Noise Generation Mechanisms Research Articles

Computational Aeroacoustics for a Cold, Non-Ideally Expanded Aerospike Nozzle

Abstract In supersonic aerospace applications, aerospike nozzles have been subject of growing interest. This study sheds light on the noise components of a cold jet exhausting an aerospike nozzle. Implicit large eddy simulations (ILES) are deployed to simulate the jet at a nozzle pressure ratio (NPR)=3. For far-field acoustic computation, the Ffowcs Williams–Hawkings (FWH) equation is applied. A mesh sensitivity study is performed and the jet instantaneous and time-averaged flow characteristics are analyzed. The annular shock structure displays short non-attached shock-cells and longer attached shock-cells. Downstream of the aerospike, a circular shock-cell structure is formed with long shock-cells. Two-point cross-correlations of data acquired at monitoring points located along the shear layers allow to identify upstream propagating waves associated to screech. Power spectral density at monitoring points in the annular shock-cell structure allows to identify its radial oscillation modes. Furthermore, a vortex sheet model is adapted to predict the annular shock-cells length and the BBSAN central frequency. High sound pressure levels (SPL) are detected at the determined BBSAN central frequencies. Finally, high SPL are obtained at the radial oscillation frequencies for the annular shock-cell structure.

Visualization of pressure fluctuation characteristics of weapon bay on unmanned aerial vehicle using delayed detached eddy simulation

In the present study CFD simulation with delayed detached eddy simulation (DDES) are performed to investigate pressure fluctuation environment on an open cavity and weapon bay of UAV. The main purpose of this study is to explore the aerodynamic noise level of complex cavities with slant ceiling and irregular length–depth ratio. First, two cases on an open cavity at Ma 0.85 are analyzed to determine the slant ceiling influence, which shows that the sound pressure levels increase by 1 dB due to the transverse flow effect. Then, a typical weapon bay on unmanned aerial vehicle is simulated to investigate the aerodynamic noise environment. The overall sound pressure level in the weapon bay is up to 157 dB, and when the Mach number turns from 0.6 to 0.8, the amplitude of all modes increases about 16–23 dB. The main focuses of this investigation are to discuss the mechanism of noise generation in a slant ceiling cavity and determine the pressure fluctuation characteristic of a typical weapon bay on unmanned aerial vehicle.Graphical abstract

Experimental Parameters Influencing the Cavitation Noise of an Oscillating NACA0015 Hydrofoil

The strong increase in anthropogenic underwater noise has caused a growing intention to design quieter ships given that ship propellers are one of the dominating noise sources along the worldwide shipping routes. This creates an imminent demand for deeper knowledge on the noise generation mechanisms of propeller cavitation. A cavitating, oscillating two-dimensional NACA0015 hydrofoil is analyzed with hydrophone and high-speed video recording as a simplified and manipulatable representative of a propeller blade in a ship’s wake field for the identification of major influencing parameters on the radiated noise. A pneumatic drive allows the application of asymmetrical temporal courses of the angle of attack, a novel amendment to the widely reported sinusoidal setups. Three different courses are tested with various cavitation numbers. The combination of a moderate angle increase and a rapid decrease is found to generate significantly higher pressure peaks compared to symmetrical angular courses. Considering that the rapid change of the angle of attack caused by the inhomogeneous wake field behind the hull is the core of the cavitation occurrence, the understanding of its influence may contribute to the design of quieter ships in the future while still allowing for the necessary high propeller efficiency.

Noise generation mechanisms of a micro-tube porous trailing edge

The noise generation and flow characteristics of a forced-transitioned NACA 0012 airfoil with a micro-tube porous trailing edge and its solid counterpart have been numerically investigated. The near-field flow dynamics and far-field noise predictions are obtained using compressible large-eddy simulations and the Ffowcs-William and Hawkings acoustic analogy, respectively. The computed far-field noise levels agree with experimental data, showing that the micro-tube structure reduces low-frequency trailing-edge noise without noticeably altering the noise directivity but induces additional high-frequency noise along the direction perpendicular to the airfoil chord. An in-depth analysis of the trailing-edge flow and surface pressure fields reveals that the noise reduction is largely linked to the ‘cross-jet-like’ flow permeation through the micro-tube structures. The interaction between the flow permeation and the boundary layer turbulence leads to a reduction in the strength of the Reynolds-stress source term in the pressure-side flow field and a reduction in the magnitude of the source terms in Amiet’s trailing-edge noise model. Spectral proper orthogonal decomposition is performed to identify flow structures that are spatially and temporally coherent within the micro-tube geometries and to link the flow features to the attenuation in noise scattering efficiency. The effect of the micro-tube structure on the wavenumber–frequency spectra of the trailing-edge surface pressure field is quantitatively analysed. Additional high-energy regions exist in the wavenumber–frequency spectra of the porous trailing edge, resulting from the turbulent eddies that permeate through the micro-tubes. Further, the high-frequency noise increase mechanism is identified as the acoustic dipole noise arising from interactions of edge-induced flow separation with the suction-side surface.

Duct modal identification considering statistical dependency via the Boltzmann machine

The fan noise is the primary component of aero-engine noise. Duct acoustic modal identification is essential for analyzing the mechanism of fan noise generation and propagation. The acoustic modal in the duct can be identified by the measurement of the microphone array mounted on the duct wall. It is generally assumed in the conventional modal identification methods that the modal coefficients are independent of each other. However, the realistic duct acoustic modal coefficients exhibit significant statistical dependence. A Boltzmann machine(BM) model considering the statistical dependence between modal coefficients is proposed for recovering the modal coefficients in this paper. The modal coefficients are estimated using a block coordinate optimization method in a Bayesian framework. Moreover, the modal Boltzmann parameters can be learned iteratively by the BM. The effectiveness of the proposed method is verified by numerical simulations and experimental tests. It turns out that the modal coefficients can be accurately identified by the proposed method, and a more significant sparsity is shown by the results. In addition, the statistical dependence between the modal coefficients can be captured by the BM.

Numerical investigation on the rotor-turbulence interaction noise

Noise from a turbomachinery component, such as compressors and turbines, is an important issue to be resolved in aerospace and ocean engineering applications. The rotor of a turbomachinery assembly usually could operate downstream to other flow structures (fuselages, stators, control surfaces, etc.) that produce either large-scale or small-scale turbulence structures, which are highly complex and are being distorted while they are ingested into the rotor, which generate the rotor-turbulence interaction noise. The presented work conducted numerical simulation research on a ten-bladed rotor ingesting different turbulence structures. The large eddy simulation (LES) and Ffowcs-Williams and Hawkings (FW-H) methods were used to predict the flow-induced noise from the rotor. The results show that the time-space evolution of the upstream turbulent structure when it passes towards the rotor. In addition, the differences in the turbulence ingestion noise at different inflow speeds and rotational speeds were studied. Furthermore, the sound generation mechanism of turbulent ingestion noise was explored using a newly proposed near-field sound source analysis method. In summary, this study can help to improve the understanding of the physical mechanisms of unsteady forces and noise generated by a rotor when it interacts with ingesting turbulent flows.

Prediction and analysis of aeroacoustic characteristics of a real-size high-speed train running in open field using compressible Large Eddy Simulation and vortex sound source

With the rapid advancement of technology, high-speed trains have been requested to increases their speeds. Aerodynamic noise generated from the external flow fields around these trains has become a critical factor to be addressed during a design phase. Accurate prediction of flow-induced noise in high-speed trains requires high-resolution sound source generation in the near-field and precise noise propagation analysis in the acoustic field without numerical dissipation. This necessitates the appropriate design of grids, taking into account aerodynamic noise generation and propagation in conjunction with the length scales of the relevant components of a real-size train. To tackle this challenge, the present research employs a three-dimensional compressible Large Eddy Simulation (LES) technique with high-resolution grids to simultaneously calculate the external flow and acoustic fields of a real-size high-speed train consisting of five cars running in an open field. A comprehensive analysis is conducted to evaluate the contributions of major components, namely, a cap, pantograph, bogie, intercoach, and HVAC cover, which are well-known as significant contributors to high-speed train flow-induced noise. The study utilizes the vortex sound source approach to analyze the generation mechanism of flow-induced noise for each component, providing valuable insights that could potentially aid in reducing aerodynamic noise.

A review of sound radiated by breaking ocean waves

“Physical Mechanisms of Noise Generation by Breaking Waves—A Laboratory Study” was published over 35 years ago by Mike Banner and Doug Cato. This seminal work describes a laboratory experiment demonstrating that the dominant source of breaking wave noise above a few hundred Hz is associated with the formation of bubbles and coalescing or splitting bubbles. This observation opened the door to explaining the properties of ambient sound under wind-driven seas and remotely monitoring air entrainment by breaking waves. Progress on these topics since Banner and Catos’ experiment will be reviewed and outstanding issues discussed. [Work supported by the US Office of Naval Research Ocean Acoustics program (Grant No. N00014-21-1-2316).]

Aeroacoustics of finite wall-mounted square cylinders in pressure gradient flows

Flow-induced noise produced by finite wall-mounted cylinders (FWMCs) is a major noise source for aircraft landing gear, rail pantographs and submarine appendages. These applications often encounter flows with various pressure gradients, however, there is little information in the literature on the effects of pressure gradients. Our recent work has demonstrated that the presence of a pressure gradient can significantly affect the near-wake flow structures of a square FWMC with a low aspect ratio of 2.4, thereby suppressing/enhancing the vortex-shedding tones. The current study extends this work to square FWMCs with varying aspect ratios, focusing on the role that aspect ratio plays in the noise generation of square FWMCs in pressure gradient flows. Experiments were undertaken using the open-jet pressure-gradient test rig in the UNSW anechoic wind tunnel, where the square FWMC model was immersed in flows with favourable-, near-zero-, and adverse-pressure gradients at a width-based Reynolds number of 28800. The square FWMC model was installed on a traversing system to realize the change in cylinder aspect ratio. Inside the test model, a series of channels and surface pressure taps were created to measure the unsteady surface pressure using a remote microphone technique. Far-field noise and unsteady surface pressure signals were simultaneously acquired to characterize the combined effects of aspect ratio and pressure gradient on the far-field noise production and reveal the noise generation mechanisms.

Experimental investigation on the hydro-acoustic characteristics of tandem cylinders with equal diameters

The investigation of the hydro-acoustic characteristics of tandem cylinders has a great importance because they occur in a variety of maritime applications that include submarines, offshore structures and seabed pipelines systems. Better understanding of the noise generation mechanism of tandem cylinders provides great contribution to reduce noise levels for many engineering applications. In this study, the hydro-acoustic characteristics of tandem cylinders with same diameter were experimentally investigated for five different distance ratios at Reynolds number of 1.6 × 104. The diameter of the front and rear cylinders used in hydro-acoustic experiments is 0.02 m. It is observed that the main peak spectrum becomes narrower as the distance ratio increases. The width of the main peak spectrum also decreases as the flow regime transition from the reattachment regime to the separation regime. An increase in the distance ratio is found to correspond to an increase in the main peak frequency. The maximum sound pressure level is observed to attain its peak value at the critical distance ratio of 3.0. Notably, the maximum sound pressure level exhibits a substantial increase of approximately 6 dB as the distance ratio increases from 1.5 to 3.0, followed by a subsequent decrease of approximately 1 dB between the distance ratios of 3.0 and 6.0.

Triple Decomposition of Unsteady Flow in a Low-Speed Mixed-Flow Fan

Numerous studies (including theoretical modeling, numerical simulation, and experimental measurement) have confirmed that wake interaction is a primary factor affecting tonal and broadband noise emitted by aeroengine fans. The knowledge and techniques derived from these studies have often been applied to analyze and control the aerodynamic noise of low-speed axial/mixed-flow fans used in ventilation and cooling applications. Despite the structural similarity between aeroengine fans and axial-flow fans for ventilation and cooling, the differences in their rotor–stator spacing and operating parameters raise the question of whether or not they share completely identical or approximately similar unsteady flow and noise generation mechanisms. In this paper, we employ a large-eddy simulation to investigate the low-speed unsteady flow inside a mixed-flow fan. The triple decomposition method is used to analyze the spatiotemporal characteristics of the time-averaged, ensemble-averaged, and stochastic fluctuation velocities and pressures in both stationary and rotating coordinate systems. The results show that the unsteady flow and noise generation mechanisms inside the fan investigated in this paper are significantly different from those of aeroengine fans, mainly due to two aspects. First, the wake effect on the velocity field of the fan studied in this paper is almost negligible, and the potential interaction is the primary source of the nonuniform spatial distribution of the time-averaged velocity field and the periodic velocity disturbance. Second, the turbulent energy spectrum of the stochastic fluctuation velocity is greatly distinct from the classic turbulence spectra in the low-frequency bands.

Acoustic resonances in an automotive HVAC outlet

In order to better understand the noise generation mechanism of air outlets in an HVAC (Heating Ventilation Air Conditioning) system of a car, investigations were performed in a generic geometry representing a typical air outlet of an automotive car. A hybrid aeroacoustic simulation based on a Large-Eddy Simulation of the incompressible flow coupled with a solver for the acoustic perturbation equations is backed up by an experiment to study the interaction between a throttle valve (used to control the flow rate) and fins (used to guide the flow). Several sound generation mechanisms are identified, of which the so-called Parker-β mode is the most prominent in the sensible frequency range. Both the experimental and simulation results indicate a whistling noise at certain gap lengths between the throttle valve and fins at the frequency of the Parker-β mode. Two different levels of resonance are identified of which one leads to feedback from the acoustic mode to the flow field and one works without feedback. We demonstrate that the latter is established by an efficient excitation of the Parker-β mode by the vortex shedding from the throttle valve if the vortex shedding frequency is close enough to the acoustic frequency.

Objective assessment on earmuff behavior in windy condition

A low frequency noise is always heared when wearing earmuff in windy condition, the noise is more than 40 dB higher than background noise at 50 Hz and most of energy is below 200 Hz at wind speed of 3.5 m/s. An objective measurement methodology was proposed to investigate low frequency noise when wearing earmuff in windy condition. Firstly, taking vehicle wind noise generation and transmission mechanism into cabin as an example, the mechanism of low frequency noise generation inside earmuff by wind was investigated, and the sources generated by wind and paths that transmit sources into earmuff were all listed. Then dedicated experiments were designed and conducted to separate each source and path contribution, and each contribution to noise inside earmuff were calculated and analyzed quantitatively with dedicated experiment results, it is found that the turbulent pressure on outside surface of earmuff transmitted through earmuff shell is the dominant contribution. Finally, porous material is effective to eliminate turbulent pressure and was attached to outside of earmuff shell as a potential passive earmuff anti-wind solution, experiment results showed good anti-wind performance.

Installed Jet Noise Analysis Using a Coupled LES/APE High-Order Method

AbstractAs the bypass-ratio of aeroengines continues to grow, the extra noise generated by the interaction of the jet with the wings becomes more relevant. Understanding the underlying mechanisms of noise generation and predicting the far-field noise of this type of configurations is therefore important for the development of the next-generation ultra-high-bypass-ratio engines. In this paper, a two-step LES/APE coupled framework has been applied to investigate these effects on a simplified configuration. The flow and acoustic results are compared to experimental data measurements available in the literature, achieving a good agreement. The installation effects and sound sources are also studied by analyzing the near-field of the LES/APE simulations.

Research on the Friction Noise Generation Mechanism and Suppression Method of Submarine Rubber-Based Propeller Bearings-A Review.

This article introduces the main mechanisms of friction noise generated by submarine rubber-based propeller bearings and analyzes their respective scope of application and limitations. Then, the research on suppressing friction noise through the optimization of the structure and improvement of materials of rubber-based propeller bearings is discussed. Finally, the article summarizes a promising research direction aimed at eliminating friction noise in submarine rubber-based propeller bearings. By improving the structure and materials, the friction noise of propeller bearings can be effectively suppressed, thereby improving the deterrence and stealth performance of submarines.

New Approaches for Analysis of Jet Noise Data

Despite the significant advances in Computational Aeroacoustics for predicting jet noise, there is still a considerable need for experimental data. However, traditional measures of the directivity of Overall Sound Pressure Levels or the spectral distribution of the acoustic energy yield limited understanding of the nature of the sources. There is hence a need to use different approaches to the analysis of jet acoustics, among them applications of statistical methods such as auto correlation, self-correlation etc to understand the noise sources. In this paper, the acoustic characteristics of supersonic and high subsonic jets from nozzles of different exit geometries are measured and analyzed using these methods. The results show that these statistical methods of analysis can reveal the presence of two distinct noise sources in the flow and are applicable to both subsonic and supersonic flows. The information can be used to determine the appropriate model for the data and is useful for forming new theories about the noise generation mechanism in turbulent jets.

Acoustic and psychoacoustic characterisation of small-scale contra-rotating propellers

This paper reports an experimental investigation of noise radiation and psychoacoustic evaluation of contra-rotating propellers with different blade counts over multiple emission angles and thrust settings. A series of aeroacoustic and psychoacoustic tests were performed with a contra-rotating propeller rig at the University of Salford, UK, where both propellers’ blade count and their axial separation z were varied at static conditions and constant thrust. The sounds were analysed using conventional acoustic metrics, such as Sound Pressure Level (SPL) and Sound Power Level for tonal, broadband, and overall noise, and using the psychoacoustic Sound Quality Metrics (SQMs) Loudness, Fluctuation Strength, Roughness, Tonality, and Impulsiveness. Configurations with axial separation-to-diameter ratio z/D between 0.2–0.4 exhibited a balance between tonal and broadband noise generation mechanisms, resulting in reduced noise levels and reduced annoyance for on-axis observers. The number of blades played a secondary role on determining noise levels: higher blade counts resulted in lower overall noise levels at larger axial separations, although at shorter distances (z/D<0.2) increasing the blade count led to higher noise levels. The SQMs Loudness, Fluctuation Strength, and Tonality were found to also reach a minimum at z/D=0.3, although increasing the number of propeller blades led to a decrease in Loudness but not in Fluctuation Strength and Tonality. The dominant noise generation mechanisms observed here present a directivity null perpendicular to the propellers’ axis, and no clear trends were observed for acoustic and psychoacoustic metrics at this emission angle. A listening experiment was set up to validate the results of the acoustic and psychoacoustic analysis, and its results suggest that the reported noise annoyance is driven by Loudness, Fluctuation Strength and Tonality metrics (p<0.01) at on-axis radiation, while at 90° emission angle, Loudness (p<0.01) and Tonality (p<0.05) are the main contributors to noise annoyance.

Noise Reduction on a Low Reynolds Number Propeller

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.

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/.

Motion-Induced Noise Mechanism Analysis and Reduction Algorithm of ELF Magnetic Receiving Antennas

Motion-induced noise is the main noise that limits extremely low-frequency communication for both shore-based and deep-sea submarines. This manuscript starts from the generation mechanism of motion-induced noise in magnetic antennas, and focuses on the causes of motion-induced noise in magnetic antennas on fixed platforms. The theoretical conclusion reveals that the vibration of magnetic antennas will induce their internal motion-induced noise in either differential or integral form, indicating a clear linear relationship between the vibration and motion-induced noise of magnetic antennas. Based on the generated principle of motion-induced noise presented in this manuscript, corresponding noise reduction algorithms are proposed, and an effective way to apply adaptive filtering algorithm in motion-induced noise cancellation is also presented to accurately reduce this type of noise. In different types of experiments on motion-induced noise testing, the noise reduction results obtained by different noise reduction algorithms demonstrate that the method proposed in this manuscript can achieve effective cancellation of motion-induced noise in magnetic antennas. This experimental result further confirms the research conclusions in this manuscript and provides a feasible approach for practical motion-induced noise cancellation.