27,992 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Probabilistic framework to evaluate the resilience of engineering systems using Bayesian and dynamic Bayesian networks
Get PDFResilience indicators are a convenient tool to assess the resilience of engineering systems. They are often used in preliminary designs or in the assessment of complex systems. This paper introduces a novel approach to assess the time-dependent resilience of engineering systems using resilience indicators. A Bayesian network (BN) approach is employed to handle the relationships among the indicators. BN is known for its capability of handling causal dependencies between different variables in probabilistic terms. However, the use of BN is limited to static systems that are in a state of equilibrium. Being at equilibrium is often not the case because most engineering systems are dynamic in nature as their performance fluctuates with time, especially after disturbing events (e.g. natural disasters). Therefore, the temporal dimension is tackled in this work using the Dynamic Bayesian Network (DBN). DBN extends the classical BN by adding the time dimension. It permits the interaction among variables at different time steps. It can be used to track the evolution of a system's performance given an evidence recorded at a previous time step. This allows predicting the resilience state of a system given its initial condition. A mathematical probabilistic framework based on the DBN is developed to model the resilience of dynamic engineering systems. Two illustrative examples are presented in the paper to demonstrate the applicability of the introduced framework. One example evaluates the resilience of Brazil. The other one evaluates the resilience of a transportation system.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Integral Design & Managemen
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Resourcefulness quantification approach for resilient communities and countries
Get PDFAvailability of resources is one of the primary criteria for communities to attain a high resilience level during disaster events. This paper introduces a new approach to evaluate resourcefulness at the community and national scales. Resourcefulness is calculated using a proposed composite resourcefulness index, which is a combination of several resourcefulness indicators. To build the resourcefulness index, resourcefulness indicators representing the different aspects of resourcefulness are collected from renowned literary publications. Every indicator is assigned a measure to make it quantifiable. Time-history data for the measures are needed to perform the analysis. While these data could be obtained from different sources, acquiring a full set of data is quite challenging. Hence, to account for missing data, the Multiple Imputation (MI) and the Markov Chain Monte Carlo (MCMC) data imputation methods are adopted. The data are then normalized, assigned weights, and aggregated to obtain the resourcefulness index. A case study is performed to demonstrate the applicability of the approach. The resourcefulness indexes of two countries, namely the United States and Italy, are evaluated. Results show that resourceful communities/countries are more resilient during disaster events as they have more tools to come up with solutions. It is also shown that knowing the current resourcefulness level helps in better identifying what aspects should be improved.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Integral Design & Managemen
A 2 h periodic variation in the low-mass X-ray binary Ser X-1
No full textSpectroscopy of the low-mass X-ray binary Ser X-1 using the Gran Telescopio Canarias have revealed a ?2 h periodic variability that is present in the three strongest emission lines. We tentatively interpret this variability as due to orbital motion, making it the first indication of the orbital period of Ser X-1. Together with the fact that the emission lines are remarkably narrow, but still resolved, we show that a main-sequence K dwarf together with a canonical 1.4 M? neutron star gives a good description of the system. In this scenario, the most likely place for the emission lines to arise is the accretion disc, instead of a localized region in the binary (such as the irradiated surface or the stream-impact point), and their narrowness is due instead to the low inclination (?10°) of Ser X-1
Self-archiving practice and the influence of publisher policies in the social sciences
No full textAuthors in different disciplines exhibit very different behaviours on the so-called ‘green’ road to open access, i.e. self-archiving. This study looks at the self-archiving behaviour of authors publishing in leading journals in six social science disciplines. It tests the hypothesis that authors are self-archiving according to the norms of their respective disciplines rather than following self-archiving policies of publishers, and that, as a result, they are self-archiving significant numbers of publisher PDF versions. It finds significant levels of
self-archiving, as well as significant self-archiving of
the publisher PDF version, in all the disciplines
investigated. Publishers’ self-archiving policies have
no influence on author self-archiving practice
Author Classifications of O*NET Individual Work Activities (IWA)
No full text<p>Author Classifications of O*NET Individual Work Activities (IWA) used in "Innovations and Economic Output Scale with Social Interactions in the Workforce"</p>
Thermal boundary layer equation for turbulent Rayleigh-Benard convection
No full textWe report a new thermal boundary layer equation for turbulent Rayleigh-Benard convection for Prandtl number P>1that takes into account the effect of turbulent fluctuations. These fluctuations are neglected in existing equations, which are based on steady-state and laminar assumptions. Using this new equation, we derive analytically the mean temperature profiles in two limits: Pr~1 and Pr>>1. These two theoretical predictions are in excellent agreement with the results of our direct numerical simulations for Pr=4.38 and Pr=2547.9, respectively
Analysis of P&O algorithm efficiency under variable irradiance conditions
No full textFight against climate change is facilitated through energy transition which entails changing the fossil fuel based energy generation to sustainable resources such as solar PV. However, PV installations require extensive resources for their development and construction, so it is essential that the efficiency of energy conversion is high. A very important factor to ensure PV system efficiency is the use of maximum power point tracking (MPPT). MPPT controls the system's operating point so that at each instance power drawn from a PV module or array is the maximum power available. This is realised by employing a DC-DC converter controlled by an MPPT algorithm. The most widely used algorithm due to its simplicity and cost of implementation is the Perturb and Observe (P&O) algorithm. However, its performance is not ideal in both steady-state and dynamic conditions. The algorithm's operation depends on the P&O parameters and conditions in which the algorithm operates are influenced by irradiance variability.There are several studies on P&O algorithm efficiency and irradiance variability separately, however, a link between these two topics has not been explored. Thus, this thesis aims to bridge the gap and its purpose is to determine the relationship between P&O algorithm efficiency and irradiance variability. This was carried out by use of real 3-second irradiance data from Oahu, Hawaii, modelling the power output of a PV module and obtaining the operating point of the system by implementing a P&O algorithm without assuming any physical properties of a DC-DC converter. Then using operating point and maximum power point power values, 3 s averages of the P&O algorithm efficiency were computed, and variability metrics were calculated using the original irradiance data. Finally, an exploratory analysis of the prepared dataset was performed to determine how the P&O algorithm varies when exposed to different irradiance variability. Additionally, a sensitivity analysis was deemed necessary to examine efficiency dependence on P&O algorithm parameters - sampling interval and perturbation amplitude - in different cases of irradiance variability.The results of this research have shown variations of P&O efficiency values on different time scales as well as in different variability conditions. Each day of the year was classified by occurring irradiance variability. It was confirmed that efficiency depends on the variability class of the day with the lowest efficiencies down to 99.8 % found for highly variable days. Monthly P&O efficiency was determined to be affected by the number of days from each class. P&O efficiency was highest when the least days in the month were classified as highly variable. Monthly efficiencies were found to be above 99.9 %. To describe the relationship between variability and P&O efficiency, variability metrics were selected and computed for 1 min periods. Major scattering of data was found when 1 min averaged efficiency was plotted against any of the variability metrics. Magnitudes of 1 min average efficiency were determined to be mostly above 95 %. To reduce scattering, average efficiencies that could be expected in bins of variability metrics were determined, and in such a case, a general decrease in efficiency was observed with increasing magnitudes of variability metrics. Additionally, polynomial fits through the data were produced to provide functions with which P&O efficiency could be approximated when variability described by a metric is known. Lastly, P&O efficiency sensitivity to its parameters study has shown that in most cases efficiency is more sensitive to perturbation amplitude, however, in high variability, sampling interval and perturbation amplitude must be sufficiently small (ΔV≤1% of Voc(STC), Ta≤10 ms) for better performance of the algorithm.Electrical Engineering | Sustainable Energy Technolog
Convective–reactive nucleosynthesis of K, Sc, Cl and p-process isotopes in O–C shell mergers
Get PDF© 2017 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. We address the deficiency of odd-Z elements P, Cl, K and Sc in Galactic chemical evolution models through an investigation of the nucleosynthesis of interacting convective O and C shells in massive stars. 3D hydrodynamic simulations of O-shell convection with moderate C-ingestion rates show no dramatic deviation from spherical symmetry. We derive a spherically averaged diffusion coefficient for 1D nucleosynthesis simulations, which show that such convective-reactive ingestion events can be a production site for P, Cl, K and Sc. An entrainment rate of 10-3M⊙s-1features overproduction factors OPs≈ 7. Full O-C shell mergers in our 1D stellar evolution massive star models have overproduction factors OPm> 1 dex but for such cases 3D hydrodynamic simulations suggest deviations from spherical symmetry. γ - process species can be produced with overproduction factors of OPm> 1 dex, for example, for130, 132Ba. Using the uncertain prediction of the 15M⊙, Z = 0.02 massive star model (OPm≈ 15) as representative for merger or entrainment convective-reactive events involving O- and C-burning shells, and assume that such events occur in more than 50 per cent of all stars, our chemical evolution models reproduce the observed Galactic trends of the odd-Z elements
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