Probabilistic analysis of a two-unit system with a warm standby subject to preventive maintenance and a single service facility

The object of this paper is to carry out the availability and the reliability analysis of a two-unit system with a warm standby having a single service facility for the performance of preventive maintenance and repair. The failure times, the repair times, the inspection times and the preventive maintenance times of the main unit and of the standby one are assumed to be arbitrarily distributed. The system is characterized by the probability of its being in the up or the down state. Explicit expressions for the mean down time of the system and for the mean time to system failure are obtained. Some previous results are derived from this work as special cases.     

Markedly increased tissue concentrations of 7-dehydrocholesterol combined with low levels of cholesterol are characteristic of the Smith-Lemli-Opitz syndrome

The Smith-Lemli-Opitz syndrome is an autosomal recessive birth defect (frequency 1:20,000-1:40,000) that results in profound mental retardation, physical deformities, and failure to thrive. It is characterized biochemically by low plasma cholesterol and greatly elevated levels of two dehydrocholesterols, one of which is the cholesterol precursor 7-dehydrocholesterol. To determine whether the block in cholesterol biosynthesis affects tissue sterols, we assayed several organs from two affected individuals, a female who died at 27 hours and a 20-week male fetus. Cholesterol concentrations in abdominal wall, adrenal gland, and kidney from two or three unaffected fetuses, who served as controls, averaged 2.0, 1.5, and 1.4 mg/g wet weight, compared to 0.08, 0.44, and 0.14, respectively, for the homozygous fetus. Cerebral cortex cholesterol concentrations were 2.2 mg/g for two 20-22-week fetal controls but only 0.21 and 0.09 mg/g, respectively, for the homozygous child and fetus. Similarly, tissue cholesterol levels were abnormally low in the homozygous child being less than 1 mg/g in liver, adipose, thymus, muscle, and adrenal and 6.2 mg/dl in plasma. Dehydrocholesterols could not be detected by conventional means in any controls but were elevated enough in tissues from affected individuals to make total sterol concentrations nearly normal. These results suggest that a defect in 3 beta-hydroxysterol delta 7-reductase leads to both a profound lack of cholesterol and its replacement by dehydrocholesterols. Such a combination may be lethal in the most severely affected individuals.     

On the formation and stability of dislocation patterns—III: Three-dimensional considerations

A model is proposed to describe the occurrence of the vein structures and the nucleation of persistent slip bands in fatigued metals as a cooperative phenomenon of motion and interaction of dislocation populations. It is based on the competition between diffusive-like mobilities for random or stress induced dislocation motion and cubic-like non-linearities for the pinning of stress driven dislocations by immobile dipoles. It describes the coexistence of rod-like vein structures and ladder-like persistent slip bands in three dimensions, integrating the qualitative picture of pattern formation in one and two dimensions given earlier in Parts I and II.     

An approximation method for queues in series with blocking

An approximation method for the analysis of queues in series with exponential servers, Poisson arrivals and blocking, is presented. The analysis considers cells consisting of three nodes with revised arrival and service processes. A recursive scheme is developed for the calculation of the steady state probability distributions of the number of items/customers at each node, and also the joint steady state probability distributions of the number of items for triplets of nodes are provided. This approximation method seems to yield encouraging results when compared with those obtained by the other methods and simulation.     

The turnover times and pool sizes of photosynthetic hydrogen production by green algae

An investigation of the turnover times of photobiological production of hydrogen gas by green algae indicate that the photoreactions associated with molecular hydrogen production have promising properties for solar energy conversion and storage. Our results indicate that (a) the intrinsic kinetic rate capability of the hydrogen photoapparatus in green algae can keep pace with the incidence rate of light quanta, even in full sunlight; (b) the photogenerated electrons for hydrogen production probably lie in the mainstream of the electron transport chain of photosynthesis.These results have been obtained by performing the first measurements on the turnover times and pool sizes of photosynthetic hydrogen production. For the three species of green algae studied, the turnover times range from 0.1 to 3 ms. The turnover time for photosynthetic hydrogen production is, therefore, comparable to that for oxygen production.Rapid multiple flash experiments have been performed which indicate that the immediate source of reductant for photosynthetic hydrogen production is derived from a pool of 5–20 equivalents, depending on the alga. This pool is probably the plastoquinone pool linking the two photosystems of photosynthesis.     

Microwave tomography: an algorithm for cylindrical geometries

The letter presents an efficient algorithm in cylindrical coordinates for microwave diffraction tomography. In comparison with algorithms in cartesian co-ordinates, the mechanical rotation of the object is avoided and higher-quality images are obtained with similar processing time.     

The Fermilab Collider Detector Facility Data Acquisition System

A detector is being constructed at Fermilab to investigate antiproton - proton collisions at up to 2 TeV center of mass energies. The detector will have approximately 75,000 channels of electronics with an expected occupancy of about 10% for typical events. The ultimate raw event rate is expected to be 50 KHz, with events being written to magnetic tape at a rate of 5 Hz. We are designing a FASTBUS based data acquisition system incorporating multiple processors running concurrently to do the necessary data compaction and filtering. Numerous local intelligences allow the system to be broken into independent subsystems for checkout and calibration.     

Simple finite element‐based computation of distance functions in unstructured grids

A distance field is a representation of the closest distance from a point to a given surface. Distance fields are widely used in applications ranging from computer vision, physics and computer graphics and have been the subject of research of many authors in the last decade. Most of the methods for computing distance fields are devoted to Cartesian grids while little attention has been paid to unstructured grids. Finite element methods are well known for their ability to deal with partial differential equations in unstructured grids. Therefore, we propose an extension of the fast marching method for computing a distance field in a finite element context employing the element interpolation to hold the Eikonal property (∥?φ∥ = 1). A simple algorithm to develop the computations is also presented and its efficiency demonstrated through various unstructured grid examples. We observed that the presented algorithm has processing times proportional to the number of mesh nodes. Copyright © 2007 John Wiley & Sons, Ltd.