Ph.D. Theses
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Browsing Ph.D. Theses by Author "Anlaş, Günay."
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Item 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.Item 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.Item Crack initiation and growth in shape memory alloys(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Mutlu, Fatma.; Anlaş, Günay.The aim of this thesis is to study the crack initiation and growth in shape memory alloys (SMAs), with emphasis given to martensitic transformation in front of the crack tip and non- transforming phase interactions along the crack path. In the first part, the effect of phase transformation on the fracture of polycrystalline NiTi SMAs is examined to introduce the complexities encountered in transforming materials. Fracture experiments are conducted at two different loading rates, and crack initiation and growth are monitored: In all samples, the crack starts and grows at a straight angle from the notch. Later, the J-Integral is evaluated experimentally and numerically. At a low loading rate the driving mechanism for resistance to fracture is phase transformation, but at a higher loading rate heat dissipation becomes more prominent due to higher magnitude of latent heat release. In the second part, crack initiation and growth in single-crystal CoNiAl SMAs are investigated using cyclic loading experiments. In crack growth tests, cracks initiate and grow at an angle similar to the martensite band angle of tensile specimens. Crack initiation angles of superelastic CoNiAl samples are calculated analytically and numerically, and the results are found to be in good agreement with the experiments. Stress intensity factors (SIFs) and crack growth rates (da/dN) are calculated for growing cracks, the behavior of which are investigated by examining the test samples with an optical microscope: When non-transforming secondary phase regions are encountered, cracks slow down/stop and SIFs increase/decrease. Overall, it is found that the non-transforming phase acts as a barrier against crack growth and improves the fracture resistance of the SMA. The results of the study underline the importance of including the effects of phase transformation and non- transforming microstructural barriers for a better understanding of the fracture behavior of SMAs.Item 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.Item 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.Item Failure criteria for functionally graded materials and application of GTN model using finite elements(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2010., 2010.) Oral, Alpay.; Anlaş, Günay.Functionally Graded Materials (FGMs) are special composites with a point to point continuous property variation. In this failure of laboratory scale FGMs is modeled using Gurson - Tvergaard -Needleman (GTN) model. Stress, energy, strain based (e.g. MTS, G, S criteria) and cohesive zone models that are used for failure modeling of FGMs are reviewed. GTN model originally used for failure of homogeneous materials is studied in detail. Because it is extremely difficult, if not possible, to obtain a closed form GTN yield function for a non homogeneous material, numerical implementation of GTN model is considered, and Abaqus is used for computational analyses. The validity of results are first checked by resolving a problem from literature using Abaqus. GTN model is numerically implemented to two different FGM specimens to study and predict failure. One of the FGM specimens is titanium monoboride / titanium (TiB / Ti) single edge notched bending (SENB) specimen, and the other one is a gradually ultraviolet irradiated polyethylene carbon monoxide (ECO) co-polymer single edge notched tension (SENT) specimen. It is concluded that GTN model is promising for failure simulations of FGMs with a proper selection of model parameters.Item 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.Item 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.Item Optimization and application of stirling engine for waste heat recovery from a heavy-duty truck engine(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Güven, Metin.; Anlaş, Günay.In this thesis, the use of the Stirling engine for recovery of waste heat from a heavy-duty truck diesel engine is studied in detail: First, the maximum theoretical non-dimensional shaft work for three types of Stirling engines is calculated, and out from the three types are compared by optimizing selected parameters in the shaft work equation using a first-order analysis. The results show that the Beta-type Stirling engine is more effective than the Alpha and Gamma types with higher power density. Results are then used for a preliminary design of the waste heat recovery (WHR) system from the exhaust gases of a commercial truck. Calculations carried out with exhaust gas temperatures measured on-road tests show that the WHR system can provide 1% of internal combustion engine (ICE) power output, corresponding to 3 kW, offering about 0.4% reduction in fuel consumption. Second, a dynamic analysis is performed on the Beta-type engine using a second-order analysis, considering size constraints. Four Beta-type Stirling engines that use the exhaust gas recirculation (EGR) cooler or the tailpipe as a heat source are simulated. The dynamic analysis shows that the Stirling engine WHR system does not generate a reasonable power output because of the low heat transfer rate from the truck’s exhaust gas. Therefore, it does not appear feasible for use in commercial vehicle applications.Item Prediction of dynamic force characteristics od radial tires using finite element and experimental techniques(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2008., 2008.) Alkan, Veysel.; Anlaş, Günay.A tire is one of the most important components of a vehicle. To predict its dynamic force characteristics, a detailed finite element model is constructed. In the finite element analysis of the tire, nonlinear stress-strain relationship of rubber, the reinforcements of the tire and contact between the tire and ground are modeled. First, a static tire model is constructed. Its vertical force-deflection characteristics on a surface with and without cleat and the pressure distribution over the cross section are obtained. Then, dynamic force characteristics of the tire are predicted. In the dynamic analysis of the tire, lateral and vertical force characteristics are examined, tire enveloping characteristics at low speed are investigated. Experimental studies are performed to validate finite element model results. All experiments are conducted using the Flat-bed Tire Test Machine at the University of Michigan Transportation Research Institute (UMTRI). Model and experimental results are compared to each other and it is concluded that to some extent, there is a good correlation between them. Error tables are also given to show the accuracy range of the proposed model.Item Theoretical and numerical analysis of fracture of shape memory alloys(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2014., 2014.) Hazar, İbrahim Selçuk.; Anlaş, Günay.; Moumni, Ziad.The subject of this thesis is theoretical and numerical analysis of the fracture of SMAs. First, the size of the martensitic region surrounding the tip of an edge-crack in a SMA plate is calculated analytically using the transformation function proposed by Zaki and Moumni (Zaki and Moumni, 2007) together with crack tip asymptotic stress equations. Transformation regions calculated analytically and computationally are compared to experimental results available in the literature (Robertson et al., 2007). Second, fracture parameters such as stress intensity factors (SIFs), J-integrals, energy release rates, crack tip displacements and T-stresses are evaluated. The objective at this point is to understand the effect of phase transformation on fracture behavior of an edge-cracked Nitinol plate under Mode I loading. In the FE analysis of the edge-cracked plate under Mode I loading, the ZM model as well as the built-in model (Auricchio et al., 1997) are used. Third, steady-state crack growth in an SMA plate is analysed. To this end, Mode I steady-state crack growth in an edge-cracked Nitinol plate is modeled using a non-local stationary method to implement the ZM model in Abaqus. The effects of reorientation of martensite near the crack tip, as a result of non-proportional loading, on fracture toughness is also studied. Finally, phase transformation regions are calculated analytically around the tip of an SMA specimen under Mode III loading. The analytical derivations are carried out first using a method proposed by Moumni (Moumni, PhD Thesis, ́ Ecole Nationale des Ponts et Chausśees, 1995) which relies on mapping the equations of the boundary value problem to the so-called “hodograph” plane. The influence of coupling on the extent of the phase transformation regions and on temperature distribution within the material is then investigated numerically.