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1.
By making a comparison between the acoustic equations and the
2-dimensional (2D) Maxwell equations, we obtain the material
parameter equations (MPE) for acoustic elliptical cylindrical
cloaks. Both the theoretical results and the numerical results
indicate that an elliptical cylindrical cloak can realize perfect
acoustic invisibility when the spatial distributions of mass density
and bulk modulus are exactly configured according to the proposed
equations. The present work is the meaningful exploration of
designing acoustic cloaks that are neither sphere nor circular
cylinder in shape, and opens up possibilities for making complex and
multiplex acoustic cloaks with simple models such as spheres,
circular or elliptic cylinders. 相似文献
2.
A three-dimensional (3D) spherical acoustic cloak is designed using an acoustic layered system, which can hide an object from
the detection of acoustic wave in arbitrary direction. The cloak is constructed from multilayered concentric spherical shells
filled with homogeneous isotropic materials. Based on spherical wave expansion method, we confirm that significant low-reflection,
acoustic-shadow-reducing, and wavefront-bending effects in 3D space can be achieved by the proposed cloak. The angle distribution
of the scattered wave is further evaluated by the far-field scattering pattern. In addition, the cloak is demonstrated to
work efficiently in a wide bandwidth in which the cloaking efficiency decreases with increasing frequency. This study may
be helpful to design high-performance 3D acoustic cloaks for broadband acoustic waves in all incidence directions. 相似文献
3.
Transformation optics offers remarkable control over electromagnetic fields and opens an exciting gateway to design 'invisible cloak devices' recently. We present an important class of two-dimensional (2D) cloaks with polygon geometries. Explicit expressions of transformed medium parameters are derived with their unique properties investigated. It is found that the elements of diagonalized permittivity tensors are always positive within an irregular polygon cloak besides one element diverges to plus infinity and the other two become zero at the inner boundary. At most positions, the principle axes of permittivity tensors do not align with position vectors. An irregular polygon cloak is designed and its invisibility to external electromagnetic waves is numerically verified. Since polygon cloaks can be tailored to resemble any objects, the transformation is finally generalized to the realization of 2D cloaks with arbitrary geometries. 相似文献
4.
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. 相似文献
5.
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. 相似文献
6.
M. Ramadan W. Akl T. Elnady A. Elsabbagh 《Applied Physics A: Materials Science & Processing》2011,103(3):641-644
A finite-element model for three-dimensional acoustic cloaks in both cylindrical and spherical coordinates is presented. The
model is developed through time-harmonic analysis to study pressure and velocity field distributions as well as the cloak’s
performance. The model developed accounts for the fluid-structure interaction of thin fluid-loaded shells. A plane strain
model is used for the thin shell. Mechanical harmonic excitation is applied to the fluid-loaded shell to investigate the effect
of mechanical oscillation of the shell on the performance of the acoustic cloak. In developing this model, a deeper insight
into the acoustic cloak phenomena presented by Cummer and Shurig in 2007 is presented. Different nonlinear coordinate transformations
are presented to study their effect on the acoustic cloak performance. 相似文献
7.
Due to the correspondence of the acoustic equations to Maxwell??s equations of one polarization in two dimensions, we exploit theoretically the acoustic counterpart of the recently proposed remote invisibility cloak. The cloak consists of a circular cylindrical core with designed bulk moduli, and an ??anti-object?? embedded inside a shell with anisotropic mass densities. The material parameters of the cloaking shells are obtained by using the coordinate transformation method. The essence of the new design of cloaks relies on the ability that the cloaked object is no longer deafened by the cloaking shell, which is verified by both the far-field and near-field full-wave finite-element simulations in two dimensions. 相似文献
8.
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. 相似文献
9.
We propose a specific transformation in cloaking to make an acoustic sensor undetectable, in which the cloaking shell consists of complementary media with single-negative acoustic parameters instead of double-negative ones, and is proved to be a magnifying superlens. Moreover, the acoustical parameters of the cloak are completely independent of those of the host material as well as the cloaked object. This may significantly facilitate the experimental realization of acoustic cloaks and is of fundamental importance in a wide range of acoustics, optics, and engineering applications. 相似文献
10.
11.
Thermal concentrators and cloaks with ellipsoidal shapes are designed by utilizing the transformation thermotics method and finite element simulations.The thermal conductivities for the concentrator and cloak are directly derive in Cartesian coordinates.The simulation results show that the ellipsoidal thermal concentrator can focus heat flux into a central region and that the ellipsoidal thermal cloak can guide heat flux around the cloaked region smoothly without disturbing the external temperature distribution and heat flux.The present method can be extended to design arbitrarily shaped thermal metadevices with novel properties. 相似文献
12.
A general method to design a 2D dissimilar cloak for irregular regions is presented by operating a nonlinear transformation in polar coordinates. The material parameters avoid discontinuities while the thickness of the cloak shell is effectively limited in elongated directions. Full-wave simulations of an elliptic cloak, a rectangular cloak, and an arbitrary-shape cloak are performed to verify the validity of the design. Both the material parameters and the scattering widths of different models are calculated and illustrated for comparison. This method provides a possible approach for designing complex shaped cloaks. 相似文献
13.
Improving the performance of acoustic invisibility with multilayer structure based on scattering analysis 下载免费PDF全文
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. 相似文献
14.
Manipulating thermal conductivities are fundamentally important for controlling the conduction of heat at will. Thermal cloaks and concentrators, which have been extensively studied recently, are actually graded materials designed according to coordinate transformation approaches, and their effective thermal conductivity is equal to that of the host medium outside the cloak or concentrator. Here we attempt to investigate a more general problem: what is the effective thermal conductivity of graded materials? In particular, we perform a first-principles approach to the analytic exact results of effective thermal conductivities of materials possessing either power-law or linear gradation profiles. On the other hand, by solving Laplace's equation, we derive a differential equation for calculating the effective thermal conductivity of a material whose thermal conductivity varies along the radius with arbitrary gradation profiles. The two methods agree with each other for both external and internal heat sources, as confirmed by simulation and experiment. This work provides different methods for designing new thermal metamaterials (including thermal cloaks and concentrators), in order to control or manipulate the transfer of heat. 相似文献
15.
《中国物理快报》2016,(4)
The concentrically layered thermal cloaks with isotropic materials could realize the equivalent thermal cloaking effect with Pendry's cloak,while the effectiveness is scarcely investigated quantitatively.Here we examine the cloaking effectiveness quantitatively by evaluating the standard deviation of the temperature difference between the simulated plane with the layered thermal cloak and Pendry's thermal cloak.The design rules for the isotropic materials in terms of thermal conductivity and layer thickness are presented.The present method could quantitatively evaluate the cloaking effectiveness,and could open avenues for analyzing the cloaking effect,detecting the(anti-) cloaks,etc. 相似文献
16.
17.
It was proposed that perfect invisibility cloaks can be constructed for hiding objects from electromagnetic illumination [J. B. Pendry, D. Schurig, and D. R. Smith, Science 312, 1780 (2006)10.1126/science.1125907]. The cylindrical cloaks experimentally demonstrated [D. Schurig, Science 314, 977 (2006)10.1126/science.1133628] and theoretically proposed [W. Cai, Nat. Photon. 1, 224 (2007)10.1038/nphoton.2007.28] have however simplified material parameters in order to facilitate easier realization as well as to avoid infinities in optical constants. Here we show that the cylindrical cloaks with simplified material parameters inherently allow the zeroth-order cylindrical wave to pass through the cloak as if the cloak is made of a homogeneous isotropic medium, and thus visible. To all high-order cylindrical waves, our numerical simulation suggests that the simplified cloak inherits some properties of the ideal cloak, but finite scatterings exist. 相似文献
18.
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. 相似文献
19.
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. 相似文献