椭圆柱体对任意入射极化的电磁波的散射

基于电磁场的多尺度变换理论,得到了以导体椭圆柱为例的电磁波任意极化,任意垂直入射到目标上的散射场的解析式.将所得结果应用于计算圆柱目标的散射场,结果与文献完全一致.对椭圆柱体的散射宽度随入射波方位角,电磁波频率以及目标尺寸的变化进行了仿真.结果表明,在垂直于电磁波的方向上观测时,椭圆柱的垂直尺寸对散射有较大的影响,电磁波水平极化时散射最强.所用算法适用于介质椭圆柱等目标的散射特性研究,所得结果为目标尺寸、形状遥感电磁测量等实际应用提供了理论依据.     

Diffraction of a Plane Electromagnetic Wave by an Infinite Dihedral Wedge with a Magnetodielectric-Coated Cylinder at the Apex

We present a technique for obtaining a rigorous solution to the problem of diffraction of a plane electromagnetic wave by an infinite dihedral wedge with a magnetodielectric-coated cylinder at the apex. In the case of an E-polarized incident wave, we obtain a closed-form solution of the problem and present the results of exact calculations of the far-zone diffracted field for wide ranges of the structure parameters. In particular, it is shown that if the wedge apex is loaded with only a magnetodielectric cylinder, then the diffracted field for the very long-wavelength incident wave is not affected by the dielectric permittivity of the cylinder and the wave-incidence angle, but is affected only by the opening angle of the wedge, the diameter of the dielectric coating in terms of the free-space wavelength, the magnetic permeability of the coating, and the electric properties of the surrounding medium. If a half-plane or a wedge-shaped part are added to a single cylinder, then the backscattering coefficient of such a structure decreases, but the discovered resonance variation in the backscattering coefficient of the cylinder in the long-wavelength range becomes more pronounced.     

Damped waves in a cylinder shadow

The sound field in the region of a deep shadow behind an impedance cylinder is analyzed for the case of plane wave diffraction by the cylinder surface. The main part of the field is represented by a cylindrical wave that has a complex index determined from the boundary conditions and decays with the angular coordinate. An expression for the amplitude of this wave is determined by extracting it from the total field formed in the shadow region. It is demonstrated that this wave approximates more closely the total field behind the cylinder, as compared to the least damped wave in the field representation obtained on the basis of the Watson transform. A way to improve the sound barriers is indicated.     

Diffraction of a plane electromagnetic wave by a slot in a conducting screen with a transverse dielectric layer

The problem of diffraction of a plane electromagnetic wave by a slot in a planar perfectly conducting arbitrary thick screen with an infinite planar dielectric layer passing through the slot transversely to the screen is solved rigorously. In each of the field existence domains (two domains on either side of the screen and the interior of the slot), the solution is represented as an expansion in piecewise harmonic or exponential modes that allow for reflection and refraction at the boundaries of the dielectric layer. It is found that a set of functions describing such modes is complete enough to construct a solution satisfying all boundary conditions of the diffraction problem. The procedures of solution construction for the case at hand and for the same diffraction structure without the dielectric layer are compared.     

Circumferential resonance modes of solid elastic cylinders excited by obliquely incident acoustic waves

When an immersed solid elastic cylinder is insonified by an obliquely incident plane acoustic wave, some of the resonance modes of the cylinder are excited. These modes are directly related to the incidence angle of the insonifying wave. In this paper, the circumferential resonance modes of such immersed elastic cylinders are studied over a large range of incidence angles and frequencies and physical explanations are presented for singular features of the frequency-incidence angle plots. These features include the pairing of one axially guided mode with each transverse whispering gallery mode, the appearance of an anomalous pseudo-Rayleigh in the cylinder at incidence angles greater than the Rayleigh angle, and distortional effects of the longitudinal whispering gallery modes on the entire resonance spectrum of the cylinder. The physical explanations are derived from Resonance Scattering Theory (RST), which is employed to determine the interior displacement field of the cylinder and its dependence on insonification angle.     

Electromagnetic wave diffraction by axisymmetric diaphragms in the millimeter wavelength range

An automated radiophysical complex operating in the millimeter wavelength range is presented. We have measured the spatial amplitude and phase distributions of the electromagnetic field in its two orthogonal components for the case of diffraction of electromagnetic waves incident upon an object at near-grazing angles. Thw wave diffraction at the aperture and circular slot in the metal shield is investigated. The directivity patterns and transmission coefficient as functions of the relative aperture diameter and circular slot are given. The extremum in the dependence of the maximum angle on the width of the circular slot is found.Institute of Radiophysics and Electronics, Ukrainian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 7, pp. 719–729, July, 1995.     

Acoustic scattering by a rigid elliptic cylinder in a slightly viscous medium

A complete solution is obtained for the two-dimensional diffraction of a time-harmonic acoustic plane wave by an impenetrable elliptic cylinder in a viscous fluid. Arbitrary size, ellipticity, and angle of incidence are considered. The linearized equations of viscous flow are used to write down expressions for the dilatation and vorticity in terms of products of radially and angular dependent Mathieu functions. The no-slip condition on the rigid boundary then determines the coefficients. The resulting computations are facilitated by recently developed library routines for complex input parameters. The solution for the circular cylinder serves as a guide and a differently constructed solution for the strip is also given. Typical results in the "resonant" range of dimensionless wave number, displaying the surface vorticity and the far-field scattering pattern are included, with the latter allowing comparison with the inviscid case.     

Scattering of a plane electromagnetic wave by an infinite wedge with a truncated circular cylinder coated coaxially with a magnetodielectric layer

We formulate and solve the problem of scattering of a plane electromagnetic wave by an infinite, perfectly conducting wedge with a truncated, perfectly conducting circular cylinder which is coated coaxially with a magnetodielectric layer and located along the wedge edge. The rigorous solution is obtained and reduced to a system of linear algebraic equations of the second kind for unknown coefficients of Fourier expansions of the scattered field. The results of calculating the scattered field in the far zone with a specified accuracy are presented for the case of an H-polarized wave. It is shown that for certain values of the electric radius of the cylinder, the backscattering cross section of such a structure has pronounced maxima.     

Radiation force and torque on a two-dimensional circular cross-section of a non-viscous eccentric layered compressible cylinder in acoustical standing waves

The purpose of this study is to develop an analytical formalism and derive series expansions for the time-averaged force and torque exerted on a compound coated compressible liquid-like cylinder,insonified by acoustic standing waves having an arbitrary angle of incidence in the polar(transverse)plane.The host medium of wave propagation and the eccentric liquid-like cylinder are non-viscous.Numerical computations illustrate the theoretical analysis with particular emphases on the eccentricity of the cylinder,the angle of incidence and the dimensionless size parameters of the inner and coating cylindrical fluid materials.The method to derive the acoustical scattering,and radiation force and torque components conjointly uses modal matching with the addition theorem,which adequately account for the multiple wave interaction effects between the layer and core fluid materials.The results demonstrate that longitudinal and lateral radiation force components arise.Moreover,an axial radiation torque component is quantified and computed for the non-absorptive compound cylinder,arising from geometrical asymmetry considerations as the eccentricity increases.The computational results reveal the emergence of neutral,positive,and negative radiation force and torque depending on the size parameter of the cylinder,the eccentricity,and the angle of incidence of the insonifying field.Moreover,based on the law of energy conservation applied to scattering,numerical verification is accomplished by computing the extinction/scattering energy efficiency.The results may find some related applications in fluid dynamics,particle trapping,mixing and manipulation using acoustical standing waves.     

高能表面电弧放电控制圆柱激波实验

采用风洞实验和高速纹影系统研究高能表面电弧等离子体激励控制圆柱激波. 在Ma=2的超声速风洞中, 分别放置了带有10, 15, 20 mm这3个不同高度圆柱的实验模型, 对比分析了不同高度圆柱的初始流场特征, 以及高能表面电弧放电的放电电容、直流源电压和圆柱高度对圆柱脱体激波控制的影响. 实验结果表明, 高能表面电弧等离子体激励诱导的冲击波和热气团与激波产生相互作用, 激波形状改变, 强度削弱. 圆柱高度越高其上方的弓形激波角越大, 在施加等离子体激励后, 弓形激波角减小, 激波强度减弱; 放电电容和直流源电压对激波控制效果的影响均呈正相关关系; 随着圆柱高度的增加, 控制效果减弱、有效控制作用时间缩短.      

Gauss声束对离轴椭圆柱的声辐射力矩

利用部分波展开法求解得到了Gauss声束入射下刚性和非刚性椭圆柱的声散射系数,推导了一般情况下的声辐射力矩表达式.在此基础上,通过一系列数值仿真详细分析了离轴距离、入射角度和束腰半径对声辐射力矩的影响.结果表明:正向与负向声辐射力矩均可以在一定条件下存在;低频情况下刚性椭圆柱比非刚性椭圆柱更容易产生较强的声辐射力矩;特定频率的入射声场可以激发出非刚性椭圆柱不同阶的共振散射模式,因而非刚性椭圆柱的声辐射力矩峰值与频率的关系更密切;增加束腰半径有利于扩大散射截面,进而增加椭圆柱的声辐射力矩.该研究结果预期可以为利用声辐射力矩实现粒子的可控旋转和流体黏度的反演提供一定的理论指导.     

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.     

Diffraction of a plane wave incident at a small angle to the axis of a strongly elongated spheroid

The high-frequency acoustic field in the half-shadow region on the surface of a strongly elongated body of rotation is studied. The asymptotic formulas obtained earlier for axial incidence of a plane wave are generalized to the case of incidence of a wave at a small angle to the body axis. The results of calculations are presented. Analysis has been performed on how the angle of incidence and the parameter describing the degree of elongation of the body affect the character of the field distribution over the surface.     

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为优化表面波放电的狭缝天线阵设计,给表面波放电装置的设计提供理论依据,对狭缝天线布局进行了理论和仿真计算分析。结果表明,宽缝辐射的电磁场比纵缝强,同相激励天线辐射的电磁场比异相激励强,更容易击穿空气产生等离子体。基于磁场强度同相分布规律对已经采用的纵缝天线阵进行改进,纵缝按照两侧交互并间隔半个波导波长布置,各个纵缝均为同相,产生等离子体效果更好。     

Excitation and propagation of non-axisymmetric guided waves in a hollow cylinder

Excitation and propagation of non-axisymmetric guided waves in a hollow cylinder is studied by using the normal mode expansion method (NME). Different sources such as angle beam, tube end excitation with normal beam, and comb transducer possibilities are discussed based on the derivations of the NME method. Numerical calculations are focused on the case of angle beam partial loading. Based on the NME method, the amplitude coefficients for all of the harmonic modes are obtained. Due to the difference of phase velocities for different modes, the superimposed total field varies with propagating distances and hence makes particle displacement distribution patterns (angular profile) change with distance. This varying non-axisymmetric angular profile of guided waves represents a nonuniform energy distribution around the hollow cylinder and thus has an impact on the inspection ability of guided waves. The angular profiles of an angle beam source are predicted by theory and then verified by experiments. The predicted angular profiles also provide information for determining the transducer location to find defects in a certain position on the hollow cylinder.     

A numerical simulation of surface wave excitation in a rectangular planar-type plasma source

The principle of surface wave plasma discharge in a rectangular cavity is introduced simply based on surface plasmon polariton theory. The distribution of surface-wave electric field at the interface of the plasma-dielectric slab is investigated by using the three-dimensional finite-difference time-domain method (3D-FDTD) with different slot-antenna structures. And the experimental image of discharge with a novel slot antenna array and the simulation of the electric field with this slot antenna array are both displayed. Combined with the distribution of surface wave excitation and experimental results, the numerical simulation performed by using 3D-FDTD is shown to be a useful tool in the computer-aided antenna design for large area planar-type surface-wave plasma sources.     

Acoustic radiation pressure on a rigid cylinder: an analytical theory and experiments

Simple and general analytical expressions are derived for the acoustic radiation force on a long rigid cylinder with a small diameter, whose axis is perpendicular to the wave propagation. Results are expressed in terms of the time-averaged densities of kinetic and potential energies of the incident sound field. For the case of a standing-wave field, which was used in these experiments, the theoretical results agree well with the experimental observations.     

Diffraction of a Plane Electromagnetic Wave by an Infinite Double-Ridge Wedge with a Dielectric Cylinder at the Crest

We reduce the considered problem to solving a matrix equation of the second kind for unknown coefficients of expansion of a diffracted field into a Fourier–Bessel series. This expansion was obtained by imposing boundary conditions on the diffracted field with the subsequent re-expansion of the field function over basis functions in a given interval. The expansion coefficients were determined analytically in the case where the electric diameter of the cylinder is less than unity as well as numerically with a high accuracy by solving the obtained matrix equation using the reduction method. We derived expressions for the pattern of the far-zone field scattered by the studied structure and the backscattering cross section and give exact numerical results for the case of an E-polarized incident wave.     

Pure elastic resonance scattering of an obliquely incident plane acoustic wave by a submerged infinite cylinder

I.IntroductionTheResonanceScattcringThcory(RST)asanimportantmethodtoana1yzeacousticscat-teringofsubmergede1asticbodieswascxtcnsivelyuscdforcy1indrica1andspherica1geometries,inc1udingsolidstructuresandho11owoncs.RSTismain1yappliedtostudytheresonancescat-teringspcctra(oracousticspcctra)ofanc1astictargctcxcitedby'theinciderttacousticwavesoastOidentifythetargct.Tocxtracttheresonancescattcringspcctra,anappropriatebackgroundsuchasrigidbackgroundorsoftonemust-besubtractcd.Expcrimenta1ly,theReson…     

Laser Generation of Surface Waves on Cylinder with a Slow Coating

An analytical model of acoustic field excited by a pulsed-laser line source on a coated cylinder is presented. Surface wave dispersive behaviours for a cylinder with a slow coating are analysed and compared with that of a bare cylinder. Based on this analysis, the laser-generated transient response of the perturbed Rayleigh wave and the higher modes of steel cylinder with a zinc coating are calculated from the model using residue theory and FFT technique. The theoretical result from the superposed waveform of the perturbed Rayleigh wave and higher modes agree well with the waveform obtained in experiment. The results show that the model and numerical method provide a useful technique for quantitatively characterizing coating parameters of coated cylinder by the laser generated surface waves.