391 research outputs found
Hydrothermal Scheduling in the Continuous-Time Framework
Continuous-time optimization models have successfully been used to capture
the impact of ramping limitations in power systems. In this paper, the
continuous-time framework is adapted to model flexible hydropower resources
interacting with slow-ramping thermal generators to minimize the hydrothermal
system cost of operation. To accurately represent the non-linear hydropower
production function with forbidden production zones, binary variables must be
used when linearizing the discharge variables and the continuity constraints on
individual hydropower units must be relaxed. To demonstrate the performance of
the proposed continuous-time hydrothermal model, a small-scale case study of a
hydropower area connected to a thermal area through a controllable high-voltage
direct current (HVDC) cable is presented. Results show how the flexibility of
the hydropower can reduce the need for ramping by thermal units triggered by
intermittent renewable power generation. A reduction of 34% of the structural
imbalances in the system is achieved by using the continuous-time model.Comment: Accepted for publication through the Power Systems Computation
Conference 202
Superconducting Transition Temperature in Heterogeneous Ferromagnet-Superconductor Systems
We study the shift of the the superconducting transition temperature in
ferromagnetic-superconducting bi-layers and in a superconducting film supplied
a square array of ferromagnetic dots. We find that the transition temperature
in these two cases change presumably in opposite direction and that its change
is not too small. We extend these results to multilayer structures. We predict
that rather small external magnetic field Oe can change the
transition temperature of the bilayer by 10% .Comment: 9 pages, 2 figure
Associations between general self-efficacy and health-related quality of life among 12-13-year-old school children: a cross-sectional survey
<p>Abstract</p> <p>Background</p> <p>While research on school children's health has mainly focused on risk factors and illness, few studies have examined aspects of health promotion. Thus, this study focuses on health promotional factors including general self-efficacy (GSE) and health-related quality of life (HRQOL). GSE refers to a global confidence in coping ability across a wide range of demanding situations, and is related to health. The purpose of this study was to examine associations between GSE and HRQOL, and associations between HRQOL and socio-demographic characteristics. Knowledge of these associations in healthy school children is currently lacking.</p> <p>Methods</p> <p>During 2006 and 2007, 279 school children in the seventh grade across eastern Norway completed a survey assessing their GSE and HRQOL. The children were from schools that had been randomly selected using cluster sampling. T-tests were computed to compare mean subscale values between HRQOL and socio-demographic variables. Single and multiple regression analyses were performed to explore associations among GSE, HRQOL and socio-demographic variables.</p> <p>Results</p> <p>Regression analyses showed a significant relationship between increasing degrees of GSE and increasing degrees of HRQOL. In analyses adjusted for socio-demographic variables, boys scored higher than girls on self-esteem. School children from single-parent families had lower scores on HRQOL than those from two-parent families, and children who had relocated within the last five years had lower scores on HRQOL than those who had not relocated.</p> <p>Conclusion</p> <p>The strong relationship between GSE and HRQOL indicates that GSE might be a resource for increasing the HRQOL for school children.</p
Faraday rotation spectra of bismuth-substituted ferrite garnet films with in-plane magnetization
Single crystalline films of bismuth-substituted ferrite garnets have been
synthesized by the liquid phase epitaxy method where GGG substrates are dipped
into the flux. The growth parameters are controlled to obtain films with
in-plane magnetization and virtually no domain activity, which makes them
excellently suited for magnetooptic imaging. The Faraday rotation spectra were
measured across the visible range of wavelengths. To interprete the spectra we
present a simple model based on the existence of two optical transitions of
diamagnetic character, one tetrahedral and one octahedral. We find excellent
agreement between the model and our experimental results for photon energies
between 1.77 and 2.53 eV, corresponding to wavelengths between 700 and 490 nm.
It is shown that the Faraday rotation changes significantly with the amount of
substituted gallium and bismuth. Furthermore, the experimental results suggest
that the magnetooptic response changes linearly with the bismuth substitution.Comment: 15 pages, 6 figures, published in Phys. Rev.
Collective dynamics of colloids at fluid interfaces
The evolution of an initially prepared distribution of micron sized colloidal
particles, trapped at a fluid interface and under the action of their mutual
capillary attraction, is analyzed by using Brownian dynamics simulations. At a
separation \lambda\ given by the capillary length of typically 1 mm, the
distance dependence of this attraction exhibits a crossover from a logarithmic
decay, formally analogous to two-dimensional gravity, to an exponential decay.
We discuss in detail the adaption of a particle-mesh algorithm, as used in
cosmological simulations to study structure formation due to gravitational
collapse, to the present colloidal problem. These simulations confirm the
predictions, as far as available, of a mean-field theory developed previously
for this problem. The evolution is monitored by quantitative characteristics
which are particularly sensitive to the formation of highly inhomogeneous
structures. Upon increasing \lambda\ the dynamics show a smooth transition from
the spinodal decomposition expected for a simple fluid with short-ranged
attraction to the self-gravitational collapse scenario.Comment: 13 pages, 12 figures, revised, matches version accepted for
publication in the European Physical Journal
Serbian KINDL questionnaire for quality of life assessments in healthy children and adolescents: reproducibility and construct validity
<p>Abstract</p> <p>Background</p> <p>The KINDL questionnaire is frequently used to evaluate quality of life (QOL) and the impacts of health conditions on children's everyday living. The objectives of this study were to assess the reproducibility and construct validity of the Serbian KINDL for QOL assessments in healthy children and adolescents.</p> <p>Methods</p> <p>Five hundred and sixty-four healthy children and adolescents completed the KINDL. Reproducibility was analyzed using the intraclass correlation coefficient (ICC). Confirmatory factor analysis (CFA) was performed to assess the structure of the KINDL construct validity.</p> <p>Results</p> <p>The intraclass correlation coefficients ranged from 0.03 to 0.84 for the subscales and total score. A second order CFA model as originally hypothesized was tested: items (24), primary factors (six subscales), and one secondary factor (QOL). The fit indexes derived from a CFA failed to yield appropriate fit between the data and the hypothesized model.</p> <p>Conclusion</p> <p>Majority of the subscales and total KINDL possess appropriate reproducibility for group comparisons. However, a CFA failed to confirm the structure of the original measurement model, indicating that the Serbian version should be revised before wider use for QOL assessments in healthy children and adolescent.</p
Free energy of colloidal particles at the surface of sessile drops
The influence of finite system size on the free energy of a spherical
particle floating at the surface of a sessile droplet is studied both
analytically and numerically. In the special case that the contact angle at the
substrate equals a capillary analogue of the method of images is
applied in order to calculate small deformations of the droplet shape if an
external force is applied to the particle. The type of boundary conditions for
the droplet shape at the substrate determines the sign of the capillary
monopole associated with the image particle. Therefore, the free energy of the
particle, which is proportional to the interaction energy of the original
particle with its image, can be of either sign, too. The analytic solutions,
given by the Green's function of the capillary equation, are constructed such
that the condition of the forces acting on the droplet being balanced and of
the volume constraint are fulfilled. Besides the known phenomena of attraction
of a particle to a free contact line and repulsion from a pinned one, we
observe a local free energy minimum for the particle being located at the drop
apex or at an intermediate angle, respectively. This peculiarity can be traced
back to a non-monotonic behavior of the Green's function, which reflects the
interplay between the deformations of the droplet shape and the volume
constraint.Comment: 24 pages, 19 figure
Global Characterisation of Coagulopathy in Isolated Traumatic Brain Injury (iTBI): A CENTER-TBI Analysis
Background - Trauma-induced coagulopathy in patients with traumatic brain injury (TBI) is associated with high rates of complications, unfavourable outcomes and mortality. The mechanism of the development of TBI-associated coagulopathy is poorly understood.
Methods - This analysis, embedded in the prospective, multi-centred, observational Collaborative European NeuroTrauma Effectiveness Research in Traumatic Brain Injury (CENTER-TBI) study, aimed to characterise the coagulopathy of TBI. Emphasis was placed on the acute phase following TBI, primary on subgroups of patients with abnormal coagulation profile within 4 h of admission, and the impact of pre-injury anticoagulant and/or antiplatelet therapy. In order to minimise confounding factors, patients with isolated TBI (iTBI) (n = 598) were selected for this analysis.
Results - Haemostatic disorders were observed in approximately 20% of iTBI patients. In a subgroup analysis, patients with pre-injury anticoagulant and/or antiplatelet therapy had a twice exacerbated coagulation profile as likely as those without premedication. This was in turn associated with increased rates of mortality and unfavourable outcome post-injury. A multivariate analysis of iTBI patients without pre-injury anticoagulant therapy identified several independent risk factors for coagulopathy which were present at hospital admission. Glasgow Coma Scale (GCS) less than or equal to 8, base excess (BE) less than or equal to − 6, hypothermia and hypotension increased risk significantly.
Conclusion - Consideration of these factors enables early prediction and risk stratification of acute coagulopathy after TBI, thus guiding clinical management
Acute psychiatric admissions from an out-of-hours Casualty Clinic; how do referring doctors and admitting specialists agree?
BACKGROUND: Over the last decades there has been an increasing pressure on the acute psychiatric wards in Norway. The major contributor to psychiatric acute admissions at the University Hospital of North Norway in the city of Tromsø in 2001 was the GP-based Tromsø Casualty Clinic, only open out-of-hours. We explored all acute psychiatric referrals from Tromsø Casualty Clinic in 2001. The purpose of the study was to characterize the admissions and assess the agreement between the referring doctors and the hospital specialists according to the need for hospitalization, agreement on application of the law and the diagnostic evaluation to assess whether the admissions were appropriate. METHODS: Retrospective, record based, descriptive study comprising 101 psychiatric acute referrals from the Tromsø Casualty Clinic to the psychiatric acute wards at the University Hospital of North Norway. RESULTS: The specialists accepted all referrals except one, they mostly agreed upon the diagnoses suggested by the referring doctors and they mostly confirmed the application of the law. Seventy-five percent of the admissions took place during weekends, public holidays or nighttimes. Diagnoses of psychoses or suicidal attempts accounted for 76 % of the total referrals. Substance abuse was noted for 43 %, and in 22 % of all admissions the patients had stopped taking their psychopharmacological medication. The police assisted the referring doctors in one third of all admissions, and was the legal representative in 52 out of 59 involuntary admissions. Thirty percent of the admissions were first- time admissions. Thirty-two percent of the hospital stays lasted for three days or less. Median length of stay was 6.5 days. CONCLUSION: The casualty clinic physicians and the hospital specialists mostly agreed in their evaluation of patients indicating that most of the admissions were appropriate. The police was more often involved in the involuntary admissions than intended in the law. The proportion of patients with substance abuse was significant. Alternative treatment strategies should be developed for non-psychotic patients in need of short-term stays
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