100,398 research outputs found

    Nonlinear seismic analysis of a reactor structure impact between core components

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    The seismic analysis of the FFTF-PIOTA (Fast Flux Test Facility-Postirradiation Open Test Assembly), subjected to a horizontal DBE (Design Base Earthquake) is presented. The PIOTA is the first in a set of open test assemblies to be designed for the FFTF. Employing the direct method of transient analysis, the governing differential equations describing the motion of the system are set up directly and are implicitly integrated numerically in time. A simple lumped-nass beam model of the FFTF which includes small clearances between core components is used as a "driver" for a fine mesh model of the PIOTA. The nonlinear forces due to the impact of the core components and their effect on the PIOTA are computed

    Conservation Assessment for Sword Moss (Bryoxiphidium norvegicum (S.E. Bridel) W. Mitten)

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    ID: 8945; issued December 31, 2002INHS Technical Report prepared for USDA, Forest Service Vienna Ranger District Shawnee National Fores

    Gymnostoma tasmanianum sp nov., a fossil Casuarinaceae from the Early Oligocene of Little Rapid River, Tasmania, Australia

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    Int. J Plant Sci. 164(4):629–634. 2003. © 2003 by The University of ChicagoMacrofossils of Casuarinaceae from Early Oligocene sediments at Little Rapid River, Tasmania, are assigned to a new species, Gymnostoma tasmanianum. This is only the second species of fossil Gymnostoma to be formally described and the first species of Gymnostoma to be described from Tasmania. The species is known from both vegetative and reproductive organs. The new taxon is distinguished from other known species by its small stomata, short article length and tooth width, small length:width ratio of articles, glabrous articles, stomata in two to five rows, and teeth elongate with acute apices and sinuses. The fossil record shows that Gymnostoma once had a much wider distribution in Australia than its current occurrence in far north Queensland.Greg Guerin and Robert S. Hil

    Leaf cuticular morphology links Platanaceae and Proteaceae

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    Int. J. Plant Sci. 166(5):843–855. © 2005 by The University of Chicago.The leaf cuticular morphology of extant species of Platanus was investigated using light and scanning electron microscopy. All species are shown to possess trichome bases of the same type as those commonly found in Proteaceae. Of particular significance are compound forms that consist of an annular surface scar associated with more than one underlying epidermal cell. These are found on the adaxial leaf surfaces of all species of Platanus and are also clearly evident on the abaxial surface of Platanus orientalis. This type of trichome base is therefore interpreted as the first detected nonreproductive morphological synapomorphy linking Proteaceae and Platanaceae. Also, the laterocytic, sometimes paracytic, or anomocytic arrangement of subsidiary cells in Platanus is distinct from the general state in Proteaceae, which is brachyparacytic and presumably derived. In Bellendena, possibly the most basal genus of extant Proteaceae, subsidiary cell arrangements resemble those of Platanus. These results are discussed with respect to leaf fossil records of Proteales, where it is concluded that the combination of brachyparacytic stomata and compound trichome bases is strong evidence for Proteaceae.Raymond J. Carpenter, Robert S. Hill, and Gregory J. Jorda

    A modern control theory based algorithm for control of the NASA/JPL 70-meter antenna axis servos

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    A digital computer-based state variable controller was designed and applied to the 70-m antenna axis servos. The general equations and structure of the algorithm and provisions for alternate position error feedback modes to accommodate intertarget slew, encoder referenced tracking, and precision tracking modes are descibed. Development of the discrete time domain control model and computation of estimator and control gain parameters based on closed loop pole placement criteria are discussed. The new algorithm was successfully implemented and tested in the 70-m antenna at Deep Space Network station 63 in Spain

    On a class of linearizable Monge-Amp\`ere equations

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    Monge-Amp\`ere equations of the form, uxxuyy−uxy2=F(u,ux,uy)u_{xx}u_{yy}-u_{xy}^2=F(u,u_x,u_y) arise in many areas of fluid and solid mechanics. Here it is shown that in the special case F=uy4f(u,ux/uy)F=u_y^4f(u, u_x/u_y), where ff denotes an arbitrary function, the Monge-Amp\`ere equation can be linearized by using a sequence of Amp\`ere, point, Legendre and rotation transformations. This linearization is a generalization of three examples from finite elasticity, involving plane strain and plane stress deformations of the incompressible perfectly elastic Varga material and also relates to a previous linearization of this equation due to Khabirov [7]

    A new algorithm for modeling friction in dynamic mechanical systems

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    A method of modeling friction forces that impede the motion of parts of dynamic mechanical systems is described. Conventional methods in which the friction effect is assumed a constant force, or torque, in a direction opposite to the relative motion, are applicable only to those cases where applied forces are large in comparison to the friction, and where there is little interest in system behavior close to the times of transitions through zero velocity. An algorithm is described that provides accurate determination of friction forces over a wide range of applied force and velocity conditions. The method avoids the simulation errors resulting from a finite integration interval used in connection with a conventional friction model, as is the case in many digital computer-based simulations. The algorithm incorporates a predictive calculation based on initial conditions of motion, externally applied forces, inertia, and integration step size. The predictive calculation in connection with an external integration process provides an accurate determination of both static and Coulomb friction forces and resulting motions in dynamic simulations. Accuracy of the results is improved over that obtained with conventional methods and a relatively large integration step size is permitted. A function block for incorporation in a specific simulation program is described. The general form of the algorithm facilitates implementation with various programming languages such as FORTRAN or C, as well as with other simulation programs

    SU(3)/SU(2): the simplest Wess-Zumino-Witten term

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    The observation that SU(3)/SU(2) ~ S^5 implies the existence of a particularly simple quantized topological action, or Wess-Zumino-Witten (WZW) term. This action plays an important role in anomaly cancellation in extensions of the Standard Model electroweak sector. A closed form is presented for the action coupled to arbitrary gauge fields. The action is shown to be equivalent to a limit of the WZW term for SU(3) x SU(3) / SU(3). By reduction of SU(3) x U(1) / SU(2) x U(1) to SU(2) x U(1) / U(1), the construction gives a topological derivation of the WZW term for the Standard Model Higgs field.Comment: 11 pages, 1 figure; references adde

    A new state space model for the NASA/JPL 70-meter antenna servo controls

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    A control axis referenced model of the NASA/JPL 70-m antenna structure is combined with the dynamic equations of servo components to produce a comprehansive state variable (matrix) model of the coupled system. An interactive Fortran program for generating the linear system model and computing its salient parameters is described. Results are produced in a state variable, block diagram, and in factored transfer function forms to facilitate design and analysis by classical as well as modern control methods
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