Browsing by Subject "Electromagnetic waves--Scattering."

Now showing 1 - 8 of 8

Open Access
Accurate and efficient solutions of electromagnetic problems with the multilevel fast multipole algorithm
(2009) Ergül, Özgür Salih
The multilevel fast multipole algorithm (MLFMA) is a powerful method for the fast and efficient solution of electromagnetics problems discretized with large numbers of unknowns. This method reduces the complexity of matrix-vector multiplications required by iterative solvers and enables the solution of largescale problems that cannot be investigated by using traditional methods. On the other hand, efficiency and accuracy of solutions via MLFMA depend on many parameters, such as the integral-equation formulation, discretization, iterative solver, preconditioning, computing platform, parallelization, and many other details of the numerical implementation. This dissertation is based on our efforts to develop sophisticated implementations of MLFMA for the solution of real-life scattering and radiation problems involving three-dimensional complicated objects with arbitrary geometries.
Open Access
Application of characteristic basis function method for scattering from and propagation over terrain profiles
(2009) Yağbasan, Atacan
A computationally efficient hybrid method, that combines the characteristic basis function method and the physical optics as well as the forward backward method, is applied for the solution of integral equations used to investigate the electromagnetic scattering from and propagation over large scale rough terrain problems. The method utilizes high-level basis functions defined on macro-domains (named as blocks) namely characteristic basis functions that are constructed by aggregating low-level basis functions (i.e., conventional sub-domain basis functions). In the construction of the abovementioned characteristic basis functions, forward backward method as well as the physical optics approach (when applicable) are used. The conventional characteristic basis function method originally developed by Prakash et al. is slightly modified to handle large terrain problems, and is further embellished by accelerating it and by reducing its storage requirements via the use of an extrapolation procedure. Numerical results for the induced currents, total fields and path loss are presented and compared with either measured or previously published reference solutions to assess the efficiency and the accuracy of the method. Besides, certain parametric studies and convergence tests have been carried out.
Open Access
Effective preconditioners for iterative solutions of large-scale surface-integral-equation problems
(2010) Malas, Tahir
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.
Open Access
The examination of new equivalent edge currents in the prediction of high frequency backscattering from flat plates
(1991) Oğuzer, Taner
Equivalent edge currents based on the geometrical theory of dilfraction (GTD) have been utilized for the prediction of electromagnetic scattering from edged bodies. These equivalent currents are use Keller’s diffraction coefficient and therefore not valid for arbitrary aspect of observation. More general expressions for equivalent edge currents are later obtained by Michaeli. Those expressions become infinite at certain observation directions. These infinities are later eliminated by the same author for the fringe component of the equivalent currents l)y choosing a skew coordinate system on the half plane to be used for the asymptotic integration. A similar approach is employed here to eliminate the infinities in the physical optics(PO) component of the equivalent edge currents. It is also shown that the radiation from the fringe and PO equivalent currents is unique and yields the GTD field. The fringe and PO equivalent currents are then applied to the backscattering problems from the perfectly conducting square and triangular plates. The higher order interactions between the edges are also included into the analysis. Some improvements are obtained over the previous solutions.
Open Access
Fast algorithms for large 3-D electromagnetic scattering and radiation problems
(1997) Şendur, İbrahim Kürşat
Some interesting real-life radiation and scattering problems are electrically very large and cannot be solved using traditional solution algorithms. Despite the difficulties involved, the solution of these problems usually offer valuable results that are immediately useful in real-life applications. The fast multipole method (FMM) enables the solution of larger problems with existing computational resources by reducing the computational complexity and the memory requirement of the solution without sacrificing the accuracy. This is achieved by replacing the matrix-vector multiplications of O(N^) complexity by a faster equivalent of complexity in each iteration of an iterative scheme. Fast Far-Field Algorithm(FAFFA) further reduces 0{N^) complexity to 0{N^·^). A direct solution would require 0{N^) operations.
Open Access
Numerical study of plane wave scattering from cylindrical cavity-backed apertures with outer or inner material coating
(1993) Çolak, Dilek
In this thesis, a dual-series-based solution is obtained for the scattering of a time harmonic plane wave from a cavity-backed aperture(CBA) which is formed by a slitted infinite circular cylinder coated with absorbing material. The material coating can be done on the inner or outer surface of the cylinder. For both cases, numerical results are presented for the radar cross section (RCS) and comparisons of the suppression of RCS are given for two different realistic absorbing materials. Finally, the dependence of RCS on the thickness of the absorbing layer and on the aspect angle of the screen are presented numerically. To the best of our knowledge, this is the first study made so far to solve the problems of CBAs with material coating inside or outside with this approach.
Open Access
Out-of-core implementation of the parallel multilevel fast multipole algorithm
(2013) Karaosmanoğlu, Barışcan
We developed an out-of-core (OC) implementation of the parallel multilevel fast multipole algorithm (MLFMA) to solve electromagnetic problems with reduced memory. The main purpose of the OC method is to reduce in-core memory (primary storage) by using mass storage (secondary storage) units. Depending on the OC implementation, the in-core data may be left in one piece or divided into partitions. If the latter, the partitions are written out into mass storage unit(s) and read into in-core memory when required. In this way, memory reduction is achieved. However, the proposed method causes time delays because reading and writing large data using massive storage units is a long procedure. In our case, repetitive access to data partitions from the mass storage increases the total time of the iterative solution part of MLFMA. Such time delays can be minimized by selecting the right data type and optimizing the sizes of the data partitions. We run the optimization tests on different types of mass storage devices, such as hard disks and solid state drives. This thesis explores OC implementation of the parallel MLFMA. To be more precise, it presents the results of optimization tests done on different partition sizes and shows how computation time is minimized despite the time delays. This thesis also presents full-wave solutions of scattering problems including hundreds of millions of unknowns by employing an OC-implemented parallel MLFMA.
Open Access
Solution of electromagnetic scattering problems involving curved surfaces
(1997) Sertel, Kubilay
The method of moments (MoM) is an efficient technique for the solution of electromagnetic scattering problems. Problems encountered in real-life applications are often three dimensional and involve electrically large scatterers with complicated geometries. When the MoM is employed for the solution of these problems, the size of the resulting matrix equation is usually large. It is possible to reduce the size of the system of equations by improving the geometry modeling technique in the MoM algorithm. Another way of improving the efficiency of the MoM is the fast multipole method (FMM). The FMM reduces the computational complexity of the convensional MoM. The FMM has also lower memory-requirement complexity than the MoM. This facilitates the solution of larger problems on a given hardware in a shorter period of time. The combination of the FMM and the higher-order geometry modeling techniques is proposed for the efficient solution of large electromagnetic scattering problems involving three-dimensional, arbitrarily shaped, conducting suriace scatterers.