695 research outputs found
Development of a reduced biodiesel surrogate fuel model for multi- dimensional CFD simulations
This work reports the development of a reduced biodiesel surrogate fuel model for multi-dimensional CFD simulations. The model is derived using an integrated kinetic mechanism reduction scheme and the final chemistry comprises only 83 species. The model is first validated in zero-dimensional (0-D) chemical kinetic calculations under a wide range of auto-ignition and jet-stirred reactor (JSR) conditions. The computed ignition delays (ID) and species profiles are in well agreement with those of the detailed model. Besides, the experimental species profiles of rapeseed methyl ester (RME) oxidation in a JSR are also reasonably reproduced. Subsequently, the fidelity of the model is further assessed in two-dimensional (2-D) CFD simulations of a constant-volume combustion vessel with respect to the experimental results of soy-methyl ester (SME) combustion. Comparisons
of the computations with the experimental data reveal that ID, lift-off lengths (LOL) and soot volume fractions
are reasonably well replicated by the model. Successively, the applicability of the reduced model to serve as a
universal surrogate model for other biodiesel feed-stocks, such as palm-methyl ester (PME) and sunflower-methyl
ester (SFME), is investigated in both 0-D and 2-D simulations. The compositions of the reduced model are varied according to the saturation/unsaturation levels in each fuel. In this work, it is demonstrated that the reduced model can potentially be used to predict the reactivity of biodiesel feed-stocks with low degree of saturation (≤30%) in both kinetic and CFD spray simulations
Can Short-Range Interactions Mediate a Bose Metal Phase in 2D?
We show here based on a 1-loop scaling analysis that short-range interactions
are strongly irrelevant perturbations near the insulator-superconductor (IST)
quantum critical point. The lack of any proof that short-range interactions
mediate physics which is present only in strong coupling leads us to conclude
that short-range interactions are strictly irrelevant near the IST quantum
critical point. Hence, we argue that no new physics, such as the formation of a
uniform Bose metal phase can arise from an interplay between on-site and
nearest-neighbour interactions.Comment: 3 pages, 1 .eps file. SUbmitted to Phys. Rev.
Topological Orbifold Models and Quantum Cohomology Rings
We discuss the toplogical sigma model on an orbifold target space. We
describe the moduli space of classical minima for computing correlation
functions involving twisted operators, and show, through a detailed computation
of an orbifold of by the dihedral group how to compute
the complete ring of observables. Through this procedure, we compute all the
rings from dihedral orbifolds; we note a similarity with rings
derived from perturbed series superpotentials of the classification
of minimal models. We then consider and show how the
techniques of topological-anti-topological fusion might be used to compute
twist field correlation functions for nonabelian orbifolds.Comment: 48 pages, harvmac, HUTP-92/A06
Green’s function method to the ground state properties of a two-component Bose–Einstein condensate
The elementary excitation spectrum of a two-component Bose–Einstein condensate is obtained by Green’s function method. It is found to have two branches. In the long-wave limit, the two branches of the excitation spectrum are reduced to one phonon excitation and one single-particle excitation. With the obtained excitation spectrum and the Green’s functions, the depletion of the condensate and the ground state energy have also been calculated in this paper
Crossover and scaling in a two-dimensional field-tuned superconductor
Using an analysis similar to that of Imry and Wortis, it is shown that the
apparent first order superconductor to metal transition, which has been claimed
to exist at low values of the magnetic field in a two-dimensional field-tuned
system at zero temperature,can be consistentlyinterpreted as a sharp crossover
from a strong superconductor to an inhomogeneous state, which is a weak
superconductor. The true zero-temperature superconductor to insulator
transition within the inhomogenous state is conjectured to be that of randomly
diluted XY model. An explaination of the observed finite temperature
approximate scaling of resistivity close to the critical point is speculated
within this model.Comment: 5 pages, 2 figures, corrected and modified according to referee
Report
Transport Properties near the z=2 Insulator-Superconductor Transition
We consider here the fluctuation conductivity near the point of the
insulator-superconductor transition in a system of regular Josephson junction
arrays in the presence of particle-hole asymmetry or equivalently homogeneous
charge frustration. The transition is characterised by the dynamic critical
exponent , opening the possibility of the perturbative
renormalization-group (RG) treatment. The quartic interaction in the
Ginzburg-Landau action and the coupling to the Ohmic heat bath, giving the
finite quasiparticle life-time, lead to the non-monotonic behavior of the dc
conductivity as a function of temperature in the leading logarithmic
approximation.Comment: Revised version for publication. To appear in PR
Phase Diagram of a Spin Ladder with Cyclic Four Spin Exchange
We present the phase diagram of the Heisenberg model on the two leg
ladder with cyclic four spin exchange, determined by a combination of Exact
Diagonalization and Density Matrix Renormalization Group techniques. We find
six different phases and regimes: the rung singlet phase, a ferromagnetic
phase, two symmetry broken phases with staggered dimers and staggered scalar
chiralities, and a gapped region with dominant vector chirality or collinear
spin correlations. We localize the phase transitions and investigate their
nature.Comment: 4 pages, 6 figures, REVTeX 4, published versio
Hyperbolic phase and squeeze-parameter estimation
We define a new representation, the hyperbolic phase representation, which enables optimal estimation of a squeeze parameter in the sense of quantum estimation theory. We compare the signal-to-noise ratio for such measurements, with conventional measurement based on photon counting and homodyne detection. The signal-to-noise ratio for hyperbolic phase measurements is shown to increase quadratically with the squeezing parameter for fixed input power
On the 3-particle scattering continuum in quasi one dimensional integer spin Heisenberg magnets
We analyse the three-particle scattering continuum in quasi one dimensional
integer spin Heisenberg antiferromagnets within a low-energy effective field
theory framework. We exactly determine the zero temperature dynamical structure
factor in the O(3) nonlinear sigma model and in Tsvelik's Majorana fermion
theory. We study the effects of interchain coupling in a Random Phase
Approximation. We discuss the application of our results to recent
neutron-scattering experiments on the Haldane-gap material .Comment: 8 pages of revtex, 5 figures, small changes, to appear in PR
On the existence of a Bose Metal at T=0
This paper aims to justify, at a microscopic level, the existence of a
two-dimensional Bose metal, i.e. a metallic phase made out of Cooper pairs at
T=0. To this end, we consider the physics of quantum phase fluctuations in
(granular) superconductors in the absence of disorder and emphasise the role of
two order parameters in the problem, viz. phase order and charge order. We
focus on the 2-d Bose Hubbard model in the limit of very large fillings, i.e. a
2-d array of Josephson junctions. We find that the algebra of phase
fluctuations is that of the Euclidean group in this limit, and show
that the model is equivalent to two coupled XY models in (2+1)-d, one
corresponding to the phase degrees of freedom, and the other the charge degrees
of freedom. The Bose metal, then, is the phase in which both these degrees of
freedom are disordered(as a result of quantum frustration). We analyse the
model in terms of its topological excitations and suggest that there is a
strong indication that this state represents a surface of critical points, akin
to the gapless spin liquid states. We find a remarkable consistency of this
scenario with certain low-T_c thin film experiments.Comment: 16 pages, 2 figure
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