Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis Experimental and Numerical Analysis of the Strain Rate Dependent Compressive Strength of a Cellular Concrete(Izmir Institute of Technology, 2019) Akyol, Burak; Güden, Mustafa; Taşdemirci, AlperExperimental and numerical quasi-static and high strain rate tests, including compression, indentation and direct impact, were performed on a cellular concrete in order to investigate the effect of strain rate on the compressive strength. The results of compression tests indicated three distinct regions of the compressive strength dependence on strain rate. A relatively lower strain rate dependent compressive stress was found in the quasi-static strain rate-regime, 2x10-3-2x10-1 s-1, a relatively high strain rate dependent compressive stress in the dynamic strain rate-regime, 180-103 s-1 and a cut-off strength above 103 s-1. The dynamic increase factor (DIF=dynamic/static fracture strength) varied between 1 and 2.5 from quasi-static to dynamic strain rate-regime with a sharp increase after about 100 s-1. The indentation tests using 25 and 30 mm-diameter indenters in the quasi-static strain rate-regime (uniaxial state of strain) and resulted in moderate DIF values (1-1.13), very similar with those of the quasi-static compression tests (1-1.15). In the indentation tests, the DIF values significantly and also confirmed the numerically determined DIF values of concrete at 1000 s-1 (~1.30) without radial and axial inertia. The compression and direct impact tests in the Split Hopkinson Bar (SHPB) set-up were implemented numerically in LS-DYNA using an anisotropic strain rate insensitive material model, MAT_096 (MAT BRITTLE DAMAGE). The stress readings were performed at the specimen different locations of the SHPB and indicated that radial and axial inertia were dominant between 1 and 30 m s-1 (30-1000 s-1).Master Thesis Numerical Modeling of Jet Grouting Cells To Reduce Liquefaction(Izmir Institute of Technology, 2019) Gürbüz, Çağdaş; Ecemiş Zeren, NurhanThe importance of the preservation of historical and culturally important buildings is essential nowadays. While improving the performance of the buildings under dynamic loadings, it is essential to evaluate and improve the subsoil conditions. It is evident, that strengthening of the building will not provide the desired performance, if serious ground problems such as liquefaction are not eliminated during earthquake loading. In this study, liquefaction evaluation of the foundation soil of a historical building (Vali Konagi), which is in the Konak district of İzmir province, has been carried out. The simplified liquefaction assessment results based on the standard penetration tests showed that under 0.45g loading, the liquefaction problem could be observed. Therefore, soil improvement is necessary for the upper profile beneath this historical building. The jet grout cells, which is a new method were suggested as a soil improvement technique against the liquefaction of the soil below the building. The parameters related to the jet grout cells were determined, and the improved soil status was analyzed. The numerical analyses of the liquefaction investigation at unimproved and improved soil were compared by finite difference program FLAC-2D. The constitutive model (UBCSand), which can simulate liquefaction was used in the program. As a result, it is observed that; by using jet grout cells liquefaction was not triggered and deformations were kept under control.Master Thesis Finite Element Based Stabilized Methods for Time Dependent Convection-Diffusion Equation and Their Analysis(Izmir Institute of Technology, 2016) Yılmaz, Kemal Cem; Tanoğlu, GamzeThis study is focused on a Fourier stability and accuracy analysis of the time integration algorithms using generalized trapezioidal family of methods of scalar unsteady convection–diffusion equation with periodic boundary conditions. The discretization in space dimension is performed by standard Galerkin finite element formulation for low Peclet numbers and stabilized finite element formulation for large Peclet numbers. The stability analysis is performed namely by von-Neumann stability analysis. Accuracy is measured in terms of damping errors and phase speed errors. The behaviour of these temporal errors of the particular time stepping algorithms, i.e. forward Euler, Crank-Nicolson and backward Euler methods are compared with each other. Particular attention is given to the stabilized finite element formulation, that is the case where we consider high Peclet numbers. For this case, it is concluded that the Crank-Nicolson time stepping represents a better approximate solution compared to the other time integrators on transport process of an initial wave profile. Finally, at the end of the study, we derive a stabilization parameter under a particular condition on Courant number, which provides the relative phase speed error being almost equivalent to its optimal level, that is, the waves with different Fourier modes propagate almost in the same speed. Theoretical results are confirmed by a number of numerical experiments.Master Thesis Convergence Analysis and Numerical Solutions of the Fisher's and Benjamin-Bono Equations by Operator Splitting Method(Izmir Institute of Technology, 2014) Zürnacı, Fatma; Tanoğlu, GamzeThis thesis is concerned with the operator splitting method for the Fisher’s and Benjamin-Bono-Mahony type equations. We showthat the correct convergence rates inHs(R) space for Lie- Trotter and Strang splitting method which are obtained for these equations. In the proofs, the new framework originally introduced in (Holden, Lubich, and Risebro, 2013) is used. Numerical quadratures and Peano Kernel theorem, which is followed by the differentiation in Banach space are discussed In addition, we discuss the Sobolev space Hs(R) and give several properties of this space. With the help of these subjects, we derive error bounds for the first and second order splitting methods. Finally, we numerically check the convergence rates for the time step ∆t.Master Thesis Experimental and Numerical Analysis of Heat Transfer Performance of Off-Set Strip Fins(Izmir Institute of Technology, 2009) Durmaz, Gürcan; Özkol, Ünver; Özkol, ÜnverThe aim of this study is to computationally and experimentally investigate the heat transfer and pressure drop characteristics of an offset-strip fin. In the present study, experiments are conducted at the range of Reynolds number from 150 to 3500 and a 3-D numerical domain, which is investigated as a conjugate problem, is created for finite volume computations. The computations are conducted by assuming that the flow in the offset-strip fin channels is steady and laminar at the range of Reynolds numbers from 200 to 5000. In this thesis, the effects of the flow behaviors in the offset strip fin channels on Colbourn j factor, which is the non-dimensional form of heat transfer coefficient, and fanning friction f factor, which is the non-dimensional form of pressure drop, are investigated. Also, the heat transfer boundary conditions and the Prandtl numbers of the fluids are kept different for these fins in order to see the effect of those.The effect of Prandtl number is investigated by using air, 0.707 < Pr < 0.71 and water, 2 < Pr < 4.35 and ethylene glycol, 94 < Pr < 138. The effect of the thermal boundary conditions is investigated by using constant heat flux and uniform temperature. Moreover, all results are compared with Kays and London.s experiments (1964) and also the results of Manglik and Bergles.s correlations (1995). The results show a very good agreement between the results of Kays and London (1964) and of Manglik and Bergles.s correlations (1995). It is also observed that results obtained from the two alternatives for the thermal boundary condition are very close to each other. According to obtained results, it is concluded that our computational results from laminar flow assumption and experiments are reliable at almost all the range of Reynolds numbers studied.Master Thesis Numerical Analysis of Finned Downhole Heat Exchangers: a Parametric Study(Izmir Institute of Technology, 2002) Alpay, Selda; İlken, ZaferThis study investigates the performance of an U-type Downhole Heat Exchanger (DHE) with a new pipe arrangement, where circular fins are fitted around the legs of the DHE.In the present work the heat transfer performance of optimized DHE with circular fins is investigated and compared with that of optimized with bare type DHE. This study numerically models a well with a DHE to determine the heat flow that can be extracted by the DHE. A DHE program is written in BASIC language to investigate the heat transfer rate both for bare and finned type DHEs. In order to verify the accuracy of this program comparisons are made with an experimental work for bare type DHEs. Also a computational fluid dynamics program, FLUENT, is used to study fluid and heat flow processes in the well and DHE systems. The results of the FLUENT program are also used to compare the DHE program. The simulations carried out also enable us to determine the influence of the design parameters of the finned type DHEs. Some examples of optimized geometries are finally shown and discussed.Master Thesis Operator Splitting Methods for Non-Autonomous Differential Equations(Izmir Institute of Technology, 2011) Korkut, Sıla Övgü; Tanoğlu, GamzeIn this thesis, convergency and stability analysis are studied for the non-autonomous differential equations. Not only classical operator splitting methods; Lie Trother splitting, symmetrically weighted splitting and Strang splitting but also iterative splitting method which is recent popular technique of operator splitting methods are considered. We concentrate on how to improve the operator splitting methods with the help of the Magnus expansion. In addition, we construct a new symmetric iterative splitting scheme. Then, we also study its convergence properties by using the concepts of stability, consistency and order. For this purpose, we use C0 semigroup techniques. Finally, several numerical examples are illustrated in order to confirm our theoretical results by comparing the new symmetric iterative splitting method with frequently used operator splitting methods.