Zheng Chang

Beijing Institute Of Technology, Beijing, Beijing Shi, China

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Publications (13)22.55 Total impact

  • Zheng Chang, Xiaoning Liu, Gengkai Hu, Jin Hu
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    ABSTRACT: Elastic ray theory is a high frequency asymptotic approximation of solution of elastodynamic equation, and is widely used in seismology. In this paper, the form invariance under a general spatial mapping and high frequency wave control have been examined by transformation method. It is showed that with the constraint of major and minor symmetry of the transformed elastic tensor, the eikonal equation keeps its form under a general mapping, however, the transport equation loses its form except for conformal mapping. Therefore, the elastic ray path can be controlled in an exact manner by a transformation method, whereas energy distribution along the ray is only approximately controlled. An elastic rotator based on ray tracing method is also provided to illustrate the method and to access the approximation.
    The Journal of the Acoustical Society of America 10/2012; 132(4):2942-5. · 1.65 Impact Factor
  • Zheng Chang, Gengkai Hu
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    ABSTRACT: A conformal mapping is proposed to design with transformation method broadband elastic wave omnidirectional absorbers. This conformal mapping transforms the infinity of the virtual space into the origin of the physical space and keeps the transformed material isotropic. The material realization of such elastic wave absorber is proposed and validated by numerical simulation. Different from the existing procedure based on Hamiltonian optics, the proposed method is a more general one and can be applied to any physical process where the transformation method is valid. It also includes the existing techniques as the special cases for electromagnetic and acoustic waves.
    Applied Physics Letters 07/2012; 101(5). · 3.79 Impact Factor
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    Zheng Chang, Gengkai Hu
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    ABSTRACT: A general conformal mapping is proposed to design omnidirectional broadband wave absorbers by transformation method. When applied to electromagnetic (EM) and acoustic waves, the existing material parameters of the EM and acoustic omnidirectional absorbers, which are previously obtained by Hamiltonian optics and geometry acoustics, can be recovered. In addition, magnetic and mass-density-controlled omnidirectional absorbers for EM and acoustic waves can also be designed, respectively. We then apply the conformal mapping to design an omnidirectional elastic wave absorber, the corresponding material realization of such elastic absorber is also proposed and validated by numerical simulation.
    05/2012;
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    Zheng Chang, Jin Hu, Xiaoning Liu, Gengkai Hu
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    ABSTRACT: A transformation method based on elastic ray theory is proposed to control high frequency elastic waves. We show that ray path can be controlled in an exact manner, however energy distribution along the ray is only approximately controlled. A numerical example of an elastic rotator is provided to illustrate the method and to access the approximation. The proposed theory may be found potential applications in seismic wave protection and structure health monitoring.
    10/2011;
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    ABSTRACT: A broadband omnidirectional electromagnetic absorber is constructed with traditional materials including silicon powder, polyethylene and a mixture of water and ethanol. Variation of the dielectric constant in the radial direction of the device is realized by gradually changing the fraction of silicon powder and polyethylene, and the absorbing core is made by mixing water and ethanol. By placing the device into a parallel metallic board and under electromagnetic illumination, we demonstrate that the trajectories of the incident waves fall into the core at both 9.0 and 10.0 GHz, as predicted by numerical simulation. Since the device is constructed without a resonant element, it can be expected to have a broadband nature.
    Journal of optics 07/2011; 13(8):085103. · 1.99 Impact Factor
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    Zheng Chang, Jin Hu, Gengkai Hu, Ran Tao, Yue Wang
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    ABSTRACT: Design of functional devices with isotropic materials has significant advantages, as regards easy fabrication and broadband application. In this letter, we present a method to derive isotropic transformation material parameters for elastodynamics under local conformal transformation. The transformed material parameters are then applied to design a beam bender, a four-beam antenna and an approximate carpet cloak for elastic wave with isotropic materials, validated by the numerical simulations.
    Applied Physics Letters 03/2011; 98(12):121904-121904-3. · 3.79 Impact Factor
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    Zheng Chang, Jin Hu, Geng-Kai Hu
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    ABSTRACT: Transformation method provides an efficient way to control wave propagation by materials. The transformed relations for field and material during a transformation are essential to fulfill this method. We propose a systematic method to derive the transformed relations for a general physic process, the constraint conditions are obtained by considering geometrical and physical constraint during a mapping. The proposed method is applied to Navier’s equation for elastodynamics, Helmholtz’s equation for acoustic wave and Maxwell’s equation for electromagnetic wave, the corresponding transformed relations are derived, which can be used in the framework of transformation method for wave control. We show that contrary to electromagnetic wave, the transformed relations are not uniquely determined for elastic wave and acoustic wave, so we have a freedom to choose them differently. Using the obtained transformed relations, we also provide some examples for device design, a concentrator for elastic wave, devices for illusion acoustic and illusion optics are conceived and validated by numerical simulations. KeywordsWave control–Elastic wave–Acoustic wave–Electromagetic wave–Local affine transformation
    Acta Mechanica Sinica 12/2010; 26(6):889-898. · 0.69 Impact Factor
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    Jin Hu, Zheng Chang, Gengkai Hu
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    ABSTRACT: By idealizing a general mapping as a series of local affine ones, we derive approximately transformed material parameters necessary to control solid elastic waves within classical elasticity theory. The transformed elastic moduli are symmetric, and can be used with Navier's equation to manipulate elastic waves. It is shown numerically that the method can provide a powerful tool to control elastic waves in solids in case of high frequency or small material gradient. Potential applications can be anticipated in nondestructive testing, structure impact protection, petroleum exploration and seismology.
    Physical review. B, Condensed matter 08/2010; 84.
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    ABSTRACT: Recently, there are emerging demands for isotropic material parameters, arising from the broadband requirement of the functional devices. Since inverse Laplace's equation with sliding boundary condition will determine a quasi-conformal mapping, and a quasi-conformal mapping will minimize the transformation material anisotropy, so in this work, the inverse Laplace's equation with sliding boundary condition is proposed for quasi-isotropic transformation material design. Examples of quasi-isotropic arbitrary carpet cloak and waveguide with arbitrary cross sections are provided to validate the proposed method. The proposed method is very simple compared with other quasi-conformal methods based on grid generation tools.
    Optics Express 03/2010; 18(6):6089-96. · 3.55 Impact Factor
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    ABSTRACT: In a recent paper, Chen et al. [Opt. Express 17, 3581 (2009)] develop an approach to design invisible cloaks with controllable constitutive parameters by adjusting the constant k in the Helmholtz's equation. In this comment, we discuss the limitation of the free parameter k in designing cloaks. It is found that the real constant k can be chosen only as limited values in order to avoid the singular material parameters.
    Optics Express 02/2010; 18(4):3917-8. · 3.55 Impact Factor
  • Jin Hu, Zheng Chang, Gengkai Hu
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    ABSTRACT: By considering the space mapping from local point of view and adopting local affine transformation, we derive the transformed relation necessary to control elastic waves in context of Navier's equation. The obtained elastic transformed materials possess a symmetric stiffness tensor and are validated numerically by a rotator and carpet cloak for elastic waves. The method provides a powerful tool to control elastic waves in solid, with potential applications to nondestructive testing, structure impact protection and seismology.
    01/2010;
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    ABSTRACT: The deformation method of transformation optics has been demonstrated to be a useful tool, especially in designing arbitrary and nonsingular transformation materials. Recently, there are emerging demands for isotropic material parameters, arising from the broadband requirement of the designed devices. In this work, the deformation method is further developed to design quasi-isotropic/isotropic transformation materials. The variational functional of the inverse Laplace's equation is investigated and found to involve the smooth and quasi-conformal nature of coordinate transformation. Together with the sliding boundary conditions, the inverse Laplace's equation can be utilized to give transformations which are conformal or quasi-conformal, depending on functionalities of interest. Examples of designing an arbitrary carpet cloak and a waveguide with arbitrary cross sections are given to validate the proposed idea. Compared with other quasi-conformal methods based on grid generation tools, the proposed method unifies the design and validation of transformation devices, and thus is much convenient. Comment: 8 pages, 4 figures
    Optics Express 12/2009; · 3.55 Impact Factor
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    ABSTRACT: In this paper, we utilize the deformation transformation optics (DTO) method to design electromagnetic beam bender, which can change the direction of electromagnetic wave propagation as desire. According to DTO, the transformed material parameters can be expressed by deformation tensor of the spatial transformation. For a beam bender, since the three principal stretches at each point induced by the spatial transformation are independent to each other, there are many possibilities to simplify the transformed material parameters of the bender by adjusting the stretches independently. With the DTO method, we show that the reported reduced parameters of the bender obtained by equivalent dispersion relation can be derived as a special case. An isotropic bender is also proposed according to this method, and it is fabricated by stacking dielectric materials in layered form. Experiments validate the function of the designed isotropic bender for a TE wave; it is also shown that the isotropic bender has a broadband with low loss, compared with the metamaterial bender. The isotropic bender has much easier design and fabrication procedures than the metamaterial bender. Comment: 9 pages, 9 figures
    12/2009;