Experiments on the interaction of internal waves with the semi-submersible platform in a stratified fluid

对密度分层流体中内波与半潜平台的相互作用问题 进行了模型试验研究. 采用摇板方法进行了内波造波试验,对内波波长、周期和波高进行了 测量分析,获得了内波波高和波长与周期之间的相关关系. 利用激光和倾角仪方法对半潜平 台的纵荡和纵摇运动响应进行了测量分析,获得了平台纵荡运动及纵摇角幅值与内波周期之 间的相关关系. 结果表明,在半潜平台的设计与应用中,内波对其运动响应的影响是不可忽 视的. 特别地,发现了在内波周期的某个范围内, 半潜平台的纵荡和纵摇都会出现 倍频周期运动响应的现象.     

薄膜涂层材料中的广义瑞利波

基于弹性动力学的线性理论，建立了涂层材料中广义瑞利波传播的理论分析模型，并 且由波动方程和边界条件推导了波的频散方程．分析了慢层和快层对相速度频散的影响，给 出了不同层厚-波长比和不同涂层-基体密度比情况下广义瑞利波相速度的理论解．算例分 析分别比较了慢层和快层结构中波的相速度、群速度，以及随深度衰减的位移与应力振 幅．另外，相速度曲线和位移振幅曲线与文献中给出的结果吻合，验证了理论模型和分析过 程的正确性．     

Peristaltic transport of a Jeffrey fluid under the effect of magnetic field in an asymmetric channel

The peristaltic flow of a Jeffrey fluid in an asymmetric channel is studied under long wavelength and low Reynolds number assumptions. The fluid is electrically conducting by a transverse magnetic field. The channel asymmetry is produced by choosing the peristaltic wave train on the walls to have different amplitudes and phase. The flow is investigated in a wave frame of reference moving with the velocity of the wave. The expressions for stream function, axial velocity and axial pressure gradient have been obtained. The effects of various emerging parameters on the flow characteristics are shown and discussed with the help of graphs. The pumping characteristics, axial pressure gradient and trapping phenomenon have been studied. Comparison of various wave forms (namely sinusoidal, triangular, square and trapezoidal) on the flow is discussed.     

Barotropic planetary waves on a random bottom

The propagation of planetary waves on an irregular seabed is studied, with the roughness modelled as a stochastic process in one coordinate. Khasminskii's limit theorem is used to determine the probability density of the wave process. The mean wave is found to have an effectively changed wave number and to decay exponentially in the direction of phase propagation. The effect of the bottom roughness is strongly dependent on the wavelength.     

Mesh effects on the computation of small amplitude water waves

Computational difficulties arise in the non-linear free-surface problem for water waves both at large amplitudes when the crest becomes nearly singular and at small amplitudes when the wave is very close to the alternative uniform flow solution. Since the limiting wavelengths for small amplitude waves are known from the Stokes linearized theory, these are used in checking results for finite-amplitude programs. When Southwell and Vaisey¹ first tried this, their methods gave an unexplained overestimate, by 6 per cent, of the limiting wavelength. This paper shows how coarse mesh effects can create such an overestimate, gives very accurate solutions at small amplitudes and considers accuracy in relation to the mesh for short and long waves.     

Rayleigh lamb waves in micropolar isotropic elastic plate

The propagation of waves in a homogeneous isotropic micropolar elastic cylindrical plate subjected to stress free conditions is investigated. The secular equations for symmetric and skew symmetric wave mode propagation are derived. At short wave limit, the secular equations for symmetric and skew symmetric waves in a stress free circular plate reduces to Rayleigh surface wave frequency equation. Thin plate results are also obtained. The amplitudes of displacements and microrotation components are obtained and depicted graphically. Some special cases are also deduced from the present investigations. The secular equations for symmetric and skew symmetric modes are also presented graphically.     

Axisymmetric free vibrations of infinite micropolar thermoelastic plate

The propagation of axisymmetric free vibrations in an infinite homogeneous isotropic micropolar thermoelastic plate without energy dissipation subjected to stress free and rigidly fixed boundary conditions is investigated. The secular equations for homogeneous isotropic micropolar thermoelastic plate without energy dissipation in closed form for symmetric and skew symmetric wave modes of propagation are derived. The different regions of secular equations are obtained. At short wavelength limits, the secular equations for symmetric and skew symmetric modes of wave propagation in a stress free insulated and isothermal plate reduce to Rayleigh surface wave frequency equation. The results for thermoelastic, micropolar elastic and elastic materials are obtained as particular cases from the derived secular equations. The amplitudes of displacement components, microrotation and temperature distribution are also computed during the symmetric and skew symmetric motion of the plate. The dispersion curves for symmetric and skew symmetric modes and amplitudes of displacement components, microrotation and temperature distribution in case of fundamental symmetric and skew symmetric modes are presented graphically. The analytical and numerical results are found to be in close agreement.     

Axisymmetric free vibrations of infinite thermoelastic plate

The propagation of axisymmetric free vibrations in an infinite homogeneous isotropic micropolar thermoelastic plate without energy dissipation subjected to stress free and rigidly fixed boundary conditions is investigated. The secular equations for homogeneous isotropic micropolar thermoelastic plate without energy dissipation in closed form for symmetric and skew symmetric wave modes of propagation are derived. The different regions of secular equations are obtained. At short wavelength limits, the secular equations for symmetric and skew symmetric modes of wave propagation in a stress free insulated and isothermal plate reduce to Rayleigh surface wave frequency equation. The results for thermoelastic, micropolar elastic and elastic materials are obtained as particular cases from the derived secular equations. The amplitudes of displacement components, microrotation and temperature distribution are also computed during the symmetric and skew symmetric motion of the plate. The dispersion curves for symmetric and skew symmetric modes and amplitudes of displacement components, microrotation and temperature distribution in case of fundamental symmetric and skew symmetric modes are presented graphically. The analytical and numerical results are found to be in close agreement.     

Scattering coefficients for a sphere in a visco-acoustic medium for arbitrary partial wave order

Analytical solutions are reported for the scattering coefficients of a solid elastic sphere suspended in a viscous fluid for arbitrary partial wave order. Expressions are derived for incident compressional and shear wave modes, taking into account the viscosity of the surrounding fluid and resultant wave mode conversion. The long compressional wavelength limit is employed to simplify the derivation, whereas no restriction is placed on the shear wavelength in the fluid compared to the particle dimension. The analytical approximations are compared with numerical results obtained from matrix inversion of the boundary equations and agree within the validity domain of the solutions.     

Random walk methods and wave diffraction

This paper develops an approach to problems of wave diffraction that combines the physical clarity of the ray method with the versatility of direct numerical methods. First it addresses scalar problems with general linear first-order boundary conditions, and then it considers problems formulated in adjacent domains with imposed interface conditions. We start by following closely the scheme of the ray method, but instead of looking for approximate expressions for the amplitudes of the Liouville decomposition we obtain their exact representations as the mathematical expectations of some functionals on the space of Brownian trajectories. The obtained solutions provide direct improvements of the ray method approximations to the exact solutions, and they are shown to admit efficient numerical evaluations.     

Temperature and velocity flow fields measurements using ultrasonic computer tomography

This paper presents a measurement method to determine the velocity flow field and the temperature in a cross-section of an aerosol chamber by means of the ultrasonic computer tomography. The required high measurement resolution of the propagation time of the ultrasonic impulse through the medium is obtained by a special signal processing technique. Since the propagation direction of a sonic wave in a non-stationary medium is not straight-lined, a ray linking procedure was developed that traces the rays. The combination of the precise propagation time measurement, the ray linking method and a vector algebraic reconstruction technique leads to a␣computer tomographic measurement system for the determination of the temperature distribution and the velocity flow field in a cross-section. Received on 9 June 1997     

局部屏蔽法在X—Ray残余应力测量中的应用

本文所述的局部屏蔽法即是在Ｘ射线测量过程中的下遮挡法，即是将被测部位暴露出来，而将其余部位进行遮挡以对Ｘ射线进行屏蔽。局部屏蔽法是解决应力梯度测量中的一种有效方法，本文解决了遮挡法的关键问题并取得了良好的效果     

小波变换在随机海浪及相关课题中的应用与前景

扼要介绍了小波变换的基本原理及其信号处理功能,综述了应用这一技术处理非平稳过程随机海浪,确定波群和异常波特性,判断波浪破碎,进行入反射分离,分析海洋和湖泊流场以及研究岸滩演变的时空变化过程与气象、海象的相关关系等方面的研究成果.讨论了这一技术在波浪动力特性认识、随机海浪的实验室模拟、波浪与建筑物相互作用研究方面的应用前景.      

The thickness of detonation waves visualised by slight obstacles

The reflection of detonation waves from slight obstacles, which hardly disturb the wave propagation, is observed by time-resolved schlieren photography. The following stoichiometric mixtures are used: pure and argon-diluted hydrogen-oxygen, hydrogen-air, acetylene-oxygen, and acetylene-air. Initial pressures are varied such that cell widths range from 1.4 up to 108 mm, which is twice the side length of the square cross-section of the tube. The trajectories of the incident and the reflected waves in the x,t-plane are used to determine lower limit values for the wave thickness. The considerable influences of the obstacle shape and of the evaluation method on the results are discussed in detail, and error sources are analyzed. The method has been improved since a previous publication by the authors. The ratio of the lower limit values to the cell width spreads from 0.4 to 0.8 in the medium cell size range. It decreases with increasing marginality and seems to increase at small scale. A unique correlation between the lower limit value and the tube diameter, both referred to the cell size, that was proposed earlier in the literature has to be refused. The velocities of the reflected waves are presented as additional information on the post-detonation wave state. The sonic transition is discussed theoretically, enhancing the stream tube model, and practically, based on detailed observations for marginal detonations.Received: 29 April 2003, Accepted: 20 October 2003, Published online: 3 February 2004PACS: 47.40.-xCorrespondence to: M. Weber     

Finite viscoplastic deflections of an impulsively loaded plate by the mode approximation technique

The application of the mode approximation technique to a fully clamped circular plate is here described. Mode solutions for finite deflections are obtained from a sequence of instantaneous modes. Master solutions for chosen initial velocity amplitudes are constructed in nondimensional form. These depend weakly on a parameter of viscoplastic material behavior and size of structure, and so can be applied to a variety of loadings and structures. Finding each instantaneous mode shape and acceleration constitutes an eigenproblem, solved by finite elements with iterations. Comparisons with recent tests on steel and titanium plates are discussed in some detail.     

Free vibration of microstretch thermoelastic plate with one relaxation time

The propagation of waves in microstretch thermoelastic homogeneous isotropic plate subjected to stress free thermally insulated and isothermal conditions is investigated in the context of conventional coupled thermoelasticity (CT) and Lord and Shulman (L–S) theories of thermoelasticity. The secular equations for both symmetric and skew-symmetric wave mode propagation have been obtained. At short wavelength limits, the secular equations for symmetric and skew-symmetric modes reduce to Rayleigh surface wave frequency equation. The amplitudes of dilatation, microrotation, microstretch and temperature distribution for the symmetric and skew symmetric wave modes are computed analytically and presented graphically for different theories of thermoelasticity. The theoretical and numerical computations are found to be in close agreement.     

A vision-based hybrid particle tracking velocimetry (PTV) technique using a modified cascade correlation peak-finding method

A novel technique for particle tracking velocimetry is presented in this paper to overcome the issue of overlapping particle images encountered in the flows with high particle density or under volumetric illumination conditions. To achieve this goal, algorithms for particle identification and tracking are developed based on current methods and validated with both synthetic and experimental image sets. The results from synthetic image tests show that the particle identification algorithm is able to resolve overlapped particle images up to 50?% under noisy conditions, while keeping the root mean square peak location error under 0.07?pixels. The algorithm is also robust to the size changes up to a size ratio of 5. The tracking method developed from a classic computer vision matching algorithm is capable of capturing a velocity gradient up to 0.3 while maintaining the error under 0.2?pixels. Sensitivity tests were performed to describe the optimum conditions for the technique in terms of particle image density, particle image sizes and velocity gradients, also its sensitivity to errors of the PIV results that guide the tracking process. The comparison with other existing tracking techniques demonstrates that this technique is able to resolve more vectors out of a dense particle image field.     

Analytical Solution for Isothermal Flow in a Shock Tube Containing Rigid Granular Material

Analytical solution of shock wave propagation in pure gas in a shock tube is usually addressed in gas dynamics. However, such a solution for granular media is complex due to the inclusion of parameters relating to particles configuration within the medium, which affect the balance equations. In this article, an analytical solution for isothermal shock wave propagation in an isotropic homogenous rigid granular material is presented, and a closed-form solution is obtained for the case of weak shock waves. Fluid mass and momentum equations are first written in wave and (mathematical) non-conservation forms. Afterwards by redefining the sound speed of the gas flowing inside the pores, an analytical solution is obtained using the classical method of characteristics, followed by Taylor’s series expansion based on the assumption of weak flow which finally led to explicit functions for velocity, density and pressure. The solution enables plotting gas velocity, density and pressure variations in the porous medium, which is of high interest in the design of granular shock isolators.     

Magnetohydrodynamic peristaltic flow of a hyperbolic tangent fluid in a vertical asymmetric channel with heat transfer

In the present paper we discuss the magnetohydrodynamic (MHD) peristaltic flow of a hyperbolic tangent fluid model in a vertical asymmetric channel under a zero Reynolds number and long wavelength approximation. Exact solution of the temperature equation in the absence of dissipation term has been computed and the analytical ex- pression for stream function and axial pressure gradient are established. The flow is analyzed in a wave frame of reference moving with the velocity of wave. The expression for pressure rise has been computed numerically. The physical features of pertinent parameters are analyzed by plotting graphs and discussed in detail.