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Homogeneous Medium (homogeneous + medium)
Selected AbstractsStereoselective Disposition of the Geminal Dimethyl Group in the Cyclization of Geranyl Acetate under Zeolite Confinement ConditionsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 19 2006Constantinos Tsangarakis Abstract The stereochemistry in the acid-catalysed biomimetic cyclization of [8,8,8-D3]geranyl acetate was examined in solution and under conditions of zeolite Y confinement. In the intrazeolite reaction the gem -allylic methyl group adopts a diastereoselective disposition in the cyclization product (64,% dr). In contrast, the gem -dimethyl disposition in a homogeneous medium (ClSO3H/2-nitropropane) proceeds with negligible diastereoselectivity (dr < 5,%). The enhanced diastereoselection within the zeolite is attributed to the proximity of the nucleophilic double bond to the intermediate carbocation, as a result of confinement (entropy effect). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Effects of toroidal HVDC ground electrode on earth-return circuitsEUROPEAN TRANSACTIONS ON ELECTRICAL POWER, Issue 5 2002W. Machczy This paper presents a method of evaluation of currents and potentials, excited by conductive effects of high-voltage direct-current (HVDC) transmission system, along two parallel earth-return circuits such as pipelines and cables buried in the vicinity of a toroidal ground electrode. It is assumed that the system considered is linear and that the earth is an isotropic, homogeneous medium of finite conductivity. Conductive coupling between earth-return circuits is taken into account, whereas the reaction of the conductors' currents on the electrode current is disregarded. The transmission line model of the conductor with earth-return, a segmental linear approximation of the curve of the primary earth potential distribution along the conductor and the concept of superposition have been used in the method. It should be pointed out, that the method does not require the time consuming numerical integration. The technical applications of the method are illustrated by examples. [source] On strike-slip faulting in layered mediaGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2002Maurizio Bonafede Summary We study the effects of structural inhomogeneities on the stress and displacement fields induced by strike-slip faults in layered media. An elastic medium is considered, made up of an upper layer bounded by a free surface and welded to a lower half-space characterized by different elastic parameters. Shear cracks with assigned stress drop are employed as mathematical models of strike-slip faults, which are assumed to be vertical and planar. If the crack is entirely embedded within the lower medium (case A), a Cauchy-kernel integral equation is obtained, which is solved by employing an expansion of the dislocation density in Chebyshev polynomials. If the crack is within the lower medium but it terminates at the interface (case B), a generalized Cauchy singularity appears in the integral kernel. This singularity affects the singular behaviour of the dislocation density at the crack tip touching the interface. Finally, the case of a crack crossing the interface is considered (case C). The crack is split into two interacting sections, each placed in a homogeneous medium and both open at the interface. Two coupled generalized Cauchy equations are obtained and solved for the dislocation density distribution of each crack section. An asymptotic study near the intersection between the crack and the interface shows that the dislocation densities for each crack section are bounded at the interface, where a jump discontinuity is present. As a corollary, the stress drop must be discontinuous at the interface, with a jump proportional to the rigidity contrast between the adjoining media. This finding is shown to have important implications for the development of geometrical complexities within transform fault zones: planar strike-slip faults cutting across layer discontinuities with arbitrary stress drop values are shown to be admissible only if the interface between different layers becomes unwelded during the earthquake at the crack/interface junction. Planar strike-slip faulting may take place only in mature transform zones, where a repetitive earthquake cycle has already developed, if the rheology is perfectly elastic. Otherwise, the fault cannot be planar: we infer that strike-slip faulting at depth is plausibly accompanied by en-echelon surface breaks in a shallow sedimentary layer (where the stress drop is lower than prescribed by the discontinuity condition), while ductile deformation (or steady sliding) at depth may be accommodated by multiple fault branching or by antithetic faulting in the upper brittle layer (endowed with lower rigidity but higher stress). [source] Experimental validation of the wavefield transform of electromagnetic fieldsGEOPHYSICAL PROSPECTING, Issue 5 2002Kaushik Das The wavefield transform is a mathematical technique for transforming low-frequency electromagnetic (EM) signals to a non-diffusive wave domain. The ray approximation is valid in the transform space and this makes traveltime tomography for 3D mapping of the electrical conductivity distribution in the subsurface possible. The transform, however, imposes stringent frequency bandwidth and signal-to-noise ratio requirements on the data. Here we discuss a laboratory scale experiment designed to collect transform quality EM data, and to demonstrate the practical feasibility of transforming these data to the wavefield domain. We have used the scalable nature of EM fields to design a time-domain experiment using graphite blocks to simulate realistic field conditions while leaving the time scale undisturbed. The spatial dimensions have been scaled down by a factor of a thousand by scaling conductivity up by a factor of a million. The graphite blocks have two holes drilled into them to carry out cross-well and borehole-to-surface experiments. Steel sheets have been inserted between the blocks to simulate a conductive layer. Our experiments show that accurate EM data can be recorded on a laboratory scale model even when the scaling of some features, such as drill-hole diameters, is not maintained. More importantly, the time-domain EM data recorded in cross-well and surface-to-borehole modes can be usefully and accurately transformed to the wavefield domain. The observed wavefield propagation delay is proportional to the direct distance between the transmitter and receiver in a homogeneous medium. In a layered medium, data accuracy is reduced and, hence, our results are not so conclusive. On the basis of the experimental results we conclude that the wavefield transform could constitute a valid approach to the interpretation of accurate, undistorted time-domain data if further improvement in the transform can be realized. [source] A simplified quasi-two-dimensional model for gain optimization in carbon dioxide gasdynamic lasers (GDL)INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 11 2002A. R. Bahrampour Abstract In this paper a simplified quasi-two-dimensional model for small signal gain optimization in gasdynamic laser is introduced. In order to obtain a homogeneous medium with maximum optical gain in the active medium, by nozzle shape formation, the shock occurrence position is controlled and is postponed to some point behind the laser active medium. Then the method of calculus of variation is used to find the supersonic part of the nozzle of a gasdynamic laser with maximum gain in the active medium. The interesting result is that the supersonic part of such a nozzle consists of a wedge as the accelerating part of the nozzle, a smooth surface for the uniformization, and finally a channel for the relaxation of the medium. (The middle section is characterized as the geometrical locus of points whose characteristic curves are concurrent at a certain point.) It is also shown that, overlooking a minor difference in the gain, the nozzle can be chosen to be a shock free one with the ultimate optical uniformity. Copyright © 2002 John Wiley & Sons, Ltd. [source] An experimental investigation of buried-object imaging in a homogeneous medium using synthetic-aperture radar conceptsMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 6 2006Caner Ozdemir Abstract An experimental investigation is conducted to image buried objects. A Fourier-based imaging algorithm based on synthetic-aperture radar (SAR) concepts is applied to the scattered data from buried dielectric targets in a homogeneous medium. An experimental setup is constructed to apply the algorithm. Three-dimensional images of water bottles and plastic targets buried in sand are presented. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1209,1214, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21578 [source] | |||||||||||||