Wave Modes Research Articles

Asymptotic analysis of fluid thermodynamic behaviors in the near-critical region under highly variable physical properties

Asymptotic analysis based on the fluids governing equations with the exponential model of thermophysical properties is introduced to quantify the influence of each property on the heat and mass transfer behavior of the near-critical fluid. The one-dimensional asymptotic model finds the different behavior in boundary layers and bulk regions controlled by the diffusion and wave mode, respectively. From the asymptotic model, three characteristic parameters are found: nondimensional wave velocity for wave mode, nondimensional diffusion coefficient for diffusion mode, and nondimensional mass transport coefficient for the coupling in between. Larger thermal conductivity, fluid compressibility, and lower specific heat are found to enhance the thermal wave. However, the efficiency of heat transfer by thermal waves is irrelevant to the thermal diffusivity but related to the fluid compressibility. From the calculation of the asymptotic model for supercritical carbon dioxide (sCO2), such efficiency is 0.150, which indicates that most of the thermal energy is accumulated in the boundary layers.

A novel multi-step superposition model for the dispersion analysis of multiaxial prestressed plate-like structures

Understanding the dispersion characteristics of multi-axial prestressed plate-like structures is crucial for the engineering application of guided wave non-destructive testing. However, existing analytical and semi-analytical models neglect both constitutive and geometric nonlinearities induced by the prestress and require tedious theoretical derivation and coding, which may not achieve accurate predictions. To address these issues, a convenient model based on the multi-step superposition analysis was proposed to investigate the effects of an arbitrary multiaxial prestress on the dispersion characteristics of plate-like structures. The core idea of this model involves introducing an initial configuration to describe the static pre-deformation between undeformed and final states; then, the dynamic wave motion was superimposed on the initial pre-deformation for the eigenfrequency analysis. Additionally, the strain energy density of the hyperelastic material was implanted and set as the relevant variable between the two steps by the inheritance of the solution. This model significantly improved the accuracy compared to the conventional method, as validated through biaxial acoustoelastic experiments. Subsequently, the acoustoelastic responses of the multiaxial prestressed 6061-T6 aluminum plate were investigated. The results reveal that the non-uniform in-plane prestress leads to the coupling of guided wave modes propagating along non-principal directions. Furthermore, the effects induced by multiaxial prestress in the frequency bands with low dispersion could be decomposed into the sum of the effects induced by three uniaxial prestresses with a very small deviation. This work provides new insights into the acoustoelastic behavior and a theoretical basis to optimize the guided wave mode and frequency.

Wavelength Tuning in Resonant Cavity Interband Cascade Light Emitting Diodes (RCICLEDs) via Post Growth Cavity Length Adjustment.

We demonstrate substrate-emitting resonant cavity interband cascade light emitting diodes (RCICLEDs) based on a single distributed Bragg reflector (DBR). These devices operate in continuous wave mode at room temperature. Compared to standard ICLEDs without a cavity, we achieved an 89% reduction in the emission spectrum width, as indicated by the Full Width Half Maximum (FWHM) of 70 nm. Furthermore, we observed far-field narrowing and improved thermal stability. A single DBR configuration allows the cavity length to be adjusted by adding refractive index-matched material to the top of the epitaxial structure after epitaxial growth. This modification effectively shifts the cavity response towards longer wavelengths. We fabricated emitters comprising two cavities of different lengths, resulting in the emission of two distinct spectral lines that can be independently controlled. This dual-color capability enables one of the emission lines to serve as a built-in reference channel, making these LEDs highly suitable for cost-effective gas-sensing applications.

Deep-learning viscoelastic seismic inversion for mapping subsea permafrost

Marine seismic surveys can be used to map ice-bearing subsea permafrost on a large scale. However, current seismic processing technologies have limited capacity to image permafrost distribution at depth, mainly due to the low sensitivity of primary reflections and refractions to the velocity inversion found at the base of permafrost. Guided waves and multiples are more sensitive to the velocity variations below the top of permafrost, but they remain challenging to use in physics-based inversion approaches. A deep-learning-based seismic inversion has the potential to improve seismic imaging below the top of permafrost by automatically extracting information from all wave modes. We develop a multi-input neural network (NN) to estimate seismic velocities from marine seismic data. The network is trained on synthetic data generated from the representative distributions of the seismic properties of subsea permafrost. We find that our network can image large velocity contrasts and reversals in depth, typical of subsea permafrost. We use our network to estimate the P- and S-wave velocity and Q-factor models from a seismic line in the Beaufort Sea. The NN indicates highly discontinuous subsea permafrost with variable thickness in the area. Our work shows that deep-learning-based seismic inversion could become a cost-effective technology to map the distribution of subsea permafrost on a large scale and, more generally, high-velocity geologic layers located in shallow waters.

Investigation of Viscoelastic Guided Wave Properties in Anisotropic Laminated Composites Using a Legendre Orthogonal Polynomials Expansion-Assisted Viscoelastodynamic Model.

This study investigates viscoelastic guided wave properties (e.g., complex-wavenumber-, phase-velocity-, and attenuation-frequency relations) for multiple modes, including different orders of antisymmetric, symmetric, and shear horizontal modes in viscoelastic anisotropic laminated composites. To obtain those frequency-dependent relations, a guided wave characteristic equation is formulated based on a Legendre orthogonal polynomials expansion (LOPE)-assisted viscoelastodynamic model, which fuses the hysteretic viscoelastic model-based wave dynamics and the LOPE-based mode shape approximation. Then, the complex-wavenumber-frequency solutions are obtained by solving the characteristic equation using an improved root-finding algorithm, which leverages coefficient matrix determinant ratios and our proposed local tracking windows. To trace the solutions on the dispersion curves of different wave modes and avoid curve-tracing misalignment in regions with phase-velocity curve crossing, we presented a curve-tracing strategy considering wave attenuation. With the LOPE-assisted viscoelastodynamic model, the effects of material viscosity and fiber orientation on different guided wave modes are investigated for unidirectional carbon-fiber-reinforced composites. The results show that the viscosity in the hysteresis model mainly affects the frequency-dependent attenuation of viscoelastic guided waves, while the fiber orientation influences both the phase-velocity and attenuation curves. We expect the theoretical work in this study to facilitate the development of guided wave-based techniques for the NDT and SHM of viscoelastic anisotropic laminated composites.

Conventional and Doppler ultrasonographic diagnosis of subclinical pregnancy toxemia with altered serum cortisol and biochemical parameters in pregnant does

The pathogenesis and diagnosis of subclinical pregnancy toxemia (SCPT) remain elusive and need further investigation in pregnant does. Therefore, the aim of our study was to describe the typical properties of hepatic venous hemodynamics by Doppler ultrasonography. A total of 70 pregnant does were classified based on the blood serum concentrations of β-hydroxybutyric acid (βHBA), pregnant does were categorized into control group (βHBA concentrations <0.8 mmol/L; n = 40) and SCPT group (βHBA concentrations >0.8 mmol/L; n = 30). DRAMISKI 4vet slim diagnostic ultrasound scanner with B, M, and Doppler (color, power, pulsing wave) modes was used for diagnosis of SCPT. Total serum cortisol level was quantitative using chemiluminescent immunoassay. Serum glucose, triglycerides, cholesterol, HDL and LDL- cholesterol and LDH- cholesterol were measured by colorimetric and kinetic methods. Liver ultrasonography of does with SCPT had been shown mild fatty infiltration with rounded margin, which was characterized by hyperechoic area. There was a significant decrease in the values of portal vein diameter (PVD), portal vein area (PVA), portal mean velocity (PMV) and portal blood flow (PBF) in SCPT does compared to control pregnant does. PVD, PVA and PBF were negatively correlated with βHBA concentrations in does with SCPT (P < 0.05). PVD was inversely associated with serum cholesterol and triglycerides concentrations (P < 0.05). In conclusions, Doppler ultrasonography examinations of pregnant does with SCPT indicate abnormal hepatic variation. Reduced PVD, PVA, PMV and PBF together with increased βHBA concentrations could predict SCPT in does with fair sensitivity and specificity.

Efficient Guided Wave Modelling for Tomographic Corrosion Mapping via One-Way Wavefield Extrapolation

Mapping corrosion depths along pipeline sections using guided-wave-based tomographic methods is a challenging task. Accurate defect sizing depends heavily on the precision of the forward model in guided wave tomography. This model is fitted to measured data using inversion techniques. This study evaluates the effectiveness of a recursive extrapolation scheme for tomography applications and full waveform inversion. It employs a table-driven approach, with precomputed extrapolation operators stored across a spectrum of wavenumbers. This enables fast modelling for extensive pipe sections, approaching the speed of ray tracing while accurately handling complex velocity models within the full frequency band. This ensures an accurate representation of diffraction phenomena. The study examines the assumptions underlying the extrapolation approach, namely, the negligible reflection and conversion of modes at defects. In our tomography approach, we intend to use multiple wave modes—A0, S0, and SH1—and helical paths. The acoustic extrapolation method is validated through numerical studies for different wave modes, solving the 3D elastodynamic wave equation. Comparison with an experimentally measured single-mode wavefield from an aluminium plate with an artificial defect reveals good agreement.

Wavefield decomposition for viscoelastic anisotropic media

Separating wave modes on seismic records is an essential step in imaging of multicomponent seismic data. Viscoelastic anisotropic models provide a realistic description of subsurface formations that exhibit anisotropy of velocity and attenuation. However, mode separation has not been extended to viscoelastic anisotropic media yet. Here, we propose an efficient approach to wavefield decomposition that takes velocity and attenuation anisotropy into account. Our algorithm operates in the frequency-wavenumber domain and, therefore, is suitable for general dissipative models. We present exact equations for wavefield decomposition in arbitrarily anisotropic attenuative homogeneous media. Then the proposed approach is applied to viscoelastic constant-[Formula: see text] VTI (transversely isotropic with a vertical symmetry axis) models. Numerical examples demonstrate the accuracy and efficiency of our approach for piecewise-homogeneous media characterized by pronounced velocity and attenuation anisotropy.

Global existence and steady states of the density-suppressed motility model with strong Allee effect

Abstract This paper considers a density-suppressed motility model with a strong Allee effect under the homogeneous Neumman boundary condition. We first establish the global existence of bounded classical solutions to a parabolic–parabolic system over an $N $-dimensional $\mathbf{(N\le 3)}$ bounded domain $\varOmega $, as well as the global existence of bounded classical solutions to a parabolic–elliptic system over the multidimensional bounded domain $\varOmega $ with smooth boundary. We then investigate the linear stability at the positive equilibria for the full parabolic case and parabolic–elliptic case, respectively, and find the influence of Allee effect on the local stability of the equilibria. By treating the Allee effect as a bifurcation parameter, we focus on the one-dimensional stationary problem and obtain the existence of non-constant positive steady states, which corresponds to small perturbations from the constant equilibrium $(1,1)$. Furthermore, we present some properties through theoretical analysis on pitchfork type and turning direction of the local bifurcations. The stability results provide a stable wave mode selection mechanism for the model considered in this paper. Finally, numerical simulations are performed to demonstrate our theoretical results.

Dynamo action driven by precessional turbulence.

We reveal and analyze an efficient magnetic dynamo action due to precession-driven hydrodynamic turbulence in the local model of a precessional flow, focusing on the kinematic stage of this dynamo. The growth rate of the magnetic field monotonically increases with the Poincaré number Po, characterizing precession strength, and the magnetic Prandtl number Pm, equal to the ratio of viscosity to resistivity, for the considered ranges of these parameters. The critical Po_{c} for the dynamo onset decreases with increasing Pm. To understand the scale-by-scale evolution (growth) of the precession dynamo and its driving processes, we perform spectral analysis by calculating the spectra of magnetic energy and of different terms in the induction equationin Fourier space. To this end, we decompose the velocity field of precession-driven turbulence into two-dimensional (2D) vortical and three-dimensional (3D) inertial wave modes. It is shown that the dynamo operates across a broad range of scales and exhibits a remarkable transition from a primarily vortex-driven regime at lower Po to a more complex regime at higher Po where it is driven jointly by vortices, inertial waves, and the shear of the background precessional flow. Vortices and shear drive the dynamo mostly at large scales comparable to the flow system size, and at intermediate scales, while at smaller scales it is mainly driven by inertial waves. This study can be important not only for understanding the magnetic dynamo action in precession-driven flows, but also in a general context of flows where vortices emerge and govern the flow dynamics and evolution.

A periodic-modulation-oriented noise resistant correlation method for industrial fault diagnostics of rotating machinery under the circumstances of limited system signal availability

The periodical impulses caused by localized defects of components are the vital characteristic information for fault detection and diagnosis of rotating machines. In recent years, multitudinous spectrum analysis-based signal processing methods have been developed and authenticated as the powerful tools for excavating fault-related repetitive transients from the measured complex signals. Nonetheless, in practice, their applications can be severely confined by the constraints of limited system signal availability and incomplete information extraction under intricate noise interferences. To tackle the aforementioned issues, this paper proposes a periodic-modulation-oriented noise resistant correlation (PMONRC) method for target period detection and fault diagnosis of rotating machinery. Firstly, the envelope of raw signal is obtained via a novel sequential procedure of signal element-wise squaring, spectral Gini index-guided adaptive low-pass filtering, and signal element-wise square root computation, to highlight the modulated wave component that is more likely to be related to the potential fault-induced periods. Subsequently, a series of sub-signals, which can encode the fault-related repetitive information and enhance noise resistance, are constructed utilizing the envelope signal. Based upon the envelope signal and the obtained sub-signals, a weighted envelope noise resistant correlation function can be derived with the assistance of the L-moment ratio-based indicator and Sigmoid transformation. Finally, the specific fault type of the rotating machinery can be identified and affirmed accordingly. The proposed PMONRC method, which is nonparametric and completely adaptive to the signal being processed itself, overcomes the deficiencies of spectral analysis-based approaches, and is applicable for the engineering circumstances of system signal limitation and low signal-to-noise ratio (SNR), possessing immense practical merit. Both simulation analyses and experimental validations profoundly demonstrate that the proposed method is superior to other existing state-of-the-art time-domain correlation methods. Moreover, as an attempt as well as exemplar to apply this method, the PMONRC-based incipient fault diagnostic results of rolling bearing data from the well-known experimental platform PRONOSTIA are presented and discussed as well, to further elucidate the effectiveness and practical engineering significance of the proposed method.

Transverse cracking signal characterization in CFRP laminates using modal acoustic emission and digital image correlation techniques

The process of formation and subsequent propagation of transverse cracks in 90° plies of carbon-fiber laminated composites was studied using modal acoustic emission approach and digital image correlation techniques. The results from modal acoustic emission approach, which included a newly developed processing tool for acoustic emission waveforms, provided information for identification and subsequent characterization or localization of signals originating from transverse cracking by analysis of the separated flexural and extensional Lamb wave modes in terms of their modal parameters. The digital image correlation method served as a verification tool of the acoustic emission data outputs in the terms of activity of significant localized events originating from the formation of the transverse crack in the 90oply. This made it possible to specify more locally the accompanying activity belonging to the formation or propagation of the magistral transverse crack. The manuscript also presents results related to the evolution of flexural/extensional wave modal parameters as the function of loading force for both [0/0/0/90]S and [90/0/0/0]S panels. It can be concluded that the detection of transverse cracks requires the need for applying a more complex acoustic emission data analysis methodology, while the standard parametric analysis, including the waveform peak frequency, is not sufficient. The presented methodology may serve as a basis for development of robust analysis tool capable of detecting the investigated phenomena.

Design of millimeter wave broadband polarization reconfigurable horn antenna based on elliptical waveguide polarizer

AbstractA broadband polarized reconfigurable horn antenna for K‐band and Ka‐band is designed using a circular polarizer in the form of an elliptical waveguide and a circular waveguide flange based on a gap structure. The designed noncontact circular waveguide flange with gap structure can switch the antenna excitation signal to different modes of electromagnetic waves by mechanical rotation. When excited by different modes of electromagnetic waves, the designed elliptical waveguide circular polarizer can switch between linear polarization, left‐handed circular polarization, and right‐handed circular polarization. The antenna is analyzed in principle, modeled and simulated, and processed by three‐dimensional printing process. The measured results show that S11 ≤ −15dB, axial ratio ≤ 3 dB, gain ≥14 dBi in 37.8% relative bandwidth range (22.5–33 GHz). The proposed antenna is broadband, multipolar, and reconfigurable, which makes it have broad application prospects in the field of satellite communication and millimeter‐wave communication.

Difference between Upshear- and Downshear-Propagating Waves Associated with the Development of Squall Lines

Abstract During the development of squall lines, low-frequency gravity waves exhibit contrasting behaviors behind and ahead of the system, corresponding to its low-level upshear and downshear sides, respectively. This study employed idealized numerical simulations to investigate how low-level shear and tilted convective heating influence waves during two distinct stages of squall-line evolution. In the initial stage, low-level shear speeds up upshear waves, while it has contrasting effects on the amplitudes of different wave modes, distinguishing it from the Doppler effect. Downshear deep tropospheric downdraft (n = 1 wave) exhibits larger amplitudes, resulting in strengthened low-level inflow and upper-level outflow. However, n = 2 wave with low-level ascent and high-level descent has higher amplitude upshear and exhibits a higher altitude of peak w values downshear, leading to the development of a more extensive upshear low-level cloud deck and a higher altitude of downshear cloud deck. In the mature stage, as the convective updraft greatly tilts rearward (upshear), stronger n = 1 waves occur behind the system, while downshear-propagating n = 2 waves exhibit larger amplitudes. These varying wave behaviors subsequently contribute to the storm-relative circulation pattern. Ahead of the squall line, stronger n = 2 waves and weaker n = 1 waves produce intense outflow concentrated at higher altitudes, along with moderate midlevel inflow and weak low-level inflow. Conversely, behind the system, the remarkable high pressure in the upper troposphere and wake low are attributed to more intense n = 1 waves. Additionally, the cloud anvil features greater width and depth rearward and is situated at higher altitudes ahead of the system due to the joint effects of n = 1 and n = 2 waves. Significance Statement Squall lines are a significant source of high-impact weather events, and their development has been partially explained through linear wave dynamics. While the recurrent generation of waves during squall-line evolution has been found, the differentiation of wave behavior behind and ahead of the system, as well as its implications for storm circulation, has remained unclear. This study employs idealized simulations to reveal that during different stages of convection, low-level shear and the tilting of convective heating exert contrasting effects on wave behaviors. Moreover, various wave modes exhibit distinct responses to specific factors, and their combined effect elucidates the structural discrepancies observed both rearward and forward of the convective updraft. These findings could allow a step toward a better understanding of the intricate interaction between waves and convections.

Physics-inspired spatiotemporal-graph AI ensemble for the detection of higher order wave mode signals of spinning binary black hole mergers

We present a new class of AI models for the detection of quasi-circular, spinning, non-precessing binary black hole mergers whose waveforms include the higher order gravitational wave modes (ℓ,|m|)={(2,2),(2,1),(3,3),(3,2),(4,4)} , and mode mixing effects in the ℓ=3,|m|=2 harmonics. These AI models combine hybrid dilated convolution neural networks to accurately model both short- and long-range temporal sequential information of gravitational waves; and graph neural networks to capture spatial correlations among gravitational wave observatories to consistently describe and identify the presence of a signal in a three detector network encompassing the Advanced LIGO and Virgo detectors. We first trained these spatiotemporal-graph AI models using synthetic noise, using 1.2 million modeled waveforms to densely sample this signal manifold, within 1.7 h using 256 NVIDIA A100 GPUs in the Polaris supercomputer at the Argonne Leadership Computing Facility. This distributed training approach exhibited optimal classification performance, and strong scaling up to 512 NVIDIA A100 GPUs. With these AI ensembles we processed data from a three detector network, and found that an ensemble of 4 AI models achieves state-of-the-art performance for signal detection, and reports two misclassifications for every decade of searched data. We distributed AI inference over 128 GPUs in the Polaris supercomputer and 128 nodes in the Theta supercomputer, and completed the processing of a decade of gravitational wave data from a three detector network within 3.5 h. Finally, we fine-tuned these AI ensembles to process the entire month of February 2020, which is part of the O3b LIGO/Virgo observation run, and found 6 gravitational waves, concurrently identified in Advanced LIGO and Advanced Virgo data, and zero false positives. This analysis was completed in one hour using one NVIDIA A100 GPU.

Velocimetry of GHz elastic surface waves in quartz and fused silica based on full-field imaging of pump–probe reflectometry

This study reports an imaging method for gigahertz surface acoustic waves in transparent layers using infrared subpicosecond laser pulses in the ablation regime and an optical pump–probe technique. The reflectivity modulations due to the photoelastic effect of generated multimodal surface acoustic waves were imaged by an sCMOS camera illuminated by the time-delayed, frequency-doubled probe pulses. Moving the delay time between 6.0nsto11.5ns, image stacks of wave field propagation were created.Two representative samples were investigated: wafers of isotropic fused silica and anisotropic x-cut quartz. Rayleigh (SAW) and longitudinal dominant high-velocity pseudo-surface acoustic wave (HVPSAW) modes could be observed and tracked along a circular grid around the excitation center, allowing the extraction of angular profiles of the propagation velocity. In quartz, the folding of a PSAW was observed. A finite element simulation was developed to predict the measurement results. The simulation and measurement were in good agreement with a relative error of 2% to 5%.These results show the potential for fast and full-field imaging of laser-generated ultrasonic surface wave modes, which can be utilized for the characterization of thin transparent samples such as semiconductor wafers or optical crystals in the gigahertz frequency range.

Modal decomposition of the shedding mechanism of partial cavitation in a Venturi Tube

Abstract The present paper studies the shedding mechanism of partial cavitation in a Venturi Tube, dominated by re-entrant jet and bubbly shock mechanisms, by using two data-driven modal decomposition methods: proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). According to the snapshot data series obtained by high-speed photography, the modal decomposition and reconstruction of the grey image are carried out. The POD results indicate that the main frequency of the re-entrant jet is higher than that of the shock under the sixth-order mode, and the energy amplitude of the latter is about 20 times that of the former. Furthermore, as the cavitation number increases, the condensation shock mechanism eventually replaces the re-entrant jet mechanism. The DMD results show that the shock behaves obvious traveling wave mode, because the frequency is higher and the phase of the spatial distribution changes evenly under the fourth-order mode. POD and DMD can help to understand the shedding mechanism of partial cavitation.

Field application of near-borehole fracture acoustic imaging using a three-dimensional scanning inversion method

Borehole acoustic remote imaging can help identify and evaluate abnormal geological bodies within tens of meters of a well, which has a very important application prospect in fracture imaging. Compared to the borehole mode wave, the echo amplitude from a near-borehole fracture is considerably small, and the currently used wave field separation method cannot completely filter the borehole mode wave. Due to the influence of the large amplitude of the remaining borehole mode wave, several artifacts are often produced near the well when using traditional migration imaging. Thus, imaging and characterizing near-borehole fractures via acoustic imaging becomes difficult. To overcome this, we propose a three-dimensional scanning inversion method that attenuates the effects of remaining refracted waves on borehole acoustic imaging. We then perform data processing and imaging of field-logging data based on this method, and multimode echoes are used to identify and evaluate fractures. Finally, a comparison of the inversion results of the borehole acoustic and borehole electrical imaging showed that the three-dimensional scanning inversion method can effectively attenuate the remaining refracted wave and better realize the clear imaging of near-borehole fractures. Different modes of echoes illuminate different areas of fractures near wells. Therefore, based on scattered S-waves, the proposed imaging method can effectively detect fractures that cross-cut a well. Moreover, based on multimode waves, the method can suppress artifacts and highlight the azimuth characteristics of near-borehole fractures. Using the proposed method, high-precision positioning and fine characterization of formation fractures in near and far wells can be achieved simultaneously. Further, near-borehole fracture parameters—such as tendency, dip, and depth—can be inverted, and the results are consistent with the inversion results obtained through borehole electrical imaging. Thus, the proposed method enables the quantitative evaluation of near-borehole fractures.

Ultra-narrow dual-band perfect absorber based on double-slotted silicon nanodisk arrays

Due to its unique advantage of optical properties, nanophotonic metamaterials have gained extensive applications in perfect absorbers. However, achieving both dual-band and ultra-narrow linewidth in absorbers simultaneously remains a challenge for typical metal-dielectric based metamaterials. In this work, a dual-band ultra-narrow perfect absorber consisting of a double slotted silicon nanodisk array that located on a silver film with a silica spacer layer is proposed theoretically. By combining the hybrid mode excited by the coupling of diffraction wave mode and magnetic dipole mode with the anapole–anapole interaction, two absorption peaks can be induced in the near-infrared regime, achieving nearly perfect absorbance of 99.31% and 99.61%, with ultra-narrow linewidths of 1.92 nm and 1.25 nm respectively. In addition, the dual-band absorption characteristics can be regulated by changing the structural parameters of the as-proposed metamaterials. The as-designed metamaterials can be employed as efficient two-channel refractive index sensors, with sensitivity and figure of merit (FOM) of 288 nm RIU−1 and 150 RIU−1 for the first band, and sensitivity and FOM of up to 204 nm RIU−1 and 163.2 RIU−1 for the second band. This work not only opens up a new design idea for the realization of dual-band perfect absorber synchronously with ultra-narrow linewidth, but also provides potential attractive candidates for developing dual-frequency channel sensors.

Local plasticity-induced nonlinear surface wave scattering in thick-walled structures

Thick-walled structures (TWSs) are frequently utilized as primary load-bearing structures. The early detection of damage is crucial to ensure the operational safety and integrity of these structures. Nonlinear elastic-wave-based techniques provide a potential solution to the problem, which heavily relies on the good understanding of nonlinear wave generation and propagation characteristics. The task, however, is technically challenging due to the complex wave modes existing in a TWS. Targeting a meticulously manufactured local plasticized damage region on the surface of a TWS, which can be regarded as the precursor of incipient damage, this study investigates the scattering features of nonlinear elastic waves resulting from the interaction between the fundamental quasi-surface wave and the local plasticity. Finite element simulations and experiments show that, upon interacting with the nonlinear material zone, both nonlinear quasi-surface waves and shear bulk waves can be generated, with the latter scattered towards the opposite surface of the TWS. The scattered nonlinear shear waves exhibit a strong and predictable directivity with high nonlinear energy content, which is conducive to the detection and localization of the incipient surface damage of the TWS.