Evaluation of interface wave velocity, reflection coefficients and interfacial stiffnesses of contacting surfaces

The phase velocity of the antisymmetric-mode interface wave as well as the longitudinal and shear wave reflection coefficients have been measured for contacting poly(methyl methacrylate) (PMMA) surfaces subjected to different contact pressures. It has been found that while the reflection coefficients decrease as the contact pressure is increased, the phase velocity of the interface wave increases from that of the Rayleigh wave toward that of the bulk shear wave. From these measurements, the normal and tangential interfacial stiffnesses of the contacting PMMA surfaces have been evaluated as functions of the contact pressure. As a result, the two independent procedures to evaluate the tangential stiffness, namely, from the interface wave velocity and from the shear wave reflection measurements, have yielded mutually consistent results. Furthermore, it has been found that the tangential/normal stiffness ratio and the shear/longitudinal reflection ratio of the contact interface are consistent with the predictions of an existing theoretical model for kissing bond interfaces.     

声速测量实验中界面反射问题的探讨

探讨在声速测量实验中,声波在发射器和接收器两个端面之间反向形成极大值的位置和条件,以及声波强度随距离变化的规律。这有利于学生正确实验和理解声波特性。     

A new method in the determination of the reflection coefficient of an inhomogeneous layer

In this paper a new derivation of an explicit expression for the reflection coefficient of an inhomogeneous layer with parameters dependent on the only variable is presented. The reflection coefficient of such an inhomogeneous layer is obtained as the limit of the reflection coefficient of many homogeneous plane-parallel layers and, being determined in this way, it satisfies the differential equation of other authors. The calculations are valid for mechanical, electromagnetic and De Broglie waves. The paper is dedicated to the memory of Dr. A. Vaíek, Professor of the J. E. Purkyn University in Brno, who died on November 16, 1966.     

Acoustic method for obtaining the pressure reflection coefficient using a half-wave layer

This paper describes a method for obtaining the pressure reflection coefficient by using a half-wave layer. Two transducers were placed in reference liquid and test liquid respectively. The reference and test liquid were separated by the thin half-wave layer. Both transducers could operate in two modes. One acted as a transmitter and worked in pulse-echo mode, and the other operated in receiver mode. By adjusting the frequency of drive signal according to the thickness and material property of the half-wave layer, it was possible to generate the maximum interference signal of multiple waves reflected at the two interfaces of the layer. Therefore, the amplitude of reflection wave in steady-state depended only on the reflection coefficient at the interface between the half-wave layer and the test liquid. The effects of the signal-to-noise-ratio and the half-wave layer attenuation on the uncertainty characteristics of the pressure reflection coefficient were discussed. The experimental results showed high accuracy for measurement of reflection coefficient.     

Determination of thermal wave reflection coefficient to better estimate defect depth using pulsed thermography

Thermography is a promising method for detecting subsurface defects, but accurate measurement of defect depth is still a big challenge because thermographic signals are typically corrupted by imaging noise and affected by 3D heat conduction. Existing methods based on numerical models are susceptible to signal noise and methods based on analytical models require rigorous assumptions that usually cannot be satisfied in practical applications. This paper presents a new method to improve the measurement accuracy of subsurface defect depth through determining the thermal wave reflection coefficient directly from observed data that is usually assumed to be pre-known. This target is achieved through introducing a new heat transfer model that includes multiple physical parameters to better describe the observed thermal behaviour in pulsed thermographic inspection. Numerical simulations are used to evaluate the performance of the proposed method against four selected state-of-the-art methods. Results show that the accuracy of depth measurement has been improved up to 10% when noise level is high and thermal wave reflection coefficients is low. The feasibility of the proposed method in real data is also validated through a case study on characterising flat-bottom holes in carbon fibre reinforced polymer (CFRP) laminates which has a wide application in various sectors of industry.     

Geoacoustic inversion of short range source data using a plane wave reflection coefficient approach

Acoustic time series data were collected in a shallow, hard bottom lake environment located in central Texas using both short range (2 m) implosive data, obtained with the source and a single hydrophone located near mid-depth in the waveguide, along with longer range implosive and explosive data from a near surface source to a bottom mounted hydrophone. Matched field inversions using simulated annealing were performed with a ray trace plus complex plane wave reflection coefficient forward propagation model that was validated in previous work. Isolating bottom interacting paths to perform the inversions is shown to be essential to reduce parameter uncertainties in the hard bottom environment and enables a systematic approach to the inversions which establishes the number of layers needed to represent the lake environment. Measured transmission loss data from a towed source were compared through a RMS error analysis to modeled transmission loss, constructed with the parameters from inversions of data from several source types, to further establish the validity of the inversion approach for this environment. Geoacoustic parameters obtained by inversions of short range, low frequency impulsive data are used to predict transmission loss at longer ranges and higher frequencies. The range dependence of the global minimum is discussed.     

A simple method for determination of the reflection coefficient at the internal surfaces of turbid slabs

The total reflectance of a slab turbid material is measured with an integrating sphere. Each slab was measured with a glossy and diffusing (etched) rear surface, both with and without a fully reflecting pill placed behind it. Formulae derived show that the results, together with the refractive index, permit calculation of the reflectance of the parity diffuse radiation at the internal rear surface. The method is applied to human dental enamel.     

超声光栅测声速的方法运用到液体表面波波速的测量

理论上分析了超声光栅衍射和液体表面波光栅衍射,得到了表面波光栅衍射图样与表面波参数的解析关系.实验上得到清晰的衍射图样,利用表面波光栅衍射法测量了液体表面波波速,实验结果与理论值误差不超过1%.     

驻波法测量声速实验的讨论

根据声波在2个换能器之间多次反射和声波在空气中的衰减,研究了驻波法测量声速实验中接收器输出信号随接收器到发送器间距离的变化.理论推导结果表明:声波在一般情况下不是严格的驻波,但是当反射比接近-1时则是双曲函数形式的驻波,这与实验测量结果一致.只有接收器到发送器的距离比较远,反射比接近-1时,才能得到几乎理想的驻波.     

Modeling elastic wave forward propagation and reflection using the complex screen method.

Formulation for calculating forward propagation and reflection in a 3D elastic structure based on the complex-screen method is given in this paper. The calculation of reflections is formulated based on the local Born approximation. When using a small angle approximation, the backscattering operator reduces to a screen operator which is similar to the forward screen propagator. Combining the forward propagator and backscattering operator together, the new method can properly handle the multiple forward scattering and single backscattering in a 3D heterogeneous model. Using a dual-domain technique, the new method is highly efficient in CPU time and memory savings. For models where reverberation and resonance scattering can be neglected, this method provides a fast and accurate algorithm. Synthetic seismograms for two-dimensional elastic models are calculated with this method and compared with those generated by the finite-difference method. The results show that the method works well for small to medium scattering angles and medium velocity contrasts.     

FDTD analysis on the effect of plasma parameters on the reflection coefficient of the electromagnetic wave

In this paper, the dielectric constant of dispersive medium is written as rational polynomial function, and the relationship between D and E is derived in time-domain. It is named shift operator FDTD (SO-FDTD) method. Compared to the analytical solution, the high accuracy and efficiency of this method is verified by calculating the reflection coefficient of the electromagnetic wave through a cold plasma slab. The effect on reflection coefficient is calculated by using the SO-FDTD method. The result shows that some factors effect on reflection coefficient. They are as follows: plasma thickness, electron density, electron distribution and incident frequency. And on most conditions, parabola distribution helps reduce reflection coefficient more effectively than homogeneous distribution.     

Anomalous wave reflection at the interface of two strongly nonlinear granular media

Granular materials exhibit a strongly nonlinear behavior affecting the propagation of energy and information. Dynamically self-organized strongly nonlinear solitary waves are the main information carriers in granular chains. We report the first experimental observation of the dramatic change of solitary wave reflectivity from the interface of two granular media triggered by a magnetically induced precompression. It may be appropriate to name this phenomenon the "acoustic diode" effect. We explain this effect by the high gradient of particle velocity near the interface.     

Effect of viscosity on ultrasound wave reflection from a solid/liquid interface

A study of the simulated reflection of a wideband ultrasound shear wave from the solid/viscous fluid interface is presented. Various parameters affecting reflection factors including the material properties of the solid, fluid properties like density and viscosity, and the operating frequency are discussed. Simulated ultrasonic response waveforms are compared with the experimentally obtained data for NIST traceable calibration standards of viscosity. A good agreement was observed between the simulated and experimental waveforms at various viscosities and for different solid substrates.     

利用海洋环境噪声直接提取海底反射系数

提出了一种利用海洋环境噪声直接(非迭代)提取海底反射系数的方法,基于海洋环境噪声垂直相关函数与方向密度函数的相似性,结合海洋环境噪声射线理论,由两个垂直排列的水听器实现海底反射系数的被动获取。方法适应性的仿真分析说明较高海况下且近场无航船干扰的海洋环境噪声数据有利于方法的实现;对于两层海底,海洋环境噪声垂直相干函数经高通滤波后,由海洋环境噪声垂直相关函数对时间的导数可准确提取垂直方向沉积层反射系数;将方法用于较高风速下的实测海洋环境噪声数据处理,结果表明直接提取方法可准确获取大于临界角部分的海底反射系数。     

利用海洋环境噪声直接提取海底反射系数

提出了一种利用海洋环境噪声直接(非迭代)提取海底反射系数的方法,基于海洋环境噪声垂直相关函数与方向密度函数的相似性,结合海洋环境噪声射线理论,由两个垂直排列的水听器实现海底反射系数的被动获取。方法适应性的仿真分析说明较高海况下且近场无航船干扰的海洋环境噪声数据有利于方法的实现;对于两层海底,海洋环境噪声垂直相干函数经高通滤波后,由海洋环境噪声垂直相关函数对时间的导数可准确提取垂直方向沉积层反射系数;将方法用于较高风速下的实测海洋环境噪声数据处理,结果表明直接提取方法可准确获取大于临界角部分的海底反射系数。     

Calculation of the coefficient of reflection of an infinite phased antenna array by the mirror-iterative method

The problem of electrodynamic calculation of the coefficient of reflection of an infinite waveguide array by the mirror-iterative method is considered. As an example, calculation results for the coefficients of reflection in a linear waveguide array are presented.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 21, No. 11, pp. 1653–1661, November, 1978.     

Experimental measurement of variations in the optical reflection coefficient of water-magnetic liquid interface in an electric field,wave motion,and surface instability

A variation in the reflection coefficient of an interface of two liquids (water and magnetic liquid) in the presence of an electric field is experimentally studied. An increase in the reflection coefficient of the interface is demonstrated. A surface instability of the water-magnetic liquid interface, the wave motion at the interface, and wave interference are observed.