Search

A compact quasi-Yagi antenna with bent arms and split-ring resonators (SRRs) is proposed. Compared with traditional quasi-Yagi antennas employing straight arms, the resonant frequency of the proposed antenna could be always consistent with its center frequency, and there is no obvious frequency shift under the process of its miniaturization. The SRRs are adopted in the proposed compact antenna for a high gain of 6.58 dBi. The reliability verification of the proposed antenna radiation characteristics is further experimentally proved with the prototype measurement. The proposed quasi-Yagi antenna has an adjustable compact structure and low frequency offset and could be used in the precise point-to-point wireless communication environment.

This paper presents a H-shaped resonator metasur-face array which can generate vortex beams by using Pancha-ratnam-Berry phase theory. The proposed reflective metasurface operates within the frequency range from 8.13 GHz to 20.3 GHz and can convert the incident plane waves into vortex beams carrying orbital angular momentum, with more than 90% polarization conversion efficiency. Compared with other designs in literature, the proposed structure has the advantages of high performance, broad bandwidth and low profile. The reflective array for generating the vortex beam with the topological charge of -2 is generated by rotating the meta-atoms. The near electric field distributions, corresponding spectral analysis of reflected waves and far scattered patterns are achieved by simulation to verify the effectiveness of the proposed metasurface.

With liquid metal like lead-bismuth alloy (LBE) acting as a coolant for nuclear reactors, it is necessary to use a more accurate heat transfer relationship and a more reliable Prt model for the low Pr fluid. Because of the low Pr of liquid metal, the thermal conductivity is more dominant than the momentum transfer, which is quite different from ordinary fluids. In this case, the turbulent Prt can better reflect the heat transfer process. In this study, the Prt = A1+A2/Pr form is selected, and the corresponding coefficients are obtained by the renormalization group analysis method, then corrected by Pr. Furthermore, the applicable range and segmentation rule of the turbulent Prt model are discussed, and the obtained Prt segmentation theoretical model is written into CFD. The result shows that, compared with the previously unmodified model, the radial temperature distribution and Nusselt number (Nu) of the annular and bundle channel obtained by RANS method with the improved Prt model is in good agreement with experimental results, and the deviations are within 5%. It is proved that the turbulent Prt segmentation theoretical model proposed in this study is effective and can represent the heat transfer characteristics of liquid metal from the mechanism.

With the development of electric vehicle wireless charging technology, the transmission power and transmission efficiency of electric vehicle wireless charging system have become the focus of current research. The transmission power and efficiency of wireless charging system largely depend on the energy loss of two resonant coupling coils. The energy loss is mainly related to the structural parameters of the coupling coil and the coupling coefficient of the two coils. On this basis, the structure of the coupling coil is designed and optimized by using the finite element analysis software Maxwell, and a new combined transmitting coil structure is designed. Experiments and verification show that the coil design meets the requirements of transmission efficiency, which can provide a reference for the design of wireless charging coil of electric vehicle in the future.

Vortex beams carrying orbital angular momentum (OAM) are expected to efficiently increase channel capacity in communication sectors, so ultra-broadband vortex beam generators with high performance are important in next-generation communication systems. Based on the concept of the Pancharatnam–Berry phase, we propose a metasurface with an H-shaped resonator for realizing vortex beam generation within an ultra-wideband frequency range from 7.3 to 21 GHz (relative bandwidth = 96.8%). The designed meta-atom can acquire high efficiency (efficiency ≥ 92%) conversion from the incident plane wave to the reflected vortex wave across the entire frequency band. To verify the performance of the proposed metasurface, a reflective array consisting of 18 × 18 single-layered elements with different rotated angles is simulated for generating the vortex beam with a topological charge of ±2. Through OAM spectral analysis, the left-handed components of the reflected electric fields and mode purity of the generated vortex beams under different frequencies are achieved and discussed in detail. Then the metasurface is fabricated and measured, and the numerical and experimental results coincide well, proving the effectiveness and high performance of the proposed design.

Based on transformation optics, we propose an exterior anti-cloak design in bipolar coordinates. The anti-cloak is a three-layered structure used to cancel the function of an exterior cloak. When the exterior cloak is placed close to a macroscopic object that can be seen with human eyes, the object is hidden from the outside electromagnetic detection. However, when the exterior anti-cloak is applied, the hidden object becomes visible to the electromagnetic waves. Simulation results demonstrate excellent performance of the exterior cloak and anti-cloak. Both the cloak and anti-cloak are exterior ones, which are convenient and flexible for practical application.

Parity-time (PT)-symmetric system has many interesting phenomenons. In this talk, we focus on the transmission and scattering properties of this kind of system. For the transmission property, the concept of PT symmetry in SPPs is proposed and confirmed. By introducing periodic modulation of the effective refractive index in SPP system, the asymmetric propagation is theoretically predicted and numerically demonstrated. After validation of this concept, we apply it in two applications: PT-waveguide and PT-cloak. Both two applications further illustrate the wide applicability of this concept in SPP system and more PT-devices for SPP wave can be envisioned.

Magnetic surface polariton (MSP) is a surface electromagnetic mode which is the dual of surface plasmon polaritons and has some peculiar properties. In the present paper, based on this surface mode, we design a 2D Luneburg lens for the manipulation of magnetic surface polariton. MSP-based devices may be realized using magnetic metamaterials or composite magnetic materials.

In this review article, a brief introduction on the theory, experiments and applications of metamaterials is presented. The main focuses are concentrated on the composing meta-atoms, the method of transformation optics, the experimental demonstration of negative refraction, and the realizations of invisibility cloaks and electromagnetic black hole. At the end of this review, some typical applications of metamaterials, including high-performance antennas made of zero-refractive-index materials, inhomogeneous metamaterial lenses, and planar metasurfaces, are introduced in details.

The concept of parity-time (PT) symmetry in SPPs is proposed and confirmed for the first time in this work. By introducing periodic modulation of the effective refractive index in SPP system, the asymmetric propagation is theoretically predicted and numerically demonstrated. After validation of this concept, we apply it in two applications: PT-waveguide and PT-cloak. Both two applications further illustrate the wide applicability of this concept in SPP system.

In this paper, a novel circular slot UWB antenna with dual notched frequency band is presented and investigated. A C-shaped slot is inserted into the fed element, and a parasitic strip is printed in the circular slot, so that the proposed antenna achieves dual band- notched characteristics, respectively. The measured and simulated results show that the proposed antenna meets the requirement of wide working bandwidth of 3.1{10.6GHz with VSWR < 2, while avoiding the interference with the 3.5GHz WiMAX and 5.5GHz WLAN band. Study of transfer function (amplitude of S21/group delay) and time domain characteristic (radiated pulses/power spectrum density (PSD)) correspond well with the VSWR, which indicate the dual band notched characteristic of the antenna.

In this paper, we study metamaterial-based transparent shells, which are invisible to electromagnetic waves and flelds. More general topics, including material loss, material discretization and far fleld distribution etc. are covered in order to facilitate possible realization. We design and analyze the transparent shells using optical transformation. Unlike the widely-studied cloaking devices which make the objects inside invisible, the transparent shells physically cover and shield the devices inside without sacriflce of their electrical performance since they are transparent to incoming electromagnetic waves. Due to the simple constitutive parameters, these transparent structures could be realized using artiflcial metamaterials in a wide frequency band, which may have wide applications in civil and military areas.

Keywords: Wave propagation, Impedance matrix, Dynamic stiffness and Soil-structure Interaction. Abstract. In this paper, we use an efficient method to solve the wave numbers of transient wave propagation i n layered media. To solve those problems, the analytic solution for the interior domain is combined both a homogeneous solution and a particular solution, the exterior domain is described by a homogeneous solution only. To obtain the homogeneous solution, one has challenge to solve the complex root of the transcendental equations will be discussed. For the soil-structure interaction problem, the wave number of transient wave propagation in layered media will be used for the impedance matrix of embedded axial symmetric foundation.

In this study, the accuracy of structured and unstructured mesh CFD codes in simulating the flow around a three-element high-lift configuration (slat, main wing, and flap) is assessed, and mesh dependency and effect of turbulence models are studied. In the first part of the study, the results of two structured mesh CFD codes and an unstructured mesh CFD code using the same turbulence model are compared and discussed. A mesh refinement approach is also used to examine the dependency of numerical accuracy on the density of unstructured meshes. By properly distributing mesh points in an unstructured mesh, the detail in flow physics is obtained. It is also shown that the quantitative prediction of the vortex in the slat cove is an important factor that affects accuracy. In the second part of the study, three turbulence models are compared using one of the structured mesh CFD codes. All turbulence models can produce similar flowfields. However, several differences are seen in the separated regions, especially in the slat cove. Commonly used turbulence models, Spalart-Allmaras model and Menter’s SST model, produce similar aerodynamic forces at lower angles of attack. At higher angles of attack, the SST model gives better results for the present computations. It is found that the maximum lift and the angle at which the stall occurs are very sensitive to the turbulence model.

The effect of Gaussian white noise on a chemical wavefront is studied in a modified FitzHugn–Nagumo model by applying numerical simulations. A rotating spiral waves can be formed if the medium is excitable enough and the fronts has a free end, when the reaction diffusion system is disturbed by a certain non-zero level noise. It is counterintuitive that noise plays a constructive role in the product and propagation of single spiral waves in this letter. Weak or strong noise will make against the product and propagation of spiral waves. In a certain noise level, spiral wave can be maintained in a medium, where such spiral waves cannot be observed in the absence of the noise.

Intrinsic fluctuation of Chua system is studied with master equation method. Our results have shown that the intrinsic noise indeed exerted considerable influence on Chua system. In contrast to that of deterministic equation the patterns of time evolution and attractor have been greatly altered under the influence of intrinsic noise.

For ideal invisibility cloaks, electromagnetic radiations cannot penetrate through the cloaking shell. However, researchers have suggested that the originally hidden object with a cloaking shell is visible to the outside world when it is covered by an ``anticloak.'' In this paper, we give the first experimental verification of the ``anticloak'' for the Laplace equation, which requires negative conductivity profiles. The proposed device consists of two layers---the cloak and anticloak, which can work individually or collectively and give rise to different functions. Functionality switching among the invisibility cloak, anticloak, transparent cloak, superscatterer, and illusion is suggested and demonstrated. Based on the circuit theory, we fabricate a switching resistor network with both positive and negative resistors to emulate the cloaking and anticloaking interactions. The switchable cloaking, anticloaking, transparent, superscatterer, and illusion performances are confirmed by the experiments and/or numerical simulations.

In recent years, invisibility cloaks have received a lot of attention and interest. These devices are generally classified into two types: passive and active. The design and realization of passive cloaks have been intensively studied using transformation optics and plasmonic approaches. However, active cloaks are still limited to theory and numerical simulations. Here, we present the first experiment on active cloaking and propose an active illusion for the Laplace equation. We make use of a resistor network to simulate a conducting medium. Then, we surround the central region with controlled sources to protect it from outside detection. We show that by dynamically changing the controlled sources, the protected region can be cloaked or disguised as different objects (illusion). Our measurement results agree very well with numerical simulations. Compared with the passive counterparts, the active cloaking and illusion devices do not need complicated metamaterials. They are flexible, in-line controllable, and adaptable to the environment. In addition to dc electricity, the proposed method can also be used for thermodynamics and other problems governed by the Laplace equation.

In this work, a different kind of parity-time (PT) symmetric one-way cloak is proposed. Different from conventional PT-cloak, it enjoys the property of isotropic modulation for refractive index profiles. By combining PT-symmetry with the concept of cloaking at a distance, the dilemma of realizing anisotropic modulation is removed. This combination facilitates the practical realization of PT-symmetric one-way cloak.

We present the first experimental demonstration of a dc electric cloak for steady current fields. Using the analogy between electrically conducting materials and resistor networks, a dc invisibility cloak is designed, fabricated, and tested using the circuit theory. We show that the dc cloak can guide electric currents around the cloaked region smoothly and keep perturbations only inside the cloak. Outside the cloak, the current lines return to their original directions as if nothing happens. The measurement data agree exceptionally well with the theoretical prediction and simulation result, with nearly perfect cloaking performance. The proposed method can be directly used to realize other dc electric devices with anisotropic conductivities designed by the transformation optics. Manipulation of steady currents with the control of anisotropic conductivities has a lot of potential applications, such as electric impedance tomography, graphene, natural resource exploration, and military camouflage.

A new isotropic and homogeneous illusion device for electromagnetic waves is proposed. This single-shelled device can change the fingerprint of the covered object into another one by manipulating the scattering of the composite structure. We show that an electrically small sphere can be disguised as another small one with different electromagnetic parameters. The device can even make a dielectric sphere (electrically small) behave like a conducting one. Full-wave simulations confirm the performance of proposed illusion device.

Based on conformal mapping method, a two dimensional, multi-functional lens structure is proposed and designed in this work. The lens is an infinitely-long, gradient-index dielectric cylinder with a semi-elliptic cross-section. The lens can first be considered like a flattened Luneburg lens, which produces highly-directive electromagnetic waves by adjusting the feed position along the line connecting the two foci. It also functions like an Eaton lens. When an incoming beam impinges on the same line but outside the two foci, it will be guided through the lens structure and take a U-turn. Besides, if properly shaped, the structure can also be used as a waveguide bend. The lens can be realized using non-resonant metamaterials with inhomogeneous hole arrays. Simulation results demonstrate excellent performance of the lens and agree well with theoretical prediction. The designed lens can be used in the electromagnetic control. And it is especially useful in the real optical lens system.

A magnification device for the Laplace equation is proposed and verified in this work. Unlike designs based on transformation optics, which are hard to realize due to the complementary media needed and the anisotropy and inhomogeneity of the resultant materials, our design only requires isotropic and homogeneous materials with positive values. The method of separation of variables is utilized to realize the magnification device, and the first experiment demonstration for the device is given, where we utilize the resistor network to mimic the problem. The measurement results validate the exact magnifying property of our proposal. Our design is suitable for the time-varying fields under quasi-static conditions, it applies directly in thermodynamics, and other problems governed by the Laplace equation.

The flow field over a wing/body model of a su- personic transport (SST) at low speed and high an- gles of attack was investigated by wind tunnel ex- periments and simulated by Navier -Stokes calcula- tions. Three-component forces and pressure distri- bution on the surface were measured, flowfield survey was carried out to obtain distributions of velocity vector by PIV, and surface flow patterns were visualized by means of oilflow technique. Numerical computations were conducted to provide detail information about the flow structure and de- tail analysis of aerodynamic characteristics. Ex- periments and numerical computations were co m- pared extensively. Vortical flowfield was identified as a complex structure with vortices separated from the body, the outboard and inboard leading edges, and the wing tip. Velocity distributions and surface pressure were discussed. Aerodynamic characteris- tics were analyzed and element contributions were discussed using computational results. Computa- tions were also conducted in a simplified wind-tunnel model of the corresponding experi- ments to estimate the wall interference and compare with a classical correction method used in wind tunnel tests.

We propose and experimentally demonstrate a planar perfect relay lens using the modified Schwarz-Christoffel (SC) mapping. Based on the SC mapping, a Maxwell fisheye lens is conformally transformed into a slab exactly, realizing a perfect relay lens. Both geometrical optics and wave optics are utilized to verify the excellent imaging performance from surface to surface. The proposed relay lens is implemented using fractal metamaterials with the H-shaped particles and the experimental result shows good agreement to the numerical simulation. The proposed design has many potential applications, such as the night vision system, endoscopes, microscopes, and photocopiers.

We present a two-dimensional carpet cloak for static magnetic field, a design that renders the magnetic response of a given volume invisible from its exterior, without altering the external magnetic fields. The device is designed using transformation optics method and can be implemented with alternating superconducting and magnetic material layers. Through the proper design of the constitutive tensors and relative thicknesses of each slab, we achieve the perfect performance of invisibility. Full wave numerical simulations confirm our design.

We propose and experimentally demonstrate an ultrathin but nearly perfect dc electric invisibility cloak. In the dc limit, transformation optics reduces to transformation electrostatics. Based on a special coordinate transformation, we derive a nearly perfect dc electric cloak which is composed of homogeneous and anisotropic conducting material. Due to the homogeneity feature, the dc cloak is realized using an ultrathin dc metamaterial with only one-unit-cell thickness, which is the ultra limit for practical artificial materials. Although ultrathin, our experimental results show that the dc cloak has excellent performance with nearly perfect cloaking behaviour.

We propose, design, and implement a two-dimensional dc carpet cloak for steady electric field using the transformation optics (TO) method. Based on the circuit theory, we introduce a resistor network to mimic the resulting anisotropic conducting medium. The experimental prototype is fabricated using metal film resistors, and the measured results agree perfectly well with theoretical predictions. This study gives the first experimental verification of a dc carpet cloak, which expands the application of TO theory, and has potential applications in related areas.

It is experimentally verified that a two-dimensional planar focusing antenna based on gradient-index metamaterials has a similar performance as that of its parabolic counterpart. The antenna is designed using quasi-conformal transformation optics, and is realized with non-resonant I-shaped metamaterial unit cells. It is shown that the antenna has a broad bandwidth and very low loss. Near-field distributions of the antenna are measured and far-field radiation patterns are calculated from the measured data, which have good agreement with the full-wave simulations. Using all-dielectric metamaterials, the design can be scaled down to find applications at optical frequencies.

We propose a robust method to design arbitrarily directional cloaks by solving the Laplace’s equation. The method is based on the orthogonality between light rays and eikonal curves. By employing the analogy between light rays and electric force lines, and the equivalence between eikonal curves and isopotential lines, the original problem in the geometrical optics is converted into a boundary value problem in electrostatics. Examples are given and the simulation results validate the design. The method can also be used to design other optical-transformation devices and be extended to solve three-dimensional problems. The medium parameters designed by the method are easily realized using metamaterials or normal materials, and hence, the method can find applications in the optical and microwave engineering.

Following the theoretical work on arbitrary waveguide bends using gradient-index and isotropic materials, a 90-degree bend structure is experimentally realized using artificial metamaterials. Broadband and low-loss unit cells, the I-shaped cells, are used in the laboratory prototype. Field mapping measurement validates the design. The method can be directly and easily extended to the design of directional cloaks, microwave lens, beam shifters, and beam-steering devices.