2.5维模拟中时域有限差分近-远场变换方法

 为进行从高功率微波源到天线辐射整个系统的2.5维模拟,在近场模拟的基础上对2.5维模拟中时域有限差分(FDTD)的近-远场变换方法进行了研究。先将2.5维近场模拟区的输出边界绕对称轴旋转一周,得到封闭的3维圆柱面;然后根据等效原理由该封闭面上的FDTD近场求得等效电磁流;再根据自由空间中电流及磁流的时域辐射场公式计算出电磁流的远区辐射场。给出了2.5维模拟中TM01模式及TE01模式激励下圆锥喇叭远区辐射算例,所得计算结果与3维全电磁模拟软件CST的计算结果符合较好。     

基于半空间FDTD的近远场外推方法: TE情形

研究基于半空间时域有限差分(FDTD)计算中TE情形的远区散射场计算方法.先将半空间Green函数代入势函数,推导远区纵向磁场的计算公式,对各空间内电磁流和对应的相位进行详细分析.再计算两个半空间模型的远区散射场,结果表明:该方法是正确和有效的.针对日本96式装甲车的计算发现,单站雷达探测系统位于战车正上方时具有最好的探测效果.     

基于时域有限差分法的液晶光学特性模拟

研究了模拟光在液晶中传播的时域有限差分(FDTD)法。该方法采用完全匹配层吸收边界条件来截断计算区域,通过独立计算总场区域与散射场区域的左连接边界和右连接边界上的入射场场值来实现非周期结构的平面波波源引入,并提出了分裂场一维辅助FDTD方法来计算含有非均匀各向异性介质的边界上的入射场场值。实现了平面波源入射下具有非周期结构的液晶器件光学特性计算。对开态下扭曲向列相液晶盒的仿真结果表明,30°入射时散射区透射率的最大泄漏误差小于3%,可知该方法对非周期、非均匀各向异性介质光学特性分析是有效的。     

垂直极化平行板有界波电磁脉冲模拟器辐射近场的快速估算方法

为快速估算出垂直极化平行板有界波电磁脉冲(EMP)模拟器的时域近场,将散射传递函数法应用于该类型模拟器近场的时域计算中,即对于给定的脉冲源,先寻找有效频谱范围能覆盖该源的高斯脉冲源,并应用时域有限差分(FDTD)方法计算该高斯脉冲源激励时模拟器中测试点场的时域响应,再利用傅里叶变换、系统的传递函数及傅里叶逆变换计算得到给定脉冲源激励时各测试点场的瞬态响应。所得计算结果与直接使用给定脉冲源激励时FDTD方法的计算结果符合较好。所述方法可用于同一模拟器在不同脉冲源激励时辐射近场的快速估算,能大大减少FDTD模拟计算的次数,尤其对于中大型模拟器能有效减少计算时间和内存。     

色散周期结构的辅助场时域有限差分法分析

利用辅助场时域有限差分(FDTD)方法分析色散周期结构的斜入射问题. 主要基于Floquet定理, 在FDTD迭代式中引入辅助场量, 结合变换的周期及吸收边界条件,解决了周期结构斜入射问题的电磁特性仿真. 进一步将辅助场FDTD方法引入至色散周期结构的模拟, 结合Z变换方法,给出了基于Drude色散模型的具体迭代公式. 数值计算结果表明所构建方法的有效性及普适应.     

用基于Daubechies尺度函数的时域多分辨分析计算电磁散射

利用Daubechies尺度函数的紧支撑性，将一种新的时域多分辨分析(MRTD)算法应用到三维目标的电磁散射中并分析了其色散特性，使用MRTD/FDTD接口技术解决了连接和吸收边界的处理.算例表明，在不牺牲精度的情况下，该方法比传统的FDTD需要的网格数少，计算速度快.最后，计算了二维PBG结构的反射特性. 关键词： 时域多分辨分析 Daubechies尺度函数 电磁散射     

分析水下目标的大比例变换总场散射场源FDTD方法

为解决探测水下目标的电磁散射问题,提出大比例变换的总场-散射场源时域有限差分（FDTD）方法.该方法包含两次FDTD计算：第一次计算采用细网格得到激励源周围的近场值;第二次计算采用粗网格得到远距离的电磁场值.两次FDTD计算通过总场-散射场边界建立联系.实现细粗网格的大比例变换,例如变换比例N=10,大大节省了计算时间,降低了计算内存的消耗,提高了计算效率.通过算例验证该方法的正确性和有效性.最后,计算水下岩层中存在异常体时的电磁响应,指出当岩层中异常体电导率不同时,接收点处电磁场的幅值和相位均不相同.     

大耦合孔同轴输出腔的3维解析分析

 在高功率微波器件中，通常采用大耦合孔的同轴微波输出腔，该腔为3维结构。采用场等效原理将腔体、耦合孔及其输出负载(行波边界)进行分区,每个区域中的场由界面上的磁流密度决定。采用格林函数积分法可得每个区域中的3维场,再由各个区域的场匹配方程求得腔体的谐振频率、特性阻抗、有载品质因数、模式分布等参数。为腔体的计算和设计提供一种计算模型。     

一种新时域交替隐式差分算法在散射问题中的应用

提出一种新时域交替隐式有限差分(ADI-FDTD)算法格式. 传统ADI-FDTD算法的 场量步进方程涉及周围若干网格的较多场量，导致两个区域的步进方程处理较困难：一个是 邻近完全匹配层(PML)和散射场交界区，另一个是邻近连接边界区. 特别是后者，考虑入射 波影响需对场量所在区域判断，根据不同情况对原有方程进行修正，一维和二维散射问题 相对简单，可三维问题修正式有数十种之多而几乎无法完成. 本方法基于分裂场形式的ADI- FDTD技术，使得散射场区和PML吸收层区的表达形式完全一致，从而忽略两者差别.另 关键词： 时域交替隐式有限差分算法 电磁散射     

1维介质光栅近场及其衍射的FDTD分析

 应用FDTD方法计算了单色平面波斜入射时1维介质光栅的近场，进而求出介质光栅的衍射效率。借助于周期边界条件，整个计算区域为周期结构的一个单元。考虑入射波在两种介质界面上会产生反射和透射，故在总场边界上引入入射波、反射波和透射波。给出了光栅单元截面为矩形和梯形的算例。该方法可用于斜入射情形下具有复杂结构和任意单元截面的介质栅近场及其衍射特性分析。     

晶间界面附近区域中的弹性场——各向异性理论

根据现代关于晶间界面的研究可知,晶界和相界多具有二维周期结构。由此出发,采用Fourier级数展开的表示,在给定了界面结构,在数学上即是形式地给定了边界条件的情况下,求出了两个各向异性物相间相界附近的弹性场。并以Ni中扭转晶界为例,作了应力场的数值计算。本文的方法和方程对于研究各种类型的晶界和相界均普遍可用,为处理这类问题提供了一种有效的简捷手段。 关键词：     

New boundary conformal field theories indexed by the simply laced Lie algebras

We consider the field theory of N massless bosons which are free except for an interaction localized on the boundary of their (1+1)-dimensional world. The boundary action is the sum of two pieces: a periodic potential and a coupling to a uniform abelian gauge field. Such models arise in open-string theory and dissipative quantum mechanics, and possibly in edge state tunneling in the fractional quantized Hall effect. We explicitly show that conformal invariance is unbroken for certain special choices of the gauge field and the periodic potential. These special cases are naturally indexed by semi-simple, simply laced Lie algebras. For each such algebra, we have a discrete series of conformally invariant theories where the potential and gauge field are conveniently given in terms of the weight lattice of the algebra. We compute the exact boundary state for these theories, which explicitly shows the group structure. The partition function and correlation functions are easily computed using the boundary state result.     

Continuous‐wave vertically emitting photonic crystal terahertz laser

Compact semiconductor light sources with high performance continuous‐wave (CW) and single mode operation are highly demanded for many applications in the terahertz (THz) frequency range. Distributed feedback (DFB) and photonic crystal (PhC) quantum cascade (QC) lasers are amongst the leading candidates in this field. Absorbing boundary condition is a commonly used method to control the optical performance of a laser in double‐metal confinement. However, this approach increases the total loss in the device and results in a large threshold current density, limiting the CW maximum output power and operating temperature. In this letter, a robust surface emitting continuous‐wave terahertz QC laser is realized in a two‐dimensional PhC structure by a second order Bragg grating extractor that simultaneously provides the boundary condition necessary for mode selection. This results in a 3.12 THz single mode CW operation with a 3 mW output power and a maximum operation temperature (T_max) of 100 K. Also, a highly collimated far‐field pattern is demonstrated, which is an important step towards real world applications.     

基于时空滤波理论的低频声场干涉结构研究

基于时空滤波理论, 采用相干函数、功率响应函数、扩展分布函数、双频函数四个系统函数较波导不变量更全面地描述了低频声场的相干结构. 理论分析、仿真研究和海试数据处理都验证了低频声场存在稳定的干涉结构, 当目标由远及近、又由近及远做匀速直线运动时, 相干函数(LOFAR图)的干涉结构为一簇类双曲线; 功率响应函数的干涉图表征了简正波群时延的差分, 体现了其频散特性; 扩展分布函数能反映LOFAR图上干涉条纹的斜率大小; 而双频函数能体现相慢度的差分. 各个系统函数均能特别突出干涉结构的某方面的特征, 各有特色.     

Computational and experimental investigation of the fields generated by a 1-3 piezocomposite transducer

Large-scale three-dimensional numerical simulations using the finite-difference time domain technique are used to compute the continuous wave fields associated with a composite transducer. The interior of the transducer is made of a periodic array of square rods. This lattice causes elastic wave Bragg diffraction similar to electrons in a periodic lattice. A low frequency mode shape is assumed for the rods. This prescribed motion includes longitudinal and transverse components. It is shown that the transverse motion in the rod gives rise to shear waves causing standing waves (lateral resonances) in the polymer regions. This is also confirmed by experimental results presented here and other independent analytical and experimental work. The full-scale numerical simulation is performed on a large parallel supercomputer and permits modeling of not only the composite transducer but the radiated pressure from near to far field. In addition, cover plates and edge effects are included, unlike analytical treatments. Although only mechanical effects are included, the wave propagation approach captures many essential features.     

A finite element scheme to study the nonlinear optical response of a finite grating without and with defect

We present a simple numerical scheme based on the finite element method (FEM) using transparent-influx boundary conditions to study the nonlinear optical response of a finite one-dimensional grating with Kerr medium. Restricting first to the linear case, we improve the standard FEM to get a fourth order accurate scheme maintaining a symmetric-tridiagonal structure of the finite element matrix. For the full nonlinear equation, we implement the improved FEM for the linear part and a standard FEM for the nonlinear part. The resulting nonlinear system of equations is solved using a weighted-averaged fixed-point iterative method combined with a continuation method. To illustrate the method, we study a periodic structure without and with defect and show that the method has no problem with large nonlinear effect. The method is also found to be able to show the optical bistability behavior of the ideal and the defect structure as a function of either the frequency or the intensity of the input light. The bistability of the ideal periodic structure can be obtained by tuning the frequency to a value close to the bottom or top linear band-edge while that of the defect structure can be produced using a frequency near the defect mode or near the bottom of the linear band-edge. The threshold value can be reduced by increasing the number of layer periods. We found that the threshold needed for the defect structure is much lower then that for a strictly periodic structure of the same length.     

Causal impulse response for circular sources in viscous media

The causal impulse response of the velocity potential for the Stokes wave equation is derived for calculations of transient velocity potential fields generated by circular pistons in viscous media. The causal Green's function is numerically verified using the material impulse response function approach. The causal, lossy impulse response for a baffled circular piston is then calculated within the near field and the far field regions using expressions previously derived for the fast near field method. Transient velocity potential fields in viscous media are computed with the causal, lossy impulse response and compared to results obtained with the lossless impulse response. The numerical error in the computed velocity potential field is quantitatively analyzed for a range of viscous relaxation times and piston radii. Results show that the largest errors are generated in locations near the piston face and for large relaxation times, and errors are relatively small otherwise. Unlike previous frequency-domain methods that require numerical inverse Fourier transforms for the evaluation of the lossy impulse response, the present approach calculates the lossy impulse response directly in the time domain. The results indicate that this causal impulse response is ideal for time-domain calculations that simultaneously account for diffraction and quadratic frequency-dependent attenuation in viscous media.     

Computational simulation of wave propagation problems in infinite domains

This paper deals with the computational simulation of both scalar wave and vector wave propagation problems in infinite domains. Due to its advantages in simulating complicated geometry and complex material properties, the finite element method is used to simulate the near field of a wave propagation problem involving an infinite domain. To avoid wave reflection and refraction at the common boundary between the near field and the far field of an infinite domain, we have to use some special treatments to this boundary. For a wave radiation problem, a wave absorbing boundary can be applied to the common boundary between the near field and the far field of an infinite domain, while for a wave scattering problem, the dynamic infinite element can be used to propagate the incident wave from the near field to the far field of the infinite domain. For the sake of illustrating how these two different approaches are used to simulate the effect of the far field, a mathematical expression for a wave absorbing boundary of high-order accuracy is derived from a two-dimensional scalar wave radiation problem in an infinite domain, while the detailed mathematical formulation of the dynamic infinite element is derived from a two-dimensional vector wave scattering problem in an infinite domain. Finally, the coupled method of finite elements and dynamic infinite elements is used to investigate the effects of topographical conditions on the free field motion along the surface of a canyon.     

Dielectric response of periodic systems from quantum Monte Carlo calculations

We present a novel approach that allows us to calculate the dielectric response of periodic systems in the quantum Monte Carlo formalism. We employ a many-body generalization for the electric-enthalpy functional, where the coupling with the field is expressed via the Berry-phase formulation for the macroscopic polarization. A self-consistent local Hamiltonian then determines the ground-state wave function, allowing for accurate diffusion quantum Monte Carlo calculations where the polarization's fixed point is estimated from the average on an iterative sequence, sampled via forward walking. This approach has been validated for the case of an isolated hydrogen atom and then applied to a periodic system, to calculate the dielectric susceptibility of molecular-hydrogen chains. The results found are in excellent agreement with the best estimates obtained from the extrapolation of quantum-chemistry calculations.     

Implementation of nonreflecting boundary conditions for the nonlinear Euler equations

Computationally efficient nonreflecting boundary conditions are derived for the Euler equations with acoustic, entropic and vortical inflow disturbances. The formulation linearizes the Euler equations near the inlet/outlet boundaries and expands the solution in terms of Fourier–Bessel modes. This leads to an ‘exact’ nonreflecting boundary condition, local in space but nonlocal in time, for each Fourier–Bessel mode of the perturbation pressure. The perturbation velocity and density are then calculated using acoustic, entropic and vortical mode splitting. Extension of the boundary conditions to nonuniform swirling flows is presented for the narrow annulus limit which is relevant to many aeroacoustic problems. The boundary conditions are implemented for the nonlinear Euler equations which are solved in space using the finite volume approximation and integrated in time using a MacCormack scheme. Two test problems are carried out: propagation of acoustic waves in an annular duct and the scattering of a vortical wave by a cascade. Comparison between the present exact conditions and commonly used approximate local boundary conditions is made. Results show that, unlike the local boundary conditions whose accuracy depends on the group velocity of the scattered waves, the present conditions give accurate solutions for a range of problems that have a wide array of group velocities. Results also show that this approach leads to a significant savings in computational time and memory by obviating the need to store the pressure field and calculate the nonlocal convolution integral at each point in the inlet and exit boundaries.