Scattering of electromagnetic waves by a coated not perfectly conducting sphere

We consider the low-frequency scattering problem of a plane electromagnetic wave by a sphere, which is covered by a penetrable concentric spherical shell. The medium, occupying the shell, is lossless while on the surface of the core an impedance boundary condition is satisfied. The impedance boundary condition was introduced by Leontovich (Investigations of Radiowave Propagation. Part II, Academy of Science, Moscow, 1948) and it accounts for situations where the obstacle is not perfectly conducting but the exterior field will not penetrate deeply into the scatterer. For the near electromagnetic field we obtain the low-frequency coefficients of the zeroth and the first orders while in the far field we derive the leading non-vanishing terms for the scattering amplitude and the scattering cross-section. Spherical coated obstacles are very important in applications. Small particles in biological suspensions, cells, some human organs, atmospheric particles and granules within composite materials are only a few examples of applied interest in science and technology.     

Influence of a circular crack on the electromagnetic field in a conductor

The problem of scattering of low-frequency electromagnetic fields on circular cracks is reduced by the method of small parameter to a system of hypersingular integral equations of the Newton-potential type. For the solution of these equations, we propose to use the method of orthogonal polynomials of two variables. It is shown that the electric dipole whose moment is proportional to the normal component of the sounding field is an electrodynamic analog of the crack.     

Low-frequency scattering pattern of an arbitrary penetrable obstacle

We consider the scattering of low-frequency sound waves by an arbitrary penetrable obstacle whose density and compressibility are different from those of the surrounding infinite medium. We present solutions both inside and outside the scatterer. The physical properties of the scatterer are implicit in the transition conditions at its surface and in the interior value of the wave length. This leads to a two-parameter boundary value problem which we convert into an integral equation valid both inside and outside the obstacle. This integral equation is then recast into an infinite system of algebraic equations which are judicially truncated to yield the required solutions.     

Scattering-induced changes in the degree of polarization of a stochastic electromagnetic plane-wave pulse

The scattering of a stochastic electromagnetic plane-wave pulse on a deterministic spherical medium is investigated. An analytical formula for the degree of polarization (DOP) of the scattered field in the far zone is derived. Letting pulse duration T(0) → ∞, our formula can be applied to study the scattering of a stationary stochastic electromagnetic light wave. Numerical results show that the DOP of the far zone field is closely determined by the size of the spherical medium when the incident field is a stochastic electromagnetic plane-wave pulse. This is much different from the case when the incident field is a stationary stochastic electromagnetic light wave, where the DOP of the far zone field is independent of the size of the medium. One may obtain the information of the spherical medium by measuring the scattering-induced changes in the DOP of a stochastic electromagnetic plane-wave pulse.     

基于单频数据重构散射障碍的新型线性采样方法

散射及反散射的数学理论与计算一直是应用数学领域中的重要课题，其成果在地质勘探、无损探测、医学成像等领域都具有广泛的应用．线性采样方法（LSM）是近年来反散射理论中一类非常流行的非迭代型重建算法，但是这种方式很难推广到如半空间中障碍反散射等更为复杂的问题中．本文基于单频数据研究Dirichlet障碍反散射问题的数值重建算法．通过构造带有阻尼边界条件的辅助边值问题，提出了一类新型的线性采样方法，并在理论上严格证明了该方法在任意给定的波数下重构障碍形状及位置的有效性．     

Low-frequency scattering from perfectly conducting spheroidal bodies in a conductive medium with magnetic dipole excitation

Inductive electromagnetic means that are currently employed in the exploration of the Earth’s subsurface and embedded voluminous bodies often call for an intensive use, primary at the modeling stage and later on at the inversion stage, of analytically demanding tools of field calculation. Under the aim of modeling implementation, this contribution is concerned with some interesting aspects of the low-frequency interaction of arbitrarily orientated (i.e. three-dimensional) time-harmonic magnetic dipoles, with 3-D perfectly conducting spheroidal bodies embedded in an otherwise homogeneous conductive medium. For many practical applications involving buried obstacles such as Earth’s subsurface electromagnetic probing at low-frequency or any other physical cases (e.g. geoelectromagnetics), non-axisymmetric spheroidal geometry approximates sufficiently such kind of metallic shapes. On the other hand, our analytical approach deals with prolate spheroids, since the corresponding results for the oblate spheroidal geometry can be readily obtained through a simple transformation. The particular physical model concerns a solid impenetrable (metallic) body under a magnetic dipole excitation, where the scattering boundary value problem is attacked via rigorous low-frequency expansions for the incident, scattered and total electric and magnetic fields in terms of positive integral powers of (ik), that is (ik)ⁿ for n ? 0, where k stands for the complex wavenumber of the exterior medium. The purpose of the modeling is to contribute to a simple yet versatile tool to infer information on an unknown body from measurements of the three-component electric and magnetic fields nearby. Our goal is to obtain the most important terms of the low-frequency expansions of the electromagnetic fields, that is the static (for n = 0) and the dynamic (n = 1, 2, 3) terms. In particular, for n = 1 there are no incident fields and thus no scattered ones, while for n = 0 the Rayleigh electromagnetic expression is easily obtained in terms of infinite series. Emphasis is given on the calculation of the next two non-trivial terms (at n = 2 and at n = 3) of the aforementioned fields. Consequently, those are found in closed form from exact solutions of coupled (at n = 2, to the one at n = 0) or uncoupled (at n = 3) Laplace equations and they are given in compact fashion, as infinite series expansions for n = 2 or finite forms for n = 3. Nevertheless, the difficulty of the Poisson’s equation that has to be solved for n = 2 is presented, whereas our analytical approach demands the use of the well-known cut-off method in order to obtain an analytical closed solution. Finally, this research adds useful reference results to the already ample library of scattering by simple shapes using analytical methods.     

The disturbance of a plane dyadic wave by a small spherical cavity

Dyadic scattering offers a general setting for solving wave-obstacle interaction problems in Continuum Mechanics, because it eliminates the direction of polarization from the scattering formulation. Once the dyadic problem has been solved, any classical scattering problem for the displacement field is recoverable through a contraction with the given polarization. In the present work we solve the scattering problem of a plane dyadic incident field which is disturbed by a spherical cavity in the medium of propagation. The cavity is considered to be small in the sense that its characteristic dimension is much less than the wave length of the incident field. The zeroth and the first order low-frequency approximations of the near field as well as the leading approximation of the far-field (which is of the third order) are obtained explicitly via an appropriate generalization of the Papkovich representation for dyadic fields. The leading approximation of the scattering cross-section is also provided. The results are then used to check the credibility of related vector results obtained from the Boundary Element Method and an amazing coincidence is observed, at least for small enough frequencies.     

A parallel implicit/explicit hybrid time domain method for computational electromagnetics

The numerical solution of Maxwell's curl equations in the time domain is achieved by combining an unstructured mesh finite element algorithm with a cartesian finite difference method. The practical problem area selected to illustrate the application of the approach is the simulation of three‐dimensional electromagnetic wave scattering. The scattering obstacle and the free space region immediately adjacent to it are discretized using an unstructured mesh of linear tetrahedral elements. The remainder of the computational domain is filled with a regular cartesian mesh. These two meshes are overlapped to create a hybrid mesh for the numerical solution. On the cartesian mesh, an explicit finite difference method is adopted and an implicit/explicit finite element formulation is employed on the unstructured mesh. This approach ensures that computational efficiency is maintained if, for any reason, the generated unstructured mesh contains elements of a size much smaller than that required for accurate wave propagation. A perfectly matched layer is added at the artificial far field boundary, created by the truncation of the physical domain prior to the numerical solution. The complete solution approach is parallelized, to enable large‐scale simulations to be effectively performed. Examples are included to demonstrate the numerical performance that can be achieved. Copyright © 2009 John Wiley & Sons, Ltd.     

Electromagnetic scattering by an aggregate of spheres: far field

In electromagnetic multisphere-scattering calculations the reexpansion method for seeking a single-field representation of the total scattered field is found impracticable because of severe numerical problems. We present a simple single-field expansion of the total scattered far field based on an asymptotic form of vector translational addition theorems. With this asymptotic expansion of the far field, we derive analytical expressions for the scattering properties of an arbitrary aggregate of spheres. Resulting formulas are free from numerical problems in practical applications. Theoretical predictions from this far-field solution for various aggregates of spheres that we tested agree favorably with laboratory microwave scattering measurements. Some numerical results are presented and compared with experimental data.     

Optical Conductivity and Pseudo-Momentum Conservation in Anisotropic Fermi Liquids

Umklapp scattering determines the conductivity of clean metals. In typical quasi one-dimensional Fermi liquids with an open Fermi surface, certain pseudo-momenta do not decay by 2-particle collisions even in situations where Umklapp scattering relaxes the momentum of the quasi particles efficiently. Due to this approximate conservation of pseudo-momentum, a certain fraction of the electrical current decays very slowly and a well-pronounced low-frequency peak emerges in the optical conductivity. We develop simple criteria to determine under what conditions approximate pseudomomentum conservation is relevant and calculate within in Fermi liquid theory the weights of the corresponding low-frequency peaks and the temperature dependence of the various relevant decay rates. Based on these considerations, we obtain a qualitative picture of the frequency and temperature dependence of the optical conductivity of an anisotropic Fermi liquid.     

Analytical approximations in multiple scattering of electromagnetic waves by aligned dielectric spheroids

In a dense medium, the failure to properly take into account multiple-scattering effects could lead to significant errors. This has been demonstrated in the past from extensive theoretical, numerical, and experimental studies of electromagnetic wave scattering by densely packed dielectric spheres. Here, electromagnetic wave scattering by densely packed dielectric spheroids with aligned orientation is studied analytically through quasicrystalline approximation (QCA) and QCA with coherent potential (QCA-CP). We assume that the spheroids are electrically small so that single-particle scattering is simple. Low-frequency QCA and QCA-CP solutions are obtained for the average Green's function and the effective permittivity tensor. For QCA-CP, the low-frequency expansion of the uniaxial dyadic Green's function is required. The real parts of the effective permittivities from QCA and QCA-CP are compared with the Maxwell-Garnett mixing formula. QCA gives results identical to those with the mixing formula, while QCA-CP gives slightly higher values. The extinction coefficients from QCA and QCA-CP are compared with results from Monte Carlo simulations. Both QCA and QCA-CP agree well with simulations, although qualitative disagreement is evident at higher fractional volumes.     

Linearized inversion methods for three-dimensional electromagnetic imaging in the multiple scattering regime

In optical or microwave computational tomography, the sample permittivity is reconstructed numerically from the measurements of its scattered field for various illuminations. When the light sample interaction involves multiple scattering, the relationship between the scattered field and the permittivity is non-linear and a direct reconstruction is not possible. Using a simple physical approach, adapted to the three-dimensional vectorial electromagnetic framework, we derive an iterative inversion technique, based on the linearization of the scattering operator, for imaging (possibly anisotropic) targets in the multiple scattering regime. We investigate the performances of different approximations of this operator accounting for more or less multiple scattering. Our method is applied to the reconstruction of targets in the microwave domain using experimental data.     

Influence of the electric current waveform on the dynamic response of the electrified composites

The influence of the electric current waveform (DC, AC and pulsed currents) on the dynamic electromagnetic, thermal, and impact response of the composite plate is studied. The analysis includes solving Maxwell’s equations in the electrified composite plate to determine an electric-current induced magnetic field and heat transfer equation to estimate the electric-current-induced heating. In addition, the dynamic mechanical response of the electrified composite plate subjected to impact and various electromagnetic loads (DC, AC, pulsed electric currents and a constant magnetic field) is analyzed by solving a coupled system of equations of motion and Maxwell’s equations in the composite plate. The results show that the dynamic response of the plate is highly dependent on the characteristics of the electromagnetic field, and the pulsed electromagnetic fields are most effective in reducing vibrations caused by the application of dynamic mechanical loads.     

基于前馈补偿的振动台粒子群迭代学习控制算法

针对电磁式振动台对地震波信号复现精度低及复现过程中迭代次数较多的问题,在建立准确的振动台模型的基础上,提出了基于加速度模型的前馈逆模型补偿方法,主要提高电磁式振动台低频特性.此外,针对迭代学习控制算法在振动台波形复现中收敛速度慢的问题,提出了带遗忘因子的反馈辅助PD型迭代学习算法,并用改进自适应粒子群算法,离线优化控制...     

防爆型三相异步电机中高精度多物理场耦合模型的研究和分析

防爆型电机正常工作过程中存在着电磁场、温度场、流体场以及应力场等多物理场,现阶段针对该方面的研究较多不足之处,如仅基于电磁场而实现电机的设计和制造,因此有必要对电机中的多物理场之间的耦合关系展开深入研究,满足多个技术指标要求。考虑防爆型三相异步电机实际应用性能指标要求,本文提出一种基于多物理场分析理论的防爆型三相异步电机的优化设计方法。首先,围绕电机实际安装尺寸大小,构建电机的几何模型和有限元计算模型;其次,考虑电机的磁场、热传递和流体分析等物理场之间的耦合,为满足通风冷却性能要求,得到电机的温度分布规律、电机热应力和变形分布;最后,对一台1000kW-2p-6kV防爆型三相异步电机进行样机试验,根据试验结果验证了本文提出方法的有效性。     

Scattering of electromagnetic spherical waves by a buried spheroidal perfect conductor

We examine the electromagnetic scattering of spherical waves by a buried spheroidal perfect conductor. The proposed analysis is based on the integral equation formalism of the problem and focuses on the establishment of a multiparametric model describing analytically the scattering process under consideration. Both the theoretical and the numerical treatment are presented. The outcome of the analysis is the determination of the scattered field in the observation environment along with its multivariable on several physical and geometric parameters of the system.     

Scattering of plane electromagnetic waves by a chiral-metal cylinder

The problem of scattering of E-and H-polarized plane electromagnetic waves by a metal cylinder covered with a chiral layer is solved by the method of partial regions. The scattering field is studied in the near and far zones. The correlation between the type of polarization of the incident electromagnetic wave and the magnitude of the depolarized component of the scattered field is considered.     

Development of a telepresence robot with adaptive shared controller

ABSTRACT

This study presents the design and control of a telepresence robot based on an omnidirectional mobile platform. A personal computer is used as a user interface for communication via internet to a laptop computer placed on the mobile platform. The laptop computer receives user’s commands to control the mobile robot. Collisions of robot with obstacle can still occur during remote controlling because of the limitation of a fixed camera on the robot and/or internet latency. We first use an artificial potential field to prevent these collisions, and then design an adaptive shared controller (ASC) to improve the performance. This ASC method modulates the weights of human control and assistance control online not only according to environmental factors, but also based on the user’s control performance. Meanwhile, we use the iterated closest point (ICP) algorithm to build a map for providing user functional reference. The experimental results show that ASC is efficient in both obstacle avoidance and user workload reduction.