10,844 research outputs found
Preliminary Abundance Analysis of Galactic Bulge Main Sequence, Subgiant, and Giant Branch Stars Observed During Microlensing with Keck/HIRES
We present an abundance analysis of six main sequence turnoff, subgiant, and
giant branch stars toward the Galactic bulge that were observed with Keck/HIRES
during microlensing events. This is an early look at the first detailed
chemical analysis of main sequence stars in the Galactic bulge. Lensing events
allow the effective aperture of Keck to be increased beyond its current
dimensions; although, some events still stretched its spectroscopic
capabilities. Future large telescopes with high resolution and high throughput
spectrometers will allow the study of abundances in distant stellar populations
and in less evolved stars with greater ease.Comment: 8 pages including 2 figures. To appear in SPIE proceedings on
Astronomical Telescopes and Instrumentation. Uses spie.cl
A finite excluded volume bond-fluctuation model: Static properties of dense polymer melts revisited
The classical bond-fluctuation model (BFM) is an efficient lattice Monte
Carlo algorithm for coarse-grained polymer chains where each monomer occupies
exclusively a certain number of lattice sites. In this paper we propose a
generalization of the BFM where we relax this constraint and allow the overlap
of monomers subject to a finite energy penalty \overlap. This is done to vary
systematically the dimensionless compressibility of the solution in order
to investigate the influence of density fluctuations in dense polymer melts on
various s tatic properties at constant overall monomer density. The
compressibility is obtained directly from the low-wavevector limit of the
static structure fa ctor. We consider, e.g., the intrachain bond-bond
correlation function, , of two bonds separated by monomers along the
chain. It is shown that the excluded volume interactions are never fully
screened for very long chains. If distances smaller than the thermal blob size
are probed () the chains are swollen acc ording to the classical
Fixman expansion where, e.g., . More importantly, the
polymers behave on larger distances () like swollen chains of
incompressible blobs with P(s) \si m g^0s^{-3/2}.Comment: 46 pages, 12 figure
On the Dynamics and Disentanglement in Thin and Two-Dimensional Polymer Films
We present results from molecular dynamics simulations of strictly
two-dimensional (2D) polymer melts and thin polymer films in a slit geometry of
thickness of the order of the radius of gyration. We find that the dynamics of
the 2D melt is qualitatively different from that of the films. The 2D monomer
mean-square displacement shows a power law at intermediate times
instead of the law expected from Rouse theory for nonentangled
chains. In films of finite thickness, chain entanglements may occur. The impact
of confinement on the entanglement length has been analyzed by a
primitive path analysis. The analysis reveals that increases
strongly with decreasing film thickness.Comment: 6 pages, 3 figures, proceedings 3rd International Workshop on
Dynamics in Confinement (CONFIT 2006
The Expected Duration of Gamma-Ray Bursts in the Impulsive Hydrodynamic Models
Depending upon the various models and assumptions, the existing literature on
Gamma Ray Bursts (GRBs) mentions that the gross theoretical value of the
duration of the burst in the hydrodynamical models is tau~r^2/(eta^2 c), where
r is the radius at which the blastwave associated with the fireball (FB)
becomes radiative and sufficiently strong. Here eta = E/Mc^2, c is the speed of
light, E is initial lab frame energy of the FB, and M is the baryonic mass of
the same (Rees and Meszaros 1992). However, within the same basic framework,
some authors (like Katz and Piran) have given tau ~ r^2 /(eta c). We intend to
remove this confusion by considering this problem at a level deeper than what
has been considered so far. Our analysis shows that none of the previously
quoted expressions are exactly correct and in case the FB is produced
impulsively and the radiative processes responsible for the generation of the
GRB are sufficiently fast, its expected duration would be tau ~ar^2/(eta^2 c),
where a~O(10^1). We further discuss the probable change, if any, of this
expression, in case the FB propagates in an anisotropic fashion. We also
discuss some associated points in the context of the Meszaros and Rees
scenario.Comment: 21 pages, LATEX (AAMS4.STY -enclosed), 1 ps. Fig. Accepted in
Astrophysical Journa
Are polymer melts "ideal"?
It is commonly accepted that in concentrated solutions or melts
high-molecular weight polymers display random-walk conformational properties
without long-range correlations between subsequent bonds. This absence of
memory means, for instance, that the bond-bond correlation function, , of
two bonds separated by monomers along the chain should exponentially decay
with . Presenting numerical results and theoretical arguments for both
monodisperse chains and self-assembled (essentially Flory size-distributed)
equilibrium polymers we demonstrate that some long-range correlations remain
due to self-interactions of the chains caused by the chain connectivity and the
incompressibility of the melt. Suggesting a profound analogy with the
well-known long-range velocity correlations in liquids we find, for instance,
to decay algebraically as . Our study suggests a precise
method for obtaining the statistical segment length \bstar in a computer
experiment.Comment: 4 pages, 3 figure
Effect of Localized Damages on the Free Vibration Analysis of Civil Structures by Component-Wise Approach
Refined one-dimensional (1D) models are used to carry out free vibration analysis of civil engineering structures affected by local damages. The Carrera unified formulation (CUF) provides higher-order structural models to be formulated in a compact and, eventually, hierarchical manner. In the domain of the CUF, refined 1D models characterized by three-dimensional capabilities can be realized by using various function expansions of the generalized displacement field over the cross section. Recently, a component-wise (CW) approach was introduced by using CUF. CW gives a detailed physical description of multicomponent structures, since each component can be modeled with its geometrical and mechanical characteristics; that is, no reference surfaces, axes, or homogenization techniques are used. In the present work, combinations of quadratic Lagrange elements are used to describe the beam theory kinematics. This approach enables the highly-accurate analysis of complex civil structures such as truss structures, industrial buildings, and a multifloor building. In this context, damage scenarios are introduced with no loss of accuracy in the mathematical formulation by deteriorating the single component of the structure. Effects of damages are, thus, evaluated by free vibration analyses
Free-vibration analysis of space vehicle structures made by composite materials
This work investigates the effects of composite materials and non-structural masses on the dynamic behavior of space structure components and whole space vehicle. A refined one-dimensional model has been used in the analyses, and the effects of composite materials and of the fuel mass introduced as non-structural masses have been considered. The adopted refined one-dimensional Finite Element Model has been developed using the Carrera Unified Formulation. This numerical tool allows to develop a variable kinematic displacement field over the beam cross-section, that is, a set of Lagrange (LE) expansions polynomials was adopted for the cross-sectional displacement field approximation. The use of such one-dimensional models leads to the so-called component-wise (CW) approach in which stiffeners and plate are modeled using the same one-dimensional kinematic. Static and free vibration analysis of space structural components and complete space structures have been performed. Both compact and thin-walled structural configurations have been considered. The results have been assessed using analytical solutions or refined three-dimensional Finite Element Models. Composite materials and non-structural masses, e.g. the fuel mass or payload, have been included in the analysis. The results show the capability of the present model to provide a quasi three-dimensional solution with a low computational cost. The refined kinematic allows composite materials to be investigated accurately
High-fidelity modeling approaches for the analysis of reinforced structures using one-, two- and three-dimensional elements
The present paper proposes a method for analyzing reinforced thin-walled structures based on high-order one-, two- and three-dimensional finite elements (FE). Refined finite elements are developed in the domain of the Carrera unified formulation (CUF). The node-dependent kinematic approach (NDK), which allows to connect in an easy manner elements with incompatible kinematics, has been used to connect elements with different dimensions without the need of ad hoc connection techniques. The formulation ensures the continuity of the displacement at the interface preventing the onset of singularities that lead to inaccurate results when beam, plate and solid elements have to be coupled to solve complex structures. The effectiveness of the present method has been confirmed by comparing the results with those from literature and with those obtained using commercial finite element codes. Static and free-vibration analyses of reinforced panels have been carried out to demonstrate the capabilities of the present models. The results show that the limits of classical structural models can be easily overcome using the present approach, and at the same time, a quasi three-dimensional solution can be obtained with a large computational cost saving
Sequential pivotal mechanisms for public project problems
It is well-known that for several natural decision problems no budget
balanced Groves mechanisms exist. This has motivated recent research on
designing variants of feasible Groves mechanisms (termed as `redistribution of
VCG (Vickrey-Clarke-Groves) payments') that generate reduced deficit. With this
in mind, we study sequential mechanisms and consider optimal strategies that
could reduce the deficit resulting under the simultaneous mechanism. We show
that such strategies exist for the sequential pivotal mechanism of the
well-known public project problem. We also exhibit an optimal strategy with the
property that a maximal social welfare is generated when each player follows
it. Finally, we show that these strategies can be achieved by an implementation
in Nash equilibrium.Comment: 19 pages. The version without the appendix will appear in the Proc.
2nd International Symposium on Algorithmic Game Theory, 200
Frictional melting of peridotite and seismic slip
The evolution of the frictional strength along a fault at seismic slip rates (about 1 m/s) is
a key factor controlling earthquake mechanics. At mantle depths, friction-induced melting
and melt lubrication may influence earthquake slip and seismological data. We report on
laboratory experiments designed to investigate dynamic fault strength and frictional melting
processes in mantle rocks. We performed 20 experiments with Balmuccia peridotite in a
high-velocity rotary shear apparatus and cylindrical samples (21.8 mm in diameter) over a
wide range of normal stresses (5.4–16.1 MPa), slip rates (0.23–1.14 m/s), and displacements
(1.5–71 m). During the experiments, shear stress evolved with cumulative displacement in
five main stages (stages 1–5). In stage 1 (first strengthening), the coefficient of friction m
increased up to 0.4–0.7 (first peak in friction). In stage 2 (abrupt firstweakening), m decreased
to about 0.25–0.40. In stage 3 (gradual second strengthening), shear stress increased toward
a second peak in friction (m = 0.30–0.40). In stage 4 (gradual second weakening), the
shear stress decreased toward a steady state value (stage 5) with m = 0.15. Stages 1 and 2 are
of too short duration to be investigated in detail with the current experimental configuration.
By interrupting the experiments during stages 3, 4, and 5, microstructural (Field Emission
Scanning Electron Microscope) and geochemical (Electron Probe Micro-Analyzer and
Energy Dispersive X-Ray Spectroscopy) analysis of the slipping zone suggest that second
strengthening (stage 3) is associated with the production of a grain-supported melt-poor
layer, while second weakening (stage 4) and steady state (stage 5) are associated with the
formation of a continuous melt-rich layer with an estimated temperature up to 1780 C.
Microstructures formed during the experiments were very similar to those found in natural
ultramafic pseudotachylytes. By performing experiments at different normal stresses
and slip rates, (1) the ‘‘thermal’’ (as it includes the thermally activated first and second
weakening) slip distance to achieve steady state from the first peak in strength decreased
with increasing normal stress and slip rate and (2) the steady state shear stress slightly
increased with increasing normal stress and, for a given normal stress, decreased with
increasing slip rate. The ratio of shear stress versus normal stress was about 0.15, well below
the typical friction coefficient of rocks (0.6–0.8). The dependence of steady state shear stress
with normal stress was described by means of a constitutive equation for melt lubrication.
The presence of microstructures similar to those found in natural pseudotachylytes and the
determination of a constitutive equation that describes the experimental data allows
extrapolation of the experimental observations to natural conditions and to the study of
rupture dynamics in mantle rocks
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