15,491 research outputs found
Introduction to Systems Approach
Main aspects of systems theory are outlined. Emphasis is on the interface of between time and systems - natural or artificial
Precise time and time interval (PTTI), an overview
Present applications of precise time and frequency (T/F) technology can be grouped as follows: (1) Communications systems which require T/F for time division multiplexing and for using spread spectrum techniques. (2) Navigation systems which need T/F for position fixing using a timed signal. (3) Scientific-Metrological applications which use T/F as the most precisely reproducible standard of measurement. (4) Astronomical-Space applications which cover a variety of the most demanding applications such as pulsar research, Very Long Baseline Interferometry (VLBI) and laser/radar ranging. In particular, pulsar time-of-arrival measurements require submicrosecond precision over a period of one-half year referred to an extraterrestrial inertial system, and constitute the most stringent requirements for uniform timekeeping to date
Semiflexible polymers under external fields confined to two dimensions
The non-equilibrium structural and dynamical properties of semiflexible
polymers confined to two dimensions are investigated by molecular dynamics
simulations. Three different scenarios are considered: The force-extension
relation of tethered polymers, the relaxation of an initially stretched
semiflexible polymer, and semiflexible polymers under shear flow. We find
quantitative agreement with theoretical predictions for the force-extension
relation and the time dependence of the entropically contracting polymer. The
semiflexible polymers under shear flow exhibit significant conformational
changes at large shear rates, where less stiff polymers are extended by the
flow, whereas rather stiff polymers are contracted. In addition, the polymers
are aligned by the flow, thereby the two-dimensional semiflexible polymers
behave similarly to flexible polymers in three dimensions. The tumbling times
display a power-law dependence at high shear rate rates with an exponent
comparable to the one of flexible polymers in three-dimensional systems.Comment: Accepted for publication in J. Chem. Phy
Negative association in uniform forests and connected graphs
We consider three probability measures on subsets of edges of a given finite
graph , namely those which govern, respectively, a uniform forest, a uniform
spanning tree, and a uniform connected subgraph. A conjecture concerning the
negative association of two edges is reviewed for a uniform forest, and a
related conjecture is posed for a uniform connected subgraph. The former
conjecture is verified numerically for all graphs having eight or fewer
vertices, or having nine vertices and no more than eighteen edges, using a
certain computer algorithm which is summarised in this paper. Negative
association is known already to be valid for a uniform spanning tree. The three
cases of uniform forest, uniform spanning tree, and uniform connected subgraph
are special cases of a more general conjecture arising from the random-cluster
model of statistical mechanics.Comment: With minor correction
Conformations, hydrodynamic interactions, and instabilities of sedimenting semiflexible filaments
The conformations and dynamics of semiflexible filaments subject to a
homogeneous external (gravitational) field, e.g., in a centrifuge, are studied
numerically and analytically. The competition between hydrodynamic drag and
bending elasticity generates new shapes and dynamical features. We show that
the shape of a semiflexible filament undergoes instabilities as the external
field increases. We identify two transitions that correspond to the excitation
of higher bending modes. In particular, for strong fields the filament
stabilizes in a non-planar shape, resulting in a sideways drift or in helical
trajectories. For two interacting filaments, we find the same transitions, with
the important consequence that the new non-planar shapes have an effective
hydrodynamic repulsion, in contrast to the planar shapes which attract
themselves even when their osculating planes are rotated with respect to each
other. For the case of planar filaments, we show analytically and numerically
that the relative velocity is not necessarily due to a different drag of the
individual filaments, but to the hydrodynamic interactions induced by their
shape asymmetry.Comment: 9 pages, 7 figures in Soft Matter (2015
Migration of semiflexible polymers in microcapillary flow
The non-equilibrium structural and dynamical properties of a semiflexible
polymer confined in a cylindrical microchannel and exposed to a Poiseuille flow
is studied by mesoscale hydrodynamic simulations. For a polymer with a length
half of its persistence length, large variations in orientation and
conformations are found as a function of radial distance and flow strength. In
particular, the polymer exhibits U-shaped conformations near the channel
center. Hydrodynamic interactions lead to strong cross-streamline migration.
Outward migration is governed by the polymer orientation and the corresponding
anisotropy in its diffusivity. Strong tumbling motion is observed, with a
tumbling time which exhibits the same dependence on Peclet number as a polymer
in shear flow.Comment: 6 pages, 7 figures, accepted by EP
External gates and transport in biased bilayer graphene
We formulate a theory of transport in graphene bilayers in the weak momentum
scattering regime in such a way as to take into account contributions to the
electrical conductivity to leading and next-to-leading order in the scattering
potential. The response of bilayers to an electric field cannot be regarded as
a sum of terms due to individual layers. Rather, interlayer tunneling and
coherence between positive- and negative-energy states give the main
contributions to the conductivity. At low energies, the dominant effect of
scattering on transport comes from scattering within each energy band, yet a
simple picture encapsulating the role of collisions in a set of scattering
times is not applicable. Coherence between positive- and negative-energy states
gives, as in monolayers, a term in the conductivity which depends on the order
of limits. The application of an external gate, which introduces a gap between
positive- and negative-energy states, does not affect transport. Nevertheless
the solution to the kinetic equation in the presence of such a gate is very
revealing for transport in both bilayers and monolayers.Comment: 6 pages, accepted for publication in Physical Review
A preliminary discussion of gravitational physics experiments for the Spacelab era
An overview of past, present, and proposed future experiments in gravitational physics is given. These experiments are concerned with the measurement of relativistic gravity effects to test theories of gravitation. Certain experiments which could be performed on shuttle and Spacelab missions and the potential of Spacelab for gravitation physics research are discussed
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