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ÖgeRamjet motorlu füzelerde ses üstü hava alığı performans analizi(İTÜ Lisansüstü Eğitim Enstitüsü, 2025)In the prepared thesis study, supersonic air intake performance analysis was performed in ramjet engine missiles. In air-breathing engines, the air required for combustion must be delivered to the combustion chamber at certain pressures and flow rates. Some engines require air intakes to provide this condition. Especially if aviation engines are considered, turbojet, turbofan, turboprop, ramjet and scramjet engines require air intakes to regulate the flow. Parameters such as the speed, flight altitude and position on the missile are limiting for their shapes and sizes. For example, since civil aviation aircraft fly at subsonic speeds, they have different geometry than the air intakes used in ramjet engine aircraft. Ramjet engine missile have convergent divergent air intakes due to their supersonic flight. The convergent divergent structure reduces the supersonic flow to subsonic speeds. There is no turbine after the combustion chamber in ramjet engines. Combustion occurs at subsonic speeds. The flow after the combustion chamber passes through the convergent divergent nozzle and provides thrust at supersonic speeds. Ramjet air intakes generally operate in the 1-5 Mach range. Flows of Mach 5 and above are considered hypersonic. Scramjet engines are generally used in hypersonic missiles. Ramjets can be axially symmetric or rectangular in shape. In this study, validation studies, parametric analysis and performance analyses were performed for an air flow with a quadrilateral structure. The parameters that determine the performance in air intakes are mass flow recovery, pressure recovery and stability. The required mass flow and pressure for an air breathing engine must be provided with stable flow. If the required flow and pressure are not provided, the engine may shut down. This situation is of vital importance for the aircraft. In this study, the parameters affecting air intake performance were determined. These parameters are compression ratio, valve closure ratio, ramp angle and number, speed and altitude, diffuser angle and bleed geometry. Ramp angles reduce the flow to the sound speed level by creating oblique shocks. The normal shock occurring in the throat region reduces the flow to subsonic speeds. The angled diffuser ramp located in the subsonic speed region reduces the speed and increases the pressure with its expanding structure. Bleed is the opening that tries to increase the performance by providing air discharge to the atmosphere in the air inlet line in ramjets. Bleed structures can be in different sizes and positions. The bleed structure is usually located close to the throat as it tries to keep the normal shock in the throat. While causing a loss of flow, it provides total pressure recovery by keeping the normal shock in the throat. The condition where the normal shock is right in the throat is accepted as the critical condition. In the condition called subcritical, the normal shock approaches the entrance to the air inlet and this situation can go as far as buzzing. The buzzing event can be accepted as the situation where sufficient mass flow is not taken into the air inlet or the flow does not fill the air inlet. Another situation is the supercritical condition. In the supercritical condition, the normal shock approaches the diffuser region. This situation also causes a loss in total pressure as in the subcritical condition. The compression ratio determines the lower limit of the air inlet cruising speed. The air inlet should be in the self-starting region in the cruising speed operating range. The self-starting region is determined by the Kantrowitz curve. Ramjet engine missile should be released into the atmosphere in the cruising speed range or they can start working after being brought to the operating speed with different thrust systems. Ramp angles can be determined by oblique shock equations. The shock angle and post-shock flow parameter ratios can be defined by oblique shock equations. In this study, it is aimed to reveal the air intake performance analysis and parametric effects by performing CFD (Computational Fluid Dynamics) analyses with the determined geometry. With CFD analyses, predictions can be obtained about the air inlet performance to be designed. The fact that it is difficult to access supersonic wind tunnels or that they can be costly to use also directs CFD methods. CFD analyses can also be run to extract critical conditions for cost-effective tests. This study aims to contribute to the literature with a 2D CFD model. First of all, the CFD model created was validated by taking the tests included in the verification study as reference. In this model, the analysis was accepted as 2D steady flow. Different solution networks and different turbulence models were used during validation. Mesh independence analysis was performed with 8 different meshes with the number of elements ranging from 78000 to 504000. In the turbulence sensitivity analysis, solutions were made with k-ω SST, k-epsilon and Spalart-Allmaras models. RANS solution was used in the analyses. The scheme was solved as coupled while discretization was performed in the 2nd order. The analyses were continued with the k-ω SST model, which was the most compatible with the validation test data. The k-ω SST model gave successful results in modeling the low boundary layer thickness and the experienced shock structures at high speeds. The shock angles and post-shock velocities obtained in the 2D CFD model were compared with the analytical calculation and similar results were obtained. The missile speed of 2.5 Mach was accepted as the validation case. The mass flow recovery and total pressure recovery curve in the test data were used for validation. In addition, time-dependent analyses were solved in order to determine whether the flow would be accepted as steady flow. Solutions were obtained at 10 ms intervals and it was seen that the flow reached the steady flow form at the end of 40 ms. Similar results were seen when the analyses were repeated at 3 different vane closure ratios. In the analyses performed, when bleed was added to the air intake, the total pressure recovery increased while the flow rate was lost. While the bleed provides air flow to the atmosphere, it tries to keep the normal shock in the throat. In high closure ratios, in the absence of bleed, high back pressure pushes the normal shock to the air intake intake and causes buzzing, while the bleed structure pulls the normal shock to the throat, causing a loss of flow rate. Therefore, high pressure recovery values can be achieved without buzzing. The flight envelope determined for the designed air intake is in the range of 2.5-3.5 Mach. Self-starting speed is 1.72 Mach. Performance analysis was performed at different speeds. Mass flow recovery increases at high speeds. High total pressure values were defined for high speeds in the wind tunnel tests in the verification study. However, the total pressure values in front of the air intake valve did not change much. The highest total pressure values were seen at the 3 Mach condition. Cowl Lip angle, defined as the third ramp angle and determining the air intake compression ratio, was solved parametrically and the analysis results were examined. The compression ratio determines the self-starting speed. According to the analysis results, as the cowl angle increases, the mass flow recovery and pressure recovery decrease since the intake area decreases. However, if the cowl angle decreases too much, the compression ratio increases and the area ratio decreases. The decrease in the area ratio may cause exit from the self-starting region. Finally, the analyses were solved with the k-ω SST model with the assumption of steady flow in 3D. Validation conditions were also provided in the 3D model. However, 2D analyses were continued due to their lower cost and compatibility with mass flow recovery, total pressure recovery and oblique shock analytical calculations. If the entire study is evaluated, it has been determined that flow regime prediction can be made with 2D CFD analyses in rectangular air intakes and cost-effective solutions can be found.
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ÖgeEksenel kompresörlerde oluk genişliği ve en-boy oranının aerodinamik performansa etkisinin HAD ile analizi(İTÜ Lisansüstü Eğitim Enstitüsü, 2025)Türbomakinelerde verimi ve operasyonel kararlılığı sınırlandıran en temel sorunlardan biri, rotor kanat ucu bölgesinde meydana gelen ikincil akışlar ve buna bağlı olarak ortaya çıkan stall eğilimidir. Kanat ucu boşluğu, mekanik toleransları güvence altına almak amacıyla gerekli olmakla birlikte, bu boşlukta oluşan basınç farkı nedeniyle gelişen sızıntı akışı ve vorteksler kompresör performansında ciddi kayıplara yol açmaktadır. Literatürde bu tip kayıpları azaltmaya yönelik çeşitli pasif kontrol yöntemleri önerilmiş olup, özellikle çevresel oluk uygulamaları son dönemde öne çıkmıştır. Ancak oluk geometrisinin, özellikle genişlik ve en boy oranı gibi parametrelerinin kompresör performansı üzerindeki etkileri hakkında sistematik ve doğrulanmış sayısal ya da deneysel çalışmalar yetersizdir. Bu tez kapsamında, kanat ucu bölgesinde sızıntı akışlarını kontrol etmek ve stall marjını iyileştirmek amacıyla dokuz farklı oluk konfigürasyonu oluşturularak parametrik HAD analizleri gerçekleştirilmiştir. Oluk genişliği yüzde 3, yüzde 6 ve yüzde 9; en boy oranı ise 1, 1.5 ve 2 değerlerine ayarlanarak farklı konfigürasyonlar tasarlanmış ve bunlar oluksuz referans geometriyle karşılaştırılmıştır. Sayısal çalışmalar için NASA tarafından deneysel verileri mevcut olan Rotor 37 eksenel kompresör modeli referans alınmıştır. Deneysel veriler özgün bir test düzeneğinde elde edilmiş olup, HAD modellemesinin doğruluğunu ve çözüm ağı bağımsızlığını sağlamak amacıyla temel kıyaslama aracı olarak kullanılmıştır. Analizlerde SST k-ω türbülans modeli tercih edilmiş ve yaklaşık 1.5 milyon elemandan oluşan çözüm ağı ile kritik bölgelerde düşük y+ değerleri hedeflenmiştir. Parametrik çalışmalar sonucunda, özellikle yüzde 3 ve yüzde 6 genişlikte ve düşük ya da orta en boy oranına sahip oluk konfigürasyonlarının stall marjını yüzde 15 ila 19 aralığında artırdığı ve verimde ise yüzde 0.1'den daha düşük seviyede kayıplara yol açtığı belirlenmiştir. Buna karşılık, yüzde 9 genişlikteki veya yüksek en boy oranına sahip oluk konfigürasyonlarında ana akışın oluk bölgesinde bozulduğu, uç bölgede düşük enerjili alanların ve vortex yapılarının büyüdüğü; buna bağlı olarak hem stall margininde hem de verimde belirgin düşüşler yaşandığı tespit edilmiştir. Akış görselleştirmeleri, olukların uç bölgesinde oluşan sızıntı akışlarını ve vorteks yapısını baskılayarak daha kararlı bir akış rejimi oluşturduğunu göstermiştir. Q-kriteri analizlerinde de, oluklu ve oluksuz geometrilerdeki vorteks yapılarının boyut ve şiddetindeki değişimler karşılaştırılmıştır. Sonuçlar, optimum oluk tasarımının pasif bir kontrol yöntemi olarak kompresörlerin operasyonel sınırlarını genişletebileceğini ve çok düşük verim kaybı ile daha kararlı çalışma aralığı sağlayabileceğini göstermiştir. Bu çalışma, ileride oluk lokasyonu, genişliği, derinliği ve sayısı gibi parametrelerin optimizasyonu ile ileri düzey sayısal ve deneysel yöntemlerle araştırılmasının önünü açacak bir temel sağlamıştır
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ÖgeAxisymmetric drop shape analysis for investigating surface tensions in pendant and sessile drops(Graduate School, 2025-06-04)Surface tension plays a critical role in numerous interfacial phenomena across scientific and industrial applications, making its accurate measurement essential for advancing material and fluid characterization. This study presents a comprehensive framework for surface tension measurement based on the Axisymmetric Drop Shape Analysis (ADSA) method. It combines a low-cost experimental setup with a custom-developed open-source software solution. The aim of this thesis is to improve measurement accuracy of the low-cost drop shape tensiometers while enhancing accessibility and reproducibility for researchers. A precision-controlled ADSA system is designed and assembled for both sessile and pendant drop configurations, incorporating a high-resolution imaging module, an environmental test chamber and a modular droplet dispensing unit. The imaging setup includes a DSLR camera and adjustable LED backlighting to ensure sharp contrast and edge clarity for drop profile acquisition. Additionally, a pitch–yaw tilt correction platform is integrated to minimize substrate inclination, ensuring axisymmetric droplet shapes crucial for accurate surface tension analysis. During the experimental setup assesment, a continous improvement has been done on identifying and minimizing key sources of error such as optical distortion, camera misalignment, and calibration inaccuracies. A combination of distortion grid correction, stable lighting conditions, and geometric alignment procedures were implemented to ensure the fidelity of drop imaging. Following the experimental setup design, a Python-based software was developed to automate the surface tension measurement process. This tool incorporates advanced image processing steps, including noise filtering, Canny edge detection, and apex detection. Subsequently, the Young–Laplace equation was numerically solved, and theoretical profiles were optimized to best fit the experimental drop contours. The software's modular structure allows researchers to adapt it for various drop types and experimental conditions, and its open-source nature promotes transparency and collaboration within the scientific community. To validate the accuracy and functionality of the system, the surface tension of water–ethanol mixtures varying %0-50 wt was measured. The obtained values showed strong agreement with those reported in the literature which confirmed the system's reliability and precision. This work offers a significant contribution to the field of interfacial science by providing a reliable, cost-effective, and reproducible approach to surface tension measurement. The ADSA platform developed here enables researchers to conduct detailed analyses with high accuracy, while its open-source software lays the groundwork for further improvements. Future research directions include enhancing the system's capability for dynamic measurements and expanding its application to complex fluids and surfactant-laden interfaces. In summary, this study delivers an integrated experimental-computational solution for surface tension analysis supporting the advancement of interfacial engineering and materials research.
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ÖgeAerodynamic performance enhancement of a 27-inch APC propeller through geometric modifications(Graduate School, 2025-06-12)Propellers are undergoing a significant resurgence in aviation, vital for Unmanned Aerial Vehicles (UAVs), general aviation, and emerging Electric Vertical Take-Off and Landing (eVTOL) aircraft. The design of a propeller—encompassing its dimensions, material, and particularly its blade geometry—is paramount for vehicle efficiency, stability, endurance, and mission success. Inefficient propellers lead to increased energy consumption and environmental impact. This thesis focuses on enhancing the propulsive efficiency of propellers through systematic, computationally-driven geometric modifications. The primary research objective was to develop and apply a methodology for improving propeller propulsive efficiency by parametrically altering key blade geometric parameters. Specifically, this study investigated the effects of static pitch, blade twist distribution, and airfoil thickness distribution using Computational Fluid Dynamics (CFD) simulations, with all analyses conducted at a fixed rotational speed of 3000 RPM across a range of advance ratios. The methodology commenced with selecting the APC 27x13E propeller as the baseline, chosen for the availability of its geometric data and published performance metrics. The blade geometry was reconstructed into a 3D CAD model using a workflow involving a Julia script and OpenVSP. An initial CFD setup using the k-ω SST turbulence model on a half-domain showed that very fine meshes (around 15.7 million cells) were needed for high accuracy but were computationally prohibitive for extensive parametric studies. Consequently, a turbulence model evaluation was performed using a full-domain model. The Realizable k-ε model with standard wall functions, applied to a mesh of approximately 2.45 million cells, was selected as the final CFD setup. This configuration predicted thrust and torque with error of approximately 6% and 5.5% respectively, when compared to vendor data, offering a suitable balance between accuracy and computational efficiency for the subsequent comparative analyses. Geometric manipulation strategies involved three campaigns. First, static pitch variants (7, 10, 13, 16, and 19-inch pitch for the 27-inch diameter propeller) were generated by adjusting local blade angles according to blade angle equation for static pitch propellers. The 13-inch baseline was found to offer a good overall performance profile across a wider range of advance ratios and was selected for further modifications. Second, four twist angle distribution variants were created (mild/aggressive root untwist with tip overtwist, and mild/aggressive root overtwist with tip untwist), keeping the blade angle at the 75% span station identical to the baseline. Third, four airfoil thickness distribution variants were developed (mild/aggressive uniform thickness changes, and mild/aggressive tapered thickness changes), anchoring the thickness at the 75% span station to the baseline value. Aerodynamic results indicated distinct impacts from each modification type. For static pitch, higher pitch propellers generally yielded better efficiency at higher advance ratios. The twist angle modifications showed the most promising results for efficiency enhancement. Specifically, the B1 variant (mild root overtwist, mild tip untwist) achieved a peak efficiency of approximately η = 0.70 at J ≈ 0.5, a modest gain of about 1.8% over the baseline's peak. However, this variant demonstrated a more substantial relative improvement at higher advance ratios; for instance, at J ≈ 0.6, its efficiency was approximately η = 0.62, a significant gain of over 23% compared to the baseline's η = 0.50 at that condition. In contrast, modifications to the airfoil thickness distribution were generally detrimental, reducing aerodynamic efficiency at moderate to high advance ratios compared to the baseline, suggesting the baseline's thickness profile was already well-optimized. This thesis successfully demonstrated a systematic CFD-driven approach for evaluating and improving propeller aerodynamic performance. The findings highlight that carefully considered twist distribution modifications can yield significant relative efficiency gains, particularly in specific segments of the operational envelope, which is valuable for UAV designers. Limitations include the steady-state MRF approach and the assumption of rigid blades. Future work could involve multi-parameter optimisation, the application of CFD-based adjoint methods for finer refinement of promising designs like the B1 twist variant, aero-acoustic analysis, and experimental validation.
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ÖgeEffects of inflow perturbations generated with hydrodynamic stability concept on the time dependent flow development(Graduate School, 2025-06-16)This study investigates the influence of inflow conditions on the development of turbulence in axially rotating pipe flow, with the aim of improving our understanding of turbulence onset in transitional regimes and generation of proper inflow conditions. The flow configuration is motivated by both fundamental interest and practical relevance in rotating machinery and pipe transport systems, where inflow disturbances and swirl can significantly affect transition dynamics. The analysis begins with the spatial inviscid hydrodynamic stability problem, solved for six configurations involving distinct mean axial velocity profiles, both with and without rotation. The parallel-shooting method is employed to compute eigenvalues and mode shapes. The inviscid results reveal both stable and unstable wave-like solutions, with mode shapes resembling Bessel functions in simpler cases. The propagation direction and growth rates of these modes seemed to vary with rotation and velocity profile. To capture viscous effects, the spatial viscous stability problem is then solved for two physically relevant cases, laminar axial profile without swirl and a turbulent profile with swirl. The viscous spectrum reveals two mode families, one converging to inviscid modes with slightly different dispersion characteristics, and a second "viscous subset" exhibiting distinct spatial structures and primarily downstream propagation. These include wall and centre modes, whose spatial coherence decreases with increasing Reynolds number and frequency. No unstable viscous modes are observed within the investigated parameter space. Large Eddy Simulations (LES) are performed using OpenFOAM with a radius-based Reynolds number of 2500 and a swirl number of 0.5, employing the Smagorinsky subgrid-scale model. Inflow perturbations are constructed from hydrodynamic stability modes, selected based on orthogonality to ensure a representative and non-redundant perturbation basis. A control case without inflow perturbations is also simulated. The results show that inflow conditions derived from viscous stability theory significantly improve the accuracy of turbulent statistics. Specifically, turbulence characteristics begin to converge approximately 40 pipe diameters downstream when perturbations are imposed, in contrast to the delayed and less realistic transition observed in the unperturbed case. Overall, this study demonstrates that accurately prescribed inflow perturbations, grounded in linear stability theory, can substantially enhance the fidelity of LES in transitional pipe flows with swirl. These findings underscore the importance of coupling theoretical stability analysis with numerical simulations to better predict and control turbulence onset.