Displacement Of Medium Research Articles

Miscible porous media displacements in the quarter five-spot configuration. Part 2. Effect of heterogeneities

Direct numerical simulations are employed to investigate the coupling between the viscous fingering instability and permeability heterogeneities for miscible displacements in quarter five-spot flows. Even moderate inhomogeneities are seen to have a strong effect on the flow, which can result in a complete bypass of the linear growth phase of the viscous fingering instability. In contrast to their homogeneous counterparts (cf. Part 1, Chen & Meiburg 1998), heterogeneous quarter five-spot flows are seen to exhibit a more uniform dominant length scale throughout the entire flow domain. In line with earlier findings for unidirectional displacements, an optimal interaction of the mobility and permeability related vorticity modes can occur when the viscous length scale is of the same order as the correlation length of the heterogeneities. This resonance mechanism results in a minimal breakthrough recovery for intermediate correlation lengths, at fixed dimensionless flow rates in the form of a Péclet number Pe. However, for a constant correlation length, the recovery does not show a minimum as Pe is varied.Confirming earlier observations, the simulations show a more rapid breakthrough as the variance of the permeability variations increases. However, this tendency is far more noticeable in some parameter regimes than in others. It is furthermore observed that relatively low variances usually cannot change the tendency for a dominant finger to evolve along the inherently preferred diagonal direction, especially for relatively small correlation lengths. Only for higher variances, and for larger correlation lengths, are situations observed in which an off-diagonal finger can become dominant. Due to the nonlinear nature of the selection mechanisms at work, a change in the variance of the heterogeneities can result in the formation of dominant fingers along entirely different channels.

Miscible porous media displacements in the quarter five-spot configuration. Part 1. The homogeneous case

A detailed two-part computational investigation is conducted into the dynamical evolution of two-dimensional miscible porous media flows in the quarter five-spot arrangement of injection and production wells. High-accuracy direct numerical simulations are performed that reproduce all dynamically relevant length scales in solving the vorticity–streamfunction formulation of Darcy's law. The accuracy of the method is demonstrated by a comparison of simulation data with linear stability results for radial source flow.Within this part, Part 1 of the present investigation, a series of simulations is discussed that demonstrate how the mobility ratio and the dimensionless flow rate denoted by the Péclet number Pe affect both local and integral features of homogeneous displacement processes. Mobility ratios up to 150 and Pe-values up to 2000 are investigated. For sufficiently large Pe-values, the flow near the injection well gives rise to a vigorous viscous fingering instability. As the unstable concentration front approaches the central region of the domain, nonlinear interactions between the fingers similar to those known from unidirectional flows are observed, such as merging, partial merging, and shielding, along with secondary tip-splitting and side-branching instabilities. At large Pe-values, several of these fingers compete for long times, before one of them accelerates ahead of the others and leads to the breakthrough of the front.In contrast to unidirectional flows, the quarter five-spot geometry imposes both an external length scale and a time scale on the flow. The resulting spatial non-uniformity of the potential base flow is observed to lead to a clear separation in space and time of large and small scales in the flow. Small scales occur predominantly during the early stages near the injection well, and at late times near the production well. The central domain is dominated by larger scales.Taken together, the results of the simulations demonstrate that both the mobility ratio and Pe strongly affect the dynamics of the flow. While some integral measures, such as the recovery at breakthrough, may show only a weak dependence on Pe for large Pe-values, the local fingering dynamics continue to change with Pe.The increased susceptibility of the flow to perturbations during the early stages provides the motivation to formulate an optimization problem that attempts to maximize recovery, for a constant overall process time, by employing a time-dependent flow rate. Within the present framework, which accounts for molecular diffusion but not for velocity-dependent dispersion, simulation results indeed indicate the potential to increase recovery by reducing the flow rate at early times, and then increasing it during the later stages.

Social Impact Research

One measure of the impact of the personal computer is in terms of its time displacement of other activities. Mass media generally are considered a prime candidate given the on-line profusion of information and entertainment services. This article compares the daily mass media habits of heavy, light, and nonusers of personal computers and on-line services based on 1994 and 1995 national surveys conducted by the Times-Mirror Center for the People and the Press. No significant or consistent evidence of time displacement of mass media was found. Indeed, heavier computer usage was associated with significantly greater use of the print media and movies, even after multivariate adjustment. It is concluded that at this stage of diffusion, personal computers may have more in common with "time-enhancing" home appliances (e.g., automobile, washing machine) than they do with television. Nonetheless, more complete time diary studies are needed to identify the full temporal effects of the personal computer, much as they did for television.

The step-loading model of subsidence induced by ground-water level changes with time : Lay Seng Yaw & Shieh Ming Jyh, in: Land subsidence. Proc. international symposium, The Hague, 1995, ed F.B.J. Barends & others, (IAHS; Publication, 234), 1995, pp 313–321

This paper tries to develop a step-loading model for simulating the complex stress of soil deposits, and utilizes the Biot consolidation theory and superposition concept to analyse subsidence phenomena induced by changes in groundwater level with time. Finally, the in situ investigation data of Wang-Kon, which is located in the coastal region of Chang-Hwa, Taiwan, will be checked in order to prove the usefulness of this model. INTRODUCTION The most serious subsidence problems at Taiwan coastal areas is due to overpumping groundwater. The groundwater levels fluctuate repeatedly, because of fluctuations in the amount of groundwater withdrawal, and fluctuations in rainfall. This study looks at the temporal effects on subsidence of the repeated processes of loading, unloading, reloading on soil deposits and the cyclic changes in groundwater levels. Studies of surface consolidation settlement induced by groundwater pumping can be divided into two categories, one is the decoupled approach method based on Terzaghi's (Terzaghi, 1943; Taylor, 1948) consolidation theory, the other is the coupled approach method based on Biot's (Biot, 1941, 1955) consolidation theory. The former first looks at the pore water pressure distribution, then calculates the strain of the soil deposits and total settlement of the ground surface by applying the concept of effective stress. The latter is a couple theory by adopting the displacement of porous media and the pore water pressure as the basic variables, which is based on the interaction of pore water and porous media. This couple consolidation theory is generally recognized to be more reasonable. Therefore, this paper not only uses the couple consolidation theory of Biot to study the subsidence problem, but also develops a step-loading model to simulate the complex stress behaviour of soil deposits due to changing water levels, in order to analyse the effects on land subsidence with time. BIOT COUPLE CONSOLIDATION THEORY The analytical model of land subsidence in this paper is based on the soil couple consolidation theory. The couple consolidation theory concerning soil was firstly presented by Biot (1941), then Verruijt (1969), Bear & Corapcioglu (1981) and Corapcioglu & Bear (1983) etc. studied this theory and derived consolidation models from the viewpoint of flowing groundwater. Their basic assumptions: 314 Lay Seng Yaw and Shieh Ming Jyh (a) fully saturated, (b) pore water is compressible, (c) solid grain is incompressible, (d) follows Darcy's law, (e) isotropic and homogeneous, (f) small strains, (g) linearity of stress-strain relations, (h) the main factor to affect porosity is the effective stress of medium. According to the above assumptions, the basic equation of this coupled consolidation theory can be expressed as follows: GVu+x 1 G ds 2v dx dx •EE =o GV«V + _ ^ _ ^ £ ^ =o y l-2vdy dy in these relations, GVV -kVp + s, V G ds _ l-2vdz ds 0dp dt dt = dldx + d dp _ dz

Entropy in the phenomenon of granular media displacement

The displacements of a loose medium in the gravitational field are discussed regarding it as a discrete medium. For the given boundary conditions the elements of this medium may form various configurations in the course of the displacements Boltzmann's concept of entropy has been adopted in the form of a functional depending on the density function of the medium elements in the configurations that are being formed. For the boundary conditions assumed in the form of the so-called Dirac's function the maximum value of entropy has been determined which corresponds to the greatest probability of the occurrence of configurations of the elements forming the discrete medium. It can be found that the density function satisfying this condition has the form of Gauss function. The measurements of displacements of a loose medium confirm this result. These displacements satisfy Boltzmann's universal principle according to which during the displacements of a loose medium there are formed configurations in which the entropy attains its maximum value. This accounts for the wide application of Oauss function as the so-called fundamental solution in the integral formulae describing the displacements of the carth's crust in regions where it underwent destruction as a result of underground mining exploitation.

A miniature flow cell designed for rapid exchange of media under high-power microscope objectives.

The design, fabrication and use of a flow cell that allows rapid displacement of media viewed by short working distance, high power objectives are described. The cell has a small internal depth (about 0.04 cm), small volume (about 0.05 ml), and is chemically inert. It has been tested extensively in studies of tethered bacteria.

Soil-Foundation-Structure-Interaction

A review of the results of the long-term research work is presented. The theoretical basis of this study is the three phase structural concept. Structural and mathematical modeling of phase components, — namely, soil, foundation, and superstructure — is covered. The elastoplastic halfspace approach is adequate to simulate the state of displacement of soil media. A three-dimensional structural analysis is required to describe the stiffness response. Tranversal flexural rigidity of the structure is derived from the height. Since soil and concrete have viscoelastic-plastic properties, loading history according to the erection process and time history following consolidation phenomena, have been investigated. Local measurements carried out on full scale models have shown a close correlation with the calculated results.