High Frequency Broadband Noise Research Articles

Experimental characterisation of rotor noise in tandem configuration

This paper presents a comprehensive investigation into the noise radiation characteristics of two rotors in a tandem configuration. Through an extensive experimental campaign, the study explores the effect of the separation distance between two rotors on the noise directivity patterns, spectral characteristics and temporal features of the radiated noise under both hovering and edge-wise flight conditions. The directivity patterns of the overall sound pressure level obtained from the polar and side arrays demonstrate a dependence on the separation distance and advance ratio. Spectral characteristics elucidate the influence of separation distance on tonal and broadband energy content. While the high-frequency broadband noise increases for both hovering and edge-wise flight similarly, the low-frequency broadband energy strongly depends on the advance ratio and the rotor separation distance. The results reveal a distinct envelope in the design space, where the radiated broadband noise for tandem configurations can be minimized, offering potential insights for noise reduction strategies. Additionally, the wavelet analysis for tandem configurations reveals that the emitted noise at the first and second blade passing frequency is strongly intermittent with amplitude modulation, which shall be considered for annoyance perception.

Assessment and mitigation of the rotor noise for future urban air mobility vehicles

Multi-rotor configurations are often adopted in urban air mobility (UAM) designs. The aerodynamic noise of rotors is a significant source of the UAM vehicles, requiring systematic measurements and mitigation. To this end, a large-scale test rig is designed and built to enable the acoustic measurement of UAM rotor blades. The test rig is placed in an anechoic chamber with the size of 8.1 m (L) × 6 m (W) × 5.1 m (H). The aerodynamic force and noise of a pair of custom-designed blades with a radius of 0.5 m are measured. The rotational speed ranges from 600 to 1800 revolutions per minute. A zig-zag turbulator is applied on the blade surface to trip the boundary layer. The effect of the turbulator on rotor noise is negligible compared to the untreated case, indicating that the blade boundary layer is fully developed. Particulate roughness is then implemented on the blade surface to explore the potential of noise reduction for turbulent blade boundary layers. Results show that a reduction of 3 dB in high-frequency broadband noise can be achieved upstream and downstream of the rotor at 1800 RPM when the particulate roughness is applied on the leading-edge region of the blades.

Aeroacoustic radiation of low Reynolds number rotors in interaction with beams.

The radiation characteristics of rotor-beam interaction noise are studied experimentally for low Reynolds number small-scale rotors in interaction with beams of different shapes, sizes, and downstream positions. The number of blades ranges from two to four. For the two-bladed rotor, the presence of the beam has no effect on the mean aerodynamic performance. Moreover, the blade passing frequency (BPF) and the high frequency broadband noise (BBN) appear not to be affected by the presence of the beam. On the contrary, the magnitude of the 2×BPF-25×BPF harmonics increases up to 30 dB compared to the case without beam, with an envelope consisting of two humps: one centered around 5×BPF and another around 20×BPF-25×BPF. For the first hump, a dipole-like pattern with minimal amplitude aligned with the beam can be observed, whereas another dipole-like pattern is observed for the higher frequency hump, but with a minimal amplitude over all the rotor disk plane. Compared to the two-bladed rotor, the presence of the beam has an effect on the mean aerodynamic performance of the three- and four-bladed rotors, increasing both the torque and the thrust at iso-rotational speed. This change leads to a change in the directivity of the BPF tone that decreases at a latitude angle of θ=0° and increases at a latitude angle of θ=40°. Moreover, the same two competing humps are observed on the BPF harmonics envelope. Interestingly, the frequency range over which an amplification of the harmonic magnitude is observed seems not to be influenced by the number of blades. Finally, the magnitude of the low frequency hump increases with the beam diameter, the rotational speed, and the number of blades but decreases with the rotor-beam distance. That of the high frequency hump increases also with the rotational speed and the number of blades, but not anymore with the beam diameter, and reaches a maximum value when the rotor-beam distance is at an intermediate distance of L = 25 mm. This hump is also influenced, to a lesser extent, by the shape of the beam. The two different evolutions permit us to conclude that the noise generation mechanisms leading to the two humps must be different. Scaling laws of the acoustical energy are derived for all those parameters. As already done for previous experiments without beam, all of the results are made available as an open database, at https://dataverse.isae-supaero.fr/.

Computational Aeroacoustics Study of Propellers with Vibrational Motion

In this work, we conduct a numerical investigation of aerodynamics and aeroacoustics of propeller blades with vibrations, which could occur in practical unmanned aerial vehicles as the blades are often made of lightweight materials. The simulations resolve the sound generation from the unsteady turbulent flows using an acoustic-wave preserved artificial compressibility method. Then, the sound projection to the far-field observers is made using an integral solution of the Ffowcs-Williams and Hawkings equation. The study shows that periodic blade vibrations with small amplitudes can lead to aerodynamic thrust fluctuations. The blade vibration also affects the generation of tip vortices and the near-blade flow structures due to the periodic change of the effective angle of attack. Consequently, significant tonal noise at the harmonics of rotational frequency is produced, and the noise can propagate to both upstream and downstream directions of the rotor disc plane. A noise source analysis is performed to identify the contribution of different noise components. Results show that the extra tonal noise is mainly caused by the Doppler effect due to the blade axial motion and the influence of the thrust fluctuations. Moreover, the study also suggests that the high-frequency broadband noise seems to be insensitive to the blade vibration.

Improving Accuracy of Airfoil Trailing Edge Noise Models with Turbulent Flow Anisotropy for Cambered Airfoil

In turbomachinery applications such as wind turbines, and aircraft airframe, flow-induced noise and vibration are significant noise sources. There are five main types of airfoil self-noise, with trailing edge noise (TEN) being the most dominant high-frequency broadband noise. The Amiet TE noise model is a low-order analytical model that requires the surface pressure (SP) near the trailing edge for TEN modelling. The accuracy with which the SP spectrum is calculated determines the accuracy of the TEN modelling. The SP spectrum is calculated using the physics-based TNO-Blake (TB) Model and its recently produced improved version. Most studies have focused on using the TB model for symmetric airfoil, with little attention paid to modelling the TEN of cambered airfoil. We use TB model variants on the SP spectrum prediction in this study, which are enforced with the Amiet TEN model for far-field noise modelling. The SP models' source comes from a high-fidelity Large-Eddy-Simulation simulation (LES) based on CABARET method. The simulation is carried out using the NACA65410 airfoil with a chord length of 0.3m and an angle of attack of 0,2, 4, 6, and 8 degrees at a Reynolds number of 5.2e5. The SP spectrum and velocity spectrum used to improve the TEN model's accuracy for cambered airfoil.

Lattice-Boltzmann calculations of rotor aeroacoustics in transitional boundary layer regime

The goal of this paper is to demonstrate the predictive capabilities of the Lattice-Boltzmann/Very-Large-Eddy-Simulation CFD solver SIMULIA PowerFLOW® when a transitional flow past a small drone rotor blade is simulated, without using any numerical or physical turbulence triggering system. A recently developed variant of PowerFLOW VLES model is validated against measurements performed at Delft University of Technology. A 2-bladed propeller of 0.3m diameter is considered, which is operated at 4000 and 5000 RPM and two different advance ratios, 0.0 and 0.6, for each angular velocity. In such conditions, corresponding to a Reynolds number based on the chord at 75% of the tip radius ranging between 7⋅104 and 9⋅104, the presence of a laminar separation bubble was experimentally observed and related to the occurrence of a high-frequency broadband noise hump in the far-field noise spectra. The numerical results are in a good agreement with the loading, noise and PIV experimental measurements, demonstrating the capability of the new PowerFLOW VLES model to predict boundary layer flow transitional phenomena such as laminar separation, reattachment and transition to turbulence, as well as the corresponding broadband trailing-edge noise radiation.

Requirements for Drone Operations to Minimise Community Noise Impact.

The number of applications for drones under R&D have growth significantly during the last few years; however, the wider adoption of these technologies requires ensuring public trust and acceptance. Noise has been identified as one of the key concerns for public acceptance. Although substantial research has been carried out to better understand the sound source generation mechanisms in drones, important questions remain about the requirements for operational procedures and regulatory frameworks. An important issue is that drones operate within different airspace, closer to communities than conventional aircraft, and that the noise produced is highly tonal and contains a greater proportion of high-frequency broadband noise compared with typical aircraft noise. This is likely to cause concern for exposed communities due to impacts on public health and well-being. This paper presents a modelling framework for setting recommendations for drone operations to minimise community noise impact. The modelling framework is based on specific noise targets, e.g., the guidelines at a receiver position defined by WHO for sleep quality inside a residential property. The main assumption is that the estimation of drone noise exposure indoors is highly relevant for informing operational constraints to minimise noise annoyance and sleep disturbance. This paper illustrates the applicability of the modelling framework with a case study, where maximum A-weighted sound pressure levels and sound exposure levels as received in typical indoor environments are used to define drone-façade minimum distance to meet WHO recommendations. The practical and scalable capabilities of this modelling framework make it a useful tool for inferring and assessing the impact of drone noise through compliance with appropriate guideline noise criteria. It is considered that with further refinement, this modelling framework could prove to be a significant tool in assisting with the development of noise metrics, regulations specific to drone operations and the assessment of future drone operations and associated noise.

Experimental investigation of low Reynolds number rotor noise.

In this paper, an experimental characterisation of low Reynolds number rotors is performed in an anechoic room. Two commercially available two-bladed rotors as well as four three-dimensional (3D)-printed rotors with different numbers of blades (from two to five) are tested. The latter have canonical geometry, with an NACA0012 blade section profile, extruded in the radial direction with constant chord and constant 10° pitch. The experimental setup and the 3D printing strategy are first validated using results from the literature for the commercially available rotors. For all the tested rotors, four noise characteristics are analysed: the overall sound pressure level (OASPL), the amplitude of the blade passing frequency (BPF), and the amplitude of its first harmonic and the high-frequency broadband noise. For all the rotors, an increase in all noise characteristics is observed with the rotational speed (rpm) for all directivity angles. Moreover, an interesting change of pattern is observed for the amplitudes of the BPF and of its first harmonic, with, in the vicinity of the rotor plane, a minimum value for low rpm and/or high number of blades, and a maximum value for high rpm and/or low number of blades. This change in directivity leads to a similar change of directivity of the OASPL. For the broadband noise, a dipole-like pattern is obtained with a minimum value at θ=-10°, i.e., aligned with the trailing edge and thus indicating the generation of trailing edge noise. Finally, scaling laws that characterise the amplitude of the different noise components with respect to the rpm are proposed.

Drone Noise Emission Characteristics and Noise Effects on Humans-A Systematic Review.

The number of operations of Unmanned Aerial Vehicles (UAV), commonly referred to as “drones”, has strongly increased in the past and is likely to further grow in the future. Therefore, drones are becoming a growing new source of environmental noise pollution, and annoyance reactions to drone noise are likely to occur in an increasing share of the population. To date, research on drone noise emission characteristics, and in particular also on health impacts, seems scarce, but systematic overviews on these topics are missing. The objective of this study was to establish a systematic literature review on drone noise emissions and noise effects on humans. The paper presents the methodology of the systematic reviews performed separately for noise emission and noise effects, assembles current literature, gives an overview on the state of knowledge, and identifies research gaps. Current literature suggests that drone noise is substantially more annoying than road traffic or aircraft noise due to special acoustic characteristics such as pure tones and high-frequency broadband noise. A range of open questions remains to be tackled by future studies.

Noise control for high-lift devices by slat wall treatment

A novel wall treatment method, termed as the Slat Cove Wavy Wall (SCWW), is proposed to improve both the aero-acoustic and the aerodynamic performances of high-lift devices. By this treatment, a wavy pattern is imposed on the wall in the slat cove. In the present work, slats with three different SCWWs are designed and applied to a straight wing composed by 30P30N airfoil. Numerical simulations and analyses are conducted to unveil the underlying mechanisms of noise reduction. The unsteady flow field and the far-field noise are predicted by a hybrid SA-IDDES method followed by the integration of the Ffowcs Williams and Hawkings equation. The modified high-lift device is proved to be able to improve the aero-acoustic performance by reducing the low-frequency components of the radiated noise. Meanwhile, the aerodynamic performance is maintained or improved under proper design parameters. The boundary layer separation in the slat cove is accelerated by the SCWW, which transforms low-frequency periodic vortex shedding to high-frequency turbulent flow. This results in the sound energy shift from low-frequency noise to high-frequency broadband noise. The SCWW method is proved to be effective in redistribution of boundary vorticity flux on the slat lower surface. To reveal the mechanism of the SCWW's drag and noise reduction, the vortex dynamic processes near the slat lower surface in terms of the Lamb vector divergence are further investigated. It is found that the Lamb vector divergence is directly related to both the noise source and the drag. The interaction area of positive and negative Lamb vector divergence can be balanced and decreased by SCWW, resulting in the drag and noise reduction. As expressed by the vortex dynamic processes, this feature implies the SCWW method is of great potential in noise attenuation.

Application of Leading-Edge Tubercles to Enhance Propeller Performance

Aircraft propellers are known to experience flow separation at various flight conditions leading to efficiency variations from approximately 40–85%. By controlling flow separation, higher propeller efficiency and improved performance might be possible. In this paper a bioinspired passive flow control strategy that improves propeller performance is used by introducing protuberances, or tubercles, in the leading-edge region of a propeller blade. Experiments also revealed apparent improvements to propeller acoustic emissions. Propeller performance and noise signatures, evaluated against a reference propeller, are presented from experiments involving different propeller configurations modified with various combinations of tubercle geometry. Specific tubercle configurations exhibit improvements in efficiency up to 6% as well as redistributed directional and high-frequency broadband noise characteristics.

Zamana Bağlı Kavite Akışı Aeroakustiğinin Büyük Girdap Simülasyonu ile Sayısal Olarak İncelenmesi

Flow along a cavity is of special interest for researchers due to the occurrence of free shear layer flow and related high levels of sound and pressure forces. In this study, turbulent flow along an open cavity at a low inlet Mach number (Ma = 0.034) is modelled by Large Eddy Simulations (LES). The velocity profiles at various stations inside the open cavity are compared to available experimental data. It is found that LES results agree with experimental data and detects the transient pressure change in the flow field satisfactorily. Transient pressure data in the flow field is evaluated in acoustic analogy. The noise generated by the cavity is compared with the established Rossiter modes and is found to be reasonable. To create an effect on the sound pressure levels (SPL), a small obstacle with quadrilateral cross section is immersed in the shear layer at three different locations. This causes that the SPL peaks are reduced compared to the case without any obstacle. Thus, cavity-induced noise form specific frequencies are redistributed to high frequency broadband noise as a result of this passive flow control method.

Phylogeny and acoustic signal evolution of a pure tone song katydid Pseudophyllus titan (Orthoptera: Tettigoniidae) based on the complete mitogenome

Ensifera is an extremely diverse group in the order Orthoptera, and it has been the subject of considerable research, especially in terms of phylogeny and bioacoustics. Katydids are known for using high-frequency broadband signals whereas most cricket songs are pure tones with a frequency range of 2–8 kHz. Pseudophyllus titan is a special katydid emits low-frequency pure tone songs similar to that of crickets. Acoustic communication of Ensifera were widely studied, but song evolution could deserve new examination taking into account the newly available phylogenies. To examine the song evolution of P. titan in the Ensifera, the mitogenomes of P. titan and three other ensiferan species (Sphagniana ussuriana, Oecanthus sinensis, Truljalia hibinonis) were obtained by high-throughput sequencing. In addition, the phylogeny of Ensifera was reconstructed including 63 in-group taxa, and the divergence time was further estimated for major ensiferan lineages. Lastly, the evolutions of song frequency were evaluated based on an ancestral character state reconstruction (ACSR). Moreover, we have studied the calling songs of 78 katydids and performed ACSR based on five genes. All phylogeny analyses confirmed that the superfamily Schizodactyloidea was placed basally to the non-grylloid clade. P. titan was in the Pseudophyllinae clade, and this clade had a distant relationship form other lineages in Phaneropteridae. The ACSR showed that P. titan evolved low-frequency pure tone songs independently after ancestors of katydids evolved high-frequency broadband noise. Referring to the geologic time scale, we suggest that the song frequency evolution in the Ensifera to some extent caused by adaptations to the changing habitat under conditions of climate change and plant evolution.

Surface roughness effect on rotor broadband noise

The change of helicopter rotor broadband noise due to different surface roughness during ice accretion is investigated. Comprehensive rotor broadband noise measurements are carried out on rotor blades with different roughness sizes and rotation speeds in two facilities: the Adverse Environment Rotor Test Stand facility at The Pennsylvania State University, and the University of Maryland Acoustic Chamber. In both facilities, the measured high-frequency broadband noise increases significantly with increasing surface roughness height. Rotor broadband noise source identification is conducted and the broadband noise related to ice accretion is thought to be turbulent boundary layer-trailing edge noise. Theory suggests turbulent boundary layer-trailing edge noise scales with Mach number to the fifth power, which is also observed in the experimental data confirming that the dominant broadband noise mechanism during ice accretion is trailing edge noise. A correlation between the ice-induced surface roughness and the broadband noise level is developed. The correlation is strong, which can be used as an ice accretion early detection tool for helicopters, as well as to quantify the ice-induced roughness at the early stage of rotor ice accretion. The trailing edge noise theories developed by Ffowcs Williams and Hall, and Howe both identify two important parameters: boundary layer thickness and turbulence intensity. Numerical studies of two-dimensional airfoils with different ice-induced surface roughness heights are conducted to investigate the extent that surface roughness impacts the boundary layer thickness and turbulence intensity (and ultimately the turbulent boundary layer-trailing edge noise). The results show that boundary layer thickness and turbulence intensity at the trailing edge increase with the increased roughness height. Using Howe’s trailing edge noise model, the increased sound pressure level of the trailing edge noise due to the increased displacement thickness and normalized integrated turbulence intensity are 6.2 and 1.6 dB for large and small accreted ice roughness heights, respectively. The estimated increased sound pressure level values agree reasonably well with the experimental results, which are 5.8 and 2.6 dB for large and small roughness height, respectively.

Evaluation of the cavitation inception speed of the ship propeller using acceleration on its adjacent structure

Cavitation noise is strongly restricted by the Navy because this noise can be detected by the sonar systems installed on enemy ships to locate a ship. When cavitation is started, the high-frequency broadband noise increases modulated with shaft rotation and propeller blade passing frequencies. Therefore, the Cavitation inception speed (CIS) is typically detected using the Detection of envelope modulation on noise (DEMON) analysis to identify the underwater propeller noise. Since the CIS varies based on the ship’s condition and sea state, realtime CIS monitoring is required. In this paper, a CIS detection method that evaluates the acceleration on the hull adjacent to the propeller was proposed and verified through experimentation.

Arctic underwater noise transients from sea ice deformation: Characteristics, annual time series, and forcing in Beaufort Sea.

A 13-month time series of Arctic Ocean noise from the marginal ice zone of the Eastern Beaufort Sea is analyzed to detect under-ice acoustic transients isolated from ambient noise with a dedicated algorithm. Noise transients due to ice cracking, fracturing, shearing, and ridging are sorted out into three categories: broadband impulses, frequency modulated (FM) tones, and high-frequency broadband noise. Their temporal and acoustic characteristics over the 8-month ice covered period, from November 2005 to mid-June 2006, are presented and their generation mechanisms are discussed. Correlations analyses showed that the occurrence of these ice transients responded to large-scale ice motion and deformation rates forced by meteorological events, often leading to opening of large-scale leads at main discontinuities in the ice cover. Such a sequence, resulting in the opening of a large lead, hundreds by tens of kilometers in size, along the margin of landfast ice and multiyear ice plume in the Beaufort-Chukchi seas is detailed. These ice transients largely contribute to the soundscape properties of the Arctic Ocean, for both its ambient and total noise components. Some FM tonal transients can be confounded with marine mammal songs, especially when they are repeated, with periods similar to wind generated waves.

A hybrid active/passive exhaust noise control system for locomotives

A prototype hybrid system consisting of active and passive components for controlling far-field locomotive exhaust noise has been designed, assembled, and tested on a locomotive. The system consisted of a resistive passive silencer for controlling high-frequency broadband noise and a feedforward multiple-input, multiple-output active control system for suppressing low-frequency tonal noise. The active system used ten roof-mounted bandpass speaker enclosures with 2-12-in. speakers per enclosure as actuators, eight roof-mounted electret microphones as residual sensors, and an optical tachometer that sensed locomotive engine speed as a reference sensor. The system was installed on a passenger locomotive and tested in an operating rail yard. Details of the system are described and the near-field and far-field noise reductions are compared against the design goal.

Broadband Noise Sources of an Experimental Wind Turbine Rotor Blade

While researches to reduce a broadband noise from wind turbine rotor blade had been conducted successfully, a generation mechanism of the noise particularly from around the tip area is yet to be elucidated and thus a further research for the noise reduction is expected. For this purpose, we had developed microphone array systems to evaluate acoustic performances of the 15 m diameter experimental wind turbine rotor blade and conducted the acoustic tests of the blade. Results of the tests revealed that the blade emits strong high frequency broadband noise from its extreme tip and the mid frequency noise from the trailing edge. This result suggests that the generation mechanism of the high frequency noise is believed due to the stream wise vortices shed directly from the extreme tip which is different from results of wind tunnel tests of three dimensional airfoils.

Noise reduction of an experimental pulse combustion work station

Pulse combustors are pulsating acoustical systems which radiate high‐intensity radiated noise. The Forbes energy engineering experimental pulse combustor was operated in a ‘‘high pressure mode’’ at a fundamental frequency of 50 Hz and an output between 330 000 and 390 000 BTU/h. The noise levels of the baseline configuration, measured around the exhaust outlet, had an SPL over 110 dB at 50 Hz and a series of high intensity narrow‐band components up to 2 kHz. The A‐weighted broadband SPL in the range of 100 dB(A) exceeded damage/risk criterion by as much as 25 dB. The noise reduction goals corresponded to NC50 and SIL of 50 dB. The approach consisted of three parts: (1) A cowl silencer installed at the outlet vent for control of mid to high frequency broadband noise; (2) a custom designed tuned low‐frequency muffler installed as part of the exhaust decoupler; and (3) thermoacoustic cladding applied to the structure. The silencer provided a reduction of 20 dB(A) in the mid‐frequency range and the muffler resulted in narrow‐band reductions of 25 to 40 dB at the lowest frequencies and 5 to 15 dB up to 2 kHz. The results, including sound intensity mapping, are discussed in terms of the goals, design modifications, and operational factors. [Work supported by The Gas Research Institute.]

Effect of blunt trailing edge on rotor broadband noise

The production of high-frequency broadband noise by turbulent vortex shedding from rotor blades with blunt trailing edges is investigated analytically. The derivation of the governing equations, analogous to that of Kim and George (1982) for boundary-layer/trailing-edge noise, is explained, and numerical results are compared with the experimental data of Hubbard et al. (1981) and Lowson et al. (1972) in graphs. It is shown that vortex-shedding noise is a significant component of blunt-trailing-edge rotor broadband noise and that the analytical method employed gives reasonable predictions. The need for a better empirical expression for the normalized spectrum and for more measurements of surface pressure fluctuations near blunt trailing edges is indicated.