Normal Incident Wave Research Articles

P-wave attenuation and dispersion in a fluid-saturated rock with aligned rectangular cracks

It is well known that mesoscopic-scale wave-induced fluid flow (WIFF) between cracks and surrounding porous matrix is one of significant mechanisms in causing seismic dispersion and attenuation. Previous theoretical models that were used to interpret the interaction between cracks and passing waves assumed the cracks to be circular or slit-like. However, in many actual cases, the cracks can have a shape more close to a rectangle. In this paper, we develop an effective medium model to estimate the P-wave attenuation and velocity dispersion in a saturated porous rock containing a random distribution of permeable, aligned, rectangular cracks of infinitesimal thickness. This is done by combining the far-field displacement representation of the solution of a normally incident P wave scattered by a single crack and Foldy's scattering theorem. The effective low-frequency P-wave velocity predicted by the present model is asymptotically consistent with the Gassmann's theory. It is shown that thorough knowledge of the crack shape is important for understanding seismic signatures. Specifically, the WIFF relaxation frequency is mainly determined by the crack intermediate dimension, while it is insensitive to the crack largest dimension. Moreover, P-wave velocity of the rectangular crack model can be quite different from those of the circular crack model and the slit crack model, although the overall frequency-dependent trends of the velocities for different models are very similar. Analogously to the circular crack model and the slit crack model, at the normal incidence of a fast-P wave, the WIFF is found to dominate the wave attenuation and velocity dispersion. These results are helpful to estimate formation parameters from different frequency components of observed seismic data.

Emerging crescentic patterns in modelled double sandbar systems under normally incident waves

Abstract. Double sandbar systems often characterize the surf zone of wave-dominated beaches and display a variety of poorly explained spatial configurations. Here, we explore the morphodynamic stability of double-barred beaches using a model based on linear stability analysis. Surf zone hydrodynamics are described by coupling depth and wave-averaged conservation of mass and momentum with the wave energy and phase equations, including roller dynamics. A simplified sediment transport formulation relates flow to seabed changes. Under normal wave incidence, an alongshore uniform coast with a cross-shore profile characterized by the presence of two sandbars can be unstable, thereby resulting in the development of crescentic and/or rip channel patterns. Our study demonstrates that sandbar coupling can be either in phase (highs and lows of both sandbars are at the same alongshore position) or out of phase (highs and lows of one sandbar correspond to lows and highs of the other sandbar). In line with observations, results of simulations show a large variability in the possible emerging bottom patterns. Our analysis indicates that modes of which the amplitude of the inner sandbar perturbation is larger than that of the outer sandbar are dominant for large height–depth differences between the two sandbar crests and small offshore wave heights. Patterns related to the outer sandbar dominate for small values of the difference in sandbar depth. For intermediate differences between the two sandbar depths, patterns on both longshore bars appear to be fully coupled (similar growth rates and strongly correlated pattern shapes). For relatively large waves and large depth over the outer sandbar, patterns can also develop close to the shoreline and/or inner surf zone together with patterns in the inner or outer sandbar.

Experimental studies of scattering indicatrixes of radar absorbing materials

There is some kind of materials widely used in microwave engineering that known as radar absorbers. Whether it is the aircraft coating, the anechoic chamber interior or waveguide feed load. Anechoic chamber absorbers are typically characterized by reflectivity at normal incidence of electromagnetic wave. The study of the angular dependence of reflectivity for several absorber samples and the appraising of the impact of their combination on the anechoic chamber performance are the main aims of present work. For this purpose the experimental studies were carried out in an antenna measuring and computing complex based on the anechoic chamber. Obtained results may be useful in the anechoic chamber design and construction stages as well as the modernization of existing ones. It can also be used for modelling of the internal electromagnetic field distribution of indoor field test ranges.

A Low-Cost, High-Efficiency and Full-Metal Reflectarray Antenna With Mechanically 2-D Beam-Steerable Capabilities for 5G Applications

This article presents a low-cost, high-efficiency, and full-metal reflectarray (RA) antenna with mechanical beam-steerable capabilities. A unit cell (UC) implemented by a metal cylinder with a cuboid-shaped notch is proposed to achieve a 1 bit reflection phase (0 and $\pi $ ) for the transverse electric (TE) and transverse magnetic (TM) normal incidence waves. The proposed UCs can be employed to construct an RA antenna with 2-D beam-steerable abilities. For demonstration, the RA antennas with six different beam directions are presented as examples, by simply adjusting the rotations of the UCs. The impedance matches of the RA antenna are all below −10 dB from 22 to 33 GHz, and the measured radiation patterns of the RA antenna are all highly consistent with the simulated counterparts for the six different beam direction scenarios. A 1.5 dB gain drop bandwidth of approximately 20% from 24.7 to 30 GHz and a peak realized gain of 18.9 dBi at 26 GHz are experimentally obtained for the RA antenna with radiation patterns at broadside. Compared to the substrate-based UCs loaded with p-i-n diodes, the proposed UCs have full-metal structures without using any active RF components and dielectric substrates, leading to the high total efficiency of the proposed RA antenna. Due to the low-cost, high-efficiency, and high-power handling properties, the proposed RA antenna can provide a fixed or scanning beam and is a good candidate for 5G millimeter-wave communication applications.

Wideband Angular Stable Absorber Based on Spoof Surface Plasmon Polariton for RCS Reduction

A wideband absorber with high angular stability based on spoof surface plasmon polariton (SSPP) is proposed in this letter. The absorber consists of a metal ground and an SSPP structure that comprises nine incurved metallic strips loading lumped resistors. It realizes wideband absorption in the range of 6.95−17.4 GHz (factional bandwidth of 85.8%) for the normal incident wave and maintains high absorptivity when the incident angle tilts to 60° and 65° for transverse electronic (TE) and transverse magnetic (TM) waves, respectively. As a reflector of a Vivaldi antenna, the proposed absorber reduces the bistatic radar cross section (RCS) effectively and does not decrease the performance of the antenna. Prototypes of the absorber and antenna are fabricated and measured. The simulated and measured results have a good agreement, which verifies the wide-angle absorption of the proposed absorber and its good work in bistatic RCS reduction.

Creation of metasurface from vertically aligned carbon nanotubes as versatile platform for ultra-light THz components

Here a simple and reproducible method for obtaining terahertz metasurfaces formed from multiwall carbon nanotubes (MWCNTs) is presented. The metasurfaces were obtained from a vertically aligned array of MWCNTs using a laser engraving technique followed by polymer covering. The structures under study demonstrate frequency-selective reflection in terahertz range following the Huygens–Fresnel formalism. For a normal incidence of the electromagnetic wave, the model for numerical calculation of backscattering from the metasurfaces was proposed. Lightweight and compact MWCNT-based metasurfaces are capable to replace conventional pyramidal absorbers and could serve as a versatile platform for scalable cost-efficient production of ultra-light electromagnetic components for THz applications.

Research of the Sound-Absorbing Structureshape Influence on the SAS Acoustic Efficiency in the Nonlinear Operation Mode

This research is focused on the acoustic efficiency of various shapeHelmholtz resonators on non-linear operating modes, with an acoustic pressure level of 130 dB. In this work physical and mathematical models for predicting the effective acoustic properties of SAS cells are formulated. Numerical studies have been performed to assess the acoustic efficacy of honeycomb and cone-shaped sound-absorbing structures samples in an interferometer with a normal incidence of a sound wave in a nonlinear operation mode. The validation of the developed mathematical models was carried out. A comparison of the numerical calculation results with the results of laboratory studies has been carried out. According to the results of the conducted computing experiments, the most effective resonators were identified, and recommendations on the SAS design based on the most effective resonators were formulated.

Ultrathin Perfect Absorbers for Normal Incident Waves Using Dirac Cone Metasurfaces With Critical External Coupling

We propose an ultrathin perfect absorber for normal incident waves using a Dirac cone metasurface with the thickness of 1/38 free-space wavelength. First, we prepare Dirac cone metasurfaces consisting of periodic metallic chain patterns on a dielectric substrate having the $\Gamma $ -point frequencies of approximately 30 GHz. The perfect absorber is designed by adjusting the external coupling of the Dirac cone metasurfaces with their chain densities. Based on the one-port resonator model, we clarify that the perfect absorber is realized at the critical coupling condition with identical internal and external $Q$ -factors. We experimentally demonstrate a perfect absorption with the reflectance as small as −37 dB by the metasurface with the critical external coupling.

Epsilon near zero metasurface for ultrawideband antenna gain enhancement and radar cross section reduction

A non-resonant metamaterial unit cell is proposed to design a metasurface for ultrawideband (UWB) gain enhancement and radar cross section (RCS) reduction of an UWB antenna. Epsilon near zero (ENZ) property and negative refractive index of metamaterial is achieved to ensure low loss and amplification of electromagnetic waves (EM) passing through it. The proposed metamaterial is designed on a 6 mm × 6 mm FR4 substrate with dielectric constant 4.4, height 1.6 mm and loss tangent 0.01. The unit cell is simulated with periodic boundary condition to get the properties of periodically arranged unit cells in one plane called metasurface. A Metasurface is designed by planar arrangement of 5×5 unit cells of proposed metamaterial unit cell and is kept at height of 2 mm above an ultrawideband planar microstrip antenna. Antenna system with metasurface has physical and electrical dimensions of 32 × 32 × 5.2 mm3 and 0.34λ0×0.34λ0×0.05λ0 respectively, where λ0 is the free-space wavelength at 3.2 GHz. The bandwidth of UWB patch antenna (3.1–10.6 GHz) is unaffected by the presence of metasurface. The analysis of radiation mechanism shows the phase difference of EM waves passing through metasurface with and without reflection is 300°–400°. This indicates emergence of coherent waves from metasurface and contributes in gain enhancement. Maximum gain is 5.7 dB at 6.8 GHz with radiation efficiency of 88.5% and maximum gain enhancement is 10.1 dB at 9.8 GHz. The reflection phase analysis of normal incident wave predicts the frequency for maximum RCS reduction as 6.4 GHz. Maximum RCS reduction of antenna with application of metasurface is 21 dB at 6.7 GHz. Significant gain enhancement and reduction of RCS is achieved for ultrawideband. The proposed antenna system metasurface with enhanced gain and reduced RCS is a good candidate for application in stealth and military platforms.

Infrared localized surface plasmon dark multipole modes generated on subwavelength corrugated metal disks

Terahertz (THz) radiation is very promising for chemical and biological sensing. One of the ways to realize a THz sensor is exciting multipole localized surface plasmon resonances (LSPRs) on subwavelength corrugated metal disks. In this paper, numerical modeling and analysis of LSPRs on a periodic array of such structures were made. At normal incidence of the incoming wave, only dipole resonances can be excited. An additional C-shaped resonator placed in the vicinity of the disk produces dark multipole modes (quadrupole, hexapole, octupole, and decapole ones) due to hybridization of these modes with the brigth dipole mode of the C resonator. Besides, multipole modes can be excited at oblique illumination of the structure. With increase in the incident angle, the multipole resonances become stronger. The high Q-factor resonance modes observed for PEC disks are wider or disappeared in case of Drude gold permittivity.

A switchable terahertz reflectarray consisting of crossed metal dipoles set in a graphene grid

A switchable reflectarray antenna composed of crossed metal dipoles mounted on a graphene grid is proposed for switchable radiation patterns at 1 THz. For an individual radiating element, the single graphene square loop works together with the metal dipoles to offer a corresponding phase response. As an entirety, the grid formed by the attached graphene loops is connected to a particular bias electrode for switchable operation. Taking the practical manufacturing technique into account, the graphene-oxide-silicon structure is used as the bias electrode. The simulated phase response shows that a phase range over 330° is obtained by changing the dimension of the metal dipoles from 11 μm to 70 μm. The simulated field distributions and radiation patterns demonstrate that the proposed structure can deflect the normal incident plane waves in the predesigned direction when it is biased in a specific state. Once the bias is set to zero, incident plane waves are scattered into different directions.

Locally programmable metamaterial elements using four-dimensional printing

Within the area of acoustic metamaterials, the field of tunable acoustic structures has gained a large amount of interest for their ability to adapt to a wide range of frequencies. In this work, we explore the intersection of tunable acoustic structures and additive manufacturing by 4D printing a metamaterial element whose acoustic response can be locally, actively tuned via resistive heating. Contrary to existing, thermally tunable structures, which rely on ambient heating for elastic modulation, the 3D printed structure presented here can be precisely controlled to selectively soften or stiffen only desired areas of the structure. We demonstrate this by tuning the acoustic response of membrane structures since many acoustic metamaterial phenomena utilize membrane geometries. Specifically, we present three geometries of tunable membranes, a structure which decreases frequency response upon heating, a structure which increases frequency response upon heating, and a structure which exhibits non-axisymmetric acoustic mode response from normal incident plane waves.

Simultaneous Perfect Bending and Polarization Rotation of Electromagnetic Wavefront Using Chiral Gradient Metasurfaces

We introduce chiral gradient metasurfaces that allow perfect transmission of all the incident wave into a desired direction and simultaneous perfect rotation of the polarization of the refracted wave with respect to the incident one. Besides using gradient polarization densities which provide bending of the refracted wave with respect to the incident one, using metasurface inclusions that are chiral allows the polarization of the refracted wave to be rotated. We suggest a possible realization of the proposed device by discretizing the required equivalent surface polarization densities realized by proper helical inclusions at each discretization point. By only using a single optically thin layer of chiral inclusions, we are able to unprecedentedly deflect a normal incident plane wave to a refracted plane wave at $45^{\circ}$ with $72\%$ power efficiency which is accompanied by a $90^{\circ}$ polarization rotation. The proposed concepts and design method may find practical applications in polarization rotation devices at microwaves as well as in optics, especially when the incident power is required to be deflected.

Design and equivalent circuit model extraction of a broadband graphene metasurface absorber based on a hexagonal spider web structure in the terahertz band.

In this paper, an ultra-wideband terahertz absorber is designed utilizing a graphene-based metasurface. The absorber is composed of three layers including the graphene metasurface, Topas-cyclic olefin copolymer dielectric substrate, and a gold ground layer. The particle swarm optimization algorithm and interpolate quasi-Newton optimization are utilized to find an optimized structure with the widest bandwidth. Full-wave simulations verify achieving absorbance of more than 90% in an extremely wide frequency band within the range of 1 THz to 3.5 THz (fractional bandwidth = 111%) under illumination of a normal incident wave. The proposed structure is polarization insensitive up to a polarization angle of 75°, while the performance of the absorber (absorbance level and bandwidth) is almost fixed for incident angles $ \theta $θ up to 60°. Moreover, the switching capability of the structure from reflection ($ {\gt} 92\% $>92%) to absorption ($ {\gt} 90\% $>90%) is investigated. The equivalent circuit model is extracted for the designed absorber, and the corresponding result is compared to that of the full-wave simulation, which confirms the validity of the extracted circuit.

Multifunctional linear‐polarized terahertz focusing metasurface

AbstractIn this article, we propose a multifunctional linear‐polarized terahertz focusing metasurface enabling terahertz focusing at different focal distances for efficient terahertz wave‐front manipulation. Due to the subtle resonance response of two orthogonal electric dipoles in each unit structure, the change in length will contribute to the phase shift depending on the polarization direction of the normal terahertz wave, correspondingly, phase distributions on the metasurface can be efficiently controlled in both vertical and horizontal directions. Numerical simulations demonstrate that two different focal distances can be achieved under a normal incidence wave with different polarization. The one focal distance is 12 mm under normally incident plane wave with x polarization, while another focal distance is 4.4 mm under y‐polarized component at 0.30 THz. This design method can significantly expand the capabilities of metasurfaces in controlling EM waves and open a pathway in designing multifunctional meta‐devices to satisfy diversified application demands.

Experimental and Numerical Assessment of Local Resonance Phenomena in 3D-Printed Acoustic Metamaterials

Abstract The so-called locally resonant acoustic metamaterials (LRAMs) are a new kind of artificially engineered materials capable of attenuating acoustic waves. As the name suggests, this phenomenon occurs in the vicinity of internal frequencies of the material structure and can give rise to acoustic bandgaps. One possible way to achieve this is by considering periodic arrangements of a certain topology (unit cell), smaller in size than the characteristic wavelength. In this context, a computational model based on a homogenization framework has been developed from which one can obtain the aforementioned resonance frequencies for a given LRAM unit cell design in the sub-wavelength regime, which is suitable for low-frequency applications. Aiming at validating both the proposed numerical model and the local resonance phenomena responsible for the attenuation capabilities of such materials, a 3D-printed prototype consisting of a plate with a well selected LRAM unit cell design has been built and its acoustic response to normal incident waves in the range between 500 and 2000 Hz has been tested in an impedance tube. The results demonstrate the attenuating capabilities of the proposed design in the targeted frequency range for normal incident sound pressure waves and also establish the proposed formulation as the fundamental base for the computational design of 3D-printed LRAM-based structures.

Topographic and coastline influences on surf Eddies

Rip currents arise from the momentum flux associated with depth-induced breaking of incident surface gravity waves on beaches with irregular bottom topography. In 3D simulations with an idealized topographic configuration, uniform density, and a steady incident wave field, a striking enhancement of transient surf eddy variability occurs over irregular topography compared to smooth topography, especially at nearly normal incident wave angles. Alternatively, with highly oblique incident waves, transient shear instability is observed over smooth topography whereas for irregular topography, standing rip eddies are generated. In the presence of larger-scale coastline variations, mega-rip circulations emerge, and they are especially strong for embayments. In all cases with significant transient surf-eddy activity, the eddy-induced mean horizontal transport is a non-trivial component of the total mean transport, and sometimes it is much larger than the gravity-wave Stokes drift. Typically the transport by the eddy-induced flow partially cancels the Eulerian transport by the time- and/or alongshore-averaged currents, but not by enough to reverse the “residual” pattern of total transport.

On the influence of lateral cells and local perforation of resonance liner samples on the accuracy of determining their acoustic characteristics in experimental studies

When determining the acoustic characteristics of a single-layer sample of a honeycomb liner by experiments in interferometers with normal wave incidence, a significant effect of incomplete lateral cells of the sample on acoustic characteristics was revealed. The experiments were carried out on two interferometers with channel diameters of 30 and 50 mm in the frequency range 500-3500 Hz at sound pressure levels of 120-155 dB. Based on the results of the experiments, an assessment was made of the effect on the acoustic characteristics of the percentage of perforation, which is the same both for one honeycomb cell and for the entire SAS sample. Based on the results of the research, recommendations were made to improve the accuracy of obtaining the acoustic characteristics of samples of honeycomb liner when testing them in interferometers of normal wave incidence.

Reconfigurable Frequency Selective Rasorber Covering Extremely Wide Transmission Frequency Range

Rasorbers with reconfigurable transmission frequency are of great interest over the past few years. However, existing devices exhibit a limited number of operation modes and cover a narrow transmission frequency range, making them unacceptable for wideband applications. In this work, a high-performance reconfigurable rasorber with mechanically-controlled frequency selective surface (FSS) and an electronically-tuned resistive layer is proposed. Theoretical analysis based on transmission line theory and numerical simulations is performed to investigate the performance of the proposed device. The rasorber were fabricated and free-space measurements were also carried out to validate the simulation results. The device can operate between single-, dual- and triple-passband modes by mechanically tuning the FSS under a fixed external bias voltage. Under single-passband mode, the transmission frequency can also be tuned continuously through simultaneously controlling the mechanically-tunable FSS and electronically-tunable resistive layer. A minimum insertion loss as low as 0.16 dB at 4.03 GHz is achieved from the measurement results. The maximum measured fractional absorption bandwidth is 87.9%, comparable to the state-of-art maximum absorption bandwidth of existing reconfigurable rasorber. The measured overall fractional transmission frequency range under normal incidence wave is 63.9%, far beyond that of any other existing reconfigurable counterparts.

Experimental Study of the Cells Interaction on Linear and Nonlinear Operating Modes

In this article, the results of SAS samples laboratory research on an interferometer with a normal sound wave incidence at sound pressure levels of 100 and 130 dB are presented. The mutual influence of prismatic resonators was revealed, which leads to a decrease in the resonators group acoustic efficiency.