PhD Dissertations

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Adaptive MIMO free space optical communication systems
Nouri, Hatef; Uysal, Murat; Uysal, Murat; Demiroğlu, Cenk; Durak, Kadir; Baykaş, T.; Güçlüoğlu, T.; Department of Electrical and Electronics Engineering; Nouri, Hatef
Free space optical (FSO) enjoys the high data rate of optical spectrum and have also the flexibility of RF links. FSO systems provide many advantages to the line of sight wireless communication technology. With the recent increasing interest on this promising technology, there is a need for a comprehensive understanding of system limitations which is mainly due to atmospheric conditions. In the first part of this research, we use our custom design atmospheric channel emulator for FSO system evaluations in a controlled environment and experimentally investigate the performance of FSO links. Specifically, we investigate the geometric loss, absorption loss, different weather conditions (like foggy and rainy), different beam shapes, and atmospheric turbulence using the atmospheric chamber. Atmospheric turbulence is a significant impairment in FSO channels which results in random fluctuations in the received signal level. By generating a desired level of atmospheric turbulence in the chamber, we investigate effect of wavelength and aperture averaging on the performance of FSO systems. Aperture averaging extracts inherent receive diversity gains and can be used as an effective fading mitigation technique. Furthermore, multiple apertures systems are also adopted in practical FSO systems to mitigate the turbulence induced fading effects and offer dramatic performance improvements in terms of link reliability (via diversity gain) and data rates (via multiplexing gain). On the other hand, the turbulence induced fading is characterized as very slow-varying, hence reliable feedback would be possible and adaptive transmission can be implemented in practical FSO systems and brings a noticeable performance improvement. Although the literature for adaptive transmission of RF systems is mature, it has been recently applied to SISO FSO systems and its direct application to MIMO FSO systems is challenging. Aiming to fill research gaps in this growing field, this work develops a framework for practical FSO systems with adaptive MIMO architectures. A MIMO system over a frequency-flat, log-normal or Gamma-Gamma slow-fading channel is considered in our work. In MIMO FSO systems, the space-time transmission strategy can also be adjusted, introducing a new dimension for adaptation. This means that practical MIMO link adaptation algorithms must also provide a dynamic adaptation between diversity and multiplexing modes of operation which needs a fundamental understanding of diversity-multiplexing tradeoff (DMT) under log-normal fading channels. Although there has been a growing interest on the study of DMT, the existing works are mostly restricted to the outcomes reported for Rayleigh, Rician, and Nakagami fading channels. In the next part of this research, we investigate the optimal tradeoff in the presence of log-normal fading channels. We derive the outage probability expression, and then present the asymptotical DMT expression. We further investigate DMT for finite SNRs and demonstrate convergence to the asymptotical case. Next, we suggest a framework for practical MIMO FSO system with adaptive architectures and shows how to use this framework to increase either link reliability (via diversity gain) and or data rates (via multiplexing gain). To illustrate our approach, we consider three MIMO transmission mapping matrices which includes: Matrix A (multiplexing), employing only spatial multiplexing; Matrix B (diversity), exploiting only diversity; and Matrix C (hybrid), combining diversity and spatial multiplexing. We first obtain expressions for the outage capacity of these matrices as the metric to maximize the rate of system for a fix target outage probability. Limiting the adaptation modes to a small subset is the key of adaptive strategy. The spatial adaptation can be combined with conventional adaptive modulation and coding (AMC) to give the optimal system performance. Particularly, we consider multiple-input single-output (MISO) and single-input multiple-output (SIMO) FSO systems with pulse position modulation (PPM) and pulse amplitude modulation (PAM). We propose three adaptive algorithms where the modulation size and/or transmit power are adjusted according to the channel conditions. We formulate the design of adaptive algorithms to maximize the spectral efficiency under peak and average power constraints while maintaining a targeted value of outage probability. In conclusion, this work propose a promising progress to overcome the main impairments (fog attenuations and turbulence induced fading) of the FSO links in four ways: 1) by examining the channel and proposing novel models and characteristics for atmospheric attenuations 2) by taking advantage of aperture averaging and wavelength dependency trade-off 3) by investigating and proposing spatial adaptation in MIMO FSO links and 4) by employing adaptive modulation and power control scenarios and approving the promising performance of adaptive system.
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Application of chemical mechanical polishing process on titanium based medical implants
(2017-05) Özdemir, Zeynep; Başım, Gül Bahar; Başım, Gül Bahar; Yaralıoğlu, Göksenin; Erkol, Güray; Sanyal, R.; Nizamoğlu, S.; Department of Mechanical Engineering; Özdemir, Zeynep
Biomaterials are commonly used as implant materials in the body for dental prostheses, orthopedic applications, heart valves and catheters. Based on the research studies conducted up to date, titanium and its alloys are known to be the most biocompatible materials due to their surface properties as well as extraordinary mechanical properties. Processing methods for the implant materials also affect the surface properties and may lead to contamination that can lessen the biocompatibility and after implantation may cause infection on patients which can be up to 4% in numbers. Changing the surface roughness and forming a surface oxide film have been implemented through various methods in the literature to increase of the biocompatibility and to ensure bio-inertness to the implant material. Sand blasting and chemical etching methods are commonly used for patterning the titanium surfaces to alter the surface roughness which can cause surface contamination. However, the other alternative methods such as high temperature plasma coating and laser patterning are costly. In this dissertation, Chemical Mechanical Polishing (CMP) process is established as an alternative technique to the existing methods in the literature in order to change the implant material surface properties. CMP process is one of the methods used in the semiconductor industry to ensure surface planarization through simultaneous mechanical and chemical actions. The abrasive particles in the polishing slurries provide the mechanical effect during the process enabling nanometer level erosion and cleaning the implant from any potential contamination during its machining. The chemical components of the slurry including the stabilizers, pH adjusters and oxidizers, on the other hand, help form a passive oxide film coating the surface. Generally, CMP is used to form very smooth surfaces but it has been demonstrated that by changing the slurry particle size and the pad material properties, it is possible to generated controlled roughness on the polished surface as well. The protective nature of the generated oxide film enables planarization in semiconductor applications. In implant applications of CMP, it is believed to help reduce the contamination on the surface of the bio-implants in the body environment and reducing the infection risk by stopping the chemical reactions in-vivo. It has been shown in the literature that the application of CMP on Ti films has been successful in terms of creating a smooth surface and a TiO2 oxide film. However, its native oxide film after CMP has not been characterized fully for its protective nature other than the passivating properties of the Ti/TiN films in semiconductor CMP applications. Titanium oxide film is known to promote the biocompatibility, cell adhesion, formation of hydroxyapatite layers. Yet, the oxide films obtained by artificial oxidation methods result in thick films and have porous structures. Therefore, in this study, CMP process has been applied to the Ti plates synergistically to remove the potentially contaminated surface layers and induce controlled roughness on the implant surfaces. In addition, the treated surface oxide layers have been characterized for the nature of the metal oxide layers in terms of their self-protective properties. Furthermore, biocompatibility of the CMP implemented surfaces have been evaluated through cell growth and infection resistance capabilities through biofilm analyses and optimal surface parameters were determined according to the desirability of the surface responses which help promote the cell behavior.In terms of carrying the results of this dissertation to the future studies, development of a 3 dimensional CMP process considering the 3-D nature of the implants is the most important necessity. The application of the 3-D CMP process on the implant surfaces is believed to be both an economical and more effective method on structuring the surface of the titanium based bio-implants. It is aimed to further develop a CMP driven surface nano-structuring methodology to create engineered surfaces on the Ti based bio-implants with self-protective surfaces to minimize chemical and bacterial reactivity, while promoting their biocompatibility through simultaneous surface patterning.
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Application of large-scale optimization methods in scheduling and routing problems
Elyasi, Milad; Özener, Okan Örsan; Özener, Okan Örsan; Yanıkoğlu, İhsan; Ekici, Ali; Yakıcı, E.; Duran, S.; Department of Industrial Engineering; Elyasi, Milad
In this thesis, we consider three different applications of large-scale optimization methods. We focus on the blood donation tailoring problem under uncertain demand in the first problem. In the second one, we propose a model for hybrid manufacturing consisting of flexible manufacturing systems and typical manufacturing machines. In the last one, we consider a two-echelon vehicle routing problem for last-mile delivery of groceries. In the first part of the thesis, we propose a stochastic scenario-based reformulation of the blood donation management problem that adopts multicomponent apheresis and utilizes donor pool segmentation as here-and-now and wait-and-see donors. The donation pool segmentation enables more flexible donation schedules than the orthodox donation approach because wait-and-see donors may adjust their donation schedules according to the realized values of demand over time. We propose a column generation approach to solve the associated multi-stage stochastic donation tailoring problem for realistically sized instances. The second part considers a flexible/hybrid manufacturing production setting with typically dedicated machinery to satisfy regular demand and a flexible manufacturing system to handle surged demand. We model the uncertainty in demand using a scenario-based approach and allow the business to make here-and-now and wait-and-see decisions exploiting the cost-effectiveness of the standard production and responsiveness of the flexible manufacturing systems. We propose a branch-and-price algorithm as the solution approach. Our computational analysis shows that this hybrid production setting provides highly robust response to the uncertainty in demand, even with high fluctuations. In the third part, we propose a \textit{two-echelon vehicle routing problem} (2E-VRP) under consideration of a heterogeneous fleet of vehicles and different customer types. In our model, unlike the previous studies in the literature, not only do the large vehicles visit the pre-assigned points, called satellites, to refill the smaller vehicles, but they also deliver items to the customers. On the other hand, smaller vehicles are responsible for the customers with small size demands and can get refilled whether at the depots or satellites. We propose a branch-and-price algorithm as the solution approach and obtain promising results in comprehensive numerical studies that prove its versatility.
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Applications of robust optimization in logistics and production planning
Avishan, Farzad; Yanıkoğlu, İhsan; Yanıkoğlu, İhsan; Özener, Okan Örsan; Özener, Başak Altan; Yakıcı, E.; Yavuz, T.; Department of Industrial Engineering
We analyze three applications of the robust optimization approach in this thesis. The initial part is dedicated to planning a dairy production and distribution problem considering uncertain demand. The second part presents an adjustable robust optimization approach for relief distribution in a post-disaster scenario where travel times are uncertain. Lastly, in the third part, we tackle the electric bus scheduling problem that incorporates uncertainty in both travel times and energy consumption. The first part of this thesis investigates a robust dairy production and distribution planning problem that considers the complexities of dairy production, including perishability, sequence dependence, and demand uncertainty. To tackle this uncertainty, we introduce an adjustable robust optimization approach that generates a robust and Pareto-efficient production and distribution management plan. This approach provides decision-making flexibility by allowing for adjustments based on the actual demand observed over a multi-period planning horizon. The effectiveness of the proposed method is evaluated through extensive Monte Carlo simulation experiments. Additionally, we conduct a case study to demonstrate how the adjustable approach outperforms the static robust approach in terms of the objective function value and solution performance. In the second part, we present an adjustable robust optimization approach for relief supply distribution in the aftermath of a disaster. The approach generates routes for relief logistics teams and determines the service times for visited sites to distribute supplies, taking into account the uncertainty of travel times. The model allows for adjustments to service decisions based on real-time information, resulting in robust solutions for the worst-case scenario of travel times but also more flexible and less conservative than those of static robust optimization. Due to the computational complexity of solving resulting models, we propose heuristic algorithms as an alternative solution approach. Using 2011 earthquake data from the Van province of Turkey, we have also demonstrated the effectiveness of our approach. In the last part, we investigate a scheduling problem for electric fleets faced with uncertain travel times and energy consumption. We propose a mixed-integer linear programming model to optimize electric fleet purchasing and charging operation costs, utilizing robust optimization to address uncertainty. A new uncertainty set is introduced to control the robustness of the solution. The model determines the required number of buses to cover all trips, schedules the trips, and designs charging plans for the buses. We evaluate the effectiveness of the model through extensive Monte Carlo simulations. Additionally, we present a case study on the off-campus transport network at Binghamton University to demonstrate the real-world applicability of the model and solution approach.
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An assessment model for participatory architecture: the example of the solidarity architecture exhibition groups
Terlemez, Ali Kemal; Hacıhasanoğlu, Orhan; Hacıhasanoğlu, Orhan; Turgut, Hülya; Uluengin, Demet Mutman; Birer, E.; Gökşin, Z. A.; Department of Design, Technology and Society; Terlemez, Ali Kemal
The relationship between participation and architecture has been discussed for more than fifty years, along with human-oriented design approaches. Scientists from the field of architecture and other related disciplines have been tackling the concept of participatory architecture and its various aspects through several methods and theories. This thesis aims to present a model by setting forth multiple dimensions of participatory architecture. The purpose of this model is to evaluate participatory architecture, making use of a multi-dimensional perspective to participatory architecture practices. Individuals need to have a certain level of say in the emergence of the place where they spend their daily lives, state their ideas and requests, and experience the space they use to take ownership of the area and establish a bond. It is an accepted fact that the user exists not as a subject but as an object within the architectural product produced in conventional architecture and construction processes. However, architectural figures, groups, and collectives also produce products applying alternative models contrary to these processes. Architectural forms that offer examples by adopting understandings that incorporate users through various tactics, methods and designs can be seen in different geographies. This thesis study focuses on the projects that incorporate users in processes through alternative architectural products. The thesis study model searches for the potentials, opportunities, and threats in participatory architecture. In this context, the theoretical and practical background of the literature research was carried out primarily to understand the concept of participation and its dimensions related to architecture. The inferences made here reveal the multiple dimensions of participatory architecture. These dimensions are levels and processes, players and participation techniques. The study addresses the applied processes conducted by the Solidarity Architecture Exhibition Groups (SAEx) that have compiled alternative architecture practices employed in Turkey in recent years. Conducted by quantitative techniques, the thesis uses a summative evaluation methodology. It builds a three-stage model for method of analysis. Participation ladder analysis (1), stakeholder typology analysis (2) and participatory techniques analysis (3) have helped interpret the architecture activities carried out with participants. In light of the evaluation, this thesis discusses the issue of participatory architecture using various observations made in the processes. This discussion describes a guiding method for participatory architecture. Data collected from primary and secondary sources are tested using a structured evaluation form to carry out the analysis. The structured evaluation form is a systematic examination that questions the existence of the specified parameters. The resulting findings provide information about the strengths and weaknesses of participatory architecture and its place in the Turkish world of architecture. The participant discusses architecture in the light of various inferences observed in the evaluation processes. This discussion describes a guiding participatory method of architecture. Chapter 1 provides detailed information on the problems, hypothesis, research questions, aims of the research, the scope of the research, and the methods used to conduct the research. Chapter 2 describes the concept of participation and its relations with architecture as a profession based on theoretical and practical studies. Chapter 3 describes the methodology of the study, the level and process relationship, players and participation methods described as multiple dimensions of participation are classified based on references from the literature. A new analysis model is introduced according to this classification. In line with the parameters determined by the three-stage analysis method, data collection criteria from primary and secondary sources related to the production of SAEx groups determined as case studies are specified within the scope of the study. Chapter 4 focuses on the data collected in 28 projects of the SAEx groups in line with the parameters determined in the previous chapter and various findings collected applying the three-stage analysis method. The results obtained are based on these findings. The last chapter discusses the relationship between participation and architecture, first in SAEx and then in Turkey and the global context, in the light of findings and results.
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Augmenting occupant thermal experience with cyber-physical-social systems : a case study on adaptive vents
(2020-06-11) Keskin, Cem; Mengüç, Mustafa Pınar; Mengüç, Mustafa Pınar; Ertunç, Özgür; Başol, Altuğ Melik; Kayakutlu, G.; Yurtseven, M. B.; Department of Mechanical Engineering; Keskin, Cem
Buildings use more than 30% of global energy, while 36% of this energy comes from direct combustion of fossil fuels. Hence, buildings constitute a considerable share of CO2 emissions and has a large contribution to climate change related problems. The highest share (almost half) of the total energy consumption in buildings is due to heating, cooling and air conditioning (HVAC). Particularly, the increasing demand for space cooling is pushing this share up. Thus, HVAC systems are considered as one of the key contributors of the total energy consumption and the adverse environmental impacts of buildings. Recent developments in technology offer promising tools to leverage interactions between occupants and the building systems. The energy efficiency of HVAC operations is not only a technical concern, what is more is that it requires to modifications considering human behavior. The convergence of physical systems in built environment with the information and communication technologies, as well as the human dimension makes it necessary to co-evaluate these distinct realms during the design and operation of building systems. The cyber-physical-social systems (CPSS) approach has a promising potential to facilitate this interdependence. A novel HVAC interface is presented in this thesis study. An Adaptive Vent System (AVS), is proposed to enable localized and customized thermal management in built environment for a better thermal experience of occupants and higher level of energy efficiency for building operators. The system is designed as a CPSS and composed of: (i) a novel diffuser design with individually operable flaps, (ii) thermal agents, (iii) a user interface, and (iv) a control and communication unit. It enables asymmetric air-inlet to manage indoor temperature distribution and aims to match varieties in temperature with the differences in occupant demands. The system is intended to decrease the conditioned air volume that results in HVAC energy efficiency without sacrificing occupant comfort. The design and operation of complex building energy systems like AVS is a challenging task. In order to assist this process, a methodological framework is presented that outlines the development process, from the early definition to prototyping and performance analysis. The framework is based on a novel CPSS modeling approach that combines hybrid dynamic modelling (for cyber and physical aspects) with human behavior modeling (for social aspects). A prototype of the proposed AVS was deployed in an office of an academic building in order to conduct experiments and validate CPSS and computational fluid dynamics (CFD) models. Both experiments and simulation based assessments of system operation shows that system can generate temperature difference between opposite sides of a room in a controllable manner. It is also shown that this difference results in considerable change in thermal sensation of occupants and can pave the way for energy savings with localization inside the rooms. The core reason for efficient HVAC operation with AVS is the usage of less amount of thermally conditioned air by minimizing the conditioning of unnecessary parts of the room. Moreover, the system leverages occupant interaction by delivering advanced control options (enhanced perceived control) for more customized practices. The interplay between the building energy management and the localization and customization of thermal management can maximize demand flexibility of buildings, that is a key concern for demand side management and energy planning. Hence, the new AVS system is to leverage overall thermal experience in built environment in an energy efficient way and help decreasing the environmental impact of buildings.
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Automated maintenance support for data-tier software
Ersoy, Ersin; Sözer, Hasan; Sözer, Hasan; Özener, Okan Örsan; Kıraç, Mustafa Furkan; Aktaş, M. S.; Kaya, K.; Department of Computer Science; Ersoy, Ersin
Data-tier software includes the data model and business logic of enterprise systems, and it is subject to long-term maintenance. Even though the user interface of these systems can be completely replaced, data-tier software usually evolves for decades. The number of domain experts with extensive knowledge about the overall software diminishes in time and applying extensions or changes becomes increasingly effort-consuming and error-prone for new developers. In this thesis, we introduce techniques and tools to provide automated maintenance support for data-tier software. These techniques and tools aim at reducing effort and the number of errors specifically for three challenging maintenance tasks: i) correct placement of a new object like a stored procedure in data-tier software, ii) evaluating the impact of changing database tables on software modules, and iii) evaluating the impact of table extensions on other tables of the same database. The first task is important because introducing a new object to data-tier software should not hamper its modular structure. This structure is defined by the allocation of objects among a set of schemas. Therefore, we introduce an approach and a tool to automatically predict the correct placement of new objects. We extract dependencies among various types of objects (database types, sequences, tables, procedures, functions, packages, and views) that are already placed in schemas. These dependencies are used for training an artificial neural network model, which is then used for prediction. Our industrial case studies show that our approach can reach an accuracy of 89%, whereas the baseline approach using coupling and cohesion metrics can reach 57.4% accuracy at most. There are already several techniques and tools for supporting the second task of analyzing the impact of changes in the data model on the source code. However, they fall short to analyze dynamically created SQL statements, queries on multiple tables, and other types of statements that allow data manipulation in PL/SQL, which is a commonly used language for developing data-tier software. We introduce techniques and a tool to parse both the data model and the source code (i.e., PL/SQL functions and procedures) taking part in all the schemas of a given database. Then, a dependency model is created based on queries and manipulation of database tables. Unlike prior studies, our tool can analyze queries that are created dynamically and that involve multiple tables as well as PL/SQL-specific features. We use the derived dependency model to estimate effort for two different common refactoring types on real systems. We observe high consistency between the automated estimations and manual estimations. The third task is concerned with the impact of changing tables on other tables of the same database. There are only a few studies that focus on this concern. Moreover, these studies consider the impact of deletion and modification of columns in database tables only. To address this limitation, we introduce an approach and a tool for automatically detecting the impact of data model extensions on the data model itself. We employ Siamese networks to detect similarities among database tables and such, to learn implicit relations among them. Table similarities are used as the basis for identifying potential impact. We develop another tool as the baseline, which employs the cosine similarity metric to measure similarity among database tables. Results obtained with Siamese networks turned out to be better than the baseline, achieving the mean F1 score of 96.1%.
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Automated refinement of models for model-based testing
(2017-07) Gebizli, Ceren Şahin; Sözer, Hasan; Aktemur, Barış; Uğurdağ, Hasan Fatih; Briand, L.; Yılmaz, C.; Department of Computer Science; Gebizli, Ceren Şahin
Model-Based Testing (MBT) enables automatic generation of test cases based on models of a system. It has been successfully applied in various application domains, each of which might introduce specific challenges. In this dissertation, we introduce methods and tools for addressing some of these challenges for the consumer electronics domain. In particular, we focus on the testing of Digital TV systems as our case study. We identified the following 3 problems in this context: i) Models of the system are created based on requirement specifications, which are often incomplete and imprecise. Therefore, these models are subject to accidental omissions of certain system behavior. As a result, critical faults can be left undetected by the generated test cases. ii) Resources are extremely limited in the consumer electronics domain. It is not feasible to attain an extensive coverage of test models. iii) A product family in consumer electronics often includes hundreds of systems. The set of features can highly differ among these systems. Therefore, the MBT process and modeling must be flexible to systematically manage variability and increase the amount of reuse for test models. To tackle the first problem, we introduce an approach and tool for automatically extending test models based on a set of collected execution traces. These traces are collected during Exploratory Testing (ET) activities. Several critical faults were detected in 3 case studies after generating test cases based on extended models. These faults were not detected by the initial set of test cases. They were also missed during the ET activities. As a solution for the second problem, we iteratively update test models in 3 steps to focus the test case generation process only on execution paths that are liable to highly severe failures. We use Markov Chains as test models, in which transitions among states are annotated with probability values. First, we update these values based on usage profile. Second, we perform an update based on fault likelihood that is estimated with static code analysis. Our third update is based on error likelihood that is estimated with dynamic analysis. We generate and execute test cases according the updated values after each iteration of updates. New faults can be detected after each iteration. To address the variability problem, we document variations among tested systems explicitly with a feature model. We map optional and alternative features in the feature model to a set of states in the test model. Transition probabilities in the test model are updated according to the selected features so that the generated test cases focus only on these features. This approach facilitates the reuse of a test model for many systems.
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Avrupa Birliği mülteci hukuku reformunda düzensiz göçmenlerin statüleri
Parlak, Türkan Melis; Caşın, Mesut Hakkı; Caşın, Mesut Hakkı; Ünver, Yener; Çağıran, M. E.; Göktuna, Hamdi Deniz Ege; Atladı, Ramazan Barış; Department of Private Law; Parlak, Türkan Melis; Parlak, Türkan Melis
Göç tarihi insanlık tarihi kadar eskidir. Homo sapiensten günümüz modern insanına değin en temel gayesi hayatta kalmak olan insanoğlu, kıt kaynaklara sahip dünyada sonsuz ihtiyaçlarını karşılamaya çalışmıştır. Bunun sonucunda değişik nedenlerle -ekonomik, sosyal, doğal veya siyasi- insanlar, yaşadıkları toprakları zorunlu veya gönüllü olarak terk ederek, başka ülkelere göç etmektedirler. Kadim zamanlarda iç savaşlarla başlayan bu göç dalgası, yıllar geçtikçe uluslararası ve politik bir hal almıştır. Bu nedenle de göç ve mülteci hukuku günümüzde uluslararası hukukun en temel meselelerinden biri haline gelmiştir. Avrupa ülkeleri gelişmiş ekonomileri ve yüksek refah seviyeleri nedeniyle göçmenler için adeta bir cazibe merkezidir. İkinci Dünya Savaşı'nın ardından göçmenleri kalkınma sürecinin bir parçası olarak gören ve "açık kapı" politikası benimseyen Avrupa ülkeleri, 1970'li yıllarla beraber yaşanan ekonomik krizlerin etkisiyle sınırlarını göçmenlere kapatarak "kapalı kapı" dönemine geçmiştir. Schengen Antlaşması ile iç sınırların kaldırılması, güçlendirilmiş dış sınır kontrolleri şeklinde karşımıza çıkmaktadır. 11 Eylül saldırılarının ardından başta Avrupa Birliği ülkeleri olmak üzere tüm dünyanın "yabancı" ve "güvenlik" algısı değişmiştir. Saldırıların ardından düşman ceza hukukunun yarattığı psikolojik etkiyle toplanan ve değişen "yabancı" ve "güvenlik" algısı ile perçinlenen "istenmeyen" olma durumuna, AB'nin bulduğu en efektif çözüm geri kabul antlaşmalarıdır. Düzensiz göç sorunu geri kabul antlaşmaları ile, kaynak ve transit ülkelerle iş birliği yapılarak AB'nin iç sorunu haline gelmeden çözülmeye çalışılmıştır. Ancak artan bu güvenlik yaklaşımı ve zorlayıcı sınır politikaları nedeniyle uluslararası koruma arayan kişiler, düzensiz bir şekilde sınırları geçmeye çalışmaktadırlar. Tehlike arz eden bu yolculuklar esnasında Akdeniz ve Ege Denizi adeta bir ölüler denizi haline gelmiş bulunmaktadır. Bu durum geri göndermeme ilkesini tartışmalı hâle getirmektedir. Kişilerin statüsüne bakılmaksızın sadece insan olmasından ileri gelen birtakım hakları bulunmaktadır. Tüm devletler, insan hakları hukuku ve göç hukuku metinlerinde düzenlenen bu hakları yetki alanları altında olan herkese sağlamakla yükümlüdürler. Sığınma hakkı bağlamında tüm devletler, düzensiz göçmenlerin uluslararası koruma prosedürlerine erişimini sağlamalı ve göçmenleri düzensiz girişi nedeniyle cezalandırmamalıdır. Ancak AB ülkeleri, çeşitli nedenlerle kişilere sığınma hakkı sağlamak hususunda isteksiz davranmakta ve sınırlarını korumak amacıyla düzensiz göçmenlerin insan haklarına gereken saygıyı göstermemektedir. Birlik, sınırlarını ve "Avrupa" kimliğini korumak pahasına ayrımcılık yapmamak, eşitlik ve temel insan haklarına saygı gösterilmesi gibi temel değerlerine aykırı hareket etmekte ve önceliğini güvenliğe vermektedir. "Avrupa Birliği Mülteci Hukuku Reformunda Düzensiz Göçmenlerin Statüleri" başlıklı çalışma ile Avrupa Birliği'nin değişen göç politikalarının düzensiz göçmenlerin haklarını nasıl etkilediği açıklanmaya çalışılmıştır. Bu bağlamda çalışmanın birinci bölümünde birçok bilim dalının göç olgusuyla ilgilenmesi ve hepsinin farklı perspektiften konuya yaklaşması nedeniyle oldukça benzer özellikler göstermesine rağmen bazı nüanslar barındıran göç kavramı ile göçe ilişkin temel kavramlar açıklanmıştır. Bunun yanı sıra göçmenleri göçe iten nedenler, göç teorileri, göç hukukunun tarihsel gelişimi ve düzensiz göçmenliği besleyen göçmen kaçakçılığı suçuna da bu bölümde yer verilmiştir. Çalışmanın ikinci bölümünde, birinci bölümde ele alınan göçün temel kavramları, nedenleri ve tarihsel gelişimi bilgilerinden yola çıkılarak Avrupa Birliği özelinde göç hukukunun esasları ve gelişimi konusu irdelenmiştir. Bu bölümde, Avrupa'nın yeniden yapılanma sürecinde benimsediği "açık kapı" politikası döneminden, göçmenleri adeta bir sorun olarak gören "kapalı kapı" döneminde değin göç ve göçmenlere yönelik yaklaşımı uluslararası antlaşmalar aracılığıyla incelenmiştir. Ayrıca bu bölümde Avrupa Birliği'nin ortak göç çalışmaları ile EURODAC, FRONTEX, EUROSUR gibi düzensiz göçmenleri önlemek amacıyla oluşturulan kurumlara da değinilmiştir. Avrupa Birliği'nin 2000'li yıllarda benimsediği politika ile artan düzensiz göç sorununa kaynak ve transit ülkeleri de dahil ederek çözüm yolu bulma süreci bölümde irdelenmiştir. Bu bağlamda geri kabul antlaşmaları ele alınmıştır. Tezin son bölümünde ise sığınma hakkı, sığınma hakkını teminat altına alan hukuki kaynaklar ve düzensiz göçmenlerin uluslararası koruma başvurusu sonuçlanana ve geri gönderilmesine değin sahip olduğu haklar uluslararası yargı kararları ışığında ele alınmıştır.
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Blockchain-based authentication and authorization for software defined networks
Latah, Majd; Kalkan, Kübra; Çakmakçı, Kübra Kalkan; Arı, İsmail; Alagöz, F.; Levi, A.; Department of Computer Science
Software-defined networking (SDN) is a novel networking paradigm that allows a simple and flexible management of the underlying forwarding devices through a centralized controller. However, SDN suffers from different security issues that may paralyze the whole network when the controller is under attack. Blockchain (BC) is considered a new technology that provides a decentralized distributed ledger, which can be used to protect the SDN controller from other malicious components in the network. In this thesis, we investigate the integration between SDN and BC technology. We focus on BC-enabled authentication and authorization for SDNs. First, we propose, DPSec, a blockchain-based data plane authentication protocol for SDNs. Second, we improve the performance of BC-enabled SDN by proposing a component-wise waiting time approach. We also utilize lattice-based signatures and Key Encapsulation Methods (KEMs) to improve the security of BC-SDN. Third, we introduce, HostSec, a blockchain-based approach that provides mutual host-controller, Packet-In/Packet-Out and host-host authentication for SDNs. Fourth, we propose, SDN-API-Sec, a blockchain-based access control method for cross-domain SDNs by utilizing BC smart contracts. The results suggest a trade-off between security and latency.
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Channel modeling and characterization for visible light communications: indoor, vehicular and underwater channels
(2018-06) Miramirkhani, Farshad; Uysal, Murat; Tekin, Ahmet; Uğurdağ, Hasan Fatih; Başar, Ertuğrul; Baykaş, Tuncer; Department of Electrical and Electronics Engineering; Miramirkhani, Farshad
Despite the increasing attention on visible light communications (VLC) systems, there is a lack of proper visible light (VL) channel models. This is a serious concern since channel modeling is the very first step for efficient, reliable, and robust VLC system design. This dissertation focuses on channel modeling and characterization study for indoor, vehicular and underwater VLC. Our study is based on Zemax®; a commercial optical and illumination design software. Although the main purpose of such software is optical system design, we take advantage of the ray tracing features of this software which allows an accurate description of the interaction of rays emitted from the lighting source within a specified confined space. The simulation environment is created in Zemax® and enables us to specify the geometry of the environment, the objects within as well as the specifications of the sources (i.e., LEDs) and receivers (i.e., photodiodes). For a given number of rays and the number of reflections, the non-sequential ray tracing tool calculates the detected power and path lengths from source to detector for each ray. These are then imported to Matlab® and processed to yield the channel impulse response (CIR). In contrary to existing works which are mainly limited to ideal Lambertian sources and purely diffuse reflections, our approach is capable to obtain CIRs for any non-ideal sources as well as specular and mixed specular-diffuse reflections. Furthermore, we can precisely reflect the presence of objects and wavelength-dependent reflection characteristics of surface materials in channel study. In the first part of this thesis, we propose a realistic indoor channel modeling approach and carry out a detailed channel characterization study. We also investigate the effect of user mobility and receiver orientation on CIRs. In the second part of this thesis, we present VLC channel models for vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) taking into account the asymmetrical pattern of headlamp and street lights, reflections from road surfaces and weather conditions. We further develop a closed-form path loss expression for V2V VLC channel for different weather conditions. In the last part of this thesis, we carry out a detailed underwater optical channel modeling and characterization study taking into account the reflection characteristics of the sea surface and sea bottom as well as the water characteristics, i.e., extinction coefficient, and scattering phase function of particles. We develop a closed-form path loss expression as an explicit function of water type, beam divergence angle and receiver aperture diameter and validate the accuracy of the proposed expression through Monte Carlo simulation results.
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Channel modeling for vehicular visible light communication
Eldeeb, Hossien Badr Hossien; Uysal, Murat; Uysal, Murat; Demiroğlu, Cenk; Durak, Kadir; Erküçük, S.; Başar, E.; Department of Electrical and Electronics Engineering; Eldeeb, Hossien Badr Hossien
The demand for vehicular communication systems has increased since they are considered the key enabling technology for Intelligent Transportation Systems (ITSs). Vehicular communications allow information sharing between the vehicles and the infrastructures, which have great potential in improving road safety, traffic ﬂow, and passenger comfort along the road. The resulting vehicular connectivity forms are Vehicle-to-Vehicle (V2V), Vehicle-to-Infrastructure (V2I), Infrastructure-to-Vehicle (I2V), and Vehicle-to-Pedestrian (V2P), commonly referred to as Vehicle-to-Everything (V2X) communication. Most of the research efforts and standardization activities on V2X communication have focused on Radio Frequency (RF) technologies. However, limited RF bands allocated for V2X networks can suffer high interference levels in heavy traffic, and channel congestion might be particularly problematic for delay-sensitive safety functionalities. To address such issues, Visible Light Communication (VLC) has been proposed as an alternative or complementary vehicular access solution to RF-based V2X communications. VLC is based on the principle of modulating the intensity of the Light-Emitting-Diode (LED) and enables the dual use of LED for both illumination and communication purposes. The ubiquitous availability of LED-based streetlights, traffic lights, and automotive exterior lighting positions VLC as a potential wireless connectivity solution for vehicular networks. Vehicular VLC has received increasing attention recently in several aspects such as physical layer design, upper layer network protocols, and integration with RF-based solutions for hybrid systems. As in any other communication system, channel modeling plays a critical role in VLC system design and optimization. Earlier results in the literature have focused on indoor channel modeling. Those results do not apply to vehicular VLC systems since they exhibit inherently different characteristics to indoor counterparts. For example, the ideal Lambertian model, used for indoor LED luminaries, fails to match the illumination characteristics of automotive Headlights (HLs), Taillights (TLs), traffic lights, and streetlights. In addition, the effects of road reﬂectance, road type, weather conditions, the orientation of the user/vehicle equipment and infrastructures, receiver aperture size, and the sunlight might strongly affect the link performance of vehicular VLC systems. Despite the increasing attention on indoor VLC channel models, there is a lack of realistic vehicular VLC channel models. Motivated by these, we provide in this dissertation a comprehensive channel modeling study for different vehicular VLC links. In the first part of this dissertation, we explain our channel modeling approach, which builds upon advanced non-sequential ray tracing features of OpticStudio software. This approach allows the integration of any realistic light source radiation pattern. It can handle a large number of reﬂections for better accuracy. Since this is a simulation-based channel modeling methodology, it must be validated, at least once, against the measurements in a real scenario. Therefore, we carry out measurements and simulations for the same cases to validate our channel modeling approach considering both the Line of Sight (LoS) and the Non-Line of Sight (NLoS) cases. Such a realistic channel modeling approach is then adopted to model the V2V VLC channel when two vehicles travel in the same lane and by utilizing the Headlights (HLs) of the source vehicle as wireless transmitters. The most recent proposed channel path loss model was a linear function of transmission distance applicable only for ranges less than 20 meters. Therefore, we propose a new path loss expression that takes the form of a negative exponential function and provides an excellent match to simulation results for large transmission ranges and under different weather conditions. This expression is then utilized to derive the achievable transmission distance for a targeted data rate while satisfying a given value of Bit Error Rate (BER). We then consider V2V based Taillights (TLs) and derive a new path loss model that works for measured TL radiation patterns of different commercial car models. Utilizing the derived path loss model, we further derive a closed-form expression for the maximum transmission distance under the target BER value. Furthermore, the Root-Mean-Square (RMS) delay spread is investigated by considering different V2V density scenarios. In the above points, the effect of both the lateral shift between the two vehicles, the exact geometry of the vehicle's HLs, and the receiver aperture on the path loss model of the V2V system is excluded. Therefore, to address such shortcoming, we develop a closed-form path loss expression for the V2V VLC system as a function of link distance, lateral shift between the two vehicles, weather type, transmitter beam divergence angle, and receiver aperture diameter. While several vehicular VLC efforts investigated the V2V link channels while only a few attentions were paid to V2I/I2V channels, and by considering very idealistic assumptions. Motivated by this, we consider the I2V VLC links where the traffic lights and streetlights are deployed as wireless transmitters. First, we model the I2V based on a commercial traffic light and derive a closed-form expression of the path loss model as a function of both longitudinal and lateral shift distances. Then, we model the I2V system with street light transmitters and derive a closed-form expression for channel path loss as a function of pole spacing, the height of both the lighting pole and the vehicle, the longitudinal and lateral distance between the vehicle and the pole, and the aperture size of the Photodetector (PD). The effect of these transceivers and infrastructure parameters on the system average error rate performance is also investigated considering the mobility of vehicular communication, which makes the path loss no longer deterministic. We further model the reverse channel link, i.e., the V2I system, where the vehicle communicates with an infrastructure pole deploying its HLs as transmitters, and three PDs located within the traffic pole to act as receivers. Based on the Channel Impulse Responses (CIRs), obtained from the ray tracing, we introduce an expression for the achievable capacity considering the effect of propagation environment and the LED non-linear characteristics. In most V2V and I2V VLC works, the most common underlying assumption is using one or two PDs. That might be sufficient for establishing a connection between two vehicles cruising in the same straight lane or between the vehicles and infrastructure with clear LoS. To position VLC as a strong candidate for vehicular connectivity, it is essential to realize multi-directional reception in various deployment scenarios supporting V2V and I2V links. To address this of practical relevance, we investigate the channel modeling of multi-directional coverage for vehicular VLC systems in different road types and traffic scenarios. We quantify the capability of receiving signals in several cases including the V2V connectivity (with HLs and TLs) and the I2V connectivity (with traffic and streetlights). We further quantify the contribution of individual PDs to elaborate on the usage cases of each PD. In the last part of this dissertation, we explore the vehicular VLC as a wireless connectivity solution to enable outdoor broadcasting for public safety systems. We utilize the ubiquitous streetlights as wireless transmitters taking into account the fundamental differences imposed by the outdoor medium and lightning infrastructure. These include the effect of the asymmetrical pattern of streetlights, the orientation of the user equipment, the weather condition, and the solar irradiance. We consider two broadcasting scenarios; VLC broadcasting in the roadway and VLC broadcasting in the sidewalk path, and obtain the received Signal-to-Noise Ratios (SNRs) for all links under the mobility condition.
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Consumer studies about purchase, repeat purchase and word-of-mouth after deep discount online
(2015-06) Arı, Ela; Pauwels, Koen; Pauwels, Koen; Börü, D.; Doğan, Kutsal; Atakan, Şükriye Sinem; İyigün, N. Ö.; Department of Business; Arı, Ela
Daily deal sites are e-commerce businesses that offer deep discounts (e.g., 50%, 90%) for products and services to consumers through local service providers. These sites increase consumer's buying power and attract more customers for local service providers. The fact that most deal sites keep half of the revenue from each consumer makes this model unprofitable for service providers. To increase the viability of this business model, after purchasing from deal sites, consumers would need to repurchase at full and/or different discount rates from service providers and generate positive Word of Mouth (WOM). For that aim, three studies were conducted. The first study is a survey conducted in Turkey, which examines the consumer motivations based on McClelland Need Theory. The results revealed that need for achievement affects deal purchases while need for group affiliation drives the intention to repurchase at full price and need for power increases the generation of WOM. The second study addresses consumer motivations through a multi-method approach, combining in-depth interviews with an experimental study. The results revealed that price discount and distance are key action variables that managers can control in order to give fewer discounts and make higher profits. Moreover, consumers with a high need for achievement are more likely to generate WOM, while those with a high need for affiliation create more electronic WOM (eWOM). The third study is an experimental study, which reveals that satisfaction is a primary driver for repurchase and WOM. Furthermore, consumer characteristics as coupon proneness and price quality schema are key characteristics for both deal sites and service providers.
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Control of solenoid-based injectors with different nozzle types to achieve soft-landing and mass flow rates
Qureshi, Muhammad Sarmad; Bebek, Özkan; Bebek, Özkan; Adam, Evşen Yanmaz; Uğurlu, Regaip Barkan; Erbatur, K.; Şafak, K. K.; Department of Electrical and Electronics Engineering
Electromagnetic (EM) solenoid actuator has recently gained attention in research because of their cost-effectiveness, compact size, and low heat dissipation. Due to these advantages, these devices are used in a variety of applications including injector systems in automobiles. However, a major drawback of using such devices is their unfavorable design to attach any physical sensor, which gives rise to their uncontrollable nature. Due to the discrete actuation properties, the solenoid actuator generates unwanted noise at the time of closing, mechanical wear, and tear, and lack of control over the actuator reduces its practical adaptability. The main focus of this dissertation is the development of open-loop control approaches to achieve low seating velocities, referred to as soft-landing, and to regulate mass flow rates using solenoid-based injectors. \\ Prior to further research, different control algorithms were evaluated for tracking performance, and the better-performing control algorithm was selected. A novel open-loop control methodology to achieve lower seating velocities (soft-landing) for solenoid-based injector systems has been proposed, to reduce impact noise and mechanical deterioration. Moreover, a quantification of the attributes to achieve soft-landing has been investigated, for the generic application of the control law on any solenoid-based injector, without any sensory feedback. In addition, the mass flow rate control for the solenoid-based injector in the open-loop environment that is, without the use of any physical sensor feedback is investigated. A novel robust control approach for tracking mass flow rate reference profiles using a solenoid-based injector has been presented. The proposed control approach successfully tracked the desired mass flow rate reference profiles, to be used in various automotive and chemical injection applications.
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Corporate sustainability interactions and corporate financial performance
(2017-11) Uşar, Damla Durak; Denizel, Meltem; Soytaş, Mehmet Ali; Denizel, Meltem; Ekici, Özgün; Özen, Ulaş; Akkan, C.; Sayın, S.; Department of Business; Uşar, Damla Durak
There is a large amount of empirical literature focused on the relationship between corporate sustainability and corporate financial performance. The literature considers firm specific aspects affecting the link but omits the influence of the competition. I view the sustainability decisions of companies as strategic decisions and use the Stackelberg game to model the effect of competition and sustainability spillovers on sustainability interactions. I rely on computational methods to identify the conditions, when the first mover advantage or the second mover advantage arises. I turn to empirical methods in order to validate the inferences made from the analytical model and estimate the parameters of the discrete choice model using the social performance ratings from MSCI KLD 400 Social Index and financial information from Wharton Research Data Services' COMPUSTAT dataset. Although the interdependence of discrete entry decisions can pose identification and estimation problems, I provide empirical evidence that the effect of competition on the likelihood of entry into the sustainability market dominates the effect of spillover. Furthermore, this finding is more profound for first time entrants.
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Cumhuriyet Savcısının etkin soruşturma yapma görevi
(2020-04) Kaya, Asım; Ünver, Yener; Ünver, Yener; Centel, N.; Çakmut, Özlem Yenerer; Kartal, P.; Department of Public Law; Kaya, Asım
Çalışmamızın inceleme konusunu "Cumhuriyet Savcısının Etkin Soruşturma Yapma Görevi" oluşturmaktadır. Bu kapsamda birinci bölümde; "etkinlik", "yeterlilik", "Cumhuriyet savcısı" gibi temel kavramlar öncelikle incelenmiştir. İlişkili kurumlardan ise "adil yargılanma hakkı", "hak arama hürriyeti" ve "adli kolluk" gibi konularla birlikte Cumhuriyet savcısının vermiş olduğu kararlara değinilmiştir. Kavramsal ve kurumsal değerlendirmeler ile etkin soruşturma görevinin amacı ve niteliği incelenerek konu bakımından literatüre katkı sağlaması ve yeni bakış açıları geliştirilmesi amaçlanmıştır. İkinci bölümde, Ceza Muhakemeleri Usulü Kanunu ve Ceza Muhakemesi Kanunu dönemine ilişkin değerlendirmeler yapılmıştır. Meselenin her iki kanun dönemi esas alınarak incelenmesi neticesinde, geçmişten günümüze kadar geçen sürede "etkin soruşturma"nın geçirdiği safahat ve geldiği aşama tespit edilmiştir. Her iki dönemin taşıdığı ortak ilke ve kurallar ile farklılıkların tespiti sorunlara yönelik getirilen çözüm önerileri bakımından önem arz etmektedir. Türk Ceza Muhakemesi Hukuku sistemi bakımından etkin soruşturmanın özellikleri, Cumhuriyet savcısının bağımsızlığı, tarafsızlığı ve reddi ile ilgili tartışmalar ortaya konmuştur. Devamında etkin soruşturmanın koruma tedbirleri ve soruşturma işlemleri ilişkisi incelenerek bu kapsamda; arama, elkoyma, iletişimin denetlenmesi, teknik araçlara izleme gibi koruma tedbirleriyle birlikte olay yeri inceleme konusu detaylı olarak irdelenmiştir. Zira "koruma tedbirleri" ve "soruşturma işlemleri" ile etkin soruşturma ilişkisi önem arz ettiğinden doktrin ve yargı kararlarıyla birlikte değerlendirme yapılmıştır. Üçüncü bölümde ise "ispat külfeti", "hukuka aykırı delil" kavramı, "ifade alma" ve "etkin soruşturma" ilişkisi üzerinde durulduktan sonra etkin soruşturmaya ilişkin genel, özel ve yapısal sorunlar incelenmiştir. Bu çerçevede etkin soruşturmanın sonuçları üzerinde değerlendirmelerde bulunulmuştur.
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Cyclic and corrosion behavior of nickel titanium shape memory alloys modified with various biocompatible coatings
Şimşek, Görkem Muttalip; Yapıcı, Güney Güven; Yapıcı, Güney Güven; Başol, Altuğ Melik; Bebek, Özkan; Yılmazer, H.; İpekoğlu, M.; Department of Mechanical Engineering
Metallic materials including stainless steels, cobalt-chromium based alloys, commer- cial titanium, Ti6Al4V, and nickel-titanium (NiTi) shape memory alloys (SMA) have long been considered to be the dominant source of implant materials in the medical industry. Among all, the practical use of NiTi SMAs is fascinating due to their ex- traordinary behaviors, which are entirely new compared to other conventional metallic materials. However, a major problem associated with the use of NiTi for in-vivo ap- plications is the potential risk of Ni release due to the highly corrosive environment of the human body. There have been many attempts to overcome such difficulties and to understand the corrosion mechanisms for conventional NiTi implant materials during the last decade through simulations or in-vivo and in-vitro experimental studies. Within this context, the effect of heat treatment parameters on the mechani- cal properties of NiTi shape memory alloy in wire form is investigated since heat treatment can strongly influence the mechanical properties of shape memory alloys. Detailed experiments were planned and utilized to examine the following properties as a function of heat treatment condition; phase transformation temperature, move- ment and repeatability, cyclic behavior, corrosion resistance, and biocompatibility. All experimental setups were custom designed and manufactured based on specific test requirements. Corrosion and cyclic experiments were performed in Ringer solu- tion and Simulated Body Fluid (SBF) to better understand the response of NiTi in a human body environment. Besides heat treatment parameters, the effect of biocompatible layer on the func- tional behavior of NiTi was also investigated since the method was found highly promising by several research groups. In this dissertation, CaP and PVA based hy- drogel coatings were applied on NiTi SMAs in wire form via the dip coating method. However, bioceramics and biopolymers possess poor mechanical properties that con- stitutes a drawback. The main contribution of the dissertation is combining the superior mechanical properties of NiTi with the excellent biocompatibility of certain polymers and ceramics to develop a new type of implant for various medical appli- cations. The present work also demonstrates for the first time the effect of NaOH pre-treatment on the wire form HA coated NiTi. These efforts show that deposition of biocompatible layer on metallic surfaces may act as physical or chemical barriers and inhibit the ion release from the surface in a highly corrosive environment. Be- sides, the influence of biocompatible layer on the cyclic behavior of NiTi was also investigated with CaP and hydrogel coated NiTi samples. Cyclic experiments were performed in different environmental conditions including dry condition, SBF and Ringer's solutions with different frequencies to better understand the NiTi response in a human body environment. Finally, this dissertation investigates the mathematical modeling of NiTi corro- sion mechanisms. Even though the biomaterials were tested in the experiments for corrosion, it is hard to experimentally predict all the situations that can occur in the body. The reasons for incomplete experimental testing include stochastic nature of the corrosion process, the need for long-term data collection, and the different re- sponses of different patients to the same biomaterials. In this manner, mathematical modeling of the corrosion process was structured with selected parameters such as pH, temperature, and difference between potential of the metal and solution by uti- lizing Cellular Automata (CA) to simulate the corrosion behavior of uncoated and coated NiTi wires. It was concluded that the developed model accurately captures the corrosion progress development in response to changes of different environmental parameters.
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Developing a methodology for the design and optimization of the pressure-swirl atomizers
Nural, Ozan Ekin; Ertunç, Özgür; Ertunç, Özgür; Mengüç, Mustafa Pınar; Başol, Altuğ Melik; Güngör, A. G.; Uzol, O.
Atomization is the process of disintegration of bulk liquid into smaller droplets and has been an almost century-long research topic of fluid dynamics. Devices used for atomization are called atomizers and many different types of atomizers using different strategies to achieve atomization have been developed. Pressure-swirl atomizer is one of the most widely used types of atomizer due to its simplicity and ability to achieve a wide range of droplet sizes and coverage area. Even though the geometry of the pressure-swirl atomizer is simple, its internal flow field includes complex phenomena such as turbulence, liquid/gas interface, recirculation zones, and instabilities. Modeling the performance of pressure-swirl atomizer have been challenged by many researchers, and different models have been developed. Out of these models, 2D and 3D numerical simulations were proposed to be the most accurate ones, even though the accuracy of 2D simulations was questioned by many researchers. Other models, 1D models, and semi-empirical correlations also exist in the literature yet the accuracy of the latter is reported to be low. Models that could be used for the optimization of pressure-swirl atomizers, where it will be used to perform thousands of calculations while having low error values are lacking in the literature. Even though numerical modeling can result in accurate predictions, computation time prevents it to be used in optimization calculations. This study presents the developed models that could be used for such calculations and present their accuracy in comparison with conducted experiments and simulations. In this study, first, inlet modeling of the 2D simulations is inspected. It was shown that the accuracy of the 2D simulations can be improved drastically by adjusting the inlet velocity components to include the effect of flow deformation that occurs due to tangential ports. It is shown that a model developed in an earlier study can be used to describe this deformation and calculate the inlet velocity components. Later, existing semi-empirical models in the literature along with the experimental data are presented and inspected. Overall 1,777 experimental data points, obtained from 34 different studies, are provided and cataloged. It is seen that, when the accuracy of semi-empirical correlations is evaluated in a global range, rather than in the range that they are developed at, error values increase significantly. A new set of semi-empirical correlations that are globally more accurate are obtained using experimental data. However, even with the developed correlations, error values in the calculation of pressure difference and droplet sizes are still higher than 50 %. Due to the lack of the desired level of accuracy in the semi-empirical modeling, boundary layer modeling approach is adopted for the description of the internal flow field of the pressure-swirl atomizer. Two different models, one for the straight sections (swirl chamber and orifice) and another for the convergent or divergent sections (convergent section or trumpet), are obtained. Evaluation of the model for the straight section is done with the comparison of the experimental data of open-end pressure swirl atomizers. Comparison of convergent or divergent section models is done with the 3D simulation of the unique atomizer geometry that is developed in the framework of this dissertation. Finally, the accuracy of the combined model is evaluated with the experimental data of close-end pressure-swirl atomizers. A large number of 3D full geometry simulations are conducted to reason the coupling of internal flow dynamics with the uniformity of the pressure-swirl atomizer spray. These simulations are done with atomizer geometry having a unique shape, and later three of the geometries are selected to be manufactured. Of the selected geometries, two have uniform sprays, while the third one has a non-uniform spray. Due to the unique shape of the inlets, these atomizers are manufactured with the method called Laser Lithography, and the manufacturing tolerance of the geometries was less than 1 $\mu$m. Experiments of the atomizers are done with Laser Induced Fluorescence method at the radial plane, and obtained results showed a non-uniform internal flow results in a spray where cluster of droplets with large diameters exist. A semi-empirical model for the description of the non-uniformity is obtained, and comparison with the existing model and experimental data in the literature is done. Spray of the pressure-swirl atomizer takes different shapes as the pressure and flow rate through the atomizer is increased, and the final form of the spray is called fully-developed spray. Development of the spray of pressure-swirl atomizers is also modeled in this study. For this purpose, both experiments conducted in the framework of this study and experimental data obtained from the literature are used. A model that is based on the bulk Reynolds and Weber numbers is obtained using the gradient-descent method, and comparisons with the existing correlations in the literature are done. Droplet diameter modeling is done by utilizing the model called Linear Instability Sheet Atomization (LISA), which is often used by commercial CFD codes. This model is analyzed by examining the equations, and eight different correlations, differing in the simplifications, are obtained. Evaluation of these correlations is done by making comparisons with the experimental data of this study, which is obtained with the Shadowgraphy method. All of the droplet diameter experiments are conducted in the primary break-up region of the atomization. Comparisons have shown that when the break-up length of the atomizer can be modeled accurately, the LISA model can be used to estimate the representative droplet diameter. For the description of the droplet diameter distribution, a new model is obtained for the calculation of the spreading parameter. This parameter is often taken as constant in the commercial CFD codes, yet it was shown that it significantly affects the obtained distributions. Comparison of obtained model with the model of commercial CFD codes is also presented. Finally, combination of the obtained models and their order of use for performing an optimization study is presented. It was also presented that the obtained model can perform the calculation of single geometry in 20.0 seconds with a single core, whereas 2D and 3D simulations require 208 core-hour and 2,880 core-hour, respectively. It can be seen that the developed model is 37,440 times faster than the 2D simulation and 518,440 times faster than the 3D simulations.
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Development of a single-chip visible light communication receiver
Kısacık, Rifat; Uysal, Murat; Uysal, Murat; Durak, Kadir; Poyrazoğlu, Göktürk; Pusane, A. E.; Altunbaş, İ.; Department of Electrical and Electronics Engineering; Kısacık, Rifat
The VLC has drawn the attention in the last decade, which is simply based on the data transmission over the visible light (400nm-700nm). It is considered as an alternative to RF-based technologies and offers unlicensed frequency spectrum to the users. Despite the widely available optical bandwidth, LEDs that are used as a light source in VLC impose a limitation in data rate due to their limited bandwidth, which changes between a few hundred kHz and a few MHz. At this point, the equalization is one the most used method to achieve higher data rates in VLC system. There have been a growing works focusing on the equalization performed for VLC system. However, most of them offer an improvement in data rate at the discrete element level. They are not integrated and consume higher power. The works, which offer a solution at the chip level, are limited. Additionally, a monolithic optoelectronic receiver that can be operated with LEDs with different bandwidths and perform the equalization for the employed LED has been not investigated recently. This work is concentrated on the design and implementation of a monolithic optoelectronic receiver that includes a photodiode with the area of 300 umx 300 um, a TIA, an adjustable equalizer controlled by a switching mechanism, and an output buffer. The designed optoelectronic receiver is implemented in 130 nm CMOS technology and tested in an experimental setup. It is employed with three different phosphorescent white LEDs and for each of the employed LEDs around 20 times improvement in data rate to their bandwidth is achieved at a distance of 2 meters. The implemented chip has an area of 0.6 mm^2 and consumes around 2.05 mW.
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Development of a solid state spot welding technique for the manufacturing of detect free joints
(2019-06) Bajilane, Isam Jabbar Ibrahim; Yapıcı, Güney Güven; Yapıcı, Güney Güven; Başol, Altuğ; Şendur, Polat; Tezel, Y. Ş.; İpekoğlu, M.; Department of Mechanical Engineering; Bajilane, Isam Jabbar Ibrahim
Development of a Solid State Spot Welding Technique for the Manufacturing of Defect Free Joints.--Friction stir spot welding (FSSW) processing is a recently developed process for joining hard weldability materials, which has expanded into automotive applications by using the concept of light alloys. Thus, by reducing its fuel consumption, FSSW as a solid-state welding method does not need to melt the workpieces used, which has attracted the attention of automotive manufacturing companies around the world. Although considerable research has been conducted to observe the advantages of the FSSW process, rather more attention has been paid to solving the probe hole (keyhole) defect, which appears at the weld spot center in the welds as a result of the pin of the welding tool after joining process is complete. This study investigates the fabrication of flat friction stir spot welds without the keyhole by using a newly developed FSSW process, which uses the intermediate layer (IL) part, and tracks the mechanical properties of the fabricated welds. The welds produced by the Intermediate Layer FSSW (IL-FSSW) process showed an excellent appearance with no large distortion resulting from the welded sheets. The top surface of the spot weld showed a smooth, flat surface. Regardless of the use of different welding parameters, the appearances of all the spot welds were comparable. It is considerable that no keyhole is formed in comparison with the conventional FSSW welds. It was shown that using the IL part improved the lap shear failure force (LSFF). Some of the welded samples, which were about two folds relative to the maximum value of the American Welding Society (AWS) welds quality requirements. In order to understand the effect of the welding parameters on the tensile behavior, design of experiment (DOE) were utilized to optimize the results of the tensile test. The optimization work indicated that the LSFF increases linearly with the increasing plunging depth. The analysis of the variance (ANOVA) statistical method in respect to LSFF indicates that the tool rotation speed was the most significant parameter, whereas the plunging feed rate was the lower one in this regard. The fatigue tests were conducted under T6 and annealing (O) conditions and over Nf of the lap welded samples of the dissimilar Al 6061/Al 2024 and similar Al 6061/Al 2024 alloys, which were determined for all the conditions. In all the samples, the annealing treatment was observed to have a negative effect on the Nf under high applied loads. However, no heat treatment effect was clearly observed under the low load levels, except in Al 2024, in which the annealing treatment had a positive effect on the Nf. In terms of the Al/Cu welds, the flat weld spots without the keyhole were produced successfully. The results revealed that there was very little difference in the LSFF achieved with the IL-FSSW using the pinless tool when compared to the welds conducted with the conventional FSSW using a tool with a pin. The X-ray diffraction (XRD) analyses showed that the Al2Cu and Al4Cu9 phases formed as a result of the peritectic reactions at the interface of the sheets within the weld nugget. The Vickers examination showed distinctly different microhardness levels up to 575 Hv, which are superior to that of the base metal corresponding to the hard intermetallic compounds formed in the weld nugget. Two finite element models were built to simulate the IL-FSSW process, i.e. thermal and mechanical. The temperature distributions obtained from the thermal model were compared with the experimental measurements with decent agreement. The mechanical model is utilized to predict the strength of the joints in conjunction with the experimental values from shear-tensile tests, providing satisfactory results for demonstrating the trends in the mechanical behavior of various joints.

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