The boundary element method applied to a moving free boundary problem

In this paper a boundary problem is considered for which the boundary is to be determined as part of the solution. A time‐dependent problem involving linear diffusion in two spatial dimensions which results in a moving free boundary is posed. The fundamental solution is introduced and Green’s Theorem is used to yield a non‐linear system of integral equations for the unknown solution and the location of the boundary. The boundary element method is used to obtain a numerical solution to this system of integral equations which in turn is used to obtain the solution of the original problem. Graphical results for a two‐dimensional problem are presented. Published in 1999 by John Wiley & Sons, Ltd.     

Automatically exploiting implicit parallelism in Java

In this paper we show how implicit parallelism in Java programs can be made explicit by a restructuring compiler using the multi-threading mechanism of the language. In particular, we focus on automatically exploiting implicit parallelism in loops and multi-way recursive methods. Expressing parallelism in Java itself clearly has the advantage that the transformed program remains portable. After compilation of the transformed Java program into byte-code, speedup can be obtained on any platform on which the Java byte-code interpreter supports the true parallel execution of threads. Moreover, we will see that the transformations presented in this paper only induce a slight overhead on uni-processors. © 1997 John Wiley & Sons, Ltd.     

Advanced space solar cells

This paper will present a brief overview of the status of research and development of advanced space solar cells from a variety of materials. Most of the investigations at present are focused on binary, ternary and quaternary III–V semiconducting compounds such as InP, GaInAs and GaInP₂. Growth techniques used for producing laboratory cells include liquid-phase epitaxy (LPE), organometallic vapor phase epitaxy (OMVPE), molecular beam epitaxy (MBE) or any of a number of variations of these techniques, such as atomic layer epitaxy (ALE), etc. Gallium arsenide is at present the only commercially available III–V compound solar cell. Compound III–V multiple bandgap cells are now under development in a jointly sponsored NASA/Air Force manufacturing technology demonstration program. In general, the decision to use a particular cell technology in space is determined by several factors, emphasis on any particular one depending on the mission environment: some are related to the properties of the photovoltaic material itself, such as efficiency and resistance to radiation damage, and some are related to details of the cell structure and associated materials, such as survivability under repeated thermal cycling and resistance to atomic oxygen erosion. The impact of these requirements on cell material selection and structural design is briefly discussed. This paper was produced under the auspices of the US Government and is therefore not subject to copyright in the USA     

Fixed-structure robust controller synthesis via decentralized static output feedback

This paper describes and illustrates a unified methodology for robust, fixed-structure controller synthesis. The approach is based upon direct fixed-structure controller synthesis using a decentralized static output feedback formulation as a general framework for representing a large class of controller structures. Scaled Popov bounds for the real structured singular value are used to account for real parameter uncertainty and provide the means for optimizing a worst-case ℋ︁₂ cost bound with respect to the free parameters of the controller. Quasi-Newton optimization algorithms are used to solve the resulting numerical optimization problem. Initial stability multiplier and scaling matrices needed in scaled Popov synthesis are obtained by solving an LMI feasibility problem. Using both centralized and decentralized controller structures, numerical results are obtained for a 16th-order acoustic duct model with uncertain damped natural frequencies and for a two-dimensional beam-spring model with uncertain actuator locations. © 1998 John Wiley & Sons, Ltd.     

Characterisation of Commercial Vodkas by Solid-Phase Microextraction and Gas Chromatography/Mass Spectrometry Analysis

A reliable method for the characterisation of vodkas has been developed by application of solid-phase microextraction (SPME) and gas chromatography/mass spectrometry (GCMS). Sixty-four samples of commercial Canadian and American vodkas, and one sample each of Japanese and German vodkas, were investigated in this study. The procedure yielded reproducible chromatographic profiles. The detected components were ethyl esters of C₈ to C₁₈ fatty acids at μg litre⁻¹ levels, various additives and contaminants. Distinctive profiles were observed for various brands. Markers for differentiating between Canadian and American vodkas were identified. The results of this study indicate that vodkas are differentiated by the combined effects of the original raw materials, any added substances, and differences in processing.     

Characterization of nonionic and cationic amine-functional polymers by aqueous SEC-MALLS

Molecular weight, root-mean-square radius, and conformational information obtained from aqueous size exclusion chromatography (SEC) and multiangle laser light-scattering (MALLS) detection are examined for amine-functional polymers. These include homopolymers and copolymers of poly(ethenylformamide) and cationic poly(vinylamine). Light-scattering studies for poly(ethenylformamide) employing 15 angles of detection are used to calculate molecular weights as high as 700,000 daltons with a root-mean-square radius of nearly 50 nm. Calculated conformational coefficients indicate a random-coil configuration at low molecular weights and a more compacted conformation at high molecular weights. Molecular weights and root-mean-square radius values for cationic poly(vinylamine) are determined using a three-angle MALLS detector. Root-mean-square radius is strongly dependent on the ionic strength of the mobile phase. Cationic poly(vinylamine) exhibits a rod-like conformation at both low and high ionic strength. Explanations are proposed for this behavior. © 1996 John Wiley & Sons, Inc.     

An analysis of failure-time distributions for product design optimization

Analysis of experimental data is often a problem facing design and manufacturing engineers. Many experiments are run for the express purpose of making a decision between manufacturing process or material alternatives. Statisticians recommend replication in experimental design; however, methods of analysing experimental data, as presented in a majority of engineering curricula, generally review only the most basic situations (checking for a statistically significant difference between the means or variances of two samples, for example). If means and variances change with time, group comparisons may require more sophisticated analyses. This paper presents one method that takes into account shifts in group means and variances over time. Resistance temperature sensor drift data generated from six different design configurations are used as an illustration. This procedure takes into account all drift path information from multiple sensors in multiple groups.     

Effect of silane coupling agent on the chemorheological behaviour of epoxidised natural rubber filled with precipitated silica

Results of measurements of physical properties and solvent swelling of the extrudates indicate that epoxidised natural rubber (ENR) interacts chemically with precipitated silica when the mix of the two was extruded at 150–170°C in a Monsanto Processability Tester (MPT). The extent of interaction between the rubber and the filler depends on the extrusion time, the volume fraction of the filler, the shear rate and the addition of the silane coupling agent, namely N-3-(N-vinyl benzyl amino) ethyl-γ-amino propyl trimethoxy silane monohydrochloride. The activation energy of the chemical interaction between ENR and silica decreases on the addition of the silane coupling agent.     

Task-Related Differences in the Cuticular Hydrocarbon Composition of Harvester Ants, Pogonomyrmex barbatus

Colonies of the harvester ant, Pogonomyrmex barbatus, perform a variety of tasks. The behavior of an individual worker appears to depend on its recent history of brief contacts with ants of the same and other task groups. The purpose of this study was to determine whether task groups differ in cuticular hydrocarbon composition. We compared the cuticular hydrocarbon composition of ants collected under natural conditions as they performed one of three tasks: patrolling (locating food sources), foraging, or nest maintenance. Task groups differed significantly in the relative proportions of classes of hydrocarbon compounds, as well as in individual compounds. Relative to nest maintenance workers, foragers and patrollers had a higher proportion of straight-chain alkanes relative to monomethylalkanes, dimethylalkanes, and alkenes. There was no significant difference in the chain length of n-alkanes among the task groups. Foragers did not differ in hydrocarbon composition from patrollers. Colonies differed significantly from one another in hydrocarbon composition, but task groups differed in consistent ways from colony to colony, suggesting that the mechanism responsible for task-related hydrocarbon composition was the same in all colonies. P. barbatus workers switch tasks during their lifetimes, suggesting that cuticular hydrocarbon composition changes during adulthood as well. Nest maintenance workers are probably younger than foragers and patrollers and perform very little of their work outside of the nest. Task-related hydrocarbon differences detected here may be associated with worker age, and/or the abiotic characteristics (temperature, humidity, and ultraviolet light) of the interior and exterior work environments.     

Producing titanium powder by continuous vapor-phase reduction

One of the goals of the U.S. Department of Energy’s Albany Research Center is to reduce the cost of titanium parts by developing a continuous titanium process. In this work, titanium powder was produced by feeding liquid TiCl₄, with argon as a carrier gas, and magnesium wire into a shaft reactor at 1,000°C. The magnesium and TiCl₄ vaporized and reacted to produce a mixture of titanium, MgCl₂, and magnesium powder. Ti/Mg/MgCl₂ powder was removed from the argon gas stream by an electrostatic precipitator, and the titanium powder was separated from the magnesium and MgCl₂ by either vacuum distillation or leaching. Vacuum distillation produced sintered titanium powder with lower oxygen levels, but unacceptably high levels of magnesium and chlorine. Leached powder was spherical and free-flowing with low levels of magnesium and chlorine, but the oxygen content was no lower than 0.82%. The high oxygen content of the leached powder is caused by surface oxidation of the submicrometer titanium powder. For more information, contact S.J. Gerdemann, Albany Research Center-Department of Energy, 1450 Queen Avenue S.W., Albany, Oregon 97321-2198; (541) 967-5964; fax (541) 967-5868; e-mail gerdeman@alrc.doe.gov.