8,023 research outputs found
Application of quasi-homogeneous anisotropic laminates in grid-stiffened panel design
Composite laminates are derived for standard configurations with quasi-homogeneous anisotropic properties, whereby in-plane and out-of-plane stiffness properties are concomitant. Dimensionless parameters, and their relationship to the well-known ply- orientation-dependent lamination parameters, are also developed from which the elements of the extensional and bending stiffness matrices are readily calculated for any fiber/resin properties. The definitive list of laminate configurations for up to 21 plies is presented, together with graphical representations of the lamination parameter design space for standard ply orientations +45, -45, 0 and 90 degrees. Finally, the potential of quasi-homogeneous anisotropic laminates as an optimum design solution for anisogid structures is explored for cases where buckling and strength constraints are both active
Robust evolution system for Numerical Relativity
The paper combines theoretical and applied ideas which have been previously
considered separately into a single set of evolution equations for Numerical
Relativity. New numerical ingredients are presented which avoid gauge
pathologies and allow one to perform robust 3D calculations. The potential of
the resulting numerical code is demonstrated by using the Schwarzschild black
hole as a test-bed. Its evolution can be followed up to times greater than one
hundred black hole masses.Comment: 11 pages, 4 figures; figure correcte
No-go theorem for bimetric gravity with positive and negative mass
We argue that the most conservative geometric extension of Einstein gravity
describing both positive and negative mass sources and observers is bimetric
gravity and contains two copies of standard model matter which interact only
gravitationally. Matter fields related to one of the metrics then appear dark
from the point of view of an observer defined by the other metric, and so may
provide a potential explanation for the dark universe. In this framework we
consider the most general form of linearized field equations compatible with
physically and mathematically well-motivated assumptions. Using gauge-invariant
linear perturbation theory, we prove a no-go theorem ruling out all bimetric
gravity theories that, in the Newtonian limit, lead to precisely opposite
forces on positive and negative test masses.Comment: 19 pages, no figures, journal versio
Excision boundary conditions for black hole initial data
We define and extensively test a set of boundary conditions that can be
applied at black hole excision surfaces when the Hamiltonian and momentum
constraints of general relativity are solved within the conformal thin-sandwich
formalism. These boundary conditions have been designed to result in black
holes that are in quasiequilibrium and are completely general in the sense that
they can be applied with any conformal three-geometry and slicing condition.
Furthermore, we show that they retain precisely the freedom to specify an
arbitrary spin on each black hole. Interestingly, we have been unable to find a
boundary condition on the lapse that can be derived from a quasiequilibrium
condition. Rather, we find evidence that the lapse boundary condition is part
of the initial temporal gauge choice. To test these boundary conditions, we
have extensively explored the case of a single black hole and the case of a
binary system of equal-mass black holes, including the computation of
quasi-circular orbits and the determination of the inner-most stable circular
orbit. Our tests show that the boundary conditions work well.Comment: 23 pages, 23 figures, revtex4, corrected typos, added reference,
minor content changes including additional post-Newtonian comparison. Version
accepted by PR
A Capstone Course in Speech-Language Pathology
Capstone courses are considered a high impact practice with the potential to positively affect student learning. This article describes the design and implementation of a capstone in speech-language pathology designed to round out the undergraduate experience by shifting the learning focus away from disorder characteristics and onto individuals living with disabilities. The course is designed to teach students compassion, self-evaluation, and strategies for personal growth
Are gauge shocks really shocks?
The existence of gauge pathologies associated with the Bona-Masso family of
generalized harmonic slicing conditions is proven for the case of simple 1+1
relativity. It is shown that these gauge pathologies are true shocks in the
sense that the characteristic lines associated with the propagation of the
gauge cross, which implies that the name ``gauge shock'' usually given to such
pathologies is indeed correct. These gauge shocks are associated with places
where the spatial hypersurfaces that determine the foliation of spacetime
become non-smooth.Comment: 7 pages, 5 figures, REVTEX 4. Revised version, including corrections
suggested by referee
Towards a Singularity-Proof Scheme in Numerical Relativity
Progress in numerical relativity has been hindered for 30 years because of
the difficulties of avoiding spacetime singularities in numerical evolution. We
propose a scheme which excises a region inside an apparent horizon containing
the singularity. Two major ingredients of the scheme are the use of a
horizon-locking coordinate and a finite differencing which respects the causal
structure of the spacetime. Encouraging results of the scheme in the spherical
collapse case are given.Comment: 9 page
Lens Galaxy Properties of SBS1520+530: Insights from Keck Spectroscopy and AO Imaging
We report on an investigation of the SBS 1520+530 gravitational lens system
and its environment using archival HST imaging, Keck spectroscopic data, and
Keck adaptive-optics imaging. The AO imaging has allowed us to fix the lens
galaxy properties with a high degree of precision when performing the lens
modeling, and the data indicate that the lens has an elliptical morphology and
perhaps a disk. The new spectroscopic data suggest that previous determinations
of the lens redshift may be incorrect, and we report an updated, though
inconclusive, value z_lens = 0.761. We have also spectroscopically confirmed
the existence of several galaxy groups at approximately the redshift of the
lens system. We create new models of the lens system that explicitly account
for the environment of the lens, and we also include improved constraints on
the lensing galaxy from our adaptive-optics imaging. Lens models created with
these new data can be well-fit with a steeper than isothermal mass slope (alpha
= 2.29, with the density proportional to r^-alpha) if H_0 is fixed at 72
km/s/Mpc; isothermal models require H_0 ~ 50 km/s/Mpc. The steepened profile
may indicate that the lens is in a transient perturbed state caused by
interactions with a nearby galaxy.Comment: 12 pages, 10 figures, submitted to Ap
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