1,351 research outputs found
A summary of recent NASA/Army contributions to rotorcraft vibrations and structural dynamics technology
The requirement for low vibrations has achieved the status of a critical design consideration in modern helicopters. There is now a recognized need to account for vibrations during both the analytical and experimental phases of design. Research activities in this area were both broad and varied and notable advances were made in recent years in the critical elements of the technology base needed to achieve the goal of a jet smooth ride. The purpose is to present an overview of accomplishments and current activities of govern and government-sponsored research in the area of rotorcraft vibrations and structural dynamics, focusing on NASA and Army contributions over the last decade or so. Specific topics addressed include: airframe finite-element modeling for static and dynamic analyses, analysis of coupled rotor-airframe vibrations, optimization of airframes subject to vibration constraints, active and passive control of vibrations in both the rotating and fixed systems, and integration of testing and analysis in such guises as modal analysis, system identification, structural modification, and vibratory loads measurement
Effect of reheating on electroweak baryogenesis
The latent heat released during the expansion of bubbles in the electroweak
phase transition reheats the plasma and causes the bubble growth to slow down.
This decrease of the bubble wall velocity affects the result of electroweak
baryogenesis. Since the efficiency of baryogenesis peaks for a wall velocity
, the resulting baryon asymmetry can either be enhanced or
suppressed, depending on the initial value of the wall velocity. We calculate
the evolution of the phase transition taking into account the release of latent
heat. We find that, although in the SM the baryon production is enhanced by
this effect, in the MSSM it causes a suppression to the final baryon asymmetry.Comment: 4 pages, 3 figures. References added. Revised version to be published
in Phys.Rev.
Estimation of GRB detection by FiberGLAST
FiberGLAST is one of several instrument concepts being developed for possible inclusion as the primary Gamma-ray Large Area Space Telescope (GLAST) instrument. The predicted FiberGLAST effective area is more than 12,000 cm2 for energies between 30 MeV and 300 GeV, with a field of view that is essentially flat from 0°â80°. The detector will achieve a sensitivity more than 10 times that of EGRET. We present results of simulations that illustrate the sensitivity of FiberGLAST for the detection of gamma-ray bursts
Flavour-Dependent and Basis Independent Measures of R Violation
We construct lepton flavour-dependent and basis-independent measures of
R-parity violation in the minimal supersymmetric extension of the Standard
Model (MSSM). We work in the context of exact supersymmetry, neglecting the
effects of Higgs vacuum expectation values and soft supersymmetry-breaking
terms. We devote particular attention to appropriate choices of flavour
eigenstates, and to the counting and enumeration of R-violating invariants in
two- and three-generation models. We also make an indicative application of our
results to derive possible basis-independent cosmological upper bounds on
flavour-dependent violation of R parity.Comment: 20 pages, Latex, requires axodraw.st
Cosmology of Brane Models with Radion Stabilization
We analyze the cosmology of the Randall-Sundrum model and that of compact
brane models in general in the presence of a radius stabilization mechanism. We
find that the expansion of our universe is generically in agreement with the
expected effective four dimensional description. The constraint (which is
responsible for the appearance of non-conventional cosmologies in these models)
that must be imposed on the matter densities on the two branes in the theory
without a stabilized radius is a consequence of requiring a static solution
even in the absence of stabilization. Such constraints disappear in the
presence of a stablizing potential, and the ordinary FRW
(Friedmann-Robertson-Walker) equations are reproduced, with the expansion
driven by the sum of the physical values of the energy densities on the two
branes and in the bulk. For the case of the Randall-Sundrum model we examine
the kinematics of the radion field, and find that corrections to the standard
FRW equations are small for temperatures below the weak scale. We find that the
radion field has renormalizable and unsuppressed couplings to Standard Model
particles after electroweak symmetry breaking. These couplings may have
important implications for collider searches. We comment on the possibility
that matter off the TeV brane could serve as a dark matter candidate.Comment: 35 pages, Late
Could the cosmic acceleration be transient? A cosmographic evaluation
A possible slowing down of the cosmic expansion is investigated through a
cosmographic approach. By expanding the luminosity distance to fourth order and
fitting the SN Ia data from the most recent compilations (Union, Constitution
and Union 2), the marginal likelihood distributions for the deceleration
parameter today suggest a recent reduction of the cosmic acceleration and
indicate that there is a considerable probability for . Also in contrast
to the prediction of the CDM model, the cosmographic
reconstruction permits a cosmic expansion history where the cosmic acceleration
could already have peaked and be presently slowing down, which would imply that
the recent accelerated expansion of the Universe is a transient phenomenon. It
is also shown that to describe a transient acceleration the luminosity distance
needs to be expanded at least to fourth order. The present cosmographic results
depend neither on the validity of general relativity nor on the matter-energy
contents of the Universe.Comment: 11 pages, 4 figures. Accepted for publication in Classical and
Quantum Gravit
Development and testing of a fiber/multianode photomultiplier system for use on FiberGLAST
A scintillating fiber detector is currently being studied for the NASA Gamma-Ray Large Area Space Telescope (GLAST) mission. This detector utilizes modules composed of a thin converter sheet followed by an x, y plane of scintillating fibers to examine the shower of particles created by high energy gamma-rays interacting in the converter material. The detector is composed of a tracker with 90 such modular planes and a calorimeter with 36 planes. The two major component of this detector are the scintillating fibers and their associated photodetectors. Here we present current status of development and test result of both of these. The Hamamatsu R5900-00-M64 multianode photomultiplier tube (MAPMT) is the baseline readout device. A characterization of this device has been performed including noise, cross- talk, gain variation, vibration, and thermal/vacuum test. A prototype fiber/MAPMT system has been tested at the Center for Advanced Microstructures and Devices at Louisiana State University with a photon beam and preliminary results are presented
1951: Abilene Christian College Bible Lectures - Full Text
Delivered in the Auditorium of Abilene Christian College
Abilene, Texas
February 18-22, 1951
Price, $3.00
FIRM FOUNDATION PUBLISHING HOUSE
Austin, Texa
First-order cosmological phase transitions in the radiation dominated era
We consider first-order phase transitions of the Universe in the
radiation-dominated era. We argue that in general the velocity of interfaces is
non-relativistic due to the interaction with the plasma and the release of
latent heat. We study the general evolution of such slow phase transitions,
which comprise essentially a short reheating stage and a longer phase
equilibrium stage. We perform a completely analytical description of both
stages. Some rough approximations are needed for the first stage, due to the
non-trivial relations between the quantities that determine the variation of
temperature with time. The second stage, instead, is considerably simplified by
the fact that it develops at a constant temperature, close to the critical one.
Indeed, in this case the equations can be solved exactly, including
back-reaction on the expansion of the Universe. This treatment also applies to
phase transitions mediated by impurities. We also investigate the relations
between the different parameters that govern the characteristics of the phase
transition and its cosmological consequences, and discuss the dependence of
these parameters with the particle content of the theory.Comment: 38 pages, 3 figures; v2: Minor changes, references added; v3: several
typos correcte
1954: Abilene Christian College Bible Lectures - Full Text
Preface
The 1954 Abilene Christian College Lectureship was one of the best attended and most successful in the history of the school. Considerable interest was manifested in the timely theme, âOvercoming Dangerous Tendencies,â and in the two special topics, âWays and Means of Doing Mission Work,â and âCaring For Widows and Orphans.â The reports from the mission fields were highly stimulating, and all in all, the speeches were unusually high caliber. The Panel Discussions were also on timely subjects and well presented. They received a warm response, as did also the thirty classes that were conducted each day. These classes were taught by persons expert in their particular fields, and covered a wide range of interests to the faithful, working Christian. We at Abilene Christian College predict for this book of Lectures a wide and hearty reception, and believe that its reading will issue in profit to the individual and to the church at large.
J. D. Thomas
Lectureship Directo
- âŠ