34,147 research outputs found
Unified Analysis of Cosmological Perturbations in Generalized Gravity
In a class of generalized Einstein's gravity theories we derive the equations
and general asymptotic solutions describing the evolution of the perturbed
universe in unified forms. Our gravity theory considers general couplings
between the scalar field and the scalar curvature in the Lagrangian, thus
includes broad classes of generalized gravity theories resulting from recent
attempts for the unification. We analyze both the scalar-type mode and the
gravitational wave in analogous ways. For both modes the large scale evolutions
are characterized by the same conserved quantities which are valid in the
Einstein's gravity. This unified and simple treatment is possible due to our
proper choice of the gauges, or equivalently gauge invariant combinations.Comment: 4 pages, revtex, no figure
Rotation misorientated graphene moire superlattices on Cu(111): classical molecular dynamics simulations and scanning tunneling microscopy studies
Graphene on copper is a system of high technological relevance, as Cu is one
of the most widely used substrates for the CVD growth of graphene. However,
very little is known about the details of their interaction. One approach to
gain such information is studying the superlattices emerging due to the
mismatch of the two crystal lattices. However, graphene on copper is a
low-corrugated system making both their experimental and theoretical study
highly challenging. Here, we report the observation of a new rotational Moire
superlattice of CVD graphene on Cu (111), characterized by a periodicity of
nm and corrugation of , as measured
by Scanning Tunneling Microscopy. To understand the observed superlattice we
have developed a newly parameterized Tersoff-potential for the graphene/Cu
(111) interface fitted to nonlocal van der Waals density functional theory
(DFT) calculations. The interfacial force field with time-lapsed CMD provides
superlattices in good quantitative agreement with the experimental results, for
a misorientation angle of without any further parameter
adjustment. Furthermore, the CMD simulations predict the existence of two
non-equivalent high-symmetry directions of the Moir\'e pattern that could also
be identified in the experimental STM images.Comment: 7 pages, 2 figures, 2 table
Transient analysis of unbalanced short circuits of the ERDA-NASA 100 kW wind turbine alternator
Unbalanced short-circuit faults on the alternator of the ERDA-NASA Mod-O100-kW experimental wind turbine are studied. For each case, complete solutions for armature, field, and damper-circuit currents; short-circuit torque; and open-phase voltage are derived directly by a mathematical analysis. Formulated results are tabulated. For the Mod-O wind turbine alternator, numerical calculations are given, and results are presented by graphs. Comparisons for significant points among the more important cases are summarized. For these cases the transients are found to be potentially severe. The effect of the alternator neutral-to-ground impedance is evaluated
Synchronization of the ERDA-NASA 100 LkW wind turbine generator with large utility networks
The synchronizing of a wind turbine generator against an infinite bus under random conditions is studied. With a digital computer, complete solutions for rotor speed, generator power angle, electromagnetic torque, wind turbine torque, wind turbine blade pitch angle, and armature current are obtained and presented by graphs
Cosmological perturbations in a gravity with quadratic order curvature couplings
We present a set of equations describing the evolution of the scalar-type
cosmological perturbation in a gravity with general quadratic order curvature
coupling terms. Equations are presented in a gauge ready form, thus are ready
to implement various temporal gauge conditions depending on the problems. The
Ricci-curvature square term leads to a fourth-order differential equation for
describing the spacetime fluctuations in a spatially homogeneous and isotropic
cosmological background.Comment: 5 pages, no figure, To appear in Phys. Rev.
Nonuniqueness for the kinetic Fokker-Planck equation with inelastic boundary conditions
We describe the structure of solutions of the kinetic Fokker-Planck equations
in domains with boundaries near the singular set in one-space dimension. We
study in particular the behaviour of the solutions of this equation for
inelastic boundary conditions which are characterized by means of a coefficient
describing the amount of energy lost in the collisions of the particles
with the boundaries of the domain. A peculiar feature of this problem is the
onset of a critical exponent rc which follows from the analysis of McKean (cf.
[26]) of the properties of the stochastic process associated to the
Fokker-Planck equation under consideration. In this paper, we prove rigorously
that the solutions of the considered problem are nonunique if and
unique if . In particular, this nonuniqueness explains the
different behaviours found in the physics literature for numerical simulations
of the stochastic differential equation associated to the Fokker-Planck
equation. In the proof of the results of this paper we use several asymptotic
formulas and computations in the companion paper [16].Comment: 64 pages, 1 figure. Previous version has been split into tw
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