基于遗传算法的五模材料分层优化*

由于超材料的微结构尺度不能忽略,利用超材料微结构所集成的器件等效参数不是连续分布的,而是呈离散分布的。因此在利用超材料设计声学器件的过程中,需要对器件进行分层离散化,而分层方案的选取往往会影响器件的工作效果。该文基于五模材料水下隐身衣的设计理论,提出了一种基于遗传算法的五模材料隐身衣分层优化策略,并分别对优化前和优化后的分层隐身衣做了声学仿真。仿真结果表明采用优化策略后,隐身衣背景声场的散射能量和反射能量均有大幅度降低,并且对窄带探测信号和宽带探测信号均有较好的隐身效果,因此该分层优化策略能够显著提升隐身衣的隐身效果。该文所提出的分层优化策略为提升五模材料隐身衣性能提供了理论支持,并可以进一步推广至其他超材料器件的微结构设计中。     

球形破片侵彻多层板弹道极限的量纲分析

为了研究Q235钢多层板的抗侵彻性能,进行了直径为9.45 mm的钨合金球形破片侵彻7.2 mm和(3.6+3.6)mm厚Q235钢双层板试验,获得了相应的弹道极限。在此基础上,建立数值仿真模型,研究了钨合金球侵彻接触式等厚3层、4层、5层、6层板的弹道极限。通过量纲分析方法,分析了分层数对靶板弹道极限的影响。结果表明:对于球形破片,总厚度为7.2 mm的等厚双层板的抗侵彻性能高于单层板;当分层数大于2时,接触式多层等厚靶板的弹道极限随着层数的增加而减小,即分层数越多,靶板的抗侵彻性能越低,通过量纲分析方法得到了靶板分层数与破片弹道极限的关系。研究结果可为未来装甲防护设计提供一定的参考。     

流动隐身衣的均匀化设计与减阻特性

流动隐身衣因为可以显著降低指定目标的表面阻力而备受关注.然而大多数传统流动隐身衣的设计参数为非均匀各向异性,非均匀这一限制增加了制备流动隐身衣的难度.为突破这种限制,本文采用等效介质理论与积分中值定理,将流动隐身衣所需的设计参数均匀化.通过数值模拟验证了简化后的均匀流动隐身衣具有与非均匀流动隐身衣一样的隐身效果,并且同样适用于多种流场.这种简化方法不仅可以将非均匀流动隐身衣简化为均匀流动隐身衣,更重要的是可以适用于其他领域,如光学、声学、电磁学与热学等不同领域的超材料均匀化设计,为降低超材料制备难度提供了新方法.此外,基于均匀流动隐身衣对不同流场的适用性,首次设计了一种流动伪装装置,该装置可以将原始物体所产生的流场伪装成由任意物体引起的期望流场,为实现流动伪装提供了解决方案.最后,定量对比分析了流动隐身衣的隐身与减阻性能随着雷诺数增加的变化,结果表明在非蠕动流时流动隐身衣仍然具有良好的隐身性能与较高的减阻效率.     

基于各向同性材料的层状椭圆柱形声隐身衣设计

基于等效介质理论, 提出了具有共焦层状结构的椭圆柱形声隐身衣设计方法. 理论分析与有限元数值模拟表明, 所设计隐身衣依然具有完美隐身衣典型特征, 可使刚性圆柱体散射场明显减小, 并且在隐身衣区域表现出波阵面弯曲的特性, 同时在隐身衣外部波阵面保持不变. 增加隐身衣离散层数可以拓宽其有效工作频带, 改善隐身效果. 由于是一种线变换隐身衣, 隐身效果受到了入射波方向的影响, 只有当入射波方向与椭圆长轴平行时效果最佳. 另外当椭圆柱焦距非常小的时候, 可近似认为是圆柱形隐身衣. 仿真实验结果证明了方法的正确性. 该研究为实现复杂形状声隐身衣提供了一种有效途径.     

分层吸声结构的声学设计与性能分析

本文给出了声波在多层介质中的传播方程，通过数值计算，分析了各层参数对分层吸声结构声学性能的影响规律。用参数匹配方法，对多层吸声结构各层材料的参数进行设计。结果表明，通过合理分布各层材料，多层吸声结构可以在较宽频段上达到满意的吸声性能。     

有ITO透明导电膜的平面分层介质系统的电磁波性能理论研究

理论研究了有ITO(indium tin oxide)透明导电膜的多层平面分层介质系统的电磁性能，给出的理论曲线和实测曲线符合很好.多层平面分层介质系统的电磁性能与ITO膜(方块电阻为8Ω)所在界面位置和平面分层介质系统层数及各层厚度等有关.优化设计了一种含有ITO透明导电膜的厚度仅7.35mm的四层平面分层介质系统，其在8—18GHz频段内电磁波反射性能很好.作为多层平面分层系统中的ITO导电膜，其方块电阻应低于30Ω，并且越小，其反射性能越好. 关键词： 多层平面介质系统 电磁性能 ITO透明导电膜     

不同基底石墨烯涂层的层间滑移减磨性能研究

石墨烯薄膜作为一种二维材料,是提高微/纳机电系统(MEMS/NEMS)摩擦力学性能的优异润滑剂.为了探究基底材料和石墨烯层数对其减磨性能的影响,本文通过在不同基底制备了不同层数的石墨烯涂层,利用原子力显微镜(AFM)实验和分子动力学(MD)仿真结合的方法,研究了石墨烯层数对减磨效应的影响.并且通过建立不同层数石墨烯涂层的摩擦性能分析模型,探究出石墨烯层间滑移是产生减磨的主要因素.结果表明:在不同载荷下,石墨烯涂层对硅基底和铜基底均有优异的减磨效果,摩擦力随着石墨烯层数的增加逐渐降低,当石墨烯层数大于10层时,达到最优99.3%的减磨效果.通过仿真分析发现,随着层数增加,石墨烯与基底的干摩擦转变为石墨烯的层间摩擦,并产生层间剪切滑移,石墨烯层间滑移是导致多层石墨烯优异减磨性能的主要因素.     

水中分层弹性球壳高频时域回波的声学编码研究

针对水下集群目标及敌我目标识别的难题,该文提出了一种基于水中分层弹性球壳高频时域回波的声学编码原理及方法。推导了水中4层弹性球壳目标散射声压的简正级数解,并与有限元结果进行了对比验证。通过构造高频主动声呐的探测脉冲信号,与4层弹性球壳声传递函数的简正级数解做卷积运算,获得了目标的时域回波脉冲序列。研究了分层弹性球壳的厚度、各层材料属性、排布顺序等对时域回波特征的影响规律,提出了基于时域回波特征的声学编码方法。研究表明：利用水中分层弹性球壳目标高频时域回波特征能够实现声学编码,回波结构稳定,且不受限于探测方向。通过携带或安装这种分层弹性球壳结构,有望识别水下航行体/悬浮体等目标。该文的研究对水下目标的主动探测身份识别及导航等具有一定的参考价值。     

确定三层增透膜层厚的方法

目前,在镀制多层增透膜时所用的方法是:利用真空蒸发膜料并按石英晶体振荡频率ω连续控制分层厚度 h_J(J 是从基底算起的层数)。当ω的变化达到预定值Δω_1时,就停止 J 层的淀积。在镀制时,每制备一组增透膜最好都有层厚的控制方法。这样就可调节频率Δω,以便使实际的层厚近似于技术程序所规定的最佳层厚 h_J,并且保证增透     

基于变换光学的隐身衣设计

从阐述麦克斯韦方程组在坐标变换下形式不变性出发,介绍了基于变换光学设计隐身衣的思路,并分别推导了实现二维圆柱形、椭圆柱形、正方形隐身衣的相对介电常量和相对磁导率张量在物理空间的分布公式.利用COM-SOL M ultiphysics仿真软件,给出了二维圆柱形、椭圆柱形、正方形隐身衣的隐身效果.     

Impedance-Matched Reduced Acoustic Cloaking with Realizable Mass and Its Layered Design

We present an impedance-matched reduced version of acoustic cloaking whose mass is in a reasonable range. A layered cloak design with isotropic material is also proposed for the reduced cloak. Numerical calculations from the transfer matrix methods show that the present layered cloak can reduce the scattering of an air cylinder substantially.     

Improving the performance of acoustic invisibility with multilayer structure based on scattering analysis

In this paper, acoustic scattering from the system comprised of a cloaked object and the multilayer cloak with only one single pair of isotropic media is analyzed with a recursive numerical method. The designed acoustic parameters of the isotropic cloak media are assumed to be single-negative, and the resulting cloak can reduce acoustic scattering from an acoustic sensor while allowing it to receive external information. Several factors that may influence the performance of the cloak, including the number of layers and the acoustic dissipation of the medium are fully analyzed. Furthermore, the possibility of achieving acoustic invisibility with positive acoustic parameters is proposed by searching the optimum value in the parameter space and minimizing the scattering cross-section.     

Invisibility of a metamaterial cloak illuminated by spherical electromagnetic wave

In this paper, the invisibility of a metamaterial cloak illuminated by spherical electromagnetic wave is analytically investigated based on the full wave Mie scattering model. It is shown that for a cloak with ideal parameters the scattered field intensity is zero, but for a cloak with a loss, only the backscattering is exactly zero. Moreover, in the loss case, the scattered field intensity increases as the loss increases, which is very different from that in the conventional stealth case, where the scattered field intensity decreases as the loss of coated material increases. In addition, it is shown that scattering cross-section of the cloak with perturbed parameters decreases as the thickness of the cloak decreases, which means that thinner cloak can exhibit more stable invisibility.     

基于主动声学超材料的圆柱声隐身斗篷设计研究

基于多层复合材料结构的二维声隐身斗篷设计思想, 利用主动隔膜声学空腔有效密度可以任意控制这一特性, 设计了主动声学超材料下的无限长圆柱声隐身斗篷. 给出了主动隔膜声学空腔单元的声电元件类比模拟电路图和具体的有效密度控制方法. 进行了主动声学超材料声隐身斗篷的结构建模, 并对平面入射波入射下此圆柱隐身斗篷周围声压分布场进行仿真计算. 结果表明, 平面波在一定频率范围内可以毫无阻碍地透过圆柱斗篷, 似乎不存在这种障碍物, 达到声隐身效果. 同时, 计算了主动声材料斗篷下总散射截面随频率变化曲线, 研究了此斗篷隐身效果随频率的变化特性. 本文从主动控制角度探讨实验实现隐身斗篷的技术问题, 有望给声隐身斗篷实验设计提供一条新的技术途径.     

双锥型五模材料低频声波调控及参数设计

为有效控制噪声并进行声波调控,构造了双锥区域为TC4钛合金、节点区域为硫化橡胶的六边形排列双锥五模材料,进行能带分析发现其具有较窄的低频声子带隙和单模传输区域。为提高五模材料的低频声波调控性能,设计了正方形和三角形排列构型,结果表明三角形排列的双锥五模材料带隙的频率更低,带宽更宽,且具有单模传输性能和较好的五模特性。此外分别探究了五模材料构型的材料参数(包括双锥区和节点区的密度、泊松比和杨氏模量)以及几何参数(包括双锥宽和节点半径的变化)对带隙及单模传输区域的影响,得到带隙和单模传输区的变化规律,选择密度较轻的填充材料、较小的双锥宽和较大的节点半径不仅可以提高低频声波调控性能,而且可以降低结构质量,提高结构的稳定性。该文结果对用于低频声波传播调控的五模材料的构型和参数的设计具有一定的借鉴意义。     

Cloaking an acoustic sensor with single-negative materials

In this review, a brief introduction is given to the development of acoustic superlens cloaks that allow the cloaked object to receive signals while its presence is not sensed by the surrounding, which can be regarded as “cloaking an acoustic sensor”. Remarkably, the designed cloak consists of single-negative materials with parameters independent of the background medium or the sensor system, which is proven to be a magnifying superlens. This has facilitated significantly the design and fabrication of acoustic cloaks that generally require double-negative materials with customized parameters. Such innovative design has then been simplified further as a multi-layered structure comprising of two alternately arranged complementary media with homogeneous isotropic single-negative materials. Based on this, a scattering analyses method is developed for the numerical simulation of such multi-layered cloak structures, which may serve as an efficient approach for the investigation on such devices.     

五模声学超表面理论分析与定向反射声学仿真*

以广义斯奈尔定律为理论依据,对五模声学超表面定向反射的基本原理进行了解析推导和理论分析,获得了五模超表面的理想连续物性参数分布,并给出了五模超表面尺寸设计准则;然后将超表面离散,获得离散单胞的密度和体积模量,并以此为目标进行五模微结构设计,采用均匀化理论计算微结构的等效物性参数;最后,进行了水下声场的声波定向反射调控仿真实验,研究了入射波频率对超表面定向反射性能的影响,仿真结果展现了五模超表面宽频有效的声波调控能力以及调控的可靠性和准确性。本文的研究工作为五模声学超表面的设计和物理实现提供理论指导。     

A revisit to imperfect acoustic cloak of multi-layered shell structures considering sound speed and impedance matching

An imperfect multi-layered acoustic cloak is proposed for a two-dimensional cloaking zone based on feasible material properties. In this model, the matching of sound speed and acoustic impedance has been investigated, and the effects of material and geometric properties on the imperfect cloak have been studied for better design of the imperfect cloak. The imperfect cloak could be improved using appropriate changes in the design parameters. By increasing the thickness of the high density layer and with some changes in the sound speeds between the high density and the low density layers, the imperfect cloaking model showed better cloaking performance than Cummer–Schurig cloak. Also, present results show that the sound speed matching is more important for acoustic cloaking than the impedance matching. These results can be applied as a practical design guide for two-dimensional cloaks using multilayered structures composed of naturally existing materials.     

Acoustic cloak with duplex communication ability constructed by multilayered homogeneous isotropic materials

Based on the effective medium theory, we propose a practical implementation of a cylindrical acoustic cloak with a concentric alternating multilayered structure of homogeneous isotropic materials, which can perfectly mimic the ideal radius-dependent and anisotropic ordinary lens cloak. The proposal exhibits near-ideal cloaking performance such as low-scattering and shadow-reducing in a wide frequency range, thus it can hide an object from the detection of acoustic waves. The acoustic wave can pass through the cloaking shell with an unchanged wavefront shape, which endues the cloaked object with duplex communication ability. More simulations on the acoustic far-field scattering patterns and the total scattering cross-section are performed to investigate the layer number and the frequency dependence of the cloaking effect, and the results show that the thinner layers exhibit a better cloaking effect. The proposal may significantly facilitate the experimental demonstration of the acoustic cloak.