An experimental investigation of interaction of sprays with acoustic fields

This paper presents the effect of axial acoustic fields on an air-atomized spray. In these experiments, a spray of ethanol is established in a transparent test section in which an acoustic field is set up using acoustic drivers. Visual observation shows that a high-amplitude acoustic field (160 dB) reduces the length of the spray. The spray velocity field was characterized by particle image velocimetry (PIV) measurements. The spray velocities are considerably reduced in the presence of the acoustic oscillations, indicating the presence of smaller droplets. The presence of acoustic oscillations increases the spray cone angle. The flow field in the near field of the spray was characterized. There is considerable entrainment of air into the spray in the presence of acoustic oscillations. It is seen that the acoustic velocity rather than the acoustic pressure primarily affects the spray.     

Acoustic streaming measurements in standing wave resonator using Particle Image Velocimetry

This paper experimentally investigates the measurement of acoustic streaming in a 7 m long-standing wave air-filled acoustic resonator. One can describe the acoustic streaming as a second-order steady flow, which is superimposed on the dominant acoustic velocity. It is induced by the nonlinearities of the acoustic propagation inside the resonator. The exploration of the acoustic velocity field by the synchronized PIV (stands for Particle Image Velocimetry) technique enabled to highlight and quantify these secondary flows. The PIV measurements of the acoustic velocity fields at different phases over the excitation signal period gave information on streaming profiles and the post processing applied allowed plotting the acoustic velocity over time. These results were compared to the outcome of a 2D numerical study performed with the commercial software Fluent, where good agreements were found. It indicates the ability of this method to accurately measure second order steady flow variations of the acoustic velocity field.     

考虑声辐射特性的结构优化设计

传统的结构动力优化设计没有考虑声辐射特性,而随着人们对环境舒适度要求的提高,降低声辐射水平成为人们越来越关心的问题。本文首先给出了用有限元方法计算结构动力响应,边界元法计算结构声辐射特性的方程,在此基础上重点建立了结构声辐射优化设计模型,同时用序列二次规划算法进行了优化求解。在JIFEX软件中实现了上述理论和算法,并通过优化设计的数值算例,进一步说明了本文的研究方法能够有效的降低结构声辐射水平。     

Linearized and non‐linear acoustic/viscous splitting techniques for low Mach number flows

Computation of the acoustic disturbances generated by unsteady low‐speed flow fields including vortices and shear layers is considered. The equations governing the generation and propagation of acoustic fluctuations are derived from a two‐step acoustic/viscous splitting technique. An optimized high order dispersion–relation–preserving scheme is used for the solution of the acoustic field. The acoustic field generated by a corotating vortex pair is obtained using the above technique. The computed sound field is compared with the existing analytic solution. Results are in good agreement with the analytic solution except near the centre of the vortices where the acoustic pressure becomes singular. The governing equations for acoustic fluctuations are then linearized and solved for the same model problem. The difference between non‐linear and linearized solutions falls below the numerical error of the simulation. However, a considerable saving in CPU time usage is achieved in solving the linearized equations. The results indicate that the linearized acoustic/viscous splitting technique for the simulation of acoustic fluctuations generation and propagation by low Mach number flow fields seems to be very promising for three‐dimensional problems involving complex geometries. Copyright © 2003 John Wiley & Sons, Ltd.     

Model for the Formation of Acoustic Self-Oscillations in a Chamber with a Jet Flowing Through its Nozzle

This paper describes free acoustic oscillations of gas in a chamber with a jet flowing through its nozzle in the case of nonstationary intensity component of vortex sheet flowing down from the edge of the nozzle. There is established feedback between acoustic oscillations and oscillations induced by a corresponding vortex sheet component. It is shown that, in the presence of given feedback, there could be instability of acoustic oscillations, which would result in acoustic self-oscillations in the chamber. The boundaries of the domain in which instability is formed are determined by developing a mathematical model of stable acoustic oscillations in the chamber with account for the influence of the vortex sheet.     

离心叶轮气动声场的数值计算与分析

采用SIMPLEC算法对Ghost叶轮的三维非定常流场进行了数值模拟。利用计算所得流场结果并结合Lighthill和Lowson声学方程计算了由叶片表面非定常脉动力产生的气动噪声。计算结果表明:气动噪声的峰值主要集中在基频及其谐波附近;与静止的点声源相比,运动的点声源不仅使声场存在明显的多普勒效应,还会使声场的强度产生较大的变化;但对转速恒定的旋转点声源,加速度的变化对声场的影响可以忽略;从声场的分布来看,整个旋转叶轮可以看成是一个按简谐变化的偶极子源,数值计算结果与理论分析的结果吻合良好。     

厚度振动模式下背衬结构PZT的声能特性

为了充分利用埋入混凝土中压电陶瓷所激励的声能,采用了一种在压电陶瓷圆片结构的一面设置金属背衬的方法将压电陶瓷双侧的声能集中到一侧,提高了声指向性和声能。实验表明:该方法能使同距离下压电陶瓷圆片轴向声能提高10倍以上,并且得到了声能的相对增量随背衬厚度和声激励频率之积变化的特性曲线。声信号的谱分析还表明:背衬没有改变声信号的主频和主峰区间的频率带宽;当声激励频率为谐振频率79kHz时,最优背衬厚度为2.4mm;当声激励频率f为60kHz～100kHz时,接收端声能相对较大,适合用于实际的压电埋入式超声无损检测。     

Numerical simulation of flow‐induced noise using LES/SAS and Lighthill's acoustic analogy

We present a finite element (FE) formulation of Lighthill's acoustic analogy for the hybrid computation of noise generated by turbulent flows. In the present approach, the flow field is computed using large eddy simulation and scale adaptive simulation turbulence models. The acoustic propagation is obtained by solving the variational formulation of Lighthill's acoustic analogy with the FE method. In order to preserve the acoustic energy, we compute the inhomogeneous part of Lighthill's wave equation by applying the FE formulation on the fine flow grid. The resulting acoustic nodal loads are then conservatively interpolated to the coarser acoustic grid. Subsequently, the radiated acoustic field can be solved in both time and frequency domains. In the latter case, an enhanced perfectly matched layer technique is employed, allowing one to truncate the computational domain in the acoustic near field, without compromising the numerical solution. Our hybrid approach is validated by comparing the numerical results of the acoustic field induced by a corotating vortex pair with the corresponding analytical solution. To demonstrate the applicability of our scheme, we present full 3D numerical results for the computed acoustic field generated by the turbulent flow around square cylinder geometries. The sound pressure levels obtained compare well with measured values. Copyright © 2009 John Wiley & Sons, Ltd.     

Prediction of vortex shedding from bluff bodies in the presence of a sound field

The separated flow around a rectangular cylinder, in the presence of a transverse duct resonant acoustic mode, is modelled using a vortex method. The instantaneous transfer of power between the mean flow and the acoustic field is predicted using Howe's theory of aerodynamic sound. Whether the net acoustic energy per cycle generated is positive or negative depends on the phase of the acoustic cycle at which vortex clouds arrive at the trailing edge of the cylinder.     

Acoustic Emission Analysis of Nanoindentation-Induced Fracture Events

Monitoring of acoustic emission activity is employed to characterize the initiation and progression of local failure processes during nanoindentation-induced fracture. Specimens of various brittle materials are loaded with a cube-corner indenter and acoustic emission activity is monitored during the entire loading and unloading event using a transducer mounted inside the specimen holder. As observed from the nanoindentation and acoustic emission response, there are fundamental differences in the fracture behavior of the various materials. Post-failure observations are used to identify particular features in the acoustic emission signal that correspond to specific types of fracture events. Furthermore, analysis of the parametric and transient acoustic emission data is used to establish the crack-initiation threshold, crack-arrest threshold, and energy dissipation during failure. It is demonstrated that the monitoring of acoustic emission signals yields both qualitative and quantitative information regarding highly localized failure events in brittle materials.     

高亚声速湍流喷流气动噪声数值分析

为适应航空噪声管制规定要求,发动机喷流噪声控制成为目前气动声学研究中的重要课题,预测分析喷流噪声辐射并揭示其产生机理将为噪声控制奠定基础.采用高精度并行LES（large eddy simulation）方法计算分析马赫数0.9高亚声速喷流的湍流演化和气动噪声现象.首先,仔细验证喷流LES湍流场计算保真性,并分析流场中不同尺度涡结构的演化形态.其次,利用可穿透面FW-H（Ffowcs Williams and Hawkings）方法外推喷流近场声源数据获得精确声辐射远场,进而分析声场主导声模态特性.最后,通过分析声源机制、分离声模态等方法研究势流核末端大尺度拟序涡运动演化形成的低波数波包在噪声主导声模态产生中的重要作用.数值结果表明LES结合可穿透面FW-H方法可精确预测高亚声速喷流的流场及声场特征,且数值分析揭示涡环对并形成的大尺度拟序结构在喷流中心线上沿径向融合,产生了在远场低方位角占优的主导声模态,并构成强指向性声场,噪声峰值方位角约为30°.      

Acoustomechanical constitutive theory for soft materials

Acoustic wave propagation from surrounding medium into a soft material can generate acoustic radiation stress due to acoustic momentum transfer inside the medium and material, as well as at the interface between the two. To analyze acoustic-induced deformation of soft materials, we establish an acoustomechanical constitutive theory by com-bining the acoustic radiation stress theory and the nonlinear elasticity theory for soft materials. The acoustic radiation stress tensor is formulated by time averaging the momen-tum equation of particle motion, which is then introduced into the nonlinear elasticity constitutive relation to construct the acoustomechanical constitutive theory for soft materials. Considering a specified case of soft material sheet subjected to two counter-propagating acoustic waves, we demonstrate the nonlinear large deformation of the soft material and ana-lyze the interaction between acoustic waves and material deformation under the conditions of total reflection, acoustic transparency, and acoustic mismatch.     

Q235钢细观损伤声发射源动态观测实验研究

金属材料细观损伤过程伴随有瞬态应力波的释放,利用声发射检测技术可采集到应力波的信息,但由于缺少直观可视的手段,利用声发射信息定量评价金属材料的细观损伤存在困难。本文将SEMtester1000原位拉伸试验机、MZ1000正置显微镜和PCI-2型声发射检测系统相结合,搭建了声发射源动态观测实验系统。以Q235钢可视化原位拉伸实验为例,获取了晶体的滑移和夹杂物的断裂两种典型声发射源图像及对应的声发射信号。通过对不同声发射源信号的分析可知,滑移是一个持续的过程,复杂的滑移活动会产生多类型的声发射信号。夹杂物断裂是瞬态过程,产生典型的突发型声发射信号,能量较滑移信号更高。依据声发射源动态观察结果及图像分析,建立了基于声发射机制的Q235钢细观损伤模型。     

Large Eddy Simulation of the sound field of a round turbulent jet

In this paper we will use Large Eddy Simulation (LES) to obtain the flow field of a turbulent round jet at a Reynolds number based on the jet orifice velocity of 11000. In the simulations it is assumed that the flow field is incompressible. The acoustic field of the jet is calculated with help of the Lighthill acoustic analogy. The coupling between the flow solver and the acoustic solver is discussed in detail. The Mach number used in the acoustic calculation was equal to 0.6. It is shown that the decay of the jet centerline velocity and centerline rms are in good agreement with experimental data of [12]. Furthermore, it is shown that the influence of the LES modeling on the acoustic field is very small, if the dynamic subgrid model is used.     

A theoretical and experimental study on flow characterisation in an acoustically excited chamber

Flame–acoustic wave interactions have been studied widely in the combustion community; however the whole physicochemical mechanism is still not clear. The present research aims to analyse the acoustic model inside an enclosed combustion chamber and to gain more detailed data to further study flame/acoustic interactions theoretically. The acoustic coupling can be calculated by using linear acoustic equations. The theoretical acoustic model has been developed to analyse the acoustic response for the present square tube and acoustical system. A good agreement between experimental measurements and theoretical predictions were proved by measuring the pressure and velocity fields of acoustic wave. The first four measured harmonic frequencies match well with theoretical prediction. The measured acoustic pressure node and anti-node region were also the same as theoretical prediction. However, due to the neglect of pressure drop loss in the upper end of the tube, the theoretical prediction data was from 23% to 43.3%, higher than that of the experimental measurements.     

Sources of acoustic resonance generated by flow around a long rectangular plate in a duct

This paper describes a numerical investigation of the acoustic resonance occurring at specific flow speeds when a long two-dimensional rectangular plate is placed on the centre-line of a duct. The flow Mach number is sufficiently small that the flow and acoustic fields can be modelled separately; however, the effect of the acoustic field in modifying the flow field is accounted for and, in turn, the flow field solution determines the time-dependent source distribution for the acoustic model. This allows the range of flow speeds, or equivalently the associated Strouhal numbers, where resonance is possible to be predicted. It is shown that the Strouhal number based on plate chord (or length) displays a stepping behaviour as the plate length is increased. The main source or sink region where energy is transferred between the acoustic and flow fields is shown to be immediately downstream of the trailing edge of the plate. Visualizations of the numerical solution show that the timing when the vortices enter this region relative to the phase of the acoustic cycle is crucial in determining if resonance can occur and is the cause of the observed stepwise increase. Comparison is made with previous physical experiments.     

Dislocation dynamics and acoustic emission during plastic deformation of crystals

A soliton approach to acoustic emission during plastic deformation of crystals is presented. The approach is based on a microscopic Frenkel-Kontorova model where the rigidity of the substrate is removed in order to establish the interaction mechanism between a dislocation and both longitudinal and transverse acoustic waves. It is shown that this interaction is described by a sine-Gordon-d' Alembert system. Within the framework of this system, two basic mechanisms of acoustic emission are investigated both analytically and numerically. One mechanism is related to nonstationary dislocation motion and the other one to the annihilation of dislocation kink-antikink pairs during Frank-Read source operation. In both cases, computer simulations are obtained which illustrate graphically the analytical considerations and model the acoustic radiation. The obtained results are in agreement with existing experimental data and may provide a better physical insight to the acoustic emission mechanisms during plastic deformation of crystals.     

Model study of noise field in the human chest due to turbulent flow in a larger blood vessel

An acoustic generation of noise by a larger human blood vessel and noise transmission in the thorax is modelled and studied. In making this, the random statistical nature of the noise sources, the basic vessel properties and the main features of the human chest structure are taken into account. Also the effects of changes in the basic vessel parameters are considered as a first approach to study acoustic effects of a vascular stenosis. The analysis of the resultant acoustic field reveals its similarity to the acoustic fields recorded in the appropriate experiments. The variations in the basic vessel parameters are found to cause the corresponding changes in the sound level and the production of new frequency components in the acoustic power spectrum. The acoustic power from a slightly thickened vessel is shown to be approximately proportional to the fourth power of the flow Reynolds number in the originally normal vessel and the eighth power of the ratio of the vessel diameters.     

Measurement of the acoustic velocity field of nonlinear standing waves using the synchronized PIV technique

The motion of gas within an air-filled rigid-walled square channel subjected to acoustic standing waves is experimentally investigated. The synchronized particle image velocimetry (PIV) technique has been used to measure the acoustic velocity fields at different phases over the excitation signal period. The acoustic velocity measurements have been conducted for two different acoustic intensities in the quasi-nonlinear range (in which the nonlinear effects can be neglected in comparison with the dissipation effects), and one acoustic intensity in the finite-amplitude nonlinear range (in which both the nonlinear term and the dissipative term play a role in the wave equation). The experimental velocity fields for the quasi-nonlinear cases are compared with the analytical results obtained from the time-harmonic solution of the wave equation. Good agreement between the experimental and analytical velocity fields proves the ability of the synchronized PIV technique to accurately measure both temporal and spatial variations of the acoustic velocity fields. The verified technique is then used to measure the acoustic velocity fields of the finite-amplitude nonlinear case at different phases.     

Design and analysis of nonlinear-transformation-based broadband cloaking for acoustic wave propagation

A coordinate-transformation method can be used to design invisibility cloaks for many types of waves, including acoustic waves. The traditional method for designing a cloak depends on a transformation from a virtual space to a physical space. Previous acoustic cloaks that are mainly designed with linear-transformation-based acoustics have drawbacks that acoustic wave trajectories in the cloaks cannot be controlled and tuned. This work uses a nonlinear mapping from a ray trajectory perspective to construct acoustic cloaks with tunable non-singular material properties. Use of a ray trajectory equation is a straightforward and alternate way to study propagation characteristics of different types of waves, which allows more flexibility in controlling the waves. A broadband cylindrical cloak for acoustic waves in an inviscid fluid is realized with layered non-singular, homogeneous, and isotropic materials based on a nonlinear transformation. Some advantages and improvements of the invisibility nonlinear-transformation cloak over a traditional linear-transformation cloak are analyzed. The invisibility capability of the nonlinear-transformation cloak can be tuned by adjusting a design parameter that is shown to have influence on the acoustic wave energy flowing into the region inside the cloak. Numerical examples show that the nonlinear-transformation cloak is more effective for making a domain undetectable by acoustic waves in an inviscid fluid and shielding acoustic waves from outside the cloak than the linear-transformation cloak in a broad frequency range. The methodology developed here can be used to design nonlinear-transformation cloaks for other types of waves.