Elliptic random-walk equation for suspension and tracer transport in porous media

We propose a new approach to transport of the suspensions and tracers in porous media. The approach is based on a modified version of the continuous time random walk (CTRW) theory. In the framework of this theory we derive an elliptic transport equation. The new equation contains the time and the mixed dispersion terms expressing the dispersion of the particle time steps. The properties of the new equation are studied and the fundamental analytical solutions are obtained. The solution of the pulse injection problem describing a common tracer injection experiment is studied in greater detail. The new theory predicts delay of the maximum of the tracer, compared to the velocity of the flow, while its forward “tail” contains much more particles than in the solution of the classical parabolic (advection-dispersion) equation. This is in agreement with the experimental observations and predictions of the CTRW theory.     

Tracer (self)-diffusion of potassium ion in NaCl−KCl−H2O mixtures at 25°C

Tracer (self)-diffusion coefficients of K⁺ have been measured using the diaphragm cell in seven compositions of the NaCl–KCl–H₂O system. These data complete the full set of isothermal vector transport properties for five of these compositions.     

井间示踪剂测试的数值模拟方法

根据油田实际情况，建立了三维两相（油、水）四组分（油、水、分配性及非分配性示踪剂）的数学模型，并编制成计算机软件．通过对示踪剂产出曲线的数值分析，得到地层中任意时刻的压力及剩余油饱和度分布，进而可对油藏进行精细描述．现场应用实例表明，用数值模拟方法求出的地层参数的精度较为可靠．     

同位素示踪技术在潘北矿太原组灰岩水水化学特征中的应用

潘北矿深部煤层开采工作面突水水源主要来自第四系松散层水、煤层上覆顶板砂岩裂隙水和下伏底板太原组石灰岩水、奥陶纪和寒武纪白云岩水.为了探讨太原组石灰岩水与其它水体之间的水力联系.采集潘北矿主要含水层水样品25件,采用MAT 253同位素质谱仪测试样品中氢氧同位素值,研究结果表明:(1)太原组石灰岩水主要表现为三种水质化学类型;(2)太原组石灰岩水的氢氧同位素含量差别较大;(3)太原组石灰岩水的主要补给源为地表水体和深部古水.     

基于Packet Tracer的IPSec VPN实验教学设计

针对当前网络实验教学的要求和特点,提出运用Packet Tracer软件提供的虚拟平台进行仿真实验教学设计。解决当前网络实验室建设中的重要问题。介绍了IPSec VPN的工作原理,重点探讨基于Packet Tracer的IPSec VPN实验教学的设计过程。     

Relation between the gel structure and the mobility of tracers in globular protein gels

Diffusion of fluorescent-labeled dextran with different molecular weights was investigated in β-lactoglobulin (β-lg) solutions and gels over a wide range of salt and protein concentrations at pH 7 by combining confocal laser scanning microscope (CLSM) with fluorescence recovery after photobleaching (FRAP). Effects of the protein concentration, the salt concentration and the tracer size were investigated in detail. Diffusion in turbid heterogeneous gels formed at 0.2 M NaCl depended weakly on the probe size and the protein concentration and remained close to that in unheated solutions. A strong decrease of the diffusion coefficient with increasing tracer size and protein concentration was observed in more homogeneous gels formed at lower salt concentrations. Larger dextran chains were trapped in transparent gels formed at NaCl concentration below 0.1 M. The present investigation complements an earlier study of tracer diffusion of larger spherical probes in β-lg gels using multi-particle tracking.     

PIV with volume lighting in a narrow cell: An efficient method to measure large velocity fields of rapidly varying flows

In this work we test a methodology for PIV measurements when a large field of view is required in planar confined geometries. Using a depth of field larger than the channel width, we intend to measure the in-plane variations of the velocity of the fluid averaged through the width of the channel, and we examine in which operating conditions this becomes possible. Measurements of the flow through a narrow channel by PIV are challenging because of the strong velocity gradients that develop between the walls. In particular, all techniques that use small particles as tracers have to deal with the possible migration of the tracers in the direction perpendicular to the walls. Among the complex mechanisms for migration, we focus on the so called Segré-Silberberg effect which can lead to transverse migration of neutrally buoyant tracers of finite size. We report experimental PIV measurements in a Hele-Shaw cell of 1 mm gap, which have been carried out by using neutrally buoyant tracers of size around 10 μm. By considering steady flows, we have observed, in particular flow regimes, the effect of an accumulation of the tracers at a certain distance to the wall due to the so called Segré-Silberberg effect. The particle migration is expected to occur at any Reynolds numbers but the migration velocity depends on the Reynolds number. A significant migration therefore takes place each time the observation duration is large enough compared to the migration time. For a given observation duration, the tracers remain uniformly distributed at low Reynolds numbers whereas they all accumulate at the equilibrium position at large ones. When using volume lighting, the PIV algorithm provides the average velocity of the flow through the gap at low Reynolds number, while it leads to the velocity of the flow at the equilibrium position of the tracers at large Reynolds numbers. By considering unsteady flows, we have observed that the migration does not occur if the timescale of flow variation is short compared to the time required for the parabolic flow to develop across the gap. In this case, there is no transverse velocity gradient and the PIV algorithm provides the fluid velocity. Altogether, these results allow us to propose guidelines for the interpretation of PIV measurements in confined flow, which are based on the theoretical predictions of the tracer migration derived by Asmolov [1].     

Ballistic motion of a tracer particle coupled to a Bose gas

We study the motion of a heavy tracer particle weakly coupled to a dense interacting Bose gas exhibiting Bose–Einstein condensation. In the so-called mean-field limit, the dynamics of this system approaches one determined by nonlinear Hamiltonian evolution equations. We derive the effective dynamics of the tracer particle, which is described by a non-linear integro-differential equation with memory, and prove that if the initial speed of the tracer particle is below the speed of sound in the Bose gas the motion of the particle approaches an inertial motion at constant velocity at large times.     

Upscaling,relaxation and reversibility of dispersive flow in stratified porous media

Dispersive tracer released in a unidirectional velocity field belonging to a stratified porous of finite height describes a transition, called relaxation, from a convective dominated behaviour for short times to Fickian behaviour for asymptotic long times. The temporal relaxation state of the tracer is controlled by the transverse mixing term. In most practical applications, the orders of the time and length scales of the relaxation mechanism are such that in an upscaled model of a stratified medium the dispersive flux is in a pre-asymptotic state. Explicit modelling of the relaxation of the dispersive flux in the pre-asymptotic region is required to improve the accuracy. This paper derives a pre-asymptotic one-dimensional upscaled model for the transverse averaged tracer concentration. The model generalises Taylor dispersion (Proc. R. Soc. London 219, 186–203 (1953)) and extends the method of Camacho (Phys. Rev. E 47(2), 1049–1053 (1993a); Phys. Rev. E 48 (1993b)) to dispersion tensors that may vary as function of the transverse direction. In the averaging step, the governing two-dimensional equation is first spectrally decomposed in terms of the eigenfunctions of the transverse mixing term. Next, the resulting modal relaxation equations are combined into an effective relaxation equation for the extended dispersive Taylor flux. Contrary to the one-dimensional Fickian approach, the upscaled model approximates the multi-scale relaxation behaviour as a single scale relaxation process and accounts for the partial reversibility of convective dispersion upon reversal of the flow direction. The upscaled model is evaluated against the original two-dimensional model by means of moment analysis. The longitudinal tracer variance predicted by our model is quantitatively correct in the short and long time limits and is qualitatively correct for intermediate times.     

质谱同位素稀释法测定U_3O_8标样中痕量Th

以~(230)Th作稀释剂用IDMS测定了U_3O_8标样中痕量Th。取样1g,先在9mol/l HCI体系中用阴离子交换法除去大量U及Fe、Pb等杂质,进而以CL5209型萃淋树脂分离Th。本法测Th的检出限为2.4×10~(-9)g。分析含Th 0.873 ppm 的U_3O_3试样时方法的精密度为±3.3%。