Theoretical Simulation of Experimental Observations of Surface Wave Propagation on a Fluid-Saturated Porous Material

Wave propagation in fluid-saturated porous materials presents very particular features like the appearance of a second compressional wave, the so-called slow compressional wave, in addition to the conventional P (or fast compressional) and the shear wave [1,2]. First experimental observation of the slow compressional wave was carried out by Plona in 1980 in water-saturated porous ceramics at ultrasonic frequencies [3]. In 1983 Feng and Johnson predicted the existence of a new surface mode along a fluid/fluid-saturated porous solid interface, in addition to the well-known leaky-Rayleigh and true Stoneley modes [4,5]. Feng and Johnson introduced the so-called surface stiffness, T, as a parameter which describes the boundary conditions at the interface. For a value of T=0 the pores at the surface are considered open, whereas for a value of T=∞ they are considered to be closed. However, according to the theory this new surface mode appears only when closed pores boundary conditions prevail at the interface. This last restriction renders the observation of the new mode problematic, because the extreme difficult in closing the surface pores without clogging all the pores close to the surface (e.g. by painting). In 1992 Nagy observed experimental evidence of the slow surface wave [6]. Nagy demonstrated that capillary forces can extend an ideally thin membrane over the surface pores at the interface between a porous solid saturated with a wetting fluid (e.g. water or alcohol) and a non-wetting fluid (e.g. air). Under this conditions, experimental evidence of a simple form of the new surface wave mode predicted by Feng and Johnson during alcohol saturation of a sintered glass beads specimen was obtained. However, due to problems inherent to the excitation of surface waves in fluid-saturated porous solids (e.g. extremely high attenuation, small propagation lengths, etc.) the results were not conclusive. In this work we will show that the experimental evidence of slow surface wave can be predicted by the analytical method of Feng and Johnson [5], if slight modifications are introduced into the calculation technique in order to account for some of the particular characteristics of the experiment

Surface Wave Inspection of Porous Ceramics and Rocks

The most interesting feature of acoustic wave propagation in fluid-saturated porous media is the appearance of a second compressional wave, the so-called slow compressional wave, in addition to the conventional P (or fast) wave and the shear wave [1,2]. The slow compressional wave is essentially the motion of the fluid along the tortuous paths in the porous frame. This motion is strongly affected by viscous coupling between the fluid and the solid. Therefore, both the velocity and the attenuation of the slow wave greatly depend on the dynamic permeability of the porous frame. It was not until 1980, that Plona first experimentally observed the slow compressional wave in water-saturated porous ceramics at ultrasonic frequencies [3]. Only three years later, Feng and Johnson predicted the existence of a new slow surface mode on a fluid/fluid-saturated solid interface in addition to the well-known leaky-Rayleigh and true Stoneley modes [4,5]. The slow surface mode is basically the interface wave equivalent of the slow bulk mode, but there is a catch: the surface pores of the solid have to be closed so that this new mode can be observed. Otherwise, a surface vibration can propagate along the fluid/fluid-saturated porous solid interface without really moving the fluid since it can flow through the open pores without producing any significant reaction force. All previous efforts directed at the experimental observation of this new surface mode failed because of the extreme difficulty of closing the surface pores without closing all the pores close to the surface (e. g., by painting). On the other hand, it has been recently shown that surface tension itself could be sufficient to produce essentially closed-pore boundary conditions at the interface between a porous solid saturated with a wetting fluid, such as water or alcohol, and a non-wetting superstrate fluid, like air [6]

On Nonspecular Reflection of Bounded Beams for Layered Half Spaces Under Water

We study the recently derived reflection coefficient for plane waves in a liquid that are incident on the liquid-solid interface of a solid half space which consists of a single layer of one elastic material bonded to a substrate of a different material. Plots of the magnitude of the reflection coefficient versus the incident angle are presented for several sets of material parameters and values of frequency f and layer thickness d. The use of the results presented for the study of nonspecular reflection of bounded acoustic beams is of primary interest. We therefore seek to identify all the critical incidence angles for nonspecular reflection. We also investigate, in particular, the surface wave propagation for the case of a stiff layer on a soft half space, and we find that the purely propagating mode cuts off with increasing fd (f is the frequency and d the layer thickness) when its speed reaches approximately the shear wave speed of the substrate, as reported in the literature. However, as fd increases further, a leaky mode appears that approaches the Rayleigh wave for the layer. This leaky mode is also associated with nonspecular reflection for large enough fd.</p

Generalized Lamb Modes in Fluid-Saturated Porous Plate

Since analysis by Rayleigh [1] and Lamb [2], the vibration modes for an elastic homogeneous infinite solid thin plate are well understood. These so-called “Lamb modes” result from a pure compressional wave and pure shear wave. Similarly, excitation of “leaky Lamb modes” in elastic plates immersed in a fluid, caused by incident acoustic waves, has been extensively described theoretically [3–7] and experimentally [8,9]. Results are generally presented as dispersion curves which relate the phase velocity of the mode to the product of frequency and plate thickness

Railway track possession assignment using constraint satisfaction

Resource allocation is the problem of allocating a set of resources to accomplish some task(s). Many real-world problems are resource-allocation problems, such as production planning and manpower planning. This paper reports on a case study on applying constraint-satisfaction techniques to solve a real-world resource allocation problem, referred to as the Railway Track Possession Assignment Problem, using the CHIP constraint language. The problem is to assign railway tracks to a given set of scheduled maintenance tasks according to a set of constraints. The manual problem-solving method is heuristic in nature. Experienced personnel were involved in the manual process. An expert system, called the Engineering Work Track Possession Assignment System (EWTPAS), was developed to carry out the assignment using constraint-satisfaction techniques. A new, two-phase resource allocation strategy based on constraint relaxation was developed and implemented in EWTPAS. EWTPAS succeeded in replacing the manual assignment process after test running for 1 year. EWTPAS is now in use. Besides having the advantage of being free of careless human errors, and the advantage of being independent of the availability of experienced staff, EWTPAS was found to be about 10 times more efficient than the manual method. © 1999 Elsevier Science Ltd. All rights reserved.link_to_subscribed_fulltex

A heavy duty universal direct sunlight heliodon

On a mesoscale the pore structure in natural rocks is strongly inhomogeneous. With an increase of the scale size one may find that the pore structure has preferred orientation (texture) which leads to anisotropy of permeability and tortuosity. In this paper the Biot theory was applied to an orthotropic fluid saturated porous medium. Such a medium supports four different wave types: fast quasilongitudinal, two quasishear and slow quasilongitudinal waves. The properties of the orthotropic frame are described by the nine independent elastic constants of the dry frame. Pore structure characteristics such as tortuosity, permeability and shape factor become direction-dependent and in the coordinate system collinear with the acoustical axes each of these parameters is represented by a second-rank tensor with the only non-zero elements on the diagonal. Our results show that the velocities of quasilongitudinal and two quasishear waves depend mostly on the properties of the frame and are not sensitive to the permeability and tortuosity directly (the frame stiffnesses, permeability and tortuosity are indirectly related due to dependence on pore structure; however they can formally be considered as independent parameters). Thus by measuring these velocities one can determine the frame elastic constants. The slow wave velocity, on the contrary, depends mostly on the pore geometry. Its angular dependence in a water- or air-saturated solid allows us to recover the components of the permeability and tortuosity tensors. This approach opens new possibilities for determination of such characteristics of porous materials as preferred pore orientation and tortuosity which have been previously inaccessible experimentally and thus to retrieve information about the pore structure

Intracellular Zn2+ accumulation contributes to synaptic failure, mitochondrial depolarization, and cell death in an acute slice oxygen-glucose deprivation model of ischemia

Despite considerable evidence for contributions of both Zn2+ and Ca2+ in ischemic brain damage, the relative importance of each cation to very early events in injury cascades is not well known. We examined Ca2+ and Zn2+ dynamics in hippocampal slices subjected to oxygen-glucose deprivation (OGD). When single CA1 pyramidal neurons were loaded via a patch pipette with a Ca2+-sensitive indicator (fura-6F) and an ion-insensitive indicator (AlexaFluor-488), small dendritic fura-6F signals were noted after several (∼6-8) minutes of OGD, followed shortly by sharp somatic signals, which were attributed to Ca2+ ("Ca2+ deregulation"). At close to the time of Ca2+ deregulation, neurons underwent a terminal increase in plasma membrane permeability, indicated by loss of AlexaFluor-488 fluorescence. In neurons coloaded with fura-6F and a Zn2+-selective indicator (FluoZin-3), progressive rises in cytosolic Zn2+ levels were detected before Ca2+ deregulation. Addition of the Zn2+ chelator N,N,N′,N′-tetrakis(2- pyridylmethyl)ethylenediamine (TPEN) significantly delayed both Ca2+ deregulation and the plasma membrane permeability increases, indicating that Zn2+ contributes to the degenerative signaling. Present observations further indicate that Zn2+ is rapidly taken up into mitochondria, contributing to their early depolarization. Also, TPEN facilitated recovery of the mitochondrial membrane potential and of field EPSPs after transient OGD, and combined removal of Ca2+ and Zn2+ markedly extended the duration of OGD tolerated. These data provide new clues that Zn2+ accumulates rapidly in neurons during slice OGD, is taken up by mitochondria, and contributes to consequent mitochondrial dysfunction, cessation of synaptic transmission, Ca2+ deregulation, and cell death. Copyright © 2009 Society for Neuroscience.link_to_subscribed_fulltex

Seismic Wave Attenuation In Fluid-Saturated Porous Media

Contrary to the traditional view, seismic attenuation in Biot&apos;s theory of fluid-saturated porous media is due to viscous damping of local (not global) porefluid motion. Since substantial inhomogeneities in fluid permeability of porous geological materials are to be expected, the regions of highest local permeability contribute most to the wave energy dissipation while those of lowest permeability dominate the fluid flow rate if they are uniformly distributed. This dichotomy can explain some of the observed discrepancies between computed and measured attenuation of compressional and shear waves in porous earth. One unfortunate consequence of this result is the fact that measured seismic wave attenuation in fluid-filled geological materials cannot be used directly as a diagnostic of the global fluid-flow permeability. -- 2 -- INTRODUCTION The low frequency limit of Biot&apos;s theory of fluid-saturated porous media (Biot, 1956ab) predicts that the coefficients for viscous attenuation of s..