Wigner functions for nonparaxial, arbitrarily polarized electromagnetic wave fields in free space

New representations are defined for describing electromagnetic wave fields in free space exactly in terms of rays for any wavelength, level of coherence or polarization, and numerical aperture, as long as there are no evanescent components. These representations correspond to tensors assigned to each ray such that the electric and magnetic energy densities, the Poynting vector, and the polarization properties of the field correspond to simple integrals involving these tensors for the rays that go through the specified point. For partially coherent fields, the ray-based approach provided by the new representations can reduce dramatically the computation times for the physical properties mentioned earlier.     

Fully vectorial highly nonparaxial beam close to the waist

I use the angular spectrum representation to compute exactly the Gaussian beam close to the waist (w(0)) in the case of a highly nonparaxial field (w(0)相似文献   

A singular perturbation problem in unsteady groundwater flows with a free surface

Perturbation methods are employed to study the seepage resulting from a heavy, prolonged rainfall over the slopes of a symmetric hill (two dimensions). The emergence of horizontal velocity components in the seepage is an important item of study, since such velocities tend to destabilize soil slopes. The study does establish the development of such outward horizontal components of flow. In addition, the geometry of the descending free-surface (dry-wet soil interface) also forms part of the solution. The perturbations introduced turn out to be of a “singular type”, and the techniques of matched asymptotic expansions are used in the construction of the solution.     

柔性关节空间机械臂奇异摄动自抗扰控制仿真研究

针对不同重力环境条件下考虑摩擦、关节刚度非线性与外扰影响的柔性关节空间机械臂的控制问题,提出了一种基于奇异摄动理论的自抗扰控制方法。首先建立了柔性关节空间机械臂在地面重力和空间微重力环境下的动力学模型;然后采用奇异摄动法将系统模型分为快变子系统和慢变子系统,针对快变子系统设计速度反馈控制律来抑制柔性关节的振动,针对慢变子系统设计加入前馈补偿的自抗扰控制器(ADRC)来抵抗系统的内外扰动,并对系统进行了稳定性分析;最后对设计的控制器进行了仿真验证与对比研究。仿真结果表明,采用该方法,在不同重力环境下柔性关节空间机械臂均能实现很好的轨迹跟踪和抖振抑制,且能有效抵抗内外扰动,系统具有鲁棒性。     

Application of a perturbation method to the solution of eddycurrent testing problems

This paper presents an analytical solution for the change of the impedance of a double conductor line due to eddy currents induced in presence of an infinitely long horizontal cylindrical flaw with vertically symmetric cross-section. The solution, found by a perturbation method, is applicable in the case the conductivities of the flaw and of the surrounding material do not differ by much. Numerical results for flaws of circular and elliptic cross-sections are presented     

基于多尺度方法的带颗粒阻尼悬臂梁自由振动半解析分析

被动式颗粒阻尼器原理简单、性能优越,广泛应用于结构减震领域。极端环境容易激起结构的高频振动,颗粒阻尼的非线性特性使得多模态振动的减振特性相互耦合。采用模态综合法分析自由端带集中质量块的悬臂梁、离散单元法模拟颗粒阻尼器中颗粒集的运动,基于多尺度分析的球-墙耦合模型建立了揭示结构多模态振动条件下颗粒阻尼减振机理的半解析分析模型。分析表明,颗粒阻尼器具有良好的减震效果,减振性能受结构振动模态影响很大。与单一模态振动减振性能相比,多阶模态振动同时存在时,低阶模态幅值的衰减出现了滞后,而高阶模态幅值的衰减则明显加快。因此,采用单一模态获得的等效阻尼比不足以表征复杂振动条件下颗粒阻尼器的减振性能。     

矢量法在计算空间目标地面照度中的应用

从基本辐射理论出发，推导了空间漫反射目标地面照度的计算公式，详细讲解了采用矢量法对该公式中的积分进行计算的过程，矢量法可应用于任何外形能写出函数关系的目标。以球为例，实际计算了其星等值，并将结果与其他方法计算的结果进行了比较，它们完全一致。     

Application of symmetric indirect Trefftz method to free vibration problems in 2D

A symmetric indirect Trefftz method is developed to solve the free vibration problem of a 2D membrane. It is proved that in this approach the spurious eigensolution exists, and an auxiliary matrix is constructed to help extraction of the spurious solution using the generalized singular‐value decomposition. In addition to the spurious eigensolution, this regular formulation suffers from its ill‐posed nature, i.e. the numerical instability. In order to deal with the numerical instability, the Tikhonov's regularization method, in conjunction with the generalized singular‐value decomposition, is suggested. The proposed approach has some merits when compared with other regular boundary element formulations reported so far; namely the capacity of representing eigenmodes and the ability to deal with a multiply connected domain of genus 1. Several numerical examples are demonstrated to show the validity of the current approach. Copyright © 2003 John Wiley & Sons, Ltd.     

Characterization of electromagnetic properties of polymeric composite materials with free space method

The introduction of microwave radars during the second World War altered the air defense scenario significantly, and this led to the development of the “stealth” techniques. By reducing the detectability of aircrafts or warships, of which the radar cross section (RCS) is a measure, they could evade radar detection, which affected not only the mission success rate but also survival of them in the hostile territory. In the very early stage of the research on stealth techniques, many researches were mainly concentrated on the reduction of RCS and development of radar absorbing materials (RAM), but nowadays studies on investigating the radar absorbing structures (RAS) using fiber reinforced polymeric composite materials are becoming popular research field.

In this study, electromagnetic characteristics of unidirectional E-glass fiber reinforced epoxy composites were tested with free space methods, which can overcome drawbacks of conventional cavity and waveguide methods. Complex relative permittivities of low-loss composite were measured with respect to the angle between the fiber orientation and the electric field vector of EM wave in X-band frequency range. From the experimental data, empirical relation between the dielectric properties of composites and test variable was suggested and verified.     

Structure of the singular terms in the free energy correctly reproducing the nonasymptotic corrections to the thermodynamic functions

The method for selecting the structure of the singular terms in the expression for the free energy correctly reproducing the nonasymptotic components of the thermodynamic functions is examined.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 49, No. 6, pp. 1027–1033, December, 1985.     

Simulation of failure processes with the variational multiscale method

In the present contribution a multiscale method for the modeling an simulation of failure and crack propagation is presented. Thereby, the variational multiscale method initially introduced by Hughes provides the methodological multiscale framework. The basis of this method is a decomposition of the solution into coarse scale and fine scale contributions, the latter incorporating the local behaviour. The method involves the propagation of cracks at finite strains. A twoscale approach, macro-meso, is adopted and both scales are discretized with finite elements whereby certain locality assumptions are prescribed to the mesoscopic solution. At the fine scale an evolving mesostructure induced by crack propagation is taken into account. The multiscale framework implies naturally a refined discretization in the area of the crack tip. Nevertheless, when crack propagation is modelled, the crack direction should not depend on the discretization. To prevent constant remeshing a discretization with discontinuous elements at the fine scale is applied. The applicability of the method to simulate multiscale failure processes at finite strains is demonstrated by numerical examples.     

三维自由空间中指向性信息未知偶极声源的等效源识别方法

偶极声源的指向性是影响声源识别结果的关键因素。目前,偶极声源的识别方法通常是基于声源的指向性信息先验假设,然而在实际偶极声源识别中,很难事先获得声源的指向性信息;此外,声源分布在二维平面上的假设通常不适用于实际的气动系统。为了准确识别指向性信息未知的偶极声源,并获得声源的三维成像结果,提出了一种基于加权迭代L1最小化算法的等效源方法。该方法将声源指向矢量作为未知参数,从测量声压与等效源源强的传递函数中分离出来,并通过加权迭代L1最小化算法将声源指向矢量与等效源源强一起求解出来,进而利用这些求解获得的声源信息进一步预测声场。与以往的偶极声源识别方法不同,该方法可以实现指向性信息未知偶极声源的三维成像。指向性信息未知偶极声源的三组仿真案例和自制类偶极声源的实验研究验证了该方法的有效性和鲁棒性。     

Application of a form perturbation technique to elastic problems

Summary Perturbation of the lateral surface of circular cylindrical bars subjected to torsion is analysed in two cases: i) The bar remains uniform but the cross-section becomes elliptical; ii) The cross-section remains circular but its diameter varies along the axis of the bar in a prescribed manner. In the latter case a sinoidal, wave-like, variation of the meridian of the lateral surface is specialized to a linear variation resulting in a conical form of the bar. In both cases the results agree closely with the standard solutions of the theory of torsion of cylindrical bars with an arbitrary cross-section and of bars with an axial symmetry, respectively.

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Zusammenfassung Die Störung der Mantelfläche tordierter Stäbe mit Kreisquerschnitt wird für zwei Fälle analysiert: 1. Der Stab bleibt zylindrisch, während der Querschnitt elliptisch wird; 2. der Querschnitt bleibt kreisförmig, während sich sein Durchmesser entlang der Stabachse in vorgeschriebener Weise ändert. Im zweiten Fall wird ein sinusförmiger Stablängsschnitt zu einem von konischer Gestalt spezialisiert. In beiden Fällen liegen die Ergebnisse nahe bei den Standardlösungen der Torsionstheorie zylindrischer Stäbe mit beliebigem Querschnitt bzw. achsensymmetrischer Stäbe.

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These results were obtained in the course of research supported by a grant of the National Science Foundation.     

A method and apparatus for precision measurements of the absorption coefficient of wide-aperture microwave radiators in free space

A method and apparatus for measuring the absorption coefficient χ _f (the relative radiation coefficient ε _f ) of wide-aperture radiators in free space are described. The requirements on the values of ε _f and the error Δε _f , which ensure the necessary accuracy when calibrating these radiators, are discussed. The results of measurements of ε _f and Δε _f of the radiating element of a cryogenic wide-aperture radiator in the 10–43.2 GHz frequency band are presented.     

Determinantal method for locally modified structures. Application to the vibration damping of a space launcher

In order to improve the dynamic behaviour of an existing or already designed structure, local modifications can be performed by taking advantage of the relative displacement between two points of the structure. A stiffner, damper or viscoelastic rod may be added and its effect on the initial structure must be assessed. A new formulation is developed, based on the response of the initial structure at the attachment points of the local modification. A determinantal equation results, whose roots are the eigenvalues of the modified structure. The equation is solved numerically with a dedicated algorithm and it is shown that this is faster than performing an eigenvalue problem reanalysis. The method is able to deal with both undamped and damped systems, and can handle several modifications simultaneously. It is applied on the last stage of a space launcher, along with a double modal synthesis method.