表面脉冲超声波沿曲面爬行的可视化研究

为了研究超声表面脉冲波在钢、铁等不透明曲面材料中的传播规律,我们采用和金属材料声速比较接近的光学玻璃作样品,利用研制的数字化动态光弹系统,分析了表面波和次表面波沿玻璃圆棒侧面传播时的行为,记录了不同时刻的横截面上的脉冲声场传播图像,求出了表面波和次表面波沿曲面爬行的传播速度。     

无限长粘弹性圆柱管中轴对称波的传播模式和衰减

在Kelvin-Voigt线性粘弹性模型框架内研究了无限长粘弹性圆柱管中轴对称波的传播和衰减。通过一个复根搜索程序对频散方程的求解得到无量纲化相速度频散曲线和衰减曲线。比较了引入损耗因子后粘弹性圆柱管与弹性圆柱管中轴对称波传播的特性。给出了圆柱管的外内径之比、损耗因子和材料参数对相速度和衰减的影响。分析结果表明与弹性圆柱管相比粘弹性圆柱管中轴对称波的各阶传播模式均存在衰减,高阶传播模式并无严格意义的截止频率。损耗因子对第一阶传播模式的相速度影响很小,而对衰减的影响则比较大,穿孔率对波传播的相速度和衰减有相当大的影响。     

Acoustic properties of a dielectric with cavities in a polarizing electric field

The effect of an external polarizing electric field on the shear wave propagation in a centrosymmetric crystal with electrostriction, whose body is penetrated with parallel cylindrical cavities (pores), is considered. The cavities are distributed throughout the crystal at random and with a low density. The waves are assumed to be polarized along the cavity generatrices, and the wave propagation occurs in the elastic isotropy plane, which is orthogonal to the cavity axes. The external field is assumed to be axial. Possibility of controlling the propagation of shear waves by the polarizing field is demonstrated for the case of metallized cavity surfaces.     

Lamb and torsional waves and their use in flaw detection in tubes

Five separate issues concerning Lamb wave propagation are discussed: (1) the actual motion of the particles in sheet material; (2) the mechanism of the initiation of Lamb waves in plates; (3) the effect of pulse propagation as opposed to continuous waves; (4) wave propagation on the inner and outer surfaces of tubes; and (5) the propagation of waves in tubes at large angles of incidence, beyond the Lamb wave regime.     

POSITIVE DEFINITE PROBLEM OF ENERGY DENSITY AND RADIATIVE ENERGY FLUX FOR PULSE CYLINDRICAL GRAVITATIONAL WAVE

By using the general expressions of energy momentum pseudo-tensor of the cylin-drical gravitational waves (GW) given by Rosen and Virbhadra in Cartesian coor-dinates, the concrete forms of energy density and radiative energy flux of the pulse cylindrical GW are obtained. Their physical properties, suitable range and asymp-totic behaviour are considered. It is found that: For the region in which space radial coordinates to origin are greater than the pulse width of the pulse cylindrical GW, the energy density and radiative energy flux of the outward travelling pulse cylindrical GW propagating along at light-cone are positive definite. However, for the region in which the space radial coordinates are less than the pulse width, there is no guarantee for the positive definite property of the radiative energy flux of the outward travelling wave. Moreover, we show that the asymptotic behaviour of the energy and energy flux densities of the pulse cylindrical GW and that of the Riemann curvature tensor have good self-consistancy in space like, time like and null infinity regions.     

Nonsymmetric Whistler Waves Guided by Cylindrical Ducts with Enhanced Plasma Density

We study the guided propagation of whistler waves whose fields depend on the azimuthal angle in cylindrical plasma-waveguide channels (density ducts) aligned with an external magnetic field and surrounded by a uniform magnetoplasma. The main attention is paid to ducts with enhanced plasma density. It is shown that, under certain conditions, such ducts are capable of guiding proper (eigen) modes and improper leaky modes. We present the results of analysis of the dispersion properties and field structures of nonsymmetric modes guided by cylindrical ducts in the whistler frequency range.     

Planar and nonplanar ion-acoustic envelope solitary waves in a very dense electron-positron-ion plasma

Ion-acoustic envelope solitary waves in a very dense plasma comprised of the electrons, positrons and ions are investigated. For this purpose, the quantum hydrodynamic model and the Poisson equation are used. A modified nonlinear Schrödinger equation is derived by employing the reductive perturbation method. The effects of the quantum correction and of the positron density on the propagation and stability of the envelope solitary waves are examined. The nonplanar (cylindrical/spherical) geometry gives rise to an instability period. The latter cannot exist for planar case and it affected by the quantum parameters, as well as the positron density. The present investigation is relevant to white dwarfs.     

Wave scattering effects in elastic percolation models

The study of the propagation of waves in randomly diluted models is presented. Porosity (crack-like) models are simulated by constructing typical elastic percolation networks with random microscopic heterogeneities in order to resemble rock media. Central and bond-bending forces (Born Hamiltonian) models are considered. For each experimental case, the elastic energy of the system is relaxed in equilibrium and then the model is excited by a pulse source in order to produce wave propagation. First, a review is presented of the well established velocity-porosity relationship from rock physics, which shows a linear trend from small porosities up to the critical porosity (percolation threshold) where the rocks fall apart. From the wave propagation analysis a general trend is observed for the attenuation of waves, from the small to the large porosity models, suggesting multiple scattering effects similar to those reported from effective-medium approximations of wave scattering due to random heterogeneities. Finally, the results are compared with those obtained from laboratory experiments on dry rocks with different porosities and different applied stress regimes.     

MHD cylindrical shock wave in self-gravitating gas

Summary Similarity solutions of the propagation of cylindrical blast waves through a self-gravitating polytropic gas caused by an instantaneous release of finite energy are investigated theoretically including the influence of transverse magnetic field. A comparative study of the distributions of velocity, density, mass, pressure, temperature and magnetic field has been illustrated through figures. The authors of this paper have agreed to not receive the proofs for correction.     

Helical waves of an elastic cylindrical shell

Properties of helical waves of a cylindrical shell described by Kirchhoff-Love equations are considered. The problem is reduced to the case of the propagation of plane waves in an equivalent plate. On the basis of the corresponding dispersion equation and its solution, a conclusion is made about the anisotropy of the shell properties. Dispersion curves are plotted for different angles of propagation of helical waves with respect to the shell axis. Displacements of the shell along and across the direction of wave propagation are calculated.     

Electrodynamic Model of the Gas Discharge Sustained by Azimuthal Surface Waves

Results of studying an electrodynamical model of the plasma producer based on azimuthal surface waves (ASW) propagation both in a cylindrical metal waveguide filled by magnetoactive plasma and along metal cylindrical antenna placed into the plasma are presented. Parameters of these gas discharges are studied in the two frequency ranges where propagation of the ASW is possible and also are compared. The value of plasma density that can be achieved by this producer is calculated. The spatial distribution of the wave electromagnetic fields, dependencies of their penetration depth into plasma and angular discharge length on the discharge parameters are studied analytically and numerically. Advantages of the operation in the both frequency ranges are indicated. Possibility of resonant absorption of the ASW energy due to their conversion into the bulk upper hybrid mode is examined as well.     

Pulse propagation of acoustic waves scattered in a channel

When acoustic waves are scattered by random sound-speed fluctuations in a two-dimensional channel the energy is continually transferred between the propagating modes. In the multiple- scattering region the energy flux assumes an asymptotic form in which there is equal energy flux propagating in each mode. Here we shall make use of this well known result to show how to obtain an asymptotic form for a pulse of acoustic energy propagating in the channel. In the multiple-scattering region the speed of the acoustic waves in the pulse continually changes as the energy is transferred between the modes. The process is basically a diffusion process around the mean speed of propagation. We shall first show, using physical arguments, that the diffusion coefficient is proportional to the square root of the propagation distance times the mean free path of scattering. The theory governing the acoustic propagation in the channel is formulated in terms of modal coherence equations and we shall next give a brief review of the definitions of the coherence functions and a discussion of how the equations governing the propagation of the modal coherence functions are derived. We shall then show how the pulse shape and the relevant parameters may be obtained by solving the basic modal coherence equations at large propagation distances.     

Structural and morphological characterization of CdSe:Mn thin films

The nonlinear propagation of cylindrical and spherical dust-ion-acoustic (DIA) envelope solitary waves in unmagnetized dusty plasma consisting of dust particles with opposite polarity and non-extensive distribution of electron is investigated. By using the reductive perturbation method, the modified nonlinear Schrödinger (NLS) equation in cylindrical and spherical geometry is obtained. The modulational instability (MI) of DIA waves governed by the NLS equation is also presented. The effects of different ranges of the non-extensive parameter q on the MI are studied. The growth rate of the MI is also given for different values of q. It is found that the basic features of the DIA waves are significantly modified by non-extensive electron distribution, polarity of the net dust-charge number density and non-planar geometry.     

Three Dimensional Laser Simulation Code on a Desktop Personal Computer

We present a new simulation code able to simulate the entire propagation of laser pulse, from the amplifiers level up to the focusing stage. This algorithm has some new characteristics that we intend to present. It computes the three-dimensional optical propagation equation using no approximation other than its picosecond expression. The stability has been carefully studied so that it can be applied to any geometry. This is a great improvement since, up to now only cylindrical geometry was accessible for accuracy. In this paper we also present a method using Fast Fourier Transform able to evaluate with a high accuracy, Fresnel’s distribution of a focused laser pulse. The advantages provided by our algorithm are its rapidity and its high physical understanding of the focusing phenomena.     

An Investigation of the Effects of Internal Waves on Sound Propagation in a Stratified Medium with a Sloping Bed

Internal waves usually cause temporal and spatial changes of density and consequently affect the acoustic wave propagation in the ocean. The purpose of this study is a laboratory investigation of the effects of internal waves generated by oscillation of a cylinder in a large stratified glass tank with a sloping bed on the sound waves propagation. Results showed that sound waves are affected by internal waves that depend on the slope angle to the direction of internal wave propagation angle ratio. When the ratio is subcritical or supercritical, the acoustic signal is much reduced as compared to the case with no sloped bottom. This can be explained in terms of the internal waves energy reaching the sloped bed and their reflections.     

电磁波在大面积等离子体片中传播特性的分析

脉冲磁约束线形空心阴极放电形成的大面积等离子体片可应用于等离子体天线、隐身及模拟超音速飞行器表面的等离子体鞘套. 本文首次利用实测等离子体片电子密度时空分布和横向场传播矩阵法, 研究了电磁波在等离子体片中反射率、透射率、吸收率随频率及脉冲放电时间的变化特征. 结果表明: 极化方向平行磁场的电磁波, 在小于截止频率的低频带内具有较高的反射率和吸收率, 增大电流, 反射率增加, 吸收率下降, 在大于截止频率的高频带内反射率和吸收率较低, 增大电流, 透射率下降, 吸收率升高; 极化方向垂直磁场的电磁波在高混杂谐振频率附近存在吸收率明显增强的吸收带, 谐振吸收峰值与放电电流无关; 脉冲放电期间, 电磁波的反射率、透射率与吸收率由不稳定过渡到稳定的时间约为100 μs, 过渡时间随着放电电流的增加而增大, 极化方向垂直磁场、小于截止频率的电磁波在稳定放电阶段谐振吸收较强. 本文的研究成果对利用等离子体片实现对电磁波的稳定高反射作用具有重要意义.     

周期量级飞秒脉冲电场在非线性克尔介质中的传输

采用时间转换法研究了周期量级飞秒脉冲电场在非线性克尔介质中的传输.由于周期量级飞秒脉冲电场的脉宽小于介质拉曼响应的特征时间,在传输过程中脉冲电场会发生剧烈的变形和分裂,并在频谱上观察到了强烈的拉曼感应频移和色散波.由于周期量级脉冲电场依赖于载波包络相位,发现在脉冲电场传输过程中,主脉冲电场和色散波电场的相位线性地依赖于初始脉冲的载波包络相位.     

光脉冲在密度随时间,空间变化的等离子体中的传播

本文定量地研究了光脉冲在密度随时间上升的等离子体中的频率上移,以及在密度随空间上升的等离子体中的脉宽压缩。通过求解波动方程,得到了描述光脉冲的时空演变特征的解析表达式,并在此基础上就脉冲形状、峰值位置、传播速度、脉冲宽度及光子数守恒等问题进行了讨论。     

Quantifying elasticity and viscosity from measurement of shear wave speed dispersion

The propagation speed of shear waves is related to frequency and the complex stiffness (shear elasticity and viscosity) of the medium. A method is presented to solve for shear elasticity and viscosity of a homogeneous medium by measuring shear wave speed dispersion. Harmonic radiation force, introduced by modulating the energy density of incident ultrasound, is used to generate cylindrical shear waves of various frequencies in a homogeneous medium. The speed of shear waves is measured from phase shift detected over the distance propagated. Measurements of shear wave speed at multiple frequencies are fit with the theoretical model to solve for the complex stiffness of the medium. Experiments in gelatin phantoms show promising results validated by an independent method. Practical considerations and challenges in possible medical applications are discussed.     

Nonplanar Electrostatic Solitary Waves in a Relativistic Degenerate Dense Plasma

By employing the reductive perturbation technique, the propagation of cylindrical and spherical ion acoustic solitary waves is studied in an unmagnetized dense relativistic plasma, consisting of relativistically degenerate electrons and cold fluid ions. A modified Korteweg-de-Vries equation is derived and its numerical solutions have been analyzed to identify the basic features of electrostatic solitary structures that may form in such a degenerate Fermi plasma. Different degrees of relativistic electron degeneracy are discussed and compared. It is found that increasing number density leads to decrease the amplitude the width of the ion acoustic solitary wave in both the cylindrical and spherical geometries. The relevance of the work to the compact astrophysical objects, particularly white dwarfs is pointed out.