Browsing by Subject "Convergence of numerical methods"

Now showing 1 - 19 of 19

Open Access
Analysis of the nonconcentric radome-enclosed cylindrical reflector antenna system, e-polarization case
(Taylor & Francis, 2005) Oğuzer, T.; Altintas, A.
Two-dimensional (2-D) radiation of a directive complex line source is analyzed in the presence of a perfectly conducting (PEC) reflector antenna system and nonconcentrically located dielectric radome. Similar problem was studied in the literature by using method of regularization and Green's function formulation for the H-polarization case. Here the same techniques are used for E-polarization case but in this case the scattered part of the Green's function is computed by using an FFT based algorithm. This provides us to solve the larger geometries accurately in reasonable computer times. So this approach can be considered as another alternative for the analysis of the E-polarized radome-enclosed reflector antenna system. Various numerical results are presented to support the convergence and accuracy of the technique and at the same time these results can be considered as reference data.
Open Access
Bounds on the opportunity cost of neglecting reoptimization in mathematical programming
(INFORMS, 2000) Oğuz, O.
Postoptimality or sensitivity analysis are well-developed subjects in almost all branches of mathematical programming. In this note, we propose a simple formula which can be used to get preliminary bounds on the value of this type of analysis for a specific class of mathematical programming problems. We also show that our bounds are tight.
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
Efficient analysis of large phased arrays using iterative MoM with DFT-based acceleration algorithm
(John Wiley & Sons, Inc., 2003) Ertürk, V. B.; Chou, H-T.
A discrete Fourier transform (DFT)-based iterative method of moments (IMoM) algorithm is developed to provide an O(Ntot) computational complexity and memory storages for the efficient analysis of electromagnetic radiation/scattering from large phased arrays. Here, Ntot is the total number of unknowns. Numerical results for both printed and free-standing dipole arrays are presented to validate the algorithm's efficiency and accuracy.
Open Access
Efficient use of closed-form Green's functions for three-dimensional problems involving multilayered media
(IEEE, 1994-06) Aksun, M. Irsadi; Mittra, R.
With the use of casting the spatial domain Green's functions into closed forms approach, it was demonstrated that the computational efficiency of the method of moments (MoM) for the solution of the mixed potential integral equations can be improved significantly for planar microstrip geometries. However, this approach is not effective in the improvement in the computational efficiency achieved for three-dimensional geometries in planar layered media. In this paper, discussed are the difficulties involved in using the spatial domain, closed-form Green's functions in the Method of Moments formulation for three-dimensional geometries and proposed a technique to improve the computational efficiency of the MoM.
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
Fast acceleration algorithm based on DFT expansion for the iterative MoM analysis of electromagnetic radiation/scattering from two-dimensional large phased arrays
(IEEE, 2002) Ertürk, Vakur B.; Chou, H. T.
An acceleration algorithm based on Discrete Fourier Transform (DFT) is developed to reduce the computational complexity and memory storages of iterative methods of moment (IMoM) solution to O(Ntot), where Ntot is the total number of elements in the array. As such, numerical results for free-standing dipoles obtained using IMoM-DFT approach are presented and compared with the conventional MoM solution.
Open Access
Free float and stochastic volatility: the experience of a small open economy
(Elsevier BV, 2004) Selçuk, F.
Following a dramatic collapse of a fixed exchange rate based inflation stabilization program, Turkey moved into a free floating exchange rate system in February 2001. In this paper, an asymmetric stochastic volatility model of the foreign exchange rate in Turkey is estimated for the floating period. It is shown that there is a positive relation between the exchange return and its volatility. Particularly, an increase in the return at time t results in an increase in volatility at time t+1. However, the effect is asymmetric: a decrease in the exchange rate return at time t causes a relatively less decrease in volatility at time t+1. The results imply that a central bank with a volatility smoothing policy would be biased in viewing the shocks to the exchange rate in favor of appreciation. The bias would increase if the bank is also following an inflation targeting policy. © 2004 Elsevier B.V. All rights reserved.
Open Access
Large deviations of probability rank
(IEEE, 2000) Arıkan, Erdal
Consider a pair of random variables (X,Y) with distribution P. The probability rank function is defined so that G(x|y) = 1 for the most probable outcome x conditional on Y = y, G(x|y) = 2 for the second most probable outcome, and so on, resolving ties between elements with equal probabilities arbitrarily. The function G was considered in [1] in the context of finding the unknown outcome of a random experience by asking question of the form 'Is the outcome equal to x?' sequentially until the actual outcome is determined. The primary focus in [1], and the subsequent works [2], [3], was to find tight bounds on the moments E[G(X|Y)θ]. The present work is closely related to these works but focuses more directly on the large deviations properties of the probability rank function.
Open Access
Local pinning of networks of multi-agent systems with transmission and pinning delays
(Institute of Electrical and Electronics Engineers, 2016) Lu, W.; Atay, F. M.
We study the stability of networks of multi-agent systems with local pinning strategies and two types of time delays, namely the transmission delay in the network and the pinning delay of the controllers. Sufficient conditions for stability are derived under specific scenarios by computing or estimating the dominant eigenvalue of the characteristic equation. In addition, controlling the network by pinning a single node is studied. Moreover, perturbation methods are employed to derive conditions in the limit of small and large pinning strengths. Numerical algorithms are proposed to verify stability, and simulation examples are presented to confirm the efficiency of analytic results. ï¿½ 2015 IEEE.
Open Access
Milling force modelling of multidirectional carbon fiber reinforced polymer laminates
(Elsevier, 2012) Karpat, Yiğit; Bahtiyar, O.; Deger, B.
Carbon fiber reinforced polymer (CFRP) usage in the aerospace industry has been steadily increasing due to its superior material properties such as high strength, low weight, high resistance to corrosion, and a low thermal expansion coefficient. In addition, CFRP parts are produced near-net-shape, a process that eliminates rough machining operations. However, machining operations such as drilling, side milling, and slotting are still necessary to give the CFRP parts their final shape. A majority of the studies on machining of CFRP laminates are on drilling. The number of studies on milling of CFRPs is quite limited. In this study, a mechanistic cutting force model for milling CFRPs is proposed based on experimentally collected cutting force data during slot milling of unidirectional CFRP laminates using a polycrystalline diamond cutter. Cutting force coefficients in radial and tangential directions are calculated as a function of fiber cutting angle. The mechanistic model is shown to be capable of predicting cutting forces during milling of multidirectional CFRP laminates and capable of investigating stability of machining. © 2012 The Authors.
Open Access
Neural networks for improved target differentiation and localization with sonar
(Pergamon Press, 2001) Ayrulu, B.; Barshan, B.
This study investigates the processing of sonar signals using neural networks for robust differentiation of commonly encountered features in indoor robot environments. Differentiation of such features is of interest for intelligent systems in a variety of applications. Different representations of amplitude and time-of-flight measurement patterns acquired from a real sonar system are processed. In most cases, best results are obtained with the low-frequency component of the discrete wavelet transform of these patterns. Modular and non-modular neural network structures trained with the back-propagation and generating-shrinking algorithms are used to incorporate learning in the identification of parameter relations for target primitives. Networks trained with the generating-shrinking algorithm demonstrate better generalization and interpolation capability and faster convergence rate. Neural networks can differentiate more targets employing only a single sensor node, with a higher correct differentiation percentage (99%) than achieved with previously reported methods (61-90%) employing multiple sensor nodes. A sensor node is a pair of transducers with fixed separation, that can rotate and scan the target to collect data. Had the number of sensing nodes been reduced in the other methods, their performance would have been even worse. The success of the neural network approach shows that the sonar signals do contain sufficient information to differentiate all target types, but the previously reported methods are unable to resolve this identifying information. This work can find application in areas where recognition of patterns hidden in sonar signals is required. Some examples are system control based on acoustic signal detection and identification, map building, navigation, obstacle avoidance, and target-tracking applications for mobile robots and other intelligent systems. Copyright © 2001 Elsevier Science Ltd.
Open Access
A nonlinear control scheme for discrete time chaotic systems
(The International Federation of Automatic Control, 2012-06) Morgül, Ömer
In this paper we consider the stabilization problem of unstable periodic orbits of discrete time chaotic systems. We consider both one dimensional and higher dimensional cases. We propose a nonlinear feedback law and present some stability results. These results show that for period 1 all hyperbolic periodic orbits can be stabilized with the proposed method. By restricting the gain matrix to a special form we obtain some novel stability results. The stability proofs also give the possible feedback gains which achieve stabilization. We also present some simulation results.
Open Access
A novel algorithm for DC analysis of piecewise-linear circuits: popcorn
(IEEE, 1994) Topçu, S.; Ocalı, O.; Atalar, Abdullah; Tan, M. A.
A fast and convergent iteration method for piecewise-linear analysis of nonlinear resistive circuits is presented. Most of the existing algorithms are applicable only to a limited class of circuits. In general, they are either not convergent or too slow for large circuits. The new algorithm presented in the paper is much more efficient than the existing ones and can be applied to any piecewise-linear circuit. It is based on the piecewise-linear version of the Newton-Raphson algorithm. As opposed to the Newton-Raphson method, the new algorithm is globally convergent from an arbitrary starting point. It is simple to understand and it can be easily programmed. Some numerical examples are given in order to demonstrate the effectiveness of the proposed algorithm in terms of the amount of computation. © 1994 IEEE
Open Access
On the design of dynamic associative neural memories
(IEEE, 1994) Savran, M. E.; Morgül, Ö.
We consider the design problem for a class of discrete-time and continuous-time neural networks. We obtain a characterization of all connection weights that store a given set of vectors into the network; that is, each given vector becomes an equilibrium point of the network. We also give sufficient conditions that guarantee the asymptotic stability of these equilibrium points.
Open Access
Plasmon and grid resonances in the electromagnetic scattering by finite grids of silver nanowires
(IEEE, 2010) Natarov, D. M.; Benson, T. M.; Altıntaş, Ayhan; Sauleau, R.; Nosich, A. I.
The problem of the H-polarized wave scattering by finite chains of circular nanowires is considered. A two-dimensional diffraction problem with rigorous boundary conditions is solved by partial separations of variables method using local polar coordinates of each scatterer. The obtained results demonstrate convergence of the algorithm and good agreement with data known for the conducting and dielectric cylinders. Plasmonic and grid resonances are found and calculated for grids from a big number of silver nanowires. This opens a way to the accurate numerical simulation of various finite configurations of wires met in today's nano and microsize photonic devices.
Open Access
Resolution enhancement of low resolution wavefields with POCS algorithm
(The Institution of Engineering and Technology, 2003) Çetin, A. Enis; Özaktaş, H.; Özaktaş, Haldun M.
The problem of enhancing the resolution of wavefield or beam profile measurements obtained using low resolution sensors is addressed by solving the problem of interpolating signals from partial fractional Fourier transform information in several domains. The iterative interpolation algorithm employed is based on the method of projections onto convex sets (POCS).
Open Access
State feedback stabilization of switched systems with delay: trajectory based approach
(IEEE, 2017) Mazenc, F.; Ahmed, Saeed; Özbay, Hitay
We present a new trajectory based approach for state feedback stabilization of switched linear continuous-time systems with a time-varying input delay. In contrast with finding classical common Lyapunov function or multiple Lyapunov functions for establishing the stability of the closed-loop switched system, the new trajectory based approach relies on verifying certain inequalities along the solution of a supplementary system. This study does not make any assumption regarding the stabilizability of all of the constituent subsystems of the switched system. Moreover, no assumption is needed about the differentiability of the delay and no constraint is imposed on the upper bound of the delay derivative. Finally, an illustrative example is included to illustrate the applicability of our results.
Open Access
Theoretical investigation on exact blind channel and input sequence estimation
(IEEE, Piscataway, NJ, United States, 1999) Özdemir, A. K.; Arıkan, Orhan
Recent work on fractionally spaced blind equalizers have shown that it is possible to exactly identify the channel and its input sequence from the noise-free channel outputs. However, the obtained results are based on a set of over-restrictive constrainst on the channel. In this paper it is shown that the exact identification can be achieved in a broader class of channels.