Normal Incident Wave Research Articles

An Efficient Integral Equation Approach to Study Wave Reflection by a Discontinuity in the Impedance-Type Surface Boundary Conditions

We investigate the interaction of obliquely incident waves by a discontinuity in the surface of water whose bottom is composed of non-dissipative porous medium. The discontinuity arises when two inertial surfaces with different surface densities float on the surface of the water, thereby giving rise to two different impedance-type surface boundary conditions. The two-dimensional problem is solved by employing Green’s second identity which leads to two coupled Fredholm integral equations in the potential function and the velocity across the vertical line below the point of discontinuity. Expressions for the reflection and the transmission coefficients are determined in terms of the solutions of the integral equations. The numerical results for the reflection coefficient are validated against the well known results for the normal incidence of the surface waves and rigid bottom available in the literature. The reflection coefficient is also analyzed for different values of the surface densities of the inertial surfaces, porous effect parameter and the angle of incidence of the surface waves. It is observed that the porous bottom causes resonance in the reflection coefficient for a particular wave frequency. Graphs for the surface profiles are also presented which reveal that the presence of inertial surface reduces the heights of the waves in a significant amount.

SIBC incorporated in conformal FDTD to efficiently simulate the thin conductive layer of periodic structure

In this study, surface impedance boundary condition (SIBC) incorporated in conformal finite-difference time-domain (CFDTD) to efficiently simulate the thin conductive layer of periodic structure is proposed. The method is based on the vector fitting (VF) technique to approximate the frequency-dependent surface impedance of the thin conductive layer by a series of partial fractions in terms of real or complex conjugate pole-residue pairs. A discrete-time equation of SIBC is then derived and incorporated in CFDTD update equations by Laplace transform and time central difference methods. The proposed method takes the following advantages, (i) the time step is no longer bounded by the fine resolution of the thin layer; (ii) The complicated recursive convolution approach is avoided, and the approximation of the frequency-dependent surface impedance of the thin conductive layer can be easy to handle with the VF method. This method is numerically verified for the time- and frequency-dependent scattering characteristics of several periodic structures behaved under normal and oblique incident waves.

Effective-surface modeling of infinite periodic metascreens exhibiting the extraordinary transmission phenomenon

The consistent analysis of metallic screens perforated with subwavelength apertures of diverse geometrical patterns (metascreens) by means of a systematic methodology is presented in this paper. The principal concept of the new technique stems from the efficient characterization—via the use of effective surface parameters—of the metascreen’s complementary array of scatterers (i.e., a metafilm) and the proper utilization of the Babinet duality principle. To this end, a previously developed model for metafilms is carefully extended (for normal plane wave incidence) in order to apply to frequencies where the extraordinary transmission phenomenon typically occurs. General-purpose analytical formulas for the reflection and transmission coefficients of an arbitrary metascreen are also derived as part of this generalized framework. Finally, the predictions for the extraordinary transmission spectra through various metascreens operating in the microwave regime, as obtained via two distinct realizations of the proposed formulation, are compared with the results of a commercial computational package in order to verify the precision of the developed technique and provide instructive physical insights.

Experimental realization of acoustic metasurface with double-split hollow sphere

Abstract We experimentally present an acoustic metasurface (AMS) with sub-wavelength thickness based on the meta-molecule consisting of eight different sized double-split hollow spheres (DSHSs). By designing the discontinuous phase profile covered 2 π span induced by the DSHSs, the AMS can manipulate the reflected acoustic waves in a way that could not be imitated by natural materials. Both simulations and experiments show that the AMS can realize anomalous reflection, i.e., a normal incident wave can be reflected into an oblique direction. Moreover, the reflection angle can be flexible controlled by mechanically tuning the spatial distance of the DSHSs in the AMS, which is consistent with the generalized Snell׳s law.

Dielectric structure design for microwave cloaking considering material properties

To hide a metallic object from outside observers, we designed cloaking structures to manipulate the propagation path of the microwave at a specific frequency. Dielectric materials are used to realize the cloaking effect and their electromagnetic properties are evaluated to reduce the size of the cloaking structure as well as to take the loss effect into account. We used the structural design method based on the phase field method to control the electric field flow around a target object for cloaking. It is aimed to minimize the scattered electric field measured at the region located behind the target object for a normal incident wave in the X-band frequency range. Numerical examples are given to verify the suggested design process and its results.

Dissipation of Impact Stress Waves within the Artificial Blasting Damage Zone in the Surrounding Rocks of Deep Roadway

Artificial explosions are commonly used to prevent rockburst in deep roadways. However, the dissipation of the impact stress wave within the artificial blasting damage zone (ABDZ) of the rocks surrounding a deep roadway has not yet been clarified. The surrounding rocks were divided into the elastic zone, blasting damage zone, plastic zone, and anchorage zone in this research. Meanwhile, the ABDZ was divided into the pulverizing area, fractured area, and cracked area from the inside out. Besides, the model of the normal incidence of the impact stress waves in the ABDZ was established; the attenuation coefficient of the amplitude of the impact stress waves was obtained after it passed through the intact rock mass, and ABDZ, to the anchorage zone. In addition, a numerical simulation was used to study the dynamic response of the vertical stress and impact-induced vibration energy in the surrounding rocks. By doing so, the dissipation of the impact stress waves within the ABDZ of the surrounding rocks was revealed. As demonstrated in the field application, the establishment of the ABDZ in the surrounding rocks reduced the effect of the impact-induced vibration energy on the anchorage support system of the roadway.

Analysis of TLM air-vent model applicability to EMC problems for normal incident plane wave

Analysis of TLM air-vent model applicability to EMC problems for normal incident plane wave

On the Acoustical Absorption of a Sandwich Plates with Perforated Material by the Transfer Matrix Method

In this paper, the acoustical absorption of sandwich plates with perforated material, for normal incidence wave to layers, is studied by using the transfer matrix method. The effect of acoustical, mechanical and geometrical parameters of the plies is analyzed. In the first part, the results of the acoustical absorption coefficient for a perforated plate are compared to those of fibrous open perforated material. In the second part the influence of the addition of skins on both sides of the layer is studied. The third part relates to the study of sensitivity of various parameters of a sandwich plate in perforated material. Finally, the effect of the number of reasons is studied. The obtained results are compared with an analytical and experimental results found in the literature. This leads to conclude about the acoustical performances of the sandwich plates with perforated material.

Stochastic seismic wave interaction with a rock joint having Coulomb-slip behavior

Seismic wave interaction with a slippery rock joint with an arbitrary impinging angle is analytically studied based on the conservation of momentum on the wave fronts. Based on the displacement discontinuity method, the wave propagation equations are derived for incident P- and S-waves. By comparison, the calculated transmission and reflection coefficients for normal incident waves are the same as the existing results, which proves the wave propagation equation obtained in the paper is correct. The wave propagation derived in the context can be applied to incident waves with different waveforms. Stochastic seismic waves are then used to analyze the seismic wave interaction with the slippery rock joint, where the stochastic seismic waves are generated from frequency spectra. The parametric studies are carried out to investigate the effect of type, intensity and impinging angle of the incident seismic waves on the wave propagation across the slippery rock joint.

Analyzing bull’s eye structures by a vertical mode expansion method with rotational symmetry

A bull’s eye structure is a metallic film with a circular subwavelength aperture surrounded by concentric annular grooves. It is an important structure for realizing applications of the extraordinary optical transmission phenomenon. The structure is invariant under rotations about the central axis perpendicular to the film. In this paper, an efficient numerical method is developed for analyzing bull’s eye and other structures that consist of different annular regions where the material properties are one-dimensional. The method is a new variant of the recently developed vertical mode expansion method which combines field expansions in one-dimensional eigenmodes with various techniques for solving scalar two-dimensional Helmholtz equations. The method exploits the rotational symmetry by solving the different Fourier components separately. For normal incident waves, the method is particularly efficient, since it is only necessary to solve one Fourier mode. The method is used to analyze bull’s eye structures with different configurations. In particular, we found that the normalized transmission coefficient can be larger than 52 for a bull’s eye structure with 22 grooves.

Planform and mobility in the Meaípe-Maimbá embayed beach on the South East coast of Brazil

The Meaípe-Maimbá embayed beach (MMEB) on the south-east coast of Brazil has been subject to anthropogenic pressures since the 70's. In this study we discuss the adequacy and contribution of the parabolic planform model to determine the planform and variability of the MMEB, taking into consideration variation in wave conditions. The role of different controlling conditions on the planform variability is analyzed, as well as the morphological and planform mobility. MMEB exhibited a new configuration in response to the construction of a harbor, which interrupted the longshore sediment transport. After four decades, three particular morphodynamic sectors have been recognized along the beach. The central sector is more exposed to normal wave incidence and cross-shore processes predominate. The northern and southern sectors are influenced by wave diffraction processes around the headlands and port, respectively. In the northern sector, the presence of secondary headlands and inner islands imposed a geomorphological control on beach morphology and coastal processes. The use of the parabolic planform model provided useful insights for the assessment of potential planform mobility, since the decadal shoreline evolution combined with beach profiles and sediment characteristics allowed understanding of the beach mobility processes and supported the interpretation of modeling results.

Robustness of plasmonic angular momentum confinement in cross resonant optical antennas

Using a combination of photoemission electron microscopy and numerical simulations, we investigated the angular moment transfer in strongly enhanced optical near-fields of artificially fabricated optical antennas. The polarization dependence of the optical near-field enhancement has been measured in a maximum symmetric geometry, i.e., excitation by a normal incident planar wave. Finite-difference time-domain simulations for the realistic antenna geometries as determined by high-resolution electron microscopy reveal a very good agreement with experimental data. The agreement confirms that the geometrical asymmetries and inhomogeneities due to the nanoscale fabrication process preserve the circular polarization in the gap regions with strong near-field enhancement.

Transmission of light through double gold nanobars embedded in split ring pair array

The optical transmission of a normal incident wave through double nanobars embedded in split ring pair (DNSRP) array was investigated by using the finite difference time domain (FDTD) method. It is demonstrated that, compared to double nanobars embedded in perfect ring pair structure, the transmission spectra of DNSRP array showed more dips and peaks. Along with the changes of slit widths, distances between the double nanobars and the environmental dielectric constant, the optical and plasmonic characteristics of the DNSRP array were tuned. Based on the electric field distributions, we discuss the physical mechanism of light passing through the DNSRP array.

2D modeling of turbulence wave number spectra reconstruction from radial correlation reflectometry data

Computational analysis of radial correlation reflectometry in the framework of a model accounting for 2D probing wave propagation and scattering is performed. The procedure of the turbulence wavenumber spectra reconstruction from radial correlation reflectometry data previously proposed in the case of normal incidence of probing wave onto the 1D plasma is justified for the real geometry of a reflectometry experiment at large and modest scale fusion plasmas.

Optimization of parabolic bars for maximum Bragg resonant reflection of long waves

This paper presents an analytical solution for the problem of the long wave reflection by a series of artificial bars with parabolic configuration in terms of the associated Legendre functions. It is shown that both the reflection and transmission coefficients depend solely upon the number of bars, the dimensionless bar height, the dimensionless bar width and the dimensionless bar distance. Particularly, under the Bragg resonance condition, i.e., the distance between two adjacent bars is about half of the wavelength of the normal incident waves, the analytical solution for the peak Bragg resonant reflection is obtained, which reveals that the peak Bragg resonance depends upon the number of bars, the dimensionless bar height and the dimensionless bar width. Based on this solution, the optimization of the parabolic bars is made to obtain the maximum Bragg resonance and a group of optimal curves, which may be very useful in the design of Bragg breakwaters with parabolic bars.

On- and off-optical-resonance dynamics of dielectric microcylinders under plane wave illumination

We explore the on-resonance and off-resonance optical response of dielectric cylinders excited by normal incident plane waves. Both the analytical method, based on Mie theory, and the numerical method, implemented with the spectral element method, are undertaken in the study. We demonstrate that the whispering gallery mode characteristic of resonance behavior is strongly dependent on the refractive index and radius changes. Detuning of either parameter deteriorates the resonance action and creates yet another exciting phenomenon known as photonic nanojets. The subwavelength light focusing property can be associated with nanojets, and engineering the parameters yields strong field confinement and slowly diffracting beam propagation. The current work investigates the optical properties of the dielectric microcylinders at close proximity to the resonance condition. Both strong field focusing associated with photonic nanojets and enhanced field localization linked with the resonance condition are desired for photon manipulation scenarios in nanophotonics.

A multiband THz bandpass filter based on multiple-resonance excitation of a composite metamaterial

We present a systematic numerical study on a metal-dielectric-metal (MDM) sandwich structure for multiple resonance transmission in terahertz (THz) region. The designed structure consists of periodic square close ring array on both side of a flexible dielectric substrate, exhibits a multiband transmission, with low average insertion loss, steep skirts and high out-of-band rejection. In addition, due to its rotationally symmetric structure, this filter is polarization-insensitive for normal incidence of the electromagnetic waves, keeping highly transmission at a wide range of incident angles for transverse electric waves and transverse magnetic waves. The metamaterial structure can be utilized as a desirable multiband filter with many practical applications, especially for THz communication, spectroscopic detection and phase imaging.

Radar cross section reduction of microstrip antenna based on wide-band metamaterial absorber

We design a wide-band metamaterial absorber (WBMA) with stable polarization and wide incident angle by loading a lumped resistance. The full-width at half-maximum of the absorber is 98%, the bandwidth of absorbing rate of more than 90% reaches 10.42 GHz, and the peak absorbing rate is 99.9% in the normal wave incidence. Loading the WBMA around the microstrip antenna by sharing the same substrate and ground plane, we fabricate the WBMA antenna, whose radar cross section (RCS) sharply decreases in the wide frequency range. Simulation and experimental results show that the antenna's radiation pattern is almost unchanged: just only the former gain improves 0.53 dB after loading the WBMA. Under different polarized waves, the antenna's monostatic RCS reduction is more than 3 dB within the working frequency band and beyond the working frequency band (6.95-17.91 GHz), the maximum value is 21.2 dB. The bistatic RCS decreases significantly from -48° to 48° at the middle working frequency (8 GHz), which well achieves the antenna stealth at the wide-band frequency and wide angle.

Dynamics of supramolecular structures induced in liquid crystals

A theory is experimentally validated for the first time, which describes (within nonequilibrium hydrodynamics) the properties of supramolecular spatially periodic structures in a planar layer of a cholesteric liquid crystal at ultrasound intensities exceeding the threshold of their formation in a frequency range where the wavelength is larger than the cholesteric helix pitch. Distortions in the layer texture in planar and wedgeshaped capillaries with planar boundary conditions at normal incidence of longitudinal waves have been observed for the frequency range of 1–4 MHz. The domain periods at the threshold of the effect and above it are determined for layers 20–100 μm thick at helix pitch values of 2–10 μm. The dependences of the domain period on the ultrasound intensity and the preliminary layer extension have been established. A model constructed with allowance for structural relaxation processes and nonlinear relaxation phenomena is tested. It is shown that a complete description of the effect, which is consistent with the experimental data, can be obtained only using approach.

Circularly polarized wave reflection focusing metasurfaces

The phase profiles of the reflected circularly polarized waves can be freely manipulated by virtue of a co-polarization reflective metasurface. Based on the co-polarization reflective metasurface, a circularly polarized wave reflection focusing metasurface can be achieved, it can make the reflected waves focus at a focal spot under the normal incidence of circularly polarized plane waves. In this paper, a reflection focusing metasurface is designed. It is found that around the central frequency f=16 GHz, the reflected waves focus on a focal spot above the metasurface with a focal distance L=200 mm under the normal incidence of right-handed circularly polarized waves. However, in the case of normal incidence of left-handed circularly waves, the reflected waves focus on an imaginary focal spot below the metasurface with the focal distance L=-200 mm. The beam-width at the focal spot and focal depth are also calculated by using CST Microwave Studio. The simulation results indicate that the beam-width at the focal spot is approximately equal to the operating wavelength. Therefore, the circularly polarized wave reflection focusing metasurface has a good performance for focusing the reflected waves. In addition, the proposed focusing metasurface displays the advantages of the long focal depth and the broad operating bandwidth.