Ph.D. Theses

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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.
A framework for the analysis of coupled-physics models using adaptive multi-level techniques
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2010., 2010.) Turan, Erhan.; Ecder, Ali.
This object of this study is to develop a computational framework to analyze coupled-physics problems within the context of multi-level methods. Adaptive solution strategies in conjunction with Newton-Krylov and Domain Decomposition Methods are used to investigate different problems. Two model coupled-physics problems are selected for simulation: a fluid-structure interaction problem and a multiphase flow problem. First problem is on the deformation of a bimetallic strip exposed to natural convection. Two non-conforming and overlapping domains are created to handle the changes on the boundaries so that the deflection of the solid is applied only some portion of the fluid region. Displacements on the strip are calculated using decoupled thermoelasticity with plane strain assumption. In the second problem, collapse of a water column into the air is modeled. The interface is tracked using the Volume of Fluid method and the results are compared against experimental studies. To let the physics interact with each other and to unify different numerical solution methods, a solver called DEMONA (Decomposition Enhanced Mechanics Optimized Numerical Analysis) is developed which is verified on numerous benchmark problems. A new technique, based on an idea to reduce the solution sets is implemented into the solver, as well. With this methodology, the unknowns are filtered using various reduction criteria which are either applied in run-time or decided prior to the computations so that a specific solution approach is employed. Consequently, an adaptive strusture is attained and different solution techniques are allowed to be tested with a single model definition.
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.
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.
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.
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.
Closed-loop actuator and sensor location selection strategies for flexible structures
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2008., 2008.) Güney, Murat.; Eşkinat, Eşref.
In this thesis, the problem of finding the optimal actuator and sensor locations for vibration control of a flexible structure is studied. An iterative search strategy is used, where the closedloop criteria are selected as the optimization metric. During iterations approximate coprime or low-authority H-infinity-controllers are designed, or alternatively quasi-controller method (residual deformations norm minimization) is used, which does not directly calculate controller but obtains some norms that instead approximate the closed-loop behavior well. In applications with controllers, the controller design is simplified by introducing simple approximate Algebraic Riccati Equation solutions and their derivatives, which are obtained by converting the state space descriptions of the physical system with signal weights into state space representations with decoupled block diagonal state matrices. Hence, based on such approximate solutions, it is possible to design computationally less complex controllers with less computational effort. Since for gradient based search techniques, the partial derivatives of the closed-loop criteria are required, Finite Element sensitivity analysis is utilized. The partial derivatives of the mass, stiffness and electromechanical coupling matrices are defined. Then, the partial derivatives of the open-loop and controller matrices are introduced. For plates with piezoelectric patches, the minimization procedure is enriched with constrained techniques, where Finite Element discretization is done automatically at iteration regarding the constraints. The modified constrained optimization technique is based on Zoutendijk's method and introduces constraints to avoid mesh generation of badly scaled finite elements
Computational and experimental investigation of low frequency noise in passenger vehicles
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2016., 2016.) Oktav, Akın.; Anlaş, Günay.; Yılmaz, Çetin.
In this study, low frequency noise characteristics of passenger vehicles are addressed. Vehicle noise variability and dominant paths that cause low frequency booms are investigated. To diagnose the cause of variability, a systematic approach is proposed, where all steps are explained brie y. Current practice of experimental transfer path analysis is discussed in the context of trade-o s between accuracy and time cost. An overview of methods, which propose solutions for structure borne noise, is given, where assumptions, drawbacks and advantages of methods are stated theoretically. Applicability of methods is also investigated, where the engine induced structure borne noise of the sedan studied is taken as a reference problem. Sources of measurement errors, processing operations that a ect results and physical obstacles faced in the application are analyzed. E ects of damping, reasons and methods to analyze them are discussed in detail. In this regard, a new procedure, which increases the accuracy of results, is also proposed. Coupled vibro-acoustic response of the sedan is analyzed, and the e ect of folding rear seat aperture is studied. An analytical solution is proposed to calculate acoustic eigenfrequencies. Then, uncoupled acoustic eigenfrequencies of the actual cavity, where trunk and cabin cavities are connected through the aperture are computed. It is shown that planar acoustic eigenfrequencies of the sedan can approximately be calculated using the analytical solution proposed. To further clarify the impact of folding rear seat aperture, coupled vibro-acoustic response of the sedan is analyzed through di erent case studies. Experimental modal analysis studies are carried out to update the computational model. The updated model is then used in modi cation prediction studies.
Convergence acceleration procedures for the computation of 2-D transonic flows
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2014., 2014.) Türk, Uğur; Ecder, Ali.
This study addresses a novel adaptive time stepping procedure, which leads to selection of larger time steps allowed by the physics of the problem. Information about the gradients of the flow variables can be regarded as an indicator for determining proper amount of time step, in which the system evolved. The signals from the pressure sensors, which act according to the pressure gradients, are chosen as a measure to determine the magnitude of the local CFL number. Thus, the aimed methodology for the selection of the local time step with the use of Pressure Sensor introduces optimal time steps to the implicit solution method by accounting for the pressure gradient in the solution domain, such that sharp pressure gradients encourages small time steps and vice versa. To illustrate the effect of proposed procedure, Newton Krylov (NK), with implicit pseudo time stepping method, has been employed to solve the compressible Euler equations for steady transonic case by turning on the pressure switch. Numerical experiments show that the introduced adaptive time stepping procedure decreases the computation time and the number of iterations, effectively. Additionally, a comparison study on the performances of Newton Krylov (NK) and nonlinear multigrid (FMGFAS) methods are presented. The longer computation time required by NK can be a result of the requirement of Newtons method for a better initial guess. When the free stream values are used as initial guess, a more sophisticated method for time step selection is needed for a better NK performance especially at the start up phase.
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.
Creating multi-touch haptic feedback on an electrostatic tactile display
(TThesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2017., 2017.) Sarkandi, Gholamreza Ilkhani.; Samur, Evren.
This study is composed of two main parts. In the ﬁrst part, we present single touch electrostatic tactile display analysis and modelling together with some prelim inary observation on conventional rendering methods. Data-driven texture rendering method is introduced and examined in two steps. First, accelerations occurring due to sliding a tool on three diﬀerent surfaces are measured, and then the collected data are replayed on an electrostatic tactile display. Second, data from the Penn Haptic Texture Toolkit (HaTT) are used to generate virtual textures on the same tactile dis play. Psychophysical experiments are carried out for both steps. The results show that the virtual textures generated using the data-driven method are mostly similar to the real textures in comparison to conventional method of rendering. Together with the supporting results from the multidimensional scaling (MDS) analysis, it is shown that the data-driven method is a viable solution for realistic texture rendering. In the second part, we propose a method and present a tactile display prototype to create multi-touch haptic feedback using electrostatic attraction. The method relies on applying high-voltage AC signals on certain orthogonal electrode lines resulting in perceivable changes of friction at the intersection points. Generated surface friction on the prototype is measured with a planar tribometer. Results show that multiple localized friction spots can be generated with the proposed method. A user study is also performed to test the prototype in a multi-touch scenario where a virtual tex ture is explored by two ﬁngers simultaneously. Quantitative and qualitative analyses demonstrate the feasibility of creating multi-touch haptic feedback on an electrostatic tactile display.
Damage phenomenon in highly filled elastomers
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2017., 2017.) Tunç, Birkan.; Özüpek, Şebnem.
The complete pathway to construct a constitutive model well suited for nite element analysis of highly lled elastomeric materials undergoing large deformation and damage was studied. The e ects of viscoelasticity, temperature, superimposed pressure, cyclic loading and damage in the form of interface debonding were included in the model. Damage initiation and evolution criteria were de ned, and the softening e ect of damage on the stress response was modelled. A robust numerical algorithm was developed and implemented as a user material into a commercial nite element software. The model parameters were determined for a set of solid propellant test data. Using the calibrated constitutive model a systematic veri cation and validation procedure of the implementation was carried out. Homogeneous and inhomogeneous deformation states were considered and various element types were investigated. Model predictions at various loading rates, temperatures and superimposed pressure levels were compared to test data not used in the calibration. Three dimensional stress analysis of a solid rocket motor subjected to cyclic temperature loading was successfully completed. The constitutive model has good predictive capabilities for moderate loading rates, wide range of superimposed pressure levels and cyclic loading. At high loading rates and cold temperatures the model overpredicts the stress response. The implementation is stable and robust in terms of convergence. It is therefore concluded that the constitutive model can be readily used for stress analysis of highly lled elastomeric media with general geometry and loading.
Design and analysis of three-dimensional phononic band gap structures with embedded intertial amplification mechanisms
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2015., 2015.) Tanıker, Semih.; Yılmaz, Çetin.
In this study, three-dimensional (3D) phononic band gap structures are investigated. First, in nite and nite periodic simple cubic, body centered cubic and face centered cubic lattices with and without inertial ampli cation mechanisms are considered. These 3D lattices are modeled with mass and spring elements that are parametrically varied to observe their e ects on band gap (stop band) limits. When inertial ampli cation mechanisms are used in the in nite periodic lattices, wide low frequency band gaps are generated. Moreover, wide and deep phononic gaps are obtained by using moderate amount of unit cells in the case of nite periodic lattices. Then, 3D phononic band gap structures are formed using distributed parameter inertial ampli cation mechanisms. The resonance and antiresonance frequencies that characterize the rst vibration stop band of the building block mechanism are obtained analytically and by nite element method. The mechanism is optimized to yield wide vibration stop bands in an octahedron and a 2 3 array of octahedrons. Furthermore, these structures are manufactured using a 3D polymer printer and their experimental frequency responses are obtained. Structural damping is added to the nite element model in order to match the resonant peak magnitudes of the numerical and experimental frequency response results. Moreover, a new inertial ampli cation mechanism is designed by adding constraining beams that reduce the degree of freedom of the initial mechanism. Consequently, ultra wide band gaps at low frequencies are attained. To sum up, it is demonstrated that the 3D structures built with inertial ampli cation mechanisms are capable of isolating excitations in longitudinal and two transverse directions in a very wide frequency range.
Design of composite structures for minumum weight
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2010., 2010.) Akbulut, Mustafa.; Sönmez, Fazıl Önder.
The goal of this study is to develop methodologies to optimize the structural design of composite materials to achieve the minimum weight. Mainly three different problem areas were considered. Firstly, weight minimization of laminated composite plates subjected to in-plane loading; secondly, as an extension of the first problem, weight minimization of laminated composites subjected to in-plane and out-of-plane loads; thirdly, optimal design of laminated composite plates with notches using progressive failure approach. Considering that composite materials are generally used in applications where weight is critical and considering their high cost, in this study, designs with minimum material use were aimed. Fiber orientation angle and number of plies in each lamina were used as design variables. The maximum stress and Tsai-Wu criteria were used individually or together to predict static failure. Different geometries and loading conditions were considered. Because the problems considered in this study contain numerous local optimums, a global search algorithm, Simulated Annealing, was used as the optimization algorithm. A number of modifications were proposed to improve the reliability of the algorithm.
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
Design optimization of structures under fatigue loading
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2009., 2009.) Ertaş, Ahmet Hanifi.; Sönmez, Fazıl Önder.
Many of the engineering failures are caused by fatigue. Fatigue failure can be defined as the tendency of a material to fracture by means of progressive brittle cracking under cyclic stresses. Because mechanical components usually experience cyclic loading during their operation, fatigue failure prevention is the foremost design requirement. Fatigue strength, thus structural performance, can greatly be increased through design optimization. Hence in this study, the general objective is to maximize the fatigue life of structures. Firstly, high cycle fatigue assessment models for homogeneous materials were investigated. Then, they were applied to predict fatigue lives of spot welded specimens. Reliability of these models depends on accurate calculation of the cyclic stress and strain states within the structure. For this purpose, a nonlinear finite element analysis was carried out taking into account plastic deformations, residual stresses developed after unloading, and contacting surfaces. Among the general purpose fatigue models, Coffin - Manson and Morrow’s mean stress models were found to correlate best with the experimental data. After that, a parametric study on fatigue strength of spot weld joints was conducted. The design variables considered in this study that were known to affect the strength of spot weld joints were sheet thickness, spot weld nugget diameter, number of spot welds, and the joint type as exemplified in tensile shear (TS), modified tensile shear (MTS), coach peel (CP), and modified coach peel (MCP) specimens. The results provide designers with some guidelines to foresee the impact of design changes on fatigue strength of spot weld joints. Secondly, a methodology was proposed to find the optimum locations of spot welds and the optimum overlapping length of the joined plates for maximum fatigue life. Minimum weld-to-weld and weld-to-edge distances recommended by the industry were considered as side constraints. The total strain life equation was used to predict the fatigue life. In order to use this model, the strain state in the structure developed under cyclic loading was calculated. Nelder-Mead (Sequential Simplex) was employed as the search algorithm in the optimization procedure. A number of problems were solved to demonstrate the effectiveness of the proposed method. Finally, a methodology was developed to optimize composite laminates subject to inplane loads for maximum fatigue life. For this purpose, a parametric fatigue life prediction model, proposed by Fawaz and Ellyin, was coupled with a global optimization technique called Direct Simulated Annealing (DSA). Fiber orientation angles were chosen as design variables. A computer code was developed in ANSYS parametric language and results were obtained for different configurations and loading conditions.
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.
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.
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.
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.

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