3,178 research outputs found

### A PIV and CFD analysis of natural convection ice melting

The melting of a vertical ice cylinder in water is investigated in this paper. The experiments were carried out in a water-filled cylindrical Perspex barrel with adiabatic walls for Rayleigh numbers of 0.22x108 and 0.475x108. The ice crystal is suspended in the water and experimental images of the natural convection melting process were obtained using both shadowgraphy and particle image velocimetry (PIV) techniques. This data is compared with a numerical model which attempts to capture the melt-front on a fixed computational grid. The numerical model takes into account the density inversion effects in the water. The results show the applicability of PIV to this type of flow and demonstrate a simple numerical model to effectively resolve the melting phenomenon

### Minimum Wages and Teen Employment: A Spatial Panel Approach

The authors employ spatial econometrics techniques and Annual Averages data from the U.S. Bureau of Labor Statistics for 1990-2004 to examine how changes in the minimum wage affect teen employment. Spatial econometrics techniques account for the fact that employment is correlated across states. Such correlation may exist if a change in the minimum wage in a state affects employment not only in its own state but also in other, neighboring states. The authors show that state minimum wages negatively affect teen employment to a larger degree than is found in studies that do not account for this correlation. Their results show a combined direct and indirect effect of minimum wages on teen employment to be -2.1% for a 10% increase in the real effective minimum wage. Ignoring spatial correlation underestimates the magnitude of the effect of minimum wages on teen employment.minimum wage, teen employment, spatial econometrics

### Parametrization of Realistic Bethe-Salpeter Amplitude for the Deuteron

The parametrization of the realistic Bethe-Salpeter amplitude for the
deuteron is given. Eight components of the amplitude in the Euclidean space are
presented as an analytical fit to the numerical solution of the Bethe-Salpeter
equation in the ladder approximation. An applicability of the parametrization
to the observables of the deuteron is briefly discussed.Comment: LaTeX, 11 pages, 2 Postscript figures; Text of the Fortran program is
available from the author by reques

### A simple model for the microscopic effective pairing interaction

The microscopic effective pairing interaction in the $^1S_0$-channel is
investigated for two different values of the chemical potential $\mu$ starting
from the separable form of the Paris NN-potential. It is shown that, within a
high accuracy, this effective interaction can be approximated by the off-shell
free T-matrix taken at the negative energy $E=2\mu$.Comment: LaTeX, 8 pages, 6 ps-figure

### Particle Accelerators in the Hot Spots of the Radio Galaxy 3C445 Imaged With the VLT

Hot spots (HSs) are regions of enhanced radio emission produced by supersonic
jets at the tip of the radio lobes of powerful radiosources. Obtained with the
Very Large Telescope (VLT), images of the HSs in the radio galaxy 3C445 show
bright knots embedded in diffuse optical emission distributed along the post
shock region created by the impact of the jet into the intergalactic medium.
The observations reported here confirm that relativistic electrons are
accelerated by Fermi-I acceleration processes in HSs. Furthermore, both the
diffuse emission tracing the rims of the front shock and the multiple knots
demonstrate the presence of additional continuous re-acceleration processes of
electrons (Fermi-II).Comment: 7 pages (latex format), 3 ps figures. Full text and PDF available at
Science web site: http://www.sciencemag.org/cgi/content/full/298/5591/19

### First NACO observations of the Brown Dwarf LHS 2397aB

Observations of the standard late type M8 star LHS 2397aA were obtained at
the ESO-VLT 8m telescope ``Yepun'' using the NAOS/CONICA Adaptive Optics
facility. The observations were taken during the NACO commissioning, and the
infrared standard star LHS 2397aA was observed in the H, and Ks broad band
filters. In both bands the brown dwarf companion LHS2397aB was detected. Using
a program recently developed (Bouy et al., 2003) for the detection of stellar
binaries we calculated the principal astrometric parameters (angular binary
separation and position angle P.A.) and the photometry of LHS 2397aA and LHS
2397aB. Our study largely confirms previous results obtained with the
AO-Hokupa'a facility at Gemini-North (Freed et al., 2003); however a few
discrepancies are observed.Comment: 5 page

### Skyrmions and the Nuclear Force

The derivation of the nucleon-nucleon force from the Skyrme model is
reexamined. Starting from previous results for the potential energy of
quasistatic solutions, we show that a calculation using the Born-Oppenheimer
approximation properly taking into account the mixing of nucleon resonances,
leads to substantial central attraction. We obtain a potential that is in
qualitative agreement with phenomenological potentials. We also study the
non-adiabatic corrections, such as the velocity dependent transition
potentials, and discuss their importance.Comment: 24 pages, UPR-0124M

### Surface behaviour of the pairing gap in semi-infinite nuclear matter

The $^1S_0$-pairing gap in semi-infinite nuclear matter is evaluated
microscopically using the effective pairing interaction recently found
explicitly in the coordinate representation starting from the separable form of
the Paris NN-potential. Instead of direct iterative solution of the gap
equation, a new method proposed by V.A.Khodel, V.V.Khodel and J.W.Clark was
used which simplifies the procedure significantly. The gap $\Delta$ obtained in
our calculations exibits a strong variation in the surface region with a
pronounced maximum near the surface.Comment: 9 pages, 2 ps figure

### Comment on Neutron-Proton Spin-Correlation Parameter A_{ZZ} at 68 Mev

We present two arguments indicating that the large value for the $\epsilon_1$
mixing parameter at 50 MeV, which the Basel group extracted from their recent
$A_{zz}$ measurement, may be incorrect. First, there are nucleon-nucleon (NN)
potentials which predict the $\epsilon_1$ at 50 MeV substantially below the
Basel value and reproduce the Basel $A_{zz}$ data accurately. Second, the large
value for $\epsilon_1$ at 50 MeV proposed by the Basel group can only be
explained by a model for the NN interaction which is very unrealistic (no
$\rho$-meson and essentially a point-like $\pi NN$ vertex) and overpredicts the
$\epsilon_1$ in the energy range where it is well determined (150--500 MeV) by
a factor of two.Comment: 6 pages text (LaTex) and 2 figures (paper, will be faxed upon
request), UI-NTH-930

### Minimum Wages and Teen Employment: A Spatial Panel Approach

The authors employ spatial econometric techniques and Annual Averages data from the U.S. Bureau of Labor Statistics for 1990-2004 to examine how changes in the minimum wage affect teen employment. Spatial econometric techniques account for the fact that employment is correlated across states. Such correlation may exist if a change in the minimum wage in a state affects employment not only in its own state but also in other, neighboring states. The authors show that state minimum wages negatively affect teen employment to a larger degree than is found in studies that do not account for this correlation. Their results show a combined direct and indirect effect of minimum wages on teen employment to be -2.1% for a 10% increase in the real effective minimum wage. Ignoring spatial correlation underestimates the magnitude of the effect of minimum wages on teen employment

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