Phd Degree / Doktora

Permanent URI for this collectionhttps://hdl.handle.net/11147/2869

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Author: search.filters.author.Yılmaz, OğuzHas files: Yes

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  • Doctoral Thesis
    Direct and Interior Inverse Generalized Impedance Problems for the Modified Helmholtz Equation
    (01. Izmir Institute of Technology, 2022) Özdemir, Gazi; Ivanyshyn Yaman, Olha; Yılmaz, Oğuz
    Our research is motivated by the classical inverse scattering problem to reconstruct impedance functions. This problem is ill-posed and nonlinear. This problem can be solved by Newton-type iterative and regularization methods. In the first part, we suggest numerical methods for resolving the generalized impedance boundary value problem for the modified Helmholtz equation. We follow some strategies to solve it. The strategies of the first method are founded on the idea that the problem can be reduced to the boundary integral equation with a hyper-singular kernel. While the strategy of the second approach makes use of the concept of numerical differentiation, the first approach treats the hyper singular integral operator by splitting off the singularity. We also show the convergence of the first method in the Sobolev sense and the solvability of the boundary integral equation. We give numerical examples which show exponential convergence for analytical data. In the second part of this work, we take into account the inverse scattering problem of reconstructing the cavity’s surface impedance from sources and measurements positioned on a curve within it. For the approximate solution of an ill-posed and nonlinear problem, we propose a direct and hybrid method which is a Newton-type method based on a boundary integral equation approach for the boundary value problem for the modified Helmholtz equation. As a consequence of this, the numerical algorithm combines the benefits of direct and iterative schemes and has the same level of accuracy as a Newton-type method while not requiring an initial guess. The results are confirmed by numerical examples which show that the numerical method is feasible and effective.
  • Doctoral Thesis
    Asymptotic Behaviour of Gravity Driven Free Surface Flows Resulting From Cavity Collapse
    (Izmir Institute of Technology, 2020) Fetahu, Elona; Yılmaz, Oğuz
    In this thesis, the gravity driven potential flows that result from cavity collapse are studied. Initially, the collapse of a vertical cylindrical cavity of circular cross sections surrounded by a liquid region is examined for two different situations. In the first one the cavity has same depth as the fluid and in the second one the cavity starts from the free surface and has less depth than the fluid. The problem is formulated by using a small parameter that represents the short duration of the stage. The first problem, as the radius and the centre of the cavity approach infinity, reduces to the classical two-dimensional dam break problem solved by Korobkin and Yilmaz (2009). The singularity of the radial velocity at the bottom circle is shown to be of logarithmic type. In the second problem, where the cavity is less deep than the fluid, the flow region is separated into two regions: the interior one, which is underneath the cylindrical cavity and above the rigid bottom, and the exterior one, which is the rest of the flow. The corresponding new problems are solved separately and then the coefficients are found by applying the matching conditions at the interface, where the fluid radial velocities and pressures coincide. On the limiting case, the problem reduces to the two-dimensional dam break flow of two immiscible fluids by Yilmaz et al. (2013a). Singularity at the bottom circle of the cavity is observed, which is of the same type as in the latter paper. Next, a third problem studies the gravity driven flow caused by the collapse of a rectangular section of a vertical plate. During the early stage, the flow is described by the velocity potential. Attention is paid to determining the velocity potential and free surface shapes. The solution follows the Fourier series method in Renzi and Dias (2013) and the boundary element method in Yilmaz et al. (2013a). Singularity is observed at the side edges and lower edge of the rectangular section. The horizontal velocity of the initially vertical free surface along the vertical line of symmetry of the rectangle is the same to the one in the two-dimensional problem Korobkin and Yilmaz (2009). The singularities observed in these problems lead to the jet formation for the initial stage. The methods applied in these computations are expected to be helpful in the analysis of gravity-driven flow free surface shapes. This thesis is a contribution towards the 3-D generalizations of dam break problems.
  • Doctoral Thesis
    Short Time Behaviour of Dam Break Flow Involving Two Liquids
    (Izmir Institute of Technology, 2018) Isıdıcı Demirel, Damla; Yılmaz, Oğuz
    The two dimensional dam break problem for wet bed case is investigated. The leading order and the second order problem are stated in nondimensional form. Solution to the leading order problem by using three different methods is given and explained in detail. Both Fourier series method and Galerkin method have difficulties on its own because of the singularity at the triple point. Although the singularity is ignored in Galerkin method, the method does not work except for the interface. Thus conformal mapping techniques is preferred because of the convenience and the strength of the complex analysis. The velocity profiles at whole boundary are obtained by using this conformal mapping. The second order solution of velocities are also obtained by using the same conformal mapping. On the other hand, the domain decomposition method (DDM) is applied for the second order dam break problem of dry bed case. The leading order solution helped to determine the suitable parameters for DDM. The leading order and second order solution of the free surfaces give a more realistic shape using the Lagrangian solution at the upper corner point. We assume this work contains useful and applicable methods in it for gravity driven flows and it will wake up different perspectives in readers mind.
  • Doctoral Thesis
    Solution of Maxwell Equations on Deformed Spherical Domains: Applications To the Scattering Problems
    (Izmir Institute of Technology, 2015) Ateş, Barış; Yılmaz, Oğuz
    In the present work, firstly we consider analytic solution of the Maxwell’s Equations in the vacuum in the presence of conducting deformed spherical body. Deformation is made in the normal direction of sphere with a small perturbation parameter and arbitrarily chosen smooth deformation function f ( ; φ). The azimuthal and polar angle dependence of the function is preserved till the end. Using the Debye Potentials the solution in the exterior domain of deformed conducting spherical body is given. In addition to this, scattering of electromagnetic plane waves from non-spherical dielectric and conducting objects are considered. In order to find scattered and transmitted fields, in contrast to common use of vector wave functions and their orthogonality properties, the scalar functions and orthogonalities of Associated Legendre Polynomials are used. All the surface integrals are evaluated analytically. The corrections to the coefficients of scattered and transmitted fields up to the second order are obtained and expressed in terms of the Clebsch-Gordon coefficients.
  • Doctoral Thesis
    Vortex Dynamics in Domains Whith Boundaries
    (Izmir Institute of Technology, 2011) Tülü, Serdar; Yılmaz, Oğuz
    In this thesis we consider the following problems: 1) The problem of fluid advection excited by point vortices in the presence of stationary cylinders (we also add a uniform flow to the systems). 2) The problem of motion of one vortex (or vortices) around cylinder(s). We also investigate integrable and chaotic cases of motion of two vortices around an oscillating cylinder in the presence of a uniform flow. In the fluid advection problems Milne-Thomson's Circle theorem and an analyticalnumerical solution in the form of an infinite power series are used to determine flow fields and the forces on the cylinder(s) are calculated by the Blasius theorem. In the "two vortices-one cylinder" case we generalize the problem by adding independent circulation k0 around the cylinder itself. We then write the conditions for force to be zero on the cylinder. The Hamiltonian for motion of two vortices in the case with no uniform flow and stationary cylinder is constructed and reduced. Also constant Hamiltonian (energy) curves are plotted when the system is shown to be integrable according to Liouville's definition. By adding uniform flow to the system and by allowing the cylinder to vibrate, we model the natural vibration of the cylinder in the flow field, which has applications in ocean engineering involving tethers or pipelines in a flow field. We conclude that in the chaotic case, forces on the cylinder may be considerably larger than those on the integrable case depending on the initial positions of the vortices, and that complex phenomena such as chaotic capture and escape occur when the initial positions lie in a certain region.