42,486 research outputs found
A dynamical symmetry breaking model in Weyl space
The dynamical process following the breaking of Weyl geometry to Riemannian
geometry is considered by studying the motion of de Sitter bubbles in a Weyl
vacuum. The bubbles are given in terms of an exact, spherically symmetric thin
shell solution to the Einstein equations in a Weyl-Dirac theory with a
time-dependent scalar field of the form beta = f(t)/r. The dynamical solutions
obtained lead to a number of possible applications. An important feature of the
thin shell model is the manner in which beta provides a connection between the
interior and exterior geometries since information about the exterior geometry
is contained in the boundary conditions for beta.Comment: 18 pages, RevTex, to be published in J. Math. Phy
Nondestructive test of regenerative chambers
Flat panels simulating internally cooled regenerative thrust chamber walls were fabricated by electroforming, brazing and diffusion bonding to evaluate the feasibility of nondestructive evaluation techniques to detect bonds of various strength integrities. Ultrasonics, holography, and acoustic emission were investigated and found to yield useful and informative data regarding the presence of bond defects in these structures
A Blockchain-based Approach for Data Accountability and Provenance Tracking
The recent approval of the General Data Protection Regulation (GDPR) imposes
new data protection requirements on data controllers and processors with
respect to the processing of European Union (EU) residents' data. These
requirements consist of a single set of rules that have binding legal status
and should be enforced in all EU member states. In light of these requirements,
we propose in this paper the use of a blockchain-based approach to support data
accountability and provenance tracking. Our approach relies on the use of
publicly auditable contracts deployed in a blockchain that increase the
transparency with respect to the access and usage of data. We identify and
discuss three different models for our approach with different granularity and
scalability requirements where contracts can be used to encode data usage
policies and provenance tracking information in a privacy-friendly way. From
these three models we designed, implemented, and evaluated a model where
contracts are deployed by data subjects for each data controller, and a model
where subjects join contracts deployed by data controllers in case they accept
the data handling conditions. Our implementations show in practice the
feasibility and limitations of contracts for the purposes identified in this
paper
Nondestructive tests of regenerative chambers
The capabilities and limitations of nondestructive evaluation methods were studied to detect and locate bond deficiencies in regeneratively cooled thrust chambers for rocket engines. Flat test panels and a cylinder were produced to simulate regeneratively cooled thrust chamber walls. Planned defects with various bond integrities were produced in the panels to evaluate the sensitivity, accuracy, and limitations of nondestructive methods to define and locate bond anomalies. Holography, acoustic emission, and ultrasonic scan were found to yield sufficient data to discern bond quality when used in combination and in selected sequences. Bonding techniques included electroforming and brazing. Materials of construction included electroformed nickel bonded to Nickel 200 and OFHC copper, electroformed copper bonded to OFHC copper, and 300 series stainless steel brazed to OFHC copper. Variations in outer wall strength, wall thickness, and defect size were evaluated for nondestructive test response
Two-stage clustering in genotype-by-environment analyses with missing data
Cluster analysis has been commonly used in genotype-by-environment (G x E) analyses, but current methods are inadequate when the data matrix is incomplete. This paper proposes a new method, referred to as two-stage clustering, which relies on a partitioning of squared Euclidean distance into
two independent components, the G x E interaction and the genotype main effect. These components are used in the first and second stages of clustering respectively. Two-stage clustering forms the basis for imputing missing values in the G x E matrix so that a more complete data array is available for other GxE analyses. Imputation for a given genotype uses information from genotypes with similar interaction profiles. This imputation method is shown to improve on an existing nearest cluster method that confounds the G x E interaction and the genotype main effect
Cartesian Bicategories II
The notion of cartesian bicategory, introduced by Carboni and Walters for
locally ordered bicategories, is extended to general bicategories. It is shown
that a cartesian bicategory is a symmetric monoidal bicategory
A Coronal Hole's Effects on CME Shock Morphology in the Inner Heliosphere
We use STEREO imagery to study the morphology of a shock driven by a fast
coronal mass ejection (CME) launched from the Sun on 2011 March 7. The source
region of the CME is located just to the east of a coronal hole. The CME ejecta
is deflected away from the hole, in contrast with the shock, which readily
expands into the fast outflow from the coronal hole. The result is a CME with
ejecta not well centered within the shock surrounding it. The shock shape
inferred from the imaging is compared with in situ data at 1 AU, where the
shock is observed near Earth by the Wind spacecraft, and at STEREO-A. Shock
normals computed from the in situ data are consistent with the shock morphology
inferred from imaging.Comment: to appear in The Astrophysical Journa
Environmental protection of titanium alloys at high temperatures
Various concepts were evaluated for protecting titanium alloys from oxygen contamination at 922 K (1200 F) and from hot-salt stress-corrosion at 755 K (900 F). It is indicated that oxygen-contamination resistance can be provided by a number of systems, but for hot-salt stress-corrosion resistance, factors such as coating integrity become very important. Titanium aluminides resist oxygen ingress at 922 K through the formation of alumina (on TiAl3) or modified TiO2 (on Ti3Al, TiAl) scales. TiAl has some resistance to attack by hot salt, but has limited ductility. Ductile Ti-Ni and Ti-Nb-Cr-Al alloys provide limited resistance to oxygen ingress, but are not greatly susceptible to hot-salt stress-corrosion cracking
Weyl geometry approach to describe planetary systems
In the present work we show that planetary mean distances can be calculated
through considering the Weyl geometry. We interpret the Weyl gauge field as a
vector field associated with the hypercharge of the particles and apply the
gauge concept of the Weyl geometry. The results obtained are shown to agree
with the observed orbits of all the planets and of the asteroid belt in the
solar system, with some empty states.Comment: 7 pages, no figure
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