Browsing by Subject "Numerical methods"

Now showing 1 - 20 of 50

Open Access
Algebraic acceleration and regularization of the source reconstruction method with the recompressed adaptive cross approximation
(IEEE, 2014) Kazempour, Mahdi; Gürel, Levent
We present a compression algorithm to accelerate the solution of source reconstruction problems that are formulated with integral equations and defined on arbitrary three-dimensional surfaces. This compression technique benefits from the adaptive cross approximation (ACA) algorithm in the first step. A further error-controllable recompression is applied after the ACA. The numerical results illustrate the efficiency and accuracy of the proposed method. © 2014 IEEE.
Open Access
Analysis of an arbitrary profile reflector antenna having resistive-type surface-H-polarization case
(IEEE, 2008-06-07) Oǧuzer, T.; Altıntaş, Ayhan; Nosich, A. I.
A regularization-based numerical solution is obtained for arbitrary-shape conic section profile reflector antenna in 2-D, for the H-polarization case. New point is that the reflector surface is assumed a resistive-type material. The problem is treated by reducing the singular integral equation obtained from the boundary condition to the dual series equations and application of the Riemann Hilbert Problem (RHP) technique. The resulting matrix equation has regularized form. Sample numerical results are obtained for various values of the eccentricity of the conic section contour of reflector and the resistivity of its surface. © 2008 IEEE.
Open Access
Analysis of an arbitrary profile reflector antenna system having resistive type surface–E-polarization case
(IEEE, 2006-06) Oǧuzer, T.; Altıntaş, Ayhan; Nosich, A. I.
The regularized solution is performed for arbitrary shape conic section profile geometry. In this case the reflector surface is taken as made up of resistive type material. The problem is formulated depending on the circular In periodicity and then Fourier series coefficients of the surface current density are obtained. The resultant matrix equation is in the regularized form. Then the various numerical results are obtained for different eccentricity factor of the conic section and the resisitvity of the reflector surface. © 2006 IEEE.
Open Access
Analysis of continuous feedback Markov fluid queues and its applications to modeling Optical Burst Switching
(IEEE, 2013) Yazıcı, Mehmet Akif; Akar, Nail
Optical Burst Switching (OBS) has been proposed as a candidate technology for the next-generation Internet. In OBS, packets are assembled into a burst, and a burst control packet is sent in advance to inform and reserve resources at the optical nodes in the path of the burst. In this study, we analyze the horizon-based reservation scheme in OBS using Markov fluid queues. First, we provide a solution to continuous feedback Markov fluid queues, then we model the horizon-based reservation scheme as a continuous feedback Markov fluid queue and numerically study it. We provide numerical examples to validate our model and its solution technique as well as to obtain some insight on the horizon-based reservation mechanism. © 2013 IEEE.
Open Access
Birleşik sezim ve kestirim sistemlerinin gürültü ile geliştirilmesi
(IEEE, 2014-04) Akbay, Abdullah Başar; Gezici, Sinan
Belirli koşullar altında, optimal olmayan bazı sezici ve kestiricilerin performansını girdilerine gürültü ekleyerek geliştirmek mümkündür. Bu çalışmada, birleşik bir sezim ve kestirim sisteminin gürültü eklenerek geliştirilmesi incelenmektedir. Sistem performansının maksimizasyonu bir optimizasyon problemi olarak tanımlanmaktadır. Optimal toplanır gürültü dağılımının istatiksel özellikleri belirlenmektedir. Sistem performansının gürültü ile iyileştirilemeyeceği bir koşul elde edilmektedir.Önerilen optimizasyon probleminin, bir doğrusal programlama (DP) problemi olarak yaklaşımı sunulmaktadır. Bir sayısal örnek üzerinde, kuramsal bulguları desteklemek amacıyla, gürültü eklenmiş sistem ile orijinal sistemin performansları karşılaştırılmaktadır.
Open Access
Block SOR preconditioned projection methods for Kronecker structured Markovian representations
(SIAM, 2005) Buchholz, Peter; Dayar, Tuğrul
Kronecker structured representations are used to cope with the state space explosion problem in Markovian modeling and analysis. Currently, an open research problem is that of devising strong preconditioners to be used with projection methods for the computation of the stationary vector of Markov chains (MCs) underlying such representations. This paper proposes a block successive overrelaxation (BSOR) preconditioner for hierarchical Markovian models (HMMs1) that are composed of multiple low-level models and a high-level model that defines the interaction among low-level models. The Kronecker structure of an HMM yields nested block partitionings in its underlying continuous-time MC which may be used in the BSOR preconditioner. The computation of the BSOR preconditioned residual in each iteration of a preconditioned projection method becomes the problem of solving multiple nonsingular linear systems whose coefficient matrices are the diagonal blocks of the chosen partitioning. The proposed BSOR preconditioner solves these systems using sparse LU or real Schur factors of diagonal blocks. The fill-in of sparse LU factorized diagonal blocks is reduced using the column approximate minimum degree (COLAMD) ordering. A set of numerical experiments is presented to show the merits of the proposed BSOR preconditioner.
Open Access
Coherent energetic interfaces accounting for in-plane degradation
(Springer Netherlands, 2016) Esmaeili, A.; Javili, A.; Steinmann, P.
Interfaces can play a dominant role in the overall response of a body. The importance of interfaces is particularly appreciated at small length scales due to large area to volume ratios. From the mechanical point of view, this scale dependent characteristic can be captured by endowing a coherent interface with its own elastic resistance as proposed by the interface elasticity theory. This theory proves to be an extremely powerful tool to explain size effects and to predict the behavior of nano-materials. To date, interface elasticity theory only accounts for the elastic response of coherent interfaces and obviously lacks an explanation for inelastic interface behavior such as damage or plasticity. The objective of this contribution is to extend interface elasticity theory to account for damage of coherent interfaces. To this end, a thermodynamically consistent interface elasticity theory with damage is proposed. A local damage model for the interface is presented and is extended towards a non-local damage model. The non-linear governing equations and the weak forms thereof are derived. The numerical implementation is carried out using the finite element method and consistent tangents are listed. The computational algorithms are given in detail. Finally, a series of numerical examples is studied to provide further insight into the problem and to carefully elucidate key features of the proposed theory. © 2016, Springer Science+Business Media Dordrecht.
Open Access
Comparative evaluation of absorbing boundary conditions using Green's functions for layered media
(IEEE, 1995) Aksun, M. İrşadi; Dural, G.
Absorbing boundary conditions are comparatively studied using the Green's functions of the vector and scalar potentials for multilayer geometries and general sources. The absorbing boundaries are introduced as additional layers with predefined reflection coefficients into the calculation of the Green's functions. The Green's functions are calculated using different reflection coefficients corresponding to different absorbing boundaries and compared to those obtained with no absorbing boundary. This approach provides an absolute measure of the effectiveness of different absorbing boundaries.
Open Access
Comparative study of acceleration techniques for integrals and series in electromagnetic problems
(IEEE, 1995-06) Kinayman, Noyan; Aksun, M. I.
Most of the electromagnetic problems can be reduced down to either integrating oscillatory integrals or summing up complex series. However, limits of the integrals and the series usually extend to infinity. In addition, they may be slowly convergent. Therefore, numerically efficient techniques for evaluating the integrals or for calculating the sum of infinite series have to be used to make the numerical solution feasible and attractive. In the literature, there are a wide range of applications of such methods to various EM problems. In this paper, our main aim is to critically examine the popular series transformation (acceleration) methods which are used in electromagnetic problems and compare them by numerical examples.
Open Access
Design considerations for MMIC distributed amplifiers
(IEEE, 1994) Ergun, Şanlı; Atalar, Abdullah
The bandwidth of the input artificial line in a distributed amplifier is the main band limiting factor. By choosing this impedance properly the bandwidth of a distributed amplifier can be maximized. A four section GaAs MESFET distributed amplifier is designed using this strategy. The fabricated MMIC amplifier gives satisfactory performance. By adding proper length of series transmission lines in the drain side, the gain and the gain flatness of the amplifier can be further improved. This fact is presented via simulation results. The superior gain potential of cascode connected FETs is also demonstrated.
Open Access
Digital computation of the fractional Fourier transform
(Institute of Electrical and Electronics Engineers, 1996-09) Özaktaş, Haldun M.; Arıkan, Orhan; Kutay, M. A.; Bozdağı, G.
An algorithm for efficient and accurate computation of the fractional Fourier transform is given. For signals with time-bandwidth product N, the presented algorithm computes the fractional transform in O(NlogN) time. A definition for the discrete fractional Fourier transform that emerges from our analysis is also discussed.
Open Access
A distance-limited continuous location-allocation problem for spatial planning of decentralized systems
(Elsevier, 2017) Gokbayrak, K.; Kocaman, A. S.
We introduce a new continuous location-allocation problem where the facilities have both a fixed opening cost and a coverage distance limitation. The problem has wide applications especially in the spatial planning of water and/or energy access networks where the coverage distance might be associated with the physical loss constraints. We formulate a mixed integer quadratically constrained problem (MIQCP) under the Euclidean distance setting and present a three-stage heuristic algorithm for its solution: In the first stage, we solve a planar set covering problem (PSCP) under the distance limitation. In the second stage, we solve a discrete version of the proposed problem where the set of candidate locations for the facilities is formed by the union of the set of demand points and the set of locations in the PSCP solution. Finally, in the third stage, we apply a modified Weiszfeld's algorithm with projections that we propose to incorporate the coverage distance component of our problem for fine-tuning the discrete space solutions in the continuous space. We perform numerical experiments on three example data sets from the literature to demonstrate the performance of the suggested heuristic method.
Open Access
E-polarized beam scattering by an open cylindrical PEC strip having an arbitrary "conical-section" profile
(John Wiley & Sons, Inc., 2001) Oğuzer, T.; Nosich, A. I.; Altintaş, A.
Two-dimensional (2-D) scattering of waves by a conducting strip with a canonical profile is simulated in the E-polarization case. This analysis is performed by reducing a singular integral equation (IE) to the dual-series equations, and making their analytical regularization. Furthermore, the incident field is taken as a complex source point (CSP) beam. This is an extension of our previous studies about circular and parabolic reflector antennas. The algorithm features are demonstrated. Far-field characteristics are presented for quite large-size curves strips of elliptic, parabolic, and hyperbolic profiles.
Open Access
Effect of the off-focus shift of the feed on the radiation characteristics of a 2-D parabolic reflector antenna
(IEEE, 2010) Oğuzer, T.; Altıntaş, A.; Nosich, Alexander
The parabolic reflector antennas are widely used in the telecommunication systems and generally have large aperture sizes like 50λ to 80λ and larger. Their reliable full-wave analysis with the conventional Method of Moments (MoM) or with the other numerical methods is difficult because of inaccessible speed and accuracy. This statement is valid both for 3D and 2D reflector antennas in both polarizations. The Method of Analytical Regularization (MAR) constitutes an alternative solution compared to the ordinary MoM, which can provide only 1-2 digit accuracy. It provides finer accuracy within a reasonable computation time because the computational error can be decreased simply by increasing the matrix size in MAR. We have previously developed this method for the accurate simulation of the arbitrary conical section profile 2D reflector antennas, and the corresponding codes have provided us with accurate benchmark data. Here we study a similar problem however with the feed simulated by Complex Source Point (CSP) source located at an off-focus point on the symmetry axis of a front-fed reflector antenna. The numerical results are presented for the radiation characteristics including the forward and backward directivities and the radiation patterns in all directions. © 2010 IEEE.
Open Access
Efficient analysis of large finite arrays via mom formulation with dft based acceleration algorithms
(2003) Çivi, Ö. A.; Ertürk, Vakur B.; Chou, H.-T.
A DFT based acceleration algorithm is combined with iterative methods to accelerate the computation of Method of Moments (MoM) analysis of electromagnetic radiation/scattering from large, finite free-standing and printed elements of phased arrays. Computational complexity of this approach is O(Ntot), where Ntot is the number of unknowns. Numerical results are presented to validate the efficiency and accuracy of the method.
Open Access
Efficient and accurate EM simulation technique for analysis and design of MMICs
(John Wiley & Sons, Inc., 1997) Kınayman, N.; Aksun, M. I.
A numerically efficient technique for the analysis and design of MMIC circuits is introduced and applied to some realistic problems. The formulation is based on the method of moments (MoM) in the spatial domain, and utilizes closed-form Green's functions. Incorporating the closed-form Green's functions into the MoM has resulted in an efficient and accurate CAD algorithm. This is because use of the closed-form Green's functions not only eliminates the calculation of the spatial-domain Green's functions, but also makes it possible to evaluate the MoM matrix elements analytically We have demonstrated the application of this method here for some stripline and microstrip geometries, and compared the results with those obtained from commercial EM software, em (Sonnet Software, Inc.)
Open Access
Electric breakdown in polycrystalline semiconductors with highly nonlinear I-V characteristics: Simulations for simple barrier height models
(1997) Yildirim H.E.; Tanatar, Bilal; Canessa, E.
An extension of the Canessa and Nguyen binary model for the nonlinear current-voltage (I-V) characteristics of polycrystalline semiconductors, based on the electrical properties of individual grains, is presented. Simple analytical models for the nonuniform distribution of barrier heights at grain boundaries are assumed. The set of nonlinear Kirchhoff equations, that determine the macroscopic current across the specimen, and the nonlinearity coefficient α are solved numerically. The applied voltage dependence of the barrier height models gives α values reaching ≈ 50, indicating high nonlinearity as required by potential commercial applications. © Tübi̇tak.
Open Access
Electromagnetic imaging of three-dimensional dielectric objects with Newton minimization
(IEEE, 2014) Etminan, Aslan; Sadeghi, Alireza; Gürel, Levent
We present a general framework for detecting the shape and electrical properties of unknown objects by using the Newton minimization approach for solving inverse-scattering problems. This procedure is performed by evolving an initial-guess object iteratively until the cost function decreases to a desired value. Rapid convergence of this method is demonstrated by some numerical results.
Open Access
Extension of forward backward method with DFT based acceleration algorithm for the efficient analysis of large periodic arrays with arbitrary boundaries
(IEEE, 2003) Çivi, Ö. A.; Chou, H. T.; Ertürk, Vakur B.
An extension of Forward-Backward (FB) with Discrete Fourier Transform (DFT) based acceleration approach is presented. This is given to provide a relatively efficient analysis of EM radiation/scattering from an electrically large, planar, periodic, finite arrays with arbitrary boundaries, such as arrays with circular or elliptical boundaries. It is shown that only very few significant DFT terms are sufficient to provide accurate results.
Open Access
Fast multipole method in layered media: 2-D electromagnetic scattering problems
(IEEE, 1996) Gürel, Levent; Aksun, M. İrşadi
In this study, the Fast Multipole Method (FMM) is extended to layered-media problems. As an example, the solution of the scalar Helmholtz equation for the electromagnetic scattering from a two-dimensional planar array of horizontal strips on a layered substrate is demonstrated.