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Distribution by Scientific Domains

Physics and Astronomy	40%

Selected Abstracts

Multigrid convergence of inviscid fixed- and rotary-wing flows

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 2 2002
C. B. Allen
Abstract The affect of multigrid acceleration implemented within an upwind-biased Euler method is presented, and applied to fixed-wing and rotary-wing flows. The convergence of fixed- and rotary-wing computations is shown to be vastly different, and multigrid is shown to be less effective for rotary-wing flows. The flow about a hovering rotor suffers from very slow convergence of the inner blade region, where the flow is effectively incompressible. Furthermore, the vortical wake must develop over several turns before convergence is achieved, whereas for fixed-wing computations the far-field grid and solution have little significance. Results are presented for single mesh and two, three, four, and five level multigrid, and using five levels a reduction in required CPU time of over 80 per cent is demonstrated for rotary-wing computations, but 94 per cent for fixed-wing computations. It is found that a simple V-cycle is the most effective, smoothing in the decreasing mesh density direction only, with a relaxed trilinear prolongation operator. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Self-organized InAs quantum dots grown in a V-groove InGaAs quantum wire

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 9 2007
Chang-Sik Son
Abstract Self-organized InAs quantum dots (QDs), by the Stranski,Krastanow mode, have been grown by using low-pressure metalorganic chemical vapor deposition on V-groove GaAs substrates. By adjusting the flow rate of AsH3 during the growth of InAs QDs, a one dimensional InAs QD array was successfully formed along the [01] direction only at the bottom of V-grooves. No QDs were observed on the sidewalls and the surface of the mesa top. The InAs QDs took on an oval shape. They were spatially well-isolated along the [01] direction with a line density of 3 × 103 cm,1. These low-density InAs QDs are expected to be used in areas of quantum information and quantum computing. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Measurement of AC current using an optical fibre sensor

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2007
C. H. Cheng
Abstract In the power industry, current is measured for metering and protection purposes. For said measurement, we demonstrated an optical fibre sensor using a single Fibre Bragg Grating (FBG) for measuring AC current for this paper. The sensor head is based on an FBG encased in a polymer-half-field metal cylinder, embedded in a magnetic material at the measuring point with the characteristics of an all-optical high pressure sensitivity. The operating mechanism is that the sensor can be attracted by the induced magnetic force created by the solenoid along one radial direction only, and responds to an axial force on the magnetic rod attached to the round plate, creating an axial attraction on the FBG. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Homologous protein import machineries in chloroplasts and cyanelles,

THE PLANT JOURNAL, Issue 4 2005
Jürgen M. Steiner
Summary The cyanelles of the glaucocystophyte alga Cyanophora paradoxa resemble endosymbiotic cyanobacteria, especially in the presence of a peptidoglycan wall between the inner and outer envelope membranes. However, it is now clear that cyanelles are in fact primitive plastids. Phylogenetic analyses of plastid, nuclear and mitochondrial genes support a single primary endosymbiotic event. In this scenario, cyanelles and all other plastid types are derived from an ancestral photosynthetic organelle combining the high gene content of rhodoplasts and the peptidoglycan wall of cyanelles. This means that the import apparatuses of all primary plastids, i.e. those from glaucocystophytes, red algae, green algae and higher plants, should be homologous. If this is the case, then transit sequences should be similar and heterologous import experiments feasible. Thus far, heterologous in vitro import has been shown in one direction only: precursors from C. paradoxa were imported into isolated pea or spinach chloroplasts. Cyanelle transit sequences differ from chloroplast stroma targeting peptides in containing in their N-terminal domain an invariant phenylalanine residue which is shown here to be crucial for import. In addition, we now demonstrate that heterologous precursors are readily imported into isolated cyanelles, provided that the essential phenylalanine residue is engineered into the N-terminal part of chloroplast transit peptides. The cyanelle and likely also the rhodoplast import apparatus can be envisaged as prototypes with a single receptor/channel showing this requirement for N-terminal phenylalanine. In chloroplasts, multiple receptors with overlapping and less stringent specificities have evolved, explaining the efficient heterologous import of native precursors from C. paradoxa. [source]

Construction and application of covariance functions with variable length-fields

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 619 2006
Gregory Gaspari
Abstract This article focuses on construction, directly in physical space, of three-dimensional covariance functions parametrized by a length-field, and on an application of these functions to improve the representation of the Quasi-Biennial Oscillation (QBO) in the Goddard Earth Observing System, Version 4 (GEOS-4) data assimilation system. The covariance functions are obtained by fusing collections of auto-covariance functions having different constant length-scales with their associated cross-covariance functions. This construction yields covariance functions with length-scales that can vary arbitrarily over any finite partition of the spatial domain. A simple, and also motivating application of these functions is to the case where the length-scale varies in the vertical direction only. The class of covariance functions with variable length-fields constructed in this article will be called multi-level to associate them with this application. The multi-level covariance functions extend well-known single-level covariance functions depending only on a constant length-scale. Generalizations of the familiar first-and third-order autoregressive covariances in three dimensions are given, providing multi-level covariances with zero and four continuous derivatives at zero separation, respectively. Multi-level piecewise rational covariances with two continuous derivatives at zero separation are also provided. Multi-level power-law covariances are constructed with continuous derivatives of all orders. Additional multi-level covariance functions are constructed using the Schur product of single- and multi-level covariance functions. A multi-variate, multi-level power-law covariance with a large troposphere-to-stratosphere length-field gradient is employed to reproduce the QBO from sparse radiosonde wind observations in the tropical lower stratosphere. This covariance model is described along with details of the assimilation experiments. The new covariance model is shown to represent the vertical wind shear associated with the QBO much more effectively than the multi-variate, multi-level covariance model in the baseline GEOS-4 system. Copyright © 2006 Royal Meteorological Society [source]