1,282 research outputs found
Flow in a slowly-tapering channel with oscillating walls
The flow of a fluid in a channel with walls inclined at an angle to each other is investigated at arbitrary Reynolds number. The flow is driven by an oscillatory motion of the wall incorporating a time-periodic displacement perpendicular to the channel centreline. The gap between the walls varies linearly with distance along the channel and is a prescribed periodic function of time. An approximate solution is constructed assuming that the angle of inclination of the walls is small. At leading order the flow corresponds to that in a channel with parallel, vertically oscillating walls examined by Hall and Papageorgiou \cite{HP}. A careful study of the governing partial differential system for the first order approximation controlling the tapering flow due to the wall inclination is conducted. It is found that as the Reynolds number is increased from zero the tapering flow loses symmetry and undergoes exponential growth in time. The loss of symmetry occurs at a lower Reynolds number than the symmetry-breaking for the parallel-wall flow. A window of asymmetric, time-periodic solutions is found at higher Reynolds number, and these are reached via a quasiperiodic transient from a given set of initial conditions. Beyond this window stability is again lost to exponentially growing solutions as the Reynolds number is increased
Nucleotide Frequencies in Human Genome and Fibonacci Numbers
This work presents a mathematical model that establishes an interesting
connection between nucleotide frequencies in human single-stranded DNA and the
famous Fibonacci's numbers. The model relies on two assumptions. First,
Chargaff's second parity rule should be valid, and, second, the nucleotide
frequencies should approach limit values when the number of bases is
sufficiently large. Under these two hypotheses, it is possible to predict the
human nucleotide frequencies with accuracy. It is noteworthy, that the
predicted values are solutions of an optimization problem, which is commonplace
in many nature's phenomena.Comment: 12 pages, 2 figure
Prominent bulk pinning effect in the MgB_2 superconductor
We report the magnetic-field dependence of the irreversible magnetization of
the recently discovered binary superconductor MgB. For the temperature
region of , the contribution of the bulk pinning to the
magnetization overwhelms that of the surface pinning. This was evident from the
fact that the magnetization curves, , were well described by the
critical-state model without considering the surface pinning effect. It was
also found that the curves at various temperatures scaled when the field
and the magnetization were normalized by the characteristic scaling factors
and , respectively. This feature suggests that the
pinning mechanism determining the hysteresis in is unique below .Comment: 4pages and 4 figures. Phys. Rev. B (accepted
Spontaneous chiral symmetry breaking in the linked cluster expansion
We investigate dynamical chiral symmetry breaking in the Coulomb gauge
Hamiltonian QCD. Within the framework of the linked cluster expansion we extend
the BCS ansatz for the vacuum and include correlation beyond the
quark-antiquark paring. In particular we study the effects of the three-body
correlations involving quark-antiquark and transverse gluons. The high momentum
behavior of the resulting gap equation is discussed and numerical computation
of the chiral symmetry breaking is presented.Comment: 13 pages, 9 figure
On practical problems to compute the ghost propagator in SU(2) lattice gauge theory
In SU(2) lattice pure gauge theory we study numerically the dependence of the
ghost propagator G(p) on the choice of Gribov copies in Lorentz (or Landau)
gauge. We find that the effect of Gribov copies is essential in the scaling
window region, however, it tends to decrease with increasing beta. On the other
hand, we find that at larger beta-values very strong fluctuations appear which
can make problematic the calculation of the ghost propagator.Comment: 15 pages, 5 postscript figures. 2 Figures added Revised version as to
be published in Phys.Rev.
Modeling the Longitudinal Asymmetry in Sunspot Emergence -- the Role of the Wilson Depression
The distributions of sunspot longitude at first appearance and at
disappearance display an east-west asymmetry that results from a reduction in
visibility as one moves from disk centre to the limb. To first order, this is
explicable in terms of simple geometrical foreshortening. However, the
centre-to-limb visibility variation is much larger than that predicted by
foreshortening. Sunspot visibility is also known to be affected by the Wilson
effect: the apparent dish shape of the sunspot photosphere caused by the
temperature-dependent variation of the geometrical position of the tau=1 layer.
In this article we investigate the role of the Wilson effect on the sunspot
appearance distributions, deducing a mean depth for the umbral tau=1 layer of
500 to 1500 km. This is based on the comparison of observations of sunspot
longitude distribution and Monte Carlo simulations of sunspot appearance using
different models for spot growth rate, growth time and depth of Wilson
depression.Comment: 18 pages, 10 figures, in press (Solar Physics
Gauge and Scheme Dependence of Mixing Matrix Renormalization
We revisit the issue of mixing matrix renormalization in theories that
include Dirac or Majorana fermions. We show how a gauge-variant on-shell
renormalized mixing matrix can be related to a manifestly gauge-independent one
within a generalized scheme of renormalization. This
scheme-dependent relation is a consequence of the fact that in any scheme of
renormalization, the gauge-dependent part of the mixing-matrix counterterm is
ultra-violet safe and has a pure dispersive form. Employing the unitarity
properties of the theory, we can successfully utilize the afore-mentioned
scheme-dependent relation to preserve basic global or local symmetries of the
bare Lagrangian through the entire process of renormalization. As an immediate
application of our study, we derive the gauge-independent renormalization-group
equations of mixing matrices in a minimal extension of the Standard Model with
isosinglet neutrinos.Comment: 31 pages, LaTeX, uses axodraw.st
Search for R-Parity Violating Decays of Scalar Fermions at LEP
A search for pair-produced scalar fermions under the assumption that R-parity
is not conserved has been performed using data collected with the OPAL detector
at LEP. The data samples analysed correspond to an integrated luminosity of
about 610 pb-1 collected at centre-of-mass energies of sqrt(s) 189-209 GeV. An
important consequence of R-parity violation is that the lightest supersymmetric
particle is expected to be unstable. Searches of R-parity violating decays of
charged sleptons, sneutrinos and squarks have been performed under the
assumptions that the lightest supersymmetric particle decays promptly and that
only one of the R-parity violating couplings is dominant for each of the decay
modes considered. Such processes would yield final states consisting of
leptons, jets, or both with or without missing energy. No significant
single-like excess of events has been observed with respect to the Standard
Model expectations. Limits on the production cross- section of scalar fermions
in R-parity violating scenarios are obtained. Constraints on the supersymmetric
particle masses are also presented in an R-parity violating framework analogous
to the Constrained Minimal Supersymmetric Standard Model.Comment: 51 pages, 24 figures, Submitted to Eur. Phys. J.
Measurement of the Hadronic Photon Structure Function F_2^gamma at LEP2
The hadronic structure function of the photon F_2^gamma is measured as a
function of Bjorken x and of the factorisation scale Q^2 using data taken by
the OPAL detector at LEP. Previous OPAL measurements of the x dependence of
F_2^gamma are extended to an average Q^2 of 767 GeV^2. The Q^2 evolution of
F_2^gamma is studied for average Q^2 between 11.9 and 1051 GeV^2. As predicted
by QCD, the data show positive scaling violations in F_2^gamma. Several
parameterisations of F_2^gamma are in agreement with the measurements whereas
the quark-parton model prediction fails to describe the data.Comment: 4 pages, 2 figures, to appear in the proceedings of Photon 2001,
Ascona, Switzerlan
- …