4,498 research outputs found
Supersonic combustor modeling
The physical phenomena involved when a supersonic flow undergoes chemical reaction are discussed. Detailed physical models of convective and diffusive mixing, and finite rate chemical reaction in supersonic flow are presented. Numerical algorithms used to solve the equations governing these processes are introduced. Computer programs using these algorithms are used to analyze the structure of the reacting mixing layer. It is concluded that, as in subsonic flow, exothermic heat release in unconfined supersonic flows retards fuel/air mixing. Non mixing is shown to be a potential problem in reducing the efficiency of supersonic as well as subsonic combustion. Techniques for enhancing fuel/air mixing and combustion are described
A variance-minimization scheme for optimizing Jastrow factors
We describe a new scheme for optimizing many-electron trial wave functions by
minimizing the unreweighted variance of the energy using stochastic integration
and correlated-sampling techniques. The scheme is restricted to parameters that
are linear in the exponent of a Jastrow correlation factor, which are the most
important parameters in the wave functions we use. The scheme is highly
efficient and allows us to investigate the parameter space more closely than
has been possible before. We search for multiple minima of the variance in the
parameter space and compare the wave functions obtained using reweighted and
unreweighted variance minimization.Comment: 19 pages; 12 figure
Gaussian phase-space representations for fermions
We introduce a positive phase-space representation for fermions, using the
most general possible multi-mode Gaussian operator basis. The representation
generalizes previous bosonic quantum phase-space methods to Fermi systems. We
derive equivalences between quantum and stochastic moments, as well as operator
correspondences that map quantum operator evolution onto stochastic processes
in phase space. The representation thus enables first-principles quantum
dynamical or equilibrium calculations in many-body Fermi systems. Potential
applications are to strongly interacting and correlated Fermi gases, including
coherent behaviour in open systems and nanostructures described by master
equations. Examples of an ideal gas and the Hubbard model are given, as well as
a generic open system, in order to illustrate these ideas.Comment: More references and examples. Much less mathematical materia
Crystallization and characterization of Y2O3-SiO2 glasses
Glasses in the yttria-silica system with 20 to 40 mol pct Y2O3 were subjected to recrystallization studies after melting at 1900 to 2100 C in W crucibles in 1 and 50 atm N2. The TEM and XRD results obtained indicate the presence of the delta, gamma, gamma prime, and beta-Y2Si2O7 crystalline phases, depending on melting and quenching conditions. Heat treatment in air at 1100 to 1600 C increased the amount of crystallization, and led to the formation of Y2SiO5, cristabalite, and polymorphs of Y2Si2O7. Also investigated were the effects of 5 and 10 wt pct zirconia additions
Quantum many-body simulations using Gaussian phase-space representations
Phase-space representations are of increasing importance as a viable and
successful means to study exponentially complex quantum many-body systems from
first principles. This review traces the background of these methods, starting
from the early work of Wigner, Glauber and Sudarshan. We focus on modern
phase-space approaches using non-classical phase-space representations. These
lead to the Gaussian representation, which unifies bosonic and fermionic
phase-space. Examples treated include quantum solitons in optical fibers,
colliding Bose-Einstein condensates, and strongly correlated fermions on
lattices.Comment: Short Review (10 pages); Corrected typo in eq (14); Added a few more
reference
Quantum squeezing of optical dissipative structures
We show that any optical dissipative structure supported by degenerate
optical parametric oscillators contains a special transverse mode that is free
from quantum fluctuations when measured in a balanced homodyne detection
experiment. The phenomenon is not critical as it is independent of the system
parameters and, in particular, of the existence of bifurcations. This result is
a consequence of the spatial symmetry breaking introduced by the dissipative
structure. Effects that could degrade the squeezing level are considered.Comment: 4 pages and a half, 1 fugure. Version to appear in Europhysics
Letter
Bell inequalities for Continuous-Variable Measurements
Tests of local hidden variable theories using measurements with continuous
variable (CV) outcomes are developed, and a comparison of different methods is
presented. As examples, we focus on multipartite entangled GHZ and cluster
states. We suggest a physical process that produces the states proposed here,
and investigate experiments both with and without binning of the continuous
variable. In the former case, the Mermin-Klyshko inequalities can be used
directly. For unbinned outcomes, the moment-based CFRD inequalities are
extended to functional inequalities by considering arbitrary functions of the
measurements at each site. By optimising these functions, we obtain more robust
violations of local hidden variable theories than with either binning or
moments. Recent inequalities based on the algebra of quaternions and octonions
are compared with these methods. Since the prime advantage of CV experiments is
to provide a route to highly efficient detection via homodyne measurements, we
analyse the effect of noise and detection losses in both binned and unbinned
cases. The CV moment inequalities with an optimal function have greater
robustness to both loss and noise. This could permit a loophole-free test of
Bell inequalities.Comment: 17 pages, 6 figure
Assessment of practicality of remote sensing techniques for a study of the effects of strip mining in Alabama
Because of the volume of coal produced by strip mining, the proximity of mining operations, and the diversity of mining methods (e.g. contour stripping, area stripping, multiple seam stripping, and augering, as well as underground mining), the Warrior Coal Basin seemed best suited for initial studies on the physical impact of strip mining in Alabama. Two test sites, (Cordova and Searles) representative of the various strip mining techniques and environmental problems, were chosen for intensive studies of the correlation between remote sensing and ground truth data. Efforts were eventually concentrated in the Searles Area, since it is more accessible and offers a better opportunity for study of erosional and depositional processes than the Cordova Area
Antibunching photons in a cavity coupled to an optomechanical system
We study the photon statistics of a cavity linearly coupled to an
optomechanical system via second order correlation functions. Our calculations
show that the cavity can exhibit strong photon antibunching even when
optomechanical interaction in the optomechanical system is weak. The
cooperation between the weak optomechanical interaction and the destructive
interference between different paths for two-photon excitation leads to the
efficient antibunching effect. Compared with the standard optomechanical
system, the coupling between a cavity and an optomechanical system provides a
method to relax the constraints to obtain single photon by optomechanical
interaction.Comment: 7 papes, 5 figure
The triaxial ellipsoid dimensions, rotational pole, and bulk density of ESA Rosetta target asteroid (21) Lutetia
We seek the best size estimates of the asteroid (21) Lutetia, the direction
of its spin axis, and its bulk density, assuming its shape is well described by
a smooth featureless triaxial ellipsoid, and to evaluate the deviations from
this assumption. Methods. We derive these quantities from the outlines of the
asteroid in 307 images of its resolved apparent disk obtained with adaptive
optics (AO) at Keck II and VLT, and combine these with recent mass
determinations to estimate a bulk density. Our best triaxial ellipsoid
diameters for Lutetia, based on our AO images alone, are a x b x c = 132 x 101
x 93 km, with uncertainties of 4 x 3 x 13 km including estimated systematics,
with a rotational pole within 5 deg. of ECJ2000 [long,lat] = [45, -7], or
EQJ2000 [RA, DEC] = [44, +9]. The AO model fit itself has internal precisions
of 1 x 1 x 8 km, but it is evident, both from this model derived from limited
viewing aspects and the radius vector model given in a companion paper, that
Lutetia has significant departures from an idealized ellipsoid. In particular,
the long axis may be overestimated from the AO images alone by about 10 km.
Therefore, we combine the best aspects of the radius vector and ellipsoid model
into a hybrid ellipsoid model, as our final result, of 124 +/- 5 x 101 +/- 4 x
93 +/- 13 km that can be used to estimate volumes, sizes, and projected areas.
The adopted pole position is within 5 deg. of [long, lat] = [52, -6] or[RA DEC]
= [52, +12]. Using two separately determined masses and the volume of our
hybrid model, we estimate a density of 3.5 +/- 1.1 or 4.3 +/- 0.8 g cm-3 . From
the density evidence alone, we argue that this favors an enstatite-chondrite
composition, although other compositions are formally allowed at the extremes
(low-porosity CV/CO carbonaceous chondrite or high-porosity metallic). We
discuss this in the context of other evidence.Comment: 9 pages, 8 figures, 5 tables, submitted to Astronomy and Astrophysic
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