36,382 research outputs found
Leak test system
System for quantitative determination of leak rates in large pressurized compartments is described. Method uses pressure reference cylinder placed in thermal contact with internal environment of compartment. Construction of equipment and details of operational procedure are reported. Illustration of equipment is included
Time evolution of the Partridge-Barton Model
The time evolution of the Partridge-Barton model in the presence of the
pleiotropic constraint and deleterious somatic mutations is exactly solved for
arbitrary fecundity in the context of a matricial formalism. Analytical
expressions for the time dependence of the mean survival probabilities are
derived. Using the fact that the asymptotic behavior for large time is
controlled by the largest matrix eigenvalue, we obtain the steady state values
for the mean survival probabilities and the Malthusian growth exponent. The
mean age of the population exhibits a power law decayment. Some Monte
Carlo simulations were also performed and they corroborated our theoretical
results.Comment: 10 pages, Latex, 1 postscript figure, published in Phys. Rev. E 61,
5664 (2000
A Solution of the Maxwell-Dirac Equations in 3+1 Dimensions
We investigate a class of localized, stationary, particular numerical
solutions to the Maxwell-Dirac system of classical nonlinear field equations.
The solutions are discrete energy eigenstates bound predominantly by the
self-produced electric field.Comment: 12 pages, revtex, 2 figure
Angular Correlations in Internal Pair Conversion of Aligned Heavy Nuclei
We calculate the spatial correlation of electrons and positrons emitted by
internal pair conversion of Coulomb excited nuclei in heavy ion collisions. The
alignment or polarization of the nucleus results in an anisotropic emission of
the electron-positron pairs which is closely related to the anisotropic
emission of -rays. However, the angular correlation in the case of
internal pair conversion exhibits diverse patterns. This might be relevant when
investigating atomic processes in heavy-ion collisions performed at the Coulomb
barrier.Comment: 27 pages + 6 eps figures, uses revtex.sty and epsf.sty,
tar-compressed and uuencoded with uufile
Consequences of wall stiffness for a beta-soft potential
Modifications of the infinite square well E(5) and X(5) descriptions of
transitional nuclear structure are considered. The eigenproblem for a potential
with linear sloped walls is solved. The consequences of the introduction of
sloped walls and of a quadratic transition operator are investigated.Comment: RevTeX 4, 8 pages, as published in Phys. Rev.
Effect of Enhanced Trackbed Support on Railway/Highway At-Grade Crossing Performance
This report documents the long-term performance of 89 railway/highway at-grade crossings in Kentucky and adjacent states that contain asphalt underlayment underneath the ballast. Asphalt underlayment increases load carrying capacity for trains and highway vehicles while enhancing the support structureâs waterproofing and confinement. The asphalt underlayment crossings studied were installed at sites that had historically exhibited poor performance and short service lives. Most of the crossing sites evaluated were 10-15 years old; however, their ages ranged up to 30 years. All of the crossings investigated have shown excellent performance since the installation of asphalt underlayments. No crossing failures, such as excessive settlement or mud pumping, have been noted, and the service lives of the crossing surfaces have been significantly improved. Detailed results from periodic inspections are included. A literature survey focuses on eleven public agencies and railroad companies that implement asphalt underlayments. Most of these entities install asphalt underlayments at heavy traffic/tonnage crossings that have routinely shown evidence of weak support leading to inferior performance. This survey, along with the empirical results of this study, indicates that asphalt underlayment, irrespective of material used on crossing surfaces, extends the service life of crossings and improves their performance
Railway/Highway At-Grade Crossing Surface Management: An Overview
This report draws on data from around the United States to describe the features of a successful railway/highway at-grade crossing management program; it emphasizes the highway/trackbed structures and crossing surfaces. Developing a structurally adequate crossing system is imperative, as this will produce a smooth surface and a stable highway/trackbed that prolongs crossingsâ lifespans while keeping maintenance costs low and minimizing the number of disruptions encountered by highway and railway traffic. An overview of the guidance issued by several transportation agencies and organizations to facilitate the design and construction of railway/highway at-grade crossings is included here. Equally important to developing a robust network of crossings is defining a clear division of labor between federal, state, and local transportation agencies and railroad companies. As such, this report discusses administrative procedures and state-level regulations that influence railway/highway at-grade crossing practices. This information is presented for AASHTO, AREMA, FRA, FHWA, MUTCD, as well as several states, including Illinois, Indiana, Iowa, Georgia, Michigan, and West Virginia
Experimental and Numerical Analysis of Skin-Stiffener Separation Using a Seven-Point Bend Configuration
Skin-stiffener separation in stiffened composite panels consists of a complex interaction between multiple scales of progressive damage and failure mechanisms. This work used a superposed cohesive element method of modeling resistance curve effects that represents the structural interface of a unidirectional tape skin and a fabric stiffener. Finite element models using cohesive elements with input properties obtained from tape-to-fabric interface characterization tests were compared to experimental results of a stable skin-stiffener separation characterization test using a seven-point bend fixture. This fixture deformed the stiffened panel specimens into a buckled configuration which induced mixed-mode interlaminar stress states at the skin-stiffener interface. The advantage of this fixture was the potential for more stable damage initiation and delamination growth compared to a stringer-stiffened panel under axial compression. Use of the superposed cohesive elements showed promise, but the characterization of interface material properties as input to the cohesive elements remains a crucial component to be investigated to ensure accurate prediction of skin-stiffener separation
The relativistic self-energy in nuclear dynamics
It is a well known fact that Dirac phenomenology of nuclear forces predicts
the existence of large scalar and vector mean fields in matter. To analyse the
relativistic self-energy in a model independent way, modern high precision
nucleon-nucleon () potentials are mapped on a relativistic operator basis
using projection techniques. This allows to compare the various potentials at
the level of covariant amplitudes were a remarkable agreement is found. It
allows further to calculate the relativistic self-energy in nuclear matter in
Hartree-Fock approximation. Independent of the choice of the nucleon-nucleon
interaction large scalar and vector mean fields of several hundred MeV
magnitude are generated at tree level. In the framework of chiral EFT these
fields are dominantly generated by contact terms which occur at next-to-leading
order in the chiral expansion. Consistent with Dirac phenomenology the
corresponding low energy constants which generate the large fields are closely
connected to the spin-orbit interaction in scattering. The connection to
QCD sum rules is discussed as well.Comment: 49 pages, 13 figure
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