1,397 research outputs found
Fully compressible simulation of low-speed premixed reacting flows
Low speed premixed combustion flows in industrial applications are generally simulated using the "incompressible" Navier-Stokes algorithms, which belong to the family of fractional step methods, or segregated methods. The approximations used for the combustion modelling in the framework of the segregated mathematical formulation, often represent important limitations for applying the combustion numerical simulation to a wider class of problems of engineering interest. Recent developments of preconditioning techniques allow to apply the same complete system of Navier-Stokes equations to a wide variety of fluid flow problems characterized by the whole range of Reynolds, Mach, Grashof, Prandtl and Damkoeler numbers. The present work describes the development of a fully "compressible" mathematical model for the simulation of low-speed turbulent premixed reactive flows. Issues on flow and fluid compressibility as well as on the two mathematical alternative formulations, are discussed. Also discussed are issues related to coupling the flamelet premixed combustion model (based on the solution of a transport equation for the progress variable) with one-equation turbulence models, instead of the classical two-equation K â Δ model. In this work the model by Spalart & Allmaras is used. The several advantages brought about by the use of the fully compressible formulation are discussed based on the results obtained on a test case taken from literature
Application of finite element code Q3DFL0-81 to turbomachinery flow fields
Through-flow and blade-to-blade calculations were made in association with a
number of experimental research activities at the Turbopropulsion Laboratory,
Naval Postgraduate School. The Q3DFL0-81 code was operated on an IBM 370-3033
mainframe computer. The flow through a single stage axial research compressor
was computed and compared with both probe survey and stage performance map
measurments. Swirling flow produced by a vaned out-flow generator for a
radial diffuser test facility was calculated for both large low-speed and
small-scale high-speed versions of the device. Flow through a two-dimensional
compressor cascade of "controlled-dif fusion" blade shapes was calculated and
the results compared with experimental data, and with predictions obtained
using the NASA code QSONIC.Prepared for: Naval Air Systems Command, Washington DChttp://archive.org/details/applicationoffin00schuN0001984WR 41099, N0001984WR 41134N
Registration of spatial image sequences for quantitative evaluation of free-form surfaces
This contribution describes an iterative registration method of spatial image sequences in view of accurate measurements and
3D reconstruction of free-form surfaces. Each image is represented by a set of 3D points characterizing the surface to be analyzed,
gained with a technique based on the use of a structured light. The novelty of our registration method lies in interpolation
of the imaged surfaces for the matching step and in the automated determination of the overlap region between two consecutive
images of the sequence. The use of a statistical criterion enables to discard the matchings of bad quality. The actual displacement
is computed using a least-squares technique based on unit quaternions and knowing a priori and approximately the
displacement between two positions of the sensor. Processing the whole sequence enables to express the points of all images in
a common reference frame. Results on both synthetic and real sequences assess efficiency and robustness of this registration
procedure.Cette contribution dĂ©crit une mĂ©thode itĂ©rative de recalage de sĂ©quences spatiales d'images en vue d'une mesure 3D prĂ©cise et d'une reconstruction de surfaces gauches quelconques. Chaque image est reprĂ©sentĂ©e par une collection de points 3D caractĂ©risant la surface Ă analyser, obtenue par une technique de projection de lumiĂšre structurĂ©e. La nouveautĂ© de notre mĂ©thode de recalage rĂ©side dans l'interpolation des surfaces imagĂ©es pour l'Ă©tape d'appariement et dans la dĂ©termination automatique de la zone de recouvrement entre deux images consĂ©cutives de la sĂ©quence. L'utilisation d'un critĂšre statistique permet d'Ă©liminer les appariements de mauvaise qualitĂ©. Le dĂ©placement effectif est calculĂ© par une technique de moindres carrĂ©s reposant sur les quaternions unitaires en connaissant a priori et approximativement le dĂ©placement entre deux positions du systĂšme de prise de vues. Le traitement de la sĂ©quence complĂšte permet d'exprimer les points de toutes les images dans un mĂȘme rĂ©fĂ©rentiel. Des rĂ©sultats expĂ©rimentaux sur des donnĂ©es synthĂ©tiques et rĂ©elles montrent que cette mĂ©thode de recalage est robuste et prĂ©cise
Magnetic phases of the mixed-spin Heisenberg model on a square lattice
We study the zero-temperature phase diagram and the low-energy excitations of
a mixed-spin () Heisenberg model defined on a square lattice
by using a spin-wave analysis, the coupled cluster method, and the Lanczos
exact-diagonalization technique. As a function of the frustration parameter
(), the phase diagram exhibits a quantized ferrimagnetic phase,
a canted spin phase, and a mixed-spin collinear phase. The presented results
point towards a strong disordering effect of the frustration and quantum spin
fluctuations in the vicinity of the classical spin-flop transition. In the
extreme quantum system , we find indications of a new
quantum spin state in the region Comment: 5 PRB pages, 7 figure
RVB Contribution to Superconductivity in
We view as electronically equivalent to (non-staggered) graphite
( layer) that has undergone a zero gap semiconductor to a superconductor
phase transition by a large c-axis (chemical) pressure due to layers.
Further, like the \ppi bonded planar organic molecules, graphite is an old
resonating valence bond (RVB) system. The RVB's are the `preexisting cooper
pairs' in the `parental' zero gap semiconducting (graphite) sheets that
manifests themselves as a superconducting ground state of the transformed
metal. Some consequences are pointed out.Comment: 4 pages, 2 figure, RevTex. Based on a talk given at the Institute
Seminar Week, IMSc, Madras (12-16, Feb. 2001
The connection between superconducting phase correlations and spin excitations in YBaCuO: A magnetic field study
One of the most striking universal properties of the
high-transition-temperature (high-) superconductors is that they are all
derived from the hole-doping of their insulating antiferromagnetic (AF) parent
compounds. From the outset, the intimate relationship between magnetism and
superconductivity in these copper-oxides has intrigued researchers. Evidence
for this link comes from neutron scattering experiments that show the
unambiguous presence of short-range AF correlations (excitations) in cuprate
superconductors. Even so, the role of such excitations in the pairing mechanism
and superconductivity is still a subject of controversy. For
YBaCuO, where controls the hole-doping level, the most
prominent feature in the magnetic excitations spectra is the ``resonance''.
Here we show that for underdoped YBaCuO, where and
are below the optimal values, modest magnetic fields suppress the resonance
significantly, much more so for fields approximately perpendicular rather than
parallel to the CuO planes. Our results indicate that the resonance
measures pairing and phase coherence, suggesting that magnetism plays an
important role in the superconductivity of cuprates. The persistence of a field
effect above favors mechanisms with preformed pairs in the normal state
of underdoped cuprates.Comment: 12 pages, 4 figures, Nature (in press
Collision, explosion and collapse of homoclinic classes
Homoclinic classes of generic -diffeomorphisms are maximal transitive
sets and pairwise disjoint. We here present a model explaining how two
different homoclinic classes may intersect, failing to be disjoint. For that we
construct a one-parameter family of diffeomorphisms with
hyperbolic points and having nontrivial homoclinic classes, such that,
for , the classes of and are disjoint, for , they are equal,
and, for , their intersection is a saddle-node.Comment: This is the final version, accepted in 200
Pseudogap behavior of nuclear spin relaxation in high Tc superconductors in terms of phase separation
We analyze anew experiments on the NMR in cuprates and find an important
information on their phase separation and its stripe character hidden in the
dependence of on degree of doping. In a broad class of materials
is the sum of two terms: the temperature independent one
attributed to ``incommensurate'' stripes that occur at external doping, and an
``universal'' temperature dependent term ascribed to moving metallic and AF
sub-phases. We argue that the frustrated first order phase transition in a
broad temperature interval bears a dynamical character.Comment: 5 pages, 3 figures; some comments and references added; accepted for
publication in JETP Letter
Beta decay of r-process waiting-point nuclei in a self-consistent approach
Beta-decay rates for spherical neutron-rich r-process waiting-point nuclei
are calculated within a fully self-consistent Quasiparticle Random-Phase
Approximation, formulated in the Hartree-Fock-Bogolyubov canonical
single-particle basis. The same Skyrme force is used everywhere in the
calculation except in the proton-neutron particle-particle channel, where a
finite-range force is consistently employed. In all but the heaviest nuclei,
the resulting half-lives are usually shorter by factors of 2 to 5 than those of
calculations that ignore the proton-neutron particle-particle interaction. The
shorter half-lives alter predictions for the abundance distribution of
r-process elements and for the time it takes to synthesize them.Comment: 14 pages RevTex, 10 eps figures, submitted to Phys. Rev.
Limits on different Majoron decay modes of Mo and Se for neutrinoless double beta decays in the NEMO-3 experiment
The NEMO-3 tracking detector is located in the Fr\'ejus Underground
Laboratory. It was designed to study double beta decay in a number of different
isotopes. Presented here are the experimental half-life limits on the double
beta decay process for the isotopes Mo and Se for different
Majoron emission modes and limits on the effective neutrino-Majoron coupling
constants. In particular, new limits on "ordinary" Majoron (spectral index 1)
decay of Mo ( y) and Se ( y) have been obtained. Corresponding bounds on the
Majoron-neutrino coupling constant are
and .Comment: 23 pages includind 4 figures, to be published in Nuclear Physics
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