1,244 research outputs found
A Lagrangian sub-grid model for the dispersion of clouds of tracers
No full textTurbulence models are expected to satisfy the conflicting requirements of accuracy and computational efficiency. Here we discuss a new model that was recently developed in order to accurately and efficiently describe the dynamic of a clouds of tracers particles in Large Eddy Simulations of homogeneous and isotropic turbulent flows. The models incorporates the multi-scale nature of time and space turbulent velocity correlations that are essential in order to correctly reproduce the relative dispersion of multi-particle clouds. The model can be seen as an off-grid solver for the Eulerian velocity field at the positions of a given number of Lagrangian tracers that self-consistently move with it. Extensions to non homogeneous and isotropic turbulence as well as to the dynamics of particles will be discussed
I.M. Panagiotopoulos as an autobiographer: the adventure of the hero and the personal myth of the author
No full textThe present doctoral dissertation refers to the complex and prominent image of I.M. Panagiotopoulos as a human being, which emerges through his exuberant and diverse work. This dissertation explains man with work and work with man and highlights a special aspect of I.M. Panagiotopoulos that of the pedagogue, through his pedagogical texts, his presence in the classroom and, in general, his pedagogical action. It refers to his childhood, repulsed mostly to the unconscious and to the deeper layers of his psyche. Also, the object of this study is the novel writing of I.M. Panagiotopoulos which istreated as a consequence of the denaturation of his obstructed erotic energy. Next, the two dominant impulses, Love and Death that meet in art and in life, are explored, with the impulses of Death predominant, a fact that leads to their narcissistic character, as well as to their conflicting nature. The anthropogeographical and metaphysical elements that constitute the concept and ontology of Greece are then highlighted. Then, the pedagogical work of I.M. Panagiotopoulos, his positions,contradictions and his proposals for Education is examined. I.M. Panagiotopoulos fought for a school that will offer Liberal Education, within a democratic regime that defines the equality of opportunity in learning. In addition, our national language is presented during its historical course and its expressive and iconographic boldness is highlighted. Finally, this thesis codifies the evolutionary stages of a single hero and creates a typology of his inner adventure, which is the adventure of the hero's desire. It also traces the author's personal myth on a general level, the correspondences and the dialectical relationship between the author and his work, in such a one-way way, that the work defines and composes the author and not vice versa.Η παρούσα διδακτορική διατριβή αναφέρεται στη σύνθετη κα εξέχουσα εικόνα του Ι.Μ. Παναγιωτόπουλου ως ανθρώπου, η οποία ξεπροβάλλει μέσα από το πληθωρικό και πολύμορφο έργο του. Εξηγεί τον άνθρωπο με το έργο και το έργο με τον άνθρωπο. Αναδεικνύει μια ξεχωριστή πλευρά του Ι.Μ. Παναγιωτόπουλου αυτή του Παιδαγωγού, μέσα από τα παιδαγωγικά του κείμενα, από την παρουσία του στην τάξη και γενικότερα από την παιδαγωγική του δράση. Αναφέρεται στην παιδική του ηλικία απωθημένη ως επί το πλείστον στο ασυνείδητο και στα βαθύτερα στρώματα του ψυχισμού του. Επίσης, αντικείμενο της μελέτης είναι η μυθιστορηματική γραφή του Ι.Μ. Παναγιωτόπουλου που αντιμετωπίζεται ως απόρροια μετουσίωσης της παρεμποδισμένης ερωτικής του ενέργειας. Ακολούθως, διερευνώνται οι δύο κυρίαρχες ενορμήσεις, του Έρωτα και του Θανάτου, που ανταμώνουν στην τέχνη και στη ζωή, με κυρίαρχες μάλιστα τις ενορμήσεις του Θανάτου, γεγονός που οδηγεί στον ναρκισσικό χαρακτήρα τους, καθώς και στη συγκρουσιακή τους φύση. Στη συνέχεια, αναδεικνύονται τα ανθρωπογεωγραφικά και μεταφυσικά στοιχεία που συνιστούν την έννοια και την οντολογία της Ελλάδας. Κατόπιν, εξετάζεται το Παιδαγωγικό του έργο και παρουσιάζονται οι θέσεις, οι αντιθέσεις και οι προτάσεις του Ι.Μ. Παναγιωτόπουλου για την Παιδεία. Ο Ι.Μ. Παναγιωτόπουλος αγωνίστηκε για ένα σχολείο, το οποίο θα προσφέρει Αγωγή φιλελεύθερη, μέσα σ’ ένα δημοκρατικό καθεστώς που ορίζει την ισότητα των ευκαιριών στη μάθηση. Επιπλέον, παρουσιάζεται η εθνική μας γλώσσα κατά την ιστορική της διαδρομή και αναδεικνύεται η εκφραστική και εικονοπλαστική της τόλμη. Τέλος, η παρούσα διατριβή κωδικοποιεί τα εξελικτικά στάδια ενός ενιαίου ήρωα και δημιουργεί μια τυπολογία της εσωτερικής του περιπέτειας, που είναι η περιπέτεια της επιθυμίας του ήρωα. Επίσης, ανιχνεύονται ο προσωπικός μύθος του συγγραφέα σε ένα γενικό επίπεδο, οι αντιστοιχίες και η διαλεκτική σχέση ανάμεσα στον συγγραφέα και στο έργο του, με τέτοιο μονοδρομικό τρόπο, ώστε το έργο να καθορίζει και να συνθέτει τον συγγραφέα και όχι αντίθετα
The stress connection: Neuroimaging studies of emotion circuits in social stress, personality, and stress-related psychopathology
Get PDFThe aim of this thesis was to identify the neural mechanisms that enable a person to adaptively respond to, and recover from stress, which was studied in healthy controls, in people with increased vulnerability or resilience to stress-related disorders, and in people with depression or PTSD, using magnetic resonance imaging (MRI). In most of the studies, a specific MRI method was employed, with which it is possible to assess how different brain regions communicate with each other (i.e., functional connectivity) when the brain is initiating or regulating stress responses. Structure, activity, and connectivity of the amygdala, a small brain region important for stress reactivity, was of main interest. The results show how stress influences information processing, and causes changes in the communication between brain areas, even long after the stressful event ended. Furthermore, personality dimensions associated with increased vulnerability or resilience to affective disorders were associated with changes in brain networks involved in emotion processing and regulation. Finally, smaller amygdala volumes were found in women with PTSD, while reduced integrity of affective brain networks was demonstrated in depression. Together, these results open important new avenues for future research into the short and long term effects of stress on the brain.Multivariate analysis of psychological dat
Methods and software for significant indicators determination of the natural language texts author profile
Get PDFMethods for the formation and optimization of author profiles are presented. The author profile is an image – a vector in a multidimensional space, which components are author’s texts measurements by a number of methods based on 4-grams, stemming, recurrence analysis and formal stochastic grammar. The author’s profile is a model of his language, including vocabulary, sentence syntax features. A comparative analysis of each of the methods effectiveness is carried out. By means of the genetic algorithm, a reduced profile of the author is formed. Insignificant indicators are excluded, which allows to reduce their number by 20%. The reduced author’s profile contains attributes that are significant for this author and is an effective attribution of a particular author.Prombles in programming 2023; 3: 22-2
Glass production from desert sand: Proof of concept and characterisation
No full textFor in-situ resource utilisation start-up Maana Electric, an investigation was undertaken to determine whether a cover glass for solar panels can be produced using only desert sand as the raw material. During this investigation, the composition of desert sand, melt formation, processing temperatures, and mechanical and optical properties were considered. The composition of 18 desert sands was analysed by means of Xray fluorescence and estimations of mineralogical composition were made, after which an attempt was made to melt the unmodified sand samples in a microwave furnace built for the purpose. Melt formation was further observed by melting binary combinations of store bought minerals that were found in the desert sands. The composition data and modelling of temperature-viscosity curves were employed to explore lowering the practical melting of the sand point by modification of the composition through benificiation. `Synthetic benificiated desert sand' was produced and melted based on the results. Glass samples produced were characterized using Xray fluorescence, visual inspection, optical spectrometry, and fracture mirror analysis. It was found that about half of the desert sand samples assessed contain over 90 wt% silica, making it less feasible for use as raw material for glass due to high melting temperatures and/or large waste streams from benificiation, while sands containing larger fractions of carbonates and/or feldspars will form a melt at less than 1650 degrees Celsius if the SiO2 content is less than 55 wt%. Transmission of 85 % of ~550 nm wavelength light was shown to be possible for desert sand glass of 3 mm thickness if Fe2O3 content is lower than 0.1 wt%, while for the same transmission in the complete effective spectrum of silicon based solar cells the iron content needs to be lowered further. Known absorbing species such as Cr2O3, NiO and CuO were detected in desert sand in trace amounts, but were not present in the synthetic mixtures, the influence of these contaminants on transmission requires further research. Mechanical analysis was inconclusive due to a limited number and low quality of the samples produced, but a review of the literature implies that a Young's modulus of >70 GPa and flexural strength of >45 MPa are attainable in a glass produced from desert sand components.Materials Science and Engineerin
Altered levels of coagulation factors in bleeding and thrombosis
Get PDFBlood coagulation is a complex system in which the proteins of the coagulation cascade play an important role. Aberrations in pro- or anticoagulant protein levels may be at the basis of coagulation-related pathological events, such as bleeding or thrombosis. Here the relations of altered coagulation factor levels and venous thromboembolism, the tissue factor threshold which needs to be overcome to initiate coagulation and the in vitro validation of APC-resistant FV as a possible alternative treatment of factor XI deficiency are explored. Cannegieter, S.C.; Cate, H. ten; Veer, C. van 'tLUMC / Geneeskund
Information about the Golden Horde in the Saltuk-name of the Ottoman author Ebul-Khayri Rumi
Get PDFThe purpose of the study is to analyze information on the history of the Golden Horde from the multi-volume work of Ebul-Khayri Rumi “Saltuk-name”, an Ottoman author of the 15th century.
Research materials. Ottoman historical works contain quite original information about the Golden Horde and the Tatar khanates. Their information allows us to reveal previously unknown pages of the history of the Golden Horde period. One of such important and early works is the three-volume work “Saltuk-name” by Ebul-Khayri Rumi, an Ottoman author of the 15th century, describing the legendary history of Sary-Saltuk, who was engaged in the spread of Islam, including in the lands of the Golden Horde, primarily in Crimea. In Russian-language historiography, this author is little known and his information has not been used for scientific research. The relevance of the article is explained by the need to introduce into scientific circulation the information of the “Saltuk-name” about the Golden Horde.
Results and scientific novelty. The study is the first attempt at a systematic analysis of Ebul-Khayri Rumi’s Saltuk-name in Russian. The novelty of the study is the introduction of specific information from the Ottoman historical work and its interpretation into scientific circulation. This study provides an important impetus for the introduction of information from the works of Ottoman authors who wrote about the Golden Horde into active scientific circulation
Measurements of fluid flow in weld pools
No full textUnderstanding the fluid flow in weld pools contributes significantly toward controlling the heat distribution in the base material and the mass distribution of molten base and additive materials. Currently, most investigations focus primarily on numerical models, due to the experimental difficulties associated with the challenging environment in and around the weld pool. Numerical simulations based on computational fluid dynamics are currently addressing the dynamic behaviour of weld pools, such as melting, solidification, temperature and velocity fields; these results provide information about conditions inside the weld pool that is impossible to gain experimentally. However, there is still a shortage of experimental validation to support these models. In this thesis, experimental methods are applied to study the fluid flow of the weld pool in both arc and laser generated weld pools. The general conclusions from the experimental study provides some suggestions for understanding the fluid flow in the weld pool, such as flow asymmetry, rotational flow and interface instabilities. For example, the interface instabilities can affect the energy and mass distribution. Describing these instabilities numerically requires three dimensional flow models. Moreover, the current experimental results question the validity of works where the surfactant concentration is assumed to be constant in the weld pool without consideration of dynamic diffusion and convection. This work has focused on experimental measurement of the fluid dynamics of weld pools, and current results provide a number of validation cases for further numerical simulations.Materials Science & EngineeringMechanical, Maritime and Materials Engineerin
Microstructural Evolution during High-Frequency Post Weld Impact Treatments for High-Strength Steels
No full textMaterial Science and EngineeringMechanical, Maritime and Materials Engineerin
Investigation and control of factors influencing resistance upset butt welding.
No full textThe purpose of this work is to investigate the factors influencing the resistance upset butt welding process to obtain an understanding of the metal behaviour and welding process characteristics, so that new automotive steels can be welded with reduced development time and fewer failures in production. In principle the welding process is rather simple, the materials to be joined are clamped between two electrodes and pressed together. Because there is an interface present with a higher resistance than the base material, heat will be generated at the joint position once the welding current and voltage are applied. As soon as the material is warm and soft enough, an extra (upsetting) pressure is applied and a weld is formed. For making wheel rims, after welding, the upset is removed by means of chisels and shear cut- ting the edges of the weld, followed by rolling the remaining material into the rim. After the upset has been removed the wheel rim is cold formed in several steps to its final shape. For the purpose of this research 2.0 mm thick S460MC and 1.65 mm thick HR45 steel were examined at a test welding facility at Fontijne Grotnes BV. Both steels were specially developed for cold forming and deep drawing purposes and have a mainly ferritic microstructure. An initial research into the crack formation in wheel rims during production was performed, proving that there were no hard and crack susceptible microstructures present in the weld area of the researched materials. The idea that the wheel rims crack due to for example to the presence of bainite and martensite was rejected. There were two main types of cracks discovered, large cracks which are formed due to initial misalignment of the sheets, grease and/or oil at the interface during welding and an uneven heat distribution during welding. The other type of cracks were micro-cracks, which originate from notches introduced during the trimming and rolling of the wheel rim edges after welding. A good alignment of the chisels and cutting tools is necessary to minimise the amount of material that is pushed into the wheel rim Because it was not feasible to do welding experiments in a full wheel rim welding machine at a production facility, the Fontijne Holland Welderr was employed, this is a scaled down version of a production welding machine, but without the material feed or post weld handling facilities. During welding it was observed that the weld sample did not heat up evenly, the emitted light at the interface was brighter at some places than at others. Also after welding the oxidation of the joint area showed that there was a non-uniform heating, with warmer and colder areas. This non-uniformity results in non-uniform weld characteristics and weld quality differences over the length of the weld. Tensile tests of samples taken both from the edges and the centre of welded samples showed that the maximum tensile strength can be as high as 95% of the tensile strength of the base material. However, the strain to failure of the welded samples was only a maximum of 60% with a 30% difference between the central test sample and the edge samples. Erichsen testing was also employed to provide an indication of weld quality. Only the centre of samples was tested due to the way the Erichsen tester is constructed. The Erichsen height was upto 85% of the base material height for the welded samples and it was found that if there are no edge defects present, the Erichsen test is a quick and valid weld quality test. Due to observed non-uniform weld quality and heating of the weld area, the focus of the research shifted to understanding why there was a non-uniformity present. Temperature measurements both by thermal camera and thermocouples showed that there is indeed a non-uniform temperature distribution present during welding over the joint interface. The outer edges of the joint line have higher temperatures than the central part of the weld. Local difference can be as high as 300 K over just 20 mm distance. Also there are hot spots present at either end of the joint line approximately 10 mm from the edges. These hot spots can give rise to local melting in steels with low melting temperature segregation bands. The causes of the uneven temperature distribution were sought. Employing local current measurements during welding was not possible, however when the welding process is just started and there has been no signifcant heating of the weld sample, it is possible to link the voltage distribution directly to the current density distribution. Measuring voltage distributions is relatively easy, voltage probes were spot welded to the surface of the weld samples and measurements showed that there was a non- uniform voltage distribution present during welding. This non-uniformity had the same distribution as the temperature distribution, low voltages at the places were the temperatures were low and high voltage signals where the temperatures were high. Because the non-uniform voltage distribution was measured for both 1 and 2-piece samples (samples with and without an interface) it can be concluded that the boundary conditions governed by the welding equipment have a major influence. The influence of the clamping system and the contact surface of the electrodes were researched to determine their its influence on the heating of the weld area. It was shown that the clamping pressure of the samples was not high enough to prevent slip of the samples during welding and that the samples had to be positioned in the centre of the electrodes to minimise non-uniform heating. At the places of the highest local pressure, the contact resistance is the lowest and heating will consequently be slower at those locations. The highest edge temperatures were measured at the far end of the electrodes, where the lowest clamping pressures are expected. To obtain more uniform heating it is necessary to obtain a more uniform current distribution. This can be accomplished by changing the contact interface between the electrode and the sample surface or the interface between the weld samples. Adapting the electrodes has a small influence but will also result in longer welding times due to a decrease in contact surface area (decrease of 10% results in a 5% increase in weld time). The interface change can decrease the welding time by up to 20% due to a local faster initial heating. Unfortunately the change in interface surface resulted in less repeatable welds. Residual stress in the welded samples was also examined. Both samples with the upset present as well as samples where the upset was removed were measured. The samples with the weld present showed tensile stresses at the centre of the joint line, but compressive stresses at the edges of the sample. When the upset was removed the overall stress state of the sheet became more neutral, the compressive and tensile stress peaks become lower. The stress distribution was explained conceptually with the aid of a modified bar model. Modelling with both MS.Marc and ABAQUS was employed in this research to obtain a better understanding of the complex electrical, thermal and mechanical relationships as observed during real experiments. Three different models were developed, a thermal model, a thermo-mechanical model and a local strain model. The local strain model showed that the Erichsen testing method is a valid method for weld quality assessment. The modelled total strain observed during flaring of wheel rims was lower than the modelled and measured strain during Erichsen testing. Both the thermal and thethermo-mechanical models are predictive. The first one is based on input parameters directly coming from measurements during welding experiments. The thermo-mechanical model is based on the temperature dependent material parameters. The initial thermal model was designed to determine if the non-uniform current density distribution, as measured during experiments, could result in a non-uniform heating of the weld area. The model did not include any temperature dependent parameters nor a mechanical coupling. This showed that the non-uniform current input resulted in a non-uniform heating of the sample. By obtaining a more uniform current distribution over the length of the joint area, a more uniform heating can be obtained. It was also indicated that around contaminations at the interface, either conductive or insulating, a locally higher temperature exists. The mechan- ical model is a more realistic model as it includes the upsetting force. The model predicts the formation of the upset when there are misalignments of the sheets during welding. Because this model includes the temperature dependent material parameters, it is possible to predict the residual stresses after welding, which are in qualitative agreement with the measured residual stresses in real welds.Materials Science and Engineering, Joining and Mechanical BehaviourMechanical, Maritime and Materials Engineerin
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