13,022 research outputs found
Tribolgy of selected ceramics at temperatures to 900 deg C
Results of fundamental and focused research on the tribological properties of ceramics are discussed. The basic friction and wear characteristics are given for ceramics of interest for use in gas trubine, adiabatic diesel, and Stirling engine applications. The importance of metal oxides in ceramic/metal sliding combinations is illustrated. The formulation and tribological additives are described. Friction and wear date are given for carbide and oxide-based composite coatings for temperatures to at least 900 C
Hardware development for the surface tension driven convection experiment aboard the USML-1 spacelab mission
The Surface Tension Driven Convection Experiment is a Space Transportation System flight experiment to study both transient and steady thermocapillary fluid flows aboard the USML-1 Spacelab mission planned for March 1992. Hardware is under development to establish the experimental conditions and perform the specified measurements, for both ground based research and the flight experiment in a Spacelab single rack. Major development areas include an infrared thermal imaging system for surface temperature measurement, a CO2 laser and control system for surface heating, and for flow visualization, a He-Ne laser and optical system in conjunction with an intensified video camera. For ground based work the components of each system were purchased or designed, and tested individually. The three systems will be interfaced with the balance of the experimental hardware and will constitute a working engineering model. A description of the three systems and examples of the component performance is given along with the plans for the development of flight hardware
On the orientational ordering of long rods on a lattice
We argue that a system of straight rigid rods of length k on square lattice
with only hard-core interactions shows two phase transitions as a function of
density, rho, for k >= 7. The system undergoes a phase transition from the
low-density disordered phase to a nematic phase as rho is increased from 0, at
rho = rho_c1, and then again undergoes a reentrant phase transition from the
nematic phase to a disordered phase at rho = rho_c2 < 1.Comment: epl.cl
Lorentz violating kinematics: Threshold theorems
Recent tentative experimental indications, and the subsequent theoretical
speculations, regarding possible violations of Lorentz invariance have
attracted a vast amount of attention. An important technical issue that
considerably complicates detailed calculations in any such scenario, is that
once one violates Lorentz invariance the analysis of thresholds in both
scattering and decay processes becomes extremely subtle, with many new and
naively unexpected effects. In the current article we develop several extremely
general threshold theorems that depend only on the existence of some energy
momentum relation E(p), eschewing even assumptions of isotropy or monotonicity.
We shall argue that there are physically interesting situations where such a
level of generality is called for, and that existing (partial) results in the
literature make unnecessary technical assumptions. Even in this most general of
settings, we show that at threshold all final state particles move with the
same 3-velocity, while initial state particles must have 3-velocities
parallel/anti-parallel to the final state particles. In contrast the various
3-momenta can behave in a complicated and counter-intuitive manner.Comment: V1: 32 pages, 6 figures, 3 tables. V2: 5 references adde
Optical-to-optical interface device
An investigation was conducted to develop an optical-to-optical interface device capable of performing real-time incoherent-to-incoherent optical image conversion. The photoactivated liquid crystal light valve developed earlier represented a prototype liquid crystal light valve device capable of performing these functions. A device was developed which had high performance and extended lifetime
Lattice Black Holes
We study the Hawking process on lattices falling into static black holes. The
motivation is to understand how the outgoing modes and Hawking radiation can
arise in a setting with a strict short distance cutoff in the free-fall frame.
We employ two-dimensional free scalar field theory. For a falling lattice with
a discrete time-translation symmetry we use analytical methods to establish
that, for Killing frequency and surface gravity satisfying
in lattice units, the continuum Hawking spectrum
is recovered. The low frequency outgoing modes arise from exotic ingoing modes
with large proper wavevectors that "refract" off the horizon. In this model
with time translation symmetry the proper lattice spacing goes to zero at
spatial infinity. We also consider instead falling lattices whose proper
lattice spacing is constant at infinity and therefore grows with time at any
finite radius. This violation of time translation symmetry is visible only at
wavelengths comparable to the lattice spacing, and it is responsible for
transmuting ingoing high Killing frequency modes into low frequency outgoing
modes.Comment: 26 pages, LaTeX, 2 figures included with psfig. Several improvements
in the presentation. One figure added. Final version to appear in Phys.Rev.
Supersymmetry algebra in N = 1 chiral supergravity
We consider the supersymmetry (SUSY) transformations in the chiral Lagrangian
for supergravity (SUGRA) with the complex tetrad following the method
used in the usual SUGRA, and present the explicit form of the SUSY
trasformations in the first-order form. The SUSY transformations are generated
by two independent Majorana spinor parameters, which are apparently different
from the constrained parameters employed in the method of the 2-form gravity.
We also calculate the commutator algebra of the SUSY transformations on-shell.Comment: 10 pages, late
Vector field models of modified gravity and the dark sector
We present a comprehensive investigation of cosmological constraints on the
class of vector field formulations of modified gravity called Generalized
Einstein-Aether models. Using linear perturbation theory we generate cosmic
microwave background and large-scale structure spectra for general parameters
of the theory, and then constrain them in various ways. We investigate two
parameter regimes: a dark-matter candidate where the vector field sources
structure formation, and a dark-energy candidate where it causes late-time
acceleration. We find that the dark matter candidate does not fit the data, and
identify five physical problems that can restrict this and other theories of
dark matter. The dark energy candidate does fit the data, and we constrain its
fundamental parameters; most notably we find that the theory's kinetic index
parameter can differ significantly from its CDM
value.Comment: 16 pages, 11 figure
Containment of a silicone fluid free surface in reduced gravity using barrier coatings
In support of the Surface Tension Driven Convection Experiment planned for flight aboard the Space Shuttle, tests were conducted under reduced gravity in the 2.2-sec Drop Tower and the 5.0-sec Zero-G facility at the NASA Lewis Research Center. The dynamics of controlling the test fluid, a 10-cSt viscosity silicone fluid in a low gravity environment were investigated using different container designs and barrier coatings. Three container edge designs were tested without a barrier coating; a square edge, a sharp edge with a 45-deg slope, and a sawtooth edge. All three edge designs were successful in containing the fluid below the edge. G-jitter experiments were made in scaled down containers subjected to horizontal accelerations. The data showed that a barrier coating is effective in containing silicone fluid under g-levels up to 10 sup -1 sub g sub 0. In addition, a second barrier coating was found which has similar anti-wetting characteristics and is also more durable
No Quantum Super-Minisuperspace with
We show that the quantum super-minisuperspace of N=1 supergravity with
has no non-trivial physical states for class A Bianchi models.
Hence, in super quantum cosmology, the vanishing of is a condition
for the existence of the universe. We argue that this result implies that in
full supergravity with there are no non-trivial physical states with
a finite number of fermionic fields. We use the Jacobson canonical formulation.Comment: 9 pages, LaTeX, CIEA-94-0
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