Direction Of Wave Propagation Research Articles

Characterization of Gravity Waves in Three Dimensions in the Daytime Thermosphere Using Combined Optical and Radio Measurements and Estimation of Horizontal Neutral Winds

AbstractGravity waves, which are considered omnipresent in the Earth's upper atmosphere, are generally investigated by monitoring the fluctuations in different atmospheric parameters. Here, we report the propagation characteristics of thermospheric gravity waves both in horizontal and vertical directions obtained using collocated optical and radio measurements from Ahmedabad, India for February 2021. The measurements of OI 630.0 nm dayglow emission rates over zenith are used to derive time periods of gravity waves. Wave number analyses of variations in the emission over a large field‐of‐view have been performed to derive gravity wave scale sizes and propagation characteristics in the horizontal direction. Time periods, horizontal scale sizes, and propagation directions are found to be in the range of 31–125 min, 78–243 km, and 203°–248° from east, respectively. Vertical wavelengths of the gravity waves are obtained from radio measurements and are in the range of 26–247 km. As the gravity wave characteristics are influenced by the ambient neutral winds, the measured gravity wave propagation characteristics in three dimensions have been used as inputs into the gravity wave dispersion relation to estimate the magnitudes of thermospheric horizontal neutral winds. These estimated daytime winds in the direction of wave propagation are found to be in the range of 1–105 ms−1, and they compare well with those measured independently from MIGHTI added HWM14 model‐derived winds. The daytime winds estimated by this approach are possibly the first of their kind as obtained from ground‐based measurements.

Wave field measurements of regular wave–monopile interaction using Free-Surface Synthetic Schlieren

Spatio-temporal wave patterns due to wave field–structure interaction can be very complex to measure and analyze when using (intrusive) point probes. Free-surface field measurements can offer much needed insight in this domain. Nevertheless, to the best of the authors’ knowledge, these methods are rarely used in experimental offshore engineering and research. In these fields, typical domain sizes are at least in the order of several m2, whereas (optical) free-surface field measurements are often not performed in domains with dimensions larger than roughly 0.5 × 0.5 m2. In the current work, the optical free-surface measurement technique named Free-Surface Synthetic Schlieren (FS-SS) is applied to measure the interaction between an incident wave field and a surface piercing cylinder, or monopile, in a domain of several m2 for the first time. The FS-SS method is validated for wave fields with wavelengths uplambda ll L to uplambda gg L, where L is the domain size in the direction of wave propagation. It is found that the incorporation of an additional water level measurement improves the agreement between intrusive wave height meters and the FS-SS measurement for large wavelengths (uplambda /L >0.5) as compared to assuming a zero-mean free-surface. Wave field–monopile interaction is measured for two values of D/lambda: D/lambda = 0.1 and D/lambda = 0.2, where D is the monopile diameter. For the case D/lambda = 0.2 the interaction wave field is analyzed by subtracting the measured wave field in the absence of a structure, from the measured interaction wave fields. The measured difference wave field reveals many interaction phenomena such as locations of amplification, both near the monopile and further away, a wake that has certain similarities with a Kelvin wake, and a circular small wavelength diffraction pattern. Additionally, the embedding of the measurements in a wave breaking regime map is presented. In this map, the applicability for certain wave conditions can be clearly visualized. It is concluded that the FS-SS method, including the proposed improvement using additional sensor data, is a useful addition to the toolbox of hydraulic engineers and researchers, and that especially the measured locations of wave amplification in the far field will not be easily detected using (arrays of) point probes.

Binary acoustic metasurfaces for transcranial focused ultrasound

Transcranial focused ultrasound is a promising technique for noninvasive and spatially targeted diagnosis and treatment of brain diseases. Acoustic lenses were designed to correct the skull-induced beam aberration, but these designs could only generate static focused ultrasound beams inside the brain. Here, we designed and 3D printed binary acoustic metasurfaces (BAMs) for skull aberration correction and dynamic ultrasound beam focusing. BAMs were designed by binarizing the phase distribution at the surface of the metasurfaces. The phase distribution was calculated based on time reversal to correct the skull-induced phase aberration. The binarization enabled the ultrasound beam to be dynamically steered along wave propagation direction by adjusting the operation frequency of the incident ultrasound wave. The designed BAMs were manufactured by 3D printing with two coding bits, a polylactic acid unit for bit “1” and a water unit for bit “0.” BAMs for single- and multi-point focusing through the human skull were designed, 3D printed, and validated numerically and experimentally. In summary, the proposed BAMs with subwavelength scale in thickness are simple to design, easy to fabric, and capable of correcting skull aberration and achieving dynamic beam steering.

Theoretical research on the transverse spin of structured optical fields inside a waveguide

Structured optical fields inside a waveguide possess the transverse spin, i.e., the spin angular momentum perpendicular to the direction of the waveguide. The physical origin of the transverse spin can be attributed to the presence of an effective rest mass of photons in guided waves, or equivalently, to the existence of a longitudinal field component, such that the transverse and longitudinal fields together form an elliptical polarization plane. In contrary to the traditional viewpoint, the transverse spin of photons in guided waves is also quantized, and its quantization form is related to the ellipticity of the polarization ellipse. The direction of the transverse spin depends on the propagation direction of electromagnetic waves along the waveguide, such a spin-momentum locking may have important applications in spin-dependent unidirectional optical interfaces. By means of a coupling between the transverse spin of guided waves and some physical degrees of freedom, one can develop an optical analogy of spintronics, i.e., spinoptics.

Research and application of Rayleigh wave extraction method based on microtremors signal analysis

This study investigates the method and application of single-channel, three-component microtremor signal co-directional Rayleigh surface-wave extraction. The research focuses on filtering linear polarization waves using polarizability wave and phase-difference filtering, which were analyzed based on both simulated data and real microtremor signals. Additionally, the study examines the use of time-frequency analysis to analyze microtremor signals and identify Rayleigh wave propagation direction. The combination of these methods leads to a set of procedures for extracting high-SNR co-directional Rayleigh surface waves from microtremor signals, which was applied to the elliptical polarizability imaging method. Results indicate that the proposed data processing process effectively filters linear polarization waves and accurately determines the propagation direction of the Rayleigh wave, leading to significant improvement in the accuracy of elliptical polarizability exploration results. This provides a reference for obtaining high signal-to-noise ratio data in microtremor Rayleigh wave seismic exploration.

Polarization-independent photonic Bragg grating filter with cladding asymmetry.

A photonic Bragg grating is a fundamental building block that reflects the direction of wave propagation through spatial phase modulation and can be implemented using sidewall corrugation. However, due to the asymmetric aspect ratio of a waveguide cross section, typical Bragg gratings exhibit a strong polarization sensitivity. Here, we show that photonic Bragg gratings with cladding asymmetry can enable polarization-independent notch filters by rotating input polarizations. Such Bragg gratings strongly couple transverse electric (TE) and transverse magnetic (TM) modes propagating in opposite directions, filtering the input signal and reflecting the rotated mode. We analyzed this polarization-rotating Bragg grating using the coupled-mode theory and experimentally demonstrated it on a silicon-on-insulator platform. Our device concept is simple to implement and compatible with other platforms, readily available as polarization transparent Bragg components.

Bandgap tuning and in-plane wave propagation of chiral and anti-chiral hybrid metamaterials with assembled six oscillators

In this paper, a hybrid acoustic metamaterial consisting of chiral and anti-chiral ligaments is proposed. The bandgap properties of the hybrid chiral and anti-chiral structure with and without oscillator intercalation are calculated using Bloch’s theorem and the finite element method, and the structure is verified and evaluated to have good vibration attenuation performance by means of transfer functions and stress clouds. By adjusting the resonant oscillator configuration and applying active strain, the structure is verified to have tunable capability, which provides a simple and easy scheme for bandgap tunability analysis. Finally, a comprehensive analysis of the structural vibration mode, phase constant plane, group velocity, phase velocity and wave propagation direction diagram lead to the conclusion that the directional and regional nature of wave propagation is responsible for the bandgap opening. The new structure in this paper provides a new way of thinking for vibration and noise reduction, and also provides a practical solution for bandgap adjustment. The comprehensive analysis of wave propagation also presents theoretical and technical support for the subsequent structure and bandgap optimization.

A novel method for sub-wavelength focusing of flexural waves

A novel method for achieving sub-wavelength focusing of reflected flexural waves in a single-phase plate is proposed in this paper. The original plane lens is divided into two adjacent lenses, and modulating the internal angle between them, more reflected wave components from broader directions can arrive at the focusing position, which leads to the sub-wavelength focusing phenomenon with the focusing size smaller than half wavelength. Firstly, Mindlin plate theory is adopted to solve the reflected wave, and lenses with different width values are designed based on phase modulation for wave focusing. After that, the sub-wavelength focusing is exemplified numerically and experimentally, with the reason explained systematically. Remarkably, the focusing size perpendicular to wave propagation direction locates (0.4λ, 0.5λ), which is controlled by the internal angle. It is demonstrated that the subwavelength focusing is general because it is still suitable when the lenses are designed via the thickness variation or other phase distribution. Additionally, the lenses exhibit broadband property, which can work in a certain frequency region centered the designed frequency. Not limited by flexural wave in plates, the design methodology in this paper can also lay the foundation for wave control in electromagnetics and acoustics.

A fault location method based on polarization analysis for coal mine

A fault is a main cause for water inrush in coal mines. The detection of faults plays an important role in the prevention and governance of water inrush in coal mines. It is hard to determine the direction of seismic wave propagation under the condition of full space of mines, leading to difficulty in accurate fault detection. This paper compares and analyzes the polarization information extraction capability of time-domain polarization analysis, frequency-domain polarization analysis, and time-frequency (TF)-domain polarization analysis, and proposes a TF-domain polarization analysis-based method for locating faults in coal mines. Firstly, the polarization analysis of signals mixed in the time domain was carried out. The results of three kinds of polarization analysis show that the TF-domain polarization analysis can accurately determine the polarization direction of multi-type signals in the case of aliasing. Secondly, a time-space-domain high-order three-dimensional three-component numerical simulation experiment was conducted. The TF-domain polarization analysis was adopted to extract the polarization information of each geophone and locate the fault. The error of the predicted fault strike was 0.16°, and the distance deviation was about 2.03%. Finally, in-situ three-component seismic signals of coal mine were used to predict the location and strike of fault. The data from on-site actual drilling verified the effectiveness of the mine fault location method based on the TF-domain polarization analysis. The predicted fault strike is consistent with the drilling data, and the distance deviation is about 5.5%.

Modulation instability analysis and optical solutions of an extended (2+1)-dimensional perturbed nonlinear Schrödinger equation

We examine an extended (2+1)-dimensional perturbed nonlinear Schrödinger equation with Kerr law nonlinearity in a nano-optical fiber with fourth-order spatial derivatives in this paper. We develop various optical solutions to the equation under study using the Jacobi elliptic expansion method. We investigate the consequences of nonlinearity and spatial dispersion in spatial directions x and y. Furthermore, by investigating the stability condition of this nonlinear equation, we derive the linear stability analysis. The innovative evolutionary behaviors in the 3-dimensional profile are provided under constraint circumstances to indicate the wave propagation direction.

Fourth-order stability analysis for capillary-gravity waves on finite-depth currents with constant vorticity

We derive a fourth-order nonlinear evolution equation (NLEE) for narrow-banded Stokes wave in finite depth in the presence of surface tension and a mean flow with constant vorticity. The two-dimensional capillary-gravity wave motion on the surface of finite depth is considered here. The analysis is limited to one horizontal dimension, parallel to the direction of wave propagation, in order to take advantage of a formulation using potential flow theory. This evolution equation is then employed to examine the effect of vorticity on the Benjamin–Feir instability (BFI) of the Stokes capillary-gravity wave trains. It is found that the vorticity modifies significantly the modulational instability and in the case of finite depth, the combined effect of vorticity and capillarity is to enhance the instability growth rate influenced by capillarity when the vorticity is negative. The key point is that the present fourth-order analysis exhibits considerable deviations in the stability properties from the third-order analysis and gives better results consistent with the exact numerical results. Furthermore, the influence of linear shear current on Peregrine breather (PB) is studied.

Reflection and transmission of plane waves in stressed media with an imperfectly bonded interface

SUMMARY The reflection and transmission of elastic waves are of great significance for predicting reservoir physical properties, interpreting seismic data and detecting crustal structures. Most studies only consider the initial vertical stress state when studying the reflection and transmission of elastic waves at imperfectly bonded interfaces, but few studies consider the influence of initial stress on boundary conditions. Moreover, the effect of initial stress on the energy distribution of elastic waves at imperfectly bonded interfaces has rarely been investigated. We propose a unified method to calculate the energy reflection and transmission coefficients for different incident waves at welded or imperfectly bonded interfaces in stressed media. The effects of initial stress on the equation of motion, the elastic properties of the medium and the boundary conditions at the interface are considered. The elastic properties of rocks under initial stress are described by acoustoelasticity theory. In addition, we define a new stress tensor to modify the linear-slip model for describing boundary conditions at the imperfectly bonded interface in the presence of initial stress. Numerical results show that the energy reflection and transmission coefficients at the non-welded interface in stressed media depend on the elastic properties of the incident and transmitted media, the initial stress, the type and magnitude of the interfacial compliance and the frequency and propagation direction of the incident wave. The initial vertical and horizontal stresses dominate the reflection and transmission coefficients at small and large angles, respectively. The discontinuity in displacement across the imperfectly bonded boundary results in the frequency dependence of the reflection and transmission coefficients. Imperfect bonding enhances P-wave and SV-wave energy reflection and weakens P-wave energy transmission. However, imperfect bonding can enhance the energy transmission of the SV wave for the imperfectly bonded interface with high contrast between tangential and normal compliance and a resonance peak appears at a specific frequency. We also notice that imperfectly bonded interfaces with interfacial compliance less than $1.0 \times {10}^{ - 11}$ m Pa−1 can be regarded as welded interfaces in the seismic frequency band (lower than 100 Hz). In addition, the initial stress greatly influences the reflection coefficients at high frequencies and the transmission coefficients at low frequencies. The initial vertical stress can reduce the energy transmission of SV waves at imperfectly bonded interfaces. In contrast, the initial horizontal stress can significantly increase the energy transmission of low-frequency SV waves and may lead to the disappearance of the resonant peak in the transmission coefficients.

Independent Manipulation of Aperture and Radiation Fields in a Transmission‐Reflection Integrated Complex‐Amplitude Metasurface

AbstractMetasurfaces have superior performance in flexible wave manipulation used for various multifunctional devices. However, they remain challenging for the completely spatial manipulation of electromagnetic waves including wave propagation direction and distance degrees of freedom. Here, a novel strategy of multiplexing metasurface with a non‐interleaved meta‐atoms array is introduced. The design fully explores wave propagation direction and distance degrees of freedom by combining irregular complex‐amplitude encoding method and transmission‐reflection selective meta‐atom. Therein, by limiting effective meta‐atoms to a specific geometry and encoding highly selective phase sequences for wavefront shaping to these meta‐atoms, the distance multiplexing properties of aperture and radiation fields can be designed to suit application needs. Also, the transmission‐reflection integrated meta‐atoms is further engineered to increase the propagation direction degrees of freedom in electromagnetic manipulation. As proof of concept, a six‐channel metasurface operating in one transmitted mode and two reflected modes are experimentally generated, enabling us to reconstruct an aperture‐field printing image and a radiation‐field holographic image for each mode simultaneously. This strategy may have an alternative route for potential applications such as data storage, image encryption, and information communication systems.

Correlation of the mechanical and structural properties of trabecular bone assessed by wave propagation and imaging methods

This study aimed to assess the correlation of the location- and direction-dependent mechanical and structural properties of trabecular bone using the wave propagation and imaging methods and to propose a non-invasive approach for the assessment of the quality of trabecular bone. Two bovine femur trabecular bone specimens were prepared and tested using the Rayleigh wave propagation tests and CT-scan imaging. The elastic modulus of the specimens was estimated in different regions of interest (ROIs) and wave propagation directions by the solution of the presented Rayleigh wave propagation model. The structural parameters of the specimens were also obtained via the processing of CT-scan images for all of the tested ROIs and directions. The estimated elastic modulus was in the range between 321and 1322 MPa for different ROIs and directions, showing significant heterogeneity and anisotropy of the bone specimens. The Pearson correlation and linear regression analyses were carried out between the obtained mechanical and structural properties, which revealed that the elastic modulus decreased with the trabecular separation and increased with the trabecular thickness and bone volume fraction. A semi-empirical relation was developed to obtain the elastic modulus of trabecular bone using the structural parameters obtained from imaging or vice versa.

Low‐Frequency Vibration and Noise Reduction and Wave Propagation Properties of Hexagonal Hybrid Metamaterials with Local Resonance Strips

Based on the idea of local resonance, a class of hybrid acoustic metamaterials with interposed resonant strips is proposed in this article, and the bandgap characteristics and transmission properties are calculated by combining Bloch's theorem and lattice theory. The new structure is verified to have low‐frequency damping and noise reduction capability through the analysis of the stress cloud diagram at specific frequencies in the bandgap range. A comprehensive analysis of vibration modes, phase constant surfaces, group velocity, phase velocity, and wave propagation direction diagram is used to explore the wave propagation and bandgap opening mechanism, providing technical support and theoretical basis for subsequent bandgap optimization and structural improvement. The results show that this structure can open multiple bandgaps in the low‐frequency range below 500 Hz, and the stepped design of the single‐cell structure and the introduction of resonant strips provide a new design idea for low‐frequency vibration and noise reduction.

Active control of wave propagation in nonlinear planar networks using piezoelectric actuation

Due to the fascinating effects of nonlinearity on the wave propagation and dynamic properties of periodic structures, nonlinear phononic crystals have been an important research topic in recent years. However, while a lot of efforts were devoted to the wave propagation analysis of nonlinear chains, 2D planar lattices with weak nonlinearities have been left relatively unexplored. Hence, this work presents an active control technique of wave propagation in nonlinear planar lattices using piezoelectric actuation. To formulate the problem, the governing equations are derived for the transverse motion of weakly nonlinear lattices with square topology considering the effects of auxiliary piezoelectric springs. The Lindstedt–Poincare perturbation method is applied to obtain semi-analytical solutions of the governing equations. Furthermore, the analytical nonlinear dispersion relations are derived for the discrete nonlinear phononic lattices with triangular topology, for the first time. The results reveal that in addition to the effect of piezoelectric springs with both positive and negative gains on the dispersion curves, the wave directionality of nonlinear planar lattices is also affected by such modifications. Hence, a tuning approach is proposed to control the directionality and isotropy of wave propagation in nonlinear planar lattices, as well as their wave attenuation performance. According to the results, piezoelectric springs with negative control gain can open new tunable ultra-low frequency stop-bands in the dispersion curves of planar nonlinear lattices.

Effect of initial stresses on the surface wave propagation in highly anisotropic piezoelectric composite media

The present work analyses the propagation phenomenon of the Rayleigh wave in a piezoelectric-orthotropic substrate beneath a piezoelectric-orthotropic layer under the influence of initial stresses. The dispersion equations of Rayleigh wave propagation on the considered composite structure are obtained by employing suitable non-traditional boundary conditions for electrically open circuits and electrically short circuits. The data of two different PVDF materials have been considered to investigate the latent characteristics of Rayleigh wave propagation in the considered structure. Numerical calculations have been carried out to study the impact of initial stresses on the phase velocity of Rayleigh wave against wave number in all the considered directions of wave propagation for both electrical cases. A comparative study has also been carried out to unfold the impact of initial stresses in all the directions of Rayleigh wave propagation in electrical open- and short-circuit cases and manifested graphically. The obtained result is well-matched with the classical case of the Rayleigh wave for validation and is discussed as one of the problem's special cases. This study may help to improve the design of electrically generating and detecting acoustic devices.

Machine learning for knowledge acquisition and accelerated inverse-design for non-Hermitian systems

Non-Hermitian systems offer new platforms for unusual physical properties that can be flexibly manipulated by redistribution of the real and imaginary parts of refractive indices, whose presence breaks conventional wave propagation symmetries, leading to asymmetric reflection and symmetric transmission with respect to the wave propagation direction. Here, we use supervised and unsupervised learning techniques for knowledge acquisition in non-Hermitian systems which accelerate the inverse design process. In particular, we construct a deep learning model that relates the transmission and asymmetric reflection in non-conservative settings and propose sub-manifold learning to recognize non-Hermitian features from transmission spectra. The developed deep learning framework determines the feasibility of a desired spectral response for a given structure and uncovers the role of effective gain-loss parameters to tailor the spectral response. These findings offer a route for intelligent inverse design and contribute to the understanding of physical mechanism in general non-Hermitian systems.

A Semi-Analytical Finite Element Framework for Lamb Waves in Soft Compressible Plates Considering Strain Stiffening Effect

This paper investigates the elastic wave propagation through soft materials that are being subjected to finite deformations. The nonlinear elastic and linearized incremental theories have been exploited to formulate governing wave equations and elastic moduli in Lagrangian space. Semi-analytical finite element (SAFE) method, a numerical approach has been formulated for computing dispersive relations of guided waves in compressible hyper-elastic plates. This framework requires finite element discretization of the cross section of the waveguide and harmonic exponential function assumes the motion along the wave propagation direction. Here, explicit phase velocity results have been shown for soft materials with a prominent stiffening effect by employing the Gent model, and these results are analyzed for elastic wave propagation through compressible materials. It has been noticed that Lamb waves have a strong dependence on the frequency-thickness product, prestretch, and direction of wave propagation. Moreover, with the strain stiffening effect, the dependence becomes stronger, especially for fundamental symmetric and anti-symmetric modes. The numerical results display that at certain prestretch the Gent material encounter snap-through instability resulting from geometrical and material nonlinearities. The influence of material properties like Gent constant and direction of wave propagation on snap-through instability has been discussed. The proposed SAFE framework reveals that finite deformations can affect elastic wave propagation through stiffness and compressibility.

Influence of Aspect Ratio on Wave Propagation in Granular Crystals Consisting of Ellipse-Shaped Particles

The behavior of waves in granular materials is very complex and closely related to the macro-micro properties such as particle shape, packing, and particle size distribution. In this study, the influence of the aspect ratio (AR) and confining stress on wave propagation in 2D granular crystals consisting of ellipse-shaped particles is investigated based on discrete element method. The energy attenuation, wave velocity, wavefront shape, and frequency dispersion are mainly focused. The results show that the energy attenuation exhibits an increasing trend with bigger confining stress and smaller AR in the direction of wave propagation. The wave velocities increase with increasing AR, and the relation between compressive wave velocities and the confining stress and AR is obtained. Wavefront has the elliptical shape which has the same AR as elliptical particles of specimen, and its analytical expression is also obtained. The dispersion is affected by AR, which is mainly reflected in the modified particle spacing in sine fitted frequency dispersion curve. Larger confining stress allows a larger maximum frequency, and produces stronger dispersion. The above conclusion can provide a reference for wave-guided granular crystals design.