Sound Generation Mechanism Research Articles

Aeroacoustic modeling of a low Reynolds number rotor in the wake of a cylindrical beam

In micro air vehicles, low Reynolds number rotors operating in close proximity to the fuselage raises the question of interaction noise as a prominent acoustic source. This paper proposes to comprehensively explore the interaction noise generation mechanisms, employing numerical simulations, experimental approaches, and analytical modelling. The focus is on scenarios where a low Reynolds number rotor is located in the wake of a cylindrical beam, and oppositely when the beam is in the wake of the rotor. In both scenarios, it is observed experimentally that the amplitudes of the BPF harmonics are increased with respect to that obtained without beam. This increase is higher when the rotor is in the wake of the beam compared to when the beam is in the wake of the rotor. The numerical simulation is shown to reproduce quite well the envelope of the amplitudes of the BPF harmonics obtained experimentally in both scenarios. Interestingly, by applying Ffowcs-Williams and Hawkings analogy on elementary surfaces, and not on the whole rotor-beam surface, the main sound generation mechanism was found to be the unsteady loading on the beam in both scenarios. Finally, an analytical modeling of the noise source mechanism is compared with experimental and numerical results.

Flow Structure and Periodic Processes in a Disc-Shaped Vortex Chamber of a Hydrodynamic Cavitator

Introduction. The essence of the acoustic – cavitation processes is that the liquid is passed through sound with a pressure at the wave surface of more than 3 bar that causes local breaks of the liquid in the vacuum phase of the wave and the collapse in the manometric phase. The opposite walls of each cavern in the collapse approach at a speed exceeding two speed of sound, due to which a high energy density is achieved at the meeting point, and what is especially valuable is the mutual transitions of energies from one form to another, unattainable under normal conditions, and, moreover, as inside cavitation area and near it. The novelty of the work is confirmed by the results of a periodic information and patent analysis, and by four patents received for inventions on the topic under consideration.Aim of the Study. The study is aimed at improving the acoustic-cavitation qualities of a disk-shaped vortex chamber used as a liquid whistle.Materials and Methods. In the study, there were used numerical modeling of flows in the FlowVision program, experimental determination of flow rates using a pitot tube, film method, removal of frequency response using SpectraPLUS 5.0, and visualization of flows and processes on optically transparent devices by the method of color indicators in stroboscopic lighting high-speed video shooting.Results. The mechanism of sound generation and noise in the flow transiting through the device has been found. The corrective effect of pump pulsations f = 300 Hz on the sound generation mechanism was revealed. The disc-shaped character of the device, which encloses the input flow in cross section from three directions, contributes to creating a more expressive acoustic signal, forming two conjugate torus vortices along the shell that ensures uniformity of the circumferential flow, attenuation of longitudinal high-frequency oscillations f = 200 kHz, and the creation of periodic zones of increased pressure along the shell. The concentrated tangential entrance to the device determines the central asymmetry of the flows in it and a number of processes that create acoustic noise.Discussion and Conclusion. The frequency of the useful acoustic signal in the vortex chamber is proportional to the speed of the transit flow, and the amplitude is proportional to the dimensions of the device. Along with the useful signal created by the interaction of the peripheral and input parts of the transit flow, noise of similar frequencies is created in the device. Other sources of noise generation are due to the presence of a concentrated tangential input. The formation of two conjugate torus vortices along the shell can be used as a means of controlling the process of interaction between parts of the transit flow. The disc-shaped vortex chamber combines the functions of sound generation and the ability to create a centrifugal field, which expands its technological capabilities.

The best whistler: A cavitating tip vortex

The discrete tone radiated from a cavitating tip vortex, known as “vortex singing,” was first recognized in 1989, but its sound generation mechanism has remained a mystery for over 30 years. In this Letter, by means of the correction for the cavitation bubble dynamics and the dispersion relation of cavity interfacial waves, we found that after the far-end disturbances propagate upstream, the whistling vortex should be triggered by near-end sound sources, the breathing mode waves. Further utilizing the theoretical solutions for singing lines and the potential singing cavitation number with frequency, we accurately identified all available tests for seeking the vortex singing over the past three decades, which not only demonstrates that the vortex singing frequency is only determined by the measurable mean cavity radius (rc), cavitation number (σ), and desinent cavitation number (σd) in experiments, but also responses to a long-standing perplexity: why such a best whistler is able to appear only within a narrow range of the cavitation number.

On the vortex dynamical contribution to the emission of infrasound from tornadoes.

Previous field experiments support the claim that a tornado can radiate discernible infrasound between 0.5 and 10 Hz. The physical mechanisms of tornado sound generation are still not fully understood, although several potential mechanisms have been proposed. In this paper, the theory of vortex sound is applied to the sound radiation from two numerical tornado simulations based on large eddy simulations. There are two different vortex-related mechanisms in distinct frequency regimes. It is found that rotation of a non-axisymmetric vorticity field produces low-frequency infrasound less than 2.0 Hz. High-frequency tornado infrasound can be attributed to more complex vortex dynamics such as vortex merging.

Investigation of flow-induced sound source generation in a centrifugal pump

Abstract This study investigates the mechanism of sound source generation in a centrifugal pump under different flow conditions. The calculated results are in good agreement with experimental data by employing the delayed detached eddy simulation and flow-acoustic coupling method to compute the flow-induced noise. Based on the vortex sound theory and vorticity binary decomposition, the sound source (STA) is divided into a shear-induced source (STS) and a rigid rotation-induced source (STR). The results show that the sound source induced by shear is the main factor. The fluctuation amplitude of STS can be nine times larger than STR at monitor point near the tongue under the design flow rate condition. There is a mutual cancellation effect between the STS and STR. The fluctuation amplitude of STA at the blade passing frequency is 86% of STS at monitor point near the tongue under high flow rate conditions, indicating that sometimes rigid rotation can suppress noise generation.

Design of osteosynthesis plate for detecting bone union using wire natural frequency

We have developed a novel osteosynthesis plate with bone union detection using a wire's natural frequency (BUDWF) to provide the quantitative result of bone union detection. The concept for detecting bone union is measuring the rate of frequency change. The frequency is measured from sound generated from the wire attached to a modified plate. The plate is modified from a Syncera ADLER B0409.10 and attached with 0.3 mm diameter 316L stainless steel wire. The sound generation mechanism was created by PEEK and installed on the plate to generate the sound. The preliminary experiments were conducted on a Sawbones tibia composite mimic. We used the cut Sawbones to create fracture samples with a 0, 0.5, 1-, 2-, and 5-mm gap representing the fractured bone with different gap sizes and prepared uncut Sawbones as a union sample. These samples were tested five times, and the sound was recorded from a condenser microphone and analyzed. We found that the BUDWF can differentiate samples with a fracture gap above 2 mm from the union sample, as the differences in the rates of frequency change between samples with a fracture gap above 2 mm and union samples were statistically significant. However, there was a limitation that the BUDWF plate was still unable to differentiate the 0 mm fracture gap and the union sample in this study.

Sound generation mechanisms in a collapsible tube.

Collapsible tubes can be employed to study the sound generation mechanism in the human respiratory system. The goals of this work are (a) to determine the airflow characteristics connected to three different collapse states of a physiological tube and (b) to find a relation between the sound power radiated by the tube and its collapse state. The methodology is based on the implementation of computational fluid dynamics simulation on experimentally validated geometries. The flow is characterized by a radical change of behavior before and after the contact of the lumen. The maximum of the sound power radiated corresponds to the post-buckling configuration. The idea of an acoustic tube law is proposed. The presented results are relevant to the study of self-excited oscillations and wheezing sounds in the lungs.

Low frequency sounds from tornadoes

It has been well established that tornadic storms can generate infrasound (i.e., sound at frequencies below human hearing) signals, but the mechanism of sound generation is still a mystery. There is speculation that these infrasound signals have the potential to serve as an alternative method for alerting and/or tracking of life-threatening tornadoes. Yet, until the mechanism for infrasound generation is understood, it remains a possibility that the infrasound signals are present due to other storm effects and would be improper to use for tornado warning or tracking. One possible explanation for the infrasound signals is an amplification of turbulent fluctuations due to latent heat production. This presentation will detail laboratory efforts to isolate noise production from turbulent structures with and without latent heat production due to condensation of water in saturated warm air. [This work was supported by the Gordon and Betty Moore Foundation, grant DOI 10.37807/gbmf11559.]

Aerodynamic and Aeroacoustic Effects of Different Transition Mechanisms on an Airfoil

The paper delves into the detailed sound generation and propagation mechanisms associated with tripping-induced flow perturbations and trailing-edge scattering using wall-resolved large-eddy simulations. Two distinct boundary-layer tripping techniques, namely a geometrically resolved stair strip and an artificially modeled trip using suction and blowing, are investigated. To facilitate comparison, the natural boundary-layer transition is also simulated as a baseline scenario. The analysis takes into account a Reynolds number of 4×105, a Mach number of 0.058, and a nonzero angle of attack of 6.25° over a NACA 0012 airfoil configuration. The mechanisms for sound generation and propagation related to trailing-edge noise remain consistent across the three transition scenarios. However, boundary-layer tripping notably leads to intricate, scenario-specific noise generation: there is an interaction between the laminar separation bubble and tripping for the stair strip case, whereas laminar boundary-layer instability is evident for the suction and blowing scenario. Aerodynamic flowfields involving acoustic noise sources, their propagating natures near the wall, and far-field acoustics are cross-examined in detail. The comprehensive analysis of observed phenomena provides valuable insights for understanding nonlinear flow and acoustic interactions relevant to airfoil noise and designing new types of trips under adverse pressure gradient flows.

Diversity of sound production and hearing in fishes: Exploring the riddles of communication and sensory biology

My research in fish bioacoustics started in the mid-1980s and comprised the diversity in sound-generating mechanisms and sound production in various contexts such as dyadic contests. I worked on European freshwater and marine fish, but mainly on tropical fish from Southeast Asia (gouramis) and the Amazonian River system (pimelodid, doradid catfishes, piranhas). The diversity of sonic organs raised the question if general patterns of innervation of sonic muscles in vocal fishes exist. Different vocal (sonic) motor nuclei were identified in the brainstem and rostral spinal cord for pectoral and swim bladder vibration mechanisms. In addition, I wanted to find out if vocal fish differ from nonvocal species in hearing abilities. Measurements applying the auditory evoked potential technique revealed a large diversity of auditory sensitivities depending on the presence or lack of accessory hearing structures independently of sound production. Furthermore, I investigated the anatomy, ultrastructure of inner ears, and hearing enhancement by auxiliary hearing structures. Recently, we studied sound-induced motion of swim bladders, auditory ossicles, and otoliths using x-ray phase contrast imaging. A large part of my studies was carried out in cooperation with laboratories in the USA, Germany, Italy, Portugal, and Brasilia which made my research interesting.

Direct aeroacoustic simulation of a flow through an expanding pipe with orifice plates

Expanding pipes with orifice plates are utilized as silencers for fluid machinery. However, tonal sounds can be generated from flows through the silencers with such a configuration. To clarify the mechanism of tonal sound generation from a flow over a circular expanding pipe with orifice plates, flow and acoustic fields were directly solved on the basis of the compressible Navier-Stokes equations. Prediction results demonstrated that tonal sounds occurred at multiple frequencies. The phase-averaged flow fields for each frequency indicated that vortex rings or spiral vortices were periodically shed in the flow around the orifice plates. Acoustic radiation occurred owing to the collision of these vortices with the orifice plates, which led to the occurrence of acoustic resonance in the expanding pipe. The predicted circumferential phase distributions of pressure fluctuations demonstrated the coexistence of different acoustic modes in the expanding pipe. The relationship between acoustic modes and vortical behaviors was presented. The effects of inflow Mach number on the vortex structures and acoustic modes for the resonance were also discussed.

Modelling of noise due to impulsive excitation using nonlinear time series analysis

Early acoustic machine design phases should target safety and comfort requirements, minimising the risk of hearing damage and noise exposure and ensuring a pleasant machine sound. Therefore, physical mechanisms of sound generation, propagation and radiation need to be modelled. Today, nonlinear physical relations in vibroacoustic models such as contact problems are mostly linearised. The aim of this paper is to increase the prediction quality of vibroacoustic models so that physical mechanisms behind sound generation due to complex physical, nonlinear phenomena can be better understood. The focus is on sound generation due to impulsive excitation, which is a significant sound generation mechanism in machinery, e.g. considering gears, forming processes (e.g. punching), or road loads of vehicles. We describe a model setup to simulate impulsive excitation on a laboratory scale, where structural vibrations can be analysed. Physical correlations including nonlinear correlations could be identified using nonlinear time series analysis. Recurrence plots will be used to identify features such as phase space contraction, e.g. to identify damping. These features form the basis to deepen the physical understanding of machinery noise, to contribute to enhanced acoustic machine designs and to enhance data-driven modelling using sparse regression in future research.

Aerodynamic sound production from flying beetles

Extensions of previous research on the aerodynamic sound production by flying beetles are presented. The topic is of interest in terms of both fundamental acoustics and the applications such as classification and tracking of invasive insect species. High speed video with synchronous acoustics recordings of the Coconut Rhinoceros Beetle (Oryctes rhinoceros) and the Oriental Flower Beetle (Protaetia orientalis) provide experimental data for the analytical and numerical models. Three dimensional, unsteady flow simulations were conducted using compressible flow software (CAESIM, Adaptive Research, Inc.) with a TVD methodology. Flapping wing motion requires mesh deformation with a rotation with a prescribed bending and also coupled rotation and translation of the wing’s hinge position. Contributions of wing beat frequency and vorticity to sound generation are examined with respect assumptions of the spatial dimensions (two or three dimensional), bending and fluid structure interactions. Underlying sound generation mechanisms at the wing tips are examined in higher fidelity. Both hovering and forward flight are investigated and compared in terms of the different flapping motion of the two species. In addition, relationships of the tones to the temporal lift and drag lift coefficients are highlighted.

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.

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.

Development of a test rig for the investigation of flow-induced mechanisms of sound generation at radial fans

A low sound pressure level is a crucial quality feature for industrial and non-industrial radial fans. The scientific literature documents numerous mechanisms of flow-induced sound generation that can occur in a radial fan. Nevertheless, not all sound generation mechanisms are considered well-understood today and thus are still dealt with in ongoing research. This work develops an experimental setup to investigate the different mechanisms of flow-induced sound generation in radial fans. The aim is to close the existing gap between available experimental research on axial and radial fans. One critical factor for the experimental investigation of flow-induced sound generation in fans is the control of inflow. Due to the design of radial fans, the fluid flow deflects from the axial supply duct to the radial direction upstream of the impeller, generating a complex flow field with limited control possibilities. Hence, we designed a test rig that improves the control of inflow conditions, aiming to resolve the superposed sound generation mechanisms in acoustic measurements. This paper presents the key features of the test rig and some first experimental results. The investigation of the distinct sound generation mechanisms is left to future work based on the introduced test rig.

Research Progress on Audible Noise Emitted from HVDC Transmission Lines

The audible noise (AN) of DC transmission lines is a crucial factor affecting environmental assessment and electromagnetic design. In recent years, the public has paid increasing attention to the audible noise after the emergence of the HVDC technique. This paper emphatically reviewed the work investigating audible noise characteristics in both time and frequency domains, especially the correlation between sound and discharge. The proposed mechanisms of DC audible noise and the mitigating techniques are summarized, including the noise generation process, the physical models, the measurement method, and the mitigating technologies. It is found that the existing literature mainly focused on the prediction and characteristics of AN, the environmental factors which influence AN, and the methods to minimize AN emission. However, existing achievements still need to be improved to fully understand the mechanism of AN generation and solve the adaptability problem of AN prediction methods. The following aspects are valuable in future research: The correlation between audible noise and other corona effects will help solve the problem of difficulty in measuring audible noise in field condition; The corona discharge mechanism and weather resistance anti-corona coating when raindrops are attached to the surface of the transmission line, which will help guide the development and application of anti-corona coatings for the transmission line. Future research should also understand the mechanism of sound wave generation when considering the space charge effect.

Feedforward Harmonic Suppression for Noise Control of Piezoelectric Synthetic Jet Actuators

Piezoelectrically driven Synthetic Jet Actuators (SJAs) are a class of pulsatile flow generation devices that promises to improve upon steady forced cooling methods in air flow generation, surface cleaning and heat transfer applications. Their acoustic emissions and vibrations, an intrinsic by-product of their operation, needs to be mitigated for applications in noise-sensitive contexts. Already used for aerodynamic control [1, 2], thrust vectoring [3], spray control [4], and heat transfer [5, 6], they are increasingly being considered for sensor lens cleaning in automobiles. In this study, the sound generation mechanisms of SJAs are discussed and an active noise reduction method is proposed and evaluated. Driven with a single frequency sinusoidal input, SJAs produce acoustic emissions at harmonic frequencies within the frequency range of speech communication. The sound pressure levels of those emissions are commensurate with that of other automotive subsystems and electronic components. Previous attempts at noise control include passive strategies, such as, nozzle design [7, 8], enclosure design [5], and mufflers. Active strategies, such as out-of-phase actuator pairs [9], and signal modulation have also been considered. Given their relative novelty, further reduction is possible at the source through the tuning of prominent tonal components of their emissions. The present study demonstrates that feedforward active control, achieved through input harmonic tuning, results in significant drops in the loudness of the radiated sound with minimal effects on flow and cleaning performance. Acoustic emissions from all prominent harmonics could be reduced using this method. Tuning is local to the location of the error microphone and further work is required to achieve global tuning. Preliminary performance investigations are presented.

Weld Defect Detection of a CMT Arc-Welded Aluminum Alloy Sheet Based on Arc Sound Signal Processing

The cold metal transfer (CMT) process is widely used in thin plate welding because of its characteristics of low heat input and stable arc. In actual production, a larger weld gap, misalignment, or other problems due to assembly error lead to serious welding defects, such as burn-through and a lack of fusion. The arc sound contains a wealth of information related to the quality of the weld. This work analyzes the mechanism of CMT arc sound generation, as well as the correlation between the time–frequency spectrum of the arc sound signal and welding quality. This paper studies the extraction of the multi-channel time–frequency spectrum of an arc sound and inputs it to a custom convolutional neural network for the CMT welding defect identification of thin aluminum alloy plates. The experimental result shows that the average accuracy of the proposed model is 91.49% in the defect identification of a CMT arc-welded aluminum alloy sheet, which is higher than that of the single-channel time–frequency convolutional neural network and other traditional classification models.

Recent Progress in Maritime Rainfall Noise

Abstract: The acoustic radiation generated by underwater marine rainfallis an important component of marine ambient noise, which is an interfering background for various hydroacoustic equipment and directly affects the performance of hydroacoustic equipment. At the same time, marine environmental noise is rich in marine information, and underwater noise from rainfall can reflect features such as rainfall intensity, raindrop particle size distribution, and wind-induced angle of entry. This paper studies the latest research progress in this field, presenting three main elements: the sound generation mechanism of individual raindrops, the research on underwater noise from rainfall, and marine rainfall noise intensity inversion. The study of underwater noise generated by rainfall can provide theoretical support for underwater noise field forecasting and marine environment monitoring and provide a reference for improving sonar performance.