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Item Open Access Characteristic lie algebra and classification of semi-discrete models(Bilkent University, 2009) Pekcan, AslıShow more In this thesis, we studied a differential-difference equation of the following form tx(n + 1, x) = f(t(n, x), t(n + 1, x), tx(n, x)), (1) where the unknown t = t(n, x) is a function of two independent variables: discrete n and continuous x. The equation (1) is called a Darboux integrable equation if it admits nontrivial x- and n-integrals. A function F(x, t, t±1, t±2, ...) is called an x-integral if DxF = 0, where Dx is the operator of total differentiation with respect to x. A function I(x, t, tx, txx, ...) is called an n-integral if DI = I, where D is the shift operator: Dh(n) = h(n + 1). In this work, we introduced the notion of characteristic Lie algebra for semidiscrete hyperbolic type equations. We used characteristic Lie algebra as a tool to classify Darboux integrability chains and finally gave the complete list of Darboux integrable equations in the case when the function f in the equation (1) is of the special form f = tx(n, x) + d(t(n, x), t(n + 1, x)).Show more Item Open Access Effective preconditioners for iterative solutions of large-scale surface-integral-equation problems(Bilkent University, 2010) Malas, TahirShow more A popular method to study electromagnetic scattering and radiation of threedimensional electromagnetics problems is to solve discretized surface integral equations, which give rise to dense linear systems. Iterative solution of such linear systems using Krylov subspace iterative methods and the multilevel fast multipole algorithm (MLFMA) has been a very attractive approach for large problems because of the reduced complexity of the solution. This scheme works well, however, only if the number of iterations required for convergence of the iterative solver is not too high. Unfortunately, this is not the case for many practical problems. In particular, discretizations of open-surface problems and complex real-life targets yield ill-conditioned linear systems. The iterative solutions of such problems are not tractable without preconditioners, which can be roughly defined as easily invertible approximations of the system matrices. In this dissertation, we present our efforts to design effective preconditioners for large-scale surface-integral-equation problems. We first address incomplete LU (ILU) preconditioning, which is the most commonly used and well-established preconditioning method. We show how to use these preconditioners in a blackbox form and safe manner. Despite their important advantages, ILU preconditioners are inherently sequential. Hence, for parallel solutions, a sparseapproximate-inverse (SAI) preconditioner has been developed. We propose a novel load-balancing scheme for SAI, which is crucial for parallel scalability. Then, we improve the performance of the SAI preconditioner by using it for the iterative solution of the near-field matrix system, which is used to precondition the dense linear system in an inner-outer solution scheme. The last preconditioner we develop for perfectly-electric-conductor (PEC) problems uses the same inner-outer solution scheme, but employs an approximate version of MLFMA for inner solutions. In this way, we succeed to solve many complex real-life problems including helicopters and metamaterial structures with moderate iteration counts and short solution times. Finally, we consider preconditioning of linear systems obtained from the discretization of dielectric problems. Unlike the PEC case, those linear systems are in a partitioned structure. We exploit the partitioned structure for preconditioning by employing Schur complement reduction. In this way, we develop effective preconditioners, which render the solution of difficult real-life problems solvable, such as dielectric photonic crystals.Show more Item Open Access Solution of electromagnetic scattering problems with the locally corrected Nyström method(Bilkent University, 2010) Kılınç, SeçilShow more The locally corrected Nystr¨om (LCN) method is used to solve integral equations with high accuracy and efficiency. Unlike commonly used methods, the LCN method employs high-order basis functions on high-order surfaces. Hence, the number of unknowns in the electromagnetic problem decreases substantially, this also reduces the total solution time of the problem. In this thesis, electromagnetic scattering problems for arbitrary, three-dimensional, and conducting geometries are solved with the LCN method. Both the electric-field integral equation (EFIE) and the magnetic-field integral equation (MFIE) are implemented. The solution time for Duffy integrals is reduced significantly by modifying the Duffy transform. Then, mixed-order basis functions are implemented to accurately represent the charge density for EFIE. Finally, both the accuracy and the efficiency (in terms of solutions times and the number of unknowns) of the LCN method are compared with the method of moments and the multilevel fast multipole algorithm.Show more Item Open Access Solution of electromagnetics problems with the equivalence principle algorithm(Bilkent University, 2010) Tiryaki, BurakShow more A domain decomposition scheme based on the equivalence principle for integral equations is studied. This thesis discusses the application of the equivalence principle algorithm (EPA) in solving electromagnetics scattering problems by multiple three-dimensional perfect electric conductor (PEC) objects of arbitrary shapes. The main advantage of EPA is to improve the condition number of the system matrix. This is very important when the matrix equation is solved iteratively, e.g., with Krylov subspace methods. EPA starts solving electromagnetics problems by separating a large complex structure into basic parts, which may consist of one or more objects with arbitrary shapes. Each one is enclosed by an equivalence surface (ES). Then, the surface equivalence principle operator is used to calculate scattering via equivalent surface, and radiation from one ES to an other can be captured using the translation operators. EPA loses its accuracy if ESs are very close to each other, or if an ES is very close to PEC object. As a remedy of this problem, tangential-EPA (T-EPA) is introduced. Properties of both algorithms are investigated and discussed in detail. Accuracy and the efficiency of the methods are compared to those of the multilevel fast multipole algorithm.Show more