1,956 research outputs found
Peer-to-Peer Planning for Space Mission Control
Planning and scheduling for space operations entails the development of applications that embed intimate domain knowledge of distinct areas of mission control, while allowing for significant collaboration among them. The separation is useful because of differences in the planning problem, solution methods, and frequencies of replanning that arise in the different disciplines. For example, planning the activities of human spaceflight crews requires some reasoning about all spacecraft resources at timescales of minutes or seconds, and is subject to considerable volatility. Detailed power planning requires managing the complex interplay of power consumption and production, involves very different classes of constraints and preferences, but once plans are generated they are relatively stable
Spatial, temporal and quantitative assessment of catalyst leaching in continuous flow
Catalyst leaching is a major impediment to the development of commercially-viable processes conducted in a liquid-phase. To date, there is no reliable technique that can accurately identify the extent and dynamics of the leaching process in a quantitative manner. In this work, a tandem flow-reactor system has been developed, which allowed us to distinguish between surface-catalyzed reactions from those occurring in solution by comparing%conversion at the exit of each reactor (S1, S2) corresponding to predominance of heterogeneous/homogeneous reactions (spatial) and two different residence times (temporal). A multiscale model is subsequently established to quantify the two types of reaction rate and simulate the catalyst leaching from a cross-coupling catalyst, PdEncat™ 30; including: (1) a multi-particle sizes model for catalyst scale; and (2) a dispersion model for reactor scale. The results show that catalyst leaching occurs via more than one process, and that the homogeneous Pd-catalyst (leached from the immobilized catalyst and dissolved in the flow) dominates the reaction and possesses a much higher activity than the heterogeneous (immobilized) Pd-catalyst. Additionally, the change of leached Pd stream inside reactors can be predicted along with the axial direction and the reaction time through the reactor-scale dispersion model
Power spectrum in the Chaplygin gas model: tachyonic, fluid and scalar field representations
The Chaplygin gas model, characterized by an equation of state of the type emerges naturally from the Nambu-Goto action of string
theory. This fluid representation can be recast under the form of a tachyonic
field given by a Born-Infeld type Lagrangian. At the same time, the Chaplygin
gas equation of state can be obtained from a self-interacting scalar field. We
show that, from the point of view of the supernova type Ia data, the three
representations (fluid, tachyonic, scalar field) lead to the same results.
However, concerning the matter power spectra, while the fluid and tachyonic
descriptions lead to exactly the same results, the self-interacting scalar
field representation implies different statistical estimations for the
parameters. In particular, the estimation for the dark matter density parameter
in the fluid representation favors a universe dominated almost completely by
dark matter, while in the self-interacting scalar field representation the
prediction is very closed to that obtained in the CDM model.Comment: Latex file, 10 pages, 18 figures in EPS forma
Revisiting the S-matrix approach to the open superstring low energy effective lagrangian
The conventional S-matrix approach to the (tree level) open string low energy
effective lagrangian assumes that, in order to obtain all its bosonic
order terms, it is necessary to know the open string (tree level)
-point amplitude of massless bosons, at least expanded at that order in
. In this work we clarify that the previous claim is indeed valid for
the bosonic open string, but for the supersymmetric one the situation is much
more better than that: there are constraints in the kinematical bosonic terms
of the amplitude (probably due to Spacetime Supersymmetry) such that a much
lower open superstring -point amplitude is needed to find all the
order terms. In this `revisited' S-matrix approach we have
checked that, at least up to order, using these kinematical
constraints and only the known open superstring 4-point amplitude, it is
possible to determine all the bosonic terms of the low energy effective
lagrangian. The sort of results that we obtain seem to agree completely with
the ones achieved by the method of BPS configurations, proposed about ten years
ago. By means of the KLT relations, our results can be mapped to the NS-NS
sector of the low energy effective lagrangian of the type II string theories
implying that there one can also find kinematical constraints in the -point
amplitudes and that important informations can be inferred, at least up to
order, by only using the (tree level) 4-point amplitude.Comment: 34 pages, 3 figure, Submitted on Aug 4, 2012, Published on Oct 15,
201
Chiral dynamics of p-wave in K^- p and coupled states
We perform an evaluation of the p-wave amplitudes of meson-baryon scattering
in the strangeness S=-1 sector starting from the lowest order chiral
Lagrangians and introducing explicitly the Sigma^* field with couplings to the
meson-baryon states obtained using SU(6) symmetry. The N/D method of
unitarization is used, equivalent, in practice, to the use of the
Bethe-Salpeter equation with a cut-off. The procedure leaves no freedom for the
p-waves once the s-waves are fixed and thus one obtains genuine predictions for
the p-wave scattering amplitudes, which are in good agreement with experimental
results for differential cross sections, as well as for the width and partial
decay widths of the Sigma^*(1385).Comment: LaTeX, 18 pages, 6 figure
Scalar models for the generalized Chaplygin gas and the structure formation constraints
The generalized Chaplygin gas model represents an attempt to unify dark
matter and dark energy. It is characterized by a fluid with an equation of
state . It can be obtained from a generalization of the
DBI action for a scalar, tachyonic field. At background level, this model gives
very good results, but it suffers from many drawbacks at perturbative level. We
show that, while for background analysis it is possible to consider any value
for , the perturbative analysis must be restricted to positive values
of . This restriction can be circumvented if the origin of the
generalized Chaplygin gas is traced back to a self-interacting scalar field,
instead of the DBI action. But, in doing so, the predictions coming from
formation of large scale structures reduce the generalized Chaplygin gas model
to a kind of quintessence model, and the unification scenario is lost, if the
scalar field is the canonical one. However, if the unification condition is
imposed from the beginning as a prior, the model may remain competitive. More
interesting results, concerning the unification program, are obtained if a
non-canonical self-interacting scalar field, inspired by Rastall's theory of
gravity, is imposed. In this case, an agreement with the background tests is
possible.Comment: Latex file, 25 pages, 33 figures in eps format. New section on scalar
models. Accepted for publication in Gravitation&Cosmolog
Observational constraint on generalized Chaplygin gas model
We investigate observational constraints on the generalized Chaplygin gas
(GCG) model as the unification of dark matter and dark energy from the latest
observational data: the Union SNe Ia data, the observational Hubble data, the
SDSS baryon acoustic peak and the five-year WMAP shift parameter. It is
obtained that the best fit values of the GCG model parameters with their
confidence level are ()
, ()
. Furthermore in this model, we can see that the
evolution of equation of state (EOS) for dark energy is similar to quiessence,
and its current best-fit value is with the confidence
level .Comment: 9 pages, 5 figure
Kahler Moduli Inflation Revisited
We perform a detailed numerical analysis of inflationary solutions in Kahler
moduli of type IIB flux compactifications. We show that there are inflationary
solutions even when all the fields play an important role in the overall shape
of the scalar potential. Moreover, there exists a direction of attraction for
the inflationary trajectories that correspond to the constant volume direction.
This basin of attraction enables the system to have an island of stability in
the set of initial conditions. We provide explicit examples of these
trajectories, compute the corresponding tilt of the density perturbations power
spectrum and show that they provide a robust prediction of n_s approximately
0.96 for 60 e-folds of inflation.Comment: 27 pages, 9 figure
Models of quintessence coupled to the electromagnetic field and the cosmological evolution of alpha
We study the change of the effective fine structure constant in the
cosmological models of a scalar field with a non-vanishing coupling to the
electromagnetic field. Combining cosmological data and terrestrial observations
we place empirical constraints on the size of the possible coupling and explore
a large class of models that exhibit tracking behavior. The change of the fine
structure constant implied by the quasar absorption spectra together with the
requirement of tracking behavior impose a lower bound of the size of this
coupling. Furthermore, the transition to the quintessence regime implies a
narrow window for this coupling around in units of the inverse Planck
mass. We also propose a non-minimal coupling between electromagnetism and
quintessence which has the effect of leading only to changes of alpha
determined from atomic physics phenomena, but leaving no observable
consequences through nuclear physics effects. In doing so we are able to
reconcile the claimed cosmological evidence for a changing fine structure
constant with the tight constraints emerging from the Oklo natural nuclear
reactor.Comment: 13 pages, 10 figures, RevTex, new references adde
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