Elektronik ve Haberleşme Mühendisliği Bölümü Yayın Koleksiyonu
Permanent URI for this collectionhttps://hdl.handle.net/20.500.12416/260
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Article Modified theory of physical optics(Optica Publishing Group, 2004) Umul, Yusuf ZiyaA new procedure for calculating the scattered fields from a perfectly conducting body is introduced. The method is defined by considering three assumptions. The reflection angle is taken as a function of integral variables, a new unit vector, dividing the angle between incident and reflected rays into two equal parts is evaluated and the perfectly conducting (PEC) surface is considered with the aperture part, together. This integral is named as Modified Theory of Physical Optics (MTPO) integral. The method is applied to the reflection and edge diffraction from a perfectly conducting half plane problem. The reflected, reflected diffracted, incident and incident diffracted fields are evaluated by stationary phase method and edge point technique, asymptotically. MTPO integral is compared with the exact solution and PO integral for the problem of scattering from a perfectly conducting half plane, numerically. It is observed that MTPO integral gives the total field that agrees with the exact solution and the result is more reliable than that of classical PO integral. (C) 2004 Optical Society of America.Article Citation - WoS: 122Citation - Scopus: 127Modified Theory of Physical Optics(Optica Publishing Group, 2004) Umul, YZA new procedure for calculating the scattered fields from a perfectly conducting body is introduced. The method is defined by considering three assumptions. The reflection angle is taken as a function of integral variables, a new unit vector, dividing the angle between incident and reflected rays into two equal parts is evaluated and the perfectly conducting (PEC) surface is considered with the aperture part, together. This integral is named as Modified Theory of Physical Optics (MTPO) integral. The method is applied to the reflection and edge diffraction from a perfectly conducting half plane problem. The reflected, reflected diffracted, incident and incident diffracted fields are evaluated by stationary phase method and edge point technique, asymptotically. MTPO integral is compared with the exact solution and PO integral for the problem of scattering from a perfectly conducting half plane, numerically. It is observed that MTPO integral gives the total field that agrees with the exact solution and the result is more reliable than that of classical PO integral. (C) 2004 Optical Society of America.Article Citation - WoS: 183Citation - Scopus: 193Analysis of Reciprocity of Cos-Gaussian and Cosh-Gaussian Laser Beams in a Turbulent Atmosphere(Optical Soc Amer, 2004) Eyyuboglu, HT; Baykal, YIn a turbulent atmosphere, starting with a cos-Gaussian excitation at the source plane, the average intensity profile at the receiver plane is formulated. This average intensity profile is evaluated against the variations of link lengths, turbulence levels, two frequently used free-space optics wavelengths, and beam displacement parameters. We show that a cos-Gaussian beam, following a natural diffraction, is eventually transformed into a cosh-Gaussian beam. Combining our earlier results with the current findings, we conclude that cos-Gaussian and cosh-Gaussian beams act in a reciprocal manner after propagation in turbulence. The rates (paces) of conversion in the two directions are not the same. Although the conversion of cos-Gaussian beams to cosh-Gaussian beams can happen over a wide range of turbulence levels (low to moderate to high), the conversion of cosh-Gaussian beams to cos-Gaussian beams is pronounced under relatively stronger turbulence conditions. Source and propagation parameters that affect this reciprocity have been analyzed. (C) 2004 Optical Society of America.Article Citation - WoS: 8Citation - Scopus: 8Comparative Analysis on Wavelet-Based Detection of Finite Duration Low-Amplitude Signals Related To Ventricular Late Potentials(Iop Publishing Ltd, 2004) Mousa, A; Yilmaz, AVentricular late potentials (VLPs) are considered as a noninvasive marker of patients with myocardial infarction, who are prone to the development of ventricular tachycardia. This paper investigates the effects of variations in physical properties of myocardial infarcts in terms of their effects on the parametric variations in VLP analysis. A sufficiently large set of signals underlining the behavior of physical parameters was employed to represent the effect of physical size, position, orientation and type of infarct. The approximated signals are variations from real electrocardiography signals by adding potentials representing late potentials based on duration, frequency, amplitude and position. The aim is not to exactly model VLP but rather to generate an approximate set of signals to examine the performance of the standard methods for different possibilities in infarct dynamics. We investigate some of the detection approaches together with their related assumptions, and try to pinpoint the drawbacks and inaccuracies of these methods and also their assumptions. The three widely accepted criteria-QRS duration, root-mean-square and duration of the signal at the end of QRS for VLP detection-were used in the investigation. Results from the application of these parameters to the set of signals are presented. In addition we investigate the physical nature of an infarct and list a number of possible reasons that might be the cause of a low success rate for the detection of additive potentials. To improve the performance of the common methods, two more wavelet transform parameters are added to those of the standard methods. The method derived from this analysis is presented as an alternative means for the detection of late signals named as delayed potentials, a more general class that includes VLP as a subset.Article Citation - WoS: 7Citation - Scopus: 7Structure and Dynamical Properties of Aun, N=12-14 Clusters: Molecular Dynamics Simulation(World Scientific Publ Co Pte Ltd, 2005) Yildirim, EK; Atis, M; Güvenç, ZBUsing molecular dynamics and thermal quenching methods on the basis of Voter-Chen version of the embedded-atom method, we have studied the melting behavior of Au-N (N = 12, 13, 14) clusters. This behavior is described in terms of overall and atom resolved root-mean-square bond-length fluctuations, specific-heat, short- and long-time average coordination numbers of each atom and short-time average temperatures of the clusters. The isomer sampling probabilities are obtained from the thermal quenching of the molten clusters, and their energy-spectrum widths are investigated. Phase change of a cluster takes place with the collective and simultaneous motion of all the atoms.Article Citation - WoS: 39Citation - Scopus: 41Modified Theory of Physical Optics Approach To Wedge Diffraction Problems(Optica Publishing Group, 2005) Umul, YZThe problem of diffraction from a perfectly conducting wedge is examined with the modified theory of physical optics (MTPO). The exact wedge diffraction coefficient is compared with the asymptotic edge waves of MTPO integral and related surface currents are evaluated. The scattered electric fields are expressed by using these current components. The total, incident and reflected diffracted fields are compared with the exact series solution of the wedge problem, numerically. (C) 2005 Optical Society of America.Article Citation - WoS: 34Citation - Scopus: 35Average Transmittance in Turbulence for Partially Coherent Sources(Elsevier, 2004) Baykal, YAverage intensity and power-transmittance in turbulence are formulated for a source with arbitrary degree of coherence (both spatial and temporal). Average power-transmittance is shown to reduce to the average intensity-transmittance if the receiver dimension is much less than the beam size. The average transmittance is found to attain its minimum value when the source is on the order of the Fresnel zone, irrespective of the degree of coherence of the source. We obtain the correct average intensity when the source is coherent, partially coherent and incoherent. Average transmittances due to turbulence are found for practical FSO (Free Space Optics) communication links. (C) 2003 Elsevier B.V. All rights reserved.Article Citation - WoS: 18Citation - Scopus: 19Reaction Dynamics of Nin (N=19 and 20) With D2: Dependence on Cluster Size, Temperature and Initial Rovibrational States of the Molecule(World Scientific Publ Co Pte Ltd, 2005) Böyükata, M; Güvenç, ZB; Özçelik, S; Durmus, P; Jellinek, JThe Ni(n)(n = 19, 20) + D2(v, j) collision systems have been studied to investigate the dependence of cluster reactivity on the cluster temperature and the initial rovibrational states of the molecule using quasiclassical molecular dynamics simulations. The clusters are described by an embedded atom potential, whereas the interaction between the molecule and the cluster is modeled by a LEPS (London-Eyring-Polani-Sato) potential energy function. Reaction (dissociative adsorption) cross-sections are computed as functions of the collision energy for different initial rovibrational states of the molecule and for different temperatures of the clusters. Rovibrational, temperature and size-dependent rate constants are also presented, and the results are compared with earlier studies. Initial vibrational excitation of the molecule increases the reaction cross-section more efficiently than the initial rotational excitation. The reaction cross-sections strongly depend on the collision energies below 0.1 eV.Article Citation - WoS: 6Citation - Scopus: 7Bombardment of Ni(100) Surface With Low-Energy Argons: Molecular Dynamics Simulations(Elsevier Science Sa, 2005) Güvenç, ZB; Hippler, R; Jackson, BResults of molecular dynamics simulations of the sputtering of Ni(100) by Ar atoms are reported. The solid is described by an embedded atom potential, and the interaction between the projectile and the metal atoms is modelled by a Morse-like function. Processes leading to Ni atom emissions from the lattice are analysed over the energy range of 70-80 eV In this energy range cluster (larger than three atoms) emission is not observed. The maximum penetration depth of Ar, the kinetic energy and angular distributions of the reflected Ar, and the sputtered Ni atoms are evaluated as functions of the impact energy and sputtering time. The computed sputtering yield is compared with the available theoretical and experimental data. (C) 2004 Elsevier B.V. All rights reserved.Article Citation - WoS: 3Citation - Scopus: 3Collisionless Fragmentation of Non-Rotating Nin (n=4-14) Clusters: a Molecular Dynamics Study(Iop Publishing Ltd, 2003) Avci, H; Çivi, M; Güvenç, ZB; Jellinek, JCollisionless fragmentation of non-rotating Ni-n (n = 4-14) clusters is studied using micro-canonical molecular dynamics (MD) computer simulations. The clusters are modelled by an embedded-atom potential energy surface. The distributions of the channel-specific fragmentation probabilities, and the global and channel-specific fragmentation rate constants are computed and analysed as functions of the internal energy and size of the clusters. The trends derived from the dynamical calculations are compared to the fragmentation energy patterns, those of the Rice-Ramsperger-Kassel (RRK) and statistical approaches. The rate constants are an order of magnitude smaller for the RRK model than with both the MD and transition-state theory approaches. The results are also compared with the other multi-channel fragmentation works.