Showing papers on "Phase velocity published in 2017"

Observation of acoustic Dirac-like cone and double zero refractive index.

Abstract: Zero index materials where sound propagates without phase variation, holds a great potential for wavefront and dispersion engineering. Recently explored electromagnetic double zero index metamaterials consist of periodic scatterers whose refractive index is significantly larger than that of the surrounding medium. This requirement is fundamentally challenging for airborne acoustics because the sound speed (inversely proportional to the refractive index) in air is among the slowest. Here, we report the first experimental realization of an impedance matched acoustic double zero refractive index metamaterial induced by a Dirac-like cone at the Brillouin zone centre. This is achieved in a two-dimensional waveguide with periodically varying air channel that modulates the effective phase velocity of a high-order waveguide mode. Using such a zero-index medium, we demonstrated acoustic wave collimation emitted from a point source. For the first time, we experimentally confirm the existence of the Dirac-like cone at the Brillouin zone centre.

Flexural Wave Propagation Analysis of Embedded S-FGM Nanobeams Under Longitudinal Magnetic Field Based on Nonlocal Strain Gradient Theory

Abstract: This paper investigates wave propagation of size-dependent functionally graded (FG) nanobeams resting on elastic foundation subjected to axial magnetic field based on the nonlocal strain gradient theory and Euler–Bernoulli beam model by using an analytical approach. The nonlocal beam model has a length scale parameter and captures the size influences. Material properties are spatially graded according to sigmoid distribution. A derivation of the governing equations for the wave propagation analysis of nanoscale S-FGM beams is conducted. Then, the dispersion relations between wave frequency and phase velocity with the wave number is investigated. It is found that wave propagation characteristics of nonlocal S-FGM beams are influenced by various parameters including length scale parameter, material graduation, elastic foundation parameters and magnetic field intensity.

Nonreciprocal propagation of surface acoustic wave in Ni /LiNbO 3

Abstract: We investigated surface acoustic wave propagation in a Ni/${\mathrm{LiNbO}}_{3}$ hybrid device We found that the absorption and phase velocity are dependent on the sign of the wave vector, which indicates that the surface acoustic wave propagation has nonreciprocal characteristics induced by simultaneous breaking of time-reversal and spatial inversion symmetries The nonreciprocity was reversed by ${180}^{\ensuremath{\circ}}$ rotation of the magnetic field The origin of the nonreciprocity is ascribed to interference of shear-type and longitudinal-type magnetoelastic couplings

Waves in Nonlocal Elastic Solid with Voids

Abstract: In this paper, the governing relations and equations are derived for nonlocal elastic solid with voids. The propagation of time harmonic plane waves is investigated in an infinite nonlocal elastic solid material with voids. It has been found that three basic waves consisting of two sets of coupled longitudinal waves and one independent transverse wave may travel with distinct speeds. The sets of coupled waves are found to be dispersive, attenuating and influenced by the presence of voids and nonlocality parameters in the medium. The transverse wave is dispersive but non-attenuating, influenced by the nonlocality and independent of void parameters. Furthermore, the transverse wave is found to face critical frequency, while the coupled waves may face critical frequencies conditionally. Beyond each critical frequency, the respective wave is no more a propagating wave. Reflection phenomenon of an incident coupled longitudinal waves from stress-free boundary surface of a nonlocal elastic solid half-space with voids has also been studied. Using appropriate boundary conditions, the formulae for various reflection coefficients and their respective energy ratios are presented. For a particular model, the effects of non-locality and dissipation parameter ( $\tau $ ) have been depicted on phase speeds and attenuation coefficients of propagating waves. The effect of nonlocality on reflection coefficients has also been observed and shown graphically.

Electrical 2π phase control of infrared light in a 350-nm footprint using graphene plasmons

Abstract: Phase velocity of graphene plasmons is electrically controlled in a set-up enabling tuning of the phase between 0 and 2π.

Nondestructive Characterization of Composite Media

Abstract: Introduction - Basic Governing Equations - Wave Surfaces - Energy Propagation - Bulk Wave Propagation Anisotropic Media - Guided Waves - Ultrasonic Measurements - Elastic Modulus Reconstruction from Ultrasonic Data - Dynamic Modulus Measurement in Anisotropic Media: Phase Velocity vs Group Velocity - Ultrasonic Modulus Measurements - Composite Microstructure Characterization

High-coupling leaky surface acoustic waves on LiNbO3 or LiTaO3 thin plate bonded to high-velocity substrate

Abstract: The propagation properties of leaky surface acoustic waves (LSAWs) and longitudinal-type LSAWs (LLSAWs) on a LiNbO3 (LN) or LiTaO3 (LT) thin plate bonded to an AT-cut quartz or c-plane sapphire (c-Al2O3) substrate with a high phase velocity were investigated. It was theoretically revealed that when the LN or LT thin-plate thickness is less than one wavelength, the particle displacement of LLSAWs was concentrated in the thin plate and the electromechanical coupling factor (K 2) was increased to two to three times that in the single substrate. Furthermore, for 36° Y-cut X-propagating LT/c-Al2O3 with an LT thin-plate thickness of 0.35 λ and X-cut 36° Y-propagating LN/c-Al2O3 with an LN thin-plate thickness of 0.19 λ, the values of K 2 for an LSAW and an LLSAW were experimentally found to increase from 5.6 and 10.4% in the single substrate to 11.5 and 19.7% in the thin-plate bonded structure, respectively.

Nonlocal strain gradient based wave dispersion behavior of smart rotating magneto-electro-elastic nanoplates

Abstract: Main object of the present research is an exact investigation of wave propagation responses of smart rotating magneto-electro-elastic (MEE) graded nanoscale plates. In addition, effective material properties of functionally graded (FG) nanoplate are presumed to be calculated using the power-law formulations. Also, it has been tried to cover both softening and stiffness-hardening behaviors of nanostructures by the means of employing nonlocal strain gradient theory (NSGT). Due to increasing the accuracy of the presented model in predicting shear deformation effects, a refined higher-order plate theory is introduced. In order to cover the most enormous circumstances, maximum amount of load generated by plate's rotation is considered. Furthermore, utilizing a developed form of Hamilton's principle, containing magneto-electric effects, the nonlocal governing equations of MEE-FG rotating nanoplates are derived. An analytical solution is obtained to solve the governing equations and validity of the solution method is proven by comparing results from present method with those of former attempts. At last, outcomes are plotted in the framework of some figures to show the influences of various parameters such as wave number, nonlocality, length scale parameter, magnetic potential, electric voltage, gradient index and angular velocity on wave frequency, phase velocity and escape frequency of the examined nanoplate.

Ambient noise tomography across Mount St. Helens using a dense seismic array

Abstract: We investigated upper crustal structure with data from a dense seismic array deployed around Mount St. Helens for two weeks in the summer of 2014. Inter-station cross-correlations of ambient seismic noise data from the array were obtained and clear fundamental mode Rayleigh waves were observed between 2.5 and 5 s periods. In addition, higher-mode signals were observed around 2 s period. Frequency-time analysis was applied to measure fundamental mode Rayleigh wave phase velocities, which were used to invert for 2-D phase velocity maps. An azimuth-dependent travel time correction was implemented to mitigate potential biases introduced due to an inhomogeneous noise source distribution. Reliable phase velocity maps were only obtained between 3 and 4 s periods due to limitations imposed by the array aperture and higher-mode contamination. The phase velocity tomography results, which are sensitive to structure shallower than 6 km depth, reveal a ~10-15% low-velocity anomaly centered beneath the volcanic edifice and peripheral high-velocity anomalies that likely correspond to cooled igneous intrusions. We suggest that the low-velocity anomaly reflects the high porosity mixture of lava and ash deposits near the surface of the edifice, a highly fractured magmatic conduit and hydrothermal system beneath the volcano, and possibly a small contribution from silicate melt.

Fundamental solutions of a multi-layered transversely isotropic saturated half-space subjected to moving point forces and pore pressure

Abstract: The steady-state dynamic response of a multi-layered transversely isotropic (TI) saturated half-space due to point forces and pore pressure moving with a constant speed is investigated in this paper. To solve this problem, the dynamic stiffness method combined with the inverse Fourier transform is employed. First, the governing equations in terms of the displacement components and pore fluid pressure are solved in the transformed domain by employing the Fourier transform. Next, the exact three-dimensional (3D) dynamic stiffness matrices for the TI saturated layer, as well as the TI saturated half-space, are constructed, and the global dynamic matrix of the problem is formulated by assembling the dynamic matrices of the discrete layers and the underlying half-space. Finally, solutions in the frequency-wavenumber domain of the displacement, pore pressure and stress are obtained through the dynamic stiffness method. The result in the time-space domain is recovered by the Fourier synthesis of the frequency responses which, in turn, are obtained by numerical integration over on one horizontal wavenumber. The accuracy of the developed formulations is confirmed by comparison with existing solutions for an isotropic and saturated medium that is a special case of the more general problem addressed. Numerical results for both low and high source velocities are presented, and the effects of moving speed, material anisotropy, permeability, surface drainage condition and TI saturated layer on the dynamic response are analyzed. It is observed that the dynamic responses reach their peak values when the source velocity is equal to or approaches the phase velocities of SH-, qP1-, qP2- and qSV- in the horizontal direction and the phase velocity of qRayleigh waves. Material anisotropy is very important for the accurate assessment of the dynamic response due to the moving point forces and pore pressure in a TI saturated medium.

High-frequency Oscillations in Small Magnetic Elements Observed with Sunrise/SuFI

Abstract: We characterize waves in small magnetic elements and investigate their propagation in the lower solar atmosphere from observations at high spatial and temporal resolution. We use the wavelet transform to analyze oscillations of both horizontal displacement and intensity in magnetic bright points found in the 300 nm and the Ca II H 396.8 nm passbands of the filter imager on board the Sunrise balloon-borne solar observatory. Phase differences between the oscillations at the two atmospheric layers corresponding to the two passbands reveal upward propagating waves at high-frequencies (up to 30 mHz). Weak signatures of standing as well as downward propagating waves are also obtained. Both compressible and incompressible (kink) waves are found in the small-scale magnetic features. The two types of waves have different, although overlapping, period distributions. Two independent estimates give a height difference of approximately 450+-100 km between the two atmospheric layers sampled by the employed spectral bands. This value, together with the determined short travel-times of the transverse and longitudinal waves provide us with phase speeds of 29+-2 km/s and 31+-2 km/s, respectively. We speculate that these phase speeds may not reflect the true propagation speeds of the waves. Thus, effects such as the refraction of fast longitudinal waves may contribute to an overestimate of the phase speed.

Acceleration of sub-relativistic electrons with an evanescent optical wave at a planar interface.

Abstract: We report on a theoretical and experimental study of the energy transfer between an optical evanescent wave, propagating in vacuum along the planar boundary of a dielectric material, and a beam of sub-relativistic electrons. The evanescent wave is excited via total internal reflection in the dielectric by an infrared (λ = 2 μm) femtosecond laser pulse. By matching the electron propagation velocity to the phase velocity of the evanescent wave, energy modulation of the electron beam is achieved. A maximum energy gain of 800 eV is observed, corresponding to the absorption of more than 1000 photons by one electron. The maximum observed acceleration gradient is 19 ± 2 MeV/m. The striking advantage of this scheme is that a structuring of the acceleration element’s surface is not required, enabling the use of materials with high laser damage thresholds that are difficult to nano-structure, such as SiC, Al2O3 or CaF2.

Two-point coherence of wave packets in turbulent jets

Abstract: Dual-plane, high-cadence, stereoscopic particle-image velocimetry of a turbulent jet is used to investigate the spatial structure of the acoustically important wave packets; these are shown to have fundamentally different coherence decay and phase speed from energy-containing eddies that dominate pointwise measurements.

Propagation of in-plane wave in viscoelastic monolayer graphene via nonlocal strain gradient theory

Abstract: The behaviors of monolayer graphene sheet have attracted increasing attention of many scientists and researchers. In this study, the propagation behaviors of in-plane wave in viscoelastic monolayer graphene are investigated. The constitutive equation and governing equation for in-plane wave propagation is developed by employing Hamilton’s principle and nonlocal strain gradient theory. By solving the governing equation of motion, the closed-form dispersion relation between phase velocity and wave number is derived and an asymptotic phase velocity can be acquired. The effects of wave number, material length scale parameter, nonlocal parameter and damping coefficient on in-plane wave propagation behaviors are discussed in the numerical studies. It is found that, when exciting wavelengths or structural dimensions become comparable to the material length scale parameters and nonlocal parameters, the scaling effects on wave propagation behaviors are significant. For nanoscaled graphene sheet, the effects of nonlocal parameter, material length scale parameter and damping coefficient on phase velocity are tiny at low wave numbers while significant at high wave numbers. The phase velocity would increase with the increase of material length scale parameter or the decrease of nonlocal parameter and damping coefficient. Furthermore, results indicate that the asymptotic phase velocity can be increase by increasing material length scale parameter or decreasing nonlocal parameter.

Surface-wave tomography of the alps using ambient-noise and earthquake phase-velocity measurements

Abstract: Kastle, E.D., El-Sharkawy, A., Boschi, L., Meier, T., Rosenberg, C., Bellahsen, N., Cristiano, L., Weidle, C.,

Gas–Liquid Two-Phase Flow Velocity Measurement With Continuous Wave Ultrasonic Doppler and Conductance Sensor

Abstract: Flow velocity is an important process parameter that quantifies the volume or mass flow rate as well as monitors the process safety. To nonintrusively measure the flow velocity of horizontal gas–liquid two-phase flow, an ultrasonic Doppler sensor and a conductance sensor with dedicated measurement models are presented. The air superficial flow velocity can be directly obtained and the water superficial flow velocity can be calculated through a two-fluid model for bubble flow and plug flow. For slug flow, a correlation between the phase velocity in slug body and overall superficial flow velocity was built based on a slug closure model. In order to eliminate the influence of the changing velocity profile in the fluid, the sample volume was designed to cover the whole pipe cross section by installing a two-chip piezoelectric transducer with 1-MHz center frequency at the bottom of the pipe. The conductance sensor provided water holdup estimate to compensate the velocity measurement model. Experiments were carried out in a 50-mm inner diameter pipe to verify the proposed sensor and model. Three flow patterns (bubble flow, plug flow, and slug flow) were generated by adjusting the inlet flow rate of the air and the water. The results show that the mean relative error can achieve within 5%.

Wave dispersion in viscoelastic single walled carbon nanotubes based on the nonlocal strain gradient Timoshenko beam model

Abstract: Based on the nonlocal strain gradient theory and Timoshenko beam model, the properties of wave propagation in a viscoelastic single-walled carbon nanotube (SWCNT) are investigated. The characteristic equations for flexural and shear waves in visco-SWCNTs are established. The influence of the tube size on the wave dispersion is clarified. For a low damping coefficient, threshold diameter for shear wave (SW) is observed, below which the phase velocity of SW is equal to zero, whilst flexural wave (FW) always exists. For a high damping coefficient, SW is absolutely constrained, and blocking diameter for FW is observed, above which the wave propagation is blocked. The effects of the wave number, nonlocal and strain gradient length scale parameters on the threshold and blocking diameters are discussed in detail.

Combined effects of reinforcement fraction and porosity on ultrasonic velocity in SiC particulate aluminum alloy matrix composites

Abstract: This work aims at applying the broadband laser-ultrasonic spectroscopy for quantitative evaluation of combined influence of the reinforcement fraction and the porosity content on the phase velocity of longitudinal acoustic waves in stir cast SiC particulate aluminum alloy matrix composites. The investigated composites contain 3.8–15.5 wt % of SiC particles with an average size of 14 μm. The growth of SiC fraction is accompanied by the growth of the total porosity content that reaches 4.7% for the maximum fraction of SiC. These combined factors cause the occurrence of dispersion of the phase velocity – it decreases in the frequency band 3–10 MHz and increases in the frequency band 20–40 MHz in comparison with that in the matrix without any reinforcement. It was found that the relative velocity dispersion increases with the growth of the porosity content within the investigated composite area regardless of the SiC fraction. The derived empirical relation between the relative velocity dispersion and the porosity content can be used for rapid quantitative porosity characterization in stir cast SiC particulate aluminum alloy matrix composites to reveal any potential areas with a reduced strength before the fabrication of composite products.

Demonstration of sub-luminal propagation of single-cycle terahertz pulses for particle acceleration

Abstract: The sub-luminal phase velocity of electromagnetic waves in free space is generally unobtainable, being closely linked to forbidden faster than light group velocities. The requirement of sub-luminal phase-velocity in laser-driven particle acceleration schemes imposes a limit on the total acceleration achievable in free space, and necessitates the use of dispersive structures or waveguides for extending the field-particle interaction. We demonstrate a travelling source approach that overcomes the sub-luminal propagation limits. The approach exploits ultrafast optical sources with slow group velocity propagation, and a group-to-phase front conversion through nonlinear optical interaction. The concept is demonstrated with two terahertz generation processes, nonlinear optical rectification and current-surge rectification. We report measurements of longitudinally polarised single-cycle electric fields with phase and group velocity between 0.77c and 1.75c. The ability to scale to multi-megavolt-per-metre field strengths is demonstrated. Our approach paves the way towards the realisation of cheap and compact particle accelerators with femtosecond scale control of particles.Controlled generation of terahertz radiation with subluminal phase velocities is a key issue in laser-driven particle acceleration. Here, the authors demonstrate a travelling-source approach utilizing the group-to-phase front conversion to overcome the sub-luminal propagation limit.

Wave propagation analysis of smart rotating porous heterogeneous piezo-electric nanobeams

Abstract: The present work is mainly focused on studying the influences of angular velocity on the wave propagation responses of functionally graded (FG) piezo-electric nanobeams. Moreover, the effects of porosity are also regarded in the wave propagation analysis of size-dependent FG beams. The distribution of electro-mechanical properties of a piezo-electric beam are precisely described employing power-law formulation. The nonlocal elasticity theory is utilized to account for the influences of small scale. Herein, a classical beam theory is expounded to derive the nonlocal governing equations of the nanobeam. Once the governing equations are completely derived, an analytical solution method is applied to obtain the dispersion relations of propagating waves. A comparison of this model with previous studies is then made to show the validity of the obtained results. Finally, the influences of various variants, such as wave number, nonlocal parameter, gradient index, electric voltage, volume fraction of porosity and angular velocity, are studied in detail to show how these parameters can affect the wave frequency, phase velocity and escape frequency of FG smart rotary porous nanobeams.