776,448 research outputs found
The use of active controls to augment rotor/fuselage stability
The use of active blade pitch control to increase helicopter rotor/body damping is studied. Control is introduced through a conventional nonrotating swashplate. State variable feedback of rotor and body states is used. Feedback parameters include cyclic rotor flap and lead-lag states, and body pitch and roll rotations. The use of position, rate, and acceleration feedback is studied for the various state variables. In particular, the influence of the closed loop feedback gain and phase on system stability is investigated. For the rotor/body configuration analyzed, rotor cyclic inplane motion and body roll-rate and roll-acceleration feedback can considerably augment system damping levels and eliminate ground resonance instabilities. Scheduling of the feedback state, phase, and gain with rotor rotation speed can be used to maximize the damping augmentation. This increase in lead-lag damping can be accomplished without altering any of the system modal frequencies. Investigating various rotor design parameters (effective hinge offset, blade precone, blade flap stiffness) indicates that active control for augmenting rotor/body damping will be particularly powerful for hingeless and bearingless rotor hubs
Stationary and transient leakage current in the Pauli spin blockade
We study the effects of cotunneling and a non-uniform Zeeman splitting on the
stationary and transient leakage current through a double quantum dot in the
Pauli spin blockade regime. We find that the stationary current due to
cotunneling vanishes at low temperature and large applied magnetic field,
allowing for the dynamical preparation of a pure spin ground state, even at
large voltage bias. Additionally, we analyze current that flows between
blocking events, characterized, in general, by a fractional effective charge
. This charge can be used as a sensitive probe of spin relaxation
mechanisms and can be used to determine the visibility of Rabi oscillations.Comment: v1: 4 pages; v2: 4 pages+ additional supplementary material, version
to appear in PR
Momentum space evolution of chiral three-nucleon forces
A framework to evolve three-nucleon (3N) forces in a plane-wave basis with
the Similarity Renormalization Group (SRG) is presented and applied to
consistent interactions derived from chiral effective field theory at
next-to-next-to-leading order (NLO). We demonstrate the unitarity of the
SRG transformation, show the decoupling of low and high momenta, and present
the first investigation of universality in chiral 3N forces at low resolution
scales. The momentum-space-evolved 3N forces are consistent and can be directly
combined with the standard SRG-evolved two-nucleon (NN) interactions for
ab-initio calculations of nuclear structure and reactions.Comment: 5 pages, 4 figure
Pattern reconstruction and sequence processing in feed-forward layered neural networks near saturation
The dynamics and the stationary states for the competition between pattern
reconstruction and asymmetric sequence processing are studied here in an
exactly solvable feed-forward layered neural network model of binary units and
patterns near saturation. Earlier work by Coolen and Sherrington on a parallel
dynamics far from saturation is extended here to account for finite stochastic
noise due to a Hebbian and a sequential learning rule. Phase diagrams are
obtained with stationary states and quasi-periodic non-stationary solutions.
The relevant dependence of these diagrams and of the quasi-periodic solutions
on the stochastic noise and on initial inputs for the overlaps is explicitly
discussed.Comment: 9 pages, 7 figure
Vortex formation in a stirred Bose-Einstein condensate
Using a focused laser beam we stir a Bose-Einstein condensate of 87Rb
confined in a magnetic trap and observe the formation of a vortex for a
stirring frequency exceeding a critical value. At larger rotation frequencies
we produce states of the condensate for which up to four vortices are
simultaneously present. We have also measured the lifetime of the single vortex
state after turning off the stirring laser beam.Comment: 4 pages, 3 figure
Thermal pump-compressor for space use Patent
Thermal pump-compressor for converting solar energ
Computational Methods for Nucleosynthesis and Nuclear Energy Generation
This review concentrates on the two principle methods used to evolve nuclear
abundances within astrophysical simulations, evolution via rate equations and
via equilibria. Because in general the rate equations in nucleosynthetic
applications form an extraordinarily stiff system, implicit methods have proven
mandatory, leading to the need to solve moderately sized matrix equations.
Efforts to improve the performance of such rate equation methods are focused on
efficient solution of these matrix equations, by making best use of the
sparseness of these matrices. Recent work to produce hybrid schemes which use
local equilibria to reduce the computational cost of the rate equations is also
discussed. Such schemes offer significant improvements in the speed of reaction
networks and are accurate under circumstances where calculations with complete
equilibrium fail.Comment: LaTeX2e with graphicx, 40 Pages with 5 embedded figures. To be
published in Computational Astrophysics, The Journal of Computational and
Applied Mathematics, eds. H. Riffert, K. Werne
Silicon Burning I: Neutronization and the Physics of Quasi-Equilibrium
As the ultimate stage of stellar nucleosynthesis, and the source of the iron
peak nuclei, silicon burning is important to our understanding of the evolution
of massive stars and supernovae. Our reexamination of silicon burning, using
results gleaned from simulation work done with a large nuclear network (299
nuclei and more than 3000 reactions) and from independent calculations of
equilibrium abundance distributions, offers new insights into the
quasi-equilibrium mechanism and the approach to nuclear statistical
equilibrium. We find that the degree to which the matter has been neutronized
is of great importance, not only to the final products but also to the rate of
energy generation and the membership of the quasi-equilibrium groups. A small
increase in the global neutronization results in much larger free neutron
fluences, increasing the abundances of more neutron-rich nuclei. As a result,
incomplete silicon burning results in neutron richness among the isotopes of
the iron peak much larger than the global neutronization would indicate.
Finally, we briefly discuss the limitations and pitfalls of models for silicon
burning currently employed within hydrodynamic models. In a forthcoming paper
we will present a new approximation to the full nuclear network which preserves
the most important features of the large nuclear network calculations at a
significant improvement in computational speed. Such improved methods are
ideally suited for hydrodynamic calculations which involve the production of
iron peak nuclei, where the larger network calculation proves unmanageable.Comment: 44 pages of TeX with 25 Postscript figures, uses psfig.sty, To appear
in the The Astrophysical Journal, April 1 1996. Complete PostScript version
of the paper is also available from
http://tycho.as.utexas.edu/~raph/Publications.htm
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