fields for different rounding radii are analyzed in conjunction with tailored, acoustically compact Green's functions to investigate the mechanisms for noise reduction. It is found that while step rounding affects both turbulence production and acoustic diffraction by the step, noise reduction is primarily due to the reduced surface diffraction effect. Additionally, a boundary-element analysis shows that the effect of step rounding on the acoustic directivity is insignificant for acoustically compact steps, but grows with increasing acoustic noncompactness of the step height.
This paper presents a study on aircraft engine fan noise scattering by the airframe structure, using a methodology in the framework of geometric acoustics with extensions to account for features that are important for aircraft noise but are absent in classic geometric acoustics. Methods to parameterize engine fan noise sources for scattering calculation are presented and the important feature of source coherence is discussed, with examples given to demonstrate its effects in experimental data and in computation. The scattering results calculated for the Boeing 787 aircraft are presented and analyzed, for both the inlet and the aft fan component, and for both the broadband and the tonal noise. Many features are shown to be consistent with, and/or provide explanations for, observations in flight test data. The methodology and capabilities described in this study are written into the NASA PAASc code and represent a necessary and significant improvement in the accuracy and capabilities for acoustic scattering prediction while meeting the rigorous requirements of aircraft system noise assessments and design studies.
This study investigates the aeroacoustic effects of harmonic gust disturbances on a five-blade electric Vertical Take-off and Landing Aircraft (eVTOL) rotor in edgewise flight. Delayed Detached Eddy Simulations (DDES) using OpenFOAM, coupled with a synthetic gust generator, are first validated against experimental results for uniform inflow conditions. The results show good agreement for broadband spectra and performance metrics, with slight underprediction of tonal levels near the blade-passing frequency (BPF). In the next step, the gust of 10% fluctuations relative to the freestream velocity is added to the flow with a frequency of 119 Hz (0.357 BPF). Increasing gust intensity to 30% of the freestream velocity has a negligible effect on mean thrust but raises root mean square (RMS) thrust fluctuations by 14%, compared to 3.9% for the gust with intensity of 10% case. Spectral analysis reveals BPF harmonic increase of 16% and 29% for the intensity of 10% and 30% gusts, respectively, while the gust-frequency response is attenuated. Acoustic results show spatially dependent changes, seen in directivity maps: small gust amplitude enhances BPF levels at most microphones, whereas larger amplitude redistributes noise, broadening high SPL regions but reducing peaks in some directions. Overall Sound Pressure Level (OASPL) shifts range from about -2 to +2 dB, depending on position, indicating that higher gust magnitudes primarily alter noise directivity rather than uniformly amplifying it. These findings highlight the importance of considering high-intensity gusts in vertiport noise assessment and support design strategies for eVTOL operations in gust-prone urban environments.
Turbulent pressure fluctuations can compromise the accuracy of far-field acoustic measurements when microphone arrays are flush-mounted on surfaces exposed to fluid flow. To address this, recent advances in acoustic metamaterials have introduced novel approaches to enhance the signal-to-noise ratio of these measurements. In this paper, new techniques for coupling the sound and flow field to a meander metasurface are discovered through computational acoustics modeling and examined through wind tunnel testing in Virginia Tech's Subsonic Modular Anechoic Research Tunnel (SMART). The results provide insight into the optimal shape for allowing sound waves into a metamaterial while attenuating turbulent boundary layer noise.
This paper presents the acoustic design of a new anechoic wind tunnel and its evaluation through simulations and experiments. An experimental method is proposed to estimate, in situ, the individual transmission losses of acoustically treated guide vanes and diffuser, without flow, from measured impulse responses. Results are compared with finite element simulations, showing comparable 1/3-octave transmission losses from 1 to 5 kHz; however, the simulations are highly sensitive to the material properties in the impedance model. The total background noise in the wind tunnel test section is estimated from transmission losses and compared with measurements at 70 m/s flow. Below 200 Hz, agreement is good; at higher frequencies, measured noise is up to 30 dB higher due to turbulent boundary layer and Kevlar roughness noise. An airfoil noise benchmark is also presented, showing a single-microphone signal-to-noise ratio up to 8 dB below 1-2 kHz.
