Superconductivity mediated by charge fluctuations in layered molecular crystals

We consider the competition between superconducting, charge ordered, and metallic phases in layered molecular crystals with the theta and beta" structures. Applying slave-boson theory to the relevant extended Hubbard model, we show that the superconductivity is mediated by charge fluctuations and the Cooper pairs have d(xy) symmetry. This is in contrast to the kappa-(BEDT-TTF)(2)X family, for which theoretical calculations give superconductivity mediated by spin fluctuations and with d(x)2(-y)2 symmetry. We predict several materials that should become superconducting under pressure.

Comparison of experimental and numerical studies of ionizing flow over a cylinder

Comparisons are made between experimental measurements and numerical simulations of ionizing flows generated in a superorbital facility. Nitrogen, with a freestream velocity of around 10 km/s, was passed over a cylindrical model, and images were recorded using two-wavelength holographic interferometry. The resulting density, electron concentration, and temperature maps were compared with numerical simulations from the Langley Research Center aerothermodynamic upwind relaxation algorithm. The results showed generally good agreement in shock location and density distributions. Some discrepancies were observed for the electron concentration, possibly, because simulations were of a two-dimensional flow, whereas the experiments were likely to have small three-dimensional effects.

Analysis of heat transfer and entropy generation for a thermally developing Brinkman–Brinkman forced convection problem in a rectangular duct with isoflux walls

Heat transfer and entropy generation analysis of the thermally developing forced convection in a porous-saturated duct of rectangular cross-section, with walls maintained at a constant and uniform heat flux, is investigated based on the Brinkman flow model. The classical Galerkin method is used to obtain the fully developed velocity distribution. To solve the thermal energy equation, with the effects of viscous dissipation being included, the Extended Weighted Residuals Method (EWRM) is applied. The local (three dimensional) temperature field is solved by utilizing the Green’s function solution based on the EWRM where symbolic algebra is being used for convenience in presentation. Following the computation of the temperature field, expressions are presented for the local Nusselt number and the bulk temperature as a function of the dimensionless longitudinal coordinate, the aspect ratio, the Darcy number, the viscosity ratio, and the Brinkman number. With the velocity and temperature field being determined, the Second Law (of Thermodynamics) aspect of the problem is also investigated. Approximate closed form solutions are also presented for two limiting cases of MDa values. It is observed that decreasing the aspect ratio and MDa values increases the entropy generation rate.

Flow and axial dispersion simulation for traveling axisymmetric Taylor vortices

Numerical experiments using a finite difference method were carried out to determine the motion of axisymmetric Taylor vortices for narrow-gap Taylor vortex flow. When a pressure gradient is imposed on the flow the vortices are observed to move with an axial speed of 1.16 +/- 0.005 times the mean axial flow velocity. The method of Brenner was used to calculate the long-time axial spread of material in the flow. For flows where there is no pressure gradient, the axial dispersion scales with the square root of the molecular diffusion, in agreement with the results of Rosen-bluth et al. for high Peclet number dispersion in spatially periodic flows with a roll structure. When a pressure gradient is imposed the dispersion increases by an amount approximately equal to 6.5 x 10(-4) (W) over bar(2)d(2)/D-m, where (W) over bar is the average axial velocity in the annulus, analogous to Taylor dispersion for laminar flow in an empty tube.

Metabolite mean transit times in the liver as predicted by various models of hepatic elimination

Predicted area under curve (AUC), mean transit time (MTT) and normalized variance (CV2) data have been compared for parent compound and generated metabolite following an impulse input into the liver, Models studied were the well-stirred (tank) model, tube model, a distributed tube model, dispersion model (Danckwerts and mixed boundary conditions) and tanks-in-series model. It is well known that discrimination between models for a parent solute is greatest when the parent solute is highly extracted by the liver. With the metabolite, greatest model differences for MTT and CV2 occur when parent solute is poorly extracted. In all cases the predictions of the distributed tube, dispersion, and tasks-in-series models are between the predictions of the rank and tube models. The dispersion model with mixed boundary conditions yields identical predictions to those for the distributed tube model (assuming an inverse gaussian distribution of tube transit times). The dispersion model with Danckwerts boundary conditions and the tanks-in series models give similar predictions to the dispersion (mixed boundary conditions) and the distributed tube. The normalized variance for parent compound is dependent upon hepatocyte permeability only within a distinct range of permeability values. This range is similar for each model but the order of magnitude predicted for normalized variance is model dependent. Only for a one-compartment system is the MIT for generated metabolite equal to the sum of MTTs for the parent compound and preformed metabolite administered as parent.

A computational study of shock speeds in high-performance shock tubes

This paper describes U2DE, a finite-volume code that numerically solves the Euler equations. The code was used to perform multi-dimensional simulations of the gradual opening of a primary diaphragm in a shock tube. From the simulations, the speed of the developing shock wave was recorded and compared with other estimates. The ability of U2DE to compute shock speed was confirmed by comparing numerical results with the analytic solution for an ideal shock tube. For high initial pressure ratios across the diaphragm, previous experiments have shown that the measured shock speed can exceed the shock speed predicted by one-dimensional models. The shock speeds computed with the present multi-dimensional simulation were higher than those estimated by previous one-dimensional models and, thus, were closer to the experimental measurements. This indicates that multi-dimensional flow effects were partly responsible for the relatively high shock speeds measured in the experiments.

Novel vascular graft grown within recipient's own peritoneal cavity

A method by which to overcome the clinical symptoms of atherosclerosis is the insertion of a graft to bypass an artery blocked or impeded by plaque. However, there may be insufficient autologous mammary artery for multiple or repeat bypass, saphenous vein may have varicose degenerative alterations that can lead to aneurysm in high-pressure sites, and small-caliber synthetic grafts are prone to thrombus induction and occlusion. Therefore, the aim of the present study was to develop an artificial blood conduit of any required length and diameter from the cells of the host for autologous transplantation. Silastic tubing, of variable length and diameter, was inserted into the peritoneal cavity of rats or rabbits. By 2 weeks, it had become covered by several layers of myofibroblasts, collagen matrix, and a single layer of mesothelium. The Silastic tubing was removed from the harvested implants, and the tube of living tissue was everted such that it now resembled a blood vessel with an inner lining of nonthrombotic mesothelial cells (the intima), with a media of smooth muscle-like cells (myofibroblasts), collagen, and elastin, and with an outer collagenous adventitia. The tube of tissue (10 to 20 mm long) was successfully grafted by end-to-end anastomoses into the severed carotid artery or abdominal aorta of the same animal in which they were grown. The transplant remained patent for at least 4 months and developed structures resembling elastic lamellae. The myofibroblasts gained a higher volume fraction of myofilaments and became responsive to contractile agonists, similar to the vessel into which they had been grafted. It is suggested that these nonthrombogenic tubes of living tissue, grown in the peritoneal cavity of the host, may be developed as autologous coronary artery bypass grafts or as arteriovenous access fistulae for hemodialysis patients.

Transport properties of strongly correlated metals: A dynamical mean-field approach

The temperature dependence of the transport properties of the metallic phase of a frustrated Hubbard model on the hypercubic lattice at half-filling is calculated. Dynamical mean-held theory, which maps the Hubbard model onto a single impurity,Anderson model that is solved self-consistently, and becomes exact in the limit of large dimensionality, is used. As the temperature increases there is a smooth crossover from coherent Fermi liquid excitations at low temperatures to incoherent excitations at high temperatures. This crossover leads to a nonmonotonic temperature dependence for the resistance, thermopower, and Hall coefficient, unlike in conventional metals. The resistance smoothly increases from a quadratic temperature dependence at low temperatures to large values which can exceed the Mott-Ioffe-Regel value ha/e(2) (where a is a lattice constant) associated with mean free paths less than a lattice constant. Further signatures of the thermal destruction of quasiparticle excitations are a peak in the thermopower and the absence of a Drude peak in the optical conductivity. The results presented here are relevant to a wide range of strongly correlated metals, including transition metal oxides, strontium ruthenates, and organic metals.

Blood vessels from bone marrow

Lengths of silastic tubing were inserted into the peritoneal cavity of rats or rabbits. By two weeks the free-floating implants had become covered by a capsule consisting of several layers of macrophage-derived myofibroblasts and collagen matrix overlaid by a single layer of mesothelial cells. The tubing was removed from the harvested implant and the tissue everted. This now resembled an artery with an inner lining of mesothelial cells (the intima), a media of myofibroblasts, and an outer collagenous adventitia. The tube of living tissue was grafted by end-to-end anastomoses into the transected carotid artery or abdominal aorta of the same animal in which the tissue had been grown, where it remained parent for four months and developed structures resembling elastic lamellae, The myofibroblasts developed a high volume fraction of myofilaments and became responsive to contractile and relaxing agents similar to smooth muscle cells of the adjacent artery wall.

An update on genetic, structural and functional studies of arylamine N-acetyltransferases in eucaryotes and procaryotes

Arylamine N-acetyltransferase (NAT) was first identified as the inactivator of the anti-tubercular drug isoniazid, The enzyme was shown to catalyse the transfer of an acetyl group from acetyl-CoA to the terminal nitrogen of the hydrazine drug. The rate of inactivation of isoniazid was polymorphically distributed in the population and was one of the first examples of pharmacogenetic variation, NAT was identified recently in Mycobacterium tuberculosis and is a candidate for; modulating the response to isoniazid, Genome sequences have revealed many homologous members of this unique family of enzymes. The first three-dimensional structure of a member of the NAT family identifies a catalytic triad consisting of aspartate, histidine and cysteine proposed to form the activation mechanism. So far, all procaryotic NATs resemble the human enzyme which acetylates isoniazid (NAT2), Human NAT2 is characteristic of drug-metabolizing enzymes: it is found in liver and intestine, In humans and other mammals, there are up to three different isoenzymes. If only one isoenzyme is present, it is like human NAT1. Human NAT1 and its murine equivalent specifically acetylate the folate catabolite p-amino-benzoylglutamate. NAT1 and its murine homologue each have a ubiquitous tissue distribution and are expressed early in development at the blastocyst stage, During murine embryonic development, NAT is expressed in the developing neural tube. The proposed endogenous role of NAT in folate metabolism, and its multi-allelic nature, indicate that its role in development should be assessed further.

Determining complicated winding patterns for shim coils using stream functions and the target-field method

In a magnetic resonance imaging equipment, gradient and shim coils are needed to produce a spatially varying magnetic field throughout the sample being imaged. Such coils consist of turns of wire wound on the surface of a cylindrical tube. Shim coils in particular, must sometimes be designed to produce complicated magnetic fields to correct for impurities. Streamline patterns for shim coils are much more complicated than those for gradient coils, In this work we present a detailed analysis of streamline methods and their application to shim coil design, A method is presented for determining the winding patterns to generate these complicated fields. (C) 2002 John Wiley & Sons, Inc.