13,670 research outputs found
Friction measuring apparatus Patent
Kinetic and static friction force measurement between magnetic tape and magnetic head surface
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Integrity static analysis of COTS/SOUP
This paper describes the integrity static analysis approach developed to support the justification of commercial off-the-shelf software (COTS) used in a safety-related system. The static analysis was part of an overall software qualification programme, which also included the work reported in our paper presented at Safecomp 2002. Integrity static analysis focuses on unsafe language constructs and “covert” flows, where one thread can affect the data or control flow of another thread. The analysis addressed two main aspects: the internal integrity of the code (especially for the more critical functions), and the intra-component integrity, checking for covert channels. The analysis process was supported by an aggregation of tools, combined and engineered to support the checks done and to scale as necessary. Integrity static analysis is feasible for industrial scale software, did not require unreasonable resources and we provide data that illustrates its contribution to the software qualification programme
Force indeterminacy in the jammed state of hard disks
Granular packings of hard discs are investigated by means of contact dynamics
which is an appropriate technique to explore the allowed force-realizations in
the space of contact forces. Configurations are generated for given values of
the friction coefficient, and then an ensemble of equilibrium forces is found
for fixed contacts. We study the force fluctuations within this ensemble. In
the limit of zero friction the fluctuations vanish in accordance with the
isostaticity of the packing. The magnitude of the fluctuations has a
non-monotonous friction dependence. The increase for small friction can be
attributed to the opening of the angle of the Coulomb cone, while the decrease
as friction increases is due to the reduction of connectivity of the
contact-network, leading to local, independent clusters of indeterminacy. We
discuss the relevance of indeterminacy to packings of deformable particles and
to the mechanical response properties.Comment: 4 pages, 3 figures. Minor changes, journal reference adde
Conformal mechanics inspired by extremal black holes in d=4
A canonical transformation which relates the model of a massive relativistic
particle moving near the horizon of an extremal black hole in four dimensions
and the conventional conformal mechanics is constructed in two different ways.
The first approach makes use of the action-angle variables in the angular
sector. The second scheme relies upon integrability of the system in the sense
of Liouville.Comment: V2: presentation improved, new material and references added; the
version to appear in JHE
Granular Pressure and the Thickness of a Layer Jamming on a Rough Incline
Dense granular media have a compaction between the random loose and random
close packings. For these dense media the concept of a granular pressure
depending on compaction is not unanimously accepted because they are often in a
"frozen" state which prevents them to explore all their possible microstates, a
necessary condition for defining a pressure and a compressibility
unambiguously. While periodic tapping or cyclic fluidization have already being
used for that exploration, we here suggest that a succession of flowing states
with velocities slowly decreasing down to zero can also be used for that
purpose. And we propose to deduce the pressure in \emph{dense and flowing}
granular media from experiments measuring the thickness of the granular layer
that remains on a rough incline just after the flow has stopped.Comment: 10 pages, 2 figure
Forward projection of transient signals obtained from a fiber-optic pressure sensor
An analytical/experimental approach is presented to reconstruct the space–time pressure field in a plane and forward project the resultant space–time pressure field using tomographic and wave vector time-domain methods. Transient pressure signals from an underwater ultrasonic planar transducer are first measured using a line fiber-optic pressure sensor which is scanned across a plane at a fixed distance z0 from the transducer. The resulting spatial line integrals in the plane are time-dependent signals which are first used to reconstruct the space–time pressure field in the plane via simply implemented tomographic methods. These signals are then used to forward project the space–time pressure field to arbitrary planes employing a wave vector time-domain method. Verification of the method is first presented using synthetic signals and the impulse response approach. An experimental verification of the approach is then presented using an ultrasonic planar transducer. The results of the projected and experimental fields are compared at various distances for synthetic signals and experimental data. Good correlation is found between the calculated, projected, and experimental data
Fermi-Walker gauge in 2+1 dimensional gravity.
It is shown that the Fermi-Walker gauge allows the general solution of
determining the metric given the sources, in terms of simple quadratures. We
treat the general stationary problem providing explicit solving formulas for
the metric and explicit support conditions for the energy momentum tensor. The
same type of solution is obtained for the time dependent problem with circular
symmetry. In both cases the solutions are classified in terms of the invariants
of the Wilson loops outside the sources. The Fermi-Walker gauge, due to its
physical nature, allows to exploit the weak energy condition and in this
connection it is proved that, both for open and closed universes with
rotational invariance, the energy condition imply the total absence of closed
time like curves. The extension of this theorem to the general stationary
problem, in absence of rotational symmetry is considered. At present such
extension is subject to some assumptions on the behavior of the determinant of
the dreibein in this gauge. PACS number: 0420Comment: 28 pages, RevTex, no figure
Axisymmetric Stationary Solutions as Harmonic Maps
We present a method for generating exact solutions of Einstein equations in
vacuum using harmonic maps, when the spacetime possesses two commutating
Killing vectors. This method consists in writing the axisymmetric stationry
Einstein equations in vacuum as a harmonic map which belongs to the group
SL(2,R), and decomposing it in its harmonic "submaps". This method provides a
natural classification of the solutions in classes (Weil's class, Lewis' class
etc).Comment: 17 TeX pages, one table,( CINVESTAV- preprint 12/93
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