2,925 research outputs found
Dynamic modelling and estimation of the error due to asynchronism in a redundant asynchronous multiprocessor system
The use of Redundant Asynchronous Multiprocessor System to achieve ultrareliable Fault Tolerant Control Systems shows great promise. The development has been hampered by the inability to determine whether differences in the outputs of redundant CPU's are due to failures or to accrued error built up by slight differences in CPU clock intervals. This study derives an analytical dynamic model of the difference between redundant CPU's due to differences in their clock intervals and uses this model with on-line parameter identification to idenitify the differences in the clock intervals. The ability of this methodology to accurately track errors due to asynchronisity generate an error signal with the effect of asynchronisity removed and this signal may be used to detect and isolate actual system failures
Conformal Carroll groups and BMS symmetry
The Bondi-Metzner-Sachs (BMS) group is shown to be the conformal extension of
Levy-Leblond's "Carroll" group. Further extension to the Newman-Unti (NU) group
is also discussed in the Carroll framework.Comment: 10 pages, no figure. Fast Track COmmunication, to appear in Class.
Quant. Gra
Conformal Carroll groups
Conformal extensions of Levy-Leblond's Carroll group, based on geometric
properties analogous to those of Newton-Cartan space-time are proposed. The
extensions are labelled by an integer . This framework includes and extends
our recent study of the Bondi-Metzner-Sachs (BMS) and Newman-Unti (NU) groups.
The relation to Conformal Galilei groups is clarified. Conformal Carroll
symmetry is illustrated by "Carrollian photons". Motion both in the
Newton-Cartan and Carroll spaces may be related to that of strings in the
Bargmann space.Comment: 31 pages, no figures. Minor misprints corrected and clarifications
added. To be published in J. Phys.
Eisenhart lifts and symmetries of time-dependent systems
Certain dissipative systems, such as Caldirola and Kannai's damped simple
harmonic oscillator, may be modelled by time-dependent Lagrangian and hence
time dependent Hamiltonian systems with degrees of freedom. In this paper
we treat these systems, their projective and conformal symmetries as well as
their quantisation from the point of view of the Eisenhart lift to a Bargmann
spacetime in dimensions, equipped with its covariantly constant null
Killing vector field. Reparametrization of the time variable corresponds to
conformal rescalings of the Bargmann metric. We show how the Arnold map lifts
to Bargmann spacetime. We contrast the greater generality of the
Caldirola-Kannai approach with that of Arnold and Bateman. At the level of
quantum mechanics, we are able to show how the relevant Schr\"odinger equation
emerges naturally using the techniques of quantum field theory in curved
spacetimes, since a covariantly constant null Killing vector field gives rise
to well defined one particle Hilbert space. Time-dependent Lagrangians arise
naturally also in cosmology and give rise to the phenomenon of Hubble friction.
We provide an account of this for Friedmann-Lemaitre and Bianchi cosmologies
and how it fits in with our previous discussion in the non-relativistic limit.Comment: 34 pages, no figures. Minor corrections, some references adde
Independent Orbiter Assessment (IOA): Assessment of the hydraulics/water spray boiler subsystem
The results of the Independent Orbiter Assessment (IOA) of the Failure Modes and Effects Analysis (FMEA) and Critical Items List (CIL) are presented. The IOA effort first completed an analysis of the Hydraulics/Water Spray Boiler (HYD/WSB) hardware, generating draft failure modes and potential critical items. To preserve independence, this analysis was accomplished without reliance upon the results contained within the NASA FMEA/CIL documentation. The IOA results were then compared to the NASA FMEA/CIL baseline with proposed Post 51-L updates included. A resolution of each discrepancy from the comparison is provided through additional analysis as required. This report documents the results of that comparison for the Orbiter HYD/WSB hardware. The IOA product for the HYD/WSB analysis consisted of 447 failure mode worksheets that resulted in 183 potential critical items being identified. Comparison was made to the NASA baseline which consisted of 364 FMEAs and 111 CIL items. This comparison produced agreement on all but 68 FMEAs which caused differences in 23 CIL items
Comparisons of elastic and rigid blade-element rotor models using parallel processing technology for piloted simulations
A piloted comparison of rigid and aeroelastic blade-element rotor models was conducted at the Crew Station Research and Development Facility (CSRDF) at Ames Research Center. A simulation development and analysis tool, FLIGHTLAB, was used to implement these models in real time using parallel processing technology. Pilot comments and quantitative analysis performed both on-line and off-line confirmed that elastic degrees of freedom significantly affect perceived handling qualities. Trim comparisons show improved correlation with flight test data when elastic modes are modeled. The results demonstrate the efficiency with which the mathematical modeling sophistication of existing simulation facilities can be upgraded using parallel processing, and the importance of these upgrades to simulation fidelity
Solenoidal versus compressive turbulence forcing
We analyze the statistics and star formation rate obtained in high-resolution
numerical experiments of forced supersonic turbulence, and compare with
observations. We concentrate on a systematic comparison of solenoidal
(divergence-free) and compressive (curl-free) forcing, which are two limiting
cases of turbulence driving. Our results show that for the same RMS Mach
number, compressive forcing produces a three times larger standard deviation of
the density probability distribution. When self-gravity is included in the
models, the star formation rate is more than one order of magnitude higher for
compressive forcing than for solenoidal forcing.Comment: 1 page, to appear in the proceedings of the IAU General Assembly
Joint Discussion 14 "FIR2009: The ISM of Galaxies in the Far-Infrared and
Sub-Millimetre", ed. M. Cunningha
`Stringy' Newton-Cartan Gravity
We construct a "stringy" version of Newton-Cartan gravity in which the
concept of a Galilean observer plays a central role. We present both the
geodesic equations of motion for a fundamental string and the bulk equations of
motion in terms of a gravitational potential which is a symmetric tensor with
respect to the longitudinal directions of the string. The extension to include
a non-zero cosmological constant is given. We stress the symmetries and
(partial) gaugings underlying our construction. Our results provide a
convenient starting point to investigate applications of the AdS/CFT
correspondence based on the non-relativistic "stringy" Galilei algebra.Comment: 44 page
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