Browsing by Author "Sheikhi, Aidin."

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    Experimentally verified numerical simulation of single crystal growth process with a low melt height and an axial vibration
    (Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2018., 2018.) Sheikhi, Aidin.; Balıkçı, Ercan.
    This Ph.D. dissertation investigates experimental and numerical crystal growth of antimony-doped germanium (Ge-Sb) single crystals. The investigation is a part of the TUBITAK project 212M030. The single crystal growth of Ge-Sb from the melt is investigated by the Vertical Bridgman (VB), Axial Heat Processing (AHP), and Axial Vibrational Control (AVC) techniques. The effects of method dependent growth parameters on the quality of the grown single crystals have been analyzed. To this end, two different pulling rates (10 mm/h and 20 mm/h), different initial melt heights (5 mm, 10 mm, 14 mm, and 58 mm), and three different sets of vibrational parameters (2 mm amplitude and 0.25 Hz frequency, 0.25 mm amplitude and 1 Hz frequency, and 0.25 mm amplitude and 25 Hz frequency) are applied in the growth of seven different crystals. It is observed that the highest single crystal length with the most homogeneous solute redistribution and the least dislocation density are achieved in an AHP crystal which is grown with the lowest pulling rate (10 mm/h). However, it is determined that an appropriate control of the vibration parameters in the AVC technique makes it possible to achieve almost the same crystal quality with doubled growth rate, so the production yield is decreased. Moreover, global and local numerical simulations are performed in order to investigate the effects of the growth parameters on the convective flow patterns. Also, results of the numerical simulations contribute to make better and more reliable interpretations of the experimental observations. The simulation results provide useful information for the experimentalists to investigate the effects of growth parameters on the temperature and solute distribution, flow pattern, and the interface shape. According to the numerical results, it is possible to clarify how the insertion of the baffle, adjusting the melt height, and optimizing the vibrational parameters of the baffle contribute the thermal and the solutal homogenization in the melt, interface stability, and consequently improved crystal quality.
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    Level set analysis of two-fluid interfacial flows
    (Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2012., 2012.) Sheikhi, Aidin.; Ecder, Ali.
    In this study incompressible immiscible two-phase ows have been analyzed in two dimensions using Level Set method. Firstly, a brief explanation about other numerical methods existing in the field of modelling interfacial two-phase flows is presented. Then, Level Set methodology and its coupling with Navier-Stokes equations are introduced in detail. Discretization method used in this study is Finite Difference method and derivations of Level Set function and convective terms are done using fth order Weighted Essentially Non-Oscillatory (WENO) scheme. In order to solve the pressure field implicitly, a projection method is applied. Afterwards, the reliability of method is tested by making comparison with Smoothed Particle Hydrodynamic (SPH) method for di erent test cases such as Rayleigh-Taylor instability, rising bubble, droplet fall, bursting bubble at free surface and droplet fall onto free surface. Effects of non-dimensional parameters like Reynolds (Re) and Eotvos (Eo) numbers on the evolution of interface in two-phase flows are investigated. Lastly, dam breaking problem is considered in order to analyze the performance of Level Set method in simulating free surface flows.