Havacılık ve Uzay Mühendisliği Anabilim Dalı
Adana Alparslan Türkeş University of Science and Technology48
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Anabilim Dalı
The effect of the riblets on flow control of NACA0018 at low reynolds number
In recent years, numerical and experimental aerodynamic studies have been focused on low Reynolds number flows in order to control turbulence transition and laminar separation bubble of unmanned aerial vehicles and wind turbines. In this thesis, numerical analysis was performed to control of flow over NACA 0018 airfoil using riblet surface at Reynolds number of Re=1x105 in order to increase aerodynamics coefficients such as lift and lift to drag ratio. The riblets were located on the suction surface of airfoil. The main parameters were selected as the height of riblet, location, gap ratio between riblets, the size of riblet region and angle of attack. The numerical analyzes were carried out using k-kL-ω transition model. The obtained results indicate that the height of riblet remarkably affects the formation of laminar separation bubble and increase the lift coefficient in comparison with clean airfoil.
Passive flow control around the airfoil with slot
In this thesis, numerical analysis was performed to control of flow over NACA 0018 airfoil with slot at variant angles of attack. The Reynolds number was set to Re=3x10^5. Inlet of slot was posiniated on the pressure side of airfoil at chordwise location of x/c=5% while outlet of slot was located on the suction side of airfoil at three different chordwise location. Numerical analyzes were carried out using two different turbulence models, K-ω SST and K-ε RNG. Obtained results from the numerical analyzes were evaluated in terms of effect of the slot on flow structure and aerodynamic forces. It was observed that the location of slot remarkably affect flow over airfoil and delays stall angle. The model 40 suppress the formation of recirculation region formed over the airfoil. Furthermore, it was observed that while the stall angle of base airfoil is α=16°, it was delayed up to α=20° for Model 40. Also, it was observed that the lift coefficient was increased by 154% for Model 40 at α=19°. On the other hand, it was observed that Model 20 significantly effective on the flow control in comparison with the base airfoil at angles of attack of α ≥13°, while it has insignificant effect at low angles of attack. Similar observation was also obtained for Model 60 at low angles of attack. According to obtained results from both turbulence models, the most efficient model was determined as Model 40 in order to control flow around the airfoil especially for post stall angles.
Fracture behavior of 3D printed PLA structures
3D Printing is gaining importance as a cost-efficient, low waste, and flexible manufacturing technology. ABS and PLA filaments are dominating the market as input materials whereas PLA is an advantageous choice thanks to its biodegradable and environmentally friendly properties. 3D printing technology offers various internal structure designs of the produced parts. The thesis aims to determine the effect of internal structure in 3D printing. The results will yield an important contribution in terms of fracture design of 3D printed PLA structures. A total of 44 parts were tested, with different nozzle sizes, filling types, filling ratios, and angles. In this way, the breaking behavior of these PLA products produced in a 3D printer, taking into account many different factors, has been examined in detail. As a result, the effect of nozzle size can be ignored. In addition, the filling ratio affected the brittleness of the part, and the filling type affected the amount of elongation. Stress intensity factors of the parts were calculated using the obtained data.
Vision-based landing site detection for a UAV: From theory to application
This study addresses the critical need to enhance the safety of multirotor UAVs during emergency flights by addressing the challenges of identifying suitable landing spots and avoiding collisions with obstacles. Drones encounter difficulties due to their elevated height, leading to potential collisions, inaccurate distance estimation, and weak radio signals. Researchers are diligently developing a cost-effective and efficient landing system to mitigate these issues and enhance overall safety. Key gaps in vision-based landing site detection for UAVs include adapting to varying environmental conditions, achieving real-time processing, precise obstacle recognition, ensuring robustness, and scalability, handling data anomalies, integrating with other sensors, obtaining diverse training data, complying with regulations, and maintaining cost-effectiveness. To address these challenges, the proposed model combines a U-Net, a deep Convolutional Neural Network (CNN) architecture, with a ResNet 34 backbone. The model builds a labelled aerial photo database using image segmentation techniques for CNN training, determines landing areas through 2D dataset image analysis, and plans paths from the UAV's position to the designated area centre using the A* algorithm. The entire research is implemented in Python, harnessing TensorFlow's capabilities. Impressively, the model achieves a training accuracy of 96.6% and a validation accuracy of 96.34% while utilizing approximately 80% of the dataset, comprising 1480 images, during the training phase, with a recorded training loss of 0.1034. These outcomes underscore the model's exceptional accuracy, establishing it as a cornerstone for subsequent landing area selection and path planning by the algorithm.
Investigation on the effects of dielectric material on the thrust performance of annular DBD plasma ion thrusters
In this thesis, the effects of dielectric material on the propulsion performance of annular DBD plasma ion thrusters are investigated. Briefly, the history of electric propulsion system, types of electric thrusters and DBD ion thrusters are given. In order to maximize the propulsion performance, geometrical parameters (annular and sinusoidal), electrical parameters and material types were considered and experiments were carried out. In all experiments, voltages of 4 kVpp, 6 kVpp and 8 kVpp were applied respectively. An electronic thrust measurement device was used to measure the thrust generated by the DBD plasma actuator during the experiments. As a result of the experiments, it is stated that the thrust value increases while the voltage is increased. Moreover, it is observed that for sine geometries with the same outer diameter, the thrust efficiency decreases as the diameter increases.
Investigation of ice accretion effects on aerodynamic performance of NACA 2415 airfoil at low reynolds number
This thesis examines the effects of three distinct ice accumulations on the aerodynamic performance of the NACA 2415 airfoil. A brief discussion is provided of icing environments, icing types and the icing effects on the airfoil and also on the aircraft components. Besides, some necessary information is provided for icing prevention (anti-icing) and de-icing techniques. The experiments are carried out for the base airfoil as well as glaze, rime, and horn ice accumulated airfoils at Re=1.2×105. For this study, force and velocity measurements are taken. Additionally, the oil and smoke-wire flow visualization techniques are also performed. The selected range of angle of attack for force measurements is defined as -7° to 23°. Additionally, considering the results of force measurements, seven distinct angles of attack values including 0°, 4°, 8°, 13°, 14°, 17° and 20° are chosen for velocity measurements and flow visualization experiments. Results showed that, CLmax of ice accreted airfoils are decreased as expected. 20%, 23% and 88% of decrease in maximum lift coefficient is observed in the GIA, RIA and HIA models, respectively. An increase in drag coefficients at α = 0° for the GIA, RIA and HIA airfoils is observed to be around 14%, 4% and 307%, respectively. A sudden increase in lift coefficient of the GIA airfoil is observed at post-stall. Moreover, a double-peak behavior is noted for the GIA, RIA airfoil and also for base airfoil in velocity measurements.
Hibrit kompozit malzemelerin balistic modellemesi
Hybrid composites have become one of the essential and highly important engineering materials for advanced and complex applications in today's industry. In this study, the behaviours of composite materials under the same ballistic effect of layer type and layer arrangement were investigated experimentally, provided that the number of layers remained the same. . In the laboratory, 10 resin-supported glass fibre and carbon doped hybrid composite samples were obtained by vacuum infusion production method. In order to examine the ballistic effects of the produced composites, shooting was carried out in the polygon in accordance with STANAG 2920 standards. As a result of these specified firing tests, the damages caused by the projectile on composite materials were analysed. In addition, it was analysed how the layer order and layer type in the material affect the projectile velocity.
Düşük reynolds sayisinda naca 0012 kanat profi̇li̇ni̇n aerodi̇nami̇k özelli̇kleri̇ üzeri̇ndeki̇ hücum kenar deği̇şi̇mi̇ni̇n etki̇si̇
In this study, it is aimed to enhance the aerodynamic performance of the NACA 0012 airfoil with the morphing leading edge method. The investigation was conducted using an airfoil with a chord length of 120 mm under Reynolds number conditions of Re = 105. In addition to the base airfoil, a MATLAB code was developed to generate modified airfoil geometries by specifying the deflection angle and the deflection initiation point (expressed as a percentage of the chord length) as input parameters. The computational analysis considered deflection angles of 2°, 4°, and 6°, initiated at 10%, 20%, and 30% of the chord length. All configurations were analyzed using ANSYS FLUENT and compared aerodynamically with the base case (0° deflection). The performance evaluation focused on maximizing lift coefficient (CL), minimizing drag coefficient (CD), and optimizing the lift-to-drag ratio (CL/CD) as a measure of aerodynamic efficiency. The results demonstrate that airfoils with 6° deflection achieved the highest aerodynamic efficiency relative to the base model in the post-stall regime (e.g., α = 18°), with observed reductions in laminar separation bubbles (LSB) and trailing-edge vortex intensity. At moderate angles of attack (α = 10°), airfoils with 4° deflection exhibited optimal performance, yielding a 73.61% improvement in CL/CD compared to the base model. While stall characteristics revealed limited improvement, all modified geometries demonstrated reduced drag compared to the base model.
The effect of partial flexibility on aerodynamic performance of finite wing
In this thesis, the aerodynamic effects of partially flexible surface coverage applied to a NACA 0018 airfoil operating in low Re regimes were experimentally investigated. The flexible surface, made of latex material, was placed along a specific chordwise region of the wing, and four different wings with varying spanwise flexibility ratios were tested. Wind tunnel experiments were conducted in the Re range of 3 x 10⁴ to 10 x 10⁴, measuring lift (CL) and drag (CD) coefficients. Additionally, surface oil and tuft flow visualization techniques were used to analyze flow structures. The findings revealed that the partially flexible surface significantly suppressed the laminar separation bubble (LSB), delayed stall, and enhanced lift coefficient, thereby improving aerodynamic efficiency, particularly at lower Re. However, the effectiveness of the flexible surface diminished as the Re increased. These results contribute to the literature by demonstrating the potential of partial flexibility in optimizing passive flow control methods for UAV applications.
Çok taneli plastisite ile çift fazlı çeliklerde lokalizasyon ve boyun verme modellenmesi
In this thesis, the effect of ferrite crystallographic orientation distribution and other microstructural parameters such as martensite morphology, martensite volume fraction, and ferrite grain size on the plastic deformation, localization, and the necking behavior of dual-phase steels are investigated. Two different type of finite element models, i.e. full size micron-scale polycrystalline samples and polycrystalline Representative Volume Element (RVEs) are built through Voronoi tessellation. Local crystal plasticity and J2 plasticity with isotropic hardening frameworks are used to model ferrite and martensite phases, respectively. This work demonstrated that the martensite morphology and the ferrite orientation distribution affect highly the formation of the shear bands and the necking location in the samples. In addition, they significantly affect stress-strain partitioning, location of the plastic localization, tensile strength and hot spots for void formation in the RVEs.
Yüksek hızlı ı̇ç ve dış akışlar ı̇çı̇n ısıl aşınmanın modellenmesı̇
In this thesis study, the ablation of graphitic materials is modeled and two different analysis tools are developed. In the first model, the computations are conducted with a decoupled approach. Secondly, a more generalized calculation method is introduced via coupling the flow field and solid conduction analysis codes. In the modeling studies, both the gas and solid domains are assumed to be axisymmetric and the physical domains are discretized with structured finite volume cells. For the flow field analyses, Godunov type approximate Riemann solvers are used for the convective flux calculations and the gradient terms of the viscous fluxes are calculated with the help of grid transformation metrics. Mass diffusion terms are included in the Navier-Stokes equations. Baldwin-Lomax, Baldwin-Barth and Spalart-Allmaras turbulence models are implemented and the results are compared for the coupled approach. A linear solver is developed for the computation of gas phase chemical reaction source terms. The effects of the solid thermal properties with temperature are not neglected in the computations. The models are tested for hypersonic air and Solid Rocket Motor (SRM) nozzle flow conditions. Wall equilibrium is assumed for graphite in air problem and an implicit solver is developed for the calculation of surface species mass fractions rather than using previously prepared lookup tables. Both equilibrium and finite rate surface thermochemistry approaches are tested for the internal flows of the SRM nozzles.
Isıl bariyer kaplamalarda oksidasyon ve plastısıte etkılı hasarın modellemesi
Computational analysis and simulation of multi-physics phenomena taking place in coating systems is still a challenging task. Specifically, for ceramic coatings used as protective systems for base materials against elevated temperatures, known as thermal barrier coating (TBC) systems, construction of continuum level models which can express coupled nonlinear phenomena has attracted great attention. Thermal stresses, oxidation, creep and numerous other mechanisms and phenomena make it even harder to model and simulate the behavior of TBCs and bring a need for the development of premier models. In this effort, a new numerical model that allows simulation of oxidation and thermally grown oxide (TGO) in bond-coat is presented. Phase-field theory is used with finite strain formulation and implemented using user element subroutine (UEL) in ABAQUS software for the finite element method. Results are compared with experimental data available for TGO in the literature. Initiation and development of damage were also modeled considering plasticity induced failure. Results were also compared with experimental data available for damage and stress development in isothermal conditions.
Helikopterler için gürültü minimal & yeşil yörünge ve uçuş profili eniyilemesi
The main aim of this study is to provide a multi-disciplinary optimization and track environment to generate acoustic optimal trajectories through waypoints that ensures the rotorcraft of interest can follow at practical effort, safety, fuel consumption and speed. Rotorcraft noise annoyance remains as a challenge to solve complex, three dimensional and coupled rotary wing aerodynamics, aeroacoustics and flight dynamics interactively. Two essential paths can be acknowledged in order to reduce annoyance. One is the more sophisticated option, optimized new rotorcraft design, whereas the other option is to benefit from the directivity characteristic of sound and perform trajectory optimization to minimize noise impact at noise sensitive premises. This study focuses on the second yet with consideration of the potential trade-offs between low noise signature and other performance parameters. Eventually, the main aim of this study is to develop a trajectory optimization and track framework for rotorcrafts providing minimal noise, low emission i.e. lower fuel consumption, safe and trackable, in other words "green" flight profiles. In this scope, a Lagrangian CFD solver specialized for rotor/propellers is developed, coupled with rotorcraft mathematical model and an aeroacoustics solver to build a high fidelity, accuracy and resolution rotorcraft comprehensive modeling environment. The developed methodology is validated with wind tunnel, whirl tower test data, PIV results and benchmark commercial tools. The developed comprehensive tool provides free flight trim, high fidelity modeling and analysis capability for conventional and unconventional rotorcraft configurations with unsteady wake dynamics covering blade-vortex, rotor-wake and rotor-rotor interactions. Further in the study, the comprehensive model is extended into a real-time computable simulation model. Then a model predictive control -an optimal control- approach is developed to simultaneously optimize the trajectory and control input to track the generated trajectory. The multi-disciplinary objective function including acoustics, performance, fuel, safety, comfort and mission concerns provides the so called "green" trajectory with reduced noise impact at desired locations. Various simulations were performed to further test the aerodynamic modeling, aeroacoustics analysis and trajectory optimization capabilities of the developed framework. It is concluded that the proof of concept, i.e. the potential of reduced noise impact and fuel consumption over the same mission through trajectory optimization, is achieved. Developed methodology can be utilized to generate optimal flight routes and procedures specific to rotorcraft configuration, which are currently rather generic for all types of rotorcrafts, especially for booming e-VTOL platforms that will mostly operate over urban areas or for re-planning of legacy flight routes
Dikey iniş ve kalkış yapabilen ve kanatlari döndürülebilen bir insansız hava aracının tasarımı ve analizi
In this study, the design and analysis of a UAV, which is capable of vertical take-off and landing using fixed six rotors placed on the tilt-wing and tilt-tail, will be explained. The aircraft has four rotors on its wing and two rotors on its tail. The main wing and horizontal tail are capable of 90° tilting. Both aerodynamic and thrust forces are used during VTOL, transition, and forward flight. Aerodynamic analysis has been performed in ANSYS Fluent v.18. A non-linear six DoF model, involving a 3D CAD model of the aircraft, has been created in MATLAB/Simulink/Simscape. The transition problem has been identified. Three types of robust controller algorithms, involving PID and LQR methods, have been implemented to overcome the challenges which have been faced while performing transition from vertical to horizontal flight phase and vice versa, and the results of each controller type have been compared concerning those criteria. Eight trim points have been identified for full mission profile, and for each trim condition, separate controllers have been designed. Gain scheduling has been employed between trim points for a smooth transition.
Doğal laminar akışlar için ayrık adjoınt tabanlı aerodinamik şekil eniyileme aracı geliştirilmesi
An adjoint-based aerodynamic shape optimization framework for natural laminar flows is developed. A Reynolds-Averaged Navier-Stokes flow solver with the Spalart-Allmaras turbulence model is coupled with the recently developed Bas-Cakmakcioglu transition model in order to predict laminar to turbulent transition onset. In the gradient-based optimization process, the sensitivity derivatives required by the optimization algorithm is obtained by the discrete adjoint method, which is developed for the in-house flow solver and implemented for natural laminar flow airfoils and wings. In the development of the discrete adjoint method, an automatic differentiation tool is employed to take the discrete derivative of the modules in the in-house flow solver heavily modified. The parametrization of the aerodynamic surface is realized by the Free-Form Deformation technique. The sensitivity derivatives with respect to design parameters, which are computed by the adjoint method, are validated with the finite-difference method. The success of the adjoint-based aerodynamic shape optimization methodology developed in this study is then demonstrated by optimizing aerodynamic characteristics of several airfoils and wings for compressible turbulent and natural laminar flows.
Bir jet eğitim uçağı kanadının dayanım ve direngenlik kısıtları altında yapısal eniyilemesi
This thesis presents structural optimization studies for a jet trainer aircraft wing structure. The main purpose is to select the most convenient rib/spar layout for T-38 wing like geometry which consists of metallic and composite components by using a finite element modeling and analysis tool of MSC NASTRAN optimization capabilities. In this study, in order to decrease the number of design variables and as well as to be able to obtain smooth thickness transitions between adjacent zones, design variable linking method is used by applying different shape functions. First of all, after creating the outer geometry in CATIA, the finite element model (FEM) is prepared using MSC PATRAN. Similarly, all design zones where design variables are free to change are designated. To be able to link the thicknesses of the components to each other with regards to their locations on the wing, all coordinates of design zones are transformed to natural coordinate system as their center points are positioned as located between 0 and 1 in span-wise direction and -0.5 and 0.5 in chord-wise direction using scripts coded in Python. Aerodynamic loads are then calculated by using MSC FLDS (FlightLoads) tool and summed in predefined monitor stations to distribute onto the structure using rigid body elements. Obtained shear force and bending moment distributions are also compared with another method called as Schrenk's Approximation. In the optimization studies, thicknesses of the metallic structures and principal composite ply thicknesses are considered as design variables. There are five design constraints which are von Mises stress for the metallic structures, failure index for strength check of composite structures, global buckling, damping and natural frequencies which are used to control the flutter speed. The objective in this research is to obtain a minimum weight in design while providing predefined constraints via investigating various design candidate geometries having different layouts but same outer geometry. Finally, by using the scripts developed within this thesis study, the design variable linking method used according to the locations of the design zones become suitable also for the Nastran Sol 200 users.
Optimizasyon yöntemleri ile helikopter manevralarının gerçekleştirilmesi
In order to certificate a helicopter, aviation safety agencies must know that all designed helicopter configurations can withstand all loads resulting from maneuvers defined in certification standards. In other words, the maneuvers defined in regulations must be performed for each appropriate combination of weight and center of gravity. Then, designers have to prove that the helicopter can fly safely across the entire flight spectrum. Therefore, load engineers perform all maneuvers defined in the helicopter usage spectrum in order to analyze all possible load values. In traditional methods, a trial and error approach is used to reach the maneuver. However, performing these maneuvers with a trial and error approach not only causes expensive computing but also requires more engineering effort. Additionally, it may cause some defects in the maneuvers. Therefore, this thesis study aims to achieve the desired helicopter maneuvers using optimization methods in order to reduce the calculation cost and engineering efforts and perform the maneuvers accurately. For this purpose, only selected maneuvers are performed in this thesis. In addition, various optimization methods in different configurations have been applied to solve these maneuvers. Thus, the most useful one among them has been decided by making the necessary comparisons. Finally, the most useful optimization method has been applied to maneuvers frequently performed by helicopters throughout its lifetime.
Mikromekanik temelli kohezif bölge elemanları yoluyla metalik malzemelerde sünek kırılma
Gaining popularity after its coupling with the finite element method, cohesive zone modelling has been used extensively to model fracture, especially in delamination problems. Its constitutive relations, i.e. traction-separation laws, are mostly derived phenomenologically without considering the physical mechanisms of crack initiation and propagation. The approach could also be used for ductile fracture where the micromechanics of the phenomenon is explained by nucleation, growth and coalescence of pores. In this context, the objective of the current thesis is to develop and implement a cohesive zone modelling framework for ductile fracture in metallic materials. In order to accomplish this, a micromechanics based traction-separation relation which considers the growth of a physical pore is developed based on the previous works in [1–3]. Tractions are directly represented as a function of pore fraction, and its evolution is driven by separations. The model is implemented as an intrinsic cohesive zone model in a two-dimensional (2D) setting. Implementation steps and methodology including the finite element framework are presented in detail for mode-I, mode-II and mixed-mode fracture cases. The derivation of the mixed-mode case leads to a yield function representation of tractions and separa-tions, instead of an explicit expression. Hence, an incremental implicit elasto-plastic numerical integration scheme is developed to solve mixed-mode system of equations. Implementation is validated by running tests with a single cohesive element. In addition, the framework is implemented as a user element subroutine in Abaqus (UEL) and the numerical simulations are conducted with compact tension (CT) and single edge notch (SEN) specimens to show the capability of the model and the influence of the micromechanical parameters such as pore size and shape on the ductile crack initiation and propagation. The work is concluded by presenting an outlook for the usage of the model in micron sized specimens where the developed micromechanical model presents a great potential in explaining certain deformation mechanisms in high strength aerospace alloys.
Jenerik bir çırpan kanat mekanizmasının aerodinamik ve atalet karakterizasyonu
The primary focus of this thesis is to conduct an aerodynamical and inertial characterization of a generic flapping wing mechanism. Firstly, a CAD model of the utilized flapping mechanism is created, and a custom test platform is developed for the performed flapping-wing test sequence. Experimental validation of the modal analysis performed for the developed test platform is presented. In the inertial force characterization study, the standalone effect of the inertial forces that occur during flapping motion on the produced total aerodynamic lift force is aimed to investigate. In this framework, the wing membrane is eliminated, and a flapping wing test is performed to measure the inertial force generated due to the motion of the prescribed kinematics in the lift direction. The dynamic calibration of the utilized sensor is carried out to ensure the fidelity of the acquired data during the flapping test. Verification studies regarding to the experimental inertial force investigation are conducted by performing theoretical and dynamic finite element analysis approaches. For the aerodynamic characterization of the proposed system, a nylon wing membrane is attached to the utilized flapping wing mechanism, and a flapping wing experiment is performed in the air medium. To verify the experimental results, a numerical validation study is conducted and, a fluid-structure interaction analysis model is presented for the corresponding model. Besides, the effect of wing-flexion on aerodynamic performance is investigated for the corresponding model. In this regard, a distinct fluid-structure interaction analysis model, including a flapping wing with high-rigidity material properties, is created to minimize the deflection that occurs on the wing structure. Experimental, theoretical, and numerical approaches conducted for the inertial and aerodynamic characterization of the utilized flapping mechanism showed great resemblance to each other. A comparative study performed for the effect of wing-flexion shows that wing elasticity enhances the thrust force for the proposed model.
Doğal dinamik kullanılarak hale yörüngede ve dünya etrafında seyır eden uzay araçları için kol uçuşu tasarımı
This thesis presents the studies performed for spacecraft formation flight design and analyses. Two main design problems are addressed in this thesis: First the formation flight design of satellites flying near Sun-Earth collinear libration points; the second the formation flight design of satellites flying at Low Earth Orbit. Thus, formation flight design near Sun-Earth L1 and L2 libration points is investigated first, where solar radiation pressure as well as the gravitational disturbances of the planets are taken into account, for different Julian dates. The periodicity of the relative motion in formation flight is taken as a design criterion and convenient initial conditions are computed for each deputy satellite for desired formation configuration. It is desired that the required formation is maintained without the need of any correction maneuvers for formation keeping. In the second part, the method presented in this thesis for formation flight design is applied to Low Earth Orbit satellites. The results are also compared to the results obtained using current methods available in the literature. Results show that proposed method gives more consistent results and provides flexibility on the orbit design for formation in terms of formation keeping and fuel consumption needs as compared with the currently available methods. Finally, it can be stated that the trajectory and orbit computations done using the method presented in this thesis provide long term formation flight for space missions at L1, L2 libration points and for Low Earth Orbit missions. The main contribution of this method is the inclusion of all disturbancing forces acting on the satellite as a time variant discrete model. The initial conditions are found iteratively that ensures the periodic trajectory. Here, the usage of time variant discrete model to obtain periodic relative motion is a feature that distinguishes the current study from the existing methods in the literature.
Deneysel sonuçları kullanarak türbülansın kuantik davranışı yaklaşımıyla bazı türbülanslı akımlarda ölçekleri değerlendirmeye yönelik bir çalışma
The knowledge of the energy contents, sizes, and lifetimes of eddies is necessary to understand the structure of turbulent flows. The spectral approach can provide the frequency-dependent energy spectrum, which helps to find out this structure. The present study focuses on finding eddy sizes or wavelengths using the frequency-dependent energy spectrum, which is consistent with the physics of turbulence. This is because the eddy size is related to wavenumber. The turbulence studies in the literature use a dispersion relation that connects frequency to wavenumber. The mentioned dispersion relation is generally used based on Taylor's hypothesis, and examples of this usage are frequently encountered in the literature. Nevertheless, this approach has some deficiencies and limitations. As a result of this, several studies in the literature have tested and made corrections to this hypothesis. Some of them have tried to find alternative methods. The present thesis study includes the application of an alternative approach named Quantic Behavior of Turbulence (QBT) proposed by Çıray. With this approach, the nature of turbulence is explained with a dual character, including particle and wavy character. The mathematical procedure offered by this approach finds wavenumber to corresponding frequency. Within this study, the first step has been an implementation of this method to a Matlab code. This computer code uses the spectral approach to obtain the spectrum in the frequency domain. After that, the wavenumbers are provided by pursuing the mathematical procedure of QBT. Then, several different types of turbulent flows have been analyzed by this code. As a consequence of these analyses, the spectrum in the wavenumber domain is obtained in accordance with the data from the literature. With this approach, results such as sizes, lifespans, and energy contents of eddies are calculated in conformity with physical behavior. It is aimed that this approach and this study can contribute to more accurate understandings and solutions of turbulence problems.
Entegre güçlendirilmiş yapısal paneller için yapay sinir ağı bazlı burkulma yükleri belirleme aracı
The sudden change in the load carrying capacity under compresive loading, called buckling, may cause catastrophic failures. Therefore, determination of the first buckling and collapse loads of structural elements is essential in preliminary design stages. Finite element (FE) analyses and structural testing are used to determine buckling characteristics of a structural element. However, in early design stages, FE analyses are time consuming and structural testing is costly. In this study, an artificial neural network tool (ANN) is used to reduce computational effort to determine buckling loads of integrally stiffened structural panels in early design stages. Reuslts of FE analyses are employed to train the ANN. Moreover, Latin Hypercube Sampling (LHS) methodology is used to reduce the number of required FE analyses to generate database that artificial neural network is based on. Finally, a Multi-fidelity sampling algorithm that uses FE models with different mesh resolutions is implemented for generation of the ANN database in order to reduce computational time spent for finite element analyses. Mean errors and fit performance model results are compared to determine accuracy of the neural network results.
Kavramsal çok pervaneli bir e-VTOL hava aracı için blended-ınverse kullanarak kontrol dağıtımı
In this thesis, the control allocation problem in a flight control system design for a multi-rotor eVTOL (electric Vertical Takeoff and Landing) aircraft is proposed. The vehicle consists of 20 identical rotors that are used as flight control actuators. The dynamic system is a MIMO (Multi Input Multi Output) system with more inputs than outputs, i.e. there are many solutions of the control problem. The objective is to find an efficient and redundant control solution that provides sufficient flight performance, handling quality, and power consumption. Developed control system algorithms are applied to a simulation model of the conceptual aircraft consisting of advanced and nonlinear dynamic components. By using this model, simulation based flight tests are conducted and the results are presented and evaluated. Also, redundancy of the control system is considered for flight safety. Therefore, various simulator based rotor failure scenarios are tested on the aircraft and the proposed method is also evaluated from a redundancy point of view. It is shown that robust and efficient control redistribution can be achieved using the proposed solution under challenging failure conditions.
Mühimmat sürüsü için güdüm yöntemlerinin değerlendirilmesi
In this study, collective flight, mission execution and decision-making algorithm for airdrop micro munitions with two different swarm algorithm alternatives are examined. Through a concept munition design is created and an aerodynamic database is prepared with the Missile-Datcom software. Six degrees of freedom flight mechanics model, subsystems of munition and environment are modeled. A linear model is obtained and controller is designed. Pole placement method is used in the controller design. Full state feedback assumption may not suitable for the actual problem. For this reason, the autopilot gains of the designed controller are converted into gain values suitable for measurements by using the projection control method. The designed autopilot tracks the commands of the swarm and proportional navigation guidance. The swarm algorithms coefficients are optimized for the nominal state by particle swarm optimization. Alternative algorithms obtained as a result of optimization are tested under various distortions by Monte Carlo analysis. Sensitivity analysis and performance evaluation are examined as well.
Sünek hasar tahmini için gözenekli bir plastisite modelinin sayısal uygulaması ve analizi
Ductile damage and fracture are known to be driven by the microvoid nucleation, growth, and coalescence. Porous micromechanical description of the ductile metals led to many phenomenological material models, which are used to predict the damage and fracture in engineering structures. In this thesis, the assessment of a rate-independent porous plasticity model is done through the representative volume element (RVE) calculations. The model is based on the formalism presented in [1] which is implemented as a user material subroutine through a prediction-correction scheme similar to a classical J2 plasticity framework. In this context, RVE's are taken from a periodic array of spherical voids surrounded by an elastoplastic matrix material with isotropic exponential hardening, and they are deformed under a constant triaxial stress state with a displacement controlled method. The implementation of the model and the method of the RVE calculations are explained in detail. Limitations of the original model are discussed, and a heuristics extensions to the constitutive framework is proposed to obtain a better fit between the porous model and the unit cell results in terms of volumetric void growth and equivalent stress-strain relation. Numerical analyses show the possibility of achieving a compact framework with a straightforward implementation that agrees well with the RVE simulations for a wide range of stress triaxiality values. The present framework is compared with the widely used Gurson-Tvergaard-Needleman (GTN) model and the differences are discussed. A simple void coalescence relation is added to this framework to simulate the final failure phase of ductile deformation. Additionally, tension simulations with smooth and blunt notched specimens are performed with the GTN model, the present porous plasticity model, and the Johnson-Cook uncoupled damage model to address the model's performance in a ductile fracture simulation. Results show that the present framework and the GTN model can yield almost identical results in notched simulations in terms of engineering stress-strain response and the porosity evolution. The thesis is concluded with an outlook and possible future improvements.
Genel maksat helikopteri için makas tipli kuyruk rotorunun aerodinamik ve aeroakustik incelemesi
The noise of the rotorcraft has become nonignorable with their increasing use in daily life. The aerodynamically active components such as the main and tail rotors are the primary sources of noise, and therefore, their evaluation during the design phase must also be carefully included. Pressure variations caused by several aerodynamic mechanisms on the rotor blades are responsible for the generation of the noise, which may be obtained by computational fluid dynamics. Along with rotor locked self pressure fields, the tail rotor being in the main rotor wake and exposed to strongly nonstationary airflow also need attention. Hence, full helicopter modeling is required to have an accurate evaluation on the tail rotor. However, this modeling and solution are costly for computational fluid dynamics. Therefore, an alternative approach for acoustic analysis is developed in this study. Accordingly, the goal of the thesis is to propose a coupling methodology between comprehensive rotorcraft analysis and an aeroacoustic solver, and to implement of the methodology for investigation of scissors tail rotor. Firstly, the comprehensive analysis approach is validated using available required power data and sectional blade airloads. Then, the proposed pressure distribution methodology is implemented to couple comprehensive analysis approach and acoustic solver. A CFD generated airfoil pressure database is used to find pressure distribution over each blade section. Additionally, this coupling is validated using sectional loads and acoustic pressure. Furthermore, aerodynamic and aeroacoustic behavior of the tail rotor are evaluated for isolated and main rotor interacted conventional tail rotor configuration. Finally, effects of vertical distance and scissors angle on aerodynamic and aeroacoustic behavior of the main rotor interacted scissors type tail rotor configuration are investigated. Results indicate that the main rotor wake has a significant impact on the tail rotor at certain flight conditions conditions, and vertical distance and scissors angle changes the oscillatory loads, and there are scissors tail rotor configurations which offers lower noise level than conventional tail rotor.
Küçük uydularda görüş tabanlı üç eksenli yönelim tahmin algoritmasının tasarımı
Equipment and budget limitations of satellite missions encourages researchers to use on-board equipment for multiple tasks. Camera stands out as an ideal equipment that can be used for multiple purposes in satellites making Earth observation. In this thesis, an attitude estimation algorithm is designed to aid the coarse attitude estimates with the attitude information obtained from Earth images. Attitude information is extracted from Earth images according to perspective geometry laws. This estimated attitude is fed into a multiplicative extended Kalman filter (MEKF). Coarse attitude information is obtained by pre-processing magnetometer and sun sensor measurements within the QUEST algorithm and also used in the measurement update process of the filter to ensure the sustainability of the attitude estimates. The algorithm is tested for a hypothetical nano satellite in different scenarios depending on the availability of Earth images at different times.
Bir çınlama plakası piroşok test sisteminin deneysel ve hesaplamalı analizi
Spacecraft are exposed to severe mechanical loads, including random and periodical vibration, acoustic loads, sinusoidal shock, and pyroshock during their trip to orbit. Such loads usually develop due to a separation event through the activation of pyrotechnic devices and might be transmitted throughout the entire structure and seriously affect electronics components' service performance. It is crucial to study whether the instruments would resist such a harsh environment. Different instruments are validated with various experimental setups, most of which are open-loop systems. Therefore, due to the vulnerability and high cost of the space instruments, the designed experimental setups should be calibrated with dummy equipment to ensure the desired shock level is applied, which takes considerable time and effort. In this context, the current study investigates the potential of the explicit finite element method in designing a ringing plate pyroshock test system and the phase of calibration tests. In order to realize this, a set of preliminary finite element simulations are performed, and the design process of the ringing plate test system is presented. Various experiments are conducted with the designed and manufactured test bench. The obtained Shock Response Spectrum (SRS) responses are fitted to the explicit finite element simulations, which show good agreement after sensitivity analysis. Then the simulations are repeated with the dummy instrument using the fitted parameters, and the potential of the numerical approach to predict a realistic response is discussed.
Kaplamalı takviyeye sahip polimer nanokompozitler için iki seviyeli bir homojenleştirme yöntemi
Polymer nanocomposites are materials which have polymer matrices and nano-scale reinforcement elements. Other than the matrix and the reinforcement element, another phase in nanocomposite systems has been observed and some of the property enhancement or deficiency has been attributed to this phase. The third phase in the composite system is known as the interphase. The local properties of polymer nanocomposites are not easy to distinguish since the length scale is very small and characterization at the nano-scale is not a simple procedure. Macroscopic behavior of nanocomposites can be deducted by homogenization of a representative volume element in many different ways. The aim of the thesis is to propose a new homogenization approach to model the polymer nanocomposites with coated inclusions. Understanding the structure of nanocomposites, formation of the interphase and different characterization techniques for the interphase are important for this aim. Interphase properties affect the overall macroscopic mechanical behavior. So it is important to model the interphase and obtain the effective composite properties accordingly. Different homogenization techniques and applications related to this problem are studied and a new approach is proposed. This method aims to model load transfer between the matrix and the reinforcement element through the interphase in a correct way. Furthermore, a computational gain is aimed with the proposed model in comparison to a full three-phase finite element simulation. Another goal is to extend the proposed method to viscoelasticity. For this purpose two-dimensional and three-dimensional finite element codes are developed for a viscoelastic homogenization framework. The formulations regarding the viscoelastic homogenization are derived and presented.
Bir iletişim uydusunda titreşim test sonuçları kullanılarak sonlu elemanlar modelinin iyileştirilmesi
In this thesis, the vibration analysis of a communication satellite is performed, and the improvement of the finite element model by using vibration test results is presented. First, the satellite finite element model is generated using MSC/PATRAN and MSC/NASTRAN commercial software. With the natural frequency and frequency response analysis, the expected frequency values and response amplitudes in accelerometers are calculated in vibration tests. The results obtained in the vibration test are compared with the analysis results using the HYPERGRAPH commercial software and their similarity with the modal assurance criteria calculation, which is developed using the FORTRAN programming language. The frequency values converged to the measured values as a result of the finite element model improvement, which is performed using error sensitivity assessment at the relevant frequencies.
Diferansiyel evrim yöntemiyle loplu helikopter nozuloptimizasyonu
The main purpose of this study is to develop a methodology for the optimization of an ejector system geometry that is used for the helicopter engine bay cooling. The performance of an ejector system is directly related to the entrainment ratio, which is the ratio of the secondary flow rate to the primary flow rate, and the turbine exit backpressure. In the literature, it is observed that the nozzle geometry used in the ejector system has a significant effect on the performance and that the most efficient geometry is reported to be the lobed shape. For this reason, the optimum lobed nozzle geometry is aimed to be obtained by changing the number of lobes, the exit diameter of the lobe, and the lobe tangent radius in this study. In addition to these parameters, the outlet diameter of the center body that is included in the nozzle structure is also considered as an optimization variable. With the help of a code written in Python, the geometry of the helicopter engine compartment was modeled with "CATIA," and a solution network was created with "Pointwise." The flow analysis of geometry is performed with "Ansys Fluent," and the entrainment ratio and turbine exit back pressure values are calculated. The differential evolution (DE) method is used as an optimization approach. As a result of the optimization study, the best geometry is obtained with the minimum lobe number. Similarly, the lobe tangent radius is close to the lower bound of the given range. The nozzle exit diameter gets approximately the middle value of the given range while the cone exit diameter having the maximum value of the given range for the best geometry. In order to assess the effect of nozzle shape on flow physics and efficiency, the best and worst geometries in all optimization processes are compared. It is observed that the swirl effect that comes from the engine turbine is highly eliminated for the best configuration. Therefore, the cooling flow can be entrained more easily. However, this decreases the mixing efficiency of the flow.The turbine exit back pressure, on the other hand, is higher for this model since the nozzle exit diameter is relatively small. In addition to the analysis of optimum geometry, effects of all otimization parameters on the performance are examined one by one. It is found that the most influential parameter in the optimization process is the lobe number. It is seen that the nozzle geometry with low-lobe numbers has better performance. On the other hand, the higher cone exit diameter affects the performance well. There is no net effect of the other parameters on the system performance.
Yaklaşma tapalı füzeler için öldürme analizi aracı
In this thesis study, a developed kill analysis tool for the missiles having proximity fuzes is presented. Some of today's missiles have proximity fuze, and this type of missiles' warhead can be detonated not when the missile hits the target but at a certain distance. The warhead consists of fragments, and the fragments scattered by the explosion's effect are intended to penetrate the target. A tool is needed for doing kill analysis in order to find the blasting time that the maximum number of fragments hit the target. It is aimed to detonate the warhead at the right time during the missile-target engagement by doing kill analyses for different engagement conditions with the help of the developed tool. Then, the kill analysis results are embedded into the missile computer before the missile is launched.
Aerodinamik kanat tasarımının SU2 ile elde edilen 3 boyutlu akış çözümleri ve cevap yüzey yöntemi kullanılarak eniyilemesi
In this study, the aerodynamic shape optimization of a wing is performed by using 3D flow solutions together with response surface methodology. The purpose of this study is to optimize the aerodynamic shape of a wing to achieve the lowest possible drag coefficient while ensuring desired maneuvering capability and lateral stability. Aerodynamic shape optimization is performed for a wing of a turboprop trainer aircraft. Optimization objective and constraints are determined according to mission requirements and the dimensions of turboprop trainer aircraft already operating. Since the objective function and the constraints consist of aerodynamic coefficients, flow solutions are obtained to calculate aerodynamic coefficients by using an open-source RANS solver (SU2). Surrogate models that relate the design parameters to be optimized to the objective function and the constraints are constructed as high-order nonlinear analytical functions with the help of response surface methodology and design of experiment techniques. In the design of the experiment, a sequential experimentation technique is used. The accuracies of the constructed surrogate models are examined to validate the models. Optimization is performed by using the surrogate models validated and the effect of the different optimization algorithms (sequential quadratic programming and interior point) and initial conditions on the optimized wing geometry are examined. Optimized wing geometry is compared with the initial geometry in terms of the objective function value and the suitability of the optimized geometry to the constraints is evaluated.
Kanat profillerinde buffet araştirmasi
The aim of the present thesis is to investigate buffet onset characteristics in two-dimensional flow. NACA0012 airfoil was chosen as the geometry in this study since the profile is frequently used for the lifting surfaces of aircraft, which are exposed to high flight loads during transonic flight and this causes buffet and flutter. In addition, it is one of the very few models which was studied for the buffet onset investigation in the literature and detailed wind tunnel test data is available. In the present study, after determining the optimum mesh resolution and the appropriate turbulence model, the results of the steady flow numerical analysis for low and high Reynolds numbers were validated with wind tunnel data. The transient flow is investigated for the buffet onset region of the NACA0012 airfoil and the results are verified with wind tunnel data. The oscillating behavior of the shock wave on the airfoil is investigated through the frequency and the Strouhal numbers of lift, drag, moment and static pressure distributions through time and frequency. The study investigates the primary effectors that stimulate buffet onset and the behavior of the flow during buffet through numerical analyses using Unsteady Reynolds-Averaged Navier-Stokes (U-RANS) Equations. The effects of Mach number, angle of attack and Reynolds number on buffet onset is investigated for the examination of buffet characteristics in detail.
Sesaltı gömülü hava alıkları için adjoint tabanlı aerodinamik şekil optimizasyonu
In this study the aerodynamic shape optimization of subsonic submerged intake is performed. The total pressure at the aerodynamic interface plane and lift are employed as objective functions. Drag is constrained and employed as penalty objective. Open-ware platforms are used including SALOME for solid modeling, GMSH for the hybrid mesh generation and SU2 for flow solutions and adjoint based shape optimization. Free form deformation box is employed for shape parameterization and surface deformation. Initially NACA intake placed on a flat plate is solved and result is compared with reference study at subsonic flow conditions. Thereafter single objective shape optimization is employed which enhances the $C_L/C_D$ from 0.08 to 0.53 at the end of design cycles. The second case is of trapezoidal-entrance intake placed in slender aerodynamic body. The results are compared with reference study and baseline case for optimization is established. The shape is optimized using multi objective function and for multiple free form deformation boxes. The intake remains flush with the body during the shape deformation process. The intake internal surface is then allowed to deform along the optimization steps. The pressure recovery increases by 3\%. The third case is similar but with circular entrance in slender body. A single free form deformation box encloses the intake region and the deformation is allowed such that optimization creates a semi-submerged shape. The total pressure recovery increases by 2\% and $C_L/C_D$ also increases by 10\%. In the last case again the trapezoidal entrance intake is employed. The optimization process is similar to the third case. The optimum shape produces a substantial deformation at the intake surface and a semi-submerged intake is finally formed. The total pressure recovery improves by 6\% and an additional $C_L/C_D$ ratio of 0.04 is achieved.
Dokuma ve çapraz-katlı bükümlü kompozit yapılardakı hasarın deneysel yöntemlerle incelenmesi
Laminated curved-shape composite parts which are used in the spar and ribs in aircraft and wind turbine blades are subjected to high interlaminar tensile and shear stresses. These stresses cause delamination and subsequent reduction in load-carrying capacity. In this study, failure mechanism of cross-ply and fabric curved composite laminates under pure transverse loading are examined experimentally using an in-house designed test fixture. Stress field over the curved beam is obtained with finite element analyses (FEA) and analytic solution where multilayered theory is used. Dynamic delamination of the cross-ply and fabric curved specimens is recorded with a high-speed camera where the failure sequence is captured and the crack tip speeds are calculated. For the cross-ply laminates, von Mises strain field is obtained with digital image correlation (DIC) method where the strain fields are found to agree well with elastic FEA up to a specific loading at which a population of matrix cracks nucleate. Detailed fractography of the tested specimens is carried out with digital microscope. Finite element analysis with 2D Hashin Failure criteria is also successfully predict the radial cracks observed in the micrographs of cross-ply curved composite laminates. However, the meandering crack path could not be properly predicted with 2D Hashin Failure Criteria. In fabric curved composite laminates, the failure is observed to occur due to inter-ply and intra-ply crack growth. In fabric laminates, the crack tip travels in a fluctuating manner at speeds reaching intersonic speeds whereas in cross-ply laminates the crack tip speed reaches about Rayleigh wave speed.
İyileştirilmiş aerodinamik performansa ve azaltılmış radar izine sahip ses altı hava alıkları için tasarım yöntemi geliştirilmesi
In this thesis, a design method for a subsonic intake with high aerodynamic performance and reduced radar signature is developed. Intakes are key components of aircrafts' propulsion systems that create open ended cavities resulting in increased radar signature. To achieve reduced radar signature characteristics, a parameterized double curved intake is proposed. Due to the nature of this multi-objective and multi-disciplinary engineering problem, surrogate based analysis and optimization approach is taken to reduce the computationally expensive and time consuming analyses. The thesis consists of three main sections. In the first section, validation studies for the intake performance evaluation methods are conducted. Mesh independence and turbulence model selection studies are carried out for the computational fluid dynamics analyses and radar cross section evaluation studies are carried out for the electromagnetic analyses. In the second section, a design problem is constructed and the verified analyses methods are used for the performance evaluation of the design points required by the surrogate model. Then the best performing design in terms of aerodynamics and radar cross section is obtained with an optimization study. In the last section, analyses with high fidelity prediction tools are conducted for the validation of the obtained design and the design method. Results showed that the surrogate based model is successful at the prediction of performance indicators and can be used for intake optimization purposes. Lastly the performance of the best design is investigated with post-process outputs revealing competitive results regarding aerodynamics and radar signature.
Jet eğitim uçağı ön gövdesinin cevap yüzey yöntemi ve genetik algoritma ile eniyilenmesi
The aerodynamic design of an aircraft's forebody geometry has a significant impact on its performance and stability. Although most studies on aircraft performance have focused on optimizing wing-like structures, the contribution of an effective forebody design to aircraft performance could be as significant as that of wing-like structures. A well-designed forebody can reduce the wave drag and improve the directional characteristics at high angles of attack. The forebody optimization of a jet trainer aircraft in terms of supersonic cruise performance and directional stability is investigated in this thesis. While doing so, two objectives are considered: the wave drag and directional stability. The response surface methodology is used to generate the aerodynamic database, and the non-dominated sorting genetic algorithm–II is used to search for Pareto-optimal solutions. The open-software flow solver SU2 is used to obtain turbulent flow solutions. It is shown that the optimization study enhances the aircraft's performance in terms of wave drag up to 2% and directional stability up to 30%.
Sınır tabakası geçişi ve boşluk akışı gürültüsünün geciktirilmiş ayrık-çevrinti benzetimine dayalı hesaplamaları
This dissertation investigates the boundary layer transition as well as the cavity flow noise prediction capabilities of a high-order in-house solver using various Delayed Detached-Eddy Simulation (DDES) frameworks. Before conducting the simulations, multiblock topology with a high-order overset grid technique is implemented into the solver, which makes mesh generation for complex geometries, such as the tunnel grids around blade sections, and cavity grids composed of two separate domains of the studied cases in this thesis. For the flow transition capability, the Baş-Çakmakçıoğlu (BCM) transition model is incorporated into DDES with a shear-layer-adapted (SLA) subgrid length scale, and applied to flowfields around a blade section and a cylinder. The results show that the BCM model captures the transition onset maintaining the laminar upstream flow while the SLA approach increases the turbulent content rapidly beyond transition. The collaboration between these two approaches enables capturing the aerodynamic coefficients of blade sections near the stall angles accurately. On the other hand, the SLA length scale is incorporated into the Improved DDES framework (IDDES-SLA) for computations of the M219 cavity flow, and its associated noise. The cavity problems are considered to have no physical lateral walls for reducing the demand for computational resources. The results show that for these specific cases the mean and turbulent flow fields could be captured reasonably without the lateral walls, when the cavity width is taken as at least one depth. In addition, unlike the standard one, the use of the SLA length scale helps capturing the Kelvin-Helmholtz instability dominated region. IDDES-SLA yields the best acoustic results among some other tested approaches, showing good agreement with reference studies. The absence of viscous lateral walls does not seem to have an impact on overall sound levels except near the front wall.
Delikli kompozit yapılarda boyutsal özelliklerin dayanım değerlerine etkisinin deneysel ve analitik olarak incelenmesi
In this thesis, we investigate the effect of size on the strength of open hole composite laminates using experimental and analytical approaches. The size effect consists of the effect of thickness and hole size while keeping width to diameter ratio constant. In the experimental part of this work, tensile tests are conducted on CFRP open hole specimens. In-situ images of the top and side surfaces of the test specimens are captured throughout the experiments and load displacement curves are acquired from the testing machine. The top surface images are captured using high resolution camera and analyzed with digital image correlation (DIC) technique. The side surface images are captured using high speed camera setup and these images and post-mortem microscope images are used to determine failure mechanisms. Finally, in agreement with the literature, size dependence is observed on a plot of failure stress vs. hole size. On the analytical part of this work, an analytical methodology is developed based on Tan's approach. The stress field is calculated using Lekhnitskii's Formulation and Classical Lamination Theory(CLT). The analytical strength prediction of the open hole composite laminate is done according to First-Ply-Fiber-Failure(FPFF) methodology. The analytical model performs well compared to other models in the literature in modeling the size dependence of the failure stress on the hole size.
Güneş yelkenli uzay aracının elastik operasyonel konsept çerçevesinde yönelim ve yörünge kontrolü
Utilization of the propellant-free thrust capability of Solar Sail Spacecraft (SSS) is addressed. For this purpose, an elliptical orbit with a very low perigee altitude and an apogee altitude high enough to have solar sail acceleration is proposed for Earth observation mission. In the mission part, regional observation tasks are carried out that take place in very Low Earth Orbit region. The orbit maintenance and control are handled in the high altitude regions. The baseline SSS design is accomplished and presented. Orbital maintenance for station keeping and orbital maneuvers for carrying out specific missions are analyzed, and capabilities of SSS for changing orbital parameters are investigated. A constellation of observation satellites is also proposed addressing the weaknesses in rapid response of SSS to mission needs. Attitude control for smooth attitude maneuvers is also addressed. It is found that the novel quaternion-based attitude tracking control approach with time dependent attitude trajectory offers smooth, jerk-free and accurate attitude tracking.
Roket tipi geometrilerin aerodinamik kararlılık türevlerinin sayısal tahmini
Prediction of aircraft aerodynamic stability derivatives is crucial for evaluating its flight dynamic characteristics. By using the open source SU2 software, steady and unsteady Reynolds-Averaged Navier-Stokes analyzes are performed in order to determine the aerodynamic stability derivatives of a missile like projectile from literature. The employed prediction methods that are based on forced oscillatory motion and differential method are particularly emphasized for roll damping and pitch damping derivatives. Also, constant roll rate analyzes are performed with the options of rigid mesh motion and rotating reference frame. Differences between them are evaluated by comparing the results obtained from these different approaches with each other and experimental data. The effects of the Spalart Allmaras and Shear Stress Transport turbulence models on steady and unsteady simulations are examined in terms of accuracy and computational cost. The effects of oscillation method parameters such as reduced frequency, time step size, and oscillation amplitude are also investigated. The numerical prediction of the aerodynamic stability derivatives shows good agreement with the experimental data.
Strake-delta kanat konfigürasyonun adjoint tabanlı aerodinamik şekil optimizasyonu
Modern fighter aircraft compulsorily demand high maneuverability capability, which is mainly provided by an improved aerodynamic performance at high angles of attack. This is achieved by mostly employing strakes and canards. In this study, adjoint-based configuration and leading edge shape optimizations of a strake on a double-delta wing configuration are performed. SU2 is employed for flow and adjoint solutions. SU2 flow solutions are first verified on solutions for adaptive grids. In the configuration optimization, the sweep angle of the strake is considered as design variable. In the leading edge shape optimization, the free-form deformation box is employed. Remarkably, it is only allowed to modify the leading edge without changing the flat strake surface. Optimization studies are performed for both inviscid and turbulent flows at 10° and 22.5° angles of attack, respectively. It is shown that the sweep angle optimization based on turbulent flow solutions improves the L/D ratio by about 8.4% at 10° angle of attack and 2.7% at 22.5° angle of attack. In addition, the leading edge shape optimization based on turbulent flow solutions improves the L/D ratio by about 4.5% at 10° angle of attack and 3% at 22.5° angle of attack.
Bir pervanenin aerodinamik yüklerin geliştirilmiş pala elemanı ve momentum teorisi yoluyla tahmini ve pervane tasarım optimizasyonu
This study focuses on accurate prediction of total forces and moments acting on a propeller in all flight conditions through Blade Element and Momentum Theory (BEMT) and design optimization of a UAV propeller. Under various flight conditions such as hover, vertical climb, and forward flight, propeller generates different aerodynamic loads in different free-stream velocities, propeller disk angles of attack, and propeller's angular speeds. For this reason, it is important to have a mathematical model that predicts all forces and moments generated by the propeller under these different flight conditions. Propeller aerodynamic loads at different flight conditions can also be found experimentally (e.g., wind tunnel or real-flight tests) or computationally (i.e., Computational Fluid Mechanics (CFD) methods) but these methods are time-consuming. As well, experimental methods are not affordable for optimization studies. The mathematical model obtained from model-based calculations of propeller aerodynamic loads is more useful compared to CFD and experimental methods. However, some assumptions in classical Blade Element Theory and assuming the induced veloctiy constant cause inaccurate prediction of the propeller's forces and moments in model-based approaches. On the other hand, the Improved BEMT (IBEMT) model proposed in this study can estimate the propeller performance in wide flight regimes from hover to forward flight for unmanned aircraft applications. It is computationally efficient in fast optimization studies. Induced velocity is calculated iteratively at each annulus of the rotor disc. Euler integration is used in the calculation of the propeller's aerodynamic loads at each blade section and azimuth angle. The improved model is validated with wind tunnel experiments and it is compared with the results of experimental data of another study and CFD result for which the geometric properties of the propeller used and operating conditions are known in detail. Besides, in this study, design optimization of a propeller is also conducted using MATLAB® Optimization Tool-Box and the IBEMT model.
Çok hafif bir uçağın statik ve dinamik aeroelastik analizleri
Aircraft design processes need to ensure that the aircraft is aeroelastically stable within its operational envelope. This thesis presents an overview of the static aeroelastic, flutter and gust response analysis of a very light aircraft. MSC.FlightLoads and MSC.Nastran are used for aeroelastic modeling and analysis. The methods to be used in the aeroelastic analysis of the VLA are tested on the AGARD 445.6 wing, and the results are in good agreement with the literature. Aeroelastic model corrections such as improvement of the aerodynamic solution, examining different aerodynamic modeling and aero-structure coupling approaches are implemented. Aerodynamic calculations are based on the Doublet-Lattice Method (DLM), which is the aerodynamic theory employed by Nastran for subsonic flows. The aerodynamic solution is improved by including the camber and the angle of incidence of the wing through the addition of an initial downwash. DLM-based loads are compared with loads obtained from computational fluid dynamics (CFD) analysis. The effects of DLM correction on the static aeroelasticity outputs are discussed. It is revealed by dynamic aeroelastic stability analysis that there is no flutter issue within the flight envelope. Matched point flutter solutions for various aileron stiffness are presented. Finally, the vertical acceleration response of the vehicle and internal structural response to the 1-cosine gust are analyzed. It is shown that the gust encountered at the cruise speed condition results in a higher vertical acceleration response than the limit maneuver load factors. Furthermore, tuned gust response analysis is conducted by tuning the gust velocity for different gust gradient lengths. Slightly higher responses than those found for the single gust gradient length required by CS-VLA [1] are captured at a shorter gradient length. Dynamic response analysis reveals that the response of the aircraft dies out in a short time and the model shows a dynamically stable behavior.
Kademeli ve eş merkezli çarpışma kutularının analizi: Tüp sayısının etkisi
In this study, stepped concentric crash tubes with circular AL6063 material are investigated using experimental and numerical methods. Crash tubes are used generally in automotive industry to as a passive safety component. In this study, feasibility of crash tubes in different areas are sought. So, it is designed almost ten times smaller than the conventional car crash tubes. The purpose of this study is to examine the energy absorption characteristics of stepped concentric crash tubes subjected to an axial impact load. The peak forces, internal energy and the total deflection are the selected parameters for the comparison of various crash tubes. The stepped concentric tubes have different lengths and different diameters, while the total mass and volume are kept constant. The area of the impacting object is taken as greater than the internal tube's cross sectional area to prevent passing the impacting object through the tube hole. First of all, a comprehensive study about articles on crash tubes presented. Almost 127 scientific articles are reviewed and tabulated information according to solution methods, geometry, material properties, impact velocity and impact mass properties. Table is given in Appendices section. In the next part, one selected model with AL6063 material is fabricated and tested at dynamic and quasi-static velocities. Dynamic tests are done by using INSTRON 8150 Drop Test Machine and quasi-static test is done by using MTS 647 Hydraulic Test Machine. Then, results of experiments compared and validated with results of analysis by using LS-DYNA, an engineering simulation software program In the second part, the effects of the number of the tubes and impact velocity are studied at high dynamic velocities by using LS-DYNA software program. To define optimal mesh quality, results of five different mesh sizes of a sample are investigated and chosen the most proper type of them to use in all analyses. After that, four different models are constituted with 3, 6, 9 and 12 stepped concentric tubes, respectively. All samples are analyzed at three different velocities: 50, 60 and 70 m/s and results are compared with one another. In the third part, stepped concentric crash tubes are modelled with aluminum foam and analyzed at 50 m/s impact velocity. Results of model with foam filled and empty tubes are compared and studied to demonstrate the advantages and disadvantages of foam material. All graphics are established using GRAPHER software program.
Teklif edilmiş sudan yer gözlem uydusu için haberleşme sistemi ön tasarımı
Sudan is one of the richest countries in the world in terms of natural resources, agricultural fields in particular. The cost of monitoring these resources from space is very high. Establishing a national satellite mission has large initial cost, but it has a large significance in the long run. SudaSat-1 is a proposed earth observation satellite mission funded by the government of Sudan to obtain high resolution images of various areas of the country and other countries as per request. Establishing reliable communication is an essential function for any spacecraft; the failure of communication means loss of mission, the goal of this thesis is to provide a preliminary design for the communication subsystem for SudaSat-1 satellite. The thesis provides a review of similar satellite missions to gain insight of the main considerations involved in the design. The review focused on the satellites with the same spatial resolution. The main characteristics of remote sensing satellites is reviewed, image acquisition modes are also explained. Satellite baseline mission design is introduced, a satellite platform is selected to satisfy overall mission requirements, imaging sensors specifications together with satellite orbit characteristics are also determined to provide the required ground sampling distance. Satellite orbit is designed and simulated using STK, orbit period and average communication time is determined. The thesis highlights mission design elements that influence the design and implementation of the communication subsystem. Communication subsystem-level requirements, functions and design components are overviewed. Components selection is based on requirements satisfaction and space heritage. Redundancy of critical components is put into consideration to increase the reliability of the subsystem. Separate downlink channels are set for imagery data (Xband) and engineering data (S-band). Satellite link power budget is calculated for both downlink channels and for uplink channel. An acceptable value of power margin is obtained. Communication subsystem architecture is presented showing the interconnection between main subsystem components. Satellite data transfer plan is determined illustrating the steps followed to transfer imagery and engineering data downlink and commands uplink
Teklif edilmiş sudan yüksek çözünürlüklü yer gözlem uydusu için uydu yönetim sistemi ön tasarımı
For Sudan as a country newly started its local researches in the field of space science, aerospace and satellite engineering, a preliminary design of a remote sensing satellite is to be made. This thesis is focusing in the satellite management subsystem and the mission design. In the beginning a review was done to the previous experiences in the field as a guide for the design, then the concept was surveyed, also the Remote Sensing basics and imaging process, the orbital dynamics and the management components and elements. A satellite platform was chosen to serve the mission, all supporting subsystems' requirements are agreed to be satisfied. A general revision for the baseline design was done. This platform is considered to be a Chinese one because the opportunity to start this project with china is relatively high, and if the project is implemented, it is most likely to be launched by a Chinese satellite launch provider. The platform is mainly made for Remote Sensing projects and it has a heritage in space. Then the mission design was started with naming the objectives and requirements for the satellite as all, from it the orbit parameters have been decided. Sensor specifications those can take the required high resolution images were selected. The operation modes have been determined to allow the calculations for the management subsystem. The satellite will be orbiting at an altitude of 650 kilometers in a polar sun-synchronous orbit. The satellite has to have the capability of taking two images per orbit. The images can be high resolution with narrow swath width or ten times low resolution with wider swath width. The architecture of the management subsystem was defined based on the mission and previous experiences. Mainly the centralized design is used. The OBC specification was set and the onboard computer software's requirements were cleared. For simplifying the design a separate computer is dedicated for the attitude determination and control operations. Telecommand types are differentiated with the priority. The data management processes were unclouded, a calculations for the data size and type was done. This allowed the minimum storage size to be known. The compression is done for all images while the encryption is an optional process. SpaceWire was picked out to be the internal bus because the high data rate needed when dealing with high resolution images taken by a high speed moving camera. The final design conclude a satellite with 1m GSD (ground sample distance) resolution for panoramic and a 4m GSD resolution for multispectral channels which is orbiting 650 km polar sun-synchronized orbit. The onboard computer has 2 GHz clock with storage can save up to 128 Gbytes. 60 Gbytes can be downloaded every pass over the main ground station. The rest of memory will be used as a redundancy storage and as a storage for images those will be downloaded to other ground stations. The main operation modes of the satellite are the acquisition mode, the battery charging mode, the recovery mode, the idle mode and the execution of a telecommand mode.