LEE- Isı Akışkan Lisansüstü Programı- Yüksek Lisans

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http://hdl.handle.net/11527/20280

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A detailed assessment of the effects of 3D radial stacking on the aerodynamic performance of NASA Stage 37 rotor blade

(Graduate School, 2024-12-27) Ülger, Furkan ; Çadırcı, Sertaç ; 503211121 ; Heat and Fluid

Axial compressors are critical components of modern engineering systems, particularly used in applications such as gas turbines, jet engines, and industrial power plants. Compressors are designed to operate efficiently by moving and compressing large volumes of gas or fluid in the axial direction. The primary function of axial compressors is to increase the pressure of the working fluid while maintaining a continuous flow, which is essential for achieving high performance in energy conversion systems. The design and operation of axial compressors are governed by the principles of aerodynamics and thermodynamics. Rotor blades, which rotate around a central axis, impart kinetic energy to the fluid, while stator blades positioned between the rotors, convert this kinetic energy into pressure energy. This staged compression process allows axial compressors to achieve high pressure ratios with minimal energy losses, making them ideal for applications requiring high efficiency. Axial compressors offer advantages such as high mass flow rates, compact designs, and adaptability to various operating conditions. However, during the design phase, factors like blade geometry, flow stability, and mechanical stresses must be carefully considered. These factors are crucial for preventing issues such as flow separation, stall, and surge. Advanced computational tools and experimental methods are frequently employed to optimize the performance of axial compressors, ensuring their efficiency and reliability in demanding applications. This thesis aims to investigate in detail the effects of modifications to the three-dimensional geometry of the rotor blade from the Stage 37 study, designed and tested by the National Aeronautics and Space Administration (NASA). These modifications are achieved by altering the stacking of the two-dimensional airfoils along the radial axis without changing their original two-dimensional design. The primary objective is to comprehensively analyze how these changes influence the performance of the rotor blade comprehensively. The flowpath of the NASA Rotor 37 blade is created using the meridional section view and dimensions provided in the NASA documentation. The airfoils of the rotor blade are also generated based on the geometric parameters shared in the same documentation. These airfoils, designed in two-dimensional sections, are stacked along the radial axis to construct the three-dimensional geometry. After stacking the generated airfoils along the radial axis, a computational mesh must be genrated for conducting CFD analyses of the resulting geometry. Ensuring both the accuracy of the analysis and the computational efficiency requires attention to several critical factors during mesh generation. In regions with complex surfaces or narrow gaps, mesh density should be increased to capture surface curvature and details effectively. However, unnecessary mesh refinement can lead to increased computational cost and time, necessitating a balance between mesh resolution and computational resources. Additionally, an appropriate fine mesh structure is essential in boundary layer regions to accurately analyse flow characteristics and properly calculate y+ values to ensure the effective application of turbulence models. Sudden transitions in mesh size and excessively high aspect ratios should be avoided, as they can adversely affect the stability and accuracy of the solution. Performing a mesh independence study is also crucial to verify that the solution is independent of the mesh configuration. Lastly, the type of mesh (structured, unstructured, or hybrid) should be selected according to the CFD software capabilities and analysis requirements. Considering these factors, the rotor geometry is meshed in ANSYS Turbogrid with a structured mesh, targeting a y+ value of 1 on the blade surface to enhance boundary layer resolution accuracy. The critical step in CFD analysis is ensuring that the results are not influenced by the chosen mesh configuration. A mesh independence study is performed by solving the same physical problem by using meshes with varying densities and comparing the obtained results. The mesh quality and density are incrementally refined in small steps, and specific parameters are recorded for each mesh configuration. In this study, isentropic efficiency and pressure ratio are used as the evaluation metrics. If the results converge to a consistent value as the mesh density increases, the solution is considered independent of the mesh. However, it is essential to balance mesh density with computational cost and time. An optimized mesh density is selected where the results exhibit negligible changes, ensuring computational efficiency without sacrificing accuracy. The mesh independence test is vital for enhancing the reliability of CFD results, particularly in applications involving complex geometries or turbulent flows. This approach improves solution accuracy while avoiding unnecessary computational expenses. Consequently, a mesh with 11.9 million elements is finalized for further analyses in this study. After completing the mesh independence study, a validation study is conducted. For the validation process, the analysis setup is prepared in the ANSYS CFX solver, and the results are compared with the experimental data of Rotor 37. Achieving accurate results during the convergence process requires not only the proper application of boundary conditions but also the selection of an appropriate solver and turbulence model. Therefore, the k-ω shear stress transport solver and the gamma-theta turbulence model, known for their better boundary layer resolution, are chosen. The validation study compares the analysis results with the performance data provided in the literature for the Stage 37 rotor blade at different rotational speeds. Sufficient accuracy is achieved for results at 90% of the design rotational speed, and the study proceeded based on these validated conditions. In this study, the radial stacking of airfoils is performed in two main approaches, each with two distinct directions, resulting in a total of four configurations. The two main stacking approaches involved creating "bow" and "full leaned" geometries. The offset directions are defined along the chordwise and chord normal direction. For each direction, the stacking is performed in two orientations: from the leading edge to the trailing edge and from the trailing edge to the leading edge along the chord, and towards positive rotation and negative rotation directions perpendicular to the chord. In the bow geometries, the hub and tip sections of the rotor blade are fixed, while the airfoil at 50% blade height is offset by a specified amount. In the fully leaned geometries, the hub section is fixed, and the tip section is offset. The offset values for the sections between the fixed and offset sections are calculated proportionally based on their relative height percentages along the blade span. This approach ensured a smooth transition of aerodynamic profiles in the modified geometries. As a result of the study, the effects of radial stacking modifications, which caused changes in the three-dimensional geometry, on performance parameters are observed. The performance parameters considered included the mass flow range, which is indirectly related to the stall margin, near stall the mass flow rate, the choking mass flow rate, the pressure ratio, and the isentropic efficiency. It is found that the choking mass flow rate is associated with the position of shocks, which is influenced by the narrowing of the flowpath. Generally, in geometries offset to upstream, the shock position is also shifted to regions with a wider passage, leading to an increase in the choking mass flow rate. Conversely, in geometries offset to downstream, the shock position shifted towards the narrowing sections of the passage, causing a decrease in the choking mass flow rate. In the bow stacking type, there is no change in the blade height. However, in the full lean geometries, where the tip section is offset, variations in blade height occurred. This change in blade height, being directly related to the energy imparted to the flow, is found to influence the pressure ratio. An increase in peak isentropic efficiency value is observed only in the geometry where the profiles are shifted chordwise from the leading edge to the trailing edge. In other stacking types, losses varied depending on the type and magnitude of the shift. Regarding the mass flow range, an increase is observed only in the full lean geometry with stacking along the chordwise from the trailing edge to the leading edge. This came with a negligible pressure loss and minimal isentropic efficiency drop. This study aims to identify methods that can minimize the time and cost of designing a new aviation engine with minimal differences from an already existing axial compressor design in the industry. By leveraging these approaches, the study seeks to streamline the design process and achieve efficient outcomes in a resource-effective manner.
Aerodynamic shape optimization of the DLR-F6 wing by using openfoam as CFD solver integrated with rsm

(Graduate School, 2023-06-15) Buluş, Halil ; Çadırcı, Sertaç ; 523201122 ; Heat Fluid

Aerodynamic shape optimization plays a critical role in aerospace engineering as it allows designers to enhance aerodynamic performance by altering the shape of a body. The ability to optimize the shape of structures like aircraft wings, wind turbine blades, and rockets can lead to increased efficiency, reduced fuel consumption, and minimized emissions. Given the pressing need to address climate change and the exponentially escalating global crisis, it's essential to prioritize sustainable solutions in every aspect of design, including minimizing the impact of aircraft emissions. This requires a primary focus on decreasing the drag and increasing the lift on the airplane, which is one of the most significant factors affecting aerodynamic performance and range. As air traffic continues to grow, the importance of aerodynamic shape optimization in reducing emissions and increasing fuel efficiency becomes increasingly clear. This thesis on the aerodynamic shape optimization of the DLR-F6 wing demonstrates an effective and a comprehensive way of an optimization process that can contribute to the ongoing research in this field. The DLR-F6 wing is a common benchmark for aerodynamic research due to its complex geometry and challenging flow characteristics. By optimizing the shape of the wing, it is aimed to improve its performance and contribute to ongoing research in this field. To ensure that the structural strength of the wing is not compromised, the optimization process also involves some considerations on various design constraints like modal frequency and mass of the wing. In the optimization process, different chord slices at various locations along the span has been taken and twisted some angles by taking their aerodynamic centers as reference. The work focuses on to determine the best improved angle sets which let the better performance on L/D value without sacrificing its structural integrity. The optimization model tree was constructed using ModeFRONTIER software, which integrated different software tools and automated the optimization process. The construction involved four main stages. Firstly, Pointwise software was used to create a new wing database by twisting at the six sliced chords with angles determined by the software. The software automatically executed all the steps to twist, create a new database, make surface mesh, and create a pre-meshed geometry for Abaqus, with the help of a journal. In the second stage, volume meshes were prepared using ANSYS Fluent Meshing, which automatically executed all the meshing processes controlled by optimization software. Thirdly, CFD analysis was conducted using OpenFOAM as the CFD solver to simulate the flow around the wing. The volume mesh created by Fluent Meshing was used as the solution cells. With the help of a function, OpenFOAM can convert a fluent mesh to foam format. To make the optimization process faster and well talent based, the HPC (High Performance Computer) was used to run in OpenFOAM. ModeFRONTIER makes an automatic connection with HPC systems based on SSH protocol and with a Linux bash script, aerodynamic analysis had been conducted. After the simulation was done, a file storing all the forces at each iteration was transferred to the host computer, and lift, drag and L/D values were computed using an inner MATLAB stage. The L/D value and lift value were set as design objectives. The purpose was the maximize these values. Since the twisting rotation angles were set as the input values, the optimization tool organizes and selects the best input values to succeed the design objectives. Lastly, Abaqus software was used to perform structural analysis to ensure the strength of the wing, with the pre-meshed geometry file directly transferred after the first stage. With an Abaqus journal, for each different design mass and modal frequency are calculated and processed with another MATLAB stage. These were selected as another design objective, with the goal being to minimize the mass and maximize the frequency value. However, 'Lift' and 'L/D' objectives were prioritized to make the optimization processes easy to go on. The optimization process was streamlined through these main stages, with simultaneous file transfers and evaluation of results made in intermediate steps. About 220 DoE (Design of Experiments) were created and evaluated. Due to the expensive CFD simulations, direct optimizations were not feasible to proceed. Therefore, RSM (Response Surface Methodology) was used to reproduce more experiments in an inexpensive way. RSM, also known as Surrogate Models, are a collection of statistical and mathematical techniques used to create, model, and analyze the relationships between input variables and output responses. By using RSM, the number of experiments needed can be reduced to obtain optimal results, saving time and resources in the optimization process. The direct optimization results were used to train the data set to build a good quality RSM. After using good RSM strategies, 1000 more experiments were created. After selecting the best design, a real analysis was conducted and showed that the RSM predicted the design output very well. The results of the optimization process were evaluated using a set of performance metrics, including the L/D ratio, maximum lift, minimum drag. Since the mass and modal frequency objectives were required to assure the structural integrity of the new design, they were not included as optimization performance metrics. The modified DLR-F6 aircraft was then compared with the original DLR-F6 aircraft using these performance metrics. The results show that the modified aircraft had a 13.15% improvement in L/D ratio and 2.24% improvement in lift compared to the original aircraft. A 3DOF flight simulation was done using MATLAB Simulink tool, with two aerodynamic databases created by sweeping 2 Mach numbers and 13 angle of attacks. One was for the airplane with the original wing, and the other was for the airplane with the optimized wing. Overall, the thesis demonstrates the effectiveness of using a multi-disciplinary optimization approach to improve the performance of complex aerodynamic shapes such as the DLR-F6 wing. The optimized wing not only shows a significant improvement in its aerodynamic performance but also maintains its structural strength. Although the flow chart may seem complex, it helps make the optimization process comprehensive and efficient. The optimization process and methodology used in this research can be applied to other complex aerodynamic shapes to improve their performance as well.
Delta kanat hücum kenarı eklentilerinin boylamsal ve yanal stabiliteye etkilerinin incelenmesi

(Lisansüstü Eğitim Enstitüsü, 2024-06-13) Ece Yalaz, Ayşe Sibel ; Çadırcı, Sertaç ; 503201127 ; Isı Akışkan

Bu tez çalışmasında savaş uçaklarında sıkça tercihe edilen delta kanat yapısına hücum kenarı eklentilerinin (strake) eklenmesi ile hava aracında meydana gelen boylamsal ve yanal stabilite etkilerinin incelemeleri yapılmıştır. İncelemeler kapsamında üç farklı referans geometri modelinde değişen hücum açısı, değişen beta açısı ve değişen aileron, flap ve rudder kontrol yüzeyi açılmalarında HAD analizleri koşturulmuş, sonuçlar üzerinde akış topoloji değerlendirmeleri ve stabilite analizleri gerçekleştirilmiştir. Şimdiye kadar üzerinde az oranda araştırma yapılmış olan strake geometrilerinin hava aracındaki kontrol yüzeyi verimliklerindeki etkisini araştırmak, stabilite ve kontrol stratejileri hakkında sonuçlar üretmek bu tezin ana amaçları arasında yer almaktadır. Çalışmalar birebir ölçekte kullanılan savaş uçağı modelinin (Lamar ve Frink, 1981) rüzgâr tüneli verileri ile doğrulanması olarak başlamıştır. Gerçekleştirilen bütün HAD analizleri sıkıştırılabilir akış kabulündeki üç boyutlu tam ölçekli modelde koşturulmuş ve RANS denklemleri hücre merkezli sonlu hacimler yöntemi ile çözülmüştür. Stabilite analizleri kapsamında, aynı zamanda tasarım parametresi olan strake referans alanının yarıya düşürülmesinin aerodinamik ve stabilitedeki etkileri de incelenmiştir. Analizler üç farklı geometri versiyonunda yapılmış olup, model üzerindeki kontrol yüzeyleri literatür referans alınarak tasarlanmıştır. Bu tez çalışmasında hava koşulu deniz seviyesi (0 ft), ideal gaz tanımlanmış ve serbest akım hızı (Ma=0.5) giriş bölgesi sınır şartı olarak verilmiştir. S0 strakesiz geometri modelini, S1 AD-14 isimli strake yapısına sahip geometri modelini ve S2 ise AD-14 strake yapısının referans alanı yarıya düşürülmüş geometri modelini içermektedir. Sonuçlar ilk olarak akış topolojisinin değerlendirilmesi üzerine kurulmuştur. Hücum açısı analizleri incelendiğinde; S0 versiyonunda ɑ=24° ve üstünde akım ayrılmalarının gittikçe şiddetlendiği ve kanat üzerinde taşıma kayıplarının yaşandığı gözlemlenmiştir. S1 ve S2 versiyonlarında ɑ=16°'den ilerideki hücum açılarında strake başlangıç bölgesinden gelişen girdabın kanat yüzeyindeki girdap ile birleştiği, kanat yüzeyindeki negatif basınç bölgesini arttırdığı görülmüştür. Strake alanının küçülmesi ile girdap şiddetinde azalma meydana geldiği görülmüştür. Beta açısının değişiminin incelendiği analizlerde strakeli versiyonların ɑ=32° ve β=10° açısındaki koşulunda strake girdabı rüzgârı önden alan bölümünde daha güçlü şekilde gelişmekte, rüzgâr arkasında kalan bölümde ise yüzeyden ayrıldığı görülmüştür. S2 versiyonu için sağ kanattaki akım ayrılmalarının S1 versiyonuna göre daha fazla olduğu gözlemlenmiştir. Ayrıca S0 versiyonunda olduğu gibi S1 ve S2 versiyonlarında da kanattan ayrılan akım, kuyruk bölgesindeki düzenli akımı gölgelemiş ve o bölgede kararsız akış rejimlerinin oluşmasına sebebiyet vermiştir. Kontrol yüzeylerinden flap ve aileronun strake entegresindeki aerodinamik etkileri incelendiğinde; kontrol yüzeyi etkinliklerinin S0 için ɑ=24° koşulunda oldukça verimsiz olduğu görülmektedir. Strake entegresi oluşan girdap gelişimi ile flap ve aileron kontrol yüzeyindeki akımın yeniden tutunduğu ve kanat yapısındaki taşımayı arttırdığı gözlemlenmiştir. Betalı koşulda rüzgârı arkadan alan kanattaki aileron yüzeyinin üzerindeki taşımanın arttığı gözlemlenmiştir. Strake referans geometrisindeki artış ile ɑ=24° hücum açısında aileron etkinliğinde artış gözlemlenmiştir. Strake geometrisinin eklenmesi ile kuyruk ve rudderdaki yanal düzlemde taşımanın artması sağlanmış, ɑ=24° ve β=10° koşulunda rudder kontrol yüzeyinde verimlikte artış sağlandığı görülmüştür. Versiyonların boylamsal stabilite analizleri yapıldığında, S0 boylamsal kararlı, S1 ɑ=0° ila ɑ=16° aralığında nötr stabilitede, ɑ=16° ila ɑ=32° aralığında ise yüksek seviyelerde boylamsal kararsız, S2 ise ɑ=0° ila ɑ=16° aralığında boylamsal kararlı durumda olduğu görülmüştür. Flap kontrol yüzeyinin ɑ=24° hücum açısında getirdiği yunuslama moment değişimi incelenecek olursa, S1 versiyonunda S0'a göre %50 artış, S2 versiyonunda ise %25 artış görülmüştür. Son olarak yüksek hücum açısındaki burun aşağı moment otoritesinin sağlanması için strake referans alanında azalma yoluna gidilebileceği gözlemlenmiştir. Versiyonların yanal stabilite analizleri yapıldığında ise aileron etkinliği S0 ile karşılaştırıldığında ɑ=24°'de S1 aileron verimi %34 oranında artarken, S2 aileron verimi yaklaşık %16 civarında artış göstermiştir. β=0° koşulunda S1 ve S2'nin rudder verimliliğinde bütün hücum açıları aralığında S0'e göre oldukça fazla iyileşme görülmüştür. Fakat beta açısının artması ile kanat girdaplarındaki kararsız yapı kuyruk akımını etkilemiş, yüksek hücum açısında rudder etkinliğinde kayıplara yol açmıştır. Strake geometrisinin kullanımı ile yanal stabilitede ɑ=24° hücum açısına kadar artış sağlanmıştır. Fakat girdap bozunmaya uğradığı ɑ=32°'de strake girdapları düzensiz yapıya bürünmüştür.
Improving the aerodynamic characteristics of the gap between the cabin and trailer of heavy-duty commercial vehicles

(Graduate School, 2023-09-14) Çil, Utku ; Çadırcı, Sertaç ; 503201121 ; Heat Fluid
Multi-objective optimization to increase the performance of a toroidal propeller

(Graduate School, 2025-01-10) Çiftçi, Ömer ; Çadırcı, Sertaç ; 503211125 ; Heat and Fluid

This thesis focuses on optimizing the performance of toroidal propellers using Computational Fluid Dynamics (CFD) and Fluid-Structure Interaction (FSI). The study aims to enhance propulsion efficiency, reduce noise emissions, and ensure structural reliability. Toroidal propellers, characterized by their closed-loop blade design, offer unique advantages over conventional propellers, including reduced tip vortex formation and improved noise performance. The research begins with an overview of the historical evolution of propeller technology, emphasizing the renewed interest in propeller-based systems due to environmental and economic considerations. It highlights the potential of toroidal propellers to integrate the benefits of tandem and contracted and loaded tip (CLT) designs, addressing common challenges such as tip vortex intensity and aerodynamic inefficiencies. The optimization methodology incorporates advanced multi-objective techniques, including differential evolution algorithms and Pareto-front analysis. Objectives include maximizing efficiency, and maintaining structural integrity. Constraints and design variables are carefully defined, and metamodeling techniques like are used to reduce computational demands. A key component of the research is the optimization process, which employs the Non-Dominated Sorting Genetic Algorithm II (NSGA-II). This algorithm is used for multi-objective optimization, balancing trade-offs between propulsion efficiency, thrust enhancement, and structural reliability. The NSGA-II method efficiently explores the design space, identifying Pareto-optimal solutions that satisfy the constraints and objectives. This approach ensures a diverse set of solutions, providing valuable insights into the trade-offs involved in propeller design. Blade parameterization and mesh generation are discussed in detail, with specific attention to the unique geometry of toroidal propellers. The research highlights the importance of accurate spatial discretization for reliable CFD and FSI analyses. The aerodynamic solver utilizes RANS-based methods to evaluate flow characteristics, while turbulence modeling techniques, including the k-omega SST model, are applied to improve prediction accuracy. FSI analysis is employed to assess the interplay between aerodynamic forces and structural responses. The finite element solver considers material properties, boundary conditions, and stress criteria to ensure structural reliability under operational loads. The results demonstrate significant improvements in performance metrics, including higher propulsion efficiency, reduced noise emissions, and enhanced structural stability. The thesis concludes that toroidal propellers represent a promising advancement in propulsion technology, capable of addressing modern aviation challenges. The findings provide a framework for future research and practical applications, contributing to the development of efficient and sustainable propeller systems. In this study, three new parameters required in the design of the toroidal propeller were determined as design variables and multi-objective optimization was performed. The effects of 3 different parameters on the target thrust increase, efficiency increase and stress reduction were examined and the findings were shared.
Numerical investigation of double layer microchannel heatsinks and performance assessment based on Taguchi method

(Graduate School, 2022-06-14) Demirsöz, Mustafa ; Çadırcı, Sertaç ; İliş Gediz, Gamze ; 503191117 ; Heat-Fluid

In this study, the performance of double-layer microchannel heat sinks was investigated by Computational Fluid Dynamics Analysis using the Taguchi Experimental Design Method. Selected parameters are Reynolds number, microchannel material, width of upper and lower channels, distance between these channels and heat flux. The effect of these parameters on the on-chip thermal resistance, the total thermal resistance and the pumping power requirement were investigated. The influence of the selected factors was investigated based on the average of the signal-to-noise (SN) ratios and the response table for the signal-to-noise ratios. Except for the total thermal resistance in the counterflow configuration, the Re number was found to be the most important factor for all other cases. Copper gives the best results due to its thermal dissipation characteristic. For the counterflow configuration, the effect of the heat flux on the thermal resistances is negligible except for the total thermal resistance. The effect of the width of the lower channel on the thermal resistances is more dominant than the upper channel. The lower and upper channel widths are equally important in terms of the required pumping power. There is an inverse relationship between channel width and required pumping power. The distance between the lower and upper channels is insignificant in terms of the required pumping power. If this distance is lower in a parallel flow configuration, on-chip thermal resistance is minimal. On the other hand, the total thermal resistance increases with this distance. As the channel widths increase, the average velocity in the channel decreases, so an increase in thermal resistance is observed.
Numerical investigation of inertial focusing of micro andnanoparticles in curvilinear microchannels

(Graduate School, 2022-01-21) Aldemir, Ahmet Turan ; Çadırcı, Sertaç ; 503181126 ; Heat Fluid

Recently, microfluidic systems have been preferred more than conventional methods due to their ease of production, economic advantages, high precision processing capabilities and ease of operation. These systems are in a situation where many disciplines such as physics, chemistry and engineering are intertwined. Microfluidic systems aim to provide manipulation of fluids moving in microchannels and to control the flow field. With the provision of this control, important developments are experienced in fields such as biomedical engineering and medicine. As an important area, there have been serious improvements in the separation of particles of different diameters in microfluidic systems recently. In the studies carried out, particles of different diameters are sent into the microchannel through from source (like a pump) and can be collected in separate beams from the channel exit. This has become an important step in the early diagnosis of deadly diseases such as cancer. The concept of separating particles of different diameters from each other in the microfluidic systems has accelerated with the developments in Lab on a Chip (LoC) and MEMS systems. Many different mechanisms have been developed for the separation of particles. Separation of particles in the microfluidic systems can be achieved either by active separation techniques, where external forces act, or passive separation techniques, where an external force is not used. In active separation, many different sources such as electrical systems, sound, optics can be used to create an external force. On the other hand, passive separation techniques use dynamics within the microchannel to separate particles. Inertial focusing, which is an important area in passive focusing, still continues to develop. The logic of inertial focusing is that forces acting on a particle moving in a fluid, either from flow, from interaction with the walls of the microchannel, or from effects at the molecular level. The most important of these forces is the lift force, which directs the particle to a certain equilibrium position. Lift force is divided into two as wall interacting lift force and shear stress lift force. The most important difference of these forces emerges when determining the equilibrium position of the particles. The lift force arising from the wall interaction pushes the particles towards the center, while the shear stress lift force directs the particles from the center towards the walls. As a result, when these forces reach equilibrium, the equilibrium position of the particle is determined. The magnitude of these forces is highly dependent on the particle diameter, so they cause the particles with different diameters to line up in different equilibrium positions, thus allowing the particles to be separated from each other. In inertial focusing, the addition of curvature to the microchannels adds a secondary effect to the flow. This phenomenon is due to the formation of eddies in the curved regions within the channel. This flow effect also causes a force called the Dean force to be added to the particles. In the studies carried out, it was observed that the focusing mechanisms of the particles improved thanks to this effect. Microchannel geometry is an important parameter that affects the mentioned forces. By changing the geometry and dimensions, it is possible to design a particle separation mechanisms that works with higher efficiency. In this context, different microchannel designs such as straight microchannel, spiral microchannel, serpentine microchannel have been studied, and detailed studies have been carried out on these effects. Studies in the field of separation of microparticles are mostly carried out using synthetic particles in order to better understand how blood and cancer cells interact with each other. Particle separation is affected by many different parameters such as flow rate, type of fluid used, particle diameter, as well as microchannel geometry. Therefore, it is very important to best understand the physics of inertial focusing in order to design the most efficient decomposition. In this study, the concept of inertial focusing is examined. Computational Fluid Dynamics (CFD) analyzes were performed on microchannels with different geometries, parametric analyzes affecting focusing were completed and the effects were examined. Within the scope of the study, flow and particle analyzes of the sunflower geometry obtained by adding serpentine regions to a conventional spiral microchannel geometry and the importance of this geometry were discussed.
Numerical investigation on hub effects of hubless-rim driven propeller

(Graduate School, 2022-12-16) Aşçı, Ali Burak ; Çadırcı, Sertaç ; 503191132 ; Heat and Fluid

Since the beginning of humanity, various methods have been developed for the interaction of countries with each other. Therefore, people have developed various methods of transportation in order to strengthen political and economic relations, explore new places and access resources in new places. Maritime transportation is one of these prominent methods to create interactions among countries. Thanks to the developments in the field of maritime transport, humanity has made significant progress in numerous fields. Ships have been a widely used vehicles for transportation and ships are generally driven by the shaft-propeller mechanism. Energy occurred from fuel, nuclear or electrical is transferred to propeller via a transmission mechanism. During this drive movement, undesirable problems such as increased fuel consumption, reduced mechanical efficiency and noise may occur and these problems may cause irreversible damage to the shaft mechanism. Besides the effects of climate change dramatically continue to increase and marine transportation sector is one of the causes to emit more CO2 in the world. Therefore, hubless Rim Driven Propellers (RDP) has been developed and used for various marine vehicles in order to prevent above-mentioned problems. The working principle of RDP can be summarized as an electric motor driving the propeller with the help of the rim. However, since hubless RDP technology is a new field of marine researches, information on how changing the dimensions of the hub affects the hydrodynamic performance of the propeller is scarce. In this master's thesis, analyzes were made for hubless RDP design with five different hub ratios (0.05, 0.1, 0.15, 0.167, 0.25) by means of various dimensionless parameters to monitor the performance effects of the propeller. Ka4-70 propeller was selected for these five designs. They were solved numerically benefiting from Unsteady Reynolds Averaged Navier-Stokes equations (URANS) and Shear Stress Transform (SST) k-ω turbulence transport equations. Numerical operations were handled on the finite volume method solver Simcenter STAR-CCM+ solver, using the Rigid Body Motion (RBM) approach. The Computational Fluid Dynamics (CFD) results have been conducted in terms of non-dimensional parameters such as thrust coefficient (KT), torque coefficient (KQ), and efficiency (η) ranging from 0.1 to 0.6 advance ratio (J) for 600 rpm. It was monitored that KT, KQ, and η increased as the hub ratio increased under a certain rotation speed until the hub ratio =0.167. Due to no experimental data for hubless RDP, validation studies were able to conduct with hub type propeller. Accordingly, an ideal propeller configuration can be determined by comparing numerical results and experimental data.
Tabak tipi girdap kırıcının tankın boşaltılması esnasındaki etkilerinin deneysel olarak incelenmesi

(Lisansüstü Eğitim Enstitüsü, 2025-01-07) Erendur, Tamer Alp ; Çadırcı, Sertaç ; 503211127 ; Isı Akışkan

Bu tezde içi sıvı dolu silindirik bir tankın tahliyesi esnasında tahliye hattına gaz girmesi olayı ve tabak tipi girdap önleyicilerin gaz girme üzerine olan etkilerinin incelenmesi gerçekleştirilmiştir. Silindir şekle sahip, alt yüzeyinin ortasında tahliye deliği bulunan bir tankta tahliyenin açılması ile hidrostatik basınç gereği debi sıfırdan başlayarak bir değere kadar hızlıca gelir. Sonrasında tankın içerisindeki sıvının azalması ile bu debi yavaş bir şekilde azalır. Deliğin çapı tankın çapından küçük olduğu için deliğin içerisinde akışkan daha hızlıdır ve tankın tahliye olması için deliğin olduğu yerde basıncın düşmesi gerekmektedir. Bu düşüş tankın tahliyesi esnasında belirli bir noktada sıvı gaz yüzeyinin bozularak gazın tahliye hattına gitmesine sebebiyet verir. Tankın tahliyesi sırasında gaz girme olayını açıklayan iki önemli parametre bulunmaktadır. Bunlardan ilki tahliye hattına gaz girişinin gerçekleştiği andaki sıvının tank tabanına olan yüksekliğidir. Bu yüksekliğe kritik yükseklik ismi verilir. Diğer önemli parametre ise gaz girme olayının gerçekleştiği süredir. Bu süre kritik süre olarak adlandırılır ve tahliyenin başladığı andan ilk gaz girişinin gerçekleştiği ana kadar ölçülür. Tez çalışmasında öncelikli olarak numerik çalışmalara yer verilmiştir. Literatürde önceden yapılan deneysel çalışmalardan biri hesaplamalı akışkanlar dinamiği çözücüsü ANSYS Fluent programı ile tekrar çözülmüş, doğrulaması yapılmış ve gaz girme olayı incelenmiştir. Numerik çalışmaların çok zaman alması, bilgisayar gücü istemesi ve deneysel çalışma imkânı olması sebebiyetiyle çalışmaya deneysel olarak devam edilmiştir. Deney düzeneğinde tankın hatta çabuk bağlantı olarak adlandırılan bağlantısı ile bağlı olması, hattın uzunluğu ve küresel vana gibi bilgisayar ortamında modellenmesi zorlu ve yüksek hesaplama gücü isteyen özellikleri sebebiyle test çalışmalarının numerik doğrulaması yapılmamıştır. Tezde kullanılan deney düzeneği 500 mm çapında ve 500 mm yüksekliğinde silindirik bir tank, tankın alt kısmın ortasında yer alan 35 mm çapında bir tahliye deliği, bu deliğin devamında borulamalar ve bir vanadan oluşmaktadır. Tankın içerisindeki sıvının tamamını dikey bir şekilde görebilecek bir kamera sabitlenmiştir. Tank üzerindeki cetvel sayesinde istenilen su seviyesinde doldurulur ve vana ile tahliye edilir. Kamera anlık olarak suyun yüksekliğini çeker. Tankın içerisindeki sıvıyı karıştırabilmek için elektrik motor ve sabitleme elemanlarından oluşan bir düzenekte bulunmaktadır. Sıvı olarak şebeke suyu kullanılmıştır. Testlere öncelikli olarak sıvının durgun olduğu koşul ile başlanmıştır. Bu koşul için 3 farklı su yüksekliği 10, 20 ve 30 cm seçilmiştir. Su istenilen seviyeye getirildikten sonra su doldurma bırakılarak suyun durgunlaşması için 1 saat beklenmiştir. Burada amaç suyun içerisinde tahliye esnasında dönmeye sebebiyet verebilecek herhangi bir hareketin kalmamasıdır. 1 saat beklemenin ardından kamera kaydı açılarak su vanası açılmıştır. Böylece suyun tahliyesi başlamıştır. Su tahliye olurken kritik yükseklik değerine gelince tankın tahliye hattına gaz girişi gözlemlenmiştir. Bu kritik yükseklik ve gaz girişinin gerçekleştiği zaman olan kritik zaman kayıt edilmiştir. Durgun halde tankta da gaz girişi gözlemlendikten sonra gaz giriş yüksekliğini düşürmek ve gaz giriş zamanını daha geç zamanlara taşımak için girdap önleyici cihazlar kullanılmıştır. 3 boyutlu yazıcıdan üretilen üstü dairesel plaka olan girdap önleyici cihazların boyutları tahliye çapının yarısından başlayarak 1,2,3 ve 4 katı olacak şekilde tasarlanmıştır. Burada amaç girdap önleyici cihazın çapının tahliye çapının oranıyla etkisini incelemektir. Tank boşken ve 5 farklı girdap önleyici cihaz tankın tahliye deliğinin üstüne yerleştirilmişken 3 farklı sıvı yüksekliğinde testler 3 tekrar olarak gerçekleştirilmiştir. Testleri 3 tekrar yapılmasının sebebi çevresel etkiler sebebiyle testin etkilenmesi ihtimalini ortadan kaldırmaktır. Yapılan 3 tekrarlı testlerden herhangi birinde bir farklılık görülmesi durumunda dördüncü bir test yapılmış ve en az 3 eş değer test sonucu elde edilmiştir. Hiçbir koşul için beşinci testin yapılmasına ihtiyaç duyulmamıştır. Sıvının durgun haliyle yapılan deney sonuçları tabak çapının artışı ile kritik yüksekliğin azaldığı, girdap önleyici cihaz kullanmanın özellikle 2 çap boyuttan sonra etkisi olduğunu ve 4 çap boyutunda girdap önleyici cihaz ile neredeyse kendi yüksekliği kadar gaz girişini engellediğini göstermektedir. Durgun halde iken yapılan testlerde literatürdeki verilere paralel olarak kritik yükseklik değerinin başlangıç su seviyesiyle kayda değer bir değişim göstermediğini göstermiştir. Tahliye esnasında tahliye edilen tankın dönmesi veya yatay kuvvetlere maruz kalması tahliye esnasında hızlı bir şekilde girdap oluşturacak ve bu girdap tankın merkezinde basınç düşüşüne sebebiyet vererek daha erken gaz girişine neden olacaktır. Bu yüzden sıvının durgun olarak başladığı koşullarda testler tamamlandıktan sonra sıvının başlangıç koşulunda açısal hıza sahip olduğu durum için testler gerçekleştirilmiştir Durgun hale benzer bir şekilde 3 farklı su seviyesinde bu işlemi gerçekleştirmek için devri voltaj ile ayarlanabilen bir elektrik motor vidalı bir mekanizmanın üzerine yerleştirilmiştir. Bu mekanizma sayesinde elektrik motorun çıkışına takılan tankın çapının yarısı uzunluğunda bir karıştırıcı plaka farklı su yüksekliklerinde suyun içerisine tam olarak yerleştirilmiştir. Testler 10, 20, 30 ve 40 devir/dakika olmak üzere 4 farklı başlangıç açısal hız koşulunda gerçekleştirilmiştir. Karıştırıcı vidalı sistem ile sıvı seviyesine kadar ayarlandıktan sonra 2 dakika boyunca belirlenen hızda sıvı karıştırılmıştır. 2 dakikalık karıştırma işlemi bittikten sonra karıştırıcının girdabı engellememesi veya sıvının açısal hızını sönümlememesi için elektrik motorun kapatılmasıyla eş zamanlı olarak karıştırıcı sıvının içerisinden çıkarılmıştır. Karıştırıcının çıkarılmasından sonra beklenmeden tahliye vanası açılmıştır. Böylece açısal hızın sıvının tank yüzeyiyle sürtünmesi kaynaklı yavaşlamasının etkisi en aza indirilmiştir. Durgun hale benzer bir şekilde kamera ile gaz girişi gerçekleşen sıvı yüksekliği ve süre kaydedilmiştir. Test sonuçlarına göre hiçbir girdap kırıcı kullanılmaz ise en ufak bir açısal hızda bile kritik yüksekliğin çok büyük ölçüde arttığı ve gaz girmesi kaynaklı tahliye sürelerinin önemli ölçüde yükseldiği gözlemlenmiştir. Sıvıya başta verilen açısal hızın artışı ile tüm girdap önleyiciler olan ve olmayan koşullarda gaz giriş yüksekliği artmıştır. Bu sonuçlar tankın maruz kaldığı kuvvetlerin gaz girişine önemli etkileri olduğunu göstermektedir. Kritik yükseklik hiçbir girdap kırıcı olmayan durumdan tahliye ile eş değer çaplı girdap kırıcıya geçince çok önemli ölçüde düşüş göstermiş olup bu girdap kırıcıdan tahliye çapının 2 katı olan girdap kırıcıya geçişte de gene önemli bir ölçüde düşüş göstermiştir. Yani girdap kırıcıların çaplarının artmasıyla oluşan iyileşme giderek azalmaktadır. 3 çaplı girdap kırıcı ile 4 çap girdap kırıcı arasındaki fark oldukça düşüktür. Bu yüzden çalışmada tahliye çapının 5 veya daha fazla katı girdap kırıcı kullanılmamıştır. Tahliye çapının 4 katı girdap kırıcı kullanılan durumda 10 ve 20 rpm karıştırma değerlerinde durgun hale benzer bir şekilde gene neredeyse kendi yüksekliğine kadar gaz girişini engellemiştir. 4D olarak adlandırılan bu girdap kırıcının düşük sarsıntılar kaynaklı oluşan girdapların oluşturduğu gaz girişini önemli ölçüde engelleyeceğini göstermektedir.

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