Derivation of a dynamic model of the kinetics of nitrogen uptake throughout the growth of lettuce : calibration and validation

A kinetic model of nitrogen (N) uptake throughout growth was developed for lettuce cultivated in nutrient solution under varying natural light conditions. The model couples nitrogen uptake with dry matter accumulation using a two-compartment mechanistic approach, incorporating structural and non-structural pools. Maximum nitrogen uptake rates are assumed to decline with shoot dry weight, to allow for the effects of plant age. The model was parameterized using data from the literature, and calibrated for differences in light intensity using an optimization algorithm utilizing data from three experiments in different growing seasons. The calibrated model was validated against the data from two independent experiments conducted under different light conditions. Results showed that the model made good predictions of nitrogen uptake by plants from seedlings to maturity under fluctuating light levels in a glasshouse. Plants grown at a higher light intensity showed larger maximum nitrogen uptake rates, but the effect of light intensity declined towards plant maturity

Rotations and Abundances of Blue Horizontal-Branch Stars in Globular Cluster M15

High-resolution optical spectra of eighteen blue horizontal-branch (BHB) stars in the globular cluster M15 indicate that their stellar rotation rates and photospheric compositions vary strongly as a function of effective temperature. Among the cooler stars in the sample, at Teff ~ 8500 K, metal abundances are in rough agreement with the canonical cluster metallicity, and the v sin i rotations appear to have a bimodal distribution, with eight stars at v sin i < 15 km/s and two stars at v sin i ~ 35 km/s. Most of the stars at Teff > 10000 K, however, are slowly rotating, v sin i < 7 km/s, and their iron and titanium are enhanced by a factor of 300 to solar abundance levels. Magnesium maintains a nearly constant abundance over the entire range of Teff, and helium is depleted by factors of 10 to 30 in three of the hotter stars. Diffusion effects in the stellar atmospheres are the most likely explanation for these large differences in composition. Our results are qualitatively very similar to those previously reported for M13 and NGC 6752, but with even larger enhancement amplitudes, presumably due to the increased efficiency of radiative levitation at lower intrinsic [Fe/H]. We also see evidence for faster stellar rotation explicitly preventing the onset of the diffusion mechanisms among a subset of the hotter stars.Comment: 11 pages, 1 figure, 1 table, accepted to ApJ

Striking Photospheric Abundance Anomalies in Blue Horizontal-Branch Stars in Globular Cluster M13

High-resolution optical spectra of thirteen blue horizontal-branch (BHB) stars in the globular cluster M13 show enormous deviations in element abundances from the expected cluster metallicity. In the hotter stars (T_eff > 12000 K), helium is depleted by factors of 10 to 100 below solar, while iron is enhanced to three times the solar abundance, two orders of magnitude above the canonical metallicity [Fe/H] ~= -1.5 dex for this globular cluster. Nitrogen, phosphorus, and chromium exhibit even more pronounced enhancements, and other metals are also mildly overabundant, with the exception of magnesium, which stays very near the expected cluster metallicity. These photospheric anomalies are most likely due to diffusion --- gravitational settling of helium, and radiative levitation of the other elements --- in the stable radiative atmospheres of these hot stars. The effects of these mechanisms may have some impact on the photometric morphology of the cluster's horizontal branch and on estimates of its age and distance.Comment: 11 pages, 1 Postscript figure, uses aaspp4.sty, accepted for publication in ApJ Letter

Massively parallel finite element simulation of compressible and incompressible flows

We present a review of where our research group stands in parallel finite element simulation of flow problems on the Connection Machines, an effort that started for our group in the fourth quarter of 1991. This review includes an overview of our work on computation of flow problems involving moving boundaries and interfaces, such as free surfaces, two-liquid interfaces, and fluid-structure and fluid-particle interactions. With numerous examples, we demonstrate that, with these new computational capabilities, today we are at a point where we routinely solve practical flow problems, including those in 3D and those involving moving boundaries and interfaces. We solve these problems with unstructured grids and implicit methods, with some of the problem sizes exceeding 5 000 000 equations, and with computational speeds up to two orders of magnitude higher than what was previously available to us on the traditional vector supercomputers

The 6 minute walk in idiopathic pulmonary fibrosis: longitudinal changes and minimum important difference

The response characteristics of the 6 minute walk test (6MWT) in studies of idiopathic pulmonary fibrosis (IPF) are only poorly understood, and the change in walk distance that constitutes the minimum important difference (MID) over time is unknown

A new mixed preconditioning method based on the clustered element-by-element preconditioners

We describe a new mixed preconditioning method for finite element computations. In the clustered element-by-element (CEBE) preconditioning, the elements are merged into clusters, and the preconditioners are defined as series products of cluster level matrices. The (cluster companion) CC preconditioners are based on companion meshes associated with different levels of clustering. For each level of clustering, we construct a CEBE preconditioner and an associatedC C preconditioner. Because these two preconditioners complement each other, when they are mixed, they give better performance. Our numerical tests, for two- and three-dimensional problems governed by the Poisson equation, show that the mixed CEBE/CC preconditioning results in convergence rates which are significantly better than those obtained with the best of the CEBE and CC methods

Point-contact investigations of challenging superconductors: two-band MgB2, antiferromagnetic HoNi2B2C, heavy fermion UPd2Al3, paramagnetic MgCNi3

An overview on recent efforts in point-contact (PC) spectroscopy of title superconductors is given. Distinct phonon features and crystalline-electric-field effects are observed in PC spectra of HoNi2B2C. Results of study of superconducting (SC) gap and excess current versus temperature and magnetic field reflecting specific multi-band electronic structure in MgB2 are presented. The nature of the extremely nonlinear I(V) curves in the antiferromagnetic (AF) and SC state are elucidated for UPd2Al3 break-junctions and MgCNi3 point contacts.Comment: 2 pages, 4 figs., presented on SCES`04 (26-30 July, Karlsruhe, Germany), accepted for publicatio