Makina Mühendisliği

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2-D mathematical modelling of hydrogen storage in metal hydrides
(Thesis (M.S.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2009., 2009.) Küçük, Gülşad.; Aksan, Emre.
In this study a numerical model which simulates heat and mass transfer processes of hydrogen absorption in LaNi5 is studied. Energy balance method is used for discretizing the heat equation. Transient heat conduction with convective boundary conditions is evaluated with explicit scheme. The analyses are conducted using MATLAB. Evaluation of temperature profiles and hydrogen mass that is being absorbed during hydriding are shown to be correlated with the experimental data and with the previous numerical studies. Since the hydriding reaction is exothermic, the rate of removal of heat released is critical in time needed to reach saturation in metal-hydride reactor. The effects of bed geometry, inlet hydrogen pressure, and metal porosity, initial bed temperature and the temperature of cooling fluid are investigated. Different cases are compared with the base case which is chosen to have similarities with conventional metal hydride tanks in reactor geometry and in thermophysical properties.
2-D modeling of compression ignition engine combustion
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2016., 2016.) Yıldırım, Cebrail.; Bedir, Hasan.
Internal combustion engines are the cornerstones of our society. They are widely used in a vast range of area from transportation to power generation. Since they are mostly petroleum-powered and emission regulations are getting tighter due to the environmental concerns, ICE research activity is vibrant today. In the present study, a multidimational CFD approach has been used to simulate a CI diesel engine combustion to investigate the e ect of injection timing and swirl ratio on combustion and emissions. AVL Fire software has been used for combustion, spray and emission modeling. Within this context, ECFM-3Z, k {u100000} ", Wave break-up, Dukowicz, extended Zeldovich and Hiroyasu approaches have been used for, in order, combustion, turbulence, spray atomization, spray evaporation, NOx and soot emission modeling. When compared with the experimental data, the simulation results show a good agreement. As for the swirl ratios, minimum NO and soot emission are achieved at SR 3.0 while the optimum bsfc and maximum indicated e ciency are found at SR 1.0. Findings of di erent injection timing are that advanced injection results in higher NO and higher soot emissions as the retarded injection provides lower NO and higher soot emissions with reference to the baseline simulation.. As a result, SR 1.0 and reference injection timing have been found to be the optimum con guration in terms of bsfc and emissions for the engine studied.
3D radial inflow turbine design and analysis for a small scale ORC application
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2017., 2017.) Sezerkan, Emre.; Bedir, Hasan.; Anlaş, Günay.
In this study, the aim is to design and analyze a single stage subsonic radial in- ow turbine for the Organic Rankine Cycle system established in Bogazici University Renewable Energy Technologies Laboratory, by using R245fa and R134a as the working uids. Complete design process of the ORC turbine, from preliminary design to 3D blade design stage, is presented. Design point of the turbine is determined according to the cycle limitations and by comparing the maximum mach number, rotational speed and mass ow rate parameters found as a result of detailed preliminary turbine design and basic cycle analysis. Results of the streamline and CFD analysis of the designed turbine are compared and discussed from the point of Mach number distribution, turbine e ciency and power output. The comparison shows that the two analyses give similar e ciencies, power outputs and Mach number distributions. Streamline analyses results show that the maximum total to static turbine e ciency and power output are 87.5 % and 3.97 kW respectively.
A computer simulation study of dislocation motion in the modified Frenkel-Kontorova model
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 1982., 1982.) Tepebağ, Bülent.; Altıntaş, Sabri.
In this thesis, the motion and vibration characteristics of edge dislocations are investigated. Topics included, are shear stress-dislocation velocity relation; effects of model parameters and phenomenological damping on the dislocation motion; motion of a dislocation in a lattice with some impurities such as one or two fixed atoms; vibration characteristics of pinned dislocations; effects of temperature, damping and geometric parameters on the vibration characteristics of dislocations. The importance of this subject is due to the major role of dislocations on the plastic deformation of materials. By the improvement of knowledge about the dislocation behaviour, it would be possible to produce more appropriate materials for engineering use. The model is a two-dimensional, atomistic one. For every atom on the lattice, the governing equation of motion is derived, and it is solved asa function of time, numerically. To perform the calculations, a computer program is used. The results obtained for several parameters are presented, and a discussion about the results is included. At the end of the thesis, a list of recommendations for a continuation of this study is given.
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 finite element analysis for flow in porous media
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 1993., 1993.) Atalık, Salim Kunt.; Örs, Haluk.
A finite element analysis for porous media is aimed. In the thesis, it is accepted that groundwater flow occurs only in the saturated region of a porous soil. The problem is formulated based on Darcy's law governing groundwater flow under the assumption that the flow is laminar and that inertial forces are negligible compared to the viscous forces. The finite element method is chosen for the solution of the problem. A finite element code is developed in FORTRAN. Finite element meshes are generated for the confined seepage and confined and unconfined aquifer problems. Head distributions for different cases are plotted. The results have been compared to those in the literature.
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.
A numerical approach for predicting hemodynamic characteristics of 3D aorta geometry under pulsatile turbulent blood flow conditions using fluid-structure interaction
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Saat, Ahmet.; Atalık, Salim Kunt.
Cardiovascular diseases are the leading cause of death all around the world and harm the society in terms of economically, socially, and psychologically. Hence diagnosing cardiovascular diseases as early as possible has become vital circumstance. Since clinicians need reliable and fast numerical approaches for their urgent pre-surgery decisions, individualised risk prediction and virtual treatment planning, CFD has become widespread in biomedical especially in cardiovascular medicine. The main aim of current study is to provide insight to hemodynamic characteristics of 3-D aorta geometry with pulsatile turbulent blood flow. In line with this purpose, blood and vessel mechanism has been evaluated through numerical fluid-structure interaction (FSI) analysis that couples computational fluid dynamics (CFD) and finite element analysis (FEA). Besides, effects of turbulence modelling, viscous effects and solid domain parameters such as artery thickness, elastic modulus and Poisson’s ratio on hemodynamic characteristics have been investigated. The investigations are carried out by using twelve turbulence models, two Non- Newtonian models and different solid domain values to compare output parameters such as oscillatory shear index, velocity field characteristics, von-Mises stress and displacement. Results have shown that SST k-omega with low-Re corrections model seem to be better capable of predicting hemodynamic characteristics. Proposed computational model can be considered as an initial work for the digital twin of cardiovascular system which is described as the realistic virtual model.
A phenomenological approach to stress corrosion cracking
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 1986., 1986.) Nehrozoğlu, Mehmet Aydemir.; Vardar, Öktem.
The applicability of the J-integral to SSC in the elasticplastic range is investigated by using precracked cantilever beam specimens of low carbon steel in 6N H2S04 solution. The correlation of J-integral with the incubation time suggests the existence of a threshold value of J.
A solution for fully developed turbulent heat transfer in a circular tube with uniform wall temperature, using a continuous velocity profile
(Thesis (M.S.) - Robert College. Faculty of Graduate School of Engineering, 1970., 1970.) Merttürk, İbrahim.; Umur, Aydın.
Heat transfer coefficients and temperature profiles for fully developed turbulent pipe flow with uniform wall temperature are predicted for P 1. The continous and differentiable velocity profile of Pai (1)1 is used to integrate the energy equation numerically. Turbulent pipe flow is assumed to be steady, and thermally and hydrodynamically fully developed. The ratio of the eddy diffusivities is taken to be unity. Axial conduction is neglected, and the flow is assumed to be axisymmetric with constant fluid properties. The results obtained are presented as curves and tables. Three correlating equations are given which predict the Nusselt number for liquid-metals and for gases.
A strain energy based multiaxial high-cycle fatigue life evaluation model for notched structures
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2020., 2020.) Gezer, Burak.; Sönmez, Fazıl Önder.
In this study, a model is presented for high-cycle fatigue-life assessment of notched engineering components subjected to constant amplitude multiaxial proportional loads. The algorithm requires only material-dependent parameters available in material data sheets. Applicability of the proposed approach is independent of any experimentally determined factor for geometry as well as loading conditions. Instead, the effects of such factors are taken into consideration in this study through a newly introduced parameter called equivalent strain energy density. The formalized fatigue design methodology estimates the fatigue life by directly using elastic stress and strain fields around the notch in conjunction with Coffin Manson-Basquin law. Stress/strain states developed in the part are determined via linear elastic finite element analysis. Predictions of the presented model are compared with many experimental results reported in the literature for specimens with various notch geometries made of different materials, including aluminium alloys and steels, subjected to different types of multi-axial loads and a remarkable level of accuracy is reached.
A study of manipulator dynamics and control
(Thesis (M.S.)- Bogazici University. Institute for Graduate Studies in Science and Engineering, 1984., 1984.) Akmehmet, Asaf.; Kuzucu, Ahmet.
In this work, the dynamics and control of a manipulator arm are investigated. Two different forms of the dynamic model of an n degree of freedom manipulator are studied and general computer oriented algorithms are developed to obtain the system matrices of the dynamic model. The algorithms are based on Newtonian mechanics; they/are recursive and independent of the manipulator configuration. For the purpose of control of a manipulator arm two different control schemes are proposed which are based on minimum energy optimal control. In the first method the system is decomposed into n subsystems and each subsystem is controlled independently while, in the second method the dynamic coupling among subsystems is taken into account. Computer simulations-are carried out for two different manipulator models in order to investigate the effectiveness of the proposed control methods, and it is seen that the proposed suboptimal adaptive feedback law gives reasonably good results.
A study on a thermal power plant modelling and control
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 1984., 1984.) Tanık, Mustafa.; Kuzucu, Ahmet.
In this study, a deterministic mathematical model of Anbarlı. Thermal Power Plant is developed from physcal laws. Resultant model is used to design the steady state regulator for 100.5 MW load level using linear optimal control theory. Although the resultant model is primarily designed for steady state control, it can be easily adapted for other Thermal Power Plant configurations and control studies. Computer simulations are carried out to investigate the dynamic behaviour of the controlled system under disturbances in generation. The simulation results indicate the feasibility of applying linear optimal control to a Thermal Power Plant based on minimization of a quadratic cost functional related to state yariables ana control inputs.
A study on parameters affecting the performance of turbulance amplifiers
(Thesis (M.S.) - Robert College. Faculty of Graduate School of Engineering, 1970., 1970.) Biringen, Sedat.; Tezel, Akın.
The turbulance amplifier is one of the two pure digital fluidic elements, and operates on the principle that a laminar jet issuing from a cylindrical tube will remain laminar up to a distance of approximately 100 times the diameter of the tube. If a second tube is placed in the jet path, a part of the laminar flmv can be captured by it, to produce an output pressure, this pressure depending on the average velocity of the jet portion intercepted. If a control tube is placed at right angles to the main jet, disturbances can be introduced into the laminar supply jet by fluid streams of lower energy issuing from the control tube. As a result of this, output pressure will decrease in the output tube. In the work that follows, various parameters, which have pronounced effects on the input-output relations in a turbulance amplifier are considered and in view of dimensional analysis a functional relation between these parameters is obtained. Investigation of jet flow phenomena is based on Schlichting's solution for circular, laminar jets in infinite flow fields; various assumptions and approximations are made to apply this solution to jet flow in a finite field turbulance amplifiers. The analytic study of mixing of jets is based on momentum conservation principle at the point of impinge of control and supply jets. Assumptions and approximations made are verified partially by a series of experiments performed on various models, and partially by experimental data available in literature.
A study on the optimum design charecteristics of compact heat-exchangers used in aircraft air-conditioning systems
(Thesis (M.S.)- Robert College. Faculty of Graduate School of Engineering, 1961., 1961.) Semerciyan, Mikayel.; Kiper, Ali M.
A study on the performance of a 1-2 parallel counterflow oil heat exchanger
(Thesis (M.S.)- Robert College. Faculty of Graduate School of Engineering, 1969., 1969.) Özsoy, Bülent.; Tezel, Akın.
A system of cracks in an elastic half-plane
(Thesis (M.S.)- Bogazici University. Institute for Graduate Studies in Science and Engineering, 1981., 1981.) Alnıak, M. Oktay.; Civelek, M. Başar.; Altıntaş, Sabri.
The plane elastostatic problem of an isotropic elastic half-plane containing an arbitrary system of cracks is considered. Fourier transform technique is employed to obtain the edge dislocation solutions. Using the edge dislocation solutions as Green's functions, the problem is formulated in terms of a system of singular integral equations. The system of singular integral equations is solved numerically for various crack combinations and external loads. The stress intensity factors and the probably cleavage angles are studied in detail.
A two parameter characterization of edge cracked NiTi shape memory alloy under plane strain conditions
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Merde, Oğuzhan Fikri.; Alkan, Sertan.
Shape memory alloys (SMAs) are metallic systems that exhibit reversible, diffusionless, martensitic phase transformation. Employing finite element analyses, the stress fields and crack tip constraints generated are examined for a NiTi SMA which exhibits superelastic behavior. For this purpose, a single edge cracked configuration satisfying plane strain conditions is subjected to uniform loading. Both pure Mode I and mixed mode (Mode I + Mode II) configurations are elaborated by changing the crack inclination angle. As a novel step, a multi-parameter fracture mechanics approach is adapted to characterize the dependence of stress field components on both asymptotic r−1/2 and radial ro terms around the crack tip. This task is accomplished by generating closed-form fitting expressions for stress components via nonlinear leastsquare regression of the full field data from finite element analyses. It has been shown that ro term plays a significant role on the stress field around the crack tip in NiTi SMAs. In characterization of crack tip constraint in NiTi, stress triaxiality parameter, Q, is utilized in the present work. To quantify the behavior of Q, the material characteristics of NiTi such as transformation start and end stresses, hardening modulus and transformation strain are varied under both pure Mode I and mixed mode configurations. The results show that martensitic transformation has an effect of stress constraint relaxation effect reflected by the decrease of Q parameter. Meanwhile promotion of transformation start stress is found to have a strong contribution in constraining crack tip, the transformation end stress is observed to have negligible effect.
Accurate thermal characterization and optimization of high lumen led array fixtures
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2020., 2020.) Kundakçıoğlu, Haluk.; Dönmezer Akgül, Fatma Nazlı.
High lumen light emitting diode (LED) arrays have been preferred in di erent applications due to their low energy consumption. To maintain the performance of LEDs, heat generated by the LED chips should be removed e ectively. Junction temperature can be estimated by thermal models utilizing constant thermal resistances provided in LED datasheets. However, constant thermal resistance used in simulations often neglect the temperature and thickness dependent thermal conductivity of chip layers, which becomes more important at higher temperatures. To include this e ect, a multiscale thermal model will be built via COMSOL. First step will include important package features such as LED array and the heat sink and the second step will include the detailed analysis of an LED chip by including temperature and thickness dependent thermal conductivity of chip. The model will be used to analyze junction temperature sensitivity. Today, heat sinks that provide passive thermal management with n structures are commonly used for e ective heat removal from LEDs. Optimization of the heat sink based on thermal needs is crucial for better design of LED arrays. The optimization model will be used to optimize the LED structure by using only three di erent inputs such as width and length of the n, and the heat that is desired to spread. And the result of these optimization are applied to the multiscale thermal model to see the real e ect of these optimizations on the junction temperature. The results of this study show that the junction temperature values with multiscale model are higher than using constant thermal resistance values from the product datasheet. In addition, to design a high lumen LED array, di erent priorities can be used according to user demand and di erent initial LED array designs are obtained from the optimization result.
Active noise control in a duct with flow
(Thesis (M.S.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2012., 2012.) Yüksel, Osman.; Yılmaz, Çetin.; Eşkinat, Eşref.
In this thesis, active noise control in a duct with ow is investigated. A one dimensional acoustic duct model, in which uid medium inside the duct has a mean ow velocity, is presented. The acoustic duct model is solved in Laplace domain and in nite dimensional system transfer functions are obtained. For controller designs, appropriate microphone and noise canceling source locations inside the duct are determined. In numerical studies, ideal boundary condition case (open end) and general boundary condition case (frequency dependent impedance end) are investigated. For these boundary conditions, low order nite dimensional transfer function approximations of actual system transfer functions are obtained. It is found that, in a selected frequency range, approximations represent actual system in a satisfactory way. By using approximated system transfer functions, low order optimal H2 and H1 controllers are synthesized via linear matrix inequalities method found in linear time invariant nite dimensional control theory. Closed loop frequency response and time domain simulations show that the controllers successfully suppress unwanted sound which propagates along the duct.

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