Role In Wave Propagation Research Articles

Simplified modeling of elastic guided waves in a pre-stressed cable

Elastic waves can be employed to probe the mechanical state of pre-stressed cables and look for defects in critical areas. Contacts between wires have been shown to play a major role in wave propagation by giving rise to tension-dependent phenomena such as anomalous transmission at certain frequencies (the "notch frequency effect"). Here, we present a simplified way of modeling the free propagation of waves at low frequencies by analytically accounting for contacts. Wires are described within beam theory and connected with tension-dependent springs derived from Hertz law. In contrast to point-wise contacts, these line-wise contacts also induce second-neighbor coupling, i.e. they connect wires that are not in direct contact one another. The simplified model is compared with a reference model based on a finite elements representation of the cross section ("SAFE" method). The range of validity in frequency is discussed. The results provide a new interpretation for the first higher order modes in and their dependence with tension in terms of contact resonances. The drastic reduction of numerical costs suggests the method could be applicable to more complex civil engineering cables, composed of many wires.

Coupled bandgaps and wave attenuation in periodic flexoelectric curve nanobeams

Curve beams are widely used in structural engineering like civil engineering and aircraft structures. Due to their coupling effects of bending, shear, and torsion, curve beams have a significant role in wave propagation fields as complete bandgaps can be generated. With the miniaturization of devices, it becomes increasingly imperative to investigate wave characteristics of curve beams at the nanoscale, taking into account the flexoelectric effect. In this study, a theoretical model for the periodic flexoelectric curve beams is established under the strain-gradient electro-elasticity theory. Subsequently, a customized state-space-based transfer matrix method for flexoelectric curve beam is specifically proposed, referred to as FCB-TMM. According to the Floquet-Bloch theorem, the dispersion relations for the periodic flexoelectric curve beams with intriguing coupling characteristics can be obtained and verified. Additionally, the influence of the flexoelectric effect and strain gradient effect on its complete bandgaps is analyzed. The results indicate that the flexoelectric effect plays a vital role in wave propagation at small scales, causing a shift in the bandgaps towards higher frequencies. Ultimately, the synergistic effects of significant geometric and material parameters combined with the flexoelectric effect on the bandgaps are systematically discussed. It is discovered that changing flexoelectric coefficients may alter the pattern of bandgap variation with these geometric and material parameters, while the influence of flexoelectric coefficients on bandgaps varies depending on the values of these parameters. Our investigation offers an insight into understanding the wave propagation of the curve nanobeams, and thus further guides the application and development of flexoelectric wave components in MEMS/NEMS.

Mode conversion and energy flux absorption in the structured solar atmosphere

Context. Structuring in the solar atmosphere, in the form of inhomogeneities transverse to the magnetic field, is believed to play a vital role in wave propagation, conversion, and absorption. Aims. We investigated the effect of transverse structuring on the processes of mode conversion and wave energy flux absorption using a 3D ideal magnetohydrodynamic simulation featuring an expanding coronal loop in a gravitationally stratified atmosphere. Methods. Multiple wave drivers were modelled. The location of the driver at the photospheric base was allowed to vary so that we could study how the driven waves interact with the transverse structuring, provided by the magnetic field, as well as with the vertical structuring due to gravity. Results. We find that the transverse structuring acts as a conduit for Alfvén wave energy flux through the transition region and into the solar corona. Moreover, in regions of strong transverse gradients, the reflection of Alfvén waves at the transition region is greatly reduced, supporting results from recent studies. Finally, we investigated the efficiency of the loop structuring at absorbing energy flux from externally driven waves and find that the loop is extremely effective at channelling wave energy flux to the loop apex in the corona; in some cases, it can absorb over a third of the externally driven wave energy flux. Conclusions. These results may have important consequences in the context of decayless loop oscillations as they suggest that the oscillations are driven by acoustic waves outside of the existing loop structure.

Nonlinear Diffusion for Bacterial Traveling Wave Phenomenon.

The bacterial traveling waves observed in experiments are of pulse type which is different from the monotone traveling waves of the Fisher-KPP equation. For this reason, the Keller-Segel equations are widely used for bacterial waves. Note that the Keller-Segel equations do not contain the population dynamics of bacteria, but the population of bacteria multiplies and plays a crucial role in wave propagation. In this paper, we consider the singular limits of a linear system with active and inactive cells together with bacterial population dynamics. Eventually, we see that if there are no chemotactic dynamics in the system, we only obtain a monotone traveling wave. This is evidence that chemotaxis dynamics are needed even if population growth is included in the system.

A Tunable Graphene 0-90° Polarization Rotator Achieved by Sine Equation Voltage Adjustment.

Polarization Manipulation has been widely used and plays a key role in wave propagation and information processing. Here, we introduce a polarization rotator in the terahertz range with a polarization conversion ratio up to 99.98% at 4.51 terahertz. It has a single graphene layer on top of the structure patterned by 45° tilted space elliptical rings. By changing the Fermi level from 0.3 ev to 0.7 ev of the graphene, we can turn the reflective light polarization direction between 0° to 90° with nearly unique magnitude. Surface currents theories and graphene characteristics clarify the relationship between polarization angle and Fermi level to be a sine equation adjusted voltage. We firstly put forward an equation to thetunable graphene changing the reflective light polarization angle. It can be widely used in measurement, optic communication, and biology. Besides, with nearly the unique reflective light in different directions, the rotator is designed into a novel radially polarization converter. The latter can be switched from radially polarized light to linearly polarized light, and vice versa, in the terahertz region.

Wave control with “Time Materials”

Phononic crystals and Metamaterials are made from assemblies of multiple elements usually arranged in repeating patterns at scales of the order or smaller than the wavelengths of the phenomena they influence. Because time and space play a similar role in wave propagation, wave propagation is affected by spatial modulation or by time modulation of the refractive index. Here we emphasize the role of time modulation. We show that sudden changes of the medium properties generate instant wave sources that emerge instantaneously from the entire wavefield and can be used to control wavefield and to revisit the way to create time-reversed waves. Experimental demonstrations of this approach will be presented More sophisticated time manipulations can also be studied in order to extend the concept of phononic crystals in the time domain.

From Loschmidt daemons to time-reversed waves.

Time-reversal invariance can be exploited in wave physics to control wave propagation in complex media. Because time and space play a similar role in wave propagation, time-reversed waves can be obtained by manipulating spatial boundaries or by manipulating time boundaries. The two dual approaches will be discussed in this paper. The first approach uses 'time-reversal mirrors' with a wave manipulation along a spatial boundary sampled by a finite number of antennas. Related to this method, the role of the spatio-temporal degrees of freedom of the wavefield will be emphasized. In a second approach, waves are manipulated from a time boundary and we show that 'instantaneous time mirrors', mimicking the Loschmidt point of view, simultaneously acting in the entire space at once can also radiate time-reversed waves.

DYNAMICS IN SUNSPOT UMBRA AS SEEN IN NEW SOLAR TELESCOPE AND INTERFACE REGION IMAGING SPECTROGRAPH DATA

We analyse sunspot oscillations using Interface Region Imaging Spectrograph (IRIS) slit-jaw and spectral data and narrow-band chromospheric images from the New Solar Telescope (NST) for the main sunspot in NOAA AR 11836. We report that the difference between the shock arrival times as measured the Mg II k 2796.35\AA\ and Si IV 1393.76\AA\ line formation levels changes during the observed period and peak-to-peak delays may range from 40~s to zero. The intensity of chromospheric shocks also displays a long term (about 20~min) variations. NST's high spatial resolution \ha\ data allowed us to conclude that in this sunspot umbral flashes (UFs) appeared in the form of narrow bright lanes stretched along the light bridges and around clusters of umbral bright points. Time series also suggested that UFs preferred to appear on the sunspot-center side of light bridges, which may indicate the existence of a compact sub-photospheric driver of sunspot oscillations. The sunspot's umbra as seen in the IRIS chromospheric and transition region data appears bright above the locations of light bridges and the areas where the dark umbra is dotted with clusters of umbral dots. Co-spatial and co-temporal data from the Atmospheric Imaging Assembly on board Solar Dynamics Observatory showed that the same locations were associated with bright footpoints of coronal loops suggesting that the light bridges may play an important role in heating the coronal sunspot loops. Finally, the power spectra analysis showed that the intensity of chromospheric and transition region oscillations significantly vary across the umbra and with height, suggesting that umbral non-uniformities and the structure of sunspot magnetic fields may play a role in wave propagation and heating of umbral loops.

On wave propagation in anisotropic elastic cylinders at nanoscale: surface elasticity and its effect

Material heterogeneity induced by a surface or interface may be neglected at macroscale since the surface-to-volume ratio is usually small. However, its effect can become significant for structures at nanoscale with a large surface-to-volume ratio. In this paper, we incorporate such surface material heterogeneity into wave propagation analysis of a nanosized transversely isotropic cylinder. This is achieved by using the concept of surface elasticity. Instead of directly using the well-known Gurtin–Murdoch (GM) surface elasticity, we develop here another general framework based on a thin layer model. A novel approach based on state-space formalism is used to derive the approximate governing equations. Three different sources of surface effect can be identified in the first-order surface elasticity, i.e., the elasticity effect, the inertia effect and the thickness effect. It is found that the derived theory becomes identical to the GM surface elasticity if the thickness effect is dropped and when the material is isotropic. The axisymmetric wave propagation in a transversely isotropic cylinder with surface effect is then studied, and an exact solution is presented. Numerical results are finally given to show that the surface effect will play a very pronounced role in wave propagation in cylinders at nanoscale.

The Alfvén resonance in pair plasmas

Waves propagating obliquely in a magnetized cold pair plasma experience an approximate resonance in the wavevector component perpendicular to the magnetic field, which is the analogue of the Alfvén resonance in normal electron-ion plasmas. Wave absorption at the resonance can take place via mode conversion to the analogue of the short wavelength inertial Alfvén wave. The Alfvén resonance could play a role in wave propagation in the pulsar magnetosphere leading to pulsar radio emission. Ducting of waves in strong plasma gradients may occur in the pulsar magnetosphere, which leads to the consideration of Alfvén surface waves, whose energy is concentrated in the region of strong gradients.

Wave propagation in an elastic tube: a numerical study

In this paper, a fluid–wall interaction model, called the elastic tube model, is introduced to investigate wave propagation in an elastic tube and the effects of different parameters. The unsteady flow was assumed to be laminar, Newtonian and incompressible, and the vessel wall to be linear-elastic, isotropic and incompressible. A fluid–wall interaction scheme is constructed using a finite element method. The results demonstrate that the elastic tube plays an important role in wave propagation. It is shown that there is a time delay between the velocity waveforms at two different locations and that the peak velocity increases while the low velocity decreases in the elastic tube model, contrary to the rigid tube model where velocity waveforms overlap each other. Compared with the elastic tube model, the increase of the wall thickness makes wave propagation faster and the time delay cannot be observed clearly, however, the velocity amplitude is reduced slightly due to the decrease of the internal radius. The fluid–wall interaction model simulates wave propagation successfully and can be extended to study other mechanical properties considering complicated geometrical and material factors.

Plasmapause and Increase in Whistler Mode Wave’s Growth

Singh et al have studied in detail the Whistler mode VLF waves intensity enhancement, by assuming change in VII , volume of flux tube, EII change due to a change of position of plasmapause. Enhancing this work, we in place of energy increase, consider the wave growth increase of whistler mode waves. The results are in agreement with reported observations and suggest that plasmapause plays an important role in wave propagation. It is shown that this guidance is possible at both the inner and outer edges of the Plasmapause and that more efficient guide occurs as the Plasmapause gradients become stronger. In the case of strong gradients, waves coming from a wide latitudes range (~8 0 ) are focused tightly about the plasmapause field lines, resulting in a wave intensity increase of about 3dB near the magnetic equator plane 12 .Since plasmapause has inner & outer edges, we can say that it has a width pp) which depends upon Kpmax values, MLT as well as L the McIlwain parameters. pp variation is discussed in next section.

Alfvén resonance absorption in electron-positron plasmas

AbstractWaves propagating obliquely in a magnetized cold pair plasma experience an approximate resonance in the wavevector component perpendicular to the magnetic field, which is the analogue of the Alfvén resonance in normal electron-ion plasmas. Wave absorption at the resonance can take place via mode conversion to the analogue of the short wavelength inertial Alfvén wave. The Alfvén resonance could play a role in wave propagation in the pulsar magnetosphere leading to pulsar radio emission. Ducting of waves in strong plasma gradients may occur in the pulsar magnetosphere, which leads to the consideration of Alfvén surface waves, whose energy is concentrated in the region of strong gradients.

Solution for the Fourth Moment Equation of Waves in Random Continuum Under Strong Fluctuations: General Theory and Plane Wave Solution

The fourth moment plays an important role in wave propagation and scattering in random media. However, in the strong scattering regimes, it remains to be solved although a rich variety of methods have been proposed. The fourth moment equation is solved using the modified Gaussian solution method proposed in this paper. The fourth moment can be considered as a sum of the Gaussian solution and non-Gaussian correction term. The Gaussian solution is the fourth moment in the saturated regime, and can be obtained as a sum of products of the second-order moments. The derived equation of the non-Gaussian correction term is solved using the Rytov approximation and the Green's function. The fourth moment solution obtained is general for without restrictions on incident wave forms. Specifically, the solution for a plane wave is derived and applied to study the amplitude scintillation and intensity correlation function of trans ionospheric radio signals. The theoretical calculation on the scintillation index agrees well with the experimental results, which confirms the fourth moment solution to some extent.

Analytical Solution to the $n$-$n$th Moment Equation of Wave Propagation in Continuous Random Media

Higher order symmetrical moments play an important role in wave propagation and scattering in random media, however it remains to be solved under strong fluctuations. In this paper, a modified Gaussian solution method is proposed for analytically solving the n-nth moment. After propagating through a random medium in the fully saturated regime, the higher order symmetrical moment of the received wave is the sum of products of the second moments, i.e., the Gaussian solution. In strong scattering regimes, the higher order symmetrical moment can be considered as a sum of the Gaussian solution and a non-Gaussian correction term, where the key issue is how to solve the derived equation of the correction term. Two methods are proposed, i.e., Green's function method and the Rytov approximation approach. Green's function method leads to a rigorous solution form, but it is complicated due to an integral equation. The approach using the Rytov approximation is found to be reasonable, as the correction is relatively small

The role of wave propagation in hydrocyclone operations II: Wave propagation in the air–water interface of a conical hydrocyclone

The air-core plays an important role in the operation of a conical hydrocyclone . In the apex, the air-core increases in diameter with the possibility of reaching a size close to the apex diameter. In this case, roping may be induced. There is a range of values for the apex to vortex ratio where roping is possible. Using some simple physical models, this work shows that perturbations propagating through the air-core interface are amplified in the direction of the apex and may be responsible for the fluctuations of the underflow that are characteristic of roping.

The role of wave propagation in hydrocyclone operations I: An axisymmetric streamfunction formulation for a conical hydrocyclone

The objective of this work is to study the role of some wavelike motion observed in the interior of a conical hydrocyclone used in mineral processing operations. In this paper, we obtained a stable numerical solution of the equations of motion, using a streamfunction-vorticity formulation on non-structured mesh. The numerical solution is obtained in a finite element context, where a stabilization is imposed through an unusual finite element method (USFEM) scheme. Finally, an adaptive mesh refinement is used for to enhance the solution.

Experimental study of wave dispersion and attenuation in concrete

Results from an experimental study concerning wave propagation in cementitious materials are presented in this paper. Narrow band pulses at several frequencies were introduced into specimens of cement paste, mortar and concrete allowing direct measurement of longitudinal wave velocities and amplitude for each frequency. It is shown that aggregate content play an important role in wave propagation increasing considerably the wave velocity, while the aggregate size seems to control the attenuation observed. Slight velocity variations observed with frequency are discussed in relation to the degree of inhomogeneity of the materials.

The role of wave propagation in the motion of an elastic body

An elementary volume of any elastic body subjected to an external force movesin accordance with Newton’s laws of motion. This behaviour also holds for thecentre of mass of an elastic body, but the means by which any portion of thebody is ‘informed’ of an imposed external force must be waves that are generatedby that force and are multiply reflected within the medium. To illustrate theirinfluence we consider the motion of an elastic bar with free ends subjected toeither an impulsive force or a constant force applied to the end at the momentt = 0. We also study motion of the bar with one fixed end due to a constant force. For this lattersituation, the free end of the bar remains in periodic motion about the position describedconventionally by Hooke’s law. In the presence of dissipation, the behaviour ofa nearly elastic bar approaches with time that of a rigid bar subjected to thesame forces. These examples of motion of an elastic bar allow us to illustrateresistance of a body to the change of its velocity (inertia) and relate it to wavepropagation.

Ultrasonic P Wave Velocity and Attenuation inPyroxenite Under 3.0 GPa up to 1170°C

Ultrasonic P wave velocity (VP) and qualityfactor (QP value, on behalf of attenuation) inpyroxenite were presented as functions of pressures (0.3-3.0 GPa) andtemperatures (20-1170°C). The experimental results show thatVP and QP depend upon pressure andtemperature. VP and QP in pyroxeniteincrease more rapidly at the pressure 0.3-1.4 GPa than those at1.4-3.0 GPa. As the temperature rising from 20°C to about1170°C at the pressure 3.0 GPa, an almost linear decrease up to11% in VP was observed, and QP drops from 243at room temperature to 68 at 1170°C with decreasing 72%. Theexperimental data indicate that the pressure and temperature inducedfabric changes and frictional sliding and dislocation in pyroxeniteplay a key role in wave propagation in rocks.