Prof. Dr. Ersin özuğurlu | Istanbul Technical University (original) (raw)

Papers by Prof. Dr. Ersin özuğurlu

Gazi University Journal of Science

Zn0.98-xMg0.02BxO nanoparticles with various dopant ratios (x = 0.00 - 0.05 with increments of 0.... more Zn0.98-xMg0.02BxO nanoparticles with various dopant ratios (x = 0.00 - 0.05 with increments of 0.01) were grown by using the sol-gel technique. The samples were synthesized and the X-ray diffraction, scanning electron microscopy, optical reflectivity, and electron dispersive analyses were used to obtain the structural, electronic, and optical properties, respectively. Williamson–Hall procedure was utilized to obtain structural properties. The energy bandgap of the particles extracted from the absorption spectra was found to be ranging between 3.23 eV and 3.28 eV and decreasing with the boron concentration. The minimum dislocation density δ and Urbach energy Eu and the maximum bandgap Eg were obtained at 1% B concentration. The refractive index calculated by Moss’s model was found to be 2.3 and the maximum bandgap energy with a value of 3.28 eV suggests that these materials can be useful for infrared applications.

Journal of Alloys and Compounds, 2019

In this work, a theoretical transmittance model has been used to analyze the optical characterist... more In this work, a theoretical transmittance model has been used to analyze the optical characteristics, namely, the refractive index, the absorption loss, and the thickness of Zn 0.99-x Cu 0.01 Co x O (x ¼ 0.01e0.05, with an increment of 0.01) thin films based on their optical transmittance data. All the thin film samples are low loss and double-facet-coated substrate systems. Our model assumed the Cauchy's dispersion equation of three-term for refractive index and Lorentzian absorption profile for the extinction coefficient due to the low loss of the samples. The equations of refractive index and extinction coefficient, and the total transmittance for the thin films were derived and fitted to the experimental data in a leastsquares method. The thicknesses of these sample films were found in the range of 338e355 nm. The effects of increasing cobalt concentration on the extinction coefficient have been observed.

Crystal Research and Technology, 2019

The effect of band gap on the structure, magnetic, and optical properties of Zn 1−x Mg x O nanoro... more The effect of band gap on the structure, magnetic, and optical properties of Zn 1−x Mg x O nanorods synthesized by hydrothermal method using varying x-values from 0.00 to 0.05 with 0.01 step increment is studied. The structural phases of Zn 1−x Mg x O samples are determined by X-ray diffraction tool. The Rietveld analysis is performed for the selected Zn 0.95 Mg 0.05 O sample and all samples' phases are found as single phase. The concentration-dependent of lattice parameters, cell volumes, microstrain, and dislocation density, locality of the atoms and their displacement, and bond length in Zn 1-x Mg x O structures are detailed. Electron Spin Resonance (ESR) measurements are performed and analyzed through concentration dependence of the g-factor and the line-widths of pike to pike (H PP) of ESR spectra. A ferromagnetic behavior of the Zn 0.95 Mg 0.05 O nanorods is observed. The optical band gaps (E g) of Zn 1-x Mg x O nanorods are obtained by the data taken from Ultraviolet-Visible (UV-VIS) diffuse reflectance spectroscopy. It is found that the E g-values increased with increasing amount of Mg elements in the structure.

AIP Conference Proceedings, 2007

ABSTRACT In this study we examine the interaction between two like‐signed quasi‐geostrophic vorti... more ABSTRACT In this study we examine the interaction between two like‐signed quasi‐geostrophic vortices containing different uniform potential vorticity. The interaction depends on 6 parameters: the potential vorticity ratio between the two vortices, their volume ratio, their individual height‐to‐width aspect ratio, their vertical offset and their horizontal separation distance. We first calculate equilibrium states using an asymptotic approach which models the vortices as ellipsoids and then determine their linear stability. It is found that two vortices can merge at farther distances when one has stronger potential vorticity than the other. This tells us that interaction between vortices having significantly different potential vorticity may be more destructive, for a given separation distance. Most of the vortex interactions for the considered parameter space are in the partial‐merger regime (where the largest vortex grows in volume) and this happens between vortices of similar po! tential vorticity.

ABSTRACT Typescript. Thesis (Ph. D.)--University of Wisconsin--Madison, 1998. Includes bibliograp... more ABSTRACT Typescript. Thesis (Ph. D.)--University of Wisconsin--Madison, 1998. Includes bibliographical references (leaves 76-78).

Applied Sciences

A Jacobian-free Newton–Krylov (JFNK) method with effective preconditioning strategies is introduc... more A Jacobian-free Newton–Krylov (JFNK) method with effective preconditioning strategies is introduced to solve a diffusion-based tumor growth model, also known as the Fisher–Kolmogorov partial differential equation (PDE). The time discretization of the PDE is based on the backward Euler and the Crank–Nicolson methods. Second-order centered finite differencing is used for the spatial derivatives. We introduce two physics-based preconditioners associated with the first- and second-order temporal discretizations. The theoretical time and spatial accuracies of the numerical scheme are verified through convergence tables and graphs that correspond to different computational settings. We present efficiency studies with and without using the preconditioners. Our numerical findings indicate the excellent performance of the newly proposed preconditioning strategies. In other words, when we turn the preconditioners on, the average number of GMRES and the Newton iterations are significantly redu...

Applied sciences, 2023

Periodic gravity-capillary waves propagating at a constant velocity at the surface of a fluid of ... more Periodic gravity-capillary waves propagating at a constant velocity at the surface of a fluid of infinite depth are considered. The surface tension is assumed to vary along the free surface. A numerical procedure is presented to solve the problem with an arbitrary distribution of surface tension on the free surface. It is found that there are many different families of solutions. These solutions generalize the classical theory of gravity-capillary waves with constant surface tension. An asymptotic solution is presented for a particular distribution of variable surface tension.

Journal of Crystal Growth, 2002

A mathematical formulation and computational techniques are presented to describe optimal control... more A mathematical formulation and computational techniques are presented to describe optimal control and design strategies for the suppression of turbulent motions in the melt and the minimization of temperature gradients in the crystal in Czochralski crystal growth processes. The methodologies developed can be used to test control mechanisms, design parameters, and optimization objectives to determine their effectiveness in improving the processes. They can also be used to effect such improvements by systematically determining optimal values of the design parameters. The controls or design parameters considered include applied magnetic fields, temperature gradients along the side wall of the crucible, and crucible and crystal rotation rates. The results show that applied magnetic fields can be very effective in reducing velocity perturbations in the melt, while side wall temperature gradients are less effective and crucible and crystal rotation rates are ineffective. The results also show that applied magnetic field and crucible and rotation rates are ineffective in reducing temperature gradients in the crystal or in the melt.

European Journal of Applied Mathematics, 2000

The distortion of a two-dimensional bubble (or drop) in a corner flow of an inviscid incompressib... more The distortion of a two-dimensional bubble (or drop) in a corner flow of an inviscid incompressible fluid is considered. Numerical solutions are obtained by series truncation. The results confirm and extend previous calculations.

The ANZIAM Journal, 2006

Two-dimensional gravity-capillary solitary waves propagating at the surface of a fluid of infinit... more Two-dimensional gravity-capillary solitary waves propagating at the surface of a fluid of infinite depth are considered. The effects of gravity and of variable surface tension are included in the free-surface boundary condition. The numerical results extend the constant surface tension results of Vanden-Broeck and Dias to situations where the surface tension varies along the free surface.

Journal of Fluid Mechanics, 2008

In this paper we systematically investigate strong interactions between two like-signed quasi-geo... more In this paper we systematically investigate strong interactions between two like-signed quasi-geostrophic vortices containing different uniform potential vorticity. The interaction depends on six parameters: the potential vorticity ratio between the two vortices, their volume ratio, their individual height-to-width aspect ratio, their vertical offset, and their horizontal separation distance. We first determine the conditions under which a strong interaction may occur. To that end, we calculate equilibrium states using an asymptotic approach which models the vortices as ellipsoids and we additionally assess their linear stability. It is found that vortices having similar potential vorticity interact strongly (e.g. merge) at closer separation distances than do vortices with a dissimilar potential vorticity. This implies that interactions between vortices having significantly different potential vorticity may be more destructive, for a given separation distance. This is confirmed by i...

Mathematical Problems in Engineering, 2009

Hirota's bilinear form for the Complex Modified Korteweg-de Vries-II equation (CMKdV-II) is d... more Hirota's bilinear form for the Complex Modified Korteweg-de Vries-II equation (CMKdV-II) is derived. We obtain one- and two-soliton solutions analytically for the CMKdV-II. One-soliton solution of the CMKdV-II equation is obtained by using finite difference method by implementing an iterative method.

In this article, we numerically solve an equation modeling the evolution of the density of glioma... more In this article, we numerically solve an equation modeling the evolution of the density of glioma in the brain-the most malignant form of brain tumor quantified in terms of net rates of proliferation and invasion. We employ a non-linear heterogeneous diffusion logistic density model. This model assumes that glioma cell invasion throughout the brain is a reaction-diffusion process and that the coefficient of diffusion can vary according to the gray and white matter composition of the brain at that location. The analysis provided in this article demonstrates that using the correct finite difference scheme can overcome the stability issues caused by the discontinuities of the diffusion coefficient. We also observe that at the steady-state these discontinuities vanish. To visualize and investigate numerically the behavior of the evolution of tumor concentration of the glioma, we calculated and plotted the number of tumor cells, the average mean radial distance, and the speed of the tumor cells along with charting the effects of net dispersal rate and net proliferation rate terms versus time for different center position values of Gaussian initial profile for each zone (gray and white matter tissues). We have proposed two numerical methods, the implicit backward Euler and the averaging in time and forward differences in space (the Crank-Nicolson scheme), both in combination with Newton's method for solving the governing equations. These methods are compared in terms of their performance in varying time-step and mesh-discretization. The Crank-Nicolson implicit method is shown to be the better choice to solve the equation.

The equation modelling the evolution of a foam (a complex porous medium consisting of a set of ga... more The equation modelling the evolution of a foam (a complex porous medium consisting of a set of gas bubbles surrounded by liquid films) is solved numerically. This model is described by the reaction-diffusion differential equation with a free boundary. Two numerical methods, namely the fixed-point and the averaging in time and forward differences in space (the Crank-Nicolson scheme), both in combination with Newton's method, are proposed for solving the governing equations. The solution of Burgers' equation is considered as a special case. We present the Crank-Nicolson scheme combined with Newton's method for the reaction-diffusion differential equation appearing in a foam breaking phenomenon.

Hirota's bilinear form for the Complex Modified Korteweg-de Vries-II equation CMKdV-II U t − 6|U|... more Hirota's bilinear form for the Complex Modified Korteweg-de Vries-II equation CMKdV-II U t − 6|U| 2 U x U xxx 0 is derived. We obtain one-and two-soliton solutions analytically for the CMKdV-II. One-soliton solution of the CMKdV-II equation is obtained by using finite difference method by implementing an iterative method.