Global-Local Finite Element Analysis

114 σ.Η αναλυτική επίλυση πολύπλοκων προβλημάτων της μηχανικής στις μέρες μας καθίσταται δυσχερής εως αδύνατη χωρίς την εφαρμογή αριθμητικών μεθόδων και τη χρήση ηλεκτρονικού υπολογιστή. Η μέθοδος των πεπερασμένων στοιχείων αποτελεί σήμερα ένα ισχυρό εργαλείο για την επίλυση τέτοιων προβλημάτων και εξελίσσεται με μεγάλη ταχύτητα τόσο σε ακαδημαϊκό όσο και σε επαγγελματικό επίπεδο. Ενδεικτικά, αν και επινοήθηκε και χρησιμοποιήθηκε για τη στατική ανάλυση φορέων, έχει καθολικότερη εφαρμογή σε μια ευρύτερη κατηγορία προβλημάτων του μηχανικού, όπως στη ρευστομηχανική, στη μεταφορά θερμότητας, στην ακουστική, στον ηλεκτρομαγνητισμό και στην εμβιομηχανική. Επιπλέον, η εξέλιξη στων Η/Υ με τις ολοένα μεγαλύτερες δυνατότητες διαχείρισης όγκου δεδομένων αλλά και με την αύξηση της ταχύτητας εκτέλεσης των αριθμητικών πράξεων κατέστησε εφικτή την επίλυση σύνθετων προβλημάτων τα οποία θεωρούνταν απροσπέλαστα πριν μερικά χρόνια. Στην κατηγορία αυτή, των προβλημάτων αυξημένου υπολογιστικού κόστους, ανήκει και η καταστατική περιγραφή πολυφασικών υλικών. Είναι γεγονός ότι το μεγαλύτερο μέρος των παραγώμενων δομικών υλικών σήμερα, παρουσιάζει κάποιο είδος ανομοιογένειας, διακριτή ή μη στην κλίμακα δομικών έργων. Χαρακτηριστικά παραδείγματα αποτελούν τα κράματα μετάλλων, τα πορώδη, τα πολυκρυσταλλικά και τα σύνθετα υλικά στα οποία το μέγεθος, το σχήμα και οι ιδιότητες των συστατικών τους μερών καθορίζουν άμεσα τη συνολική τους μηχανική συμπεριφορά. Διάφορες τεχνικές έχουν αναπτυχθεί για την προσομοίωση και την περιγραφή της απόκρισης ανομοιογενών υλικών. Η παρούσα εργασία επικεντρώνεται στη μέθοδο ομογενοποίησης πολλαπλών κλιμάκων η οποία συνίσταται στην επίλυση δύο εμφωλευμένων προβλημάτων συνοριακών τιμών, για τη μακροκλίμακα και τη μικροκλίμακα αντίστοιχα. Τα βασικά χαρακτηριστικά μιας τέτοιας μεθόδου είναι ότι - Δεν απαιτείται η περιγραφή των καταστατικών νόμων του μακροφορέα. - Παρέχει τη δυνατότητα ενσωμάτωσης μεγάλων παραμορφώσεων και στροφών τόσο στην προσομοίωση της μικροκλίμακας όσο και του μακροφορέα. - Παρέχει τη δυνατότητα λεπτομερούς προσομοίωσης των συστατικών μερών της μικροκλίμακας. - Επιτρέπει οποιαδήποτε τεχνική επίλυσης στην κλίμακα του μικροφορέα. Αναλυτικά, σύμφωνα με τη μέθοδο αυτή. υπολογίζεται το διάνυσμα ανηγμένων παραμορφώσεων σε κάθε υλικό σημείο του μακροφορέα το οποίο στη συνέχεια χρησιμοποιείται για τη μόρφωση των συνοριακών συνθηκών του αντιπροσωπευτικού μικροφορέα στο αντίστοιχο σημείο. Μετά την επίλυση του προβλήματος συνοριακών τιμών της μικροκλίμακας, το διάνυσμα των τάσεων του μακροφορέα υπολογίζεται μέσα από τη διαδικασία ομογενοποίησης του πεδίου των τάσεων και κατά τον τρόπο αυτό υπολογίζεται η σχέση τάσεων ανηγμένων παραμορφώσεων για κάθε υλικό σημείο Ωστόσο, υπάρχουν κάποιοι περιορισμοί στην εφαρμογή της εν λόγω υπολογιστικής τεχνικής. Συγκεκριμένα, παρά το ότι κατά την προσομοίωση λαμβάνονται υπ' όψην οι διάφορες παράμετροι της μικροκλίμακας όπως το ποσοστό όγκου, η κατανομή και η μορφολογία των συστατικών μερών του υλικού, τα αποτελέσματα της μεθόδου είναι ανεξάρτητα από το απόλυτο μέγεθος του αντιπροσωπευτικού όγκου της μικροκλίμακας. Παρ' όλα αυτά, η τεχνική ομογενοποίησης στα πλαίσια ανάλυσης πολλαπλών κλιμάκων αποτελεί ένα σημαντικό εργαλείο για τον υπολογισμό των καταστατικών σχέσεων πολυφασικών υλικών στα οποία είναι αδύνατη η εφαρμογή οποιασδήποτε άλλης μεθόδου.Nowadays, analysis of complicated problems in the domain of mechanics consti- tutes a hard and even impossible task without the implementation of numerical methods and the employment of computational machines. Finite element method is a powerful tool for the solution of such problems and is rapidly developed in an academic and professional sense. Even if it was developed and implemented for structural analysis, it is widely employed in several domains such as in fluid mechanics, heat transfer, acoustics and electromagnetism. Furthermore, the development of computer hardware in terms of data processing, has significantly contributed to the solution of problems that were considered inaccessible a few years ago. Most of the materials produced in industry are heterogeneous on one or another spatial scale. Typical examples include metal alloy systems, porous media and polycrystalline materials and composites. The overall response of these micro heterogeneous materials depends strongly on the size, shape properties and spatial distribution of the microstructural components. Several techniques have been developed for the prediction of the macroscopic behavior of such materials. The present work is concentrated on the first order homogenization technique in the framework of a multi-scale approach which consists of the solution of two nested boundary value problems, for the macro-scale and the micro-scale respectively. Methods of this type - Do not require any constitutive assumption with respect to the overall ma- terial behavior. - Enable the incorporation of large deformations and rotations on both micro and macrolevel. - Provide the possibility to introduce detailed microstructural information. - Allow the use of any modelling technique at the microlevel. Concretely, according to this approach, the macroscopic deformation tensor is calculated for every integration point of the macrostructure and then is used to formulate the kinematic boundary conditions for the associated microstructural representative volume element (RVE). After the solution of the microstructural boundary value problem, the macroscopic stress tensor is computed by averaging the resulting microstructural stress field over the volume of the RVE and as a result, we obtain the stress-strain relation at every macroscopic point. However, there is a major disadvantage of the existing first-order computational homogenization. More specifically, this technique can account for the volume fraction, distribution and morphology of the micro-components however, it cannot take into account the absolute size of the microstructure making it thus impossible to treat microstructural size effects. Nevertheless, computational homogenization provides a significant strategy to obtain micro-macro structure-property relations for materials for which the overall macroscopic response cannot be computed by any other method.Κωνσταντινος Ε. Τατση

CEO pay, shareholder returns, and accounting profits

We assess the impact on CEO pay (including salary, cash bonus, and benefits in kind) of changes in both accounting and shareholder returns in 99 British companies in the years 1972-89. After correcting for heterogeneity biases inherent in the standard specifications of the problem, we find a strong positive relationship between CEO pay and within-company changes in shareholder returns, and no statistically significant relationship between CEO pay and within-company changes in accounting returns. Differences between firms in long-term average profitability do appear to have a substantial effect on CEO pay, while differences between firms in shareholder returns add nothing to the within-firm pay dynamics.These findings call into question the rationale for explicitly share-based incentive schemes

L-Edge Spectroscopy of Dilute, Radiation-Sensitive Systems Using a Transition-Edge-Sensor Array

We present X-ray absorption spectroscopy and resonant inelastic X-ray scattering (RIXS) measurements on the iron L-edge of 0.5 mM aqueous ferricyanide. These measurements demonstrate the ability of high-throughput transition-edge-sensor (TES) spectrometers to access the rich soft X-ray (100-2000eV) spectroscopy regime for dilute and radiation-sensitive samples. Our low-concentration data are in agreement with high-concentration measurements recorded by conventional grating-based spectrometers. These results show that soft X-ray RIXS spectroscopy acquired by high-throughput TES spectrometers can be used to study the local electronic structure of dilute metal-centered complexes relevant to biology, chemistry and catalysis. In particular, TES spectrometers have a unique ability to characterize frozen solutions of radiation- and temperature-sensitive samples.Comment: 19 pages, 4 figure

Popularizing Party Journalism in China in the Age of Social Media: The Case of Xinhua News Agency

The phenomenon of the “popularization” of journalism has become widespread in the process of media marketization, globalization and digitalization. This phenomenon has been studied mostly in the Anglo-American context. This article instead draws attention to China, where the tendency toward popularizing (party) journalism is also occurring but taking a rather different form. It focuses on the case of Xinhua News Agency—the pioneer as well as the most representative case of traditional party journalism in the country. The article considers to what extent Xinhua’s online media content concerning the ruling party since 1949—the Communist Party of China—has been popularized both in terms of content and style. The changes to online media content made by Xinhua are indicative of the extent to which it is possible to combine the status of a state-owned central news organization with a new journalistic orientation that seeks to make the messages from and about the party more appealing to technology-savvy and entertainment-driven audiences in the new media environment in mainland China

Absence of Fast-moving Iron in an Intermediate Type Ia Supernova between Normal and Super-Chandrasekhar

In this paper, we report observations of a peculiar SN Ia iPTF13asv (a.k.a., SN2013cv) from the onset of the explosion to months after its peak. The early-phase spectra of iPTF13asv show an absence of iron absorption, indicating that synthesized iron elements are confined to low-velocity regions of the ejecta, which, in turn, implies a stratified ejecta structure along the line of sight. Our analysis of iPTF13asv's light curves and spectra shows that it is an intermediate case between normal and super-Chandrasekhar events. On the one hand, its light curve shape (B-band Δm_(15) = 1.03 ± 0.01) and overall spectral features resemble those of normal SNe Ia. On the other hand, its large peak optical and UV luminosity (M_B = -19.84 mag, M_(uvm2) = -15.5 mag) and its low but almost constant Si II velocities of about 10,000 km s^(−1) are similar to those in super-Chandrasekhar events, and its persistent carbon signatures in the spectra are weaker than those seen commonly in super-Chandrasekhar events. We estimate a ^(56)Ni mass of 0.81 ± ^(+0.10)_(-0.18) M⊙ and a total ejecta mass of 1.59^(+0.45)_(-0.12)M⊙. The large ejecta mass of iPTF13asv and its stratified ejecta structure together seemingly favor a double-degenerate origin

Reconsidering the Barefoot Doctor Programme

This paper examines the widely acclaimed Barefoot Doctor campaign in China. The Barefoot Doctor Campaign has come to symbolize the success of Chinese health care to the extent that it has become a model for WHO public health strategy. Yet little has been done to understand how or whether it worked on the ground and what difficulties and contradictions emerged in its implementation. Using previously unexplored party archives as well as newly collected oral interviews, this paper moves away from a narrow focus on party politics and policy formulation by examining the reality of health care at the local level and the challenges faced by local authorities and individuals as the campaigns evolved

First-year Sloan Digital Sky Survey-II (SDSS-II) Supernova Results: Hubble Diagram and Cosmological Parameters

We present measurements of the Hubble diagram for 103 Type Ia supernovae (SNe) with redshifts 0.04 < z < 0.42, discovered during the first season (Fall 2005) of the Sloan Digital Sky Survey-II (SDSS-II) Supernova Survey. These data fill in the redshift "desert" between low- and high-redshift SN Ia surveys. We combine the SDSS-II measurements with new distance estimates for published SN data from the ESSENCE survey, the Supernova Legacy Survey, the Hubble Space Telescope, and a compilation of nearby SN Ia measurements. Combining the SN Hubble diagram with measurements of Baryon Acoustic Oscillations from the SDSS Luminous Red Galaxy sample and with CMB temperature anisotropy measurements from WMAP, we estimate the cosmological parameters w and Omega_M, assuming a spatially flat cosmological model (FwCDM) with constant dark energy equation of state parameter, w. For the FwCDM model and the combined sample of 288 SNe Ia, we find w = -0.76 +- 0.07(stat) +- 0.11(syst), Omega_M = 0.306 +- 0.019(stat) +- 0.023(syst) using MLCS2k2 and w = -0.96 +- 0.06(stat) +- 0.12(syst), Omega_M = 0.265 +- 0.016(stat) +- 0.025(syst) using the SALT-II fitter. We trace the discrepancy between these results to a difference in the rest-frame UV model combined with a different luminosity correction from color variations; these differences mostly affect the distance estimates for the SNLS and HST supernovae. We present detailed discussions of systematic errors for both light-curve methods and find that they both show data-model discrepancies in rest-frame $U$-band. For the SALT-II approach, we also see strong evidence for redshift-dependence of the color-luminosity parameter (beta). Restricting the analysis to the 136 SNe Ia in the Nearby+SDSS-II samples, we find much better agreement between the two analysis methods but with larger uncertainties.Comment: Accepted for publication by ApJ

Factors associated with health-seeking behavior among migrant workers in Beijing, China

<p>Abstract</p> <p>Background</p> <p>Migrant workers are a unique phenomenon in the process of China's economic transformation. The household registration system classifies them as temporary residents in cities, putting them in a vulnerable state with an unfair share of urban infrastructure and social public welfare. The amount of pressure inflicted by migrant workers in Beijing, as one of the major migration destinations, is currently at a threshold. This study was designed to assess the factors associated with health-seeking behavior and to explore feasible solutions to the obstacles migrant workers in China faced with when accessing health-care.</p> <p>Methods</p> <p>A sample of 2,478 migrant workers in Beijing was chosen by the multi-stage stratified cluster sampling method. A structured questionnaire survey was conducted via face-to-face interviews between investigators and subjects. The multilevel methodology (MLM) was used to demonstrate the independent effects of the explanatory variables on health seeking behavior in migrant workers.</p> <p>Results</p> <p>The medical visitation rate of migrant workers within the past two weeks was 4.8%, which only accounted for 36.4% of those who were ill. Nearly one-third of the migrant workers chose self-medication (33.3%) or no measures (30.3%) while ill within the past two weeks. 19.7% of the sick migrants who should have been hospitalized failed to receive medical treatment within the past year. According to self-reported reasons, the high cost of health service was a significant obstacle to health-care access for 40.5% of the migrant workers who became sick. However, 94.0% of the migrant workers didn't have any insurance coverage in Beijing. The multilevel model analysis indicates that health-seeking behavior among migrants is significantly associated with their insurance coverage. Meanwhile, such factors as household monthly income per capita and working hours per day also affect the medical visitation rate of the migrant workers in Beijing.</p> <p>Conclusion</p> <p>This study assesses the influence of socio-demographic characteristics on the migrant workers' decision to seek health care services when they fall ill, and it also indicates that the current health service system discourages migrant workers from seeking appropriate care of good quality. Relevant policies of public medical insurance and assistance program should be vigorously implemented for providing affordable health care services to the migrants. Feasible measures need to be taken to reduce the health risks associated with current hygiene practices and equity should be assured in access to health care services among migrant workers.</p