The Study of Psychoacoustic Effects of Noise Emitted by the Machine Tools Structure

The main sources of noise at work, in industrial environments, are machine tools: namely, mechanical transmission structure composed of gears, bearings, electric motors drive and the cutting process. Generated vibrations are transmitted through structures, carcasses or directly to environment, developing a complex acoustic field around machine tools. In terms of occupational medicine, the noise is generated by a combination of vibrations producing sounds with different characteristics and that are produced in the workplace [2]. It is important to identify the dominant noise source, the cause of exceeding the admissible limits (87dB) and noise transmission mode to make an objective correlation with effects on humans [1]. Hearing loss can occur immediately at extreme sound levels, but, in general, the problem is noise exposure over time. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical Investigation of Sound Generation due to Laminar Flow Past Elliptic Cylinders

Numerical investigation of sound generation due to unsteady laminar flow past elliptic cylinders has been carried out using direct numerical simulation $(DNS)$ approach at a free-stream Mach number of $0.2$. Effects of aspect ratio $(0.6\le AR\le 1.0)$ and Reynolds number $(100\le Re \le 160)$ on the characteristics of radiated sound fields are analyzed. Two-dimensional compressible fluid flow equations are solved on a refined grid using high resolution dispersion relation preserving $(DRP)$ schemes. Using present $DNS$ data, equivalent noise sources as given by various acoustic analogies are evaluated. Amplitudes and frequencies associated with these noise sources are further related to characteristics of disturbance pressure fields. Disturbance pressure fields are intensified with increase in Reynolds number and aspect ratio. Thus, radiated sound power increases with increase in Reynolds number and aspect ratio. Among various cases studied here, minimum and maximum values of radiated sound power are found at $Re=120$ & $AR=0.6$ and $Re=160$ & $AR=1.0$, respectively. Directivity patterns show that the generated sound fields are dominated by the lift dipole for all cases. Next, proper orthogonal decomposition $(POD)$ technique has been implemented for decomposing disturbance pressure fields. The $POD$ modes associated with the lift and the drag dipoles have been identified. $POD$ analyses also clearly display that the radiated sound fields are dominated by the lift dipole only. Further, acoustic and hydrodynamic modes obtained using Doak's decomposition method have confirmed the patterns of radiated sound field intensities.     

On the Production of Sound by Turbulent Boundary Layer Flow over a Compliant Coating

A theory is proposed of the generation of sound by turbulentboundary layer flow over a nominally plane, compliant wall coating.It is argued that, at sufficiently low mean flow Mach numbers,the principal noise production mechanism involves the scatteringof energy into sound from the hydrodynamic region of the wallpressure fluctuations by time dependent irregularities in thefree surface of the coating induced by the turbulent flow. Theeffective noise sources are equivalent to a distribution ofaerodynamic dipoles over the surface of the coating whose axesare parallel to the wall. The far field acoustic pressure spectrum,and the wavenumber-frequency wall pressure spectrum in the acousticdomain are expressed in terms of the behaviour of the wall pressurespectrum in the hydrodynamic domain. A simple empirical modelof the hydrodynamic region, defined in terms of measurable parameters,is used to obtain explicit representations of the acoustic spectra.     

Approach for Adaptive Sound Power Minimization of Elemental Sources in Theory and Experiments

Low frequency noise is a major issue in future aircraft with counter rotating open rotor engines. Passive noise treatments are bulky and heavy at these low frequencies. That is why active approaches are considered. The well known concept of destructive interference is used by state of the art active noise cancellation methods. The active systems are located closely to the noise source to cancel it. The microphones used for pressure minimization are in the near sound field of the noise source where active and reactive sound intensity are present. The pressure based concept is minimizing just the potential part of the acoustic energy. A new approach is proposed in this paper. It minimizes directly the net flow of acoustic energy that is transmitted into an interior. Simulation results showed that a proper estimation of acoustic power output is achieved by measuring sound intensity at a single point directly on the central axis of each secondary source. The innovative energy based active control approach (TASPM) has also successfully been validated in a laboratory environment. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Aerodynamic Sound Due to a Source Near a Half-Plane

The effect of applying a Kutta-Joukowski condition at the edgeof a semi-infinite plane which is generating noise in a turbulentfluid at low Mach numbers is examined. It is found that in somecircumstances the noise is increased and the intensity of thedistant sound field may depend upon the third power of a typicalfluid velocity. When the sound field is convected the orders of magnitude ofthe acoustic far field are the same whether or not the Kutta-Joukowskicondition is applied, provided that the point of observationis not near the wake. Near the wake there is an acoustic "surface"wave which is much stronger than the distant field elsewhere.     

Numerical analysis of the error in the sound power predicted through the Lighthill acoustic analogy

This article gives rigorous numerical analysis of the error in prediction of aeroacoustic noise via Lighthill analogy. The first fundamental and intractable problem considered herein is to predict the sound power generated by aerodynamic noise on surfaces. We give a full analysis of three methods of prediction. The second fundamental difficulty in the numerical analysis of aeroacoustics is the limited regularity of the underlying turbulent flow. This is treated herein by giving a negative norm error analysis which reduces the required regularity. We also give a comprehensive analysis of a fully discrete scheme including effects of the error coming into acoustic equation from the turbulent flow simulation. © 2010 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 28: 204–234,2012     

圆形垂直浮力射流的稳定性与混合特性研究

建立浅水静止环境中圆形轴对称垂直浮力射流的k-ε模型,采用混合有限分析方法进行了数值计算．针对两种不同的流动形态:近区的混合流体以浮力表面层的形式沿径向扩散的稳定排放;近区产生旋涡,浮力热水对混合热水形成二次挟带的非稳定排放,并对稳定性判据进行了验证,最后对两种不同流动结构下的远区的混合特性进行了数值模拟,结果同Lee和Jirka的试验和理论资料均十分吻合．     

Synchronous exploration for the control of real-time external noise suppression in a three-dimensional subdomain

A mathematical (difference) model is proposed for a real-time active shielding device that shields an acoustic field in a given subdomain from the influence of sound sources located in an additional subdomain. An algorithm for computing the current control ensuring a prescribed process is based on information produced by the author’s technique of synchronous weak noise exploration. This information can be measured in real time. Active control problems for nonstationary solutions of linear difference equations in a three-dimensional domain consisting of two subdomains are studied using the difference potential method. The shape of the domain and the boundary conditions may depend on time, while the coefficients may depend on time and spatial coordinates. If the difference problem is a mathematical model of sound propagation, the goal of control is to change the acoustic field in the given subdomains, for example, to shield the acoustic field in one subdomain from the undesirable influence (noise) of sources located in the other subdomain.     

Multiscale Considerations for Sound Generation in Low Mach Number Flows

Classical approaches in aeroacoustics are mainly based on analytical solutions of the linear wave equations which are valid in the far field. The sound generation is approximated by source terms obtained from a flow simulation. This procedure designated as the ‘acoustic analogy’ was initiated in the classical work of Lighthill. In order to tackle the sound generation problem at low Mach numbers, we consider a multiple scale asymptotic analysis. As we deal with a fluid flow generating the sound itself, the asymptotic ansatz uses one time scale given by the flow convection, but two space scales due to the difference in fluid and sound velocity. The insight given by this analysis is used to obtain source terms describing the sound generation and perturbation equations for the sound propagation. Numerical results are shown for the example of a co‐rotating vortex pair.     

On a Synthetic Jet Flow

Velocity vector field of a synthetic jet has been investigated experimentally. The synthetic jet is generated by an electrodynamical system designed for control of a free shear layer. To catch unsteady periodical structures the phase average method is applied to time series of instantaneous velocities. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Aerodynamic noise from wave-turbulence interaction

In order to estimate the acoustic energy scattered when a unit volume of free turbulence, such as in free jets, interacts with a plane steady sound wave, theoretical expressions are derived for two simple models of turbulence: eddy model and isotropic model. The effect of convection by mean motion of the energy-bearing eddies on the incident sound wave and on the sound generated from wave-turbulence interaction is taken into account. Finally, by means of a representative calculation, the directionality pattern and Mach number dependence of the noise so generated is discussed.     

Hybrid computational aeroacoustics based on compressible flow data at low Mach numbers

In some practical applications (e.g. cavity with a lip), even at low Mach numbers, acoustic feedback mechanisms excite flow structures. The compressible flow simulation cannot distinguish between a pure fluid dynamic part and acoustic phenomena. With this in mind, we propose a workflow based on Helmholtz-Hodge decomposition, to extract pure source terms of the compressible flow simulation, to model the sound radiation. (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Active sound control in composite regions

In active sound control, noise shielding of a target region is achieved via additional sources (called controls) situated at the perimeter of the region. The sources protect the target region by adjusting the acoustic field near the boundary of the region. In the present paper a numerical model of active sound control based on surface potentials in 3D bounded composite regions is numerically studied. In the composite region setup, it is required that the regions be shielded from noise while allowing admissible sound that is generated in the shielded regions to be preserved. The admissible sound is usually required to propagate freely inside the protected regions or in a (selective) predetermined pattern. The adjusting approach used here does not require any knowledge of the sound sources or the properties of the propagation medium in order to obtain the controls. Moreover, the approach differs sharply from some other approaches where the detailed knowledge of the sound sources and the propagation medium is required. For the first time, numerical test cases involving both free communication and predetermined communication pattern between the regions in three dimensions are considered. In all test cases, these regions are effectively shielded from the noise while any present admissible sound is preserved. In addition, selective propagation of the admissible sound between the regions is enforced. The effect of the number of controls on their operation is also studied. Whether admissible sound is present or not, the level of noise cancellation decreases linearly as fewer controls are used. In addition to the increase in size of the interference zone, the controls become individually distinguishable.     

Numerical Prediction of Aerodynamic Noise Generated from an Aircraft in Low Mach Number Flight

The paper describes numerical prediction of aerodynamic noise generated from an Aircraft. Simulation of turbulent flow is done solving the incompressible Navier-Stokes equation, where turbulence is modeled applying the orthogonal subgrid scale (OSGS) method with dynamical subscales. Because of comparison, the same simulation is done using the LES (Large Eddy simulation). It is shown how simulation of turbulent flow affects the prediction of acoustic sources calculated using Lighthill's analogy. Translation from the time to frequency domain is done through DFT (Direct Fourier Transform), which gives smaller usage of memory. Acoustic sources are used in the inhomogeneous Helmholtz equation to simulate pressure wave propagation in the domain. It is shown that OSGS with dynamical subscales gives better representation of the spectrum. Overall, better prediction of energy transfer across large and small eddies will give better allocation and presentation of acoustics sources. These sources will change wave propagation of the pressure in the acoustic field. (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stable methods for an inverse problem in acoustic scattering by an obstacle and an inhomogeneous medium

We consider (in two-dimensional Euclidean space) the scattering of a plane, time-harmonic acoustic wave by an inhomogeneous medium Ω with compact support and a bounded obstacle D lying completely outside of the inhomogeneous medium. We show that one may determine the shape of D and the local speed of sound in Ω from a knowledge of the asymptotic behavior of the scattered wave (i.e. the far field). This is done by considering a constrained optimization problem and employing integral equation and conformal mapping techniques. By assuming a priori that the functions which determine the shape of D and the local speed of sound in Ω lie in given compact sets, we show that the problem is stable, in the sense that the solution of the inverse scattering problem depends continuously on the far field data.     

Modelling of coupled vibro-acoustic interactions in an active casing for the purpose of control

The aim of active casing is to reduce device noise generated to the acoustic environment by controlling vibrations of the walls of the device casing. The article concerns theoretical modelling of an active casing composed of six walls mounted to a rigid frame. The casing is considered as a complex coupled vibro-acoustic system, where the walls are excited by the acoustic field inside the casing, and by actuators bonded to the walls and used for active control. The acoustic field inside the casing is, in turn, formed by the noise originating from the device itself, and the secondary sound generated by all vibrating walls. The structure of such system is described in detail and, after being represented as interconnected subsystems, its state-space model is developed. The model includes fluid loading effect and imperfect fastening of wall edges to the frame. The system of coupled partial differential equations, subject to defined boundary conditions, is reformulated as an abstract first order evolution equation in an appropriate infinite dimensional function space. All assumptions and the structure of the model are chosen to make it useful for the purpose of control. In particular, the model can be used for analysis of controllability and observability of the whole casing as a vibro-acoustic system and, based on that, for optimisation of placement of actuators and structural sensors to efficiently reduce noise over required frequency band. On the basis of the obtained state equation, a block diagram of the system is constructed. It shows the structure of the whole system and the nature of interactions occurring between distinguished subsystems.     

Orbital stability of solitary waves on a ferrofluid jet

We prove the orbital stability of small-amplitude axisymmetric solitary waves on the surface of an incompressible, inviscid ferrofluid jet. The ferrofluid surrounds a current-carrying rod and is subject to the azimuthal magnetic field generated by the rod. We show that under appropriate assumptions on the magnitude of the magnetic intensity in the ferrofluid, both the trivial flow and the solitary waves with strong surface tension are conditionally orbitally stable, while the conditional orbital stability of solitary waves with near-critical surface tension can be deduced from properties of the corresponding dispersive PDE model equation. The arguments are based on the recent orbital stability results for internal waves by Chen and Walsh (2022) and an improved version of the Grillakis–Shatah–Strauss method introduced by Varholm et al. (2020).     

Effect of expiratory flow rate on the acoustic characteristics of sibilant /s/

Sibilant /s/, which is an unvoiced sound, is believed to be produced by a jet of air through a sibilant groove toward the sharp edges of the teeth. We investigated what effects the expiratory flow rate had on the acoustic characteristics of sibilant /s/ in the present study. A large eddy simulation was implemented to investigate the flow in an oral cavity model that simplified the important anatomical features of the oral cavity when sibilant /s/ was pronounced. The results illustrated that an increase in the expiratory flow rate caused turbulence to develop in the separated flow from the edge of the obstacle wall which mimics a tooth. The increase in turbulent intensity induced a dramatic elevation in Lamb-vector divergence in the flow-separation region, which was regarded as a sound source of sibilant /s/. The total power of the sound source which was located near the obstacle edge of the obstacle wall at smaller flow rates, extended downstream in the flow separation as the expiratory flow rate increased through which turbulence developed. These results demonstrated that the expiratory flow rate had a significant influence on the acoustic characteristics of sibilant /s/ particularly when the flow rate increased to cause turbulence.