State-space truncation methods for parallel model reduction of large-scale systems

We discuss a parallel library of efficient algorithms for model reduction of large-scale systems with state-space dimension up to (10⁴). We survey the numerical algorithms underlying the implementation of the chosen model reduction methods. The approach considered here is based on state-space truncation of the system matrices and includes absolute and relative error methods for both stable and unstable systems. In contrast to serial implementations of these methods, we employ Newton-type iterative algorithms for the solution of the major computational tasks. Experimental results report the numerical accuracy and the parallel performance of our approach on a cluster of Intel Pentium II processors.     

Nanostructure in a Ti alloy processed using a cryomilling technique

A nanocrystalline Ti alloy with a uniform distribution of grains was synthesized using cryogenic mechanical milling. The effects of cryomilling parameters, such as milling time and ball to powder ratio (BPR), on the particle size, grain size, chemistry, and structure of cryomilled Ti powders were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The experimental results show that nanocrystalline Ti powders with a grain size of about 20 nm can be prepared using the cryomilling technique. Compared to SPEX milling at room temperature, cryomilling led to lower contamination levels of oxygen, nitrogen, and iron in the cryomilled Ti powder. The average particle size initially increased from the original 55 μm to a maximum value of 125 μm after 2 h of milling, and then decreased to 44 μm after 8 h of milling. Both the average particle size and the grain size decreased as the BPR increased.     

A novel fully differential biopotential amplifier with dc suppression

Fully differential amplifiers yield large differential gains and also high common mode rejection ratio (CMRR), provided they do not include any unmatched grounded component. In biopotential measurements, however, the admissible gain of amplification stages located before dc suppression is usually limited by electrode offset voltage, which can saturate amplifier outputs. The standard solution is to first convert the differential input voltage to a single-ended voltage and then implement any other required functions, such as dc suppression and dc level restoring. This approach, however, yields a limited CMRR and may result in a relatively large equivalent input noise. This paper describes a novel fully differential biopotential amplifier based on a fully differential dc-suppression circuit that does not rely on any matched passive components, yet provides large CMRR and fast recovery from dc level transients. The proposed solution is particularly convenient for low supply voltage systems. An example implementation, based on standard low-power op amps and a single 5-V power supply, accepts input offset voltages up to +/-500 mV, yields a CMRR of 102 dB at 50 Hz, and provides, in accordance with the AAMI EC38 standard, a reset behavior for recovering from overloads or artifacts.     

Effect of atmosphere on the electrical properties of TiO2–SnO2 varistor systems

This work illustrates the advancement of research on TiO₂-based electroceramics. In this work will be presented that the addition of different dopants, as well as thermal treatments at oxidizing and inert atmosphere, influences of the densification, the mean grain size and the electrical properties of the TiO₂-based varistor ceramics. Dopants like Ta₂O₅, Nb₂O₅, and Cr₂O₃ have an especial role in the barrier formation at the grain boundary in the TiO₂ varistors, increasing the nonlinear coefficient and decreasing the breakdown electric field. The influence of Cr_Ti is to increase the O and O₂ adsorption at the grain boundary interface and to promote a decrease in the conductivity by donating electrons to O₂ adsorbed at the grain boundary. In this paper, TiO₂ and (Sn,Ti)O₂-based studies of polycrystalline ceramics, which show a non-linear I–V electrical response typical of low voltage varistor systems are also presented. All these systems are potentially promising for varistor applications.     

An analytical model for the oxide size in Al alloys synthesized by reactive atomization and deposition

This article presents the formulation of an analytical model to predict the size scale of oxide dispersoids in as-deposited Al alloys synthesized by reactive atomization and deposition (RAD). The proposed model formulation is primarily based on the assumption that all of the strain energy in the oxides is used to create interfaces between the oxide dispersoids and the matrix. It is also assumed that oxides are fragmented into plate-shaped dispersoids. Assuming three types of cross-sectional geometries for the oxide plates, i.e., circular (corresponding to disc-shaped oxide dispersoids), rectangular, and equilateral polygon, the following predictions are made on the basis of the analytical model. First, the curves for calculated effective cross-sectional diameters of plate-shaped oxide dispersoids vs droplet size and calculated effective volumetric diameter of plate-shaped oxide dispersoids vs droplet size can be divided into three distinct regions. These three regions are identified on the basis of two characteristic droplet sizes corresponding to the solid fraction equal to that on the deposited material’s surface ( ) and to solid fraction of 0.6 (D _0.6), respectively, at impact. Second, the velocity of individual droplets at impact has a limited effect on the calculated effective cross-sectional diameter and effective volumetric diameter. With an increase in solid fraction on the deposited material’s surface, the calculated effective cross-sectional diameter and effective volumetric diameter decrease significantly when droplet sizes are smaller than , whereas they remain almost unchanged when droplet sizes are higher than D _0.6. Third, when plate-shaped oxide dispersoids with rectangular or equilateral polygon cross-sectional geometries are chosen, the calculated effective cross-sectional diameter and effective volumetric diameter are larger than the corresponding calculated diameter and effective volumetric diameter of disc-shaped oxide dispersoids. The calculated effective cross-sectional diameters, based on different cross-sectional geometries (e.g., circular, rectangular with up to 10 of the length/width ratio, and equilateral polygon), are all in reasonable agreement with the experimentally observed effective cross-sectional diameter of oxides.     

Promoting physical activity in the Americas

Physical inactivity, obesity, and noncommunicable disease rates are rapidly climbing to epidemic proportions and are becoming the leading causes of death and disability in the Americas and globally. The causes are complex and will require a multifaceted, multisectoral approach. Recognizing this, the World Health Organization adopted a broad-ranging process to develop a Global Strategy for the Promotion of Diet, Physical Activity, and Health, as mandated by the World Health Assembly in May 2002. The results of the yearlong effort are to be presented at the World Health Assembly in May 2004.     

Events affecting levels and seasonal evolution of airborne particulate matter concentrations in the Western Mediterranean

Time series (1996-2000) of levels of PM (PM10 and TSP) and gaseous pollutants recorded in air quality monitoring networks from Northeastern Spain were interpreted using meteorological data and satellite observations of African dust plumes. The main objective of this study was to identify the processes affecting time variations of PM levels on a day-to-day and seasonal basis. From March to October PM levels at rural, urban, and industrial sites vary as a function of the concatenation of Atlantic air mass advections (Atlantic episodes with low PM levels) and regional circulations (regional events with high PM levels, very often associated with high ozone levels), which favor the aging of air masses in the Western Mediterranean basin. During these regional episodes, PM is transported from urban/industrial to rural sites by meso-scale circulations. From November to February low PM levels are recorded at rural sites, and variations in PM levels at urban/industrial sites are governed by the successive occurrence of Atlantic episodes and local urban/industrial pollution events. The African dust outbreaks take place throughout the year and may induce PM levels to increase simultaneously in large areas of the Iberian peninsula. The difference between PM concentrations measured at urban and rural sites experiences a seasonal trend similar to that of levels of NO(x) and CO, which is characterized by a winter maximum due to the higher frequency of intensive local urban pollution events. However, maximum PM levels are recorded in summer at rural sites owing to the frequent occurrence of regional episodes. Furthermore, in three years of the study period (1997, 1998, and 2000), a second-order PM maximum was also recorded at rural sites in March owing to intensive African dust outbreaks.     

Analysis of distributed genetic algorithms for solving cutting problems

In this paper, a solution to the three‐stage two‐dimensional cutting problem is presented by using sequential and parallel genetic algorithms (GAs). More specifically, an analysis of including distributed population ideas and parallelism in the basic GA are carried out to solve the problem more accurately and efficiently than with ordinary sequential techniques. Publicly available test problems have been used to illustrate the computational performance of the resulting metaheuristics. Experimental evidence in this work will show that the proposed algorithms outperform their sequential counterparts in time (high speedup with multiprocessors) and numerically (lower number of visited points during the search to find the solutions).     

Spark plasma sintering of a nanocrystalline Al-Cu-Mg-Fe-Ni-Sc alloy

The microstructure and aging behavior of a nanocrystalline Al-Cu-Mg-Fe-Ni-Sc alloy was studied. The nanocrystalline powders were produced by milling at liquid nitrogen temperature and then consolidated using spark plasma sintering (SPS). The microstructure after SPS consisted of a bimodal aluminum grain structure (coarse-grained and fine-grained regions), along with Al₉FeNi and Al₂CuMg particles dispersed throughout. The microstructure observed in the as-consolidated sample is rationalized on the basis of high current densities that are generated during sintering. Solution treatment and aging of the SPS Al-Cu-Mg-Fe-Ni-Sc sample resulted in softening instead of hardening. This observation can be explained by the reduced amount of Cu, Mg, and Si in solid solution available to form S′ Al₂CuMg due to the precipitation of Al₇FeCu₂ and Si-rich particles, and by the fact that rodlike S′ Al₂CuMg particles could only precipitate out in the coarse-grained regions, greatly decreasing their influence on the hardness. This lack of precipitation in the fine-grained region is argued to represent a new physical observation and is rationalized on the basis of physical and thermodynamic effects. The nanocrystalline SPS Al-Cu-Mg-Fe-Ni-Sc sample was also extremely thermally stable, retaining a fine-grained structure even after solution treatment at 530°C for 5 h. The observed thermal stability is rationalized on the basis of solute drag and Zener pinning caused by the impurities introduced during the cryomilling process.