3,320 research outputs found
A method of limit point calculation in finite element structural analysis
An approach is presented for the calculation of limit points for structures described by discrete coordinates, and whose governing equations derive from finite element concepts. The nonlinear load-displacement path of the imperfect structure is first traced by use of a direct iteration scheme and the determinant of the governing algebraic equations is calculated at each solution point. The limit point is then established by extrapolation and imposition of the condition of zero slope of the plot of load vs. determinant. Three problems are solved in illustration of the approach and in comparison with alternative procedures and test data
A finite element procedure for nonlinear prebuckling and initial postbuckling analysis
A procedure cast in a form appropriate to the finite element method is presented for geometrically nonlinear prebuckling and postbuckling structural analysis, including the identification of snap-through type of buckling. The principal features of this procedure are the use of direct iteration for solution of the nonlinear algebraic equations in the prebuckling range, an interpolation scheme for determination of the initial bifurcation point, a perturbation method in definition of the load-displacement behavior through the postbuckling regime, and extrapolation in determination of the limit point for snap-through buckling. Three numerical examples are presented in illustration of the procedure and in comparison with alternative approaches
Progressive Mauve: Multiple alignment of genomes with gene flux and rearrangement
Multiple genome alignment remains a challenging problem. Effects of
recombination including rearrangement, segmental duplication, gain, and loss
can create a mosaic pattern of homology even among closely related organisms.
We describe a method to align two or more genomes that have undergone
large-scale recombination, particularly genomes that have undergone substantial
amounts of gene gain and loss (gene flux). The method utilizes a novel
alignment objective score, referred to as a sum-of-pairs breakpoint score. We
also apply a probabilistic alignment filtering method to remove erroneous
alignments of unrelated sequences, which are commonly observed in other genome
alignment methods. We describe new metrics for quantifying genome alignment
accuracy which measure the quality of rearrangement breakpoint predictions and
indel predictions. The progressive genome alignment algorithm demonstrates
markedly improved accuracy over previous approaches in situations where genomes
have undergone realistic amounts of genome rearrangement, gene gain, loss, and
duplication. We apply the progressive genome alignment algorithm to a set of 23
completely sequenced genomes from the genera Escherichia, Shigella, and
Salmonella. The 23 enterobacteria have an estimated 2.46Mbp of genomic content
conserved among all taxa and total unique content of 15.2Mbp. We document
substantial population-level variability among these organisms driven by
homologous recombination, gene gain, and gene loss. Free, open-source software
implementing the described genome alignment approach is available from
http://gel.ahabs.wisc.edu/mauve .Comment: Revision dated June 19, 200
Dust sublimation by GRBs and its implications
The prompt optical flash recently detected accompanying GRB990123 suggests
that, for at least some GRBs, gamma-ray emission is accompanied by prompt
optical-UV emission with luminosity L(1-7.5eV)=10^{49}(\Delta\Omega/4\pi)erg/s,
where \Delta\Omega is the solid angle into which gamma-ray and optical-UV
emission is beamed. Such an optical-UV flash can destroy dust in the beam by
sublimation out to an appreciable distance, approximately 10 pc, and may clear
the dust out of as much as 10^7(\Delta\Omega/4\pi)M_sun of molecular cloud
material on an apparent time scale of 10 seconds. Detection of time dependent
extinction on this time scale would therefore provide strong constraints on the
GRB source environment. Dust destruction implies that existing, or future,
observations of not-heavily-reddened fireballs are not inconsistent with GRBs
being associated with star forming regions. In this case, however, if gamma-ray
emission is highly beamed, the expanding fireball would become reddened on a 1
week time scale.
If the optical depth due to dust beyond approximately 8 pc from the GRB is
0.2<\tau_V<2, most of the UV flash energy is converted to infra-red, \lambda
\sim 1 micron, radiation with luminosity \sim 10^{41} erg/s extending over an
apparent duration of \sim 20(1+z)(\Delta\Omega/0.01) day. Dust infra-red
emission may already have been observed in GRB970228 and GRB980326, and may
possibly explain their unusual late time behavior.Comment: 16 pages, including 1 figure, submitted to Ap
Radiative proton-antiproton annihilation and isospin mixing in protonium
A detailed analysis of the radiative annihilation is made in the
framework of a two-step formalism, the annihilates into meson
channels containing a vector meson with a subsequent conversion into a photon
via the vector dominance model (VDM). Both steps are derived from the
underlying quark model. First, branching ratios for radiative protonium
annihilation are calculated and compared with data. Then, details of the
isospin interference are studied for different models of the initial protonium
state and also for different kinematical form factors. The isospin interference
is shown to be uniquely connected to the mixing in the
protonium state. Values of the interference terms directly deduced from data
are consistent with theoretical expectations, indicating a dominant
component for the and a sizable component for the
protonium state. The analysis is extended to the
transition, where the large observed branching ratio remains unexplained in the
VDM approach.Comment: 34 pages, RevTeX, 2 figures, to appear in Phys. Rev. C; typos
correcte
Desynchronizing two oscillators while stimulating and observing only one
Synchronization of two or more self-sustained oscillators is a well-known and
studied phenomenon, appearing both in natural and designed systems. In some
cases, the synchronized state is undesired, and the aim is to destroy synchrony
by external intervention. In this paper, we focus on desynchronizing two
self-sustained oscillators by short pulses delivered to the system in a
phase-specific manner. We analyze a non-trivial case when we cannot access both
oscillators but stimulate only one. The following restriction is that we can
monitor only one unit, be it a stimulated or non-stimulated one. First, we use
a system of two coupled Rayleigh oscillators to demonstrate how a loss of
synchrony can be induced by stimulating a unit once per period at a specific
phase and detected by observing consecutive inter-pulse durations. Next, we
exploit the phase approximation to develop a rigorous theory formulating the
problem in terms of a map. We derive exact expressions for the phase --
isostable coordinates of this coupled system and show a relation between the
phase and isostable response curves to the phase response curve of the
uncoupled oscillator. Finally, we demonstrate how to obtain phase response
information from the system using time series and discuss the differences
between observing the stimulated and unstimulated oscillator
Non-minimal Einstein-Yang-Mills-Higgs theory: Associated, color and color-acoustic metrics for the Wu-Yang monopole model
We discuss a non-minimal Einstein-Yang-Mills-Higgs model with uniaxial
anisotropy in the group space associated with the Higgs field. We apply this
theory to the problem of propagation of color and color-acoustic waves in the
gravitational background related to the non-minimal regular Wu-Yang monopole.Comment: 14 pages, no figure
High-order phase reduction for coupled 2D oscillators
Phase reduction is a general approach to describe coupled oscillatory units
in terms of their phases, assuming that the amplitudes are enslaved. For such a
reduction, the coupling should be small, but one also expects the reduction to
be valid for finite coupling. This paper presents a general framework allowing
us to obtain coupling terms in higher orders of the coupling parameter for
generic two-dimensional oscillators and arbitrary coupling terms. The theory is
illustrated with an accurate prediction of Arnold's tongue for the van der Pol
oscillator exploiting higher-order phase reduction
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