新型椭圆形互补隐身斗篷设计

基于变换光学理论和互补媒质理论,提出了新型椭圆形互补隐身斗篷的设计方法,并得到了本构参数张量表达式,利用基于有限元算法的电磁仿真软件对该模型进行了全波仿真验证,结果证实了所得到的表达式的正确性,这种隐身斗篷不仅能够实现对位于其内部的物体隐身,而且电磁波能透进隐身斗篷内部进而可以与外界进行通信。     

二维椭圆形散射转移斗篷的设计研究

基于变换光学理论设计了椭圆形散射转移斗篷,并推导了相应的本构参数张量表达式,根据得到的本构参数,利用基于有限元算法的软件分别对椭圆形介质柱、导体柱进行了全波仿真验证.仿真结果证实了所得本构参数的正确性,验证了散射转移斗篷可以模拟任意物体,包括介质、导体的幻象,这些结果为设计隐身斗篷提供了一种新的理论途径.     

While rotating while cloaking

Invisible optical and thermal cloaking have been explored as the typical demonstrations of the transformation optics and thermotics theory. However, the existing cloaks are realized by only one-coordinate transformation, and the cloaking layout, i.e. the form of electromagnetic wave/heat passing around the invisible region, is single for a long time. Here, we propose a new rotated thermal cloak which can unify the conventional cloaking and rotating together, and realize the while-rotating–while-cloaking effect. The required anisotropic thermal conductivity tensor is deduced from the new geometric mapping. Though rotated, the heat flux can be tuned around the central invisible region perfectly by the proposed rotated thermal cloak. The underlying physics is explored by comprehensive analysis of the distribution of the thermal conductivity tensor, which is further compared with those of the conventional cloak and rotator. The experimental feasibility is also discussed by validating the practical while-rotating–while-cloaking effect through a proof-of-concept design. The proposed rotated thermal cloak is expected to extend the possibility of cloaking scheme, and open avenues for the multiple coordinate transformation in counterpart physical fields, like optics, electrics, acoustics, magnetics, mechanics, etc.     

Multi-window invisible cloaks

This paper reports that a general method of designing invisible cloaks is using variant constitutive material parameters to realize the space transformation. A hollow region can be hidden after this transformation. It was recently shown (Ma H, Qu S B, Xu Z and Wang J F 2009 \wx{Appl. Phys. Lett.}{94} 103501) that when the original point moves to the boundary of a cloak, the cloak can be designed to be open. Based on this theory, we propose multi-window invisible cloaks which can conceal a group of objects. Full wave simulations for invisible cloaks with regular and irregular shapes verified this method.     

Nonsingularity in two-dimensional cylindrical invisible cloaks

The method of designing electromagnetic invisible cloaks is usually based on the form-invariance of Maxwell's equations in coordinate transformation. The exterior boundary of a cylindrical invisible cloak is unchanged and the interior boundary is extended from that of a point to that of a cylindrical region in coordination transformation. This transformation process makes perfect cloaks, but it causes singularity in the constitutive material parameters of cloaks. This singularity makes the cloaks impossible to realize in practice. In order to remove this singularity, this paper sets a small cylindrical region replacing a point in the space transformation. The cylindrical region is so small that it does not affect the invisibility effects, but it can remove the singularity for material parameters. Full wave simulations based on the finite element method were used to verify the designed cloaks.     

A canonical relativistic approach to quantize electromagnetic field in the presence of moving magneto-dielectric media

A canonical relativistic formulation is introduced to quantize electromagnetic field in the presence of a polarizable and magnetizable moving medium. The medium is modeled by a continuum of the second rank antisymmetric tensors in a phenomenological way. The covariant wave equation for the vector potential and the covariant constitutive equation of the medium are obtained as the Euler-Lagrange equations using the Lagrangian of the total system. A fourth rank tensor which couples the electromagnetic field and the medium is introduced. The susceptibility tensor of the medium is obtained in terms of this coupling tensor. The noise polarization tensor is calculated in terms of both the coupling tensor and the ladder operators of the tensors modeling the medium.     

基于Newmark算法的任意磁化方向铁氧体电磁散射时域有限差分分析

利用坐标系转换矩阵给出实验室坐标系中饱和磁化铁氧体的频域磁导系数张量,再通过频域到时域的转换关系jω→?/?t得到一个二阶微分方程形式的时域本构关系.然后采用Newmark方法求解时域本构关系从而给出一种适用于处理任意磁化方向铁氧体电磁问题的Newmark时域有限差分算法.利用此算法计算了饱和磁化铁氧体层的反(透)射系数和饱和磁化铁氧体球的后向雷达散射截面,所获得的结果验证了此算法的正确有效性.     

Exact interior solutions in 2+1-dimensional spacetime

An exact formulation of two-dimensional chiral hydrodynamics with diffeomorphism and conformal anomalies is provided. The constitutive relation involving the stress tensor is computed. It reveals a one parameter class of solutions which is a new result. For a particular value of this parameter, the results found in the gradient expansion scheme are reproduced. Moreover, the constitutive relation is analogous to the corresponding relation for an ideal fluid, appropriately modified to include the chirality property, which has also been derived here.     

有限引导磁场下相对论环形电子注色散特性的研究

采用等效媒质处理方法来研究有限引导磁场下沿纵向运动的相对论环形电子注.首先建立运动坐标系以电子注纵向速度匀速运动,在运动坐标系中电子注可以被考虑成静止的磁化等离子体,再通过四维空间的洛伦兹变换得到电子注在静止的实验室坐标系下可以被等效为双各向异性媒质,其不仅具有张量形式的电导率和磁导率,还具有手征特性.在此基础上同时考虑了由于电子注表面波动所引起的表面电流密度.采用该方法研究了有限引导磁场下圆柱波导中沿纵向运动的相对论环形电子注,推导出该模型的色散方程,并进行了数值计算.计算结果表明该研究方法能够得到更准 关键词： 相对论环形电子注 磁化等离子体 色散特性     

考虑相变的热弹塑性本构方程及其应用

将在热加工及热处理过程中发生相变的材料考虑为热弹塑性的多相混合物,基于连续介质物理学及含内变量的不可逆热力学,推导出考虑相变的热弹塑性本构方程.根据一组不同温度水平下的高温短时拉力试验,确定本构方程中的材料参数及其随温度的变化关系.同时,提出了考虑相变的热传导方程和计及应力影响的相变动力学方程.基于这些方程编制相应有限元分析程序,用其分析计算了1Cr12WMoV不锈钢管通的焊接残余应力分布.计算所得结果与通过X射线衍射测试的实验结果具有较好的一致性. 关键词： 相变 热弹塑性 本构方程 焊接     

Non-newtonian viscosity and normal pressure associated with the flow alignment in macromolecular liquids

Non-linear constitutive laws previously obtained within the framework of irreversible thermodynamics are applied to the calculation of the flow alignment and the friction pressure tensor of macromolecular liquids. A non-newtonian viscosity is obtained which decreases with increasing magnitude of the velocity gradient. For rigid molecules it reaches a finite asymptotic value, viz. the second newtonian viscosity. The friction pressure tensor contains normal components. The magnitude of this normal pressure is closely related to the change of the viscosity caused by the velocity gradient. Flow alignment and the pressure tensor are also studied for time-dependent (alternating) velocity gradients.     

基于变换光学的椭圆形透明聚集器的设计研究

基于麦克斯韦方程组在不同坐标系下具有形式不变性以及变换光学理论, 通过设计材料的本构参数(介电常数和磁导率)来引导电磁波的传播, 提出了具有电磁透明和电磁聚集两种功能的新型电磁器件-椭圆形透明聚集器的设计方法. 电磁波透明体不会阻挡电磁波的传播并且能够与斗篷内部进行交互; 电磁波聚集器是当电磁波入射到该装置上时, 电磁波能够被设计的装置按照要求集中到一个区域或者一个点, 实现电磁波能量的集中. 本文利用压缩变换和扩展变换推导出了这种电磁器件中各层的相对介电常数和相对磁导率的张量表达式, 并利用基于有限元算法的电磁仿真软件对该电磁器件进行了全波仿真验证, 得到了入射波从各个不同方向入射时磁场z 分量的分布图, 仿真结果证实了该设计方法和电磁参数的正确性. 最后还讨论了电磁器件存在损耗时的情况, 当损耗逐渐增大时, 器件的功能在一定程度上受到了削弱. 本文的设计方法为其他新型电磁器件的设计提供了一种新的思路.     

Sphere-based calibration method for trinocular vision sensor

A new method to calibrate a trinocular vision sensor is proposed and two main tasks are finished in this paper, i.e. to determine the transformation matrix between each two cameras and the trifocal tensor of the trinocular vision sensor. A flexible sphere target with several spherical circles is designed. As the isotropy of a sphere, trifocal tensor of the three cameras can be determined exactly from the feature on the sphere target. Then the fundamental matrix between each two cameras can be obtained. Easily, compatible rotation matrix and translation matrix can be deduced base on the singular value decomposition of the fundamental matrix. In our proposed calibration method, image points are not requested one-to-one correspondence. When image points locates in the same feature are obtained, the transformation matrix between each two cameras with the trifocal tensor of trinocular vision sensor can be determined. Experiment results show that the proposed calibration method can obtain precise results, including measurement and matching results. The root mean square error of distance is 0.026 mm with regard to the view field of about 200×200 mm and the feature matching of three images is strict. As a sphere projection is not concerned with its orientation, the calibration method is robust and with an easy operation. Moreover, our calibration method also provides a new approach to obtain the trifocal tensor.     

一种有场旋转和集中功能的新型多功能电磁器件的设计与研究

基于变换光学理论设计了一种新型的多功能电磁器件—旋转集中器.它以特殊的方式引导电磁波使其传播方向在器件核心区发生指定角度的旋转,并同时实现电磁场能量向该核心区的集中.针对提出的三种等效的旋转集中器结构,分别推导了相应的本构参数表达式,并利用有限元软件对三种结构分别进行了全波仿真.仿真结果验证了本构参数表达式的正确性.这三种不同的结构中,前两种结构由三层介质构成,后一种简化为两层介质.对于给定的任意旋转角度和能量集中率,三种结构可以使电磁场发生等效的传播方向旋转和能量集中两种效果.这些结果有助于对旋转器和集中器机理的进一步理解,并为复合功能器件的设计奠定了更充分的理论基础.本文提出的旋转集中器在设计高效率接收天线和特殊电路封装互联器件等方面都有潜在的应用前景.     

Nonlinear electromechanical analysis: a constitutive theory for perovskite type ferroelectrics

A constitutive theory is established in this paper to describe the nonlinear electromechanical behavior of perovskite type ferroelectrics subjected to external stress and external electric field. In the proposed theory, each domain is considered as an inclusion. The Helmholtz free energy and Gibbs free energy of a constituent element, that are derived by using micromechanics approaches, are functionals of the orientation distribution function (ODF) that depicts the domain distribution patterns. By applying the internal variable theory and expanding ODF in Fourier series, the yielding condition, the evolution of ODF, and the constitutive relation are obtained. In terms of the simplification of the constitutive relation, theoretical predictions are compared with experimental results. There is an agreement between the theoretical and the experimental results, indicating that the theory is reasonable and applicable. It should be pointed out that the constitutive model proposed in this paper is restricted to ferroelectric materials exhibiting transformation from cubic to tetragonal only.     

Motion decomposition,frame-indifferent derivatives,and constitutive relations at large displacement gradients from the viewpoint of multilevel modeling

Widespread approaches to generalizing geometrically linear constitutive relations to the case of large displacement gradients have been considered. These approaches are based on the replacement of the material derivatives of stress and strain tensors by frame-indifferent corotational or convective derivatives. The correctness of choosing the indifferent derivatives is analyzed from a more general viewpoint of motion decomposition into rigid and strain-induced motion. It is shown that the use of the Zaremba-Jaumann derivative in constitutive relations corresponds to motion decomposition by the Cauchy-Helmholtz theorem according to which instantaneous rigid rotation of a material particle with small neighborhood is described by the vorticity tensor. The relations derived with the use of the so-called "logarithmic spin" are analyzed. It is noted that the spin tensors entering into these relations are not associated with the material fibers (in particular with the symmetry axes of anisotropic materials) during the entire studied process of deformation. Hence these spins do not describe the rotation of the reference frame (crystallographic one for metals) in which the material property tensor is defined. A new method of motion decomposition is proposed on the basis of a two-level (macro and meso) approach for single and polycrystalline metals. The mesoscopic spin is determined by the rotation rate of the corotational coordinate system associated with the crystallographic direction and crystallographic plane. Mesoscopic constitutive relations are formulated using the proposed spin. The spin of a representative macrovolume is determined by averaging the spins of the crystallites contained in this volume. This spin is used to formulate rate-type elastic constitutive equations. Examples are given to illustrate the stress state determination for loading along closed strain paths and two-segment paths for isotropic and anisotropic (with cubic symmetry, hcp) elastic materials, and an elastoviscoplastic fcc crystallite. The determination is carried out by using the corotational derivatives in the constitutive relations which are obtained by different motion decomposition methods.     

Bandwidth evaluation of dispersive transformation electromagnetics based devices

In this paper, the transient responses of some devices which are based on transformation electromagnetics are studied, such as invisible cloaks and concentrators, by using the Finite-Difference Time-Domain (FDTD) numerical technique. In particular, effects of the inherent losses as well as the coating size of the ideal cylindrical cloak on its bandwidth and cloaking performance are examined. In addition, it is demonstrated that the performance of transformation electromagnetics based devices is affected by the material parameters in the design, although they may behave nicely under monochromatic plane wave illuminations. The obtained results are of interest for the future practical implementation of these structures.     

THE EQUIVALENCE OF TWO TRANSFORMATIONS OF THE INVARIANCE OF LAGRANGIAN FUNCTION IN NON-ABELIAN GAUGE FIELD

In this paper,the general transformation of non-abelian gauge field is derived from the in variance reguirement of the effective lagrangian function.It is shown that when the group parameter θ(x) is the product of the anti-commuting ghost field and an infinitesimal number anticommuting ξ which is independent of the component indices space,the B.R.S.transformation is obtained.When θ(x) is equal to the ghost field C(x) times an infinitesimal tensor ξ_i which is commuting with respect to the upper index b,we get another transformation.Both transformations are proved to be equivalent.     

A framework for quality control and parameter optimization in diffusion tensor imaging: theoretical analysis and validation

In this communication, a theoretical framework for quality control and parameter optimization in diffusion tensor imaging (DTI) is presented and validated. The approach is based on the analytical error propagation of the mean diffusivity (D(av)) obtained directly from the diffusion-weighted data acquired using rotationally invariant and uniformly distributed icosahedral encoding schemes. The error propagation of a recently described and validated cylindrical tensor model is further extrapolated to the spherical tensor case (diffusion anisotropy approximately 0) to relate analytically the precision error in fractional tensor anisotropy (FA) with the mean diffusion-to-noise ratio (DNR). The approach provided simple analytical and empirical quality control measures for optimization of diffusion parameter space in an isotropic medium that can be tested using widely available water phantoms.