Search results for "medium"
showing 10 items of 3746 documents
Quantitative comparison of mean field mixing laws for conductivity and dielectric constants of porous media
2003
Abstract Exact numerical solution of the electrostatic disordered potential problem is carried out for four fully discretized three-dimensional experimental reconstructions of sedimentary rocks. The measured effective macroscopic dielectric constants and electrical conductivities are compared with parameter-free predictions from several mean field type theories. All these theories give agreeable results for low contrast between the media. Predictions from local porosity theory, however, match for the entire range of contrast.
Quantitative analysis of numerical estimates for the permeability of porous media from lattice-Boltzmann simulations
2010
During the last decade, lattice-Boltzmann (LB) simulations have been improved to become an efficient tool for determining the permeability of porous media samples. However, well known improvements of the original algorithm are often not implemented. These include for example multirelaxation time schemes or improved boundary conditions, as well as different possibilities to impose a pressure gradient. This paper shows that a significant difference of the calculated permeabilities can be found unless one uses a carefully selected setup. We present a detailed discussion of possible simulation setups and quantitative studies of the influence of simulation parameters. We illustrate our results b…
Numerical simulation of creeping fluid flow in reconstruction models of porous media
2002
Abstract In this paper we examine representative examples of realistic three-dimensional models for porous media by comparing their geometry and permeability with those of the original experimental specimen. The comparison is based on numerically exact evaluations of permeability, porosity, specific internal surface, mean curvature, Euler number and local percolation probabilities. The experimental specimen is a three-dimensional computer tomographic image of Fontainebleau sandstone. The three models are stochastic reconstructions for which many of the geometrical characteristics coincide with those of the experimental specimen. We find that in spite of the similarity in the geometrical pro…
On multi-scale percolation behaviour of the effective conductivity for the lattice model
2015
Macroscopic properties of heterogeneous media are frequently modelled by regular lattice models, which are based on a relatively small basic cluster of lattice sites. Here, we extend one of such models to any cluster's size kxk. We also explore its modified form. The focus is on the percolation behaviour of the effective conductivity of random two- and three-phase systems. We consider only the influence of geometrical features of local configurations at different length scales k. At scales accessible numerically, we find that an increase in the size of the basic cluster leads to characteristic displacements of the percolation threshold. We argue that the behaviour is typical of materials, w…
Local porosity theory for electrical and hydrodynamical transport through porous media
1993
The current status of local porosity theory for transport in porous media is briefly reviewed. Local porosity theory provides a simple and general method for the geometric characterization of stochastic geometries with correlated disorder. Combining this geometric characterization with effective medium theory allows for the first time to understand a large variety of electrical and hydrodynamical flow experiments on porous rocks from a single unified theoretical framework. Rather than reproducing or rephrasing the original results the present review attempts instead to place local porosity theory within the context of other current developments in theory and experiment.
A linear open-chain piperazine-pyridine ligand and its meso-helical Co complex
1998
Abstract An oligomeric ligand (HPPy) 2 P, N , N ′-bis[2-(6-( N -( N ′-(2-hydroxyethyl)piperazinyl)methyl)]piperazine, was designed to resemble structurally open-chain aza-crowns. Owing to the all- trans configuration of piperazine and pyridine free electron pairs, it should adopt a near linear overall structure in solvent. Theoretical calculations at ab initio level confirm the overall linear structure of free ligand. The crystal structure of the complex [(HPPy) 2 P][Co(NO 3 ) 2 ] 2 shows a contraction from ∼3 to 2 nm structure. Each coordination site creates either a Δ or Δ configuration around the metal ion, thus causing a ligand with an even number of coordination centres to be meso -hel…
Solid state conformational and theoretical study of complexes containing the (CxN)Pd moiety (CxN = 2-(phenylazo)phenyl-C,N and its derivatives)
2003
Palladium complexes having the 2-(phenylazo)phenyl-C,N ligand exhibit a planar chelating ring with NN and N–C distances longer and shorter respectively than those found in trans-azobenzene. The ligand is not planar upon complexation, the mean angle between the phenyl ring and the chelating one found in the Cambridge Structural Database being of 45.6°. We have quantified and characterised the kind of distortion from planar coordination around metallic centers. The method employed makes use of two improper torsion angles, tetrahedral distortion being most frequently found in phenylazophenyl palladium complexes. Crystal structures of three succinimidate complexes having the title moiety are re…
Epoxy-Acetogenins and other Polyketide Epoxy Derivatives as Inhibitors of the Mitochondrial Respiratory Chain Complex I
2000
Annonaceous acetogenins (ACG), an extensive group of cytotoxic natural products, are antitumor agents whose main mode of action is inhibition of the mammalian mitochondrial complex I. Herein we describe the importance of the different chemical groups along the alkyl chain for optimal inhibitory potency, discussing the structurally relevant factors present in these compounds. For this purpose, a series of epoxide derivatives from alpha-linolenic acid were prepared and their activity compared with that of epoxy-acetogenins and tetrahydrofuranic (THF) acetogenins isolated from Rollinia membranacea.
Steroidal saponins from two species of Dracaena.
2010
Four new steroidal saponins (1-4) were isolated from the stem and bark of two species of Dracaena: deistelianosides A and B (1 and 2) from D. deisteliana and arboreasaponins A and B (3 and 4) from D. arborea. Six known saponins and one known sapogenin were also isolated. The structures of 1-4 were established as diosgenin 3-O-[3-O-sulfate-alpha-l-rhamnopyranosyl-(1-->4)]-beta-d-glucopyranoside (1), 1-O-beta-d-xylopyranosyl-(1-->2)-[alpha-l-rhamnopyranosyl-(1-->3)]-beta-d-fucopyranosyl(23S,24S)-spirosta-5,25(27)-diene-1beta,3beta,23alpha,24alpha-tetrol 24-O-alpha-l-arabinopyranoside (2), pennogenin-3-O-alpha-l-rhamnopyranosyl-(1-->2)-[alpha-l-rhamnopyranosyl-(1-->3)]-[6-O-acetyl]-beta-d-gluc…
Intermetallic compounds of the heaviest elements: the electronic structure and bonding of dimers of element 112 and its homolog Hg
2002
Abstract Fully relativistic (four-component) density-functional calculations were performed for the element 112 dimers (112)X (X = Pd, Cu, Ag and Au) and those of its lighter homolog, Hg. A relatively small decrease of about 15–20 kJ/mol in bonding was found from the HgX to (112)X compounds. Respectively, the bond lengths were increased by 0.06 A on the average. The Mulliken population analysis has shown this effect to be a result of a decreasing contribution of the relativistically stabilized 7s-AO of element 112 to bonding. The following trend in the binding energies was predicted for (112)X as a function of X: Pd >Cu>Au>Ag, exactly as the trend obtained experimentally for adsorption of H…