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• Combination of 1D-GLL-SAFE and 2D-Gauss-SAFE to compute GW. • Compute GW in acoustoelastic and complex waveguide. • Development of freeware SAFEDC. Guided wave (GW)-based techniques have been extensively investigated and applied in material characterization, damage detection, and structural health monitoring. A comprehensive understanding of GW is the cornerstone for the development of such techniques. Based on the semi-analytical finite element (SAFE) method, an open-source dispersion calculator of GW propagating in acoustoelastic and complex waveguides with both isotropic and anisotropic material properties is developed. First, by assuming the simple harmonic motion along the propagation direction and discretizing along the thickness direction, 1D-GLL-SAFE (one-dimensional Gauss-Lobatto-Legendre SAFE) is adopted for the solution of GW in plate waveguide, which is attributed to its superior performance in terms of computational accuracy and efficiency. Different theories on acoustoelasticity are adopted to calculate GWs under loading. Then 2D-Gauss-SAFE (two-dimensional Gauss SAFE) with triangular meshes filling the cross section is adopted for GW in general waveguides considering the ease of convenience in meshing. Finally, based on the 1D-GLL-SAFE and 2D-Gauss-SAFE algorithms, an open-source tool SAFEDC (SAFE-based dispersion calculator) is developed, which not only provides the solution of GW in pre-stressed isotropic waveguide and general cross section, but also extends to GW in laminates with arbitrary layer stacking configurations and hybrid stacking including multiple materials. Most of the GW features, including phase velocity, group velocity, wave number, wave structure in terms of displacement, stress, and strain, and animation of wave propagation are all offered in SAFEDC, which helps the researchers and engineers to understand and utilize GW. [ABSTRACT FROM AUTHOR]

• The greatest pulse delay is observed for low absorption and low anisotropic scattering. • A maximum in relative pulse width is formed at some distance from the source. • A combination of optical parameters and distance results in a minimum in frequency response. • The frequency response minimum position shows monotonous dependence on anisotropy. The paper reports on the numerical study of temporal characteristics, frequency and phase responses of the photon density waves generated by a point-sized source in scattering media with different scattering anisotropy and absorption index. The study is performed with time-resolved Monte-Carlo simulations for a wide range of medium optical properties covering values typical for aerosols and sea water. We demonstrate that although the normalized pulse delay increases monotonically with distance in the course of propagation in the medium, for particular sets of optical properties its normalized width shows a non-monotonous trend with a maximum corresponding to distances where ballistic and multiply scattered photons give comparable contribution to the irradiance field. In the frequency responses of photon density waves for certain combinations of optical properties at particular distances a deep minimum is formed, also associated with the interference of partial waves. The dimensionless frequency corresponding to this minimum monotonically increases with the increase in anisotropy factor. Dependencies of local and averaged phase and group velocities on distance derived from phase responses revealed a formation of a minima in local group velocities dependencies with further increase to the average velocity values at higher distances, which is also associated with relative contribution of low- and multiply scattered photons. The revealed dependencies can be useful for verifying the analytical models of nonstationary radiation transfer and directly for assessing the achievable parameters of optical systems for various purposes operating in natural scattering media, such as the atmosphere and sea water. [ABSTRACT FROM AUTHOR]

The aim of this study is to investigate the feasibility of evaluating the degree of early fatigue in solid plates by utilizing quasi-static components (QSC) generated by primary Lamb wave (PLW) propagation. The investigation is carried out through theoretical considerations, numerical simulation, and experimental examination. The paper begins by examining the generation of QSCs by PLW propagation under group velocity matching. Subsequently, a suitable mode pair comprising primary and zero-frequency Lamb waves, which satisfy group velocity matching, is chosen for a given aluminum plate to enable the QSC to accumulate and grow with propagation distance. The aluminum plates are then subjected to tension-tension cyclic loading, and experimental measurements are performed. By measuring the amplitudes of the PLW and the generated QSC at different numbers of loading cycles (N), a quantitative relationship between the relative nonlinear acoustic parameter and N is established. The study demonstrates that the relative nonlinear acoustic parameter increases sensitively and monotonically with the increase in fatigue loading cycles. These experimental results confirm the feasibility of utilizing the group-velocity-matching QSC-based approach to evaluate the degree of early fatigue in solid plates. • Feasibility of using QSC generated by PLW under group velocity matching for early fatigue assessment in solid plates. • Quantitative relationship between relative nonlinear acoustic parameter and loading cycles established through experiments. • Consistent findings validate the practicality of using group-velocity-matching QSC-based approach for early fatigue evaluation. [ABSTRACT FROM AUTHOR]

In this paper, a tunable functionally graded metamaterial beam is designed for flexural wave attenuation by integrating piezomagnetic shunt damping system and inertial amplification mechanism. The proposed piezomagnetic shunt damping system is a complex mechanic–magnetic–electrical coupling system, which aims to transform and consume the vibration energy through local resonance to form a bandgap with tunable and strong wave attenuation ability. Based on Timoshenko beam theory, the wave equation and transmission spectrum of the metamaterial beam are derived by using transfer matrix method. The theoretical results are compared with the numerical simulation results. The results show that the resonance characteristics of piezomagnetic shunt circuit lead to negative group velocity and strong wave attenuation(−30 dB) at frequency defined by inductance and capacitance in shunt circuit. In varying frequency regimes, the coupled bandgap width can be maximized to 1925 Hz by tuning the external capacitance and coil turns. The superposition of double attenuation peaks caused by inertial amplification mechanism and local resonance coupling can be generated in the bandgap, allowing wave attenuation up to −94 dB. Subsequently, the attenuation capability of locally resonant bandgap can be significantly enhanced without changing the frequency by introducing the negative resistance into shunt circuit. Furthermore, the initiation and termination frequencies of bandgap can be flexibly designed by important material and structural parameters. This work is expected to provide valuable research ideas for the application of advanced intelligent materials in the field of vibration and noise reduction. • A novel hybrid-controlled graded metamaterial beam is proposed for bandgap tuning. • A tunable bandgap is formed based on piezomagnetic shunt damping system. • Coupled bandgaps are achieved by integrating different regulation mechanisms. • Wave transmission can be controlled by regulating significant material parameters. [ABSTRACT FROM AUTHOR]

• A model was proposed to predict velocities through a channel with a floating canopy. • New estimators of the penetration distance into canopies and bed boundary layer thickness were proposed and validated. • Predicted velocities agreed with measurements through model and real E. crassipes canopies. Floating vegetation with unanchored and connected roots can completely cover the water surface of rivers, ditches, and streams. Eichhornia crassipes is a typical macrophyte with a stable network of roots that is present in aquatic environments, forming a complicated flow velocity adjustment. This study constructed model floating canopies and real E. crassipes canopies to investigate flow velocity adjustment and verify a new proposed velocity model. Laboratory experiments showed that flow velocity was modified in both the streamwise (x) and vertical (z) directions and impacted by the mean channel velocity, canopy density, and relative flow depth. The penetration distance δ e of the Kelvin–Helmholtz vortices penetrating into the floating canopy increased to its maximum beyond the flow adjustment distance X D , where X D was the distance from the canopy leading edge to the position at which the flow was fully developed. The thickness of the bed boundary layer δ b was related to the vegetation drag (F D (x)), the bed shear stress (τ b (x)), and the height of the free-flow region below the canopy h g. New estimators for δ e and δ b were proposed and validated for floating canopies. Combined with the new estimators (δ e and δ b), flow continuity equation, momentum equation and four-layer divided method, a new analytical model was proposed to predict vertical profiles of velocity within and below a floating canopy in both the flow adjustment region (X D > x > 0) and fully developed flow region (x > X D). Fourteen groups of velocity data from this study and published literature were used to verify the proposed model. The predicted vertical profiles of velocity in flow adjustment and fully developed flow regions agreed with measurements under a wide range of vegetation and flow conditions, suggesting that the proposed model was capable of accurately predicting velocity profiles in a channel with a floating canopy. Finally, the model was validated in a channel with a real E. crassipes canopy. A comparison between predicted and measured velocities confirmed that the new model is valid for application in a channel with real floating macrophytes. [ABSTRACT FROM AUTHOR]

• A novel hybrid metamaterial structure with a wide low-frequency bandgap is proposed and studied. • The band gap and wave control characteristics of the proposed structure are studied using finite element analysis. • Discovering different combinations of inclusions in structures can provide useful guidance for achieving low-frequency bandgaps. • The total width of the bandgap frequency of the proposed structure can reach 43.99 % in the frequency range of 300 Hz. In order to achieve low-frequency vibration and noise reduction, artificial metamaterial with great potential in this field have been widely found. Based on previous studies, a new tetragonal hybrid metamaterial structure is proposed in this paper. It is exciting that the combination of inclusions with different material properties and sizes can affect the generation of band gaps. Then, based on the selected frequency, the propagation characteristics of waves in the structure were qualitatively analyzed using group velocity and phase velocity, and the potential for vibration reduction and noise reduction of the proposed structure was verified through finite element simulation of vibration in finite period structures. The results show that the proposed structure can open multiple broadband gaps, and the frequency range of the band gap can be flexibly adjusted according to needs. This study provides a new way for the design of low-frequency phononic metamaterial systems. [ABSTRACT FROM AUTHOR]

Mesoscopic numerical simulation has become an important tool in thermal management and energy harvesting at the micro/nano scale, where the Fourier's law failed. However, it is not easy to efficiently solve the phonon Boltzmann transport equation (BTE) from ballistic to diffusive limit. In order to accelerate convergence, an implicit synthetic iterative scheme is developed for the stationary phonon BTE, in which a macroscopic moment equation is invoked and solved iteratively coupled with the typical source iteration of the kinetic equation. Different from previous numerical interpolation, the phonon BTE is solved again at the cell interface along the group velocity direction within a certain length when reconstructing the interfacial phonon distribution function. Fourier stability analysis shows that the present method could converge faster than the source iteration method in the (near) diffusive regime. Numerical results prove that the present scheme can capture the ballistic-diffusive effects correctly and efficiently. The present acceleration framework could be a powerful tool for simulating practical thermal engineering problems in the future. • A synthetic iterative scheme is developed to solve the stationary phonon BTE with ab initio input. • BTE is solved again at the cell interface along the group velocity direction within a certain length when reconstructing the interfacial distribution function. • In the (near) diffusive regime, the present convergence is one to three orders of magnitude faster than the source iteration and several times faster than the previous acceleration strategy. [ABSTRACT FROM AUTHOR]

We have proposed a theoretical model for controllable optical soliton pulse formation in a magnetic-field-assisted four-level inverted Y-type atomic system. The probability amplitudes of the pertinent levels are used to analyze the contrast between self- and cross-phase modulated probe absorption signals. The absorption profiles obtained from the probability amplitudes and the zeroth-order of the pulse propagation constant in the Taylor series are appropriately congruent. The nonlinear Schrödinger equation derived from the Maxwell–Bloch equation determines the respective bright and dark soliton conditions with positive and negative values of the pulse shape parameter. It is demonstrated that the coupling laser field strengths, laser frequency detunings, and the external magnetic field's direction are the controlling parameters to produce both bright and dark optical soliton pulses. Alongside, the shape of the optical soliton pulses in self- and cross-phase modulation is influenced by the polarization angle of the probe and coupling fields. It is also found that the variation of coupling field strengths may be used to regulate the group velocity of the soliton probe pulse from superluminal to subluminal modes or vice-versa. [ABSTRACT FROM AUTHOR]

• New method for determining waveguide axial stress using guided waves. • Analytical insight into how axial stress affects guided wave dispersion properties. • In-depth investigation on selecting optimum modes for guided wave testing. • Detailed parametric study for predicting longitudinal rail stress. The determination of the longitudinal stress in continuous welded rails is essential for the safe operation of high-speed railways. Guided wave inspections have many advantages, and can provide an effective tool for non-destructive measurements of the longitudinal stress in rails. This paper provides a new method for determining longitudinal rail stress using ultrasonic guided wave dispersion properties. First, the characteristic equations of guided waves propagating in a waveguide with existing longitudinal stress are constructed, generating the dispersion properties of the waveguide such as the wavenumber, phase velocity and group velocity. The sensitivity of the dispersion properties of propagative modes to the longitudinal rail stress is investigated, which is then used for selecting appropriate modes for guided wave inspections. On the basis of the perturbation method, the relationship between the change in longitudinal rail stress and the change in dispersion properties is developed, which can be used for determining the longitudinal rail stress from dispersion properties. Furthermore, a mode selection index considering both the stress sensitivity and mode shape characteristics is employed for selecting appropriate propagative modes for guide wave inspections. Finally, a numerical example for CHN60 rail is adopted to demonstrate the effectiveness of the proposed method. From the results, the proposed method can accurately determine the longitudinal stress in the rail by using the dispersion properties available. [ABSTRACT FROM AUTHOR]

This study investigates the excitation of short-wavelength spin waves (SWs) in nanowire-based spin-Hall nano-oscillators (SHNOs). Using micromagnetic simulations, we study the wavelength of propagating SWs as a function of nanowire width, ranging from 10 to 50 nm, excited in a 1.7 nm-thick CoFeB layer using a pure spin current generated by a 5 nm thin platinum layer via the spin Hall effect. Our results showed that SWs with shorter wavelengths are excited in narrow widths at higher magnetic fields and higher currents. In addition, the wavelength of the propagating waves scales linearly with the width of the nanowire. We attribute the emission of short wavelength due to the quantization of SWs in the nanowire. By reducing the nanowire width, SWs with ultrashort wavelengths are excited at high frequencies, with larger propagation speeds and longer attenuation lengths. Our results show that ultrashort SWs with wavelengths as short as 80 nm can be generated in metallic-based SHNOs, with a group velocity of v g = 1.6 µm/ns and attenuation length of L a t t = 0.6 µm. These findings have significant implications for the scalability of magnonic devices, which could have potential applications in SW-based logic and neuromorphic computing. • Spin Hall nano-oscillators. • Propagating spin waves. • Short wavelength. • Micromagnetic simulations. [ABSTRACT FROM AUTHOR]

• The lattice thermal conductivity (κ) of uniaxial diamond has been calculated from first-principles. • The sensitivity of the κ of uniaxially strained diamond to size effects differed according to the scale. • The phonon group velocities of the TA1 and LA branches under different strains led to the relative change of the two κ at different temperatures. The miniaturization of high-frequency electronic devices has led to heat accumulation in their components. The ultrahigh thermal conductivity of diamond makes it an ideal material for heat dissipation, but factors such as temperature changes during the working process, special structures, thermal stress, and lattice mismatch can affect performance. This study investigated the effects of stress, temperature, isotope purity, and feature size on the thermal conductivity of diamond using a first-principles method. The sensitivity to size effects of the thermal conductivity of uniaxially strained diamond differed according to the scale. Under a low temperature (100 K), and with a feature size of 1 µm, the thermal conductivity of diamond with large-scale tensile strain exceeded that of a diamond with small-scale strain, which was an anomalous phenomenon. Isotope purification did not significantly affect the variation rule of the thermal conductivity of strained diamond. [Display omitted] [ABSTRACT FROM AUTHOR]

The tectonics of the Garwal-Kumaon Himalaya is characterized by thrusts, tectonic windows, and klippen. We investigate crustal shear wave velocity variations using ambient noise cross-correlation tomography. The studied region encompasses the Kali River valley in the east to Satluj valley in the west, with the adjoining Indo-Gangetic Plain in the south covering the northern part of the Delhi-Haridwar ridge. The fundamental mode group velocities of Rayleigh waves are extracted from cross-correlation data of 33 broadband seismological stations from a regional seismic network. A total of 374 dispersion curves with a period range of 4–29 s show a group velocity variation between ∼2.3 and ∼ 3.4 km/s. The shear wave velocity structure of the uppermost lithosphere down to ∼50 km obtained by non-linear inversion of the Rayleigh wave dispersion data provides new insight into the geometry of the crust. A large variation in Vs in the range of ∼2.8 to ∼4.7 km/s corresponds to a variety of changes in the tectonic deformation, structure, and crustal thickness of the Himalayan wedge. Thick low-velocity sedimentary formations are identified beneath the Indo-Gangetic Plain and the frontal Himalaya. Anomalous low Vs zones are also observed in the mid-crust beneath the higher Himalaya and southern Tibet, indicating partial melting or the presence of aqueous fluid zones. The high-velocity anomalies may be correlated with duplex structures beneath the Lesser Himalaya and with lithospheric flexure. [Display omitted] • Very low Vs in the uppermost crust beneath IGP indictes upto 10 km thick sediments. • Contrast of Vs in different depths in western and eastern parts of Central Himalaya. • Low Vs around MCT with high seismicity close and above Main Himalayan Thrust. • Possibility of flexure in the mantle below ramp structure. • Moho depth is between 40 and 47 km depth or deeper beneath study region of NW Himalaya. [ABSTRACT FROM AUTHOR]

We designed and investigated the electronic, mechanical, and thermoelectric properties of Graphene/hexagonal Boron Nitride (Gr/h-BN) heterostructure at various twisting angles based on the Ab-initio simulation. The structural stability was studied at optimized rotation angles (ϕ) = 0 ∘ , 16.10 ∘ , 21.79 ∘ , 38.21 ∘ , 43.90 ∘ and 60 ∘. The heterostructure shows semiconducting nature at ϕ = 0 ∘ , 21.79 ∘ and 38.21 ∘. These twisted heterostructures have demonstrated extraordinary mechanical properties such as Young's modulus and bulk modulus. Using the semiclassical Boltzmann transport theory, it is observed that the Seebeck coefficient, electric conductivity, and power factor at ϕ = 0 ∘ , 21.79 ∘ , 38.21 ∘ , and 60 ∘ are much higher than the values measured at ϕ = 16. 1 0 ∘ and 43.90 ∘. Moreover, at ϕ = 6 0 ∘ , the Power Factor for the n-type dopants can reach 1.37 × 10 11 W/msK 2. The lattice thermal conductivity at room temperature is found to be very low for ϕ = 16. 1 0 ∘ , 21.79 ∘ , 43.90 ∘ and 38.21 ∘ rotation angles. An ultralow lattice thermal conductivity with a value of 0.095 W/mK at 300K has been observed for 21.79 ∘ rotation angle, which is lower than other rotation angles because of very low group velocity (22.1 km/s) and short phonon lifetime (∼ 0.12 ps). The high thermoelectric performance results from an ultralow thermal conductivity arising due to the strong lattice anharmonicity. The present observations can offer significant impact on the design of high performance thermoelectric materials based on twisted van der Waals heterostructure (vdWH). [Display omitted] [ABSTRACT FROM AUTHOR]

We illuminate the crustal and upper mantle structure of the northwestern corner of South America by mapping radial and azimuthal seismic anisotropy. Radial anisotropy is inferred from the discrepancy between V SV and V SH polarized S-velocities in horizontally propagating Rayleigh and Love waves, respectively. First, short-period (7–35 s) phase and group velocities are estimated from ambient noise cross-correlations, and long-period (40–170 s) group velocities are determined from fundamental-mode, surface-wave trains from earthquake sources at regional and teleseismic epicentral distances. Dispersion velocities are then tomographically inverted to develop maps of group velocity variation for periods between 7 and 170 s including azimuthal velocity dependence. We observe that positive anisotropy in the upper crust is related to thick Quaternary sequences in sedimentary basins, while negative anisotropy is found beneath active volcanoes and igneous-metamorphic terrains. Negative anisotropy features observed in the lower crust beneath the Eastern Cordillera suggest the existence of sill-like intrusions that penetrate the lower crust, thus supporting underplating mechanisms. Our study reveals SW-NE oriented fast axes of azimuthal anisotropy beneath the Eastern Cordillera, which strongly suggest the presence of shear zones within the fold and thrust belt system. Notably, the fast axes align parallel to the regional faults in the area, indicating a significant role in controlling faulting processes. Finally, we observe a wide trench-parallel zone of azimuthal anisotropy beneath the Nazca Plate, the Panama Block, and the Caribbean, which we associate with subslab mantle flow. • Fast axes at the crustal are correlated with the direction of maximum horizontal stresses. • Trench-parallel fast directions are associated with the subslab and mantle flow in the S–N direction. • Negative anisotropy in the lower crust beneath the Cordillera supports evidence of underplating process [ABSTRACT FROM AUTHOR]

The electromagnetically induced transparent-like metamaterials have received much attention because of their excellent slow-light properties and strong nonlinear effects. They have many promising applications in novel terahertz functional devices, high-sensitivity sensors, and optical storages. In this paper, a flexible terahertz switching device based on the electromagnetically induced transparent-like effect was proposed. The device exhibited a significant slow light phenomenon without pumped laser. Meanwhile, it is demonstrated that the device has a large modulation depth and excellent tunable slow light performance with low-power pumped laser. Its amplitude modulation depth can reach 60.4% and the group delay modulation can reach 32.5 ps. And the minimum group velocity of the device in slow terahertz light can reach 0.69 × 105 m/s. Moreover, the flexible substrate of the proposed device is not easily damaged. It makes the device suited for more complex environments well. Therefore, the device will have great potential for future research on high-performance terahertz switching devices. • A terahertz device based on the EIT-like effect was proposed. • The WTE2 material enables the device to have significant modulation capability. • A small group velocity of the device in slow terahertz light was obtained. [ABSTRACT FROM AUTHOR]

Tin telluride compounds are promising thermoelectric (TE) materials on account of the excellent electronic and thermal transport properties. The present work focuses on examining the TE performance of a two-dimensional wrinkled SnTe monolayer by employing the first-principles calculations and Boltzmann transport theory. The SnTe monolayer demonstrates high mechanical, dynamic and thermal stabilities, which have been thoroughly investigated using the elastic modulus, formation energy and ab initio molecular dynamics simulations. The lattice thermal conductivities of the SnTe monolayer along the zigzag - and armchair -direction are 11.29 and 12.40 W/mK at 300K, respectively. Due to the large Grüneisen parameters, short phonon relaxation time and low phonon group velocity, the p -type SnTe monolayer exhibits high TE performance at 900 K, as characterized by a high ZT of 1.44 along the zigzag -direction. Our present study not only offers valuable insights into the TE properties of the wrinkled SnTe monolayer, but also sheds some light on the theoretical designing of the novel SnTe-based material for TE applications. • The SnTe monolayer is indirect semiconductor with wide bandgap. • The SnTe monolayer exhibits high dynamic and mechanical, and thermal stabilities. • The SnTe monolayer has excellent thermal and electronic transport properties. • The SnTe monolayer is a promising p -type thermoelectric material with optimal ZT of 1.44 at 900 K. [ABSTRACT FROM AUTHOR]

We have conducted a study on the electronic and thermoelectric characteristics of Janus AsSeX (X = Cl, Br, I) monolayers. To accomplish this, we employed a combination of density functional theory (DFT) and Boltzmann transport theory. Through our study, we observed that the Janus AsSeX monolayers possess excellent dynamic, thermal, and mechanical stabilities. Our results reveal that AsSeCl, AsSeBr, and AsSeI are indirect band gap semiconductors with bandgaps of 2.10 eV, 1.87 eV, and 1.85 eV, respectively. Following the value of the B/G and v , we observe that Janus AsSeCl displays brittle property, whereas Janus AsSeBr and AsSeI exhibit ductile characteristics. We have also calculated the lattice thermal conductivity of these materials at room temperature, which are 1.87 Wm−1K−1, 3.80 Wm−1K−1, and 2.63 Wm−1K−1 for AsSeCl, AsSeBr and AsSeI, respectively. Furthermore, we have extracted thermal transport parameters, including the phase volume space group velocity, scattering rate, Grüneisen parameter, and group velocity, and carried out an in-depth analysis of their underlying mechanisms. Finally, we predict ZT values of 1.55, 0.95, and 1.11 for the p-type doping of Janus AsSeCl, AsSeBr, and AsSeI monolayers at 700 K, indicating that Janus AsSeCl and AsSeI monolayers are potential thermoelectric materials with high ZT values. [ABSTRACT FROM AUTHOR]

• All optical velocity manipulation via a MZI incorporated with Brillouin random lasing resonance is proposed and demonstrated. • The proposed scheme manipulates the MZI transmittance and overcomes the saturation effect for a flexible light speed control. • 160 kHz sinusoidally modulated signals shows fast light propagation with group index of 0.875 over 2 km single mode fibers. We proposed and experimentally demonstrated a novel scheme of light group velocity control via a Mach-Zehnder fiber interferometer incorporated with Brillouin random lasing resonance in a half-open linear cavity. A theoretical model of the proposed Brillouin random lasing resonance-based Mach-Zehnder interferometer (BRLR-MZI) for the light group velocity manipulation was derived, predicting optical power-dependent transmittance spectrum shift as well as the enhancement of the temporal advancement. Thanks to Brillouin random lasing-induced fast light effect, the proposed scheme not only achieves a flexible all-optical manipulation of its transmittance spectrum but also overcomes the saturation effect for the light speed control based on a traditional MZI. In experimental validation, sinusoidally modulated signals with the modulation frequency of 160 kHz ultimately experienced a maximum advancement of 4031 ns (corresponding to the group index of 0.875), which exhibits an advancement improvement of 1364 ns compared with the traditional MZI-based scheme. It suggests a promising platform for exploring applications in fields of all-optical signal processing, microwave photonic, and ultra-sensitive fiber sensing. [ABSTRACT FROM AUTHOR]

We report shear wave phase and group velocity, dispersion and attenuation in oil-in-gelatin viscoelastic phantoms and in vivo liver data. Moreover, we measured the power law coefficient from each dispersion curve and used it, together with the shear wave velocity, to calculate an approximate value for attenuation that agrees with independent attenuation measurements. Results in phantoms exhibit good agreement for all parameters with respect to independent mechanical measurements. For in vivo data, the livers of 20 patients were scanned. Results were compared with pathology scores obtained from liver biopsies. Across these cases, increases in shear wave dispersion and attenuation were related to increased steatosis score. It was found that shear wave dispersion and attenuation are experimentally linked, consistent with simple predictions based on the rheology of tissues, and can be used individually or jointly to assess tissue viscosity. Thus, this study indicates the possible utility of using shear wave dispersion and attenuation to non-invasively and quantitatively assess steatosis. [ABSTRACT FROM AUTHOR]

The goal of this paper is to investigate the propagation of two-dimensional elastic Bloch waves in helical periodic structures, obeying two discrete screw symmetries about the same axis. First, a three-dimensional coordinate system is built from the two helical directions of periodicity of the problem and the radial coordinate originating from the symmetry axis. The existence of Bloch waves in bi-helical structures can be justified owing to the independence of the metric tensor of the coordinate system on both helical coordinates. Considering the elastodynamic equilibrium equations, Bloch theorem is expressed in appropriate bases to project the vector wavefields, namely the covariant/contravariant bases of the bi-helical coordinate system (or, alternatively, the cylindrical basis). From a geometrical point of view, the three-dimensional unit cell is delimited by non-plane boundaries, which must be carefully parametrized. The so-called wave finite element method is then applied to numerically solve the Bloch wave eigenproblem and the implementation of the numerical method in a bi-helical system is detailed. Owing to the cylindrical nature of the geometry, the two-dimensional propagation constants are not independent to each other. The relationship between both constants is established. The calculation of wave mode properties (wavenumbers, group and energy velocities) is performed along the helical propagation directions, as well as the straight axial and circumferential directions. Numerical validations of the overall approach are carried out for cylindrical uniform tubes and a chiral nanotube. Finally, the method is applied to a complex multi-wire structure, often encountered in energy cables, consisting of two layers of helical wires twisted in opposite directions. [ABSTRACT FROM AUTHOR]

• Rayleigh wave dispersion in anti-plans, a layer over half-space is developed. • An efficient approach, to identify the dispersion equation, group and phase velocity is proposed. • Frequency responses for the effects of porosity, orthogonality, non-homogeneity, etc. are derived. This paper is concerned with the Rayleigh wave dispersion in an incompressible functionally graded orthotropic half-space loaded by a thin fluid-saturated aeolotropic porous layer under initial stress. Both the layer and half-space have subjected to the incompressible in nature. The particle motion of the Rayleigh type wave is elliptically polarized in the plane, which described by the normal to the surface and the focal point along with wave generation. The dispersion of waves refers typically to frequency dispersion, which means different wavelengths travel at a different velocity of phase. To deal with the analytical solution of displacement components of Rayleigh type waves in a layer over a half-space, we have taken the assistance of different methods like exponential, characteristic polynomial and undetermined coefficients. The dispersion relation has been derived based upon suitable boundary conditions. The finite difference scheme has been introduced to calculate the phase velocity and group velocity of the Rayleigh type waves. We also have derived the stability condition of the finite difference scheme (FDS) for the phase and group velocities. If a wave equation has to travel in the time domain, it is necessary to achieve both accuracy and stability requirements. In such cases, FDS is preferred because of its power, accuracy, reliability, rapidity, and flexibility. The effect of various parameters involved in the model like non-homogeneity, porosity, and internal pre-stress on the propagation of Rayleigh type waves have been studied in detail. Graphical representations for the effects of various parameters on the dispersion equation have been represented. Numerical results demonstrated the accuracy and versatility of the group and phase velocity depending on the stability ratio of the FDS. [ABSTRACT FROM AUTHOR]

In the current work, we develop a higher gradient dynamic homogenization method with micro-inertia effects. To that scope, we compute the macroscopic constitutive parameters up to the second gradient, using two distinct approaches, namely the Hamilton's principle and the total internal energy formulation. Thereupon, we analyze the sensitivity of the second gradient constitutive terms on the inner material and geometric parameters for the case of composite materials with a periodic, layered microstructure. The results suggest that the significance of the second gradient terms highly depends on the differences in the geometric and material properties of the underlying microstructural phases, as well as on the deformation mode of interest. What is more, the wave propagation study indicates that different higher gradient results are obtained, depending on the formulation used and on the wavenumber range of interest. In particular, Hamilton based, second gradient models perform poorly in the low wavenumber range, compared to first gradient or to analytic, exact solutions. Contrariwise, second gradient macroscopic constitutive formulations derived using the total higher gradient internal energy outperform first gradient approaches, better approximating the wave propagation characteristics of the effective medium, as the corresponding comparison in between the frequency diagrams and phase and group velocities suggest. [ABSTRACT FROM AUTHOR]

It is known from linear theory that bottom oscillations in uniform open-channel flow can produce resonant surface waves with zero group velocity and diverging amplitude (Tyvand and Torheim 2012). This resonance exists for Froude numbers smaller than one, at a critical frequency dependent on the Froude number. This resonance phenomenon is studied numerically in the time domain, with fully nonlinear free-surface conditions. An oscillatory 2D bottom source is started, and the local elevation at resonance grows until it may reach a saturation amplitude. Four waves exist at subcritical Froude numbers, where resonance represents the third and the fourth wave merging. In the zero-frequency limit, the dispersive second and fourth wave merge into a steady wave with finite group velocity and amplitude, and no other periodic waves exist. In the time-dependent nonlinear analysis at zero frequency, a transient undular bore may emerge as the dominating phenomenon. [ABSTRACT FROM AUTHOR]

• Brain metastasis velocity describes the recurrence rate of new brain metastases after initial treatment with radiosurgery. Brain metastasis velocity is calculated by dividing the number of new brain metastases after initial radiosurgery by the time in years since first radiosurgery. • Patients can be stratified into high, intermediate, and low risk velocity groups, which corresponds with overall survival. • The current study externally validates brain metastasis velocity as a prognosticator with over 700 patients from eight institutions. • Brain metastasis velocity is being employed in clinical trials as a tool to stratify patients for salvage whole brain radiation and radiosurgery. Brain metastasis velocity (BMV) is a prognostic metric that describes the recurrence rate of new brain metastases after initial treatment with radiosurgery (SRS). We have previously risk stratified patients into high, intermediate, and low-risk BMV groups, which correlates with overall survival (OS). We sought to externally validate BMV in a multi-institutional setting. Patients from nine academic centers were treated with upfront SRS; the validation cohort consisted of data from eight institutions not previously used to define BMV. Patients were classified by BMV into low (<4 BMV), intermediate (4–13 BMV), and high-risk groups (>13 BMV). Time-to-event outcomes were estimated using the Kaplan–Meier method. Cox proportional hazards methods were used to estimate the effect of BMV and salvage modality on OS. Of 2829 patients, 2092 patients were included in the validation dataset. Of these, 921 (44.0%) experienced distant brain failure (DBF). Median OS from initial SRS was 11.2 mo. Median OS for BMV < 4, BMV 4–13, and BMV > 13 were 12.5 mo, 7.0 mo, and 4.6 mo (p < 0.0001). After multivariate regression modeling, melanoma histology (β : 10.10, SE: 1.89, p < 0.0001) and number of initial brain metastases (β : 1.52, SE: 0.34, p < 0.0001) remained predictive of BMV (adjusted R2 = 0.06). This multi-institutional dataset validates BMV as a predictor of OS following initial SRS. BMV is being utilized in upcoming multi-institutional randomized controlled trials as a stratification variable for salvage whole brain radiation versus salvage SRS after DBF. [ABSTRACT FROM AUTHOR]

• An energy flow model of two-dimensional panels in supersonic airflow is developed. • Equivalent damping due to supersonic airflow is introduced to derive the energy flow model. • The impedance of the panel in supersonic airflow is derived to calculate the input power. • Supersonic airflow leads to different equivalent damping for elastic waves propagating in different directions. • An energy finite element method for high-frequency analysis of elastic waves with different damping is developed. Many energy flow models have been proposed for high-frequency forced vibration analysis of structures. In this paper, a novel energy flow model is developed to predict the high-frequency vibration response of panels in supersonic airflow and quantify the effects of supersonic airflow on high-frequency forced vibration characteristics. The additional damping due to supersonic airflow is derived from the motion equation of a two-dimensional panel. The relationship between the wavenumber and the group velocity is introduced to describe the energy transmission property considering the effects of supersonic airflow. Then the energy density governing equation (i.e. energy flow model) is established and solved by the energy flow analysis (EFA) and the energy finite element method (EFEM). Finally, comparing the vibration responses obtained by the present energy flow model with the corresponding exact analytical solutions, the developed energy flow model is verified to be effective for high-frequency vibration analysis of panels in supersonic airflow. Furthermore, the numerical simulations indicate that supersonic airflow can affect the equivalent damping of the propagating elastic waves, and thus change the energy density distribution of the panel. [ABSTRACT FROM AUTHOR]

We study the dependence of the dispersion characteristics of quasi-particle excitations in a non-ideal one-dimensional microcavity lattice on concentrations of structural imperfections formed by uneven spacing of cavities. Two configurations are considered where either one of the sublattices contains quantum dots or the whole structure is quantum-dot-free. It is shown that an appropriate incorporation of structural defects into a one-dimensional microcavity array can be used to modulate the energy spectrum of electromagnetic excitations (and hence the optical properties) of the crystal. • Incorporation of structural defects into a one-dimensional microcavity array modifies its energy spectrum. • Group velocity of polaritonic excitations is sensitive to concentration of structural defects. • Virtual crystal approximation proves effective for description of microcavity chains. • Peculiarities of electromagnetic spectrum are manifested in the quasi-particle density of states. [ABSTRACT FROM AUTHOR]

Muscle force can be generated actively through changes in neural excitation, and passively through externally imposed changes in muscle length. Disease and injury can disrupt force generation, but it can be challenging to separate passive from active contributions to these changes. Ultrasound elastography is a promising tool for characterizing the mechanical properties of muscles and the forces that they generate. Most prior work using ultrasound elastography in muscle has focused on the group velocity of shear waves, which increases with increasing muscle force. Few studies have quantified the phase velocity, which depends on the viscoelastic properties of muscle. Since passive and active forces within muscle involve different structures for force transmission, we hypothesized that measures of phase velocity could detect changes in shear wave propagation during active and passive conditions that cannot be detected when considering only group velocity. We measured phase and group velocity in the human biceps brachii during active and passive force generation and quantified the differences in estimates of shear elasticity obtained from each of these measurements. We found that measures of group velocity consistently overestimate the shear elasticity of muscle. We used a Voigt model to characterize the phase velocity and found that the estimated time constant for the Voigt model provided a way to distinguish between passive and active force generation. Our results demonstrate that shear wave elastography can be used to distinguish between passive and active force generation when it is used to characterize the phase velocity of shear waves propagating in muscle. [ABSTRACT FROM AUTHOR]

Radial smoothing scheme based on the optical Kerr effect can add periodical spherical wavefront modulation to the laser beam and make the speckles within the focal spot in far field sweep periodically in radial direction, which improves the illumination uniformity of the laser beam on target in inertial confinement fusion facilities. In order to analyze the smoothing performance of radial smoothing scheme in a laser quad, the corresponding theoretical model was established and the temporal characteristic parameters of the pump laser were further analyzed and optimized. The results show that the fluctuation of the smoothing performance with time can slow down by appropriately adjusting the delay time between the laser beam and pump laser pulses in a laser quad. Furthermore, the different sweep period of the incoherent beams can be obtained by altering the pulse duration of pump laser pulses in a laser quad. Consequently, the group velocity of intensity envelope becomes faster with the increase of the frequency difference between incoherent beams. What' more, a better smoothing performance can be achieved by applying the appropriate spatial deflection between the laser beam and pump laser pulses in a laser quad, and the deflection angle of the pump laser in a laser quad was further optimized to improve the illumination uniformity on target significantly. Radial smoothing with obliquely incident pump laser can add a spherical wavefront to the laser beam, and further makes the speckles redistribute rapidly both in radial and transverse directions. With appropriate incident direction and angle, RS can achieve a much better final smoothing performance than 2D-SSD. [ABSTRACT FROM AUTHOR]

(a) Artificial delamination in the CFRP specimen, (b) group velocity distribution, and (c) 3D imaging in the CFRP specimen. A pulsed laser illuminates a target zone that causes rapid thermoelastic expansion, generating broadband high-frequency ultrasonic wave (photoacoustic wave, PA wave). We developed a PA microscopy (PAM) with a confocal area of laser and ultrasonic wave for applications in nondestructive testing (NDT). The synthetic aperture focusing technique (SAFT) is applied in the PAM for the three-dimensional (3D) imaging of interior flaws. Here, we report proof-of-concept experiments for the NDT of a subsurface flaw in a thin laminar material. Graphical abstract (a) shows a specimen of carbon-fiber-reinforced plastic (CFRP) with an artificial delamination. Here, it should be noted that the group velocity varies directionally due to the strong anisotropy of the CFRP specimen (see Graphical abstract (b)). By considering the group velocity distribution in the SAFT, the shape and location of the subsurface delamination were accurately estimated as shown in Graphical abstract (c). Coating the surface of the CFRP specimen with a light-absorbent material improved the amplitude of the PA wave. This finding showed that the signal-to-noise ratio of the waves scattered from the flaws can be improved. [ABSTRACT FROM AUTHOR]

• A model is proposed to predict the longitudinal profile of velocity inside vegetation patches. • The proposed analytical model was verified by twenty groups of velocity data. • The predicted velocity profiles agree well with the measured velocities. • Velocity profiles can evaluate the deposition of fine sediment inside vegetation patches. This paper proposes an analytical model for predicting the longitudinal profile of depth-averaged streamwise velocities in a channel with an emergent array of rigid cylinders. The governing equation of the analytical model was derived from the momentum equation and flow continuity equation. The longitudinal transect through a flume with a vegetation patch was divided into four regions based on two length scales of flow adjustment upstream of and inside the array, and analytical solutions were proposed for the four regions. Laboratory experiments demonstrated that the array length does not influence flow adjustments near the upstream edge of the array, so the model can predict the longitudinal profile of streamwise velocity for either short or long arrays. Twenty groups of velocity data from different sources were used to verify the proposed model. The predicted velocities agreed well with the measured velocities, indicating that the model is capable of predicting the longitudinal profiles of the velocity upstream of and inside a model patch. The predicted velocity profile can be further employed to estimate regions of enhanced or diminished deposition of fine sediment or organic matter inside model patches. [ABSTRACT FROM AUTHOR]

• Longitudinal waves in plasma formed by above-threshold ionization were studied. • Dispersion law of short wave responds to electronic sound. • Number of waves corresponds to number of photoelectron distribution energy peaks. • Frequencies of short wave exceed electronic Langmuir frequency. In the regime of above-threshold ionization of gas atom in the field of laser radiation, plasma with photoelectron distribution consisting of peaks at discrete energy values is formed. It is shown that the number of longitudinal waves in such plasma coincides with the number of peaks in the distribution function. When peaks practically don't overlap, the dispersion law of each wave in the region of short waves is determined by electrons from the corresponding peak. In this case the phase and group velocities of the waves are close to the electron velocity, which corresponds to the peak maximum. It is possible to talk about such waves as an electronic sound, since the perturbations of the electron density mainly arise due to pressure perturbations. When the peaks are narrow, but having a finite width, the Cherenkov damping of waves is exponentially small. Numerical calculations the dispersion laws for of the two and four waves in photoionized xenon plasma, in which the electron distribution function consists of two or four narrow peaks are given. [ABSTRACT FROM AUTHOR]

Abstract The present paper deals with the Shear wave propagation in a multilayered magnetoelastic anisotropic monoclinic medium with finite difference modeling to comprehend the stability criteria, phase velocity, and group velocity. Utilizing Maxwell's fundamental theory of magnetoelasticity, the problem has been constructed. Haskell's matrix technique has been utilized to obtain the frequency equation. Stability analysis has been conducted based on the finite difference technique to reduce the soaring error values and control its stability. Numerical evaluation as well as graphical representation, have been employed to enlighten the effects of different values of courant number and magnetoelasticity on the phase and group velocities. [ABSTRACT FROM AUTHOR]

• Seismic noise allows the extraction of surface waves near an underground mine. • Seismic noise methods can be used to image the vicinity of an underground mine. • Fault system affects the velocity features of the geological medium. • Seismic ambient noise analyses have the potential to act as a de-risking tool. We use a passive seismic data set recorded from June 2017 to July 2018 by twenty-one stations deployed in a five by five km grid to the south-west of the Boliden Tara Mines orebody (Eastern Ireland) to image the shallower part of the Carboniferous rock units. We cross-correlate one year of ambient noise between all station pairs to generate cross-correlation functions that are rich in surface waves. Then we investigate the locations of the noise sources by applying f-k cross-correlation beamforming. We find two persistent noise sources associated with oceanic microseisms and a cluster of anthropogenic activities, respectively. We undertake dispersion analysis to extract group velocity measurements in the 1–9 Hz band that we invert using the fast marching method to create Rayleigh wave velocity maps. This is followed by a wavelet-based sparsity constrained tomographic method to produce a 3-D shear wave velocity model down to 800 metres. The results highlight low and high-velocity anomalies related to the structural and stratigraphic heterogeneity within the Upper Dark limestone formation. In addition, the velocity gradient of our model agrees with the sonic log velocity profile available adjacent to the study area, suggesting that this sparse network can be used to obtain reliable broad-scale information and could also guide the design phase of more expensive and involved experiments, such as active seismic reflection imaging. [ABSTRACT FROM AUTHOR]

Transport equations for phonons and electrons are solved to understand the thermoelectric transport in ZnSb. Inherent poor bonding characteristics are reflected in the low frequencies and low group velocities of phonons estimated within the harmonic approximation. Low-frequency optical branches overlap with the acoustic branches, increasing the scattering phase space. Estimated mode Grüneisen parameters indicate an anharmonic system, which is also reflected in the low self-consistent phonon lifetimes. All these factors contribute to the low thermal conductivity of ZnSb. The optimum doping concentration for the thermoelectric figure of merit is approximately 5 × 1018 per cm 3. There is an asymmetry in the distribution of the density of states near the band edges, which also contributes to the hole-dominated thermoelectric transport in the intrinsic region. • Thermoelectric transport in ZnSb is studied. • Weak bonding results in low frequencies and low group velocities for phonons. • The frequency overlap between the acoustic and optical phonons increases scattering. • Low phonon lifetimes suggest a strongly anharmonic system. • Asymmetry in the density of states near the band edges contributes to hole-dominated transport. [ABSTRACT FROM AUTHOR]

The Zagros fold-and-thrust belt is the principal response to the Arabia–Eurasia continental collision, which generates significant seismic activity in Iran. Surface wave tomography can be a proper tool for crustal structure imaging in this region. We used observed dispersion curves of Rayleigh wave derived from high-quality local-earthquake waveforms to obtain 2D tomographic maps in the period range 1–28 s beneath the Zagros collision zone. Subsequently, local pseudo-dispersion curves for individual grid points are inverted for shear-wave velocity structure, in order to obtain a 3D velocity model of the crust. Our results indicate shallow low-velocity structures near the suture zone. Moreover, a low-velocity flexure shape in the south of the Zagros Mountain Front Fault could be due to the thick sedimentary basin beneath the Dezful embayment. Relatively an uplifting of the deeper structures beneath Sanandaj-Sirjan and the high Zagros might could be led to high-velocity structures at mid-crustal depths. The variation of shear-wave velocity along the Zagros indicates the inhomogeneous structures along this folding and thrust belt. [Display omitted] • A new 3D V S model of the Zagros calculated from Rayleigh waves of local earthquakes. • Low-velocity upper crustal structures are consistent with the location of the suture and thick sedimentary basins. • High-velocity in the middle crust may be associated with an uplifting of the deeper structures. [ABSTRACT FROM AUTHOR]

The existence and stability of zero-velocity solitons in a dual-core optical fiber, where one core possesses cubic–quintic nonlinearity and the other core is linear with a Bragg grating, are investigated. The frequency spectrum for the model consists of three bandgaps, of which only the central band gap is a genuine one. There are no soliton solutions in the central gap. On the other hand, the upper and lower gaps are filled with two distinct and disjoint families of soliton solutions, termed Type-1 and Type-2 solitons, that differ in their shapes and phase characteristics. When the relative group velocity in the linear core is zero, implicit analytical solutions for Type-1 and Type-2 quiescent solitons are found. For nonzero values of the relative group velocity, soliton solutions can only be found using numerical techniques. The stability of solitons has been analyzed systematically by means of numerical stability analysis. It is found that Type-2 solitons are always unstable. In the case of Type-1 solitons, stability regions are identified in the upper and lower bandgaps. The effects and interplay of the relative group velocity and the Bragg grating-induced linear coupling coefficient on the stability of the solitons are also analyzed. [ABSTRACT FROM AUTHOR]

The development of nanophononic metamaterials presents new opportunities for controlling thermal conduction. Using the theory of elasticity and the finite element method, we systematically investigated the thermal conductance of two-dimensional nanophononic metamaterials embedded with cylindrical arrays. When the sound speed of the embedded material is higher than matrix, there is a weak correlation between structural parameters and thermal conductance. However, when the nanophononic metamaterials are embedded with lower sound speed materials, they exhibit clear local resonances. The local resonance frequency can move to a higher or lower range depending on the filler radius. Local resonances reduce group velocity but greatly increase the phonon density of states, contributing to an increase in thermal conductance. Furthermore, this conclusion applies to nanophononic metamaterials with different matrix materials. • We investigate thermal transport properties of 2D nanophononic metamaterials. • Nanophononic metamaterials embedded with soft materials exhibit local resonances. • Local resonances reduce group velocity but increase the phonon density of states. • Local resonances finally lead to an increase in thermal conductance. • The frequency of these local resonances can move to a higher or lower range. [ABSTRACT FROM AUTHOR]

Guided waves, especially Lamb waves, propagating in plate-like structures with constant thicknesses have been investigated intensively in the past decades in the context of structural health monitoring (SHM) and non-destructive testing (NDT). However, structures with variable thicknesses are also very common in practice, such as ship hulls whose thickness varies for satisfying various load conditions, and defect identification for such structures is also of importance. In this paper, we compare the propagation characteristics of both Lamb modes and SH 0 mode in variable thickness plates and propose to use the SH 0 mode for defect identification in such waveguides because of its non-dispersive property. First, the non-dispersive property of the SH 0 mode in waveguides with variable thickness is demonstrated in theory. Then, a wave prediction method is adopted to illustrate the differences between the Lamb modes and the SH 0 mode when propagating in waveguides with constant or variable thickness. Finally, experiments are conducted on a variable thickness plate to verify the non-dispersive property of the SH 0 mode and further compare the results with those of the Lamb modes. Recently developed thickness-poled omnidirectional shear horizontal piezoelectric transducers (OSH-PTs) are used to produce pure SH 0 waves. It is observed that the velocity of the Lamb modes may change significantly and the group velocity of the S 0 mode even decreases by 55.1% during propagation, which hampers Lamb modes from defect identification in variable thickness plates. By contrast, the SH 0 mode is non-dispersive thus promising for damage detection, and its effectiveness for defect localization in such structures is validated experimentally. [ABSTRACT FROM AUTHOR]

We consider a higher-order nonlinear Schrödinger amplitude equation with real and imaginary Raman gain terms. This equation is shown to govern the propagation of femtosecond pulses in optical fiber media. We investigate on the modulational instability of continuous wave solution of this amplitude equation and show that the gain exist in both the anomalous and normal dispersion regimes, provided the complex intrapulse Raman terms are considered. However, when the dissipation induced by Raman process in the optical fiber is neglected; periodic bright solitary waves can only be identified in the anomalous dispersion regime. Nonlinear periodic trains of optical pulses are analytically obtained using the ansatz technique which incorporates the direct integral method. The observation of an optical shock front moving at the group velocity, is attributed to the Raman dissipation process in the optical fiber. Finally, results of numerical simulations equally underscores the influence of Raman gain terms on the evolution of optical soliton trains. [ABSTRACT FROM AUTHOR]

In this paper, a single-phase quadrangular star-shaped ligamentous heterostructure is constructed, and a two-phase structure is formed by attaching a resonator at the center of the structure. The lattice theory and finite element modeling are combined to analyze the band structure of the two structures, and the additional resonator at the center of the structure improves the bandgap range. Based on this, the bandgap can be widened and reduced by adjusting the resonator size. In the structural vibration modal analysis, it is explained that the coupling of traveling wave and vibration can open the omnidirectional bandgap, which provides guidance for the subsequent structural optimization. In addition, the relationship between bandgap opening and directional propagation of elastic waves is verified by a comprehensive analysis of group velocity, phase velocity, and other means. Finally, the transmission characteristics and stress distribution of the structure within the finite period structure are investigated, and it is further verified that the structure has a strong attenuation capability to vibration. This structure has good vibration isolation capability in a wide frequency range and has potential for application in complex vibration control. • Calculation of structural bandgap using Bloch's theorem and lattice theory. • Analysis of bandgap properties of different component structures. • Evaluation of vibration attenuation by transfer function and stress cloud map. • Analysis of the bandgap opening mechanism by vibration modes of the structure. • Analysis by group and phase velocity, iso-frequency and wave propagation maps. [ABSTRACT FROM AUTHOR]

• Anisotropic wave skew effects can cause an apparent offset in guided wave velocities. • Difference from theory of UD CFRP plate experiments and FE simulations. • Reduced accuracy for damage detection and localization in composite structures. • Skew angle correction for point source excitation, implemented experimentally. Guided ultrasonic wave propagation in anisotropic structures results in directional dependency of velocity and wave skewing effects that can impact the accuracy of damage detection. Phase and group velocities of the A 0 guided wave mode, propagating in a unidirectional carbon fiber reinforced laminate, were investigated experimentally and through finite element analysis. A correction for the significant offset in phase and group velocities due to wave skewing effects is illustrated for both point and short line sources, achieving good agreement with theoretical calculations assuming planar wave fronts. The influence of the line excitation length on velocity measurements is discussed. [ABSTRACT FROM AUTHOR]

• The effects of phonon renormalization induced by the fourth-order anharmonicity and 4-phonon scattering are fully considered when calculating the KXY thermal transport properties. • We verified the microscopic origin of heat transport from the microscopic point of view of the bonding strength between atoms and combined with the results of phonon group velocity, scattering rate, Grüneisen parameter, and mean free path. • We capture heat-carrying high-frequency optical phonons that make an anomalous contribution to heat transport. • We verified and predicted the mechanical properties of KXY. [Display omitted] In this work, we systematically studied the thermal transport properties of ternary zintl compound KXY using self-consistent phonon theory combined with compressive sensing technology and the Boltzmann transport equation. The effects of quartic anharmonic induced phonon renormalization and 4 - phonon scattering on the results are fully considered. Vibrational mode visualization reveals that the anomalous contribution of optical phonons to lattice thermal conductivity in KCaBi originates from high-speed out-of-phase vibrations of Ca atoms. We analyzed the microscopic mechanism of heat transport from the perspective of atomic bonding combined with the results of group velocity, scattering rate, Gr u ¨ neisen parameter, mean free path, etc. In addition, we verified and predicted the mechanical properties in KXY by calculation. [ABSTRACT FROM AUTHOR]

Utilizing the first-principle techniques combined with the Boltzmann transport equation, we investigate the thermal transport properties of monolayer GeS. The calculated lattice thermal conductivity κ L is fairly low, e.g., 11.76 (12.53) Wm−1K−1 at 300 K, along the x (y) axis, which is much lower than the values for phosphorene, monolayer MoS 2 , graphene, etc. Our analysis reveals that the low lattice thermal conductivity is resulted from the strong three-phonon scattering process and the low phonon group velocity. Due to the weak anisotropy of GeS, we study the contribution of each phonon branch to the heat transport along x and y axis, and find that the longitudinal acoustic (LA) mode contributes most of the total κ L along x axis, while the transverse acoustic (TA) branch has the largest contribution to the total κ L along y axis. Additionally, the size dependence of thermal conductivity with respect to the nanoribbon width is also studied for the design of nanometer devices. • The lattice thermal conductivity of monolayer GeS is anisotropic. • The thermal conductivity of monolayer GeS is lower than those of most 2D materials. • The low thermal conductivity is resulted from the strong three-phonon scattering. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the wave behavior of the octa-chiral lattices with local resonators and tunable wave propagation properties with different chiral geometrical dimensions. The octa-chiral lattices are assembled with the repeat unit cells and the unit cell contains a ring and eight slender elastic massless ligaments which are rigidly connected to the ring. The ring has a heavy disk with its surrounding soft elastic annulus. The dynamic equations of the lattices are established with the principle of the Lagrangian method. The wave behaviors of the lattices are calculated by solving the eigenvalue problem governing the wave harmonic propagation with the application of the Bloch's theorem. By calculating the band structures and the phase and group, we investigate the effects of the chiral geometry and resonators on the formation of the low-frequency band-gaps and the directional frequency-dependent energy flow. The phase and group velocities show that the maxima wave propagation speeds are along the ±45°. While, the phase velocities and the group velocities trend to be identical with the increase of the chiral angles. The findings suggest that we can tune the anisotropic wave behavior to the isotropic wave characterization with special structural design on the chiral geometry. [ABSTRACT FROM AUTHOR]

Corrugated and dielectric structures have been widely used for producing terahertz radiation source. Recently, a novel scheme of sub-terahertz free electron laser (FEL) from a metallic pipe with corrugated walls driven by a non-ultra-relativistic (< 10 MeV) picosecond electron beam was proposed and analyzed by using the Vlasov–Maxwell equations which can only provide crude estimates of the expected parameters of the FEL. It offers a promising candidate for compact accelerator-based high power terahertz source which can be realized with relatively low energy and low peak-current electron beams. In this paper, we propose a terahertz Cherenkov free electron laser scheme by using the cylindrical dielectric loaded waveguide (DLW) instead, and the new device scheme is studied numerically using both one-dimensional (1D) numerical model and particle in cell (PIC) simulations. The 1D model is developed in specialty for the longitudinal beam dynamics and electromagnetic simulations of the new Cherenkov FEL scheme. The basic idea of the approach is to use the analytic expressions of the longitudinal mode fields steady-state solution using Green function) to compute the wakefields at each time step using the macro particle currents as sources, while the Lorentz equations of the particles can be integrated numerically. The transitive effects in electromagnetic field (related to the group velocity of the wakefield) are modeled only approximately even in 1D case. It allows fast and accurate time-dependent calculation of the interaction between the wakefields and the electron beam. Good agreement is obtained comparing with particle in cell (PIC) simulation results. [ABSTRACT FROM AUTHOR]

This study aims to study the radial wave dispersion of anisotropic solid sphere. In order to include the size-dependent effects, the nonlocal strain gradient theory in conjunction with three dimensional (3D) elasticity theory is used. This model presents both stiffness softening and hardening mechanisms of nanostructures using a nonlocal stress field parameter and also a strain gradient length scale parameter. The nanoparticles are made of different crystallinity (i.e. gold, beryllium oxide, rutile, and hematite), while the equations of wave motion are obtained based on mentioned theories in which an analytic technique is utilized to find the wave frequency, phase velocity and group velocity as a function of wave number. The reliability of present approach is verified for radial vibration by comparing obtained results with those provided in literature. It is demonstrated that, radius of nanoparticle, small-scale parameters, wave number, and crystallinity of nanoparticle have a remarkable effect on radial wave characteristics of anisotropic solid sphere. In addition, the possibility of replacing an anisotropic model with isotropic ones is studied. • Nonlocal strain gradient 3D spherical model is developed. • Wave propagation behavior of different anisotropic nanoparticles is studied. • Phase velocities of nanoparticles decreased by increasing radius of the nanoparticle. • Softening stiffness and hardening stiffness effects are presented. [ABSTRACT FROM AUTHOR]

We propose new forms of low-order absorbing boundary conditions (ABC) for time-dependent elastic waves in isotropic and anisotropic media. The configuration considered is that of a two-dimensional elastic waveguide. The ABC shares with the widely-known Lysmer–Kuhlemeyer (LK) boundary condition the ease of implementation, especially in a finite-element (FE) setting, while offering gains in accuracy over the LK-like condition (or the LK condition in the isotropic case). The new conditions are proved to be energy-stable, even in the case when inverse modes are present, for which the normal components of the phase and group velocities have an opposite direction. The developed ABCs are particularly convenient to use in a Finite Element setting as they preserve the symmetry and positivity of the formulation and are simple to implement. The ABC features several parameters, which are optimized for accuracy. The optimization criterion is based on the notion of the energy-rate reflection coefficient. We test the performance of the scheme via a numerical example. While the gain in accuracy is found to be moderate (around 20% relative to the LK condition), the main result of this investigation lies in the development of ABCs for anisotropic elastodynamics that are proved to be stable from the outset, and are amenable for accuracy optimization. We plan to design in a future study analogous stable and optimized high-order ABCs using the methodology developed here. [ABSTRACT FROM AUTHOR]