Ph.D. Theses
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Browsing Ph.D. Theses by Author "Ecder, Ali."
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Item 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.Item 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.Item Numerical analysis of surface-driven non-isothermal viscoelastic flow(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2010., 2010.) Kaptan, Yalın.; Ecder, Ali.; Atalık, Salim Kunt.The numerical investigations of the moving edge non-isothermal viscoelastic flows are simulated by using two example problems (lid driven cavity (LDC) and rotating disc in a cylindrical enclosure (RDCE) flows) in this study. The viscoelastic behavior of the fluids is modeled by adopting three differential constitutive relations namely Upper Convected Maxwell (UCM), Oldroyd B and Giesekus models. The comparisons reveal that the Giesekus model is the most realistic one and the maximum Weissenberg number limit is higher compared to the others. Two separate solvers are used in the simulations; PETSc and IN-GMRES solvers. PETSc code is used as a solver for the Newtonian flows and a benchmark tool for the Krylov subspace methods and preconditioners. PETSc analyses reveal that BiCGStab with ILU(5) preconditioning is the most effective solver in the simulations of the Newtonian flows. IN-GMRES solver is used to simulate the non-isothermal viscoelastic flows and it is based on the matrix free preconditioned inexact Newton-Krylov methods. To obtain higher Weissenberg number limits in the simulations, the numerical tools such as the continuation, the upwind differencing scheme, the higher order discretization schemes, the slanted stencils and similar others are implemented in the IN-GMRES algorithm. In the non-isothermal part of the study, besides the advection and diffusion, the viscous dissipation is also included and it is understood that the viscous dissipation is very important in simulations of non-Newtonian flows. The viscosity is modeled as temperature dependent by adopting the approximate Arrhenius formulation and it is realized that the viscosity changes can alter the flow field. The effects of the Reynolds number, the Weissenberg number, the Prandtl number, the Brinkman number, aspect ratio and some of the material parameters are documented within this study.Item Numerical investigation of high Knudsen number flow in rectangular enclosures(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2006., 2006.) Orhan, Mehmet.; Ecder, Ali.; Tezel, Akın.Nowadays, enlightening unknown aspects of rarefied gas flow is one of the critical issues of fluid dynamic research to ensure correct and proper operations of manyMicro- Electro-Mechanical-Systems (MEMS). Thermally driven motion of rarefied gases is gaining in importance to develop Knudsen compressors having better performance or to improve single crystal growth processes. Therefore, accurate prediction of the physics lying behind the thermal creep in the transition regime as well as slip flow regime is one of the main motivations of this study. The other emphasis is possible flow instability of the rarefied gases in enclosures. For this purpose, an asymptotic approximation has been performed in the first part of the study to find analytical solutions. In the second one, linear disturbance theory of hydrodynamic stability has been applied to the problem to determine bounds of instabilities. Analytical solutions of two-dimensional stability analysis have been introduced. Critical states have been identified for different models and for varying Knudsen numbers. More generally, eigen-spectrum of the perturbation equations has been identified in three-dimensions. At the last part, by applications of an artificial viscosity scheme, a computer program has been constructed to solve Burnett and also Navier-Stokes equations. Mechanisms of the thermal creep flow have also been verified by inspecting stress tensors of Burnett equations. Most importantly, the insufficiency and the failure of Navier-Stokes equations for the creeping flows have been proved. Moreover, it has been shown that Burnett equations can correctly model such creeping flows.