Ultrasonic plate waves in paper

A theoretical and experimental study of ultrasonic plate waves in machine-made paper is described. The paper is assumed to behave as a homogeneous orthotropic plate. The dispersion equation for orthotropic plate waves in principal directions is then developed analytically. It is shown that at low frequencies the orthotropic dispersion equation depends only on in-plane parameters. Using appropriate elastic constants, a computer is used to construct the dispersion curves and describe the internal motion for plate waves in paper. Finally, a portion of the dispersion curve is checked experimentally.     

Nonlinear lamb waves in a metal plate with defects

Experimental results on the propagation of finite-amplitude Lamb waves in a solid plate made of polycrystalline aluminum alloy with defects are presented. The Lamb waves are recorded and visualized using a scanning laser vibrometer. The dependences of the higher harmonic amplitudes, both averaged over the plate surface and measured at a point far from the defect and at the site of the defect, on the fundamental frequency amplitude are studied. A threshold character of the higher harmonic generation and a power-law behavior of their amplitudes are revealed, the latter feature being unconventional for the nonlinearity associated with the anharmonicity of the crystal lattice of the material. The possibility of locating the distribution of individual defects from the measured spatial distribution of structural nonlinearity in the material under study is experimentally demonstrated. The results of the experiments are explained in terms of the bilinear medium model.     

Low-frequency asymptotics of complex dispersion curves for lamb waves in layered elastic plate

An asymptotic and iterative method is proposed to calculate complex dispersion curves for isotropically layered plates. At the first stage, a dispersion equation is derived in explicit form and its limiting form is obtained for the static case. Passages to the limit of coinciding materials or vanishingly small layer thicknesses are investigated. Specific case of materials with coinciding shear moduli is analyzed in detail. Asymptotics of static roots is deduced for a large value of the root magnitude, the error of asymptotics is estimated, and an iterative method is proposed for calculating exact root values. Long-wave asymptotics of dispersion curves is derived, and it is shown that every complex dispersion curve has a long flat initial segment. Asymptotics is the more accurate, the lower the frequency is and the higher the number of the curve is. Exact values of wave numbers on the dispersion curve are also evaluated by another iterative procedure. Examples of calculating the dispersion curves are presented and the efficiency of the algorithm is shown.     

Ultrasonic engine based on traveling waves in a plate

Velocity dispersion curves and phase relationships between the surface displacement components are calculated in the case of waves propagating in XZ-, YZ-, and ZY-cut LiNbO₃ plates. The dispersion curves agree with experimental frequency dependences of the excitation efficiency for various modes in the plates. The displacements of ZnS microparticles placed on the surface of the plates qualitatively agree with the analytical phase relationships between the displacement components. Results obtained indicate that the plate may be used as an ultrasonic engine capable of transferring microparticles in various physical and industrial processes.     

铝板结构兰姆波阵列瞬时相位包络成像及补偿*

针对板结构中多裂纹缺陷的识别和定位问题,提出了瞬时相位全聚焦成像的包络算法,同时提出了一种基于激励信号脉冲时长的波速补偿方法,通过数值仿真验证补偿后的瞬时相位包络全聚焦成像对多缺陷的识别和定位能力。与幅值全聚焦成像相比,可以有效减小噪声、直达波、端面回波引起的虚像,提高缺陷的识别率。与相位全聚焦成像相比,可以实现缺陷处的单峰值聚焦,更便于缺陷的定位。实验结果表明,所提出的波速补偿瞬时相位包络全聚焦成像在 的铝板上对实现了5个缺陷的成像和定位,其定位的平均误差为12.24 mm。     

Noise in a coupling electromagnetic detecting system for high frequency gravitational waves

This paper discusses the basic categories of noise in detecting high frequency gravitational waves in the microwave band (～0.1--10 GHz), which contain shot noise from the laser and the thermal radiation photons, thermal noise from statistical fluctuation of the thermal photons and fluctuation of the temperature, radiation press noise on the fractal membrane, the noise caused by the scattering of the Gaussian Beam (GB) in the detecting tube and noise in the microwave radiometers. The analysis shows that a reasonable signal-to-noise ratio may be achieved for a detecting device with the fixed power of GB (10⁵ W), only when the temperature of the environment is no more than T=1 K, and the optimal length of the microwave radiometers is about 0.3 m.     

Nonlinear Landau damping of high frequency waves in non-Maxwellian plasmas

Space plasmas often possess non-Maxwellian distribution functions which have a significant effect on the plasma waves.When a laser or electron beam passes through a dense plasma,hot low density electron populations can be generated to alter the wave damping/growth rate.In this paper,we present theoretical analysis of the nonlinear Landau damping for Langmuir waves in a plasma where two electron populations are found.The results show a marked difference between the Maxwellian and non-Maxwellian instantaneous damping rates when we employ a non-Maxwellian distribution function called the generalized(r,q)distribution function,which is the generalized form of the kappa and Maxwellian distribution functions.In the limiting case of r=0 and q→∞,it reduces to the classical Maxwellian distribution function,and when r=0 and q→κ+1,it reduces to the kappa distribution function.     

Low frequency surface waves in the presence of an external high frequency electric field

The problem of surface sound propagation in the presence of external high frequency dipole electric field is investigated using Vlasov-Poisson equations. The structure of the electric field of these surface waves is also determined. It has been found that the ion sound waves that exist in the presence of external high frequency waves are true surface waves while those without the external field are quasi-surface waves as the former waves decay within one wavelength away from the surface.     

基于空耦超声Lamb波的金属板状结构内部缺陷检测方法

针对接触式超声检测方法在金属板结构内部缺陷实际工程检测中存在的环境要求高、效率低、操作难度高等问题,提出了空耦超声Lamb波检测方法,该方法能更好地适应现场应用环境,提高检测效率,减少传感器数量。通过有限元仿真和实验分析比较了空耦超声检测与接触式超声检测两种方法接收到的信号和成像效果。结果表明:有限元仿真和实验中,空耦超声检测方法对缺陷位置的定位误差分别为2 mm和3.6 mm,接触式检测方法对缺陷位置的定位误差分别为2 mm和11.3 mm,空耦检测具有较高的定位精度;单侧激励条件下,适合采用A0模态Lamb波对板内缺陷进行检测;空耦超声检测可以通过调整信号接收角度接收单一模态Lamb波,避免伪像产生。该方法为后续金属板状结构内部缺陷的空耦超声检测提供参考。     

Direct electromegnetic generation of high frequency acoustic waves in semiconductor superlattices

The periodic space charge layers formed in semiconducting superlattice structures can be used for direct electromagnetic generation of high frequency acoustic waves. The mechanism for and resonant nature of the acoustic generation are discussed.     

Modeling of a high frequency ultrasonic transducer using periodic structures

Solidly mounted integrated transducers with a Bragg cell inserted between the piezoelectric film and the substrate are investigated for high frequency ultrasonic applications. A numerically stable recursive one dimensional transmission/reflection model was used to analyze the behavior of the periodic structure. This theoretical analysis includes the study of the influence of the acoustic properties of the constitutive layer, the effect of the number of cells and their arrangement. A 35 MHz integrated transducer consisting in a PZT ceramic laid down on a Au/PZT Bragg cell deposited on a porous substrate was fabricated and characterized. Both theoretical and experimental results highlight the interest of using a periodic structure for high frequency ultrasonic applications.     

Electromagnetic generation of high frequency longitudinal sound waves via deformation potential coupling

The possibility of generating high frequency longitudinal sound waves via deformation potential coupling in semi-metals and semi-conductors is examined and the conversion efficiency is derived. The latter has its maximum value for sound wavelengths near the Fermi-Thomas screening length of the carriers.     

超声导波管道缺陷定位方法研究

该文提出了一种基于多换能器的超声导波管道缺陷定位方法,利用两个环形换能器阵列分别接收与缺陷相关的反射信号和透射信号,实现管道缺陷轴向及周向定位。首先利用变分模态分解法对信号进行滤波,在滤除噪声的基础上对信号进行小波变换,获得缺陷反射的导波时频信息,根据反射波的渡越时间计算缺陷在管道轴向的位置。在缺陷的周向定位上,提出一种损伤指数迭代方法,根据透过缺陷后导波的损伤指数来计算缺陷的周向位置及周向长度。仿真和实验结果表明,该文所采用的方法能够较为准确地测量出管道上缺陷的位置及周向长度。     

A digital frequency meter for MM waves

This is a brief account of a design of the digital frequency meter applied to mm wave techniques. In measurement of frequency by the method of oscillatory absorption, it has two harmonic mixing systems instead of the usual single harmonic mixing system.     

An investigation of generalized lamb waves using ultrasonic reflectivity measurements

Ultrasonic critical angle reflectivity method is used to measure the phase velocity of generalized Lamb waves in cadmium and zinc platings. In addition to the free propagating modes, leaky-wave modes are also observed. Leaky waves are considered using principles familiar from the discussion of surface waves in anisotropic media and a relatively good agreement is obtained between the theoretical computations and measured data.     

A novel method of high speed frequency shift keying modulation in mm waves

To place a high speed switching diode in the cavity of the mm wave scillator has the behavior equivalent to an inductive post. The on-and-off switching of the diode will give rise to the change of inductance, hence the change of parameters of the cavity, and the osaillating frequency. In this article, the basic principle and the measured result are given. It is found that this is an efficient method in high speed frequency shift keying (FSK) modulation for a mm wave transmitter in communication.     

Ion acoustic solitary waves in the presence of a high frequency electric field

The propagation of a small but finite amplitude ion acoustic wave in the presence of a radio frequency (frequency below electron plasma frequency) electric field is investigated. The velocity, the amplitude and the width of the solitary wave are found to decrease with increasing intensity of the RF field.     

Graph models for waves in thin structures

A brief survey on graph models for wave propagation in thin structures is presented. Such models arise in many areas of mathematics, physics, chemistry and engineering (dynamical systems, nanotechnology, mesoscopic systems, photonic crystals etc). Considerations are limited to spectral problems, although references to works with other studies are provided.     

Graph models for waves in thin structures

Abstract

A brief survey on graph models for wave propagation in thin structures is presented. Such models arise in many areas of mathematics, physics, chemistry and engineering (dynamical systems, nanotechnology, mesoscopic systems, photonic crystals etc). Considerations are limited to spectral problems, although references to works with other studies are provided.