The parallel version of precondition techniques is developed for matrices arising from the Galerkin boundary element method for two-dimensional domains with Dirichlet boundary conditions. Results were obtained for implementations on a transputer network as well as on an nCUBE-2 parallel computer showing that iterative solution methods are very well suited for a MIMD computer. A comparison of numerical results for iterative and direct solution methods is presented and underlines the superiority of iterative methods for large systems.

Electromagnetic pulse simulators are essential for testing the ability of electronic devices to withstand high intensity electromagnetic ends. This work presents the analysis of various parallel plate transmission lines used in electromagnetic pulse simulators. Numerical techniques are developed to obtain the characteristic impedance, field map, and cut-infrequencies for the higher order modes of these transmission lines as exact analytical methods are not available for the determination of these quantities. The field map and characteristic impedance are computed using both finite element method (FEM) and boundary element method (BEM). Cut-off frequencies of the higher order modes are computed using FEM. The analysis presented can handle very general transmission line geometries. This analysis is useful for the design of electromagnetic pulse simulators.

[Truncated abstract] The study reported in this thesis extends the scaled boundary finite-element method to firstorder and second-order wave diffraction problems. The scaled boundary finite-element method is a newly developed semi-analytical technique to solve systems of partial differential equations. It works by employing a special local coordinate system, called scaled boundary coordinate system, to define the computational field, and then weakening the partial differential equation in the circumferential direction with the standard finite elements whilst keeping the equation strong in the radial direction, finally analytically solving the resulting system of equations, termed the scaled boundary finite-element equation. This unique feature of the scaled boundary finite-element method enables it to combine many of advantages of the finite-element method and the boundaryelement method with the features of its own. ... In this thesis, both first-order and second-order solutions of wave diffraction problems are presented in the context of scaled boundary finite-element analysis. In the first-order wave diffraction analysis, the boundary-value problems governed by the Laplace equation or by the Helmholtz equation are considered. The solution methods for bounded domains and unbounded domains are described in detail. The solution process is implemented and validated by practical numerical examples. The numerical examples examined include well benchmarked problems such as wave reflection and transmission by a single horizontal structure and by two structures with a small gap, wave radiation induced by oscillating bodies in heave, sway and roll motions, wave diffraction by vertical structures with circular, elliptical, rectangular cross sections and harbour oscillation problems. The numerical results are compared with the available analytical solutions, numerical solutions with other conventional numerical methods and experimental results to demonstrate the accuracy and efficiency of the scaled boundary finite-element method. The computed results show that the scaled boundary finite-element method is able to accurately model the singularity of velocity field near sharp corners and to satisfy the radiation condition with ease. It is worth nothing that the scaled boundary finite-element method is completely free of irregular frequency problem that the Green's function methods often suffer from. For the second-order wave diffraction problem, this thesis develops solution schemes for both monochromatic wave and bichromatic wave cases, based on the analytical expression of first-order solution in the radial direction. It is found that the scaled boundary finiteelement method can produce accurate results of second-order wave loads, due to its high accuracy in calculating the first-order velocity field.

The main concern of this Dissertation is focused on the derivation of novel integral formulation for simple problems. This alternative integral representations display a rapid decay as the complex parameter involved tends to infinity and are therefore suitable for numerical computations and for the study of the asymptotic properties of those solutions. There is also another important advantage attached to the novel formulae presented. These integral representations are useful for solving changing-type boundary value problems (such as Dirichlet data on part of the boundary and Neumann data on the complementary of the boundary). The following problems are analyzed: (a) The Laplacian operator in the interior of a Square, (b) the Laplacian operator in the interior and exterior of a Sphere and, (c) the Stokes' operator concerning the irrotational flow of an incompressible, viscous fluid. Moreover, the behaviour of the Gegenbauer functions of the first and second kind of general complex degree and order on the cut (-1, +1) are examined. / Με οδηγό μια νέα μεθοδολογία επίλυσης Μερικών Διαφορικών Εξισώσεων (ΜΔΕ), προβλήματα που σχετίζονται με την θεωρία Δυναμικού όπως επίσης και με την ροή Stokes, θα αναλυθούν. Απώτερος σκοπός αποτελεί η ανάπτυξη ολοκληρωτικών αναπαραστάσεων, η οποίες χαρακτηρίζονται από ταχεία σύγκλιση, με σκοπό να χρησιμοποιηθούν στην ασυμπτωτική μελέτη, στην αριθμητική ανάλυση όπως επίσης και στην επίλυση προβλημάτων μεικτών συνοριακών συνθηκών (π.χ. δεδομένα Dirichlet στο ένα κομμάτι του συνόρου και δεδομένα Neumann στο υπόλοιπο). Συγκεκριμένα, τα ακόλουθα προβλήματα αναλύονται: (α) Εξίσωση Laplace στο εσωτερικό ενός τετραγώνου, (β) εξίσωση Laplace στο εσωτερικό και εξωτερικό μιας σφαίρας και, (γ) εξίσωση αστρόβιλης ροής Stokes στο εσωτερικό ενός σφαιρικού κελύφους το οποίο στην συνέχεια καταλήγει, με οριακές διαδικασίες, στο εσωτερικό και εξωτερικό μιας σφαίρας. Τέλος, παρουσιάζονται αναπτύγματα και ασυμπτωτικές εκφράσεις των συναρτήσεων Gegenbauer.

The parallel version of precondition techniques is developed for matrices arising from the Galerkin boundary element method for two-dimensional domains with Dirichlet boundary conditions. Results were obtained for implementations on a transputer network as well as on an nCUBE-2 parallel computer showing that iterative solution methods are very well suited for a MIMD computer. A comparison of numerical results for iterative and direct solution methods is presented and underlines the superiority of iterative methods for large systems.

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