26,038 research outputs found
Studies of convection in a solidifying binary mixture at reduced gravity
A great deal of interest was generated recently in the possibility of producing new materials in the reduced gravity environment provided during the forthcoming missions of Spacelab. The range of possibilities extend from producing large crystals of uniform properties to manufacturing materials with unique properties. Most of these processes involve the solidification of materials from the liquid state. Convective motions within the liquid during solidification can influence the local material composite and the shape of the solid-liquid interface which may result in solids with non-uniform properties and crystal defects. The microgravity environment of Spacelab is being viewed as one in which the buoyancy forces are eliminated so that convection driven by thermal gradients does occur, resulting in an improved solidification process. However, convection may occur for other reasons and whether convection is negligible or not during solidification constitutes processing in low-gravity environment. Little information exists presently on convection during solidification under such circumstances. A continuation of an analytical investigation into the nature of convective motion in a binary liquid layer due to surface tension forces during its solidification is reported. The onset of convection will be determined through a stability analysis which is described
Development of the ARIES parachute system
The design and testing of a two-stage parachute system to recover a space telescope weighing up to 2000 pounds is described. The system consists of a 15-ft dia ribbon parachute reefed to 50% for 10 seconds and a 73-ft dia paraform or cross second stage reefed to 10% for 10 seconds. The results of eight drop tests and one operational rocket launched flight and recovery are presented. A successful operational recovery of a 1600-lb NASA space telescope was conducted. The payload was launched by a second stage Minuteman rocket to an altitude of about 300 miles above sea level
The Stability of an Isotropic Cosmological Singularity in Higher-Order Gravity
We study the stability of the isotropic vacuum Friedmann universe in gravity
theories with higher-order curvature terms of the form
added to the Einstein-Hilbert Lagrangian of general relativity on approach to
an initial cosmological singularity. Earlier, we had shown that, when ,
a special isotropic vacuum solution exists which behaves like the
radiation-dominated Friedmann universe and is stable to anisotropic and small
inhomogeneous perturbations of scalar, vector and tensor type. This is
completely different to the situation that holds in general relativity, where
an isotropic initial cosmological singularity is unstable in vacuum and under a
wide range of non-vacuum conditions. We show that when , although a
special isotropic vacuum solution found by Clifton and Barrow always exists, it
is no longer stable when the initial singularity is approached. We find the
particular stability conditions under the influence of tensor, vector, and
scalar perturbations for general for both solution branches. On approach to
the initial singularity, the isotropic vacuum solution with scale factor
is found to be stable to tensor perturbations for and stable to vector perturbations for , but is
unstable as otherwise. The solution with scale factor
is not relevant to the case of an initial singularity for
and is unstable as for all for each type of perturbation.Comment: 25 page
Cosmological models with flat spatial geometry
The imposition of symmetries or special geometric properties on submanifolds
is less restrictive than to impose them in the full space-time. Starting from
this idea, in this paper we study irrotational dust cosmological models in
which the geometry of the hypersurfaces generated by the fluid velocity is
flat, which supposes a relaxation of the restrictions imposed by the
Cosmological Principle. The method of study combines covariant and tetrad
methods that exploits the geometrical and physical properties of these models.
This procedure will allow us to determine all the space-times within this class
as well as to study their properties. Some important consequences and
applications of this study are also discussed.Comment: 12 pages, LaTeX2e, IOP style. To appear in Classical and Quantum
Gravit
The bag and size limit review: new regulations and summary of submissions
Since the new bag and size limits were introduced in December 1991, there have been many suggestions for refinement and improvement of these regulations. This paper includes the summary of submissions to Fisheries Management Paper No 52 - Review of Bag and Size Limit Proposals for Western Australian Recreational Fishers and details new regulations which will come into force on 1 July 1995
Deeply Virtual Compton Scattering on Spin-1 Nuclei
We consider the Generalized Parton Distributions for spin-1 nuclei in general
and on the deuteron in particular. We use the impulse approximation to obtain a
convolution model for them. Sum rules are used to check the validity of the
approach and to estimate the importance of higher Fock-space states in the
deuteron. Numerical predictions for the Beam Spin Asymmetry in deeply virtual
Compton scattering are presented.Comment: 4 pages, Talk given at the XVIth International Conference on
Particles and Nuclei (PANIC02), Osaka, Japan, 30 September 4 October 200
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