2,911 research outputs found
Application of finite element techniques in predicting the acoustic properties of turbofan inlets
An analytical technique was developed for predicting the acoustic performance of turbofan inlets carrying a subsonic axisymmetric steady flow. The finite element method combined with the method of weighted residuals is used in predicting the acoustic properties of variable area, annular ducts with or without acoustic treatments along their walls. An approximate solution for the steady inviscid flow field is obtained using an integral method for calculating the incompressible potential flow field in the inlet with a correction to account for compressibility effects. The accuracy of the finite element technique was assessed by comparison with available analytical solutions for the problems of plane and spinning wave propagation through a hard walled annular cylinder with a constant mean flow
The transition temperature of the dilute interacting Bose gas for internal degrees of freedom
We calculate explicitly the variation of the Bose-Einstein
condensation temperature induced by weak repulsive two-body interactions
to leading order in the interaction strength. As shown earlier by general
arguments, is linear in the dimensionless product
to leading order, where is the density and the scattering length. This
result is non-perturbative, and a direct perturbative calculation of the
amplitude is impossible due to infrared divergences familiar from the study of
the superfluid helium lambda transition. Therefore we introduce here another
standard expansion scheme, generalizing the initial model which depends on one
complex field to one depending on real fields, and calculating the
temperature shift at leading order for large . The result is explicit and
finite. The reliability of the result depends on the relevance of the large
expansion to the situation N=2, which can in principle be checked by systematic
higher order calculations. The large result agrees remarkably well with
recent numerical simulations.Comment: 10 pages, Revtex, submitted to Europhysics Letter
Acoustic properties of turbofan inlets
The acoustic field within a duct containing a nonuniform steady flow was predicted. This analysis used the finite element method to calculate the velocity potential within the duct
Multicritical behavior of two-dimensional anisotropic antiferromagnets in a magnetic field
We study the phase diagram and multicritical behavior of anisotropic
Heisenberg antiferromagnets on a square lattice in the presence of a magnetic
field along the easy axis. We argue that, beside the Ising and XY critical
lines, the phase diagram presents a first-order spin-flop line starting from
T=0, as in the three-dimensional case. By using field theory we show that the
multicritical point where these transition lines meet cannot be O(3) symmetric
and occurs at finite temperature. We also predict how the critical temperature
of the transition lines varies with the magnetic field and the uniaxial
anisotropy in the limit of weak anisotropy.Comment: 21 pages, 8 fig
Distributions of absolute central moments for random walk surfaces
We study periodic Brownian paths, wrapped around the surface of a cylinder.
One characteristic of such a path is its width square, , defined as its
variance. Though the average of over all possible paths is well known,
its full distribution function was investigated only recently. Generalising
to , defined as the -th power of the {\it magnitude} of the
deviations of the path from its mean, we show that the distribution functions
of these also scale and obtain the asymptotic behaviour for both large and
small
Breakdown of the perturbative renormalization group at certain quantum critical points
It is shown that the presence of multiple time scales at a quantum critical
point can lead to a breakdown of the loop expansion for critical exponents,
since coefficients in the expansion diverge. Consequently, results obtained
from finite-order perturbative renormalization-group treatments may be not be
an approximation in any sense to the true asymptotic critical behavior. This
problem manifests itself as a non-renormalizable field theory, or,
equivalently, as the presence of a dangerous irrelevant variable. The quantum
ferromagnetic transition in disordered metals provides an example.Comment: 4pp, 1 eps fi
Disorder-Induced Shift of Condensation Temperature for Dilute Trapped Bose Gases
We determine the leading shift of the Bose-Einstein condensation temperature
for an ultracold dilute atomic gas in a harmonic trap due to weak disorder by
treating both a Gaussian and a Lorentzian spatial correlation for the quenched
disorder potential. Increasing the correlation length from values much smaller
than the geometric mean of the trap scale and the mean particle distance to
much larger values leads first to an increase of the positive shift to a
maximum at this critical length scale and then to a decrease.Comment: Author information under
http://www.theo-phys.uni-essen.de/tp/ags/pelster_di
Anomalous Density-of-States Fluctuations in Two-Dimensional Clean Metals
It is shown that density-of-states fluctuations, which can be interpreted as
the order-parameter susceptibility \chi_OP in a Fermi liquid, are anomalously
strong as a result of the existence of Goldstone modes and associated strong
fluctuations. In a 2-d system with a long-range Coulomb interaction, a suitably
defined \chi_OP diverges as 1/T^2 as a function of temperature in the limit of
small wavenumber and frequency. In contrast, standard statistics suggest
\chi_OP = O(T), a discrepancy of three powers of T. The reasons behind this
surprising prediction, as well as ways to observe it, are discussed.Comment: 4 pp, revised version contains a substantially expanded derivatio
Searching for RR Lyrae stars in the Canis Major Overdensity
The Canis Major overdensity (CMa) was initially proposed to be the remnant of
a tidally disrupting dSph galaxy. Since its nature is still subject of debate,
the goal of the present work was to conduct a large-scale RR Lyrae survey in
CMa, in order to see if there is an overdensity of these stars. The survey
spans a total area of ~34 sq. deg. with observations in V and R filters, made
with the 1.0m Jurgen Stock Schmidt telescope at the National Astronomical
Observatory of Venezuela. Current results in a subregion, including
spectroscopic observations, show that the small number of RR Lyrae stars found
can be accounted for by the halo and thick disk components of our Galaxy.Comment: 2 pages, 1 figure, to appear in the proceedings of IAU Symposium
No241 "Stellar Populations as Building Blocks of Galaxies
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