Ph.D. Theses

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Experimental analysis and modeling of porous NiTi shape memory alloys
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Özerim, Gülcan.; Anlaş, Günay.; Moumni, Ziad.
Porosity brings new features to NiTi SMAs, and raises its potential for biomedical applications. Although different techniques are provided in the literature for manufacturing porous NiTi samples, the subject is still open to further investigation to achieve superior shape memory characteristics. Based on this, the aim of the thesis is to analyze and model the mechanical behavior of porous NiTi SMAs. First, NiTi compacts were produced using spark plasma sintering. After sintering, because the samples did not show the expected pseudoelastic behavior, they were systematically subjected to heat treatment. The transformation behavior and the phase composition were analyzed using DSC and XRD. These characterization gave an insight to the micro-structure after heat treatment. Then, instrumented micro-indentation was carried out to measure the hardness that was altered by aging. Selected samples that were tested under uniaxial compression showed an enhancement in the pseudoelasticity of the SPSed NiTi that was heat-treated. In the modeling part, a macro-scale phenomenological model is proposed for the mechanical behavior of the porous NiTi by using poromechanics. The model considers the porous medium as a skeleton that consists of a solid matrix and connected porous space. The porosity is included as an internal state variable. Both the pseudoelastic and plastic deformations were considered. The phenomenological model was implemented into Abaqus through a UMAT, and validated using experimental results available in the literature, as well as the numerical results obtained from the unit cell (UC) technique used in this study. The model proposed in this thesis represents the mechanical behavior of porous SMAs with reasonable accuracy with a significant reduction in numerical cost when compared to the UC approach. The model can be especially useful in possible biomedical applications.
Turbulent combustion modelling of fuels using LES
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Güryuva, Serdar.; Bedir, Hasan.
Combustion simulations with high fidelity turbulence models and detailed chemistry may suffer from high computational power requirements due to the combined cost of time-scale dissipation and small integration steps. Such a limitation can be avoided by employing a hybrid reaction mechanism reduction method called local self-similarity tabulation (LS2T). LS2T directly solves several dominant species reactions and incorporates the effects of other species on dominant ones by data retrieval from pre-calculated tables. This study is based on the application of the LS2T method to high fidelity 3D combustion simulations with different fuels and combustion physics. The test cases that are selected for demonstration purposes are Sandia Flame-D, the premixed methane combustion, and Sandia Spray-A, the non-premixed n-dodecane combustion. The combustion simulations use large eddy simulation (LES) as turbulence solver and transported probability density function (TPDF) for species transport, to increase the accuracy of the simulation and avoid the use of any additional reaction model. This study is the first demonstrator of LS2T approach application to 3D combustion problem with Sandia Flame-D simulation and it is also the very first Spray-A simulation that is executed using LES and TPDF in a 3D resolved domain. The report consists of wide literature research regarding the LES combustion analyses of both test cases and chemistry reduction techniques applied, a detailed theory behind all the physics solution methods used, the computational methods implemented for the study, results, and discussions of the 0D and 3D simulations. The results show that by the use of the LS2T method, it is possible to have high accuracy and generate results similar to the detailed chemistry while maintaining an acceptable computational effort.
Adaptive boundary control using backstepping for 1D variable length string-mass system under disturbances
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Szczesiak, Mateusz.; Anlaş, Günay.; Yılmaz, Çetin.
In this thesis, an adaptive boundary control using delayed control methodology for a 1D wave equation is examined. The outlined problem is applied in the control of an ideal string- mass system with constant or time-varying length. The dynamics of the system, which constitutes the basis for the control problem, is first derived using the extended Hamilton‘s Principle. The resulting wave PDE is then transformed into two decoupled hyperbolic equations using the method of characteristics. The solution of the characteristic equation allows one to project the input signal at one boundary onto the dynamics describing the other boundary. Here, the input appears with an explicit delay. If the domain is characterized by a moving boundary, i.e., the length of the string is non -constant, the delay is time-varying. The problem then becomes that of control of a linear ODE with an input delay. Afterward, the transport PDE representation is used to re-express the delay in terms of a PDE‘s boundary value re sulting in an ODE- PDE cascade system. The backstepping transformation then gives the control law and transforms the system into the target system characterized by fa vorable control properties. The only feedback required for the control is the boundary measurements. Thereafter, Lyapunov‘s theory is used in the stability analysis. Any unknown in-domain or boundary disturbances, as well as uncertain boundary parame ters, are handled using the adaptive control strategies. The dynamics of the string-mass system and the performance of the derived controllers are illustrated using numerical simulations. This is followed by a case study where the deployment and control of an underwater sensor in the presence of the water waves are simulated.
Nonlinear viscoelastic material modeling using nested linkage mechanisms
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Özcan, Mustafa Umut.; Yılmaz, Çetin.; Sönmez, Fazıl Önder.
In this study, basic linear lumped elements such as springs and dashpots are used in nested linkage mechanisms in order to simulate and predict the mechanical behaviour of nonlinear viscoelastic materials. The proposed mechanism model containing two nested linkages can show initial softening followed by hardening response under quasi-static loading, which is commonly displayed by hyperelastic materials. Hence, material nonlinearity is simulated by geometric nonlinearity of the linkage mechanism. The mechanism also displays relaxation, hysteresis, and dynamic stiffness responses of viscoelastic materials with the help of dashpot elements. By tuning the geometric parameters of the mechanism, and the stiffness and damping parameters in the system, desired viscoelastic response can be obtained. Most of the previous experimental studies in the literature considered just two of different possible test scenarios. Comparisons with the experimental results in the literature show that the nested linkage mechanism with linear springs and dashpots can successfully simulate the material response in the tests for different double combinations of quasi-static loading, ramp-and-hold loading, hysteresis, and dynamic stiffness tests. When the experimental studies in the literature are investigated, it is seen that studies investigating three different test scenarios are rare. In this thesis, these four testing scenarios are considered in the same study for model validation for the first time. These four tests are conducted on three rubber samples with different stiffness and damping characteristics. It is shown that the nested linkage mechanism model can accurately mimic the material behaviour in these four different tests with a single set of values for the design parameters. In order to evaluate the prediction capability of the nested linkage mechanism model, optimization is conducted using only two test scenarios and the responses in the other two test scenarios are validated. To further assess the prediction capability of the model, parameter values are obtained for a sample and the responses of a sample from the same material with a different size is estimated for the four test scenarios. Finally, considering the hardening behaviour of the samples, the number of parameters in the model is reduced from 8 to 5 and it is shown that the reduced model also gives quite satisfactory results.
Mechanical behavior of low density polymeric foams under multiple loading and unloading
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2008., 2008.) Öztürk, Umud Esat.; Anlaş, Günay.
In this thesis, mechanical behavior and energy absorption characteristics of low density polymeric foams under multiple loading and unloading are investigated for uniaxial and hydrostatic compression, uniaxial tension, simple shear, and cylinder and block indentation. Constitutive models and energy absorption diagrams available in literature for uniaxial compressive loading are reviewed. A new phenomenological constitutive model for accurate calculation of load, deformation, and absorbed energy is proposed for multiple loading and unloading. Results of the available and the new models are compared to those of experiments for expanded polystyrene (EPS) and polyethylene (PE) foams. A design procedure for multiple compressive loading and unloading is presented. A drop test rig for measuring uniaxial compressive behavior of foams at high loading speed and a hydrostatic compression test setup to study the mechanical behavior of foams under multiple hydrostatic loading and unloading are built. Tools to be used with Zwick Z020 universal tensile testing machine are prepared for uniaxial tension, simple shear, and cylinder and block indentation tests. Stressstrain results are presented for EPS and PE foam specimens. Finite element simulations of EPS and PE foam specimens under multiple loading and unloading for uniaxial and hydrostatic compression, uniaxial tension, simple shear, and cylinder and block indentation are performed using Abaqus finite element package for volumetric and isotropic hardening. The results of finite element simulations are compared to those of experiments.
LPV modeling and robust control of yaw and roll modes of road vehicles
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2007., 2007.) Başlamışlı, Selahattin Çağlar.; Köse, İbrahim Emre.; Anlaş, Günay.
In this thesis, the usefulness of linear parameter varying (LPV) modeling of vehicle dynamics is investigated for controller synthesis to take nonlinear tire behavior into account. The H infinity control framework is used to design combined active steering and differential controllers to improve vehicle handling during maneuvers involving large driver commanded steering angles. Two approaches are undertaken to reduce the size of the parameter set to minimize solver time during the controller synthesis step. The first approach is built on modeling tire stiffnesses as parametric uncertainties. This leads to a linear fractional transformation (LFT) model of the combined vehicle body and tire subsystems and to the design of a static state feedback controller intended to be robust against large variations in parameters. In the second approach, a rational fit is proposed for the nonlinear tire model used, and original parametric vehicle models are derived by integrating the fitting model into the equations of motion. This leads to the design of gainscheduled LPV controllers where scheduling is based on lateral and longitudinal tire slips. At small driver commanded steering angles, both controllers achieve decoupling of sideslip and yaw rate modes. However, at large driver commanded steering angles, the steering response of the first controller is observed to be unstable at the physical limit of the vehicle due to the shortcomings of the parametric uncertainty model in predicting tire behavior at large lateral slip. Meanwhile, the second controller achieves decoupling of all vehicle modes for the whole range of driver commanded steering angles up to and at the physical limit of the vehicle, revealing the importance of incorporating the tire friction circle concept into the controller synthesis.
Numerical investigation of high Knudsen number flow in rectangular enclosures
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2006., 2006.) Orhan, Mehmet.; Ecder, Ali.; Tezel, Akın.
Nowadays, enlightening unknown aspects of rarefied gas flow is one of the critical issues of fluid dynamic research to ensure correct and proper operations of manyMicro- Electro-Mechanical-Systems (MEMS). Thermally driven motion of rarefied gases is gaining in importance to develop Knudsen compressors having better performance or to improve single crystal growth processes. Therefore, accurate prediction of the physics lying behind the thermal creep in the transition regime as well as slip flow regime is one of the main motivations of this study. The other emphasis is possible flow instability of the rarefied gases in enclosures. For this purpose, an asymptotic approximation has been performed in the first part of the study to find analytical solutions. In the second one, linear disturbance theory of hydrodynamic stability has been applied to the problem to determine bounds of instabilities. Analytical solutions of two-dimensional stability analysis have been introduced. Critical states have been identified for different models and for varying Knudsen numbers. More generally, eigen-spectrum of the perturbation equations has been identified in three-dimensions. At the last part, by applications of an artificial viscosity scheme, a computer program has been constructed to solve Burnett and also Navier-Stokes equations. Mechanisms of the thermal creep flow have also been verified by inspecting stress tensors of Burnett equations. Most importantly, the insufficiency and the failure of Navier-Stokes equations for the creeping flows have been proved. Moreover, it has been shown that Burnett equations can correctly model such creeping flows.
Dynamic analysis of diesel engine crankshaft system using finite elements and multibody system simulation programs
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2008., 2008.) Yılmaz, Yasin.; Anlaş, Günay.
In this thesis, dynamic analysis of in-line six cylinder diesel engine crankshaft system is carried out using analytical and numerical methods. The dynamic analysis of the crankshaft system consists of calculation of forces, displacements and stresses over a complete engine cycle (two revolutions of the crankshaft) under steady state (constant speed) conditions with a model of the whole cranktrain. Crankshaft system consists of crankshaft, engine block, pistons, piston pins, connecting rods, flywheel, torsional vibration damper, bearings and mounts that support the engine block. The loading on the system comes from the cylinder gas pressure and inertia of crankshaft system components. In the analytical part of the study, first, the forces acting on the crankshaft system are determined. Then, main bearing loads are calculated using a statically determinate system approach for each crank throw. Finally, torsional vibration and stress analyses of the crankshaft system are performed. In the numerical analysis of the crankshaft system, Msc. Nastran and Msc. Adams programs are used. The dynamic stress distribution in the crankshaft is evaluated using a flexible crankshaft model that is obtained through finite elements and Component Mode Synthesis (CMS) technique. To study the effect of oil holes on crankshaft dynamic stresses, crankshaft models with and without oil holes are used. The effect of TV damper on crankshaft stresses is investigated. Bearings are modeled using hydrodynamic bearing models of ADAMS. Coupled axial, bending and torsional vibrations of the crankshaft system are considered. Effect of each part of the crankshaft system on crankshaft dynamic stress and vibration characteristics are investigated. A separate chapter is devoted to effects of counterweight mass and position on main bearing load and crankshaft bending stresses. In the analysis, rigid, beam and 3D solid (flexible) crankshaft models are used. Main bearing load results for rigid, beam and 3D solid models are compared and beam model is used in counterweight configuration analyses. Twelve-counterweight configurations with a zero degree counterweight angle and eight-counterweight configurations with thirty degree counterweight angle, each for 0%, 50% and 100% counterweight balancing rates, are considered. It is found that maximum main bearing load and web bending stress increase with increasing balancing rate, and average main bearing load increases with decreasing balancing rate. Both configurations show the same trend. For this specific engine, the load from gas pressure rather than inertia forces is the parameter with the most important influence on design of the crankshaft. Results of bearing loads and web bending stresses are tabulated.
Characterization of NiMnGa magnetic shape memory alloys
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2005., 2005.) Pirge, Gürsev.; Altıntaş, Sabri.
Magnetic shape memory (MSM) alloys are a new class of actuator materials withhigh actuation frequency, energy density and strain and they can be used in themanufacturing of actuators, smart structures, sensors and transducers. NiMnGa alloys experience a reversible martensitic transformation, which is a temperature-dependent phasetransformation from a highly symmetric crystallographic structure (austenite) to lowsymmetry (martensite). These materials are ferromagnetic. Ferromagnetism is aphenomenon by which a material can exhibit a spontaneous magnetization, and is one of the strongest forms of magnetization. Ferromagnetic metal alloys whose constituents arenot ferromagnetic in their pure forms are called Heusler alloys, named after Friedrich Heusler. Applying a strong magnetic field to some of the Heusler alloys may inducereorientation of martensite variants with high magneto-crystalline anisotropy energy, which leads to a net shape change of the material. In this study, the effect of alloycomposition, cutting direction and heat treatment on the microstructure, local composition,and thermal and dilatometric properties of NiMnGa alloys were investigated.Characterization tests involved various crystals, with and without post-crystal growth heat treatment, by chemical analysis, differential scanning calorimetry (DSC), dilatometry,optical microscopy, scanning electron microscopy and radiography. Metallographic studies showed that as solidified, off-stoichiometric alloys had three distinct microstructural features-a Heusler phase, a Mn rich phase and a eutectic or eutectoid region. Various heattreatment procedures were applied to successfully remove the last phase and produce MSM effect. Heat treatment was also essential for the production of a distinct martensitetransformation in DSC and dilatometry traces and a martensitic transformation to occurover a narrow temperature range. Bulk and microanalysis showed that there are significant concentration variations in the boules grown by the Bridgman method, that lead to changesin phase transformation behavior which were observed by DSC. The presence of composition variations in the boules is a major issue because of its effect on the martensitetransformation temperature. For boules with composition variations, both transformed anduntransformed regions will exist over some temperature range, degrading the performanceof any actuator made from them. Clearly, further effort on the improvement of the crystalgrowth technique is needed to remove the composition gradient and variations and toobtain a fine dendritic structure, which would be much easier to homogenize. For thecurrent growth conditions, coarse cellular structures have been obtained which showsignificant solute segregation. An increase in the thermal gradient during the directionalsolidification process resulted in a finer cellular structure.
Core-shell PVA / gelatin nanofibrous scaffolds using multinozzle aqueous electrospinning
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2019., 2019.) Şengör, Mustafa.; Bedir, Hasan.; Altıntaş, Sabri.
Biological scaffolds have been used in the reconstruction of the damaged tissues. They have similar morphology and structure to the host tissues. However, they can be produced using materials that can be harmful to humans and the environment. In this context, core-shell nano ber based sca olds, whose mechanical strengths are provided by PVA(poly vinyl alcohol) and recognition sites are provided by gelatin, were fabricated in a non-woven manner using multiple nozzles of electrospinning technique. Instead of widely used toxic, acidic or salt-based ionic solvents, deionized \water" was used as the only solvent for both polymers. Firstly, nano bers were produced from 8 % (w / w) gelatin and 8%(w / w) PVA solutions individually. Limits were determined for parameters such as voltage, feed rate, temperature and polymer concentrations. Although pure gelatin nano bers have diameters of less than 50 nm, they have beaded structure and have lower mechanical strengths. Smooth bers were obtained from 8% PVA. Fibers with PVA: gelatin core shell morphology were then produced at di erent feed rate ratios (FRR). Based on the ber diameter, the optimal FRR with a 15 kV voltage magnitude and 15 cm electrode distance was found to be 1: 1 with an average diameter of 280 nm. The ratio of 1: 3 and 1: 4 was seen as the formation of \beaded" bers and the pealing limit of gelatin over PVA, respectively. Mechanical and water resistance of the produced sca olds was further improved by cross-linking. Core - shell morphology was demonstrated by TEM, SEM, EDS analysis. The secondary structure of the gelatin from collagen and the e ects of the electrospinning were revealed by FTIR and DSC. Approximately 60% of all cross-linked sca olds were degraded in solution using lysozyme enzyme up to day 14.
Development and modeling of a colonoscopy robot
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2020., 2020.) Tutcu, Cem.; Samur, Evren.
Colorectal cancer is the second leading mortality cause among all cancer types. Similar to the other cancer types, early detection plays a vital role in the prevention of mortality. Colonoscopy is an endoscopic method that is widely used to screen colon, and remove legions, which is considered to be the most reliable method for detecting colorectal cancer. Conventional colonoscopes are propelled and positioned manually. This operation presents the risk of colon perforation, and patient discomfort due to high reaction forces applied to the colon wall. The conventional approach also often emerges the problem of colonoscope shaft looping inside the convoluted colon that causes loss of haptic feedback from the tip. Due to these post-colonoscopy complications, scans are not performed as frequently as required to mitigate the risks. In this study, a novel robotic solution is proposed for colonoscopy operations that will reduce operational risk, and improve patient comfort which will have an impact to increase colonoscopy scan rate. The robotic system also aims to provide a more ergonomic working environment for the colonoscopist to reduce long term usage complications. This thesis focuses on the colonoscopy robot development; particularly the design of an in-vivo shaft, kinematics and quasi-static modeling of the robot, and a medical application scenario. An experimental study is performed to prove navigation and position control capabilities of the system using a large scale prototype. Experiments showed that wall reaction forces are considerably lower than the conventional colonoscopy. Positioning tests have demonstrated close correlation with a model estimate up to a certain robot body length. This thesis proves the concept of a growing soft robot that can be further developed to be used in colonoscopy in future studies.
Physical based analysis and model reduction of engineering systems
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2003., 2003.) Orbak, Ali Yurdun.; Eşkinat, Eşref.; Türkay, Osman S.
There is a need for obtaining low order approximations of high order models of physical systems as low order models result in several advantages including the reduction of computational complexity and improved understanding of the original system structure. Different methods have been suggested in literature for obtaining suitable low order approximations, but these approaches do not reflect the relation between the mathematical model and the physical components of a system. In this thesis, some new approaches are provided for model reduction in the physical domain. The approaches that are presented use the idea of decomposition of physical systems, which is useful for the identification of dominant components or subsystems. The procedures are applied to the physical systems that are represented by bond graphs as they lead to better understanding of the system structure. One of the proposed methodologies exploits the idea of decomposition of physical systems. The proposed decomposition and model reduction procedures are directly implemented on the model providing a better perception of the physical model reduction and a better design point of view. As a second methodology, the determination of subsystems and/or components that influence a given eigenvalue of the overall system has been explored. A set of theorems and definitions are proposed that lead to an efficient procedure for this aim. After the calculation of eigenvectors, effect matrices are produced that indicate the relative importance of physical parameters in a selected eigenvalue. Using these matrices, an efficient physical model reduction procedure is constructed. The advantages of the presented approaches over existing methodologies are emphasized through several examples.
Shape and topology optimization of intertial amplification induced phononic band GAP structures
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2018., 2018.) Yüksel, Osman.; Yılmaz, Çetin.
Inertial ampliﬁcation is a novel phononic band gap generation method in which wide vibration stop bands can be obtained at low frequency regions. The engineering importance for this novelty comes from the fact that the phononic band gap structures can be utilized as passive vibration isolators for the low frequency range. In this thesis, primarily the research is focused on the improvements achieved on stop band widths and depths via employment of structural optimization tools. To that end, size, shape and topology optimization studies are conducted on a compliant inertial ampliﬁcation mechanism, then with these compliant unit cell mechanisms, one and two dimensional periodic structures are formed. Consequently, by means of these periodic structures, it is demonstrated that the vibration transmission is inhibited for wide ranges at low frequencies. The work comprises analytical and numerical studies and more import antly experimental validation of the results. Moreover, topology optimization studies performed during the thesis lead to the development of a new fast topology optimiz ation algorithm to obtain structures with maximized fundamental frequency, though this was not originally among the research objectives. Finally, explicit problem formu lations and a comprehensive review on topology optimization are also presented.
Integrated computational alloy design, single crystal growth, and characterization of nickel base superalloys
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2018., 2018.) Montakhabrazlighi, Mehdi.; Balıkçı, Ercan.
This dissertation is formed by a part of work conducted in a TUBITAK project (112M783). In the dissertation, first alloy design of a third generation superalloy, and then its production as single crystals, heat treatments, and creep tests are investigated. A combined Neural Network (NN) – PHAse COMPutation (PHACOMP) – CALculation of PHAse Diagrams (CALPHAD) method is applied to alloy development of single crystal Ni base superalloys with low density and high creep resistance. PHACOMP method is used for estimation of the volume fraction of the γ’ and a parameter named Md which is an index showing the propensity of the alloys towards formation of the TCP phases. Neural network is used for modeling the density and rupture time by training and testing a network with a set of the known experimental alloy compositions. Modeling results is combined with data obtained from PHACOMP to render very useful scatter plots for the effect of alloying elements; stress, temperature and volume fraction of the γ’ phase on density, rupture strength and formation of TCP phases in the Ni base single crystal superalloys. A third generation alloy (ERBALLOY) was designed and produced by two methods, vertical Bridgman (VB), and vertical Bridgman with a submerged baffle (VBSB). The effect of low melt height on solidification characteristics of the alloys is studied. Evolution of the phases is simulated by CALPHAD bases Thermo-Calc software. The solution and aging heat treatment of the alloys are modeled with Dictra and TC-Prisma software. Characterization of the microstructure is performed by optical, scanning transmission electron microscope (SEM), and electron probe microanalysis (EPMA). The creep behavior of the ERBALLOY is tested at an intermediate and a high temperature and showed reasonable agreement with NN results. The approach used in this study is in line with the Materials Genome Initiative (GMI) and Integrated Computational Materials Engineering (ICME), and it can be applied for designing low density-creep resistant single crystal superalloys for critical parts of the aircraft/gas turbine engines.
Experimentally verified numerical simulation of single crystal growth process with a low melt height and an axial vibration
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2018., 2018.) Sheikhi, Aidin.; Balıkçı, Ercan.
This Ph.D. dissertation investigates experimental and numerical crystal growth of antimony-doped germanium (Ge-Sb) single crystals. The investigation is a part of the TUBITAK project 212M030. The single crystal growth of Ge-Sb from the melt is investigated by the Vertical Bridgman (VB), Axial Heat Processing (AHP), and Axial Vibrational Control (AVC) techniques. The effects of method dependent growth parameters on the quality of the grown single crystals have been analyzed. To this end, two different pulling rates (10 mm/h and 20 mm/h), different initial melt heights (5 mm, 10 mm, 14 mm, and 58 mm), and three different sets of vibrational parameters (2 mm amplitude and 0.25 Hz frequency, 0.25 mm amplitude and 1 Hz frequency, and 0.25 mm amplitude and 25 Hz frequency) are applied in the growth of seven different crystals. It is observed that the highest single crystal length with the most homogeneous solute redistribution and the least dislocation density are achieved in an AHP crystal which is grown with the lowest pulling rate (10 mm/h). However, it is determined that an appropriate control of the vibration parameters in the AVC technique makes it possible to achieve almost the same crystal quality with doubled growth rate, so the production yield is decreased. Moreover, global and local numerical simulations are performed in order to investigate the effects of the growth parameters on the convective flow patterns. Also, results of the numerical simulations contribute to make better and more reliable interpretations of the experimental observations. The simulation results provide useful information for the experimentalists to investigate the effects of growth parameters on the temperature and solute distribution, flow pattern, and the interface shape. According to the numerical results, it is possible to clarify how the insertion of the baffle, adjusting the melt height, and optimizing the vibrational parameters of the baffle contribute the thermal and the solutal homogenization in the melt, interface stability, and consequently improved crystal quality.
Characterisation of failure in composite materials with acoustic emission and correlation with micromechanics
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2018., 2018.) Öz, Fatih Ertuğrul.; Ersoy, Nuri.
Polymer composites fail through complex damage mechanisms. It is not easy to determine stress levels for onset of various damage mechanisms with a single uniaxial tension test, since their stress-strain responses do not provide a clear yield point or stiffness degradation during loading. Acoustic Emission (AE) is an important technique used to detect damage in composite materials. An AE signal is an ultrasonic wave resulting from the sudden release of the strain energy when damage initiates and contains information about the damage mode. General conclusions in literature for the correlation of damage modes with corresponding AE characteristics are relied on interpretations rather than direct observation of damage modes. In this thesis, damage progression in Carbon Fibre Reinforced Plastic composites are investigated using AE technique. Optical instruments are used to obtain reliable correlations with damage modes and the AE events. First unidirectional laminates are tested. Artificial defects in the form of slits are incorporated at certain plies during manufacturing to stimulate damage in desired sequence. Tension tests are stopped at certain stress levels before the ultimate strength and specimen edges are investigated with optical microscope to identify damage modes and correlate with AE characteristics. Then results are compared with predictions of a progressive damage model implemented using Finite Element Micromechanical Model and a very good consistency is achieved. In the second part, Digital Image Correlation (DIC) and in-situ edge observation are applied simultaneously during the tension tests of different quasi-isotropic laminates. They provide robust evidences for damage mode correlations. The k-means++ clustering algorithm is used to group similar AE events. It is seen that damage progression and their AE characteristics change with lay-up sequence. The results obtained in this thesis put the reliability of AE based damage mode classifications, widely adopted in literature, in question and a new classification scheme is proposed.
Molecular dynamics study of water-HBN nanofluid
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2018., 2018.) Akıner, Tolga.; Ertürk, Hakan.; Mason, Jeremy K..
This study considers the molecular simulations of nanoﬂuids and the goal is to investigate the thermomechanical mechanisms in nanoscale thermal transport. The enhanced thermal conductivity and limited shear viscosity increase is the fundamental phenomena that makes nanoﬂuids as a hot research topic of the recent thermal-ﬂuid and nanoscience literature, and a potential novel complex liquids for variety of appli cations. The nanoﬂuid problem has been studied from the nanomechanical point of view and molecular dynamics simulations are used to investigate the physical aspects. A water-copper system has been modelled as a benchmark study to understand the nanocolloid concept and the capacity of existing methodologies. Green-Kubo formal ism, pure water system, thermal enhancement and viscosity increase of water-copper nanoﬂuids and Brownian motion eﬀect has been studied and compared with the exper imental results. Potential function improvement has been aimed for a water-hexagonal boron nitride system to obtain a robust mathematical foundation for the molecular dynamics simulations. Therefore, interlayer interactions of hexagonal boron nitride and interface interactions at the water-hexagonal boron nitride interface have been formulated using recent quantum simulation results and experimental data. Thermo mechanical properties of hexagonal boron nitride have been accurately estimated using simulations with derived potentials, and water-hexagonal boron nitride interfacial dy namics have been discussed for the interfacial thermal transport. A new temperature calculation algorithm for non-equilibrium simulations has been introduced and tested for rigid and ﬂexible water model. A new approach has been preliminarily developed to study the agglomeration in nanoﬂuids with orthotropic nanoparticles using simulations and experimental images.
Design of spectrally selective coatings for high efficiency power generation devices
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2017., 2018.) Khosroshahi, Ferhad Kazemi.; Ertürk, Hakan.; Mengüç, M. Pınar.
This thesis is aimed at designing and optimizing spectrally selective emitters/ ﬁlters for optical or thermal applications such as thermophotovoltaic (TPV) devices. Using spectrally selective emitters/ﬁlters in these devices is a crucial step to approach to an optimum system. Design of a spectrally selective ﬁlter based on one-dimensional Si/SiO2 layers is considered ﬁrst for improved performance of TPV devices. Spec trally selective ﬁlters transmit only the convertible radiation from the emitter as non convertible radiation leads to a reduction in cell eﬃciency due to heating. The presented Si/SiO2 based ﬁlter concept reﬂects the major part of the unconvertible range back to the emitter to minimize energy required for the process and it is adaptable to diﬀer ent types of cells and emitters with diﬀerent temperatures since its cut-oﬀ wavelength can be tuned. While this study mainly focuses on InGaSb based TPV cell, Si, GaSb, and Ga0.78In0.22As0.19Sb0.81 based cells are also examined. The simulations show that signiﬁcant enhancement in the overall system and device eﬃciency is possible by using such ﬁlters with TPV devices. In addition, graphene based spectrally selective nano structures are theoretically investigated to achieve absorption and transmission within narrow wavelength bands. Two concepts are identiﬁed to control the spectral ab sorptance and transmittance and the results showed that using these two-dimensional, multi-layered structures, with gratings and graphene layers narrow-band absorptance and transmittance can be achieved. The eﬀect of the graphene layer is identiﬁed for the emitter structure using power dissipation proﬁles. The suggested ﬁlter structure is then optimized for a TPV system and it is shown that the overall TPV system eﬃciency can be improved by using the optimized ﬁlter. The methodology described in this thesis allows for an improved emitters/ﬁlters d
Advanced coating technologies with spectral alteration for solar applications
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2018., 2018.) Yalçın, Refet Ali.; Ertürk, Hakan.
Spectrally selective coatings are used to maximize the efficiency of solar thermal systems and they are designed based on the application. This study focuses on two solar applications; solar thermal energy systems and greenhouses. For solar thermal energy systems, the coating should have high absorptance at solar wavelengths and low emittance at the infrared wavelengths, where absorber emits heat to maximize the heat transfer to the working fluid. For greenhouse applications, coating should provide high radiation at the photosynthetic spectrum and distribute light uniformly and diffusely. This study focuses on fluorescent and non-fluorescent pigmented coatings that consist of a binder and well dispersed nanometer or micrometer sized particles that are known as pigments, selected to achieve the desired spectrally selective behavior based on application. Radiative behavior of coatings depends on coating thickness, pigment size, concentration, and the optical properties of the binder and pigment materials that can be identified by modeling the radiative transfer across these coatings. Models are developed for the problems considered to solve the radiative transfer equation based on the governing physics to predict the spectral reflectance, transmittance and light distributions in conjunction with Lorenz-Mie theory and T-matrix methods that are used for predicting radiative transfer properties. These models are used to design coatings to achieve optimal behavior for considered applications. It is found that the model used for designing pigmented coatings of solar thermal systems can be very critical, and coatings must be designed using a unified model considering the effective medium theory and four flux method together with Lorentz-Mie theory. Besides, it is found that while fluorescent coatings can improve spectral distribution of irradiation for photosynthetic production, they also lead to a significant decrease in the transmittance, decreasing the irradiance when used for traditional greenhouses. However, for vertical farms it is found that using fluorescent particles in coatings both improve distribution of light and effective PAR, resulting around 35% increase in yearly crop production for lettuce.
A deterministic approach to transition to turbulance in plane shear flows
(Thesis (Ph.D.)- Bogazici University. Institute for Graduate Studies in Science and Engineering, 1999., 1999.) Atalık, Salim Kunt.; Tezel, Akın.
In this work, a parametrical study of the transition to turbulence in two-dimensional shear flows has been conducted. For this purpose, the solutions of the full two-dimensional Navier-Stokes equations have been investigated numerically using spectral methods. In parallel, a new spectral integration algorithm, called the Nonlinear Galerkin Method, stemming from dynamical systems theory and developed for the integration of dissipative evolution equations such as Navier-Stokes equations, has been tested and applied for the study cases. Different nonlinear Galerkin methods have been compared for this purpose with respect to each other in terms of convergence and efficiency and the improvements on the classical Galerkin spectral method have been shown numerically. Transition to turbulence has been analyzed by the parametrical investigation of qualitatively different solutions in the phase space of two-dimensional Navier-Stokes equations for bounded and unbounded shear flows with one nonhomogeneous direction. The applications were plane channel (Poiseuille) flow and oscillatory plane Poiseuille flow for the bounded flow case, and temporally growing mixing layer and plane jet flows for the unbounded flow case. With this work, we aim to contribute to the enlightening of the structure of the phase space of two-dimensional Navier-Stokes equations as well as to the testing of a new integration algorithm which seems to be promising in the direct numerical simulation of Navier-Stokes equations.

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