15,136 research outputs found
Performance of LI-1542 reusable surface insulation system in a hypersonic stream
The thermal and structural performance LI-1542 reusable surface insulation (RSI) tiles was investigated. The test panel was designed to represent part of the surface structure on a space shuttle orbiter fuselage along a 1250 K isotherm. Aerothermal tests were conducted at a free-stream Mach number of 6.6, a total temperature of 1820 K, Reynolds numbers of 2 millon and 5 million per meter, and dynamic pressures of 26 and 65 kPa. The RSI tiles demonstrated good thermal protection and structural integrity. High temperatures were caused by misalinement in tile height, offset the tile longitudinal alinement, and leakage around thermal seals when differential pressure existed across the panel. The damage tolerance of LI-1542 RSI appeared high. The tile coating crazed early in the test program, but this did not effect the tile integrity. Erosion of the tile edges occurred at forward-facing steps and at the ends of longitudinal gaps because of particle impacts and flow shear
Hamiltonian systems with symmetry, coadjoint orbits and plasma physics
The symplectic and Poisson structures on reduced phase spaces are reviewed, including the symplectic structure on coadjoint orbits of a Lie group and the Lie-Poisson structure on the dual of a Lie algebra. These results are
applied to plasma physics. We show in three steps how the Maxwell-Vlasov equations for a collisionless plasma can be written in Hamiltonian form relative to a certain Poisson bracket. First, the Poisson-Vlasov equations are shown
to be in Hamiltonian form relative to the Lie-Poisson bracket on the dual of the (nite dimensional) Lie algebra of innitesimal canonical transformations. Then we write Maxwell's equations in Hamiltonian form using the canonical
symplectic structure on the phase space of the electromagnetic elds, regarded as a gauge theory. In the last step we couple these two systems via the reduction
procedure for interacting systems. We also show that two other standard models in plasma physics, ideal MHD and two-
uid electrodynamics, can be written in Hamiltonian form using similar group theoretic techniques
Template-based Gravitational-Wave Echoes Search Using Bayesian Model Selection
The ringdown of the gravitational-wave signal from a merger of two black
holes has been suggested as a probe of the structure of the remnant compact
object, which may be more exotic than a black hole. It has been pointed out
that there will be a train of echoes in the late-time ringdown stage for
different types of exotic compact objects. In this paper, we present a
template-based search methodology using Bayesian statistics to search for
echoes of gravitational waves. Evidence for the presence or absence of echoes
in gravitational-wave events can be established by performing Bayesian model
selection. The Occam factor in Bayesian model selection will automatically
penalize the more complicated model that echoes are present in
gravitational-wave strain data because of its higher degree of freedom to fit
the data. We find that the search methodology was able to identify
gravitational-wave echoes with Abedi et al.'s echoes waveform model about 82.3%
of the time in simulated Gaussian noise in the Advanced LIGO and Virgo network
and about 61.1% of the time in real noise in the first observing run of
Advanced LIGO with significance. Analyses using this method are
performed on the data of Advanced LIGO's first observing run, and we find no
statistical significant evidence for the detection of gravitational-wave
echoes. In particular, we find combined evidence of the three events
in Advanced LIGO's first observing run. The analysis technique developed in
this paper is independent of the waveform model used, and can be used with
different parametrized echoes waveform models to provide more realistic
evidence of the existence of echoes from exotic compact objects.Comment: 16 pages, 6 figure
Geometric quantization of Hamiltonian actions of Lie algebroids and Lie groupoids
We construct Hermitian representations of Lie algebroids and associated
unitary representations of Lie groupoids by a geometric quantization procedure.
For this purpose we introduce a new notion of Hamiltonian Lie algebroid
actions. The first step of our procedure consists of the construction of a
prequantization line bundle. Next, we discuss a version of K\"{a}hler
quantization suitable for this setting. We proceed by defining a
Marsden-Weinstein quotient for our setting and prove a ``quantization commutes
with reduction'' theorem. We explain how our geometric quantization procedure
relates to a possible orbit method for Lie groupoids. Our theory encompasses
the geometric quantization of symplectic manifolds, Hamiltonian Lie algebra
actions, actions of families of Lie groups, foliations, as well as some general
constructions from differential geometry.Comment: 40 pages, corrected version 11-01-200
Study of instabilities and transition to turbulence in a linear hall accelerator
Magnetospheric instabilities and transition to plasma turbulence in Hall current accelerator
Dynamic quantum clustering: a method for visual exploration of structures in data
A given set of data-points in some feature space may be associated with a
Schrodinger equation whose potential is determined by the data. This is known
to lead to good clustering solutions. Here we extend this approach into a
full-fledged dynamical scheme using a time-dependent Schrodinger equation.
Moreover, we approximate this Hamiltonian formalism by a truncated calculation
within a set of Gaussian wave functions (coherent states) centered around the
original points. This allows for analytic evaluation of the time evolution of
all such states, opening up the possibility of exploration of relationships
among data-points through observation of varying dynamical-distances among
points and convergence of points into clusters. This formalism may be further
supplemented by preprocessing, such as dimensional reduction through singular
value decomposition or feature filtering.Comment: 15 pages, 9 figure
Quarks, Gluons and Frustrated Antiferromagnets
The Contractor Renormalization Group method (CORE) is used to establish the
equivalence of various Hamiltonian free fermion theories and a class of
generalized frustrated antiferromagnets. In particular, after a detailed
discussion of a simple example, it is argued that a generalized frustrated
SU(3) antiferromagnet whose single-site states have the quantum numbers of
mesons and baryons is equivalent to a theory of free massless quarks.
Furthermore, it is argued that for slight modification of the couplings which
define the frustrated antiferromagnet Hamiltonian, the theory becomes a theory
of quarks interacting with color gauge-fields.Comment: 21 pages, Late
Four-quark state in QCD
The spectra of some 0++ four-quark states, which are composed of \bar qq
pairs, are calculated in QCD. The light four-quark states are calculated using
the traditional sum rules while four-quark states containing one heavy quark
are computed in HQET. For constructing the interpolating currents, different
couplings of the color and spin inside the \bar qq pair are taken into account.
It is found that the spin and color combination has little effect on the mass
of the four-quark states.Comment: 10 pages, 4 ps figures, Late
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