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ÖgeGeometry-aware support design for additive manufacturing(Graduate School, 2025-06-12)In recent years, additive manufacturing (AM) has drawn significant attention and interest from both academia and industry due to its remarkable advantages. However, one critical challenge in AM is tunability of mechanical properties for AM parts. Therefore, this dissertation focuses on the development of systematic approaches for enhancing mechanical property estimation and improving geometric accuracy through support structure design. Specifically, the study introduces data-driven models to predict mechanical outcomes based on process parameters such as feed rate, support distance, and nozzle temperature. In parallel, novel support generation techniques—such as conformal and Pythagorean-Hodograph (PH) curve-based structures—are proposed to reduce geometric errors and optimize support material usage. The presented methods aim to improve both predictability and efficiency in AM processes, paving the way for smarter design strategies and more reliable part production. Through experimental validation and comparative analysis, this work highlights the critical role of support design in achieving functional and structurally sound AM components. This dissertation first addresses the challenge of poor bridging and mechanical performance in additive manufacturing (AM) parts by examining the influence of key process parameters and support distances. Poor bridging, which occurs when material extruded between unsupported points sags or deforms, not only affects part quality but also compromises mechanical integrity. To mitigate these issues, two estimated models (EMs) are developed to predict the ultimate stress and strain of AM parts based on feed rate, support distance, and nozzle temperature. The EMs are derived using a central composite design (CCD) method and refined through systematic sampling for improved accuracy. Extensive tensile tests are conducted to validate the models. Finally, the EMs are integrated into a process optimization framework that automatically suggests optimal process parameters to meet predefined mechanical performance criteria. This approach enhances both the reliability and efficiency of AM production by enabling data-driven tuning of manufacturing parameters. An efficient support structure design methodology is then introduced for three-axis additive manufacturing, with a particular focus on conformal print-path strategies. Recognizing the trade-off between support density and geometric accuracy, the proposed approach integrates print-path geometry into support placement, ensuring controlled support distances and minimizing the risk of unsupported features. The method has been validated across eight diverse test cases, demonstrating a notable reduction in support material usage by an average of 16\% and a significant enhancement in geometric accuracy by 73\% compared to conventional support generation techniques. These results highlight the potential of geometry-aware support planning to enhance both material efficiency and dimensional fidelity in AM processes. Finally, a novel framework is proposed for generating supports in additive manufacturing (AM) using Pythagorean Hodograph (PH) curves. PH curves offer significant advantages in AM by enabling smooth geometry transitions and demonstrating favorable offset properties, which promote uniform support spacing, minimize poor bridging effects, and reduce machine vibrations. We propose two distinct PH-based support (PHs and O-PHs) design schemes integrated with a type of Traveling Salesman Problem (TSP) based print sequencing algorithm. The results show that both O-PHs and PHs significantly improve geometric accuracy in printed parts and reduce their print times.
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ÖgeOptimization of structures in the frequency domain(Graduate School, 2024-07-05)The frequency-domain approach has been gaining great popularity over the last decades owing to the advantages it provides considering computational time and assessment of system dynamics, especially under random excitations. Nevertheless, the results achieved in the frequency domain might be difficult to apply to the physical world. On the other hand, the frequency-domain approach is getting attention in structural optimization studies. In this thesis study, structural optimization has been discussed with the frequency-domain approach, and solutions to structural optimization problems (e. g. numerical instabilities) mentioned in the literature were investigated. For this purpose, a novel constraint, which is adapted from the Nevanlinna-Pick (NP) interpolation theory, was imposed on the optimization problem. The NP interpolation theory states that the existence of a mapping function between two complex domains is contingent upon the positive definiteness of the Pick matrix. Particularly, the response of a system, which is a complex function, cannot have an independent amplitude from its derivative amplitude at certain frequency values. This mathematical theorem corresponds to a physical fact, the dissipativity of systems. In this context, Boundary Nevanlinna-Pick (B-NP) interpolation, which is a variant of NP interpolation, was set between the excitation frequency interval (or the frequency range of interest) and the complex values of the system response, which are computed through the transfer function. In other words, the transfer function was designed as a sort of mapping function (i.e., interpolant) in the NP interpolation theorem, while the frequency ranges of interest and the response values are the domains to be mapped. Hence, one cannot shape a physical system's frequency response arbitrarily at discrete frequencies. Consequently, the associated Pick matrix was formed, and its positive definiteness condition can be incorporated into the optimization problem as a non-linear constraint. In this way, a feasible design space was determined under the guidance of the B-NP constraint. When optimizing a physical system's frequency response, disregarding the NP interpolation theory may lead to numerical instability and impractical solutions. Even if the structural equations are linear, the constraints to be imposed due to natural frequencies, vibration amplitudes, fatigue damage, and stress limits transform the optimization problem into a nonlinear optimization problem in the frequency domain. Hence, achieving the optimal solutions becomes challenging, especially in the frequency domain. In this context, it is expected that the optimization capability in the frequency domain will be increased by determining the feasible regions where the results are achievable in the physical world with the help of the B-NP constraint. This goal will be achieved with the help of an interpolation method that has not been used before in any structural optimization study in the frequency domain. Furthermore, it is expected that employing constraints derived from B-NP interpolation theory leads to an efficient exploration of the frequency domain by affecting the direction and steps of the iterations during optimization. First of all, the numerical limitations of incorporating B-NP constraint into a structural optimization in the frequency domain were investigated.
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ÖgeSentetik gaz yanma karakteristiğinin incelenmesi(Lisansüstü Eğitim Enstitüsü, 2025-03-14)Teknolojik ilerleme için ihtiyaç duyulan enerji halen çoğunlukla termodinamiğin en önemli konularından biri olan hidrokarbon yanmasından elde edilmektedir. Hidrokarbon yanmasında alevin kararlılığı hem brülör geometrisinin tasarımını hem de yanmanın enerji verimliliğini etkileyen bir parametredir. Kararlı bir alev oluşturmak için literatürde kullanılan çeşitli yöntemler vardır. Bu yöntemlerden biri olan girdaplı akış, brülörlerde yanma performansını artıran ve birçok mühendislik uygulamasında kullanılan bir tekniktir. Bu çalışmada girdaplı akışa sahip bir yakıcı tasarımı yapılması amaçlanmıştır. Bu amaçla yürütülen tez çalışması, doğrulama çalışması ve yakıcı tasarımı şeklinde iki temel adım ve bunların alt adımları olarak sunulmuştur. Doğrulama adımında, Sydney girdaplı alevlerinden SM1 alevi sayısal olarak analiz edilmiştir. İlk aşamada, iki denklemli Re-Normalisation Group (RNG) k-ε ve Shear Stress Transport (SST) k-ω türbülans modelleri ile akış ve GRI 3.0 reaksiyon mekanizması ile CH4 yanmasının kimyasal reaksiyonları modellenmiş ve literatürdeki elde edilen deneysel sonuçlarla karşılaştırılmıştır. Doğrulama çalışmasının ikinci aşamasında, deneysel sonuçları en iyi tahmin eden Shear Stress Transport k-ω türbülans modelinin sayısal sonuçları, Büyük Eddy Simülasyonu (LES) türbülans modeli kullanılarak SM1 alevi için literatürde yer alan bir sayısal analizin sonuçları ile karşılaştırılmıştır. Bu iki türbülans modelinin deneysel verileri tahmin etme düzeyleri ve akış bölgesindeki davranışları incelenmiştir. Karşılaştırma sonucunda Sydney girdap alev ailesinin kararlı alevleri için Shear Stress Transport k-ω türbülans modelinin kullanılmasının yeterli olduğu görülmüştür. Yakıcı geometrisi tasarımında da Shear Stress Transport k-ω türbülans modeli kullanılmasına karar verilmiştir. Yakıcı tasarımı iki alt aşamada değerlendirilmiştir. Birinci aşamada, doğrulama adımında incelenen SM1 alevinin geometrisi temel alınmıştır. Alev, ekivalans oranı ve hava fazlalık katsayısı açısından incelenmiştir. Kararlılığını etkileyen parametreler değerlendirilmiştir. Daha sonra endüstriyel yakıcılarda kullanılan yakıcıların güç, yakıt ve hava jet hızları ve hava fazlalık katsayısı gibi değişkenleri göz önüne alınmıştır. Bu yakıcılardaki hızların ve hava fazlalık katsayılarının SM1 alevine göre oldukça düşük oldukları gözlemlenmiştir. Düşük hava hızları ve makul hava fazlalık katsayısı ile yakıcı geometrisinin nasıl davrandığı incelenmiştir. Elde edilen sonuçlar yakıcının endüstriyel şartlarda kararlı bir alev oluşturmadığını göstermiştir. İnceleme esnasında hızların ve hava fazlalık katsayısının etkileri hakkında da bilgi birikimi oluşturulmuştur. Analizlerin sonucunda, yakıtın yakıcı yüzeyine yayılmasını sağlayacak geometrik bir çözüm gerekliliği görülmüştür. Yakıcı tasarımının ikinci aşamasında, konik bir geometrik engelin yakıt jeti önüne konulmasının etkileri incelenmiştir. Geometrik engel değerlendirilirken, engelin büyüklüğü ve yakıt girişini olan uzaklığı için farklı değerler kullanılarak parametrik bir çalışma yapılmıştır. Geometrik şeklin yakıtı yakıcı üzerinde yaymasını sağlamak ve bu esnada yakıt jetinin akış ayrılması sonucu akış alanını kesmesini engellemek karşılaşılan en büyük zorluk olmuştur. Bu zorlukları aşmak için geometrik engele eklemeler yapılmış ve sonuç geometrisi oluşturulmuştur. Yapılan uzun analizlerin sonucunda çalışmada kullanılan farklı gaz bileşimleri için kararlı alev oluşmasını sağlayan bir engel geometrisi elde edilmiş ve yapılan yakıcı tasarımı olarak sunulmuştur.
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ÖgeHemodynamic characterization of heart and venous valves based on multi-phase blood flow and FSI modelling(Graduate School, 2024-11-01)This thesis explores physiological phenomena and pathological conditions through the lens of in-silico modeling, a technique that complements traditional in-vitro and in-vivo studies, which often face limitations in replicating complex biological conditions and measuring certain vital parameters. Advances in high-performance computing have significantly enhanced the capability of in-silico models, allowing for the simulation of a wide range of physiological states from rest to stress, which are challenging to achieve through conventional experimental setups. By integrating Fluid-Structure Interaction (FSI) models, this research leverages both fluid mechanics and structural dynamics to effectively simulate intricate physiological processes within the human body, addressing venous valve function, potential risks such as Deep Vein Thrombosis (DVT), and aortic stenosis, contributing to the understanding of blood flow dynamics and guiding potential advancements in therapeutic strategies. Focusing specifically on the dynamics of biological valves, the research highlights their pivotal role and the complexities involved in their dysfunction, which are often linked to severe cardiovascular conditions. The thesis progresses from simulating simple single-phase models of venous valves to more sophisticated multi-phase models, utilizing methodologies to capture the complexities of the multi-phase nature of blood. Employing advanced techniques such as the Immersed Boundary-Finite Element (IBFE) method and the Arbitrary Lagrangian Eulerian (ALE) formulation, the thesis enhances our understanding of valve dynamics under various pathological states and contributes valuable insights for future cardiovascular disease research and potential therapeutic developments. A key aspect of this thesis is the utilization of various computational indices and descriptors derived from FSI simulations, which allows for a more comprehensive understanding of the biomechanical environment surrounding venous and aortic valves. After this general explanation of the thesis scope and a brief description of the methodologies utilized, a concise summary of the individual chapters is presented below. The chapter on geometry modeling explores the complex structures of venous and aortic valves, which are critical for simulating their dynamics in computational studies. It begins by detailing the modeling of the venous valve, chosen for its relative simplicity compared to the aortic valve, and describes the modifications made to synthetic geometries to meet the requirements of both single-phase and two-phase FSI simulations. For aortic valves, the chapter discusses how synthetic geometries, adapted from Computer-Aided Design (CAD) frameworks, are further modified to incorporate representations of calcification, which significantly affects valve functionality. The severity of calcification is categorized, and the models are adjusted to reflect different grades, facilitating the analysis of its impact on valve hemodynamics. This detailed geometric modeling forms the foundation for advanced simulations, providing a deeper insight into the mechanical behavior of these valves under various pathological conditions. The next chapter elaborates on the development and implementation of single-phase FSI simulations using the IBM to model the complex interactions between fluid mechanics and structural dynamics. The focus is on Immersed Boundary Method's (IBM) application in addressing significant displacements in fluid-structure systems, utilizing a computational framework that integrates Lagrangian descriptions for structures and Eulerian descriptions for fluid dynamics. The IBM's methodology is detailed, showcasing how forces and stresses are calculated and interact within the fluid and structure domains using integral transforms with delta function kernels to couple Eulerian and Lagrangian variables. The simulation also employs constitutive laws for material behavior, including a Neo-Hookean hyperelastic model for the aortic valve, to accurately depict the elastic properties of biological tissues. Additionally, the chapter discusses the numerical schemes and boundary conditions critical for capturing the physics of fluid-structure interactions. Advanced discretization techniques for both the fluid and structure domains are employed to ensure high fidelity in the simulation. The chapter on two-phase FSI model development investigates the transition from single-phase blood flow model to more complex two-phase model, crucial for accurately simulating the dynamics of blood flow involving both Red Blood Cells (RBCs) and plasma. The ANSYS Fluent-Structural coupled module, utilizing the ALE method, is employed due to its capability for handling two distinct miscible phases, providing a more realistic representation of blood dynamics where the volume fractions of phases sum to one, ensuring the conservation of mass and momentum. Significantly, the integration of volume fraction equations, conservation laws, and specific equations for the momentum of RBC and plasma phases allow for detailed modeling of interactions within the blood. This method accounts for forces such as drag, which is essential for understanding the complex interactions in blood flow. The use of the ANSYS software for FSI models demonstrates the application of partitioned coupling techniques, maintaining computational efficiency while ensuring accurate modeling of the physics involved in fluid-structure interactions. This setup is vital for understanding the mechanical and hemodynamic behavior of cardiovascular structures like deep veins under realistic physiological conditions. Chapter six presents the hemodynamic characterization parameters, which are essential in studying the valves' function. These metrics include transvalvular, Wall Shear Stress (WSS)-based indices, and the helicity descriptors, offering valuable insights into the behavior of valves under healthy and pathological conditions. The transvalvular indices encompass Geometric Orifice Area (GOA), maximum jet velocity, kinetic energy, energy dissipation, and vorticity intensity, which are critical for assessing valve performance, particularly in the context of aortic stenosis. These indices provide a detailed understanding of the energy transport and flow patterns that influence valve function. In addition to transvalvular indices, WSS-based indices such as Time-Averaged Wall Shear Stress (TAWSS), Oscillatory Shear Index (OSI), and Relative Residence Time (RRT) are examined. They help quantify the interactions between blood flow and the vascular endothelium, shedding light on how shear environments contribute to endothelial health and cardiovascular disease progression. The chapter also introduces helicity-based descriptors, which provide a perspective on the rotational aspects of blood flow in the aortic domain. Helicity, a scalar measure of helical motion in fluid flow, is crucial for evaluating the efficiency of blood transport and identifying potential pathological developments. Six bulk flow helicity descriptors are discussed, each quantifying various aspects of flow topology and helical structure, particularly in the presence of aortic valve calcifications. These descriptors highlight the importance of helical flow patterns in understanding aortic valve performance and the associated hemodynamic consequences under both healthy and calcified conditions. Last chapter presents the critical findings from the single-phase and two-phase FSI simulations. For venous valves, the results highlight the relationship between increased flow rates and the potential for flow disturbances, particularly under conditions of elevated pressure. This includes the identification of regions prone to recirculation, which may correlate with thrombosis risk. Key transvalvular indices and WSS-based metrics provide further insights into the mechanical stresses exerted on the valve leaflets. In the case of aortic valves, the results demonstrate the impact of calcification on hemodynamic patterns. The simulations reveal how varying degrees of calcification influence energy dissipation and helicity, offering a detailed perspective on the altered flow dynamics within the valve. These findings provide a comprehensive understanding of how pathological conditions affect the biomechanical environment of both venous and aortic valves. The two-phase FSI simulations reveal distinct flow behaviors between plasma and RBCs under varying flow rates. Higher velocities lead to vortex formation, pressure fluctuations, and RBC accumulation near valve cusps, especially in low shear regions. WSS-based indices like TAWSS and OSI highlight areas prone to flow disturbances and thrombosis risk. These findings underscore the value of two-phase modeling in understanding venous function and thrombus formation.
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ÖgeKarayolu taşıtlarından kaynaklanan egzoz gazı emisyonlarının gerçek hayat seyir koşullarına göre modellenmesi(Lisansüstü Eğitim Enstitüsü, 2023-06-20)Karayolu taşıtlarından kaynaklanan kirletici emisyonlar atmosferdeki toplam emisyon salımının önemli bir kısmını oluşturmaktadır. Bu sebeple taşıtlardan kaynaklanan egzoz emisyonlarını kısıtlayan regülasyonlar oluşturulmuştur. Bu regülasyonlar, yasa koyucular tarafından güncellenerek emisyonları daha da sınırlamaktadır. Bu çalışmada son Euro emisyon regülasyonuna göre gerçek sürüş testleri yapabilmek için güzergah oluşturma metodolojisi türetilmiştir. Oluşturulan metodolojinin çıktılarına göre farklı taşıtlarla emisyon testleri yapılarak metodoloji doğrulanmıştır. Gerçekleştirilen emisyon testlerinin dinamik şartları incelenmiş ve dinamik olarak testlerin tamamlanıp tamamlanmadığı incelenmiş ve türetilen yeni bir yorumlama metodu ile sonuçlar irdelenmiştir. Ayrıca bu tez çalışmasında, İstanbul trafik akış örneklemi kullanılarak emisyon envanteri oluşturma metodolojisi oluşturulmuştur. Bu metodoloji kullanılarak genel İstanbul envanteri örneklemi için bir uygulama türetilmiş ve ayrıca Covid -19 pandemisinin trafik akışına ve emisyonlara etkisi incelenmiştir.