77 research outputs found
Superluminal Pulse Propagation in a One-sided Nanomechanical Cavity System
Full text linkWe investigate the propagation of a pulse field in an optomechanical system.
We examine the question of advance of the pulse under the conditions of
electromagnetically induced transparency in the mechanical system contained in
a high quality cavity. We show that the group delay can be controlled by the
power of the coupling field. The time delay is negative which corresponds to
superluminal light when there is a strong coupling between the nano-oscillator
and the cavity.Comment: 10 pages, 4 figures, accepted by Acta Physica Polonica
Wave Pattern Produced By An Heat Source Movıng Wıth Constant Velocıty On The Top Of An Infınıte Plate
Full text linkTez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2002Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2002Lineer, izotropik elastisite teorisi çerçevesinde sonsuz metal tabaka üzerinde sabit hız ile ilerleyen ısısal kaynağın meydana getirdiği dalga paternleri hesaplandı. Başlangıç koşullarına bağlı olan geçici etkiler ve dalgaların sönümleri ihmal edildi. Eğer ısısal kaynağın gidiş hızı, yüzey dalgalarının hızından küçükse, dispersif bükülme dalgaları meydana gelir. Dalga boyu tabakanın kalınlığından çok büyükse, dispersif bükülme dalgalarının frekansı dalga sayısının karesiyle orantılıdır. Bunun sonucu olarak dalga tepesi, gidiş yönüyle 90 derece açı yapar. Bu sonuç, yukardaki şartların getirdiği kısıtlamalar dahilinde ısısal kaynağın gidiş hızından bağımsızdır.Within the frame of linear, isotropic elasticity theory the wave pattern produced by a heat source moving with constant velocity on the top of an infinite plate is computed. The transient effects associated with the initial conditions and the damping of the waves are neglected. If the travel speed of the heat source is smaller than the velocity of the surface waves, the dispersive flexural waves will be excited. The frequency of these waves is proportional to the square of the wave number if the wavelength is much larger than the thickness of the sheet. In this limiting case it is found that the crest of the waves makes 900 degrees with the travel direction and this result is independent of the travel speed as long as the parabolic approximation remains valid for the dispersion relation of flexural waves.Yüksek LisansM.Sc
Dispersion And Waveguiding Of Ultraslow Light In Atomic Bose-Einstein Condensates
No full textTez (Doktora) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2007Thesis (PhD) -- İstanbul Technical University, Institute of Science and Technology, 2007Bose-Einstein yoğuşuk maddesini uyumlu optik bilgi işlemleri için dinamik hafıza aleti olarak araştırdık. Özellikle, yoğuşuk maddede depolama zamanı içinde çok yavaş optik vurumlar(darbeler) analiz edildi. Yüksek dereceden dağınım özellikleri hesaba katarak yoğuşuk maddede optik darbelerin ilerlemesini modelledik. Bu çalışmada atomik sistemin konuma bağlı olarak değişen yoğunluğu, sıcaklık ve atomik çarpışmaların şiddetine göre değişen yoğunluk profili de dikkate alındı. Yapılan sayısal ve analitik hesaplar ile kısa darbenin genişlemesini sıcaklığa, konuma ve atomik çarpışmalara göre analiz ettik. Bir optik darbe yoğuşuk madde içinde rezonans frekansında elektromanyetik olarak indüklenmiş saydamlık etkisi ile çok yavaş ilerletilebilir. Hızı son derece yavaş olduğundan sistem optik bilgiyi saklama için kullanılabilir. Sınırlı parametreler için uyumlu optik bilgi hafıza kapasitesi bulundu. Radyal yöndeki yoğunluk profilini dikkate alan ve fiber optikte eğimli indeks fiber profili ile yoğuşuk maddenin kırılma indisini modelledik. Ayrıca madde içinde çok yavaş optik darbeler gözlendi. Bu da rezonans olmayan elektromanyetik olarak indüklenmiş saydamlık koşulları altında gerçekleşti. Yoğunlaşmış maddenin taşıyabileceği mod sayısını hem analitik hem de sayısal olarak irdeledik. Ayrıca optik vurumların Bose-Einstein yoğuşuk (BEY) maddesinde ilerlemesinde üç boyutlu etkileri ve optik modların sayısındaki bağ koşulları incelendi. Kırılma indisi arttırılması durumunda daha çok optik modun (BEY) maddesinde taşınabileceğini gösterdik. Mod sayısının sıcaklığa bağlı değişimini inceledik. Kuvantum uyumluluk sayesinde, deneysel parametreleri kullanarak mod sayısının kontrol edilebileceğini gösterdik. Ayrıca tek ve çok mod koşulları analitik olarak bulundu. Analitik sonuçlar, sayısal sonuçlarla kontrol edildi. Ek olarak, kısa optik darbelerin Bose-Einstein yoğuşuk maddesinde ilerlemesinde ikinci dereceden dağınım etkisini ve nonlineer optik Kerr etkisini inceledik. Kerr nonlineer katsayısının darbe genişliği olan darbe üzerinde etkisi incelendi.We investigate potential of atomic Bose-Einstein condensates as dynamic memory devices for coherent optical information processing. Specifically, the number of ultra-slow pulses that can be simultaneously present within the storage time in the condensate has been analyzed. By modelling short pulse propagation through the condensate, taking into account high-order dispersive properties, constraints on the information storage capacity has been discussed. The roles of temperature, spatial inhomogeneity, the interatomic interactions and the coupling laser on the pulse shape have been pointed out. For a restricted set of parameters, it has been found that coherent optical information storage capacity would be optimized. The conditions determining the number of optical modes that can be supported by an atomic Bose-Einstein condensate are also investigated. We investigate waveguiding of ultraslow light pulses in an atomic Bose-Einstein condensate. We show that under the conditions of off-resonant electromagnetically induced transparency, waveguiding with a few ultraslow modes can be realized. Analytical results for the single and two-mode conditions are found. We show that under the conditions of off-resonant electromagnetically induced transparency, waveguiding with a few ultraslow modes can be realized. Results of numerical simulations are compared with analytical calculations of the propagation constants for a parabolic-refractive index profile. Moreover, we examine short pulse propagation through the atomic Bose-Einstein condensates taking into account Kerr nonlinearity and second-order dispersive effect. The roles of Kerr nonlinearity on the pulse shape have been examined on pulse width with 0.01 .DoktoraPh
Quality assessment of 3D building data
Full text linkThis project has been funded by Ordnance Survey Research, the research and development department of the Ordnance Survey of Great Britain, which is gratefully acknowledged. The first author, Devrim Akca, was formerly with the Institute of Geodesy and Photogrammetry of ETH Zurich, Switzerland.Three-dimensional building models are often now produced from lidar and photogrammetric data. The quality control of these models is a relevant issue both from the scientific and practical points of view. This work presents a method for the quality control of such models. The input model (3D building data) is co-registered to the verification data using a 3D surface matching method. The 3D surface matching evaluates the Euclidean distances between the verification and input data-sets. The Euclidean distances give appropriate metrics for the 3D model quality. This metric is independent of the method of data capture. The proposed method can favourably address the reference system accuracy, positional accuracy and completeness. Three practical examples of the method are provided for demonstration.This project has been funded by Ordnance Survey Research, the research and development department of the Ordnance Survey of Great Britain, which is gratefully acknowledged. The first author, Devrim Akca, was formerly with the Institute of Geodesy and Photogrammetry of ETH Zurich, Switzerlanddepartment of the Ordnance Survey of Great BritainPublisher's Versio
Enhancement of Pem Fuel Cell Performance at Higher Temperatures and Lower Humidities by High Performance Membrane Electrode Assembly Based on Nafion/Zeolite Membrane
No full textDEVRIM, YILSER/0000-0001-8430-0702This work reports the preparation of Nafion/zeolite composite membranes with different zeolite loading. The structure of the Nafion/zeolite composite membranes are investigated by Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and by thermogravimetric analysis (TGA). The introduction of zeolite particles into the Nafion matrix helps to improve the water uptake, proton conductivity and thermal stability of the nanocomposite membranes compared to the virgin Nafion membrane. The SEM analyses have proved the uniform and homogeneous distribution of zeolite in composite membranes. The composite membranes are tested in a single PEMFC with a 5 cm(2) active area operating at the temperature range of 75-120 degrees C and in humidified under 50% relative humidity (RH) and fully humidified conditions. Single PEMFC tests show that Nafion/zeolite composite membrane is more stable and also performed better than virgin Nafion membrane at low humidity condition. The results indicate the Nafion/zeolite composite membranes could be utilized as the proton exchange membranes for PEMFC. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.Atilim University Research project [ATU-LAP-SD-1314-02]This study is supported by Atilim University Research project ATU-LAP-SD-1314-02. The author gratefully acknowledge to TEKSIS (METU Technopolis, TURKEY) for their support during MEA preparation
Bose-Einstein Condensate in a Linear Trap with a Dimple Potential
No full textWe study Bose-Einstein condensation in a linear trap with a dimple potential where we model dimple potentials by Dirac delta function. Attractive and repulsive dimple potentials are taken into account. This model allows simple, explicit numerical and analytical investigations of noninteracting gases. Thus, the Schrodinger equation is used instead of the Gross-Pitaevski equation. We calculate the atomic density, the chemical potential, the critical temperature and the condensate fraction. The role of the relative depth of the dimple potential with respect to the linear trap in large condensate formation at enhanced temperatures is clearly revealed. Moreover, we also present a semi-classical method for calculating various quantities such as entropy analytically. Moreover, we compare the results of this paper with the results of a previous paper in which the harmonic trap with a dimple potential in 1D is investigated
Optomechanical cavity without a Stokes side-band
No full textWe investigate a theoretical demonstration of perfect frequency conversion in an optomechanical system in the weak coupling regime without a Stokes side-band. An optomechanical cavity illuminated by a weak probe field generates two side-modes, differentiating from the original signal by a phonon frequency. We report the presence of a special combination of parameters in the weak-coupling regime, where Stokes side-mode vanishes exactly. Only the anti-Stokes mode is observed with a few hundreds Hz spectral bandwidth of the probe field. Emergence of this special point is totally unrelated with the electromagnetically induced transparency (EIT) condition, where absorption (dip) cancellation is limited with the damping rate of the mechanical oscillator. Emergence is independent of the cavity type, i.e. single or double-sided, and takes place only for a single value of the effective coupling strength constant which is specific to the system parameters. At a specific effective coupling strength between the mirror and the cavity field, which can be tunable via the coupling field, only the anti-Stokes band is generated. At that specific coupling there appears no Stokes field. Hence, a filter, to eliminate the Stokes field, does not necessitate
Effect of Multimedia Assisted 7e Learning Model Applications on Academic Achievement and Retention in Students
No full text<p style="text-align: justify;">In the rapidly developing age of technology, the contribution of using multimedia-supported instructional materials in the field of teaching technologies to science education has been increasing steadily. The purpose of this research is to compare the multimedia learning instructional materials prepared according to the 7E learning model and the students' academic success and the effect on the reminiscence of the learned knowledge in the science course described using traditional methods and models. In this research, a quadruple pattern of solomon was used as a semi-experimental design. As a data collection tool, the Matter's Change Unit Success Test was used. The research was conducted in fall semester of 2014-2015 academic year and on 92 fifth grade students. Experimental groups were given instructional materials prepared according to 7E model with multimedia support and control groups were taught with traditional method. There was no statistically significant difference in the pre-test scores of the experimental-1 and control-1 groups according to the independent t-test results in the applied success test but there was a statistically significant difference in favor of the experimental groups according to the post-test and retention test scores. According to the analyzed results; There is a statistically significant difference between the post-test and retention test scores of the experimental groups. There is also a statistically significant difference between the post-test and retention test scores of the control groups. But this difference is in favor of experimental groups. According to the results of the study, it can be said that the learning material prepared according to the multimedia supported 7E model influences the academic achievement positively and the learned information is more memorable.</p></jats:p
Computing Reliability Indices of a Wind Power System Via Markov Chain Modelling of Wind Speed
Full text linkDEVRIM, YILSER/0000-0001-8430-0702; Eryilmaz, Serkan/0000-0002-2108-1781Statistical modelling of wind speed is of great importance in the evaluation of wind farm performance and power production. Various models have been proposed in the literature depending on the corresponding time scale. For hourly observed wind speed data, the dependence among successive wind speed values is inevitable. Such a dependence has been well modelled by Markov chains. In this paper, the use of Markov chains for modelling wind speed data is discussed in the context of the previously proposed likelihood ratio test. The main steps for Markov chain based modelling methodology of wind speed are presented and the limiting distribution of the Markov chain is utilized to compute wind speed probabilities. The computational formulas for reliability indices of a wind farm consisting of a specified number of wind turbines are presented through the limiting distribution of a Markov chain. A case study that is based on real data set is also presented.Scientific and Technological Research Council of Turkey (TUBITAK) [TUBITAK 1001-119F182]The author(s) disclosed receipt of the following finan-cial support for the research, authorship, and/or publi-cation of this article: This work was supported by The Scientific and Technological Research Council of Turkey (TUBITAK) under project TUBITAK 1001-119F182
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