Two regimes of blast wave propagation in a bubbly liquid

The evolution of a spherically symmetric layer in a two-phase bubbly liquid under the action of a blast wave is numerically studied. It is demonstrated that such a medium can feature two characteristic regimes of blast wave propagation.     

The emission of sound by statistically homogeneous bubble layers

This paper is concerned with the flow of a bubbly fluid along a wavy wall, which is one Fourier component of a linearized hydrofoil. The bubbles are dispersed, not throughout the whole of the liquid, but only over a certain distance from the wall, as occurs in practice with cavitation bubbles. Outside the bubbly regime there is pure liquid.The interface between the bubbly fluid and pure liquid fluctuates for various reasons. One of these is the relative motion between bubbles and liquid. This is considered here in detail. A calculation is made of the sound emitted by the bubbly layer into pure liquid as a result of this stochastic motion of the interface.     

基于能量重心法的列车轴承多普勒畸变故障声信号校正诊断研究

由麦克风采集的运动声源信号存在多普勒畸变现象,这极大的增加了对信号分析的难度,针对这一问题,本文提出了一种基于能量重心法的多普勒畸变校正方法,并将其应用在列车轴承故障诊断中。首先由基于能量重心法的瞬时频率（IF）估计（IFE）来获取IF向量,在莫尔斯声学理论的基础上,对IF向量进行非线性拟合,进而得到在时域进行重采样的时间间隔序列,再对原信号进行重采样处理,即可实现多普勒畸变的消除。最后本文做了一个仿真和列车轴承内外圈故障声信号的实验,分析的结果验证了此种方法的有效性。     

Stochastic resonance in metal superlattices in the presence of a transverse noise

The stochastic resonance in a metal multilayer system, which is excited by a magnetic field containing a longitudinal harmonic signal and a transverse component oriented in the plane of the structure, has been studied by numerical analysis of the dynamics of the system of coupled magnetic moments. It is established that the resonance can be controlled by changing the amplitude and frequency of the harmonic signal.     

基于固液耦合的舰炮身管模态分析

基于模态分析理论和结构耦合动力学中的固液耦合理论,考虑液体冷却层和内、外管表面之间的耦合效应,采用直接耦合法建立了含液体层的舰炮身管固液耦合计算模型,利用ANSYS软件对该模型进行了模态分析．分析结果表明,含有液体冷却层的舰炮身管固有频率降低,刚度提高,改善了火炮射击时身管的动态特性．     

Novel multi-layer polymer-metal structures for use in ultrasonic transducer impedance matching and backing absorber applications

This paper presents a novel design principle for designing multilayer polymer-metal structures which are well suited for front surface impedance conversion (matching) and for back surface acoustic absorption. It is shown that a polymer layer with an outer metal layer, when loaded by a low impedance propagation medium, acts as an efficient impedance converter. The resulting impedance seen at the inner polymer surface is increased and the structure provides the same performance as a traditional quarter-wavelength matching layer. Experimental evidence is also shown for a double-matching scheme for a lead zirconate titanate (PZT) transducer using an inner polymer-metal multilayer and an outer polymer quarterwavelength layer, resulting in a 55% bandwidth at 2.6 MHz with air backing. Also, it is theoretically shown that multiple layers of a lossy polymer adhesive-metal structure produce low propagation velocity and high absorption. Experimental proof of this ultrasonic multilayer backing absorber is provided. Design theories based on both a simplified mass and spring model and a rigorous one-dimensional wave model have been developed and show fair agreement.     

On the existence of stationary solitary waves in a dissipative bubbly liquid

The formation of a solitary wave during the propagation of acoustic perturbations in a liquid containing heat-conducting gas bubbles is revealed for the first time by means of numerical modeling. A physical mechanism explaining the existence of a nondecaying solitary wave in this dissipative medium is proposed, which is based on the reverse influence of the bubble motion dynamics on the heat balance between gas and liquid. This phenomenon is called the moving boundary effect.     

超声导波用于液层负载板损伤检测的数值模拟

液层负载薄板结构的损伤检测是结构无损检测和健康监测中的一个重要问题。为寻求合适的液层负载薄板结构损伤检测信号及分析损伤缺陷对检测信号的响应,求解了双侧液层负载薄板的频散方程,计算得到4 mm双侧水域负载铝板的频散曲线,并利用有限元模拟方法研究了缺陷位置、角度及大小对铝板中检测信号的影响。研究结果表明:中心频率小于100 kHz的S0模式漏Lamb波衰减因子趋近于0,适合长距离损伤检测。此外,当缺陷的存在造成板结构的不对称性时,声信号在缺陷处发生明显的模式转换,且转换出的A0模式信号的透射系数随缺陷位置到板厚中心距离的增加而减小,随缺陷与铝板中间面的角度和缺陷长度的增加先增大后减小,并随缺陷宽度的增加而增大。     

Mechanical properties of single cells by high-frequency time-resolved acoustic microscopy

In this paper, we describe a new, high-frequency, time-resolved scanning acoustic microscope developed for studying dynamical processes in biological cells. The new acoustic microscope operates in a time-resolved mode. The center frequency is 0.86 GHz, and the pulse duration is 5 ns. With such a short pulse, layers thicker than 3 microm can be resolved. For a cell thicker than 3 microm, the front echo and the echo from the substrate can be distinguished in the signal. Positions of the first and second pulses are used to determine the local impedance of the cell modeled as a thin liquid layer that has spatial variations in its elastic properties. The low signal-to-noise ratio in the acoustical images is increased for image generation by averaging the detected radio frequency signal over 10 measurements at each scanning point. In conducting quantitative measurements of the acoustic parameters of cells, the signal can be averaged over 2000 measurements. This approach enables us to measure acoustical properties of a single HeLa cell in vivo and to derive elastic parameters of subcellular structures. The value of the sound velocity inside the cell (1534.5 +/- 33.6 m/s) appears to be only slightly higher than that of the cell medium (1501 m/s).     

高静水压下障板样品声特性的计算与测量

为研制一种在声呐水下声系统中起到屏蔽噪声、提高声基阵空间增益作用的耐压、低频、宽带声障板,设计了声管样品,根据分层介质中的声传播理论,利用多层均匀结构的传递矩阵法计算了它在空气和水介质背衬下的声压反射系数和透射系数,在不同的背衬情况下具有明显不同的声学特性。制备了直径206mm的样品,利用杭州应用声学研究所水声驻波管和行波管测量系统测量了频率范围100Hz～4kHz、静水压范围从常压到2MPa的声学性能,测量结果和理论计算值基本吻合,声障板样品在静水压状态下具有一定的低频、宽带反声特性。     

柱坐标系下浅海传播声场时域有限差分预报方法

直角坐标系下的时域有限差分方法在水声传播仿真时由于计算量太大而较少应用。文章利用时域有限差分方法推导了柱坐标系下的声波方程的近似表达式,结合复频移完全匹配层建立浅海声传播模型,在减少计算量的同时准确预报了传播信号声压的时域波形、传播声场的时空演变过程以及频域传播损失曲线。在柱坐标系下使用时域有限差分方法仿真Pekeris传播环境声场并与简正波和波数积分模型进行对比,分析了时域有限差分水声传播模型的适用范围。结果表明,时域有限差分方法仿真浅海中近程传播声场精度较高。模型的稳定性与时间和空间网格大小有关,声源频率越高,空间和时间网格划分越小,计算量越大。数值离散带来的频散误差累积会导致远场传播声场计算不准确,因此时域有限差分水声传播模型更适用于低频中近程声场计算。     

Ultrasound transducer modeling-received voltage signals and the use of half-wavelength window layers with acoustic coupling layers

A general set of modeling equations for lossless one-dimensional multilayer ultrasound transducers is presented based on first principles. In particular, a direct relationship between ultrasound transducer results and the underlying physical principles of electroacoustics is given. As such, the model may provide better physical understanding for designers not fully versed in electrical circuits theory or in linear system analyses. The model is suitable for time-domain analysis and monofrequency design. Special attention is given to the determination of the time-dependent voltage across the receiver electrodes subject to a general voltage input, but information on any (dynamic) variable of interest is provided. The basic equations governing the dynamics of the multilayer structure acting as transmitter as well as receiver are solved by Fourier analysis and by imposing continuity of velocity and pressure between layers. Sound transmission between the two piezoelectric circuits is assumed to take place in a water bath such that the Rayleigh equation can be used to obtain the incoming pressure at the receiver aperture from the acceleration of the opposing transmitter aperture. Comparison with experimental results is possible by allowing coupling to external electric impedances. A numerical test case using a multilayered 1-MHz transducer for flow meter applications was considered and good agreement with experiments was obtained in terms of voltage signals. The transducer contains a half-wavelength stainless steel layer needed to resist corrosion, the ability to operate at temperatures in a wide range from 20 to 150 degrees Celsius, resistance to impact from flowing particles in the medium, high pressure or vacuum, and pH values up to 10 in some locations. The influence of epoxy glue and grease acoustic coupling layers-between the piezoceramics and the stainless steel layer-in the range from 1 to 70 mum was examined. It was shown that, for the same layer thickness, epoxy glue is preferred as compared with grease, both in terms of signal shapes and amplitudes. Finally, inclusion of appropriate electric impedances in the transmitter and receiver circuits is found to affect signal pulses strongly.     

Node dynamic relaxation method: principle and application

Two main methods, inactive element method and quiet element method, to simulate the process of multilayer and multipass welding were reviewed, and the shortcomings of both methods were discussed as well. Based on these analyses, a method called node dynamic relaxation method was put into forward to simulate the multilayer and multipass welding process, and the principle and application of this method were discussed in detail. The simulating results show that using the node dynamic relaxation method can decrease mesh distortion, improve calculation efficiency, and obtain good simulation results. This method can also be used in the field of simulation addition or removing materials in finite element analysis.     

Characterization of a grating-coupled liquid crystal cell using a rigorous theoretical model

Abstract

We use a rigorous differential formalism to model the optical response of a multilayer structure having both a surface-relief grating and containing uniaxial materials. The uniaxial material is, in this case, a layer of liquid crystal that has its uniaxial axis defined by its director. By fitting experimental angle dependent reflectivity data to a multilayer grating model we are able to determine the spatial profile of the liquid crystal director, and show how accurately the optical response of such a system may be modelled.     

Anomalies of the boundary reflection of ultrasound from a Maxwellian liquid

Reflection of continuous and pulsed longitudinal and transverse acoustic waves from a dissipative medium represented by a model of Maxwellian liquid in contact with a solid halfspace is considered theoretically. The substantial dependence of the modulus and phase of the reflection coefficient on both the viscosity and time of relaxation of stresses in the Maxwellian liquid is shown. Using computer programs, the acoustic pulsed signals reflected from the interface between the media and transmitted through it have been calculated. The calculations were performed for an asymmetric shape of the signal incident on the interface; this shape corresponds to that of a real signal emitted by an ultrasonic piezoceramic transducer.     

面向跨水空介质通信的雷达水表面声波提取

由于水空介质的非连续性,空中电磁波和水下声波都无法跨越该介质边界,所以无中继器跨水空介质通信一直是研究中的难点。水下声波会引起水空边界的微小振动即水表面声波(Water Surface Acoustic Wave,WSAW),而毫米波雷达能够实现对微动信号的高精度探测,因此文章提出了基于级联成像调频连续波(Frequency Modulated Continuous Wave,FMCW)雷达提取WSAW信号的方法。首先研究了WSAW的基本特征及FMCW雷达微动信号的提取原理;其次采用线性调频的水下声信号并结合匹配滤波技术来提高输出信噪比,针对大幅度自然扰动,提出了一种多角度积累结合阈值相位补偿的信号增强算法;最后在不同波高的扰动下进行实验,结果表明在1.5 cm的自然波动干扰下仍能提取WSAW信号,成像FMCW雷达偏过一定角度也能有效地进行通信,验证了文中所提方法的有效性。     

Investigation on Effect of Gravity Level on Bubble Distribution and Liquid Turbulence Modification for Horizontal Channel Bubbly Flow

Bubbly flows in the horizontal channel or pipe are often seen in industrial engineering fields, so it is very necessary to fully understand hydrodynamics of horizontal bubbly flows so as to improve industrial efficiency and to design an efficient bubbly system. In this paper, in order to fully understand mechanisms of phase distribution and liquid–phase turbulence modulation in the horizontal channel bubbly flow, the influence of gravity level on both of them were investigated in detail with the developed Euler–Lagrange two–way coupling method. For the present investigation, the buoyance on bubbles in both sides of the channel always points to the corresponding wall in order to study the liquid–phase turbulence modulation by bubbles under the symmetric physical condition. The present investigation shows that the gravity level has the important influence on the wall–normal distribution of bubbles and the liquid–phase turbulence modulation; the higher the gravity level is, the more bubbles can overcome the wall–normal resistance to accumulate near the wall, and the more obvious the liquid–phase turbulence modulation is. It is also discovered that interphase forces on the bubbles are various along the wall–normal direction, which leads to the fact that the bubble located in different wall–normal places has a different wall–normal velocity.     

Aperture size effect on ultrasonic wavefront distortion correction

The influences of aperture size on wavefront distortion correction are investigated both theoretically and numerically. A multilayer, phase-screen model is assumed to be the underlying, distorting medium. Numerical simulations were performed using three wavefront distortion correction methods: time-shift compensation (TSC), backpropagation followed by time-shift compensation (BP+TSC), and the previously proposed, multilayer, phase-screen compensation (MPSC) method. The distorted wavefronts were generated by propagating a planar wavefront through a multilayer, phase-screen model constructed with a two-dimensional (2-D) scanned map of a real abdominal slice. Performances were evaluated by L2 errors between the corrected wavefronts and the undistorted planar wavefront. Point spread functions also were calculated to evaluate the relative image quality. Theoretical analysis shows L2 error will decrease as aperture size grows when exact phase compensation (EPC) is applied, although finite errors will always exist along the edges of the corrected wavefront. Three different aperture sizes, 14.24 mm (64 elements), 28.48 mm (128 elements), and 56.96 mm (256 elements) are considered in this study. Numerical results show that the quality of wavefront with EPC is essentially limited by the aperture size, and the correction methods considered are relatively robust against the aperture size. It also shows that, for low aberration, results with MPSC and EPC are comparable. However, for high aberration, MPSC significantly outperforms EPC in suppression of L2 error and sidelobes. This study suggests that, for most medical ultrasound imaging systems, the exact structure of the distorting medium may not be necessary to be known a priori for optimal distortion correction because of the limitation imposed by finite aperture size.     

微穿孔板传递矩阵计算方法的改进及实验

传统的声电类比法对双层微穿孔板吸声结构进行计算时,忽略了空腔声质量的影响,对于空腔距离比较短,频率比较低时是适用的,但是当空腔距离比较大或频率比较高时,则存在误差。用传递矩阵法对微穿孔板吸声结构进行分析：解决了空腔声阻抗的近似计算带来的误差;对于微孔部分传递矩阵中的声阻抗计算仍然采用马大猷教授的理论,不需要引入修正参数δx。通过上述工作,进行了微穿孔板吸声结构的吸声系数的计算和相应的实验验证,理论计算结果与实验数据吻合良好。     

正弦声信号在钻柱中传播的有限差分法模拟

将钻柱简化为理想周期性管结构,利用有限差分方法,对不同频率的正弦声信号在周期性管结构中传输过程中,存在透射系数及衰减系数差异较大的现象进行数值模拟与分析,得到不同频率的正弦声信号穿过周期性管结构后信号的时域及频域分布。分别对400 Hz和500 Hz的正弦声信号在周期性管结构中传输进行了模拟。模拟结果表明：处于带通频率的正弦声信号在周期性管结构中传播波形失真小,频谱变化小,传播性能良好;带阻频率的正弦声信号在周期性管结构中传播波形失真严重,频谱变化很大,几乎不能通过。模拟结果与同类研究成果有较好的一致性。验证该模型和所采用的数值方法的有效性,为钻柱中声信号传输的数值模拟及与其类似的实际问题的数值模拟提供参考。