48,044 research outputs found
Conversion and verification procedure for goal-based control programs
Fault tolerance and safety verification of control systems are essential for the success of autonomous robotic systems. A control architecture called Mission Data System, developed at the Jet Propulsion Laboratory, takes a goal-based control approach. In this paper, a method for converting goal network control programs into linear hybrid systems is developed. The linear hybrid system can then be verified for safety in the presence of failures using existing symbolic model checkers. An example task is developed and successfully verified using HyTech, a symbolic model checking software for linear hybrid systems
Eccentric discs in binaries with intermediate mass ratios: Superhumps in the VY Sculptoris stars
We investigate the role of the eccentric disc resonance in systems with mass
ratios q greater than 1/4, and demonstrate the effects that changes in the mass
flux from the secondary star have upon the disc radius and structure. The
addition of material with low specific angular momentum to its outer edge
restricts a disc radially. Should the mass flux from the secondary be reduced,
it is possible for the disc in a system with mass ratio as large as 1/3 to
expand to the 3:1 eccentric inner Lindblad resonance and for superhumps to be
excited.Comment: 6 pages with 7 figures, accepted by MNRA
SPH simulations of irradiation-driven warped accretion discs and the long periods in X-ray binaries
We present three dimensional smoothed particle hydrodynamics (SPH)
calculations of irradiation-driven warping of accretion discs. Initially
unwarped planar discs are unstable to the radiation reaction when the disc is
illuminated by a central radiation source. The disc warps and tilts and
precesses slowly in a retrograde direction; its shape continuously flexes in
response to the changing orientation of the Roche potential. We simulate ten
systems: eight X-ray binaries, one cataclysmic variable (CV), and a `generic'
low mass X-ray binary (LMXB). We adopt system parameters from observations and
tune a single parameter: our model X-ray luminosity () to reproduce the
observed or inferred super-orbital periods. Without exception, across a wide
range of parameter space, we find an astonishingly good match between the
observed and the model . We conclude irradiation-driven warping
is the mechanism underlying the long periods in X-ray binaries. Our Her X-1
simulation simultaneously reproduces the observed , the "main-" and
"short-high" X-ray states and the orbital inclination. Our simulations of SS
433 give a maximum warp angle of , a good match to the cone
traced by the jets, but this angle is reached only in the outer disc. In all
cases, the overall disc tilt is less than \degrees{13} and the maximum disc
warp is less than and or equal to \degrees{21}.Comment: 17 pages, 14 figures, shorter abstract (24 lines limit
DNA nano-mechanics: how proteins deform the double helix
It is a standard exercise in mechanical engineering to infer the external
forces and torques on a body from its static shape and known elastic
properties. Here we apply this kind of analysis to distorted double-helical DNA
in complexes with proteins. We extract the local mean forces and torques acting
on each base-pair of bound DNA from high-resolution complex structures. Our
method relies on known elastic potentials and a careful choice of coordinates
of the well-established rigid base-pair model of DNA. The results are robust
with respect to parameter and conformation uncertainty. They reveal the complex
nano-mechanical patterns of interaction between proteins and DNA. Being
non-trivially and non-locally related to observed DNA conformations, base-pair
forces and torques provide a new view on DNA-protein binding that complements
structural analysis.Comment: accepted for publication in JCP; some minor changes in response to
review 18 pages, 5 figure + supplement: 4 pages, 3 figure
Hydrodynamic modelling of accretion flows
In the proceedings of this, and of several recent close binary conferences,
there have been several contributions describing smoothed particle
hydrodynamics simulations of accretion disks. It is apposite therefore to
review the numerical scheme itself with emphasis on its advantages for disk
modelling, and the methods used for modelling viscous processes.Comment: 3 pages, to appear in proceedings of IAU Colloquium 194: Compact
binaries in the galaxy and beyon
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Stratigraphical evidence of Elysium sea ice from HiRise images
Abstract not available
Recommended from our members
Morphological evidence for a sea-ice origin for Elysium Planitia platy terrain
Abstract not available
Comprehensive simulations of superhumps
(Abridged) We use 3D SPH calculations with higher resolution, as well as with
more realistic viscosity and sound-speed prescriptions than previous work to
examine the eccentric instability which underlies the superhump phenomenon in
semi-detached binaries. We illustrate the importance of the two-armed spiral
mode in the generation of superhumps. Differential motions in the fluid disc
cause converging flows which lead to strong spiral shocks once each superhump
cycle. The dissipation associated with these shocks powers the superhump. We
compare 2D and 3D results, and conclude that 3D simulations are necessary to
faithfully simulate the disc dynamics. We ran our simulations for unprecedented
durations, so that an eccentric equilibrium is established except at high mass
ratios where the growth rate of the instability is very low. Our improved
simulations give a closer match to the observed relationship between superhump
period excess and binary mass ratio than previous numerical work. The observed
black hole X-ray transient superhumpers appear to have systematically lower
disc precession rates than the cataclysmic variables. This could be due to
higher disc temperatures and thicknesses. The modulation in total viscous
dissipation on the superhump period is overwhelmingly from the region of the
disc within the 3:1 resonance radius. As the eccentric instability develops,
the viscous torques are enhanced, and the disc consequently adjusts to a new
equilibrium state, as suggested in the thermal-tidal instability model. We
quantify this enhancement in the viscosity, which is ~10 per cent for q=0.08.
We characterise the eccentricity distributions in our accretion discs, and show
that the entire body of the disc partakes in the eccentricity.Comment: 18 pages (mn2e LaTeX), 14 figures, 5 tables, Accepted for publication
in MNRA
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Icelandic debris flow and their relationship to martian gullies
Abstract not available
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