Vortex sheet simulation of a plane �canonical� mixing layer

Discrete vortex simulations of the mixing layer carried out in the past have usually involved large induced velocity fluctuations, and thus demanded rather long time-averaging to obtain satisfactory values of Reynolds stresses and third-order moments. This difficulty has been traced here, in part, to the use of discrete vortices to model what in actuality are continuous vortex sheets. We propose here a novel two-dimensional vortex sheet technique for computing mixing layer flow in the limit of infinite Reynolds number. The method divides the vortex sheet into constant-strength linear elements, whose motions are computed using the Biot-Savart law. The downstream far-field is modelled by a steady vorticity distribution derived by application of conical similarity from the solution obtained in a finite computational domain. The boundary condition on the splitter plate is satisfied rigorously using a doublet sheet. The computed large-scale roll-up of the vortex sheet is qualitatively similar to experimentally obtained shadow-graphs of the plane turbulent mixing layer. The mean streamwise velocity profile and the growth rate agree well with experimental data. The presently computed Reynolds stresses and third-order moments are comparable with experimental and previous vortex-dynamical results, without using any external parameter (such as the vortex core-size) of the kind often used in the latter. The computed autocorrelations are qualitatively similar to experimental results along the top and bottom edges of the mixing layer, and show a well-defined periodicity along the centreline. The accuracy of the present computation is independently established by demonstrating negligibly small changes in the five invariants (including the Hamiltonian) in vortex dynamics.

Modelling plane mixing layers using vortex points and sheets

We present here a critical assessment of two vortex approaches (both two-dimensional) to the modelling of turbulent mixing layers. In the first approach the flow is represented by point vortices, and in the second it is simulated as the evolution of a continuous vortex sheet composed of short linear elements or ''panels''. The comparison is based on fresh simulations using approximately the same number of elements in either model, paying due attention in both to the boundary conditions far downstream as well as those on the splitter plate from which the mixing layer issues. The comparisons show that, while both models satisfy the well-known invariants of vortex dynamics approximately to the same accuracy, the vortex panel model, although ultimately not convergent, leads to smoother roll-up and values of stresses and moments that are in closer agreement with the experiment, and has a higher computational efficiency for a given degree of convergence on moments. The point vortex model, while faster for a given number of elements, produces an unsatisfactory roll-up which (for the number of elements used) is rendered worse by the incorporation of the Van der Vooren correction for sheet curvature.

The effect of boundaries on the plane Couette flow of granular materials: a bifurcation analysis

The tendency of granular materials in rapid shear flow to form non-uniform structures is well documented in the literature. Through a linear stability analysis of the solution of continuum equations for rapid shear flow of a uniform granular material, performed by Savage (1992) and others subsequently, it has been shown that an infinite plane shearing motion may be unstable in the Lyapunov sense, provided the mean volume fraction of particles is above a critical value. This instability leads to the formation of alternating layers of high and low particle concentrations oriented parallel to the plane of shear. Computer simulations, on the other hand, reveal that non-uniform structures are possible even when the mean volume fraction of particles is small. In the present study, we have examined the structure of fully developed layered solutions, by making use of numerical continuation techniques and bifurcation theory. It is shown that the continuum equations do predict the existence of layered solutions of high amplitude even when the uniform state is linearly stable. An analysis of the effect of bounding walls on the bifurcation structure reveals that the nature of the wall boundary conditions plays a pivotal role in selecting that branch of non-uniform solutions which emerges as the primary branch. This demonstrates unequivocally that the results on the stability of bounded shear how of granular materials presented previously by Wang et al. (1996) are, in general, based on erroneous base states.

Growth mechanism of phases, Kirkendall voids, marker plane position, and indication of the relative mobilities of the species in the interdiffusion zone

Often, wrong conclusions about the mobilities of species are drawn from the position of the Kirkendall marker plane or voids in the interdiffusion zone. To clarify, I have discussed the growth mechanism of the phases and the position of the marker plane depending on the relative mobilities of the species. The formation of different kinds of voids in the interdiffusion zone is discussed. Further, the microstructure that could be found because of the Kirkendall effect is also explained.

Electron diffraction structure analysis: structural research with low-quality diffraction data

Electron Diffraction Structure Analysis (EDSA) with data from standard selected-area electron diffraction (SAED) is still the method of choice for structure determination of nano-sized single crystals. The recently determined heavy atom structure α-Ti2Se (Albe & Weirich, 2003) is used as an example to illustrate the developed procedure for structure determination from two-dimensionally SAED data via direct methods and kinematical least-squares refinement. Despite the investigated crystallite had a relatively large effective thickness of about 230 Å as determined from dynamical calculations, the obtained structural model from SAED data was found in good agreement with the result from an earlier single crystal X-ray study (Weirich, Pöttgen & Simon, 1996). Arguments, which support the validity of the used quasi-kinematical approach, are given in the text. The influences of dynamical and secondary scattering on the quality of the data and the structure solution are discussed. Moreover, the usefulness of first-principles calculations for verifying the results from EDSA is demonstrated by two examples, whereas one of the structures was unattainable by conventional X-ray diffraction.

Synthesis and structure of La14V6CuO36.5: a transparent Cu(I) vanadate containing [OCuO](3-) sticks

The title compound, La14V6CuO36.5, was prepared from a stoichiometric mixture of La2O3,V2O5, and CuO at 1050-1080 degreesC. The compound forms transparent, pale green crystals and was characterized by wavelength dispersive spectroscopy and single crystal X-ray diffraction. The structure contains isolated VO43- tetrahedra and [OCuO](3-) sticks dispersed in a lanthanum oxide network. Films of La14V6CuO36.5 were grown on R-plane sapphire by using pulsed laser deposition. Rutherford backscattering spectroscopic and X-ray diffraction analyses of the films showed oriented growth of the title phase, a similar to5 eV optical band gap and n-type conductivity. The compound is an example of a transparent copper(I) oxide.

A kinetic study of the phase transformations in Al2O3 and Na2O · 6Al2O3 by a combined use of 27Al MAS NMR spectroscopy and x-ray diffraction: Importance of nucleation

Phase transformations of Al2O3 and Na2O · 6Al2O3 prepared by the gel route have been investigated for the first time by 27Al MAS NMR spectroscopy in combination with x-ray diffraction. Of particular interest in the study is the kinetics of the γ → α and γ → β transformations, respectively, in these two systems. Analysis of the kinetic data shows the important role of nucleation in both these transformations.

Microstructural Studies of Ferroelectric Bi2VO5.5 Thin Films With LaNiO3 Electrodes

Thin films of Bi2VO5.5 (BVO), a vanadium analog of the n = I member of the Aurivillius family, have been prepared by pulsed laser deposition. The BVO films grow along the [001] direction on LaNiO3(LNO) and YBa2Cu3O7 (YBCO) electrode buffer layers on LaA- IO3(LAO) substrates as obtained from X-ray diffraction studies. The microstructure of the films and of the interfaces within the film and between the film and the substrate were characterized using transmission electron microscopy. The in-plane epitaxial relationship of the rhombohedral LNO on perovskite LAO was [100] LNO // [100] LAO and [001] LNO // [001] LAO. High resolution lattice images showed a sharp interface between LNO and LAO. However, the LNO film is twinned with a preferred orientation along the growth direction. The BVO layer is single crystalline on both LNO/LAO and YBCO/LAO with the caxis parallel to the growth direction except for a thin layer of about 400 Å at the interface which is polycrystalline.

Cloning, expression, purification and preliminary X-ray diffraction studies of a putative Mycobacterium smegmatis thiolase

Thiolases are important in fatty-acid degradation and biosynthetic pathways. Analysis of the genomic sequence of Mycobacterium smegmatis suggests the presence of several putative thiolase genes. One of these genes appears to code for an SCP-x protein. Human SCP-x consists of an N-terminal domain (referred to as SCP2 thiolase) and a C-terminal domain (referred as sterol carrier protein 2). Here, the cloning, expression, purification and crystallization of this putative SCP-x protein from M. smegmatis are reported. The crystals diffracted X-rays to 2.5 angstrom resolution and belonged to the triclinic space group P1. Calculation of rotation functions using X-ray diffraction data suggests that the protein is likely to possess a hexameric oligomerization with 32 symmetry which has not been observed in the other six known classes of this enzyme.

The effect of boundaries on the plane Couette flow of granular materials: a bifurcation analysis

The tendency of granular materials in rapid shear ow to form non-uniform structures is well documented in the literature. Through a linear stability analysis of the solution of continuum equations for rapid shear flow of a uniform granular material, performed by Savage (1992) and others subsequently, it has been shown that an infinite plane shearing motion may be unstable in the Lyapunov sense, provided the mean volume fraction of particles is above a critical value. This instability leads to the formation of alternating layers of high and low particle concentrations oriented parallel to the plane of shear. Computer simulations, on the other hand, reveal that non-uniform structures are possible even when the mean volume fraction of particles is small. In the present study, we have examined the structure of fully developed layered solutions, by making use of numerical continuation techniques and bifurcation theory. It is shown that the continuum equations do predict the existence of layered solutions of high amplitude even when the uniform state is linearly stable. An analysis of the effect of bounding walls on the bifurcation structure reveals that the nature of the wall boundary conditions plays a pivotal role in selecting that branch of non-uniform solutions which emerges as the primary branch. This demonstrates unequivocally that the results on the stability of bounded shear flow of granular materials presented previously by Wang et al. (1996) are, in general, based on erroneous base states.

Low temperature crystallographic data on Kevlar 49 fibres

Using X-ray diffraction data, the behaviour of Kevlar 49 fibres at low temperatures, up to -100degreesC, has been analysed. During cooling, the basal plane of the monoclinic unit cell shrinks whereas the c- (unique, chain axis) length is not significantly affected. In contrast, in the return heating cycle to ambient temperature, the basal plane expands and contraction occurs along the chain direction. The unit cell registers a reduction in volume in both the cooling and heating cycles. Conspicuously, after a cycle of cooling and heating, the unit cell does not return to its initial volume. (C) 2003 Kluwer Academic Publishers.

Ternary copper(II) complexes of thiosemicarbazones and heterocyclic bases showing N3OS coordination as models for the type-2 centers of copper monooxygenases

Six ternary copper(II) complexes of general formulation [CuLB] (1-6), where L is dianionic ONS-donor thiosemicarbazones derived from the condensation of salicylaldehyde with thiosemicarbazides and B is NN-donor heterocyclic bases like 2,2'-bipyridine, 1,10-phenanthroline and 2,9-dimethyl-1,10-phenanthroline, are prepared from a reaction of copper(II) acetate hydrate with the heterocyclic base (B) and the thiosemicarbazone (H2L) in MeOH, and structurally characterized by X-ray diffraction technique. Crystal structures of the complexes display a distorted square-pyramidal (4 + 1) coordination geometry having the ONS-donor thiosemicarbazone bonded at the basal plane. The chelating heterocyclic bases exhibit axial-equatorial mode of bonding. The complexes are one-electron paramagnetic and they show axial X-band EPR spectra in DMF-toluene glass at 77 K giving g(parallel to)(A(parallel to)) and g(perpendicular to) values of similar to2.2 (175 x 10(-4) cm(-1)) and similar to2.0 indicating a {d(x2-y2)}(1) ground state. The complexes show a d-d band near 570 nm and a charge transfer band near 400 nm in DMF. The complexes are redox active and exhibit a quasireversible Cu(II)-Cu(I) couple in DMF-0.1 M tetrabutylammonium perchlorate near 0.1 V vs. SCE. They are catalytically active in the oxidation of ascorbic acid in presence of dioxygen. The complexes with a CuN3OS coordination model the ascorbate oxidation property of dopamine beta-hydroxylase and peptidylglycine a-hydroxylating monooxygeanase. (C) 2003 Elsevier Science B.V. All rights reserved.

The effect of T-stress on plane strain dynamic crack growth in elastic-plastic materials

In this paper, the effects of T -stress on steady, dynamic crack growth in an elastic-plastic material are examined using a modified boundary layer formulation. The analyses are carried out under mode I, plane strain conditions by employing a special finite element procedure based on moving crack tip coordinates. The material is assumed to obey the J (2) flow theory of plasticity with isotropic power law hardening. The results show that the crack opening profile as well as the opening stress at a finite distance from the tip are strongly affected by the magnitude and sign of the T -stress at any given crack speed. Further, it is found that the fracture toughness predicted by the analyses enhances significantly with negative T -stress for both ductile and cleavage mode of crack growth.