Results 1 - 10 of 11
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[en] In the present work, we have studied the luminous efficacy of global solar radiation incident on vertical surfaces for clear skies (cloudless and rather clean skies) and mean hourly values of global solar radiation (i.e. averaged over each hour). Luminous efficacy models similar to those previously obtained for a horizontal surface have been developed using solar elevation as the independent variable. Thus, the hypothesis that the slope of the surface does not influence the general formulation of the model has been assumed. However, the final formulation of the models clearly depends on the insolation conditions of the inclined surface and, more specifically, on whether direct radiation is or is not incident on it. As a consequence, two different types of models have been proposed to take both insolation conditions into account. On the other hand, global luminous efficacy models have also been obtained as the best fits of the experimental data. Global illuminance has been estimated for the vertical surfaces considered, facing north, east, south and west, with the models obtained as the best fits of the experimental data and the other two types of models mentioned in the preceding paragraph. Comparison of the performance of models for global illuminance estimation has validated the initial hypothesis that the slope of the surface does not influence the general formulation of the model. The performances of the known Perez model and the proposed models are also compared
[en] Month-hour equal mean zenith luminance contours are obtained from one year of data of zenith luminance measurements for cloudless, overcast and partly cloudy skies and also when the combined data for all sky types are considered. For many hours in different months, the overcast sky luminance values are roughly about three times the cloudless sky luminance values and one and a half times the partly cloudy sky values. The dependence of month-hour equal mean zenith luminance contours on the ratio of global to extraterrestrial illuminance on a horizontal surface is also given. From equal mean zenith luminance contours, the approximate values of the mean zenith luminance for different sky conditions and different hours and months of the year can be easily obtained. Month-hour equal mean diffuse illuminance contours are obtained from diffuse illuminance measurements performed during the period 1992-1998. The dependence on solar altitude of the monthly average hourly values of diffuse illuminance is given and compared to the corresponding one obtained from data for Bet Dagan (Israel)
[en] This work presents a characterization of the luminous efficacy of the direct radiation for Florianopolis, Brazil, for which several direct luminous efficacy models available in the scientific literature were evaluated. In the first part of the work, the evaluation was restricted to clear sky conditions. The models evaluated were used with the original coefficients given by their authors and also with local coefficients obtained for Florianopolis, showing their dependence on the local characteristics. None of the models presented significantly better statistics than the others, although one of them, developed by Robledo and Soler, seems to justify better the physical behavior of the direct luminous efficacy. In the second part of the paper, three models of direct luminous efficacy for all sky types have been evaluated, the first two having been specifically developed for the direct luminous efficacy, whereas and the third was developed for the global luminous efficacy and has been adapted in this paper to characterize the direct luminous efficacy. The statistical evaluation of the three models showed that the last general model has accuracy similar to the two other ones
[en] In this paper the physics of clustering in atomic nucleus as seen from a mean field perspective will be discussed. Special attention is paid to phenomena involving octupole deformation like the α structure of "2"0Ne or the emission of heavy clusters. The stabilizing role of spin for cluster-like highly deformed states is also discussed in the case of "3"6 Ar
[en] By using the extension of the statistical Wick's theorem (Gaudin's theorem) to deal with generalized statistical density operators (those which can be expressed as the product of and operator carrying out a canonical transformations times a density operator) and using the appropriate limits we are able to rederive in a very simple way the standard generalized Wick's theorem for overlaps of mean-field wave functions. Due to the simplicity of the derivation it is now straightforward to consider more involved cases and some of them are discussed. The present derivation also allows us to obtain general and compact formulas for other particular cases of the generalized Wick theorem involving overlaps of multiquasiparticle excitations of product wave functions. The new expressions allow us to reduce the combinatorial complexity of the standard calculation of the above overlaps
[en] Two-body matrix elements of arbitrary local interactions are written as the sum of separable terms in a way that is well suited for the exchange and pairing channels present in mean-field calculations. The expansion relies on the transformation to center of mass and relative coordinate (in the spirit of Talmi's method) and therefore it is only useful (finite number of expansion terms) for harmonic oscillator single particle states. The converge of the expansion with the number of terms retained is studied for a Gaussian two body interaction. The limit of a contact (delta) force is also considered. Ways to handle the general case are also discussed.
[en] Any realistic model of r-process nucleosynthesis requires accurate predictions of nuclear masses for isotopes beyond the reach of currently available experimental facilities, thus one has to rely on theoretical nuclear masses. Self-consistent mean-field (SCMF) theories based on energy density functionals (EDF) were actively developing in the recent decades. However, due to computational complexity most of systematic surveys did not treat the odd-A nuclei at the same self-consistent level as the even-even isotopes. We performed a fully self-consistent large-scale calculation of nuclear masses using the exact blocking presciption with time-reversal symmetry breaking for odd-A nuclei within Hartree-Fock-Bogolyubov (HFB) framework with Gogny EDF. We analyse and compare the results for odd-A nuclei with the experimental values as well as commonly employed equal filling approximation.
[en] Fission dynamics properties of the Barcelona-Catania-Paris-Madrid energy density functional are explored with mean-field techniques. Potential energy surfaces as well as collective inertia relevant in the fission process are computed for several nuclei where experimental data exist. Inner and outer barrier heights as well as fission isomer excitation energies are reproduced quite well in all the cases. The spontaneous fission half-lives tsf are also computed using the standard semiclassical approach and the results are compared with the experimental data. The experimental trend with mass number is reasonably well reproduced over a range of 27 orders of magnitude. However, the theoretical predictions suffer from large uncertainties when the quantities that enter the spontaneous fission half-life formula are varied. Modifications of only a few per cent in the pairing correlation strengths strongly modify the collective inertia with a large impact on the spontaneous fission lifetimes in all the nuclei considered. Encouraged by the quite satisfactory description of the trend of fission properties with mass number, we explore the fission properties of the even-even uranium isotope chain from 226U to 282U. Very large lifetimes are found beyond A = 256 with a peak at neutron number N = 184.
[en] For the luminance measurements reported in the present work, one sensor was used to measure zenith luminance, and 34 sensors were positioned on a dome with their collimators pointing toward different positions P on the celestial sphere, which can be specified by the values of their zenith Z and azimuth θ angles. Overcast skies values were used. The final model proposed for the mean luminance distribution for overcast skies normalized to the zenith luminance L Zθ/L z has been developed in four steps. For each value of Z, the mean value of L Zθ/L z is expressed as a ratio of gradation functions that depend on θ. Only for θ = 180 deg. , a good coincidence is obtained with the model proposed by the Commission International de l'Eclairage (CIE) for a wide range of values of Z
[en] The non-perturbative method to compute Adiabatic Time Dependent Hartree Fock Bogoliubov (ATDHFB) collective inertias is extended to the Generator Coordinate Method (GCM) in the Gaussian overlap approximation (GOA) including the case of density dependent forces. The two inertias schemes are computed along the fission path of the 234U and compared with the perturbative results. We find that the non-perturbative schemes predict very similar collective inertias with a much richer structure than the one predicted by perturbative calculations. Moreover, the non-perturbative inertias show an extraordinary similitude with the exact GCM inertias computed numerically from the energy overlap. These results indicate that the non-perturbative inertias provide the right structure as a function of the collective variable and only a phenomenological factor is required to mock up the exact GCM inertia, bringing new soundness to the microscopic description of fission.