一维周期结构散射近-远场外推的两种方法

 首先讨论了自由空间中一维周期结构近-远场外推的Floquet模方法,即对FDTD计算所得散射近场中输出边界上散射场进行级数展开,求出各阶Floquet模的复数幅值,再求得远区场。接着介绍了求解一维周期结构远区散射场的周期Green函数方法,即根据FDTD计算所得散射近场中输出边界上散射场,求得等效面电磁流后,再借助周期Green函数进行外推得到远区场。两种方法均仅用一个周期单元内的散射近场进行外推。计算结果验证了上述两种方法外推的有效性。     

大型垂直极化EMP辐射波模拟器时域辐射近场的快速预估

将二维轴对称柱坐标系中基于MPI平台的并行FDTD （MPI-FDTD）方法与基于基尔霍夫表面积分表达式（KSIR）的场变换相结合,实现"MPI-FDTD+MPI-KSIR"的混合并行技术,并应用于无电阻加载的大型垂直极化电磁脉冲（EMP）辐射波模拟器时域辐射近场的快速预估.验证算例所得辐射近场不仅与商用软件及并行FDTD的整体模拟符合很好,且与并行FDTD整体模拟相比可以节约80%的计算时间.给出并行加速比和并行效率的测试结果.该方法可用于其它大型垂直极化EMP辐射波模拟器时域辐射近场和辐射远场的快速预估.     

口径天线冲激脉冲辐射场方向性的时域分析

为分析口径天线辐射场方向性,在计算等效面磁流的同时,引入等效面电流的作用,对辐射远场方向性进行了修正,解析公式更适合描述自由空间中口径天线的辐射场;在时间域对冲激脉冲口径辐射场方向性进行分析,得到以下结论:馈入脉冲宽度变宽、口径尺寸减小、口径场幅度锥削分布,以及球面波前均会降低辐射场方向性;辐射场存在时域波形延拓现象,即存在空间色散问题;球面波使口径辐射场方向性迅速恶化,在远区得到一个主轴附近峰值场强相对平坦,而在一定角度峰值场强迅速下降的场。     

JEMS-FDTD软件在飞机HIRF仿真中的应用

高强辐射场防护逐渐受到重视,并成为飞机设计和机载设备装机的必要条件。将自主研发的大规模并行三维时域全波电磁模拟软件JEMS-FDTD应用于飞机高强辐射场(HIRF)仿真。数值模拟中为保证计算精度,采用了非均匀网格及共形网格技术。根据实验条件,平面波近似与实验条件差异较大,需考虑天线近场效应,故构建了球面波等效源。给出了平面波近似及球面波近似下的仿真结果与实验结果的对比,结果显示,球面波近似与实验结果更加吻合。在完成数值模拟的基础上,采用PFC方法对仿真结果与实验结果的吻合度进行了评估,评估等级与欧盟HIRF-SE项目定级相当。     

大型双反射面天线近轴区辐射FDTD-TDPO模拟

针对大型双反射面天线尺寸过大、使用单一的时域方法无法单独模拟其远区辐射特性,以及使用单一的高频近似技术无法给出准确结果的局限,给出了用于大型双反射面天线时域辐射模拟的并行时域有限差分(PFDTD)-并行时域物理光学(PTDPO)混合方法。先创建两个用于并行计算的进程组,一个用于PFDTD计算,另一个用于PTDPO计算; 再根据PFDTD方法计算出双反射面天线馈源的近磁场,同时根据这些磁场使用基尔霍夫表面积分表达式(KSIR)并行计算出次反射面上及其附近的磁场,再使用KSIR并行计算出主反射面上的磁场; 最后使用PTDPO方法计算出双反射面天线的远区辐射场的瞬态响应。同时,还给出主反射面口径为40个波长的Cassegrain双反射面天线的算例,并对馈源相同而口径尺寸不同的双反射面天线进行近轴区的时域辐射模拟。     

碳纤维结构对其辐射光谱性质的影响研究

为了研究碳纤维类高温防隔热复合材料中热辐射传输特性及对其传热性能的影响,采用数值模拟的方法,针对2维(2-D),2.5维(2.5-D)编织的碳纤维复合材料的光谱辐射特性进行了研究。通过对比时域有限差分法(FDTD)和Mie氏理论计算的微米级尺度下单根无限长圆柱的散射强度分布,验证FDTD计算碳纤维热辐射特性的正确性。采用FDTD分析了编织方式、纤维特征尺寸、纵向层数、纤维间隔及基质折射率对其辐射性质的影响。研究发现,均质块材与实际编织结构的块材光谱辐射特性差异明显,散射强度分布差异可达2个量级。     

电磁脉冲对金属板搭接缝隙耦合效应分析

 针对在缝隙耦合研究中,对金属板接合处搭接缝隙电磁特性的讨论相对较少的情况,应用时域有限差分方法模拟了强电磁脉冲对搭接缝隙的耦合穿透过程,计算了缝隙口径面等效面磁流和进入另一空间的透射场。为便于由等效磁流外推透射场,应用等效原理将电磁流在1/4空间的辐射等效为电磁流及其镜像在自由空间的辐射,避免了计算搭接缝隙边界情形下空间格林函数的困难。外推时采用分区外推法节省了计算时间。最后通过分析耦合场的变化曲线,总结了耦合共振规律。     

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。     

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.     

JEMS-FDTD软件在运输机电磁特性仿真中的应用

复杂电磁环境的研究包括多方面内容,其中飞行器的电磁环境效应研究受到了较多的关注。随着计算机技术的飞速发展,利用数值模拟技术获得飞行器电磁特性已经成为飞行器电磁环境效应研究的一个重要手段。我们将自主研发的大规模并行三维时域全波电磁模拟软件JEMS-FDTD应用于运输机整机电磁特性仿真,计算并分析了飞机的电磁耦合散射特性,获得了飞行器在电磁脉冲照射下的时域/频域、近场/远场等电磁信息。计算中为保证精度采用了非均匀以及高阶FDTD计算技术。     

2.5维圆波导开放边界截断的修正完全匹配层方法

 在时域有限差分算法中，完全匹配层是一种高效的吸收边界条件。通过在标准完全匹配层(PML)方程中添加具有物理意义的电导率和导磁率附加项，提出了修正完全匹配层(MPML)方法，给出了圆柱坐标下3维MPML方程，并应用于2.5维圆波导开放边界的截断计算。计算结果表明，在不同频率和模式的激励下，加入平滑电导率和导磁率附加项后，与标准PML方法相比，MPML方法的相对误差降低8~10 dB。     

NUMERICAL SIMULATION OF OPEN WAVEGUIDE CONVERTERS USING FDTD METHOD

We study 3-dimensional asymmetric diffraction problems for waveguide-based electro-dynamic systems, radiating to infinite free space. For calculations we utilize the FDTD (Finite Difference Time Domain) numerical simulation method with the UPML (Unsplit Perfectly Matched Layer) absorbing boundary conditions. This paper states that the FDTD method, in spite of its relatively low calculation speed, has an approved ability of solving certain problems that cannot be solved by the other traditional numerical simulation methods (the method of integral equation, the method of scattering matrix).     

A Locally Conformal Algorithm for Handling Curved Metallic Surfaces and Its Application

This paper describes a modified locally conformal algorithm for finite-difference time-domain (FDTD) method. Fields in the entire computational domain are computed by a regular FDTD algorithm except those near curved metallic surfaces, where special techniques proposed in this paper are applied. The computation efficiency of a regular FDTD method is maintained while a high space-resolution is obtained by this new algorithm. To validate the reliability of the algorithm, coaxial continuous transverse stub arrays at millimeter wave Ka-band and microwave X-band are tested, and the simulated results show good agreement with the experimental results from an HP-8510B Network Analyzer and the simulation results from software package HFSS.     

Finite-difference time-domain modeling of curved material interfaces by using boundary condition equations method

To deal with the staircase approximation problem in the standard finite-difference time-domain(FDTD) simulation,the two-dimensional boundary condition equations(BCE) method is proposed in this paper.In the BCE method,the standard FDTD algorithm can be used as usual,and the curved surface is treated by adding the boundary condition equations.Thus,while maintaining the simplicity and computational efficiency of the standard FDTD algorithm,the BCE method can solve the staircase approximation problem.The BCE method is validated by analyzing near field and far field scattering properties of the PEC and dielectric cylinders.The results show that the BCE method can maintain a second-order accuracy by eliminating the staircase approximation errors.Moreover,the results of the BCE method show good accuracy for cylinder scattering cases with different permittivities.     

Tunable optical switch based on two orthogonal line defect waveguides

The transmission characteristics of a 2-dimensional (2D) square lattice photonic crystal waveguide (PCW) with an additional perpendicular line defect are investigated by the plane wave expansion method (PWM) and the finite difference time domain (FDTD) method. From simulation results and theoretical analysis, it is found that changing the refractive index of the dielectric columns in the additional line defect can effectively adjust the light transmission through the PCW. The working mechanism can be boiled down to either the excitation of a single defect mode or the contra-directional coupling of the two defect modes. And this mechanism can be used to design other tunable optical devices.     

电磁场2.5维时域间断有限元方法

介质沿空间固定方向均匀分布的结构在电磁导波器件中有十分广泛的应用,对这类器件的分析通常被称为2.5D电磁问题。利用器件在固定方向介质分布均匀的特点,将电磁场量沿该方向进行空间傅里叶变换,可以把对三维问题的分析转化为两维问题求解,从而极大地减小计算开销。针对传统基于差分的2.5D电磁场算法在弯曲形状逼近上有阶梯误差的缺陷,本文提出了基于三角形网格的2.5D时域间断有限元方法（DGTD）,并用它模拟了电偶极子与光纤的耦合效率和光子晶体光纤的色散特性。与基于规则网格的2.5D差分方法进行对比。结果表明,文中建立的2.5D DGTD方法对弯曲形状的模拟更加逼真,计算内存占用最大减少10.4%,计算精度最大相差0.011%,计算时间缩短74.9%,计算效率提高。     

螺旋线行波管电子效率改善的数值模拟

 运用全电磁2.5维粒子模拟软件MAGIC和3维高频结构仿真软件HFSS，模拟仿真了正-负双跳变慢波结构对螺旋线行波管电子效率的改善，从慢波系统高频参量、非同步参量、增益参量和电子注相位等角度进行了讨论。设计并加工了一支采用双跳变结构的螺旋线行波管，实测结果表明：采用双跳变结构后可将电子效率从19%提高到29%；模拟仿真方法得到的平均相对误差为5%左右。     

High-power coaxial microwave ubitron: Simulation by the particle-in-cell method

Results of complete numerical simulation of a high-power coaxial microwave ubitron are reported. The simulation uses a 2.5-dimensional numerical electromagnetic code based on the particle-in-cell method and is performed for the amplification and oscillation operating modes. The results of our numerical simulation are compared with analytical results and results following from the nonlinear theory that are obtained by partial computer simulation.     

Analysis of nano-grating-assisted light absorption enhancement in metal-semiconductor-metal photodetectors patterned using focused ion-beam lithography

We present finite-difference time-domain (FDTD) analysis results of light absorption enhancement factor dependence on the profile shape of nano-gratings etched into the surfaces of metal-semiconductor-metal photodetector (MSM-PD) structures. The MSM-PDs patterned by nano-gratings are optimized geometrically, improving the light absorption near the design wavelength through plasmon-assisted electric field concentration effects. FDTD simulation results show about 50 times light absorption enhancement prediction for 850 nm light due to improved optical signal propagation through the nano-gratings in comparison with the conventional MSM-PD designs employing only a subwavelength aperture. We also report on the nano-grating profile shapes obtained typically in our experiments using focused ion-beam lithography and discuss the dependency of light absorption enhancement on the geometric parameters of nano-gratings inscribed into MSM-PDs.     

Effects of Ion-to-Electron Mass Ratio on Electron Dynamics in Collisionless Magnetic Reconnection

A 2 1/2-dimensional electromagnetic particle-in-cell （PIC） simulation code is used to investigate electron behaviour in collisionless magnetic reconnectfon. The results show that the ion/electron mass ratio （mi/me） almost has no impact on the reconnection rate, however it can significantly affect electron behaviour in the diffusion region. For the case with larger mass ratio, the width of electron current sheet becomes smaller and the outflow region along the separatrix is smaller, hence the peak of the electron outflow speed is essentially larger. Density cavities and the parallel electric field E// along the separatrix can be found in the case with larger mass ratio, which may have significant influences on the acceleration and heating of the electrons near the X point.