187 research outputs found
Vorticity estimation utilizing the rotation of finite line segments
Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.In a fluid flow field where velocities are measured or
numerically calculated, the vorticity value at a point in the
field is usually estimated by approximating partial
derivatives of velocity with ratios of velocity differences
and spatial differences. Based on the fundamental
definition of rotation, this paper presents an alternate
approach to estimate vorticity, by using the instantaneous
relative angular velocity of finite line segments radiating
from the point to adjacent points in the fluid.
The number, length and placement of line segments are
varied to find their effects on predicted vorticity values in
four laminar flow fields with known vorticity. Equations
are also derived for a configuration consisting of four
independent finite line segments which has application to a
square numerical velocity mesh and to instrumentation such
as hot wire vorticity probes and vane vorticity meters. The
results presented are relevant to both experimental and
computational fluid mechanics.cs201
Monte Carlo Calculations for Liquid He at Negative Pressure
A Quadratic Diffusion Monte Carlo method has been used to obtain the equation
of state of liquid He including the negative pressure region down to the
spinodal point. The atomic interaction used is a renewed version (HFD-B(HE)) of
the Aziz potential, which reproduces quite accurately the features of the
experimental equation of state. The spinodal pressure has been calculated and
the behavior of the sound velociy around the spinodal density has been
analyzed.Comment: 10 pages, RevTex 3.0, with 4 PostScript figures include
Edge effects in a frustrated Josephson junction array with modulated couplings
A square array of Josephson junctions with modulated strength in a magnetic
field with half a flux quantum per plaquette is studied by analytic arguments
and dynamical simulations. The modulation is such that alternate columns of
junctions are of different strength to the rest. Previous work has shown that
this system undergoes an XY followed by an Ising-like vortex lattice
disordering transition at a lower temperature. We argue that resistance
measurements are a possible probe of the vortex lattice disordering transition
as the linear resistance with
at intermediate temperatures due to dissipation at the array
edges for a particular geometry and vanishes for other geometries. Extensive
dynamical simulations are performed which support the qualitative physical
arguments.Comment: 8 pages with figs, RevTeX, to appear in Phys. Rev.
The Relativistic Electrodynamics Least Action Principles Revisited: New Charged Point Particle and Hadronic String Models Analysis
The classical relativistic least action principle is revisited from the
vacuum field theory approach. New physically motivated versions of relativistic
Lorentz type forces are derived, a new relativistic hadronic string model is
proposed and analyzed in detail.Comment: n/
Open Issues on the Synthesis of Evolved Stellar Populations at Ultraviolet Wavelengths
In this paper we briefly review three topics that have motivated our (and
others') investigations in recent years within the context of evolutionary
population synthesis techniques. These are: The origin of the FUV up-turn in
elliptical galaxies, the age-metallicity degeneracy, and the study of the
mid-UV rest-frame spectra of distant red galaxies. We summarize some of our
results and present a very preliminary application of a UV grid of theoretical
spectra in the analysis of integrated properties of aged stellar populations.
At the end, we concisely suggest how these topics can be tackled once the World
Space Observatory enters into operation in the midst of this decade.Comment: 8 pages, 4 figures, accepted for publication in Astrophysics & Space
Science, UV Universe special issu
Calculation of the interspecies s-wave scattering length in an ultracold Na-Rb vapor
We report the calculation of the interspecies scattering length for the
sodium-rubidium (Na-Rb) system. We present improved hybrid potentials for the
singlet and triplet ground states of the NaRb
molecule, and calculate the singlet and triplet scattering lengths and
for the isotopomers NaRb and NaRb. Using
these values, we assess the prospects for producing a stable two-species
Bose-Einstein condensate in the Na-Rb system.Comment: v2: report correct units in Table captions, fix error in conclusions
for NaRb TBEC. Otherwise, more concise presentation, typos
fixed. 6 pages, 1 figur
Three-body non-additive forces between spin-polarized alkali atoms
Three-body non-additive forces in systems of three spin-polarized alkali
atoms (Li, Na, K, Rb and Cs) are investigated using high-level ab initio
calculations. The non-additive forces are found to be large, especially near
the equilateral equilibrium geometries. For Li, they increase the three-atom
potential well depth by a factor of 4 and reduce the equilibrium interatomic
distance by 0.9 A. The non-additive forces originate principally from chemical
bonding arising from sp mixing effects.Comment: 4 pages, 3 figures (in 5 files
Domain Wall Junction in N=2 Supersymmetric QED in four dimensions
An exact solution of domain wall junction is obtained in N=2 supersymmetric
(SUSY) QED with three massive hypermultiplets. The junction preserves two out
of eight SUSY. Both a (magnetic) Fayet-Iliopoulos (FI) term and complex masses
for hypermultiplets are needed to obtain the junction solution. There are zero
modes corresponding to spontaneously broken translation, SUSY, and U(1). All
broken and unbroken SUSY charges are explicitly worked out in the Wess-Zumino
gauge in N=1 superfields as well as in components. The relation to models in
five dimensions is also clarified.Comment: 27 pages, 6 figures, comments on zero modes added, a few references
adde
Structural and dynamical properties of superfluid helium: a density functional approach
We present a novel density functional for liquid 4He, properly accounting for
the static response function and the phonon-roton dispersion in the uniform
liquid. The functional is used to study both structural and dynamical
properties of superfluid helium in various geometries. The equilibrium
properties of the free surface, droplets and films at zero temperature are
calculated. Our predictions agree closely to the results of ab initio Monte
Carlo calculations, when available. The introduction of a phenomenological
velocity dependent interaction, which accounts for backflow effects, is
discussed. The spectrum of the elementary excitations of the free surface and
films is studied.Comment: 37 pages, REVTeX 3.0, figures on request at [email protected]
Monte Carlo Methods for Estimating Interfacial Free Energies and Line Tensions
Excess contributions to the free energy due to interfaces occur for many
problems encountered in the statistical physics of condensed matter when
coexistence between different phases is possible (e.g. wetting phenomena,
nucleation, crystal growth, etc.). This article reviews two methods to estimate
both interfacial free energies and line tensions by Monte Carlo simulations of
simple models, (e.g. the Ising model, a symmetrical binary Lennard-Jones fluid
exhibiting a miscibility gap, and a simple Lennard-Jones fluid). One method is
based on thermodynamic integration. This method is useful to study flat and
inclined interfaces for Ising lattices, allowing also the estimation of line
tensions of three-phase contact lines, when the interfaces meet walls (where
"surface fields" may act). A generalization to off-lattice systems is described
as well.
The second method is based on the sampling of the order parameter
distribution of the system throughout the two-phase coexistence region of the
model. Both the interface free energies of flat interfaces and of (spherical or
cylindrical) droplets (or bubbles) can be estimated, including also systems
with walls, where sphere-cap shaped wall-attached droplets occur. The
curvature-dependence of the interfacial free energy is discussed, and estimates
for the line tensions are compared to results from the thermodynamic
integration method. Basic limitations of all these methods are critically
discussed, and an outlook on other approaches is given
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