535,063 research outputs found

    Lack of continuity between Cronobacter biotypes and species as determined using multilocus sequence typing

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    The accuracy of the Cronobacter biotyping scheme was compared with the 7-loci multilocus sequence typing scheme. Biotyping did not reliably assign species level identification, as only half (17/31) of the biotype variants were unique to any of the seven Cronobacter species and the remaining biotypes were shared across the genus

    An N-body Integrator for Gravitating Planetary Rings, and the Outer Edge of Saturn's B Ring

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    A new symplectic N-body integrator is introduced, one designed to calculate the global 360 degree evolution of a self-gravitating planetary ring that is in orbit about an oblate planet. This freely-available code is called epi_int, and it is distinct from other such codes in its use of streamlines to calculate the effects of ring self-gravity. The great advantage of this approach is that the perturbing forces arise from smooth wires of ring matter rather than discreet particles, so there is very little gravitational scattering and so only a modest number of particles are needed to simulate, say, the scalloped edge of a resonantly confined ring or the propagation of spiral density waves. The code is applied to the outer edge of Saturn's B ring, and a comparison of Cassini measurements of the ring's forced response to simulations of Mimas' resonant perturbations reveals that the B ring's surface density at its outer edge is 195+-60 gm/cm^2 which, if the same everywhere across the ring would mean that the B ring's mass is about 90% of Mimas' mass. Cassini observations show that the B ring-edge has several free normal modes, which are long-lived disturbances of the ring-edge that are not driven by any known satellite resonances. Although the mechanism that excites or sustains these normal modes is unknown, we can plant such a disturbance at a simulated ring's edge, and find that these modes persist without any damping for more than ~10^5 orbits or ~100 yrs despite the simulated ring's viscosity of 100 cm^2/sec. These simulations also indicate that impulsive disturbances at a ring can excite long-lived normal modes, which suggests that an impact in the recent past by perhaps a cloud of cometary debris might have excited these disturbances which are quite common to many of Saturn's sharp-edged rings.Comment: 55 pages, 13 figures, accepted for publication in the Astrophysical Journa

    Computational Geometry Column 42

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    A compendium of thirty previously published open problems in computational geometry is presented.Comment: 7 pages; 72 reference

    L-Arabinose Isomerase Formation in a Conditional Mutant of Gene \u3cem\u3earaA\u3c/em\u3e of \u3cem\u3eEscherichia coli\u3c/em\u3e B/r

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    A temperature-sensitive mutant of Escherichia coli in which the synthesis of l-arabinose isomerase is blocked during growth at 42 C was found to possess the following properties. (i) The mutation occurred in the structural gene for the isomerase, gene araA. (ii) During growth at elevated temperatures the mutant accumulates a product which is a precursor to the active enzyme. (iii) The precursor produced at 42 C is slowly converted to active enzyme at 28 C in the absence of protein and ribonucleic acid synthesis. It is concluded that the mutation results in a change in the structure of isomerase which causes formation of active enzyme to be thermolabile at a step beyond the level of translation

    A dynamic HAZOP case study using the Texas City refinery explosion

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    © 2016 Elsevier Ltd. The catastrophic explosion that occurred at Texas City on 23 March 2005 during the start-up of the raffinate splitter resulted in an estimated 15 deaths and 180 injuries. Since the incident, several studies have investigated the root causes of the disaster. Some contributing factors to the incident include wider organisational, process safety management, and human elements. There have also been some attempts to model the sequence of events before the incident, and the consequences of the resulting fires and explosions. This study provides a dynamic model of the sequence of events leading up to the incident and replicates the reported process variables during the isomerisation unit start-up on the day of the incident. The resulting simulation model is used as the framework for a dynamic hazard and operability (HAZOP) study

    The 200 MeV Pi+ induced single-nucleon removal from 24Mg

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    Nuclear gamma-rays in coincidence with outgoing pions or protons following single nucleon removal from Mg-24 by 200 MeV pions (+) were detected with Ge(Li) detectors. Differential cross sections are reported for gamma-rays from the first excited mirror states of Na-23 and Mg-23 in coincidence with positive pions or protons detected in particle telescopes at 30, 60, 90, 120, and 150 deg; angle-integrated absolute cross sections and cross section ratios are calculated. These results are compared with the predictions of a Pauli-blocked plane-wave impulse approximation (PWIA) and the intranuclear cascade (INC) and nucleon charge exchange (NCX) reaction models. The PWIA and the INC calculations generally agree with the angular dependence of the experimental results but not the absolute magnitude. The NCX calculation does not reproduce the observed cross section charge ratios

    Quasi-static and Dynamic Behavior of Additively Manufactured Metallic Lattice Cylinders

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    Lattice structures have tailorable mechanical properties which allows them to exhibit superior mechanical properties (per unit weight) beyond what is achievable through natural materials. In this paper, quasi-static and dynamic behavior of additively manufactured stainless steel lattice cylinders is studied. Cylindrical samples with internal lattice structure are fabricated by a laser powder bed fusion system. Equivalent hollow cylindrical samples with the same length, outer diameter, and mass (larger wall thickness) are also fabricated. Split Hopkinson bar is used to study the behavior of the specimens under high strain rate loading. It is observed that lattice cylinders reduce the transmitted wave amplitude up to about 21% compared to their equivalent hollow cylinders. However, the lower transmitted wave energy in lattice cylinders comes at the expense of a greater reduction in their stiffness, when compared to their equivalent hollow cylinder. In addition, it is observed that increasing the loading rate by five orders of magnitude leads to up to about 36% increase in the peak force that the lattice cylinder can carry, which is attributed to strain rate hardening effect in the bulk stainless steel material. Finite element simulations of the specimens under dynamic loads are performed to study the effect of strain rate hardening, thermal softening, and the failure mode on dynamic behavior of the specimens. Numerical results are compared with experimental data and good qualitative agreement is observed.Comment: 20th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matte

    On Folding a Polygon to a Polyhedron

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    We show that the open problem presented in "Geometric Folding Algorithms: Linkages, Origami, Polyhedra" [DO07] is solved by a theorem of Burago and Zalgaller [BZ96] from more than a decade earlier.Comment: 6 pages, 1 figur
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