Modes Of Wave Propagation Research Articles

Broadband Seismic Isolation of Periodic Ladder Frame Structure

Abstract Ladder frame structures are used as models for multistorey buildings. These periodic structures exhibit alternating propagating and attenuating frequency bands. Of the six different wave modes of propagation, two modes strongly attenuate at all frequencies. The other four modes have nonoverlapping stop band characteristics. Thus, it is challenging to isolate such structures when subjected to broadband, multimodal base excitation. In this study, we seek to synthesize a periodic ladder frame structure that has attenuation characteristics over the maximal range of frequencies for all the modes of wave propagation. We synthesize a unit cell of the periodic structure, which comprises two distinct regions having different inertial, stiffness, and geometric properties. The eigenvalues of the transfer matrix of the unit cell determines the attenuating or the nonattenuating characteristics of the structure. A novel pictorial presentation in the form of eigenvalue map is developed. This is used to synthesize the optimal unit cell. Also, design guidelines for suitable selection of the design parameters are presented. It is shown that a large finite periodic structure comprising a unit cell synthesized using the present approach has significantly better isolation characteristics in comparison to the homogeneous or any other arbitrarily chosen periodic structure.

Understanding the guided waves propagation behavior in timber utility poles

Guided stress waves are considered one of the most efficient and reliable techniques that provide sufficient quantitative and qualitative assessment. In this study, we focused on scrutinizing the propagation behavior of guided waves in western white pine timber poles, experimentally, and numerically using COMSOL Multiphysics. Macro fiber composites (MFCs), due to their flexibility and convenience to install on curved profiles, were used to actuate and sense guided waves along the tested specimens. Various solutions for wave mode tuning and characterization have been tested for traction free and embedded boundary conditions. The behavior of propagating wave modes was analyzed and compared in the two boundary conditions tested. Also, the excitation frequency, based on the dispersion curves generated for transversely isotropic timber, was selected to ensure the presence of favorable propagating (for instance longitudinal modes) modes with minimal dispersion. Undesirable wave modes—such as flexural modes (non-axisymmetric)—were eliminated by a ring design composed of multiple MFC actuators coupled around the pole’s circumference. The remaining propagating longitudinal modes and their reflections, such as modes L(0,1) and L(0,2) propagating at nearly 1000 m/s and 800 m/s respectively, were significantly enhanced by the actuation of the ring which could be effectively used for the assessment process. The results demonstrated the complexity of the propagating modes in circular timber structures and the importance of the ring design in the excitation of the selected modes of interest and damping unwanted ones.

Propagation of Surface Waves along a Boundary of a Photorefractive Crystal with a Nonlinear Defocusing Coating

A composite waveguide structure is considered; it is a crystal with the diffusion mechanism of formation of a nonlinear photorefractive response, on the surface of which a coating with defocusing Kerr nonlinearity is deposited. It is shown that nonlinear surface extraordinary waves of two types (with different ranges of existence and field decay profiles) exist in it. The modes of nonlinear surface wave propagation, at which the field intensity can be maximum or minimum near the surface or at the crystal–coating interface, are determined.

Generalized analytical dispersion equations for guided Rayleigh-Lamb (RL) waves and shear horizontal (SH) waves in corrugated waveguides

Corrugated waveguides are periodic structures that exhibit important acoustic features. Features like acoustic bandgaps exhibit the ability to filter acoustic and ultrasonic frequencies. By varying the mean thickness, corrugation height, and periodicity, one can tune the propagating wave modes in the guide and thus can modify the bandgaps. This study aims to obtain a generalized Rayleigh–Lamb equation for corrugated waveguides such that a single equation is sufficient for both flat waveguides such as plates as well as corrugated and tapered plates. Further, the objective was to understand the effects of the corrugation height, periodicity, and mean thickness such that a physics-based predictive design of wave filters can be achieved. Instead of applying the Bloch–Floquet theorem to the displacement function directly, which is conventional, the theorem is applied to the scalar and vector wave potentials obtained from Helmholtz decomposition. The governing equations from these relationships after applying boundary conditions were then solved using a logical root-finding algorithm. To verify the generalized expressions guided wave band structure for a plate was obtained using Rayleigh–Lamb equation and compared with the generalized form setting the corrugation height to zero. The analytical solutions are then validated through a comparison with the results obtained from a rigorous finite element simulation. Finally, the effects on the propagating and evanescent wave modes due to the corrugation height and periodicity are studied for future reference.

Analysis of energy dissipation and crack evolution law of sandstone under impact load

Based on the split Hopkinson pressure bar (SHPB) laboratory tests, the dynamic mechanical properties and failure mode of sandstone are analyzed, and a SHPB numerical model is established by particle flow code (PFC). The dynamic stress equilibrium, stress wave propagation, stress-strain characteristics and failure mode are analyzed, respectively, which verifies the effectiveness of the model. Then we studied the impact failure process form both mesoscopic cracks and energy point of views. The results show that microcracks are activated in large quantities with the increasing of strain rate. When the crack density reaches a certain degree, the interaction between the cracks can not be ignored. The failure mode gradually changes from local tension–shear damage mode to axial splitting failure mode and then to crushing failure mode. During the impact failure process, the energy is mainly consumed by the generation of the cracks and the friction caused by the slip of the particles, namely, broken dissipation energy. As the impact load increases, the broken dissipation energy density shows the high–speed growth and the low–speed growth stage successively with a double exponential growth pattern. The friction energy increases continually by a certain percentage, which indicates it should be considered during the analysis of fracturing process. Moreover, the dynamic strength and fragmentation degrees are closely related to energy dissipation density.

The Complex Rayleigh Waves in a Functionally Graded Piezoelectric Half-Space: An Improvement of the Laguerre Polynomial Approach.

The research on the propagation of surface waves has received considerable attention in order to improve the efficiency and natural life of the surface acoustic wave devices, but the investigation on complex Rayleigh waves in functionally graded piezoelectric material (FGPM) is quite limited. In this paper, an improved Laguerre orthogonal function technique is presented to solve the problem of the complex Rayleigh waves in an FGPM half-space, which can obtain not only the solution of purely real values but also that of purely imaginary and complex values. The three-dimensional dispersion curves are generated in complex space to explore the influence of the gradient coefficients. The displacement amplitude distributions are plotted to investigate the conversion process from complex wave mode to propagating wave mode. Finally, the curves of phase velocity to the ratio of wave loss decrements are illustrated, which offers extra convenience for finding the high phase velocity points where the complex wave loss is near zero.

Modes of Radio Wave Propagation: Troposcatter

Modes of Radio Wave Propagation: Troposcatter

Analysis of Wave Patterns Under the Region of Macro-Fiber Composite Transducer to Improve the Analytical Modelling for Directivity Calculation in Isotropic Medium

Analytical modelling is an efficient approach to estimate the directivity of a transducer generating guided waves in the research field of ultrasonic non-destructive testing of the large and complex structures due to its short processing time as compared to the numerical modelling and experimental techniques. The wave patterns or the amplitude variations along the region of ultrasonic transducer itself depend on its behavior, excitation frequency, and the type of propagating wave mode. Depending on the wave-pattern of a propagating wave mode, the appropriate value of the amplitude correction factor must be multiplied to the amplitudes of the excitation signal for the accurate evaluation of directivity pattern of the ultrasonic transducers generating guided waves in analytical modelling. The objective of this work is to analyse the wave patterns under the region of macro-fiber composite (MFC) transducer to improve the accuracy of a previously developed analytical model for the prediction of directivity patterns. Firstly, the amplitude correction factor based on the wave patterns under the region of P1-type MFC (MFC-2814) transducer at two different frequencies (80 kHz, 3 periods and 220 kHz, 3 period) glued on 2 mm Al alloy plate has been estimated analytically in the case of an asymmetric (A0) guided Lamb wave. The validation of analytically estimated amplitude correction factor is performed by a proposed experimental method that allows analyzing the behaviour of MFC transducer under its region by gluing MFC on bottom surface and scanning the receiver on the top surface of the sample. Later on, the estimated amplitude correction factor is included in the previously developed 2D analytical model for the improvement in the directivity patterns of the A0 mode. The modified analytical model shows a significant improvement in the directivity pattern of the A0 wave mode in comparison to the results obtained by the previous model without considering the proper wave patterns. The results reveal that errors between the directivity estimated by the present modified 2D analytical model and experimental investigation are reduced by more than 58% in comparison to the previously developed analytical model.

Two‐Dimensional Full‐Wave Simulation of Whistler Mode Wave Propagation Near the Local Lower Hybrid Resonance Frequency in a Dipole Field

AbstractWe investigate the propagation of whistler mode waves near the local lower hybrid resonance (LHR) frequency in a dipole field with a two‐dimensional full‐wave model. First, we run a simulation in which a parallel whistler with frequency above the local LHR frequency is launched at the equatorial region in electron plasma. We find that whistler emission propagates along the dipole field line to a high latitude, turns quasi‐electrostatic where the wave frequency is close to the local LHR frequency, and continues to propagate until being absorbed. Then, the proton response is considered. We find that (1) a quasi‐electrostatic whistler reflects where the wave frequency is below the local LHR frequency and propagates to a larger L‐shell and lower latitude, (2) a strong standing‐wave pattern is formed in the LHR reflection region, and (3) the whistler emission turns from right‐hand circularly polarized to linearly polarized near the reflection region. Finally, we run a simulation in which a quasi‐electrostatic whistler is launched at a high latitude with a small‐scale density irregularity added as a depletion to the background plasma. We find that a small portion of quasi‐electrostatic whistler energy can be coupled to a parallel whistler, which can propagate to a much lower altitude while most of the wave energy experiences LHR reflection. Moreover, the mode coupling depends on the transverse and longitudinal sizes of the density irregularity. This makes a possible explanation of ground observations of nonducted whistler emission, which could have been reflected in the high‐latitude ionosphere and magnetosphere.

Jeans Instability of a Protoplanetary Gas Cloud with Radiation in Nonextensive Tsallis Kinetics

In the framework of Tsallis statistics, we study the effect of medium nonextensivity on the Jeans gravitational instability criterion for a self-gravitating protoplanetary cloud, the substance of which consists of a mixture of perfect gas and blackbody radiation. Generalized Jeans instability criteria have been found from the corresponding dispersion relations obtained both for a uniform cloud with radiation and for a rotating protoplanetary cloud. An integral generalized Chandrasekhar stability criterion for a gravitating spherical cloud has also been obtained. The presented results are analyzed for various values of deformation parameters $$q,$$ dimensionality $$D$$ of the velocity space and coefficient $$\beta $$, characterizing the fraction of radiation in the total pressure of the system. It is shown that radiation stabilizes the substance of nonextensive protoplanetary clouds, and for rotating clouds, the Jeans instability criterion is modified by the Coriolis force only in the transverse modes of perturbation wave propagation.

Нелинейные поверхностные волны в симметричной трехслойной структуре из оптических сред с различными механизмами формирования нелинейного отклика

The propagation of surface TM waves in a three-layer structure, which is a crystal thickness plate with a focusing nonlinearity of a Kerr type, sandwiched between uniaxial photorefractive crystals with a diffusion mechanism for the formation of nonlinearity is considered. Nonlinear surface waves with different attenuation patterns can propagate along the interfaces between the layers. Waves of one type attenuate with distance from the interfaces without oscillations into the depth of the outer layers of photorefractive crystals, and the waves of other type attenuate with oscillations. Wave profiles can have two forms of symmetry about the center of a three-layer structure: symmetric and antisymmetric. The two waves with a symmetric distribution exist with nonperiodic damping and the damping without oscillations. The waves only with antisymmetric distribution exist with nonperiodic damping and the damping without oscillations. The dependences of the propagation constant on the characteristics of the layered structure for the long-wave mode of surface wave propagation are found in an explicit analytical form. The conditions for their existence are indicated.

Распространение поверхностных волн вдоль границы фоторефрактивного кристалла с нелинейным дефокусирующим покрытием

A composite waveguide structure, which is a crystal with a diffusion mechanism for the formation of a nonlinear photorefractive response, the surface of which is coated with a non-linear Kerr defocusing type, is considered. It is shown that there exist TM polarized nonlinear surface waves of two types, which differ in the range of existence and the profile of field decay. The propagation modes of nonlinear surface waves are determined in which the field intensity can be maximum or minimum near the surface or at the boundary of the coated crystal.

Experimental Investigation of the Anomalously Low Friction Phenomena in Blocky Rock Systems

Rock masses can be regarded as a blocky rock system. After a disturbance load is applied, the anomalously low friction phenomenon may take place and cause geological disasters. A series of impact experiments on granite blocks were conducted to investigate the anomalously low friction phenomena. Vertical vibration, Fourier frequency spectrum, and horizontal motions were investigated. It can be found that the tensile phases of vertical vibration can reduce the maximum static friction force, namely, the shear strength. The quasi‐resonance operating mode of the rock blocks was observed. During the stress wave propagation, the vibration in the loading direction tends to transfer from high frequency to low frequency and the modes of stress wave propagation do not depend on disturbance energies. The observed translational and rotational motions were due to the initial shear force, which is less than the friction force with no disturbance load. Stability of the blocky rock system is very sensitive to the initial stress state. In the subcritical state, friction force reduction can easily break the equilibrium of forces along the contact surface and even a slight disturbance may make the horizontal motions happen, which may lead to geological disasters with great energy release.

Ionization wave propagation characteristics under different polarity of pulse waveforms in micro-DBD device driven by bipolar nanosecond pulse waveform

Different spatiotemporal modes of ionization wave propagation at opposite polarity of bipolar pulses in a micro-dielectric barrier discharge structure device are investigated. The device is fabricated on a heavily doped n-type silicon substrate, and a 1 cm × 1 cm square cavity is formed on the 180 μm-thick polyimide film. Different modes of ionization wave propagation determined by the polarity of bipolar pulses are observed, and the details of streamerlike mode and wavelike mode under positive and negative half cycles of pulses are investigated, respectively. The propagation speeds of streamerlike ionization waves and wavelike ionization waves are ∼120 km/s and ∼40 km/s on average and ∼150 km/s and ∼70 km/s in maximum, respectively. Different parameters of bipolar pulses, especially the rising time of pulses, are applied to the proposed device to explore the variation of ionization wave propagation properties. The results show that the modes of the ionization wave propagation are barely changed when the device is driven by different rising time pulses. However, the initial plasma generation time and propagation speed are greatly changed. With a decrease in the rising time from 400 ns to 50 ns, the initial plasma generation time is brought forward over 200 ns, and the ionization wave propagation speed is improved over 30% for both cases. The results imply great significance in the exploration of the dynamics of plasma discharge evolution and regulation of plasma discharge properties through manipulating the pulse parameters.

U-joint Double split O (UDO) shaped with split square metasurface absorber for X and ku band application

This paper reports, a metasurface absorber which is U-joint loaded Double Split O shaped with a square split patch at the center and Double Negative (DNG) property. The proposed absorber shows significant absorption for potential applications like radar signal absorption and satellite communication, remote sensing, RF shielding. The vertical U joint and split O mutually creates substantial resonance and expedite by the center square patch, to cover two consecutive X and Ku band absorption peaks at 11.56 GHz with 82.31%, 12.27 GHz with 98.91% and 14.19 GHz with 97.79%. Besides, balanced geometrical shape with insensitivity polarization for TE and TM mode wave propagation enhances the robustness and tunability of the structure. The absorption mechanism analyzed and discussed with the Finite Integration Technique (FIT). Characterization of S-parameter executed with an established method for dielectric property extraction and finally compared the proposed absorber with existing literature.

Dispersion characteristics of periodic structural systems using higher order beam element dynamics

In the current work, we elaborate upon a beam mechanics-based discrete dynamics approach for the computation of the dispersion characteristics of periodic structures. Within that scope, we compute the higher order asymptotic expansion of the forces and moments developed within beam structural elements upon dynamic loads. Thereafter, we employ the obtained results to compute the dispersion characteristics of one- and two-dimensional periodic media. In the one-dimensional space, we demonstrate that single unit-cell equilibrium can provide the fundamental low-frequency band diagram structure, which can be approximated by non-dispersive Cauchy media formulations. However, we show that the discrete dynamics method can access the higher frequency modes by considering multiple unit-cell systems for the dynamic equilibrium, frequency ranges that cannot be accessed by simplified formulations. We extend the analysis into two-dimensional space computing with the dispersion attributes of square lattice structures. Thereupon, we demonstrate that the discrete dynamics dispersion results compare well with that obtained using Bloch theorem computations. We show that a high-order expansion of the inner element forces and moments of the structures is required for the higher wave propagation modes to be accurately represented, in contrast to the shear and the longitudinal mode, which can be captured using a lower, fourth-order expansion of its inner dynamic forces and moments. The provided results can serve as a reference analysis for the computation of the dispersion characteristics of periodic structural systems with the use of discrete element dynamics.

Generalized Transfer Matrix Method for periodic planar media

Sound transmission through infinite planar multilayered structures characterized by in-plane periodicity is accurately and efficiently predicted by exploiting free wave propagation and Bloch modes. A through-thickness transfer matrix is derived for each layer by manipulating the dynamic stiffness matrix related to a finite element model of a unit cell. The transfer matrices of all the layers composing the structure account for the Bloch modes generated in heterogeneous layers. The proposed technique is equally appealing for in-plane homogeneous structures since few elements and no Bloch modes are needed in this case, ensuring high efficiency. In such a framework, the acoustic radiation or transmission of multilayered systems excited by an oblique plane wave can be assessed. The proposed approach is validated in case of structures consisting of heterogeneous layers by comparison with alternative approaches.

Combustion of Gasless Systems: Thermocapillary Convection of Metal Melt

The role of thermocapillary convection in combustion of gasless binary mixtures containing a low-melting reagent was explored by numerical modeling. Variation in relative amounts of reagents and starting sample porosity was found to change a mode of combustion wave propagation over the binary systems under consideration.

Synthetic conventional reflectometry probing of edge and scrape-off layer plasma turbulence

Microwave reflectometry techniques have been successfully applied to measure electron density fluctuations in magnetic confined fusion devices with good spatial and time resolutions. However, quantitative interpretation of turbulence measurements has driven continuous development of both analytical theory and sophisticated numerical codes in support of reflectometry. Comparisons between experimental and synthetic reflectometry have been performed previously while only recently realistic gyro-fluid simulations have been employed together with a full-wave code to simulate measured turbulence properties with reflectometry. In this work, we report on recent efforts to employ the two-dimensional full-wave code REFMUL to implement a synthetic reflectometry diagnostic to model plasma turbulence measurements on the edge and scrape-off layer (SOL) peripheral regions of fusion devices. Numerical descriptions of microscopic turbulence were obtained from both an analytical model (following a Kolgomorov-like wavenumber spectrum) and from the GEMR turbulence code based on gyro-fluid theory. Simulations of fixed frequency conventional reflectometry with ordinary mode (O-mode) wave propagation were carried out for both turbulence cases separately. Preliminary comparisons between synthetic measurements, numerical plasma characteristics, and experimental data from ASDEX Upgrade tokamak are made. As previously described in literature, regimes of linear and non-linear response occurring at low and high turbulence levels, respectively, are observed and characterized. Synthetic spectra across moderate to high turbulence levels display qualitatively good agreement with experimental data across the edge region.

Plasmonic wave propagation mode analysis of single and multi-layer graphene-pec structures

Plasmonic graphene-PEC structure is presented for THz frequency range in detailed analysis. The number of graphene layer effect on Surface Plasmon Polariton (SPP) wave behaviour is analyzed. Moreover, the graphene characteristics and propagation mode are presented. In the following, the proposed structure, graphene-PEC, analyzes in each electromagnetic field component in details. The numerical solver COMSOL Multiphysics is utilized to demonstrate that the graphene-PEC structure support quasi-TEM mode which classical approach is still true in THz band.