Wave Reflection Coefficient Research Articles

Characterization of surface crack width in plates using Rayleigh wave electromagnetic acoustic transducers

Surface crack in the plate like structures is one of the initial indication for its degradation. This indication could be very critical and requires some serious maintenance because this continuous exposure will lead to a severe damage to the safety and even to environment. To evaluate the structural performance and safety, a regular monitoring of such cracks is needed. A quantitative characterization method is utilized for the estimation of surface groove width instead of depth or length, which is previously done by many researchers, using Rayleigh waves in pitch-catch mode using electromagnetic acoustic transducers (EMATs). The method employs the experimentally determined reflection coefficient and transmission coefficient of Rayleigh waves scattered at a surface groove by a number of varying widths, which are compared to the reference curves, obtained from two-dimensional finite element method (2D-FEM) simulations for the same groove width as used in experiments. A good relation between FEM results and experimental results is found. Two different EMAT couples of different centre frequencies were employed to explore the measurement, and to compare and analyse the quantitative characterization of different groove widths. Both the FEM results and experimental results are compared and verified for the different groove widths. This comparison shows some percentage of error for the groove width ranging from 0.3 mm to 1.8 mm.

Patterns of Acoustic Wave Propagation in the Contact Layer of Press Joints

The insufficient strength of the fixed releasable press joints in the inner rings of the bearings with the pins of the wheelset axles leads failures of axle boxes. During the operation it is possible to change the bearing strength of the bearing rings, which may result in grinding of an axle pin part, fretting corrosion and ring rotation. In order to control the integrity of the ring on the axis pin, the most promising is implementing the acoustic methods. Such methods are based on the correlation between the reflection coefficient of ultrasonic waves at the boundary with the rolling surface of the ring and its stress-strain state (and, as a consequence, the integrity). The reflected ultrasonic waves contain information on the parameters of the contact liquid layer and the acoustic properties of the ring material. The goal of the research is to study the effect of the contact fluid layer between the transducer and the tested samples from different materials on the parameters of the recorded signal during ultrasonic testing. Throughout the research, an original combined transducer was manufactured. Testing of samples from various materials by ultrasonic echo method testing was carried out. The amplitude-frequency characteristics of the signals reflected from the studied samples are obtained. The reflection coefficients of longitudinal waves reflected from the boundary “transducer – test sample” at frequencies of 2.5 and 5 MHz were experimentally determined. The correlation between the reflection coefficient and the grinding degree of the transducer and the test sample is determined.

The radial Teukolsky equation for Kerr–Newman–de Sitter geometry: revisited

We use Heun-type solutions given in Suzuki et al. (Prog Theor Phys 100:491, 1998) for the radial Teukolsky equation written in the background metric of the Kerr–Newman–de Sitter geometry to calculate reflection coefficient for waves coming from the de Sitter horizon and reflected at the outer horizon of the black hole.

1Pa2-1 Quantitative evaluation of wall thinning of piping using deep neural network based on the frequency variation of the T(0,1) mode guided wave reflection coefficient

1Pa2-1 Quantitative evaluation of wall thinning of piping using deep neural network based on the frequency variation of the T(0,1) mode guided wave reflection coefficient

Effect of motions on the performance and mooring tension of a floating pontoon-type breakwater

ABSTRACT This study experimentally investigated the performance and hydrodynamic response of a flexible floating pontoon-type breakwater. The floating breakwater consists of ten rubber pontoons with flexible connections. The effect of relative breadth and relative wave height on the wave transmission, reflection and dissipation coefficients has been discussed for various assembly configurations of the floating breakwater. The results show that the flexible multi-pontoon breakwater has better performance with larger relative breadth. The configuration of continuous pontoons without gap has the best performance on wave attenuation for relatively long waves. In short waves, however, better performance was observed on the pontoon configurations of two equal sets with gap. The novelty of this study is figuring out the relationship among the wave reflection coefficient, motion and mooring tension of the breakwater. The results reveal that the variation of reflection coefficient versus relative breadth shows the same trend as that of mooring tension.

Numerical simulation of characteristics of wave propagation and reflection coefficient in a helix-acoustic black hole

A new type of the acoustic black hole beam—a helix-acoustic black hole—is proposed to overcome the spatial restriction on modular acoustic black hole structures. The modular acoustic black hole structure, consisted of a base and several number of acoustic black hole beams, has potential to apply into real engineering world. There are two main sections in an acoustic black hole beam: (1) a uniform thickness part and (2) an acoustic black hole region of which the thickness decreases according to the power-law profile. Conventional acoustic black hole beams can be ultimately assembled as 8–10 acoustic black hole beams on a modular acoustic black hole structure. In this article, a different shape of an acoustic black hole beam is newly designed to allow the assembly of more acoustic black hole beams on the modular acoustic black hole structure. The shape of the helix-acoustic black hole is such that the thickness of the acoustic black hole region smoothly decreases, just like a conventional acoustic black hole beam, as well as twisting along the longitudinal direction. The normal direction of the bottom surface in the uniform thickness is the same along the longitudinal axis in the conventional acoustic black hole beam. However, the normal direction of the helix-acoustic black hole of the bottom surface in the acoustic black hole region is different along the longitudinal direction. It is necessary to use numerical simulations to explore the performance of the helix-acoustic black hole beam because the shape of acoustic black hole region is different from the conventional one. Two types of numerical simulations were conducted: transient analysis and modal frequency analysis. From the transient analysis, the acoustic black hole effect was investigated by comparing the travel time which is dependent on the variation of the thickness. Using modal frequency analysis, the reflection coefficients between the conventional acoustic black hole beam and helix-acoustic black hole beam are also compared. It is noted that reflection coefficients were additionally compared depending on how “sharply” or “smoothly” they were twisted in the acoustic black hole region of the helix-acoustic black hole. Those results demonstrate that the helix-acoustic black hole has a dynamic characteristic similar to that of conventional acoustic black hole beams, which means that more helix-acoustic black holes can be assembled on the modular acoustic black hole structure by resolving the spatial restriction and leading to expectations of better performance.

Attenuation properties of EM wave by magnetized nonuniform plasma slab coated on perfect conductor plane

AbstractThe total amplitude reflection coefficient of EM wave in a nonuniform plasma slab coated on perfect conductor plane is newly derived by using the scattering matrix method (SMM), and the attenuation is calculated. Three types of plasma electron density profile, that is the hyperbolic, sinusoidal, and linear profiles, are used. The external magnetic field and plasma parameters such as the maximum electron density and collision frequencies are discussed to calculate the attenuation of EM wave. The calculation results show that the plasma electron density profile and maximum electron density exert a large effect to the attenuation of EM wave and the attenuation under the uniform external magnetic field is taken place by the electron cyclotron resonance absorption, the up hybrid resonance absorption and geometric resonances absorption.

Observational Evidence for the Role of Hall Conductance in Alfvén Wave Reflection

AbstractElectromagnetic energy carried by magnetohydrodynamic modes is an important mechanism in the energy transfer between the magnetosphere and the ionosphere. Alfvén waves are known to carry field‐aligned currents and thus play an important role in the dynamics of the ionosphere‐magnetosphere coupling. The role of Hall conductance in this interplay has been explored in magnetohydrodynamic models of the ionosphere but has hitherto not been observed in situ. We use 5 years of observations from the Swarm mission to shed light on this interplay. We present a high‐latitude climatology of both the measured Poynting flux and the measured Alfvén wave reflection coefficient. Our results indicate that high‐energy precipitation, which penetrates deep into the ionosphere and directly leads to strongly enhanced Hall conductance, is an important cause of positively interfering Alfvén wave reflection. We present such observational evidence and, with that, suggest that Hall conductance is substantially more important in the ionospheric wave reflection climatology than hitherto believed.

Возбуждение анизотропной импедансной метаповерхности в виде эллиптического цилиндра.

The problem of arbitrary excitation of waves by a system of external sources near an anisotropic metasurface in the form of an elliptical cylinder with a surface homogenized impedance tensor of general form is solved. The solution to the problem is written as a superposition of E- and H-waves in elliptical coordinates. The partial reflection coefficients of waves were found from the boundary conditions using the orthogonality of the Mathieu angular functions. For these coefficients, four coupled infinite systems of linear algebraic equations of the second kind are obtained. The conditions under which the solution of the excitation problem by the method of eigenfunctions is obtained in an explicit form are found and analyzed. It is shown that for this, the surface impedance tensor of a uniform metasurface must belong to a class of deviators (have zero diagonal elements). In the particular case of a mutual (most easily realized) metasurface, its impedance tensor should only be reactance. In another special case, the impedance tensor of a set of deviators describes a class of anisotropic nonreciprocal metasurfaces with the so-called perfect electromagnetic conductivity (PEMC).

Linear water wave interaction with a composite porous structure in a two-layer fluid flowing over a step-like sea-bed

ABSTRACT The present work is concerned with the interaction of oblique surface gravity waves by a simple and composite porous block of finite width placed on a multi-step bottom in a two-layer fluid. The ocean depth is taken to be finite and its bed impermeable. The problem is studied by employing linearised water wave theory and eigenfunction expansion. The dispersion relations and their roots are analysed which give a clear understanding of the phenomenon. The cases of simple and composite interface-piercing structures are taken up separately to investigate the impact of porosity in wave attenuation for surface and interface modes. Waves propagate through the porous structure with distinct eigenvalues. The appropriateness of structures of various configurations on the scattering of surface waves is investigated by examining the reflection coefficients for waves in surface and interface modes as well as their effects on the free surface and interface elevations, the wave-loads on the structure and the rigid wall supporting the structure at one end. Further, as a special case, the sea-bed preceding the step bottom is considered to be porous and its effect on reflection is examined. The investigation establishes that for a suitable configuration of the porous structure, an optimum width can be ascertained to design a breakwater of reasonable efficiency possessing characteristics of both reflection and dissipation processes. The problems are solved analytically and the results are presented in graphical form. This kind of study is likely to have immense significance for designing of different types of coastal structures with respect to reflection and dissipation of wave energy at continental shelves which is influenced by a stratified fluid, which is modelled in this work as a two-layer fluid for convenience. Comparison of present results with available results show good agreement and this points towards the effectiveness of the model described in this work.

Wave scattering by inverted trapezoidal porous boxes using dual boundary element method

A numerical model is developed based on the dual boundary element method (DBEM) to solve the problem of surface gravity waves past multiple floating inverted trapezoidal porous boxes under the framework of linear potential flow theory. The continuity of velocity and semi-empirical quadratic pressure drop equation is adopted on the porous boundary to incorporate the effect of incident wave amplitude on the energy loss coefficient. Various results by an array of inverted trapezoidal porous boxes are discussed in the form of scattering coefficients (wave reflection, transmission, and energy loss coefficients), and force coefficients (horizontal force, vertical force, and moment). The verification of the numerical model is carried out by an independently developed multi-domain boundary element method (MBEM) code, and also with results available in the literature. The study reveals that, in most cases, the long waves exhibit insignificant hindrance whereas intermediate waves delineate the harmonic oscillations and deep water waves feel maximal hindrance by inverted trapezoidal porous boxes. The wave damping of KL≥0.95 is obtained for structural width B/λ≥0.66 by a pair of identical inverted trapezoidal porous boxes. The angle of porous wings is helpful to alter the scattering coefficients and also helps in the fine-tuning of force coefficients. The angle of partially open wings within 300≤θ≤600 is suggested along with structural porosity μ=0.25 for effective constriction of incident waves. The parametric analysis will be useful in the meticulous design of cost-effective breakwaters.

Acoustic characterisation of towing tanks

Large water tanks known as towing tanks, wave basins and cavitation tunnels, are used to determine the performance of ships, boats and submarines at model-scale. They may also be used for noise measurements but suffer from the effects of reverberation, which makes it difficult to capture the free field conditions necessary for complete characterisation of the noise. This paper describes an adapted Image Source Model and its use to understand these reverberation effects. It extends the conventional Image Source Model by the use of spherical wave reflection coefficients at the tank boundaries, in addition to the inclusion of volumetric absorption. Validation of the Image Source Model is undertaken by comparison with published transfer function measurements made in the depressurized wave basin at the Maritime Research Institute Netherlands (MARIN). The relative contribution to the tank response from the direct field and free surface is examined. The tank response is found to vary with three frequency-scales (periodicities) resulting from interactions with the free surface and axial modes.

A Mathematical Model of Longitudinal Waves Incident at a Free Surface of a Pre-Stressed Dissipative Half-Space

The aim of this work is to study the behavior of reflection of a longitudinal wave at a free surface of dissipative half-space under the effects of compressive initial stresses. When a longitudinal wave is incident on the free surface of an elastic dissipative half-space, two damped waves (Primarywaves and secondary waves are reflected. Among of these waves, P-waves are affected by compressional initial stresses. The governing equation and corresponding closed-form solutions are derived based on Biot’s incremental deformation theory. The equations of motion are solved analytically and the influence of initial stress parameter on the reflection coefficient of P-wave incidents at the free surface of dissipative half-space is studied in detail. Numerical computations are performed for actual Earth crust and the results analyzing the incident of longitudinal waves are discussed and presented graphically. The analytical solutions and numerical results reveal that the compressive initial stress parameter has notable effects on the reflection coefficient of longitudinal wave incidents on the free surface of dissipative medium. In addition, it has been observed that the presence of compressive initial stresses increases the phase velocity of the longitudinal waves. To the authors’ best knowledge, effects of compressive initial stresses on the reflection coefficients of the incident longitudinal wave on a free surface of dissipative half-space have not been studied before. Since the actual Earth is subject to initial stresses due to different resources, understanding the influences of compressive initial stresses on the reflection coefficient of a longitudinal wave helps seismologists and earthquake engineers to get accurate results of the reflection coefficients of seismic waves propagation in the Earth. Thus, the present study would be useful for seismology and earthquake engineering fields and further study about the nature of seismic waves.

Hydrodyamic Efficiency of Double Pontoon Supported on Piles (Dept.C)

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Design development of porous collar barrier for offshore floating fish cage against wave action, debris and predators

This paper presents a design concept of a porous collar barrier for a novel floating open-net fish cage that is integrated with a floating spar wind turbine (referred to as COSPAR fish cage). The COSPAR fish cage has an octagonal shape with each side length of 30m. The collar barrier, having an array of rectangular cut-outs with round corners, is installed at the top portion of the cage to attenuate wave transmission inside the cage as well as to protect fish from external predators and debris. Single and double collar barrier designs corresponding to single net and double net cages are studied. The wave transmission, reflection and energy-loss coefficients of barriers are determined from numerical analysis based on the linear potential wave theory and the eigenfunction expansion method. Various underwater heights (2m ≤ h ≤ 8m) and porosity (0.25≤ ε ≤ 0.75) of the collar barriers are examined with the view to obtaining the barrier design for minimal transmission coefficient and energy-loss coefficient. Without a collar barrier, the single net and double net cage can only provide average wave transmission coefficients of 0.9 and 0.8, respectively. This study finds that the transmission coefficient could be reduced below 0.4 by having a single collar barrier with h = 4m and ε = 0.25. On the other hand, the transmission coefficient could be further reduced below 0.3 by a double collar barrier with the same h and ε. In addition, the double collar barrier gives lower energy-loss coefficient and better proofing against fish escape, biosecurity and predator intrusion than the single collar barrier. A double collar barrier design with porosity combination of ε1 = 0.25, ε2 = 0.5 is recommended for the COSPAR fish cage as it yields competitive wave scattering performances and saves collar material by 25 % when compared with the best performing porosity combination of ε1 = ε2 = 0.25.

Analytical Technique for Synthesizing a Leaky-Wave Antenna with a Semitransparent Wall of Metal Cylinders

We have developed an analytical technique for synthesizing a leaky-wave antenna in the form of an irregular hollow rectangular metal waveguide with a narrow wall in the form of an array of circular metal cylinders using an approximate expression for the reflection coefficient of a plane wave from a fine-periodic lattice of circular metal cylinders and the dispersion equation for a waveguide with a partially transparent wall. As an example of the application of this technique, two leaky-wave antennas are synthesized: with a uniform and sinusoidal distribution of the field amplitude along the antenna. We have performed electrodynamic modeling of synthesized antennas using the finite element method. For a synthesized antenna with a uniform amplitude distribution, the results of a numerical experiment are compared with the results of measurements of an experimental sample in the centimeter wavelength range.

A comparative study of the hydrodynamic characteristics of permeable twin-flat-plate and twin-arc-plate breakwaters based on physical modeling

To explore the similarities and differences between permeable twin-flat-plate (TFP) and twin-arc-plate (TAP) breakwaters in terms of their wave attenuation performance and structural behavior, the two breakwaters are compared in terms of the wave transmission coefficient, reflection coefficient, wave pressure, wave force, and flow field characteristics by physical model tests. The results show the wave transmission coefficients of the TAP breakwater are 34.5% and 25% smaller than those of the TFP breakwater in the emerged state and at the still water level, respectively. The wave reflection coefficient of the TFP breakwater shows a maximum increase of approximately 66.7% compared with the TAP breakwater. Except for the upper surface of the upper plate in the emerged state, the wave pressures on the TAP breakwater are smaller than those on the TFP breakwater by up to 62.4%. The vertical wave forces on the TAP breakwater are less than those on the TFP breakwater by up to 40.2%. The characteristics of the flow field around the TFP and TAP breakwaters are markedly different. The results of this study provide necessary theoretical guidance for engineering applications of permeable plate-type breakwaters.

Hydraulic performance and wave transmission through pile-rock breakwaters

In recent years coastal erosion in the Vietnamese Mekong Delta (VMD) is occurring at a rapid rate which is leading to considerable damage of infrastructure and the human-made environment. Various solutions have been proposed to protect the coastline in the VMD including Pile-Rock breakwaters (PRBWs). PRBWs have several advantages that make them particularly applicable to the coast of West Ca Mau and the Mekong Delta. The focus of this study is to test PRBW efficiency in the VMD by evaluating a physical experimental model followed by evaluation of observations in the field from the Ca Mau province. A series of experiments were undertaken to build an empirical formula of the wave transmission coefficient. The results show that the PRBW structure works effectively in the case of emergence when the wave transmission coefficient Kt is between 0.3 and 0.4 but in this case the wave reflection coefficient is quite large (i.e. Kr of 0.45–0.6). The experimentally obtained and measured transmission coefficient Kt show strong agreement. The analytical results show that a PRBW dissipates most of the energy for short periodic waves, however long periodic waves can still be transmitted through the PRBW. The PRBW shows significant advantages over other styles of breakwaters including an increased potential to reduce erosion, high stability, and a reduced deadweight of construction materials. These advantages mean that PRBWs have a great potential to aid restoration of mangrove forests that are sheltered behind the breakwater in the VMD region.

Reflection and transmission coefficient approximations for P, S1 and S2 waves in triclinic media

SUMMARY The conventional assumptions, in most published approximations of the reflection and transmission (R/T) coefficients of plane waves at a plane interface between two anisotropic half-spaces, confine their applications to weakly anisotropic and/or weak contrast models. We consider the horizontal interface enclosed by two triclinic half-spaces to approximate the R/T coefficients normalized by the vertical energy flux. The homogeneous background medium can be anisotropic with arbitrary symmetry to better simulate the strongly anisotropic media. The second-order approximations are proposed to accommodate the strong contrast interface. We also consider an isotropic background medium under the weak anisotropy assumption. The obtained approximations can be applied to P, S1 and S2 waves, except for the transmission coefficients between the S1 and S2 waves. The S-wave transmission coefficients are insensitive to the model parameter contrasts and predominately rely on the S-wave polarization directions in the half-spaces above and below the interface. The proposed approximations are tested numerically.

ВІДБИТТЯ УЛЬТРАЗВУКОВИХ ХВИЛЬ ВІД ДВОШАРОВОГО ПАКЕТУ ТЕКСТИЛЬНИХ МАТЕРІАЛІВ ЗІ ЩІЛЬНИМ ВЕРХНІМ ШАРОМ

Obtain and show in general terms the expression for the module of the complex reflection coefficient of ultrasonic waves from a two-layer package of textile materials with a dense first layer. The above is necessary for the possibility of research of contactless control of such materials. To analyze the process of reflection of waves from a controlled material, the method of superposition of sounding vibrations is used. In general terms, the expression for the complex reflection coefficient of ultrasonic waves from a two-layer package of textile materials with a dense first layer is shown. It is proved that taking into account the complete attenuation of the reflected waves in the layer of the first material, it is possible to simplify the basic expressions for the complex reflection coefficient from a package of textile materials. It is shown that the change in the amplitude of the reflected ultrasonic waves for the material presented in the work with a change in the thickness of its layers is very small. The change in the amplitude of the reflected waves for such materials mainly causes a dense first layer of package of textile materials. The dependences of the real and imaginary parts of the complex reflection coefficient of ultrasonic waves on a package of textile materials with a dense first layer are obtained, which will help analytically determine both the change in the amplitude of the waves and their phase shift caused by the interaction of these vibrations with the material. It is proposed to use the component expressions for the module of the complex reflection coefficient of ultrasonic waves, which will allow you to adjust the parameters of the contactless transducers for different two-layer package of textile materials with a dense surface layer. To ensure the possibility of determining a small change in the measured magnitude of the amplitude of the reflected ultrasonic waves from the two-layer package of textile materials with the first dense layer, you can use switching and modulation devices and transducers. This will eliminate the errors that may be associated with the non-identity of the measuring channels when determining a small change in the amplitude of the reflected ultrasonic waves.