Near surface flaw detection by ultrasonic critical angle imaging

Acoustic critical angle measurements usually involve broad beam or global isonification of a water-solid interface. In order to obtain local interface data, it is convenient to use a focused acoustic source with lens axis at incident angle and point-receiver at specular reflection angle . By scanning this combination at various critical angles (=_crit), interface waves are generated locally near the lens focus. These waves penetrate the solid to a depth of approximately one shear wavelength _T and are modulated by discontinuities such as flaws, inclusions, and debonds that may be present there. A radiated longitudinal wave in water accompanies these modulated interface waves and carries information on subsurface flaws to the point-detector along the direction of a specularly reflected wave. By scanning the focus-receiver combination along the interface, the receiver output may be used to form images of local subsurface detail. We show that in anisotropic materials, such as crystals, there are generally two different _crit where a radiating interface wave provides subsurface information. We also discuss special problems associated with the use of a focused acoustic source including nonlinear effects (generation of harmonics) in the water.     

Nodal DG-FEM solution of high-order Boussinesq-type equations

A discontinuous Galerkin finite-element method (DG-FEM) solution to a set of high-order Boussinesq-type equations for modelling highly nonlinear and dispersive water waves in one horizontal dimension is presented. The continuous equations are discretized using nodal polynomial basis functions of arbitrary order in space on each element of an unstructured computational domain. A fourth-order explicit Runge-Kutta scheme is used to advance the solution in time. Methods for introducing artificial damping to control mild nonlinear instabilities are also discussed. The accuracy and convergence of the model with both h (grid size) and p (order) refinement are confirmed for the linearized equations, and calculations are provided for two nonlinear test cases in one horizontal dimension: harmonic generation over a submerged bar, and reflection of a steep solitary wave from a vertical wall. Test cases for two horizontal dimensions will be considered in future work.     

Interactions of Inert Confiners with Explosives

The deformation of an inert confiner by a steady detonation wave in an adjacent explosive is investigated for cases where the confiner is sufficiently strong (or the explosive sufficiently weak) such that the overall change in the sound speed of the inert is small. A coupling condition which relates the pressure to the deflection angle along the explosive-inert interface is determined. This includes its dependence on the thickness of the inert, for cases where the initial sound speed of the inert is less than or greater than the detonation speed in the explosive (supersonic and subsonic inert flows, respectively). The deformation of the inert is then solved by prescribing the pressure along the interface. In the supersonic case, the detonation drives a shock into the inert, subsequent to which the flow in the inert consists of alternating regions of compression and tension. In this case reverberations or ‘ringing’ occurs along both the deflected interface and outer edge of the inert. For the subsonic case, the flow in the interior of the inert is smooth and shockless. The detonation in the explosive initially deflects the smooth interface towards the explosive. For sufficiently thick inerts in such cases, it appears that the deflection of the confiner would either drive the detonation speed in the explosive up to the sound speed of the inert or drive a precursor wave ahead of the detonation in the explosive. Transonic cases, where the inert sound speed is close to the detonation speed, are also considered. It is shown that the confinement affect of the inert on the detonation is enhanced as sonic conditions are approached from either side.     

Pulsed laser deposition of La0.67Ca0.33MnO3 thin films on LiNbO3 and surface acoustic wave studies

Surface Acoustic Waves on piezoelectric substrates can be used to investigate the dynamic conductivity of thin films in a non-contact and very sensitive way, especially at low conductivities. Here, we report on such surface acoustic wave studies to characterize thin manganite film like La_0.67Ca_0.33MnO₃, exhibiting a Jan Teller effect with a strong electron phonon interaction and a metal insulator transition at high temperatures.

We report on the deposition of La_0.67Ca_0.33MnO₃ on piezoelectric substrates (LiNbO₃ in different crystal cuts, employing a pulsed laser deposition technique. The structural quality of the thin films are examined by X-Ray Diffraction, Scanning Electron Microscope and Energy Dispersive X-ray spectroscopy. For the electrical characterization, we employ the surface acoustic wave technique, accompanied by conventional direct current resistance measurements for comparison.     

Truncation Approximations for Gravity-capillary Free-surface Flows

Gravity-capillary free-surface flows past disturbances in a channel of finite depth are considered. These flows are usually assumed to extend from x = −∞ to x = ∞ where the x-axis is parallel to the bottom. Many numerical schemes truncate this infinite domain to the interval −B < x < A where A and B are large positive numbers. These truncations introduce inaccuracies, especially when the effect of surface tension is included. In this paper numerical methods are presented which remove these inaccuracies. This is achieved by taking into account the contributions from −∞ to −B and from A to ∞. Explicit computations are presented for a semi-circular obstacle at the bottom of the channel.     

Basic properties of free stratified flows

Results of analytical studies of the governing equations of stratified rotating fluids based on the unification of theories of continuous and discrete groups, perturbations and modern numerical visualizations are described. Symmetries of basic systems and their simplified versions, different approximations and constitutive turbulent models are compared. A new method to calculate discrete groups analytically, which does not need a preliminary search for continuous groups, is developed. As an example of the practical use of the developed algorithm, a complete classification of cellular and roll structures of Bénard convection is presented. A complete classification of 3D periodic motions in compressible viscous stratified and rotating fluids, including regular (wave) and singular elements, is performed by perturbation methods. In all cases, in a viscous fluid, besides waves there are two different periodic boundary layers. In a homogeneous fluid the split boundary layers are merged, thus forming a joint doubly-degenerate structure. Stratification and rotation reduce the degeneration of the 3D periodic boundary layers. Calculations of a 3D periodic wave beam emitted by an oscillating part of a sloping plane are visualized by a computer-graphics method. The existence of thin extended components on the edges of the 3D wave cone is demonstrated.     

水介质受冲击后的应力波计算

从流体动力学基本理论出发,用应力波理论建立了以水作为介质的应力波传播模型,推导出介质受冲击后瞬时的应力波传播至某一点的超压值的计算方法,进行了压力的测试实验,并将计算值与实验结果进行了比较,证明该计算模型是可靠的.     

基于PT在线注入信号及神经网络跟踪小波变换模极大值变化的配电网故障定位方法

提出了一种配网单相接地在线故障定位的新方法。故障发生后从变电站母线在线向系统注入高频脉冲信号并获取线路上与注入信号相对应的反射行波,在电网结构已知的情况下,通过对反射波的分析进行故障定位。该方法不仅能检测故障点的电气距离,而且能确定故障所在的分支线路,还能通过调节注入信号的大小多次定位故障,提高定位的准确性。应用该方法进行故障定位不受接地电阻的限制。通过M atlab/S imu-link进行了建模与仿真分析,仿真结果验证了该方法的正确性。     

用于通信与环境监测的超低频系统

介绍了美、俄超低频对潜通信系统,提出除军事用途,超低频通信系统最有前景的方向是进行电磁监测,俄ZEVS系统的研究实验揭示了SLF电磁波异常与将要发生地震的一些初步规律,地震与观测的电磁场异常和视电阻率的变化有较好的对应性。利用它可以研究地下电阻率结构和空间电磁场;研究地震等引起的电磁场异常变化;用于探测地壳结构和地下资源、发现核废料、环境研究以及地震的监测和预报。具有广阔的应用前景和发展潜力。     

基于声波与振动探测的地震灾害生命搜索系统信号分析

基于声波与振动探测原理的地震生命探测系统的研制在我国灾害生命搜索救助领域中占有重要位置.对于其中数理模型的研究是研制和开发声波、振动生命探测定位仪的基础.测试实验表明,在声波与振动灾害救助系统中,所采集的信号带有大量的随机噪声,为了提高信噪比,必然要进行滤波.本文用的是小波变换的去噪方法.同时,从传感器接收到的信号可能是从多个振源发出的,这就要涉及到多源信号的分离问题.为此,引入了独立变量分析（ICA）的分离方法和傅立叶变换加以解决.当找到需要的信号后,通过相关函数分析自动获取多道信号的相对时差,进而求取振源的位置.