130 research outputs found
Consortium for materials development in space
The status of the Consortium for Materials Development in Space (CMDS) is reviewed. Individual CMDS materials projects and flight opportunities on suborbital and orbital carriers are outlined. Projects include: surface coatings and catalyst production; non-linear optical organic materials; physical properties of immiscible polymers; nuclear track detectors; powdered metal sintering; iron-carbon solidification; high-temperature superconductors; physical vapor transport crystal growth; materials preparation and longevity in hyperthermal oxygen; foam formation; measurement of the microgravity environment; and commercial management of space fluids
Model for the spatio-temporal intermittency of the energy dissipation in turbulent flows
Modeling the intermittent behavior of turbulent energy dissipation processes
both in space and time is often a relevant problem when dealing with phenomena
occurring in high Reynolds number flows, especially in astrophysical and space
fluids. In this paper, a dynamical model is proposed to describe the
spatio-temporal intermittency of energy dissipation rate in a turbulent system.
This is done by using a shell model to simulate the turbulent cascade and
introducing some heuristic rules, partly inspired by the well known -model,
to construct a spatial structure of the energy dissipation rate. In order to
validate the model and to study its spatially intermittency properties, a
series of numerical simulations have been performed. These show that the level
of spatial intermittency of the system can be simply tuned by varying a single
parameter of the model and that scaling laws in agreement with those obtained
from experiments on fully turbulent hydrodynamic flows can be recovered. It is
finally suggested that the model could represent a useful tool to simulate the
spatio-temporal intermittency of turbulent energy dissipation in those high
Reynolds number astrophysical fluids where impulsive energy release processes
can be associated to the dynamics of the turbulent cascade.Comment: 22 pages, 9 figure
Liquid storage tank venting device for zero gravity environment Patent
Venting device for liquid propellant storage tank using magnetic field to separate liquid and gaseous phase
Suppression of Shot-Noise in Quantum Cavities: Chaos vs. Disorder
We investigate the behavior of the shot-noise power through quantum
mechanical cavities in the semiclassical limit of small electronic wavelength.
In the absence of impurity scattering, the Fano factor , giving the noise to
current ratio, was previously found to disappear as more and more classical,
hence deterministic and noiseless transmission channels open up. We investigate
the behavior of as diffractive impurities are added inside the cavity. We
find that recovers its universal value provided (i) impurities cover the
full cavity so that only a set of zero measure of classical trajectories may
avoid them, and (ii) the impurity scattering rate exceeds the inverse dwell
time through the cavity. If condition (i) is not satisfied, saturates below
its universal value, even in the limit of strong scattering. Our results
corroborate the validity of the two-phase fluid model according to which the
electronic flow splits into two well separated components, a classical
deterministic fluid and a stochastic quantum-mechanical fluid. Only the latter
carries shot-noise.Comment: 6 pages, 3 figures, to appear in the proceedings of the fourth
conference on ``Unsolved Problems of Noise and Fluctuations in Physics,
Biology and High Technology'
On the dynamics of a self-gravitating medium with random and non-random initial conditions
The dynamics of a one-dimensional self-gravitating medium, with initial
density almost uniform is studied. Numerical experiments are performed with
ordered and with Gaussian random initial conditions. The phase space portraits
are shown to be qualitatively similar to shock waves, in particular with
initial conditions of Brownian type. The PDF of the mass distribution is
investigated.Comment: Latex, figures in eps, 23 pages, 11 figures. Revised versio
Limitations of the Standard Gravitational Perfect Fluid Paradigm
We show that the standard perfect fluid paradigm is not necessarily a valid
description of a curved space steady state gravitational source. Simply by
virtue of not being flat, curved space geometries have to possess intrinsic
length scales, and such length scales can affect the fluid structure. For modes
of wavelength of order or greater than such scales eikonalized geometrical
optics cannot apply and rays are not geodesic. Covariantizing thus entails not
only the replacing of flat space functions by covariant ones, but also the
introduction of intrinsic scales that were absent in flat space. In principle
it is thus unreliable to construct the curved space energy-momentum tensor as
the covariant generalization of a geodesic-based flat spacetime energy-momentum
tensor. By constructing the partition function as an incoherent average over a
complete set of modes of a scalar field propagating in a curved space
background, we show that for the specific case of a static, spherically
symmetric geometry, the steady state energy-momentum tensor that ensues will in
general be of the form
where the
anisotropic is a symmetric, traceless rank two tensor which
obeys . Such a type term is absent for an
incoherently averaged steady state fluid in a spacetime where there are no
intrinsic length scales, and in principle would thus be missed in a
covariantizing of a flat spacetime . While the significance of such
type terms would need to be evaluated on a case by case basis,
through the use of kinetic theory we reassuringly find that the effect of such
type terms is small for weak gravity stars where perfect fluid
sources are commonly used.Comment: Final version to appear in General Relativity and Gravitation (the
final publication is available at http://www.springerlink.com). 29 pages, 1
figur
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