3,178 research outputs found

    A PIV and CFD analysis of natural convection ice melting

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    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

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    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

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    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

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    The microscopic effective pairing interaction in the 1S0^1S_0-channel is investigated for two different values of the chemical potential őľ\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=2őľE=2\mu.Comment: LaTeX, 8 pages, 6 ps-figure

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

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    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

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    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

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    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

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    The 1S0^1S_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 őĒ\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

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    We present two arguments indicating that the large value for the ŌĶ1\epsilon_1 mixing parameter at 50 MeV, which the Basel group extracted from their recent AzzA_{zz} measurement, may be incorrect. First, there are nucleon-nucleon (NN) potentials which predict the ŌĶ1\epsilon_1 at 50 MeV substantially below the Basel value and reproduce the Basel AzzA_{zz} data accurately. Second, the large value for ŌĶ1\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 ŌĀ\rho-meson and essentially a point-like ŌÄNN\pi NN vertex) and overpredicts the ŌĶ1\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

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    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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