Στην παρούσα διπλωματική εργασία, ασχολούμαστε με μια καινοτόμα τεχνολογία, αυτή των υπολογιστικών πλεγμάτων (grids). Συγκεκριμένα μελετάμε την συμπεριφορά τέτοιων συστημάτων όσον αφορά τον τρόπο που εκτελούνται οι διάφορες εργασίες σε αυτά. Αρχικά λοιπόν αφότου κάνουμε μια γενική αναδρομή στο τι είναι τα grids, πώς υλοποιούνται και ποιές ανάγκες καλύπτουν, αναφερόμαστε σε ένα συγκεκριμένο υπολογιστικό πλέγμα, το οποίο είναι εγκατεστημένο στο Πανεπιστήμιο Πατρών σε χώρο του τμήματος Μηχανικών Η/Υ και Πληροφορικής και το οποίο έχει εναρμονιστεί με επιτυχία από τις αρχές του 2006 σε μια πανευρωπαϊκή δομή grids στα πλαίσια του έργου EGEE (Enabling Grids for E-science in Europe). Στο υπόλοιπο κομμάτι της μελέτης μας, ασχολούμαστε με την κατηγοριοποίηση και την μοντελοποίηση επικοινωνιακών και υπολογιστικών εργασιών στον κόμβο της Πάτρας. Αναλυτικότερα, αφού περιγράψουμε τα δομικά στοιχεία που αποτελούν τον τοπικό κόμβο και αναπτύξουμε τη μεθοδολογία για συγκέντρωση και στατιστική επεξεργασία των δεδομένων του (υπό μορφή log files), επικεντρωνόμαστε στην περιγραφή της διαδικασίας που ακολουθείται από τη διεθνή βιβλιογραφία για την κατηγοριοποίηση των παραπάνω εργασιών. Στη συνέχεια εφαρμόζουμε τα παραπάνω για την περίπτωσή μας και τέλος συγκρίνουμε τα αποτελέσματά μας με μεθόδους που προτείνονται από άλλους μελετητές για την κατηγοριοποίηση τέτοιων εργασιών. Η σύγκριση μας οδηγεί στο ασφαλές συμπέρασμα, πως το μοντέλο το οποίο προτείνουμε για την κατηγοριοποίηση των εργασιών είναι πιο αποδοτικό και πιο απλό από όσα έχουν προταθεί ως σήμερα. Η παραπάνω μελέτη βοηθά στην πληρέστερη κατανόηση της συμπεριφοράς ενός συστήματος πλέγματος, κάτι το οποίο είναι ιδιαίτερα σημαντικό κατά τον σχεδιασμό αλγορίθμων χρονοδρομολόγησης σε τέτοιου είδους συστήματα. Τελικός στόχος παραμένει η δίκαιη μεταχείριση των χρηστών και η μέγιστη αποδοτικότητα της όλης υποδομής. / -

Recently, a multi-resolution, hierarchical version of the hexagon mesh was introduced. In order for computers to display this mesh of hexagons, it must be triangulated. This thesis describes an algorithm to triangulate the multi-resolution, hierarchical hexagon mesh. The triangulation provides good triangle strips, which result in efficient rendering of the hexagon mesh, and well-proportioned triangles, which avoid rendering artifacts. This algorithm was designed for the rendering of hexagon meshes on the globe, though it currently does not support the pentagons of the truncated icosahedron. / Thesis (Master, Computing) -- Queen's University, 2009-01-21 12:11:02.532

Thesis (Ph. D.)--University of Oregon, 2004. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 101-107). Also available for download via the World Wide Web; free to University of Oregon users.

Thesis (M.S.)--Florida State University, 2004. / Advisor: Dr. Larry Dennis, Florida State University, College of Arts and Sciences, Department of Physics. Title and description from dissertation home page (viewed Aug. 27, 2004). Includes bibliographical references.

This dissertation is concerned with the behaviour of square-on-square double-layer space grids (DLSG's) and in particular their collapse mechanisms after initial buckling. A detailed review of the methods used for collapse analysis of these structures is conducted, which shows that their collapse can be of progressive nature. An outline of the remedies that have been suggested to make safe their collapse behaviour is given, and it traces the line for further study. The present investigation starts by describing the redundancy in these regular pin-jointed assemblies in terms of unit states of self-stress. This enables us to understand, by means of the extended Maxwell rule, how it is that mechanisms of collapse can occur when relatively few bars in the assembly have failed. The influence of boundary conditions on the mechanisms of collapse and its progressive nature is also investigated. This method of analysis is verified by detailed examination of several previously reported experimental results, and some conclusions can be drawn. The prestressing of statically indeterminate assemblies by means of lack-of-fit is investigated in the hope of changing their collapse characteristics. The unit state of self-stress has been used to explain a method for selecting the bars that can be shortened/lengthened in order to impose a prescribed state of self-stress, and the scope for achievable patterns of bar tensions has been described. A by-product of the investigation is an algorithm which calculates the required lack-of-fit to impose an initial state of self-stress which maximizes the load-carrying capacity of indeterminate trusses. Using the formulae derived for prestress, a statistical analysis is developed to estimate initial bar tensions due to random lack-of-fit in order to obtain a realistic estimate of the load-factor of an assembly before initial bar failure. Furthermore, we have argued and demonstrated how backlash at the bar-joint connection may have a beneficial effect in reducing substantially initial bar tensions due to the inevitable lack-of-fit on account of manufacturing errors of bars. An extensive experimental program has been conducted to verify the assumptions adopted and formulae derived in this dissertation.

A generalized grid based technique was developed for handling the relative motion of grids in CFD simulations involving rotating machineries. In the present method, the relative motion between the grid blocks is handled by splitting the cellaces at the interface and updating the grid data structure appropriately. The resulting grid will have cells and cellaces with an arbitrary number of nodes and which are stored in a cellace based data structure. The current methodology is developed for cells with any number of nodes. However, the present work supports only tetrahedral elements at the interface of the rotating grid-blocks at the beginning of the simulation. Also the present approach can handle multiple objects in the domain of interest which are rotating in arbitrary directions. The current approach was tested by rotating a generalized grid for a single un-ducted SR7 propeller with eight blades designed with 41 degrees of sweep at the tip. This was also tested for two counter rotating SR7 propellers. After every rotation the new grid was tested for negative volumes, folded cellaces, proper connectivity of nodes forming the cellaces, and for gaps. Preliminary work has been conducted to couple the grid generation strategy to a generalized grid based flow solver.

In this thesis, an approach is presented for the generation of grids suitable for the simulation of high Reynolds number viscous flows in complex three-dimensional geometries. The automatic and reliable generation of such grids is today on the biggest bottlenecks in the industrial CFD simulation environment. In the proposed approach, unstructured tetrahedral grids are employed for the regions far from the viscous boundaries of the domain, while semi-structured layers of high aspect ratio prismatic and hexahedral elements are used to provide the necessary grid resolution inside the boundary layers and normal to the viscous walls. The definition of the domain model is based on the STEP ISO standard and the topological information contained in the model is used for applying the hierarchical grid generation parameters defined by the user. An efficient, high-quality and robust algorithm is presented for the generation of the unstructured simplicial (triangular of tetrahedral) part of the grid. The algorithm is based on the Delaunay triangulation and the internal grid points are created following a centroid or frontal approach. For the surface grid generation, a hybrid approach is also proposed similar to the volume. Semi-structured grids are generated on the surface grid (both on the edges and faces of the domain) to improve the grid resolution around convex and concave ridges and corners, by aligning the grid elements in the directions of high solution gradients along the surface. A method is also developed for automatically setting the grid generation parameters related to the surface grid generation based on the curvature of the surface in order to obtain an accurate and smooth surface grid. Finally, a semi-structured prismatic/hexahedral grid generation algorithm is presented for the generation of the part of grid close to the viscous walls of the domain. The algorithm is further extended with improvements meant to increase the grid quality around concave and convex ridges of the domain, where the semi-structured grids are known to be inadequate. The combined methodology is demonstrated on a variety of complex examples mainly from the automotive and aeronautical industry.

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Electrical energy generation and distribution systems are good examples of complex systems. They include continuous, discrete, and social dynamics. They are operated by millions of human and non-human (or electro-mechanical) agents, and they show statistical properties found in other complex systems, such as power-law distributions in failure sizes. A number of recent large blackouts in Europe and North America have emphasized the societal importance of understanding these dynamics. Classical electromagnetic analysis alone frequently does not provide the insight required to characterize and mitigate risks in the electricity infrastructure. The objective of this thesis is to obtain insights into the dynamics of power grids using tools from the science of complex systems. In particular, this thesis will compare the topology, electrical structure, and attack/failure tolerance of power grids with those of theoretical graph structures such as regular, random, small-world, and scale-free networks. Simulation results in this thesis will describe the cost of the disturbances as a function of failure or attack sizes. The cost associated with network perturbations is often measured by changes on the diameter or average path length, whereas in the electricity industry, the loss of power demand (or blackout size) is the best indicator of the cost or impact of disturbances to electricity infrastructure.