Master Degree / Yüksek Lisans Tezleri
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Master Thesis A Computational Fluid Dynamics Investigation of Motion Mitigation on a Floating Object Containing Tuned Liquid Column Damper(01. Izmir Institute of Technology, 2024) Zengin, Ramazan Kadir; Özkol, ÜnverNet sıfır emisyon enerji sistemlerine yönelik küresel dönüşüm, sürdürülebilir gelişmelere ve yenilenebilir enerji alternatiflerine olan ilgiyi artırmış, rüzgâr bu bağlamda ön plana çıkmıştır. Ancak, geleneksel yöntemler derin su bölgelerindeki rüzgâr kaynaklarına erişimde zorlanmaktadır. Denizüstü Yüzer Rüzgar Türbinleri (YRT), bu engeli aşarak daha önce erişilemeyen derin su sahalarından rüzgâr enerjisi elde etmeyi mümkün kılmıştır. YRT'lerin ömrünü uzatmak için istenmeyen yük ve hareketlerin en aza indirilmesi kritik öneme sahiptir. Bu proje, OpenFOAM kullanarak YRT'lerin yüksek doğruluklu ve entegre bir simülasyon metodolojisini araştırmaktadır. Dalga üretimi ve sönümlemesi için 'sönümleme bölgesi' yöntemini kullanan waves2Foam aracı (Jacobsen vd. 2012) kullanılmış ve demirleme kuvvetleri yarı sabit katener modeli ile hesaplanmıştır. Çok fazlı simülasyon, dinamik ağ teknikleri entegre edilen waveDyMFoam çözücüsü ile gerçekleştirilmiştir. Akışkan-Yapı Etkileşimi (AYE) bağlantısı, PIMPLE metoduna dayalı, seri alt yineleme stratejisi ile sağlanmıştır. Metodoloji, Ayarlanmış Sıvı Kolon Sönümleyici (ASKS) uygulaması ile adım adım geliştirilmiş ve mevcut çalışmalarla doğrulanmıştır. Ardından, bir Dalga Enerji Dönüştürücüsü (DED) üzerinde serbest bozunma analizi yapılmış ve ASKS'nin farklı kütle oranları altında sönümleme performansı doğrulanmıştır. Serbest bozunma koşullarında %4 kütle oranlı ASKS uygulaması ile hareket azaltımı %47,80, düzenli dalga koşullarında ise %37,01 olarak elde edilmiştir. Bu metodoloji, ASKS ve Ayarlanmış Sıvı Çok Kolonlu Damper (ASÇKS) uygulamalarının dalga koşulları altında yüzer nesneler üzerindeki sönümleme performansını başarıyla göstermekte olup, YRT modellemesi için ASKS'nin güvenilir bir teknik olduğunu kanıtlamaktadır.Master Thesis Investigation of Coupling Methods for Floating Offshore Wind Turbines and Their Application for an Innovative Floating Platform(01. Izmir Institute of Technology, 2024) Değirmenci, Olcay; Özkol, ÜnverFosil yakıtların yerini alacak daha çevreci enerji kaynaklarına olan ilgi arttıkça, çeşitli enerji alanları da devreye girmektedir. Bunlardan biri ve giderek artan potansiyeli ile dikkat çeken deniz üstü yüzer rüzgâr enerjisi, önemli araştırma konularından biri olarak platformun OpenFAST gibi aero-hidro-servo-elastik programlara entegrasyonu veya benzer aero-hidro kuvvetlerinin bir arada verilmesi yöntemlerinin incelenmesini kapsamaktadır. Bu tez kapsamında daha önceden 300 kW ve 5 MW değerleri için tasarlanmış olan yenilikçi yüzer platformun 5 MW'lık NREL referans rüzgâr türbini için ölçüleri ve özellikleri analizlerde kullanıma uygun şekilde düzenlenmiştir. Literatürde yaygın olarak kullanılan spar, semi-submersible platformlara ek olarak yenilikçi platform için akuple yöntemleri incelenmiştir. Türbinlerin ve platformların sistem tepkilerini ve davranışlarını görmek için doğal periyotları ve sönüm oranlarını bulma, serbest salınım testleri, düzenli, düzensiz dalga koşullarında, sabit ve türbülanslı rüzgâr koşulları dahil çeşitli koşullarda analizler yapılmıştır. Bunun sonucunda, akuple yöntemlerinin doğrulanması, platformların birbirleriyle karşılaştırılması, yenilikçi platformun esnek ve rijit yapısındaki farklı davranışlar incelenmiştir. Bu bağlamda, OpenFAST'a girdi olarak hidrodinamik katsayı sağlayan açık kaynaklı Capytaine ve NEMOH gibi programların yanı sıra AQWA gibi ticari programların, WAMIT gibi yaygın olarak kullanılan ticari programın OpenFAST çıktılarıyla benzer sonuçlar verdiği görülmüştür. Ayrıca, F2A akuple yönteminin de bu sonuçlarla oldukça tutarlı olduğu gözlemlenmiştir. Ayrıca, platformlar karşılaştırıldığında, bağlama halatlarındaki kuvvetlerin özgün platformda diğer platformlara göre daha yüksek olmasına rağmen, özellikle sistemin kararlılığı açısından önemli bir kriter olan platformun yunuslama açısının önemli ölçüde daha düşük olması sebebiyle, kule, kanat ve sapma yatağı gibi bileşenler üzerinde diğer platformlara kıyasla daha az yük oluştuğu hesaplanmıştır.Master Thesis Investigation of the Effect of the Spacing Between Orifices on the Damping Performance in Passive-Tuned Liquid Column Dampers(01. Izmir Institute of Technology, 2024) Karakuş, Ozan; Özkol, ÜnverYapısal hareketler, sönümleyc tasarımı ve bunların lşkler yapı dnamğnn ana konularıdır. Sönümleme sstemler, oluşablecek her türlü dış uyartımı dkkate alarak yapıları stablze etmek çn tasarlanmıştır. Yapısal hareketlern azaltılması yapıların bütünlüğünü koruyablmes açısından son derece önemldr. Bu sönümleme sstemler genel olarak aktf ve pasf sönümleme sstemler olmak üzere k sınıfta toplanır. Pasf Ayarlı Sıvı Kolon Sönümleycler -TLCD- bast yapıları ve dışarıdan br enerj grş gerektrmemes nedenyle pratkte terch edlen sstemlerdr ve bu nedenle yapısal salınımların bastırılmasında etkn br şeklde kullanılablrler. Bu tezde, TLCD'nn sönüm performansı, özellkle yük kaybı katsayısının sıvı kolon performansı üzerndek etks dkkate alınarak ncelenmştr. Analtk br model üretld ve hareket denklem 1 boyutlu olarak çözüldü. İk delğn brbrne yakın olması durumunda analtk çözümlern doğrulanması ve analtk modeln yeterl olmadığı durumlarda da blg elde etmek amacıyla deneyler yapılmıştır. Yük kaybı katsayısının hesaplanması brçok çalışmada araştırılmış ve amprk formülasyonlar önerlmştr, ancak sıvı kolonun yük kaybı değer br dz deney uygulanarak elde edleblr. Ayarlanmış sıvı kolon dampernn yük kaybı değern elde etmek çn deneysel br tasarım kullanıldı. Yük kaybı katsayısının, ayarlanmış sıvı kolon damperlernn sönümleme performansı üzerndek etksn gözlemlemek çn br dz deney tamamlandı. Deneysel ve analtk sonuçlara göre sstemn sönüm oranı le dampern yük kayıp katsayısı arasında br lşk tespt edlmştr. TLCD'nn performansının maksmuma çıkarılableceğ k delk arasında optmum br aralık olduğu deneysel olarak tespt edlmştr. Ayrıca deneysel sonuçlara göre sstemn sönümleme katsayısının %10.5 oranında gelştğ not edlmştr.Master Thesis Model Design and Experimental Investigation of Floating Wind Turbine(01. Izmir Institute of Technology, 2022) Arıdıcı, Ali; Özkol, ÜnverFloating offshore wind energy has great potential (which constitutes almost 80% of total offshore wind energy) to meet electricity demand of the world at the same time to reach net-zero emission goal by 2050. Floating offshore wind turbines (FOWT) are able to achieve highest capacity factor since local effects of the offshore terrains are lesser. Thus, it receives stronger and more stable wind. On the other hand, combined hydrodynamic and aerodynamic forces with 6 degrees of freedom (DoF) bring unsteadiness and there- fore, challenges on FOWT design. Furthermore, significant rotational motions, particu- larly pitch motion, lead the turbine to transient state which can not be simulated through conventional numerical tools. Therefore, to understand the dynamics of the FOWT, it is necessary to conduct experimental studies to obtain results by considering all the param- eters. The main aim of the thesis is to investigate the dynamic response of the FOWT under the extreme wind and wave conditions. A 1/40 Froude-scaled version of the Northel POYRA P36/300 mounted on the spar-type floating platform was developed by colleagues as a part of TUBITAK (217M451) project. In this thesis, experimental studies were car- ried out in the wave flume with a wind nozzle in the hydraulic laboratory of IZTECH Civil Engineering Department. Atmospheric boundary layer (ABL) was scaled, and in- struments of the experiment were calibrated to characterize wind nozzle and wave maker, which are vital to obtaining reliable results. The wind nozzle was designed based on experimental data to reproduce correct Froude-scaled ABL.Master Thesis Investigations on Nanoscale Wetting, Fluid Transport, and Droplet Evaporation at Nanostructured Surfaces by Molecular Dynamics Simulations(01. Izmir Institute of Technology, 2021) Şatıroğlu, Ezgi; Barışık, Murat; Özkol, ÜnverThere is a significant need to understand solid-liquid interactions at nanoscale to determine the fluid behavior in several revolutionary applications. Specifically, nanoscale surface wetting, nanoscale liquid transport, and nanoscale heat transfer are the most sought-after subjects in recent scientific and industrial applications. This thesis focuses on characterization and possible control of wetting, fluid flow, and heat transfer using nanoscale surface structures. First, wetting behavior on a nanostructured surface was studied to resolve contact angle hysteresis. The droplet was found stabilized at a metastable state with a contact angle significantly different from its equilibrium value due to contact line pinning from the surface asperities. The contact angle was found to increase linearly by increasing droplet size when the droplet is pinned. However, these pinning effects become negligible, and the contact angle reaches the equilibrium value of the corresponding surface when the surface structure size becomes negligible compared to droplet size. Second, fluid flow in nanostructured nanochannels was studied to determine the transport behavior. While the slip boundary condition on a smooth surface correlated with the wetting angle, transport in a nanostructured channel remained mostly independent from wetting condition of the corresponding surface structure. Lastly, droplet evaporation over nanopatterned surfaces was investigated. When the droplet temperature reached the Leidenfrost point, a sudden increase in the interface thermal resistance was observed, which significantly decreased the heat transfer to the droplet. Increasing the size of the surface structure pushed the Leidenfrost point to higher surface temperatures. Current results contribute to various disciplines in engineering and applied sciences.Master Thesis Investigation on Fluctuating Forces on Horizontal Axis Wind Turbines Due To Tower Shadow Effect(01. Izmir Institute of Technology, 2020) Demircioğlu, Hasan Metehan; Özkol, ÜnverIn order to assess aerodynamic performance of wind turbines an in house Blade Element Momentum (BEM) code is developed in Python programming language. Blade element momentum theory is a powerful tool to predict aerodynamic performance of wind turbines and it requires relatively low computational time compared to other aerodynamic performance analysis tools. Therefore, it is frequently used by wind turbine manufacturers especially, at the initial design phase. In this method, wind turbine blade is divided into multiple stations and it is assumed that there are no aerodynamic interactions between these stations. Blade element computations are performed for a given wind speed, rotational speed, blade radius, number of blades, airfoil profiles, twist angle and chord length along the blade span. Aerodynamic data, related to airfoils are taken from experimental studies in the literature. Later, airfoil data were updated by integrating National Renewable Energy Laboratory (NREL) airfoilprep.py tool to in house BEM code. Airfoilprep.py contains stall delay model and Viterna extrapolation. Stall delay model refers to correction of 2D airfoil data to account 3D effects caused by rotation of blades. Viterna extrapolation is used to extend airfoil foil data obtained from experiments or computational tools to 360o angle of attack range since wind turbines often operate at high angles of attack values. Finally, interaction of rotor and tower to power production is implemented in BEM code. Effect of tower presence is modelled by applying potential flow to non-rotating cylinder. Lastly, results obtained from BEM simulations are compared with the data obtained from NREL's Phase VI unsteady experimental study.Master Thesis Spray Characteristics of Emulsified Biodiesel-Diesel Blends in a Constant Volume Combustion Chamber(01. Izmir Institute of Technology, 2020) Tezel, Yusufcan; Özkol, Ünver; Özkol, ÜnverOver the last decades, various studies have been carried out by the researchers to find out an alternative fuel that can overcome emission problems caused by diesel fuel which affect the environment and human health significantly. Due to emulsified biodiesel-diesel blend (EBB) fuels are being a possible alternative fuel for diesel, in this study, it was aimed to investigate the macroscopic spray parameters such as spray penetration length and cone angle of different EBB fuels, namely B20W15 and B20W5, containing 15% and 5% water by volume, respectively. In order to examine the spray characteristics of B20W15 and B20W5, the experiments were carried out by means of constant volume combustion chamber and utilizing shadowgraph technique with a high-speed camera. Experiments were performed with 600 bar and 800 bar injection pressure while the ambient pressure was 0 bar, 5 bar and 10 bar, respectively. After experiments were fulfilled, the recorded images of test fuels were processed via ImageJ program. The results showed that stability of the emulsion can be obtained when HLB value was 8 and surfactant concentration was 5% by volume. It was understood that increment in water concentration in the emulsion caused deterioration in emulsion stability while it led higher viscosity, higher density, longer spray penetration lengths and narrower cone angles. It was observed that B20W5 resulted wider spray cone angles and shorter spray penetration lengths than B20W15 under 0,5 and 10 bar chamber pressure with injections pressures of 600 and 800 bar. Also, it was understood that both EBB lead longer spray penetration lengths and narrower spray cone angles compared to reference diesel while they lead vice versa compared to reference biodiesel. The reason of spray geometry difference between diesel and EBB fuels can be associated with the higher viscosity and density of EBB fuels compared to diesel. As a result, considering that B20W5 reduce the use of fossil fuels, and no significant difference compared to diesel in terms of spray geometry, it can be said that B20W5 may be a promising alternative fuel for the futurMaster Thesis Numerical Detection of Cavitation in Plunger Valves(01. Izmir Institute of Technology, 2020) Akbulut, Bedia; Özkol, ÜnverCavitation is a very serious problem for control valves. Besides noise and vibration, cavitation can seriously damage mechanical parts. Experimental costs for cavitation tests are high, especially when the difficulty of testing large sized valves is considered. For these reasons, developing a Computational Fluid Dynamics (CFD) model can be an attractive solution for predicting cavitation. Cavitation; is a phase change event where the bubbles that occur when the fluid pressure drops below the vapor pressure seriously damage the parts of the machines such as pumps, impellers, and valves. The first aim of this research is to determine the onset of cavitation by performing two-phase CFD simulations in plunger valves. Then, several cavitation cages are connected to observe the change at the beginning of cavitation. A cavitation cage is used to protect the valve and valve disc when downstream pressure is too low. As a result of the analysis, it was observed that the cavitation delayed using the cavitation cage. The second purpose of this research is to design an interface program that will present the Loss and Flow Coefficient diagrams obtained from tests and CFD analysis to the user. Each valve has its own flow coefficient. This depends on how the valve is designed to allow flow through the valve. Therefore, the main differences between the different flow coefficients are due to the valve type and of course the valve opening position. The flow coefficient is important to choose the best valve to use in a particular application.Master Thesis Experimental Investigation of the Pressure Drop Performance and the Cavitation of a Plunger Valve(Izmir Institute of Technology, 2020) Aydeniz, Emre; Özkol, ÜnverThis thesis, realized within the scope of a TUBITAK project of DOĞUġ VANA A.ġ, is aimed to characterize the pressure drop and cavitation performance of plunger valve with various cavitation lattices in different opening ratios. Pressure drop performance and cavitation studies have been examined experimentally. Loss coefficients and flow coefficients were used to examine the pressure drop performance of four different types of cavitation lattices. According to the results of this experimental study, it was observed that the loss coefficient is almost independent of Reynolds number except for Re < 4x105. Cavitation creates vibration and noise in high-frequency ranges. Therefore, cavitation detection was examined in the high-frequency range of 5 kHz to 18 kHz and was performed using an accelerometer. Fourier Transform (FT) and Power Spectrum Density (PSD) were used to measure the energy levels of vibrations. Cavitation limits (incipient and critical cavitation) were detected by comparing PSD levels and cavitation indexes at different aperture ratios for all lattices. As a result, the safe cavitation range for the valve was determined by comparing the cavitation indexes according to the valve opening ratio. A new cavitation index estimation study was performed for the new diameter and upstream pressure values with the effect of size scaling and pressure scaling.Master Thesis Design of Centrifugal Blood Pump(01. Izmir Institute of Technology, 2020) Sarızeybek, Ceren; Özkol, ÜnverIn this thesis a maglev centrifugal blood pump is studied numerically which will be used in ECMO systems. Aim was to design a pump with 3.3 L/min at 5000 rpm having 200 mmHg head. Pump is designed by SolidWork and CFD studies are applied in Ansys Fluent. Firstly, blade discharge angle values between 20 degree and 40 degree are simulated with 5 degree intervals and 35 degree is found to be the most appropriate for hydraulic efficiency and head. Likewise, six different up-clearance values between 2.0 mm and 0.1 mm are studied to see effect of up-clearance on head and efficiency. Also to predict hemolysis for different up-clearance values, Giersiepen power law hemolysis model with Lagrangian approach is employed. 0.1 mm up-clearance is found suitable. For this up-clearance hemolysis is calculated as 4%.