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Homogeneous Media (homogeneous + media)
Selected AbstractsRequired source distribution for interferometry of waves and diffusive fieldsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2009Yuanzhong Fan SUMMARY The Green's function that describes wave propagation between two receivers can be reconstructed by cross-correlation provided that the receivers are surrounded by sources on a closed surface. This technique is referred to as ,interferometry' in exploration seismology. The same technique for Green's function extraction can be applied to the solution of the diffusion equation if there are sources throughout in the volume. In practice, we have only a finite number of active sources. The issues of the required source distribution is investigated, as is the feasibility of reconstructing the Green's function of the diffusion equation using a limited number of sources within a finite volume. We study these questions for homogeneous and heterogeneous media for wave propagation and homogeneous media for diffusion using numerical simulations. These simulations show that for the used model, the angular distribution of sources is critical in wave problems in homogeneous media. In heterogeneous media, the position and size of the heterogeneous area with respect to the sources determine the required source distribution. For diffusion, the sensitivity to the sources decays from the midpoint between the two receivers. The required width of the source distribution decreases with frequency, with the result that the required source distribution for early- and late-time reconstruction is different. The derived source distribution criterion for diffusion suggests that the cross-correlation-based interferometry is difficult to apply in field condition. [source] Seismic wave properties in time-dependent porosity homogeneous mediaGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2007G. Quiroga-Goode SUMMARY It is quantified the properties of seismic waves in fully saturated homogeneous porous media within the framework of Sahay's modified and reformulated poroelastic theory. The computational results comprise amplitude attenuation, velocity dispersion and seismic waveforms. They show that the behaviour of all four waves modelled as a function of offset, frequency, porosity, fluid viscosity and source bandwidth depicts realistic dissipation within the sonic,ultrasonic band. Therefore, it appears that there is no need to include material heterogeneity to model attenuation. By inference it is concluded that the fluid viscosity effects may be enhanced by dynamic porosity. [source] Geometrical interpretation of the multi-point flux approximation L-methodINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 11 2009Yufei Cao Abstract In this paper, we first investigate the influence of different Dirichlet boundary discretizations on the convergence rate of the multi-point flux approximation (MPFA) L-method by the numerical comparisons between the MPFA O- and L-method, and show how important it is for this new method to handle Dirichlet boundary conditions in a suitable way. A new Dirichlet boundary strategy is proposed, which in some sense can well recover the superconvergence rate of the normal velocity. In the second part of the work, the MPFA L-method with homogeneous media is studied. A systematic concept and geometrical interpretations of the L-method are given and illustrated, which yield more insight into the L-method. Finally, we apply the MPFA L-method for two-phase flow in porous media on different quadrilateral grids and compare its numerical results for the pressure and saturation with the results of the two-point flux approximation method. Copyright © 2008 John Wiley & Sons, Ltd. [source] Computer solutions of Maxwell's equations in homogeneous mediaINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 8 2003O. Pironneau Abstract This document is the material support for a talk given for JSIAM on the current methods for the computation of radar cross sections. The talk covers more than just computations of RCS and extends to any problem which involves the numerical solution of Maxwell's equations in homogeneous media. The talk is based on a review of the most recent papers in leading journals and on the author's experience. Copyright © 2003 John Wiley & Sons, Ltd. [source] Collective Optical Behavior of Cationic Water-Soluble Dendrimers,ADVANCED MATERIALS, Issue 23-24 2004S. Wang Water-soluble dendrimers containing cationic charges and optically active units on the periphery of the macromolecular structure have been designed. Collective optical behavior of the chromophores on the surface was demonstrated by fluorescence quenching and energy-transfer experiments. These water-soluble dendrimers can be used for optically amplified DNA detection (see Figure) in homogeneous media. [source] A CFL-like constraint for the fast marching method in inhomogeneous chemical kineticsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2008Ramón Escobedo Abstract Level sets and fast marching methods are a widely used technique for problems with moving interfaces. Chemical kinetics has been recently added to this family, for the description of reaction paths and chemical waves in homogeneous media, in which the velocity of the interface is described by a given field. A more general framework must consider variable velocities due to inhomogeneities induced by chemical changes. In this case, a constraint must be satisfied for the correct use of fast marching method. We deduce an analytical expression of this constraint when the Godunov scheme is used to solve the Eikonal equation, and we present numerical simulations of a case which must be enforced to obey the constraint. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Effect of chemical heterogeneity on adsorbed solute dispersion at column scaleAICHE JOURNAL, Issue 4 2008Safia Semra Abstract Chemical heterogeneity seems to be responsible for spreading increase of adsorbed solute breakthrough curves. Adsorption in fixed beds assumes chemically homogeneous media. However, this is not always true, in particular when natural sands or mixed adsorbent filters are used in drinking water purification. Neglecting eventual effect of chemical heterogeneity may engender false modeling bases. So, considering homogeneous grain size distribution, the effect of chemical heterogeneity on global dispersion in porous media has been investigated experimentally in this article at column scale. Breakthroughs of adsorbed solute showed a visible effect of chemical heterogeneity on solute global dispersion increasing. The more heterogeneous the medium, the more spread the adsorbed solute breakthrough. Reduced variance showed a linear variation with the chemical heterogeneity scale at closely constant media global capacity. A pseudo-homogeneous model has been developed to simulate experimental data by increasing dispersion parameter. © 2008 American Institute of Chemical Engineers AIChE J, 2008 [source] Radical clocks and electron transfer.JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 12 2005Comparison of crown ether effects on the reactivity of potassium, magnesium towards 1-bromo-2-(3-butenyl)benzene. Abstract The reaction of the title precursor of the aryl radical clock 1-bromo-2-(3-butenyl)benzene, 1Br, towards potassium and magnesium in THF was studied in the presence and absence of various additives, at ambient and low temperatures. The additives were cis -dicyclohexano-18-crown-6 or tert -butyl alcohol; the first one to render soluble potassium by forming its alkalide, the second to distinguish carbanionic from radical cyclization. The addition of 1Br to a THF stirred suspension of potassium pieces yields remarkably low amounts of products resulting from radical cyclization, in contrast to the amounts reported by Bunnett and Beckwith's group for the reaction in 67% ammonia,33% tert -butyl alcohol medium. The amount of cyclized products obtained with potassium pieces in THF is in the same range as that observed in the reaction of magnesium with 1Br in THF. This similarity allows us to discard the earlier triad hypothesis that we proposed to account for the unexpectedly low amounts of cyclized products of aryl halides radical clocks in Grignard reagent formation. The addition of crown ethers to the THF reaction medium induces contrasting effects for potassium and magnesium. A distinctive increase in the radical cyclization is observed for potassium, whereas the addition of crown inhibits the formation of Grignard reagent more efficiently when the solvent is diethyl ether than when it is THF. The observed effects are explained by putting in perspective the metal reactive dissolution with elementary steps occurring in the vicinity of a cathode. The reaction of potassium pieces or magnesium turnings is comparable to the heterogeneous electron transfer occurring at a cathode whereas the reaction of potassium in the presence of crown ether is comparable to homogeneous conditions of electron transfer obtained in redox catalysis. A discussion of the dianion hypothesis for the Grignard reaction of aryl halides is provided and the importance of considering the reactivity of reactive metal dissolution (or organic corrosion) in the framework of recent progress made in the modelling of electrode reactivity is emphasized. This paper shows that caution should be taken when radical clocks are used to study reactions at the metal,solution interface. More specifically, the non-observation of rearranged products from the radical clock (even for the very rapid ones) under these conditions does not necessary imply that there is no radical intermediate along the dominant reaction channel. This pattern of reactivity strongly contrasts with that usually observed when radical clocks are used in homogeneous media. The leading parameters in the rearranged/unrearranged products ratio seem to be the time that the reactive species (radical anions) created by the first electron transfer spend in the close vicinity of this surface, the rate constant of rearrangement of the radical formed by the cleavage of the radical anion and the redox properties of this radical. Copyright © 2005 John Wiley & Sons, Ltd. [source] | ||||||||||