LEE- Uçak ve Uzay Mühendisliği-Doktora

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http://hdl.handle.net/11527/19475

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A high-order finite-volume solver for supersonic flows

(Lisansüstü Eğitim Enstitüsü, 2022) Spinelli, Gregoria Gerardo ; Çelik, Bayram ; 721738 ; Uçak ve Uzay Mühendisliği

Nowadays, Computational Fluid Dynamics (CFD) is a powerful tool in engineering used in various industries such as automotive, aerospace and nuclear power. More than ever the growing computational power of modern computer systems allows for realistic modelization of physics. Most of the open-source codes, however, offer a second-order approximation of the physical model in both space and time. The goal of this thesis is to extend this order of approximation to what is defined as high-order discretization in both space and time by developing a two-dimensional finite-volume solver. This is especially challenging when modeling supersonic flows, which shall be addressed in this study. To tackle this task, we employed the numerical methods described in the following. Curvilinear meshes are utilized since an accurate representation of the domain and its boundaries, i.e. the object under investigation, are required. High-order approximation in space is guaranteed by a Central Essentially Non-Oscillatory (CENO) scheme, which combines a piece-wise linear reconstruction and a k-exact reconstruction in region with and without discontinuities, respectively. The usage of multi-step methods such as Runge-Kutta methods allow for a high-order approximation in time. The algorithm to evaluate convective fluxes is based on the family of Advection Upstream Splitting (AUSM) schemes, which use an upwind reconstruction. A central stencil is used to evaluate viscous fluxes instead. When using high-order schemes, discontinuities induce numerical problems, such as oscillations in the solution. To avoid the oscillations, the CENO scheme reverts to a piece-wise linear reconstruction in regions with discontinuities. However, this introduces a loss of accuracy. The CENO algorithm is capable of confining this loss of accuracy to the cells closest to the discontinuity. In order to reduce this accuracy loss Adaptive Mesh Refinement (AMR) is used. This algorithm refines the mesh near the discontinuity, confining the loss of accuracy to a smaller portion of the domain. In this study, a combination of the CENO scheme and the AUSM schemes is used to model several problems in different compressibility regimes, with a focus on supersonic flows. The scope of this thesis is to analyze the capabilities and the limitations of the proposed combination. In comparison to traditional implementations, which can be found in literature, our implementation does not impose a limit on the refinement ratio of neighboring cells while utilizing AMR. Due to the high computational expenses of a high-order scheme in conjunction with AMR, our solver benefits from a shared memory parallelization. Another advantage over traditional implementations is that our solver requires one layer of ghost cells less for the transfer of information between adjacent blocks. The validation of the solver is performed in different steps. We assess the order of accuracy of the CENO scheme by interpolating a smooth function, in this case the spherical cosine function. Then we validate the algorithm to compute the inviscid fluxes by modeling a Sod shock tube. Finally, the Boundary Conditions (BCs) for the inviscid solver and its order of accuracy are validated by modeling a convected vortex in a supersonic uniform flow. The curvilinear mesh is validated by modeling the flow around a NACA0012 airfoil. The computation of the viscous fluxes is validated by modeling a viscous boundary layer developing on a flat plate. The BCs for viscous flows and the curvilinear implementation are validated by modeling the flow around a cylinder and a NACA0012 airfoil. The AUSM schemes are tested for shock robustness by modeling an inviscid hypersonic cylinder at a Mach number of 20 and a viscous hypersonic cylinder at a Mach number of 8.03. Then, we validate our AMR implementation by modeling a two-dimensional Riemann problem. All the validation results agree well with either numerical or experimental results available in literature. The performance of the code, in terms of computational time required by the different orders of approximation and the parallel efficiency, is assessed. For the former a supersonic vortex convection served as an example, while the latter used a two-dimensional Riemann problem. We obtained a linear speed-up until 12 cores. The highest speedup value obtained is 20 with 32 cores. Furthermore, the solver is used to model three different supersonic applications: the interaction between a vortex and a normal shock, the double Mach reflection and the diffraction of a shock on a wedge. The first application resembles a strong interaction between a vortex and a steady shock wave for two different vortex strengths. In both cases our results perfectly match the ones obtained by a Weighted Essentially Non-Oscillatory (WENO) scheme documented in literature. Both schemes are approximating the solution with the same order of accuracy in both, time and space. The second application, the double Mach reflection, is a challenging problem for high-order solvers because the shock and its reflections interact strongly. For this application, all AUSM-schemes under investigation fail to obtain a stable result. The main form of instability encountered is the Carbuncle phenomenon. Our implementation overcomes this problem by combining the AUSM+M scheme with the formulation of the speed of sound of the AUSM+up scheme. This combination is capable of modeling this problem without instabilities. Our results are in agreement with those obtained with a WENO scheme. Both, the reference solutions and our results, use the same order of accuracy in both, time and space. Finally, the third example is the diffraction of a shock past a delta wedge. In this configuration the shock is diffracted and forms three different main structures: two triple points, a vortex at the trailing edge of the wedge and a reflected shock traveling upwards. Our results agree well with both, numerical and experimental results available in literature. Here, a formation of a vortex-let is observed along the vortex slip-line. This vorticity generation under inviscid flow condition is studied and we conclude that the stretching of vorticity due to compressibility is the reason. The same formation is observed when the angle of attack of the wedge is increased in the range of 0-30. In general, the AUSM+up2 scheme performed best in terms of accuracy for all problems tested here. However, for configurations, in which the Carbuncle phenomenon may appear, the combination of the AUSM+M scheme and the computation of the speed of sound formula of the AUSM+up scheme is preferable for stability reasons. During our computations, we observe a small undershooting right behind shocks on curved boundaries. This is imputable to the curvilinear approximation of the boundaries, which is only second-order accurate. Our experience shows that the smoothness indicator formula in its original version, fails to label uniform flow regions as smooth. We solve the issue by introducing a threshold for the numerator of the formula. When the numerator is lower than the threshold, the cell is labeled as smooth. A value higher than 10^-7 for the threshold might force the solver to apply high-order reconstruction across shocks, and therefore will not apply the piece-wise linear reconstruction which prevents oscillations. We observe that the CENO scheme might cause unphysical states in both inviscid and viscous regime. By reconstructing the conservative variables instead of the primitive ones, we are able to prevent unphysical states for inviscid flows. For the viscous flows, temporarily reverting to first-order reconstruction in the cells where the temperature is computed as negative, prevents unphysical states. This technique is solely required during the first iterations of the solver, when the flow is started impulsively. In this study the CENO, the AUSM and the AMR methods are combined and applied successfully to supersonic problems. When modeling supersonic flow with high-order accuracy in space, one should prefer the combination of the AUSM schemes and the CENO scheme. While the CENO scheme is simpler than the WENO scheme used in comparison, we show that it yields results of comparable accuracy. Although it was beyond the scope of this study, the AUSM can be extended to real gas modeling which constitutes another advantage of this approach.
A modified anfis system for aerial vehicles control

(Lisansüstü Eğitim Enstitüsü, 2022) Öztürk, Muhammet ; Özkol, İbrahim ; 713564 ; Uçak ve Uzay Mühendisliği

This thesis presents fuzzy logic systems (FLS) and their control applications in aerial vehicles. In this context, firstly, type-1 fuzzy logic systems and secondly type-2 fuzzy logic systems are examined. Adaptive Neuro-Fuzzy Inference System (ANFIS) training models are examined and new type-1 and type-2 models are developed and tested. The new approaches are used for control problems as quadrotor control. Fuzzy logic system is a humanly structure that does not define any case precisely as 1 or 0. The Fuzzy logic systems define the case with membership functions. In literature, there are very much fuzzy logic applications as data processing, estimation, control, modeling, etc. Different Fuzzy Inference Systems (FIS) are proposed as Sugeno, Mamdani, Tsukamoto, and ¸Sen. The Sugeno and Mamdani FIS are the most widely used fuzzy logic systems. Mamdani antecedent and consequent parameters are composed of membership functions. Because of that, Mamdani FIS needs a defuzzification step to have a crisp output. Sugeno antecedent parameters are membership functions but consequent parameters are linear or constant and so, the Sugeno FIS does not need a defuzzification step. The Sugeno FIS needs less computational load and it is simpler than Mamdani FIS and so, it is more widely used than Mamdani FIS. Training of Mamdani parameters is more complicated and needs more calculation than Sugeno FIS. The Mamdani ANFIS approaches in the literature are examined and a new Mamdani ANFIS model (MANFIS) is proposed. Training performance of the proposed MANFIS model is tested for a nonlinear function and control performance is tested on a DC motor dynamic. Besides, ¸Sen FIS that was used for estimation of sunshine duration in 1998, is examined. This ¸SEN FIS antecedent and consequent parameters are membership functions as Mamdani FIS and needs to defuzzification step. However, because of the structure of the ¸Sen defuzzification structure, the ¸Sen FIS can be calculated with less computational load, and therefore ¸Sen ANFIS training model has been created. These three approaches are trained on a nonlinear function and used for online control. In this study, the neuro-fuzzy controller is used as online controller. Neuro-fuzzy controllers consist of simultaneous operation of two functions named fuzzy logic and ANFIS. The fuzzy logic function is the one that generates the control signal. It generates a control signal according to the controller inputs. The other function is the ANFIS function that trains the parameters of the fuzzy logic function. Neuro-fuzzy controllers are intelligent controllers, independent of the model, and constantly adapting their parameters. For this reason, these controllers' parameters values are constantly changing according to the changes in the system. There are studies on different neuro-fuzzy control systems in the literature. Each approach is tested on a DC motor model that is a single-input and single-output system, and the neuro-fuzzy controllers' advantages and performances are examined. In this way, the approaches in the literature and the approaches added within the scope of the thesis are compared to each other. Selected neuro-fuzzy controllers are used in quadrotor control. Quadrotors have a two-stage controller structure. In the first stage, position control is performed and the position control results are defined as angles. In the second stage, attitude control is performed over the calculated angle values. In this thesis, the neuro-fuzzy controller is shown to work perfectly well in single layer control structures, i.e., there was not any overshooting, and settling time was very short. But it is seen from quadrotor control results that the neuro-fuzzy controller can not give the desired performance in the two-layered control structure. Therefore, the feedback error learning control system, in which the fuzzy controller works together with conventional controllers, is examined. Fundamentally, there is an inverse dynamic model parallel to a classical controller in the feedback error learning structure. The inverse dynamic model aims to increase the performance by influencing the classical controller signal. In the literature, there are a lot of papers about the structure of feedback error learning control and there are different proposed approaches. In the structure used in this work, fuzzy logic parameters are trained using ANFIS with error input.The fuzzy logic control signal is obtained as a result of training. The fuzzy logic control signal is added to the conventional controller signal. This study has been tested on models such as DC motor and quadrotor. It is seen that the feedback error learning control with the ANFIS increases the control performances. Antecedent and consequent parameters of type-1 fuzzy logic systems consist of certain membership functions. A type-2 FLS is proposed to better define the uncertainties, because of that, type-2 fuzzy inference membership functions are proposed to include uncertainties. The type-2 FLS is operationally difficult because of uncertainties. In order to simplify type-2 FLS operations, interval type-2 FLS is proposed as a special case of generalized type-2 FLS in the literature. Interval type-2 membership functions are designed as a two-dimensional projection of general type-2 membership functions and represent the area between two type-1 membership functions. The area between these two type-1 membership functions is called Footprint of Uncertainty (FOU). This uncertainty also occurs in the weight values obtained from the antecedent membership functions. Consequent membership functions are also type-2 and it is not possible to perform the defuzzification step directly because of uncertainty. Therefore, type reduction methods have been developed to reduce the type-2 FLS to the type-1 FLS. Type reduction methods try to find the highest and lowest values of the fuzzy logic model. Therefore, a switch point should be determined between the weights obtained from the antecedent membership functions. Type reduction methods find these switch points by iterations and this process causes too much computation, so many different methods have been proposed to minimize this computational load. In 2018, an iterative-free method called Direct Approach (DA) was proposed. This method performs the type reduction process faster than other iterative methods. In the literature, studies such as neural networks and genetic algorithms on the training for parameters of the type-2 FLS still continue. These studies are also used in the interval type-2 fuzzy logic control systems. There are proposed interval type-2 ANFIS structures in literature, but they are not effective because of uncertainties of interval type-2 membership functions. FLS parameters for ANFIS training should not contain uncertainties. However, the type-2 FLS should inherently contain uncertainty. For this reason, Karnik-Mendel algorithm is modified, which is one of the type-reduction methods, to apply the ANFIS on interval type-2 FLS. The modified Karnik-Mendel algorithm gives the same results as the Karnik-Mendel algorithm. The modified Karnik-Mendel algorithm also gives exact parameter values for use in ANFIS. One can notice that the ANFIS training of the interval type-2 FLS has been developed successfully and has been used for system control.
A multi-disciplinary design approach for conceptual sizing of advanced rotor blades

(Lisansüstü Eğitim Enstitüsü, 2022-07-19) İbaçoğlu, Hasan ; Arıkoğlu, Aytaç ; 511072102 ; Aeronautics and Astronautics Engineering

Rotorcrafts are versatile vehicles with their unique hovering flight capability. However, their forward flight speed limitations and high noise levels are shortened to their usage in much wider areas. Therefore, the rotorcraft industry working on advanced rotorcraft, which are called compound rotorcrafts, development projects increasingly to overcome these problems. The conceptual design phase is the beginning of a development project where the most critical decisions are taken in this stage. So, vehicle-level optimization algorithms are needed for decision-making to lead the project correctly. On the other hand, simplified low-level approaches must be used during conceptual design optimization because of too many design parameters to avoid impractical solution times. Furthermore, rotorcrafts with advanced rotors require advanced design approaches to obtain superior performance, structural, and noise-level characteristics. Therefore, advanced conceptual design approaches are needed to overcome this contradiction. The rotor is the most critical component, which is also the source of the most problems of a rotorcraft such as lack of performance and noise. Therefore, rotor blade optimization is the main issue in the conceptual design phase at the beginning of a project. A multidisciplinary rigid rotor blade design optimization approach that is suitable for the conceptual design, sizing, and evaluation stages of helicopter development processes is suggested. Performance, structural strength of the blade, and noise-level predictions are considered for the objective function. Blade outer surface and structure are represented by a geometrical model in which the chord, thickness ratio, chamber ratio, and twist distributions along the blade radial stations can be defined as linear or nonlinear functions. The distribution of the number of layers for both skin and spar was also defined in the presented model parametrically. Low-level but sufficient fidelity analysis methods were chosen to be able to reduce the computing time. Performance analysis and sizing of the vehicle were obtained by Blade Element Momentum Theory (BEMT) based in-house developed helicopter sizing code called ROTAP. A trim algorithm for compound helicopters that may have additional lifting surfaces and thrust components is suggested. Airfoil Characteristics are calculated by the well-known panel method code Xfoil. Both these codes are modified and embedded in the code developed for this study. Structural analysis was obtained using the 1D FEM approach. Cross-sectional properties of the composite beam are calculated by VABS and displacements under the loads are calculated by GEBT. Reduced FfowcsWilliams-Hawkings equations are used to estimate loading, thickness, and high-speed impulsive noise levels. A hybrid optimization algorithm is suggested to get optimal results. Sequential Quadratic Programming (SQP) can be used to find local optimal points. And then the global optimal point is searched by RSM over local optimal points iteratively. RSM-based surrogate modeling, evaluation, and optimization tool was also developed for manual inspection of the design space. As a case study, multi-objective aerodynamic performance optimization of aircraft propeller is performed.
A numerical approach for plasma based flow control

(Graduate School, 2023-04-05) Ata, Reşit Kayhan ; Şahin, Mehmet ; 511132114 ; Aeronautics and Astronautics Engineering

In the present study, a novel numerical method has been developed to solve incompressible magnetohydrodynamics (MHD) and electrohydrodynamics (EHD) flow problems in a parallel monolithic (fully-coupled) approach. To solve the fluid flow, incompressible Navier-Stokes equations are discretized using face/edge centered unstructured Finite Volume Method (FVM). The same formulation is used for the magnetic transport equation to model the magnetic effects. The side-centered approach, where the velocity and magnetic field components are placed at the center of each cell face while pressure and Lagrange variables are placed at the center of the control volume, provides a stable numerical algorithm without the need of modifications for pressure-velocity coupling. The discretization of both MHD and EHD equations described above results in saddle point problem in fully coupled (monolithic) form. In order to solve this problem an upper triangular right preconditioner is used and restricted additive Schwarz preconditioner with FGMRES algorithm is employed to solve the system. Domain decomposition is handled by METIS library. For these numerical algorithms PETSc software package is used. For the solution of incompressible MHD flow problems, the continuity, incompressible Navier-Stokes, magnetic induction equation are solved along with the divergence free condition of magnetic field. Due to the interaction between magnetic field and conducting fluids, Lorentz force term is added to the fluid momentum equation. For the numerical stability, a Lagrange multiplier term is used in the magnetic induction equation, which has no physical meaning nor effect on the solution. The original approach satisfies the mass conservation within each element but it is not necessarily satisfied in the momentum control volume. Two modifications are proposed as a remedy. First, the convective fluxes are computed over the two-neighbouring elements which then resulted in improved mass conservation over the momentum control volume and increased stability. The second modification applies to only two-dimensional MHD flows. The Lorentz force term in the momentum equation is replaced with $\sigma [\textbf{E} + \textbf{u} \times \textbf{B}] \times \textbf{B}$. Neglecting $\textbf{E}$ makes this term similar to mass matrix if $\textbf{B}$ is taken from the previous time step. Therefore, this modification improves the preconditioning of the monolithic approach. The developed solver is first validated for two-dimensional Hartmann flow of which the analytical solution is known. Then lid-driven cavity and backward facing step problems are investigated under external magnetic field both in 2D and 3D with insulating walls. Three-dimensional MHD flow in ducts is another case where analytic solutions exist. Both conducting and insulating wall boundary conditions are employed and validated. Finally two-dimensional flow over circular cylinder and NACA 0012 profile are investigated for vertical/horizontal external magnetic field and insulating/conducting boundaries. The eletrohydrodynamics (EHD) flow problems involve the interaction between electric field and charged particles inside the fluid. In the present study, the effect of plasma on the flow over lifting bodies is investigated and the working fluid is air, which is neutral at standard conditions. Therefore, a device called Dielectric Barrier Discharge (DBD) is used to ionize the air in a small volume near the surface. DBD consists of two electrodes separated by a dielectric layer. When a voltage is applied to the electrodes, ionization takes place. In order to simulate this phenomenon, Suzen\&Huang model is employed in which Poisson equation is solved for electric potential and charge density, separately. Once potential and charge density are known Coulumb force can be calculated and added as a body force term in the incompressible Navier-Stokes equation. The side-centered approach is used for the velocity components and pressure is placed at the element center for the momentum and continuity equations. For the solution of Poisson equation the charge density and electric potential are placed at the element center while gradients are defined at the edge centers. The solver is first applied to an EHD flow in quiescent air and compared with both experimental and numerical solutions. Then, two electrodes are placed at the bottom wall of 2D cavity with a moving lid to investigate the effect of electric field on classical cavity problem. Finally, EHD flow over NACA 0012 airfoil at angle of attacks up to $\alpha=7$ is investigated in terms of flow structure, lift and drag coefficients.
Advanced energy and exergy analysis on aircraft jet engines

(Graduate School, 2023-12-08) Fawal, Sara ; Kodal, Ali ; 511212113 ; Aeronautics and Astronautics Engineering

A comparative performance analysis for various optimization criterion functions is to be carried out for an irreversible Brayton cycle applicable to aircraft jet engines: Ramjet, Turbojet (No Afterburner), Turbojet (With Afterburner), Turbo-Ramjet. Newly defined parameters are introduced as power loss parameter (PLOS), effective power loss parameter (EPLOS) and Carnot-Brayton shape factor (CBSF) for a better assessment of the performance and power losses throughout the operation of the engine cycle. In addition, optimization functions, such as maximum power (MP), maximum power density (MPD), ecological coefficient of performance (ECOP) and ecological function (ECOL) are considered and their optimal operation conditions are compared with respect to each other. This research studied the effects on the prescribed optimization criterions targeted towards the aviation industry under variations of compressor pressure ratio θ_c, compressor and turbine efficiencies (η_c and η_t respectively), cycle temperature ratio / maximum cycle temperature, altitude and flight Mach number M_∞ where applicable with respect to the jet engine being considered. Therefore, the classical irreversible Brayton cycle is extended and applied to airbreathing engines; which included effects of all the engine components (from free stream to inlet to outlet) as part of the thermodynamic cycle model. While many researchers have carried out performance analysis for internal combustion engines including gas turbine engine, this study is an extension of the available optimization functions such as MP, MPD, ECOP and ECOL for aircraft jet engines. As mentioned, power density is defined as the ratio of power to the maximum specific volume in the cycle. Whereas ECOP is defined as the ratio of power output to the loss rate of availability and ECOL as the power output minus the loss rate of availability. In order to extend the classical irreversible Brayton cycle to airbreathing engines applicable for aircrafts, further development studies must be carried out to obtain: higher propulsion efficiency and higher ratios of power output with respect to engine weight, volume, and frontal area. The objective is to obtain a larger power output to engine size (weight) in a more thermodynamically efficient manner for a real turbojet cycle where maximum ECOP, ECOL, power density and power conditions can be used as a basis for the determination of optimal operating conditions and preliminary design constraints for real turbojet engines at flight conditions. The comparative performance analysis for various optimization criterion functions used for the aircraft engine cycle will be applied to ramjet, turbojet without afterburner and tubojet with afterburner to reach the final intended application of turboramjet engine. The turboramjet engine cycle is identified as Turbine Based Combined Cycle Engines (TBCC). Such hybrid cycle engines can be applied to UAV's, UCAV's and powering future hypersonic flight vehichles. The software to be used for the comparative performance analysis for the irreversible Brayton cycle applicable to aircraft jet engine cycles is the academic version of MATLAB 2018b provided by the MathWorks group. The emissions and radiative forcing (RF) from the aviation industry and its effects on air pollution and the ecology are an important concern, where aviation ranks as one of the top ten emitters. The major greenhouse gas emitters that contribute to RF are: carbon dioxide CO2, carbon monoxide CO, water H2O, nitrous oxide NOX, sulphur oxides SOX and volatile organic compounds VOCs. Thus, performance evaluation of aircraft propulsion systems must be assessed with respect to environmental and ecological conditions as well as power and fuel consumption considerations. Therefore, various optimization criterion functions which can be used as tools by the aviation industry to design 'new generation engines' which are economically and ecologically favourable. It is anticipated that this research would provide valuable insight in the preliminary design of airbreathing engines (Ramjet, Turbojet: No Afterburner, Turbojet: With Afterburner and Turbo-Ramjet) and set a stage for exploration towards adaptive engine components and cycles for the conception of truly intelligent engines; an engine that can assess its current operating state and work under the most efficient power regime (ECOL or ECOP or MP or MPD) to achieve the designers and engine's intended performance potential.
An ALE framework for multiphase flows

(Graduate School, 2022-08-24) Güventürk, Çağatay ; Şahin, Mehmet ; 511162103 ; Aeronautical and Astronautical Engineering

An Arbitrary Lagrangian Eulerian (ALE) framework which combines the advantages of both Lagrangian and Eulerian methods is developed to solve incompressible multiphase flow problems. The div-stable side centered unstructured finite volume formulation is used for the discretization of the incompressible isothermal Navier-Stokes equations along with the isothermal constitutive equations for Oldroyd-B and FENE-CR fluids. In this approach, the velocity vector components are defined at the mid-point of each cell face, while the pressure term and extra stress tensor are defined at element centroids. The present arrangement of the primitive variables leads to exact total mass conservation at machine precision due to the present stable numerical discretization with no ad-hoc modifications. In addition, a special attention is given to satisfy global discrete geometric conservation law (DGCL) at discrete level for the application of the interface kinematic boundary condition in order to conserve the total mass for each species for multiphase flow problems. Furthermore, the pressure field and extra stress field are treated to be discontinuous across the interface with the discontinuous treatment of density and viscosity and jump conditions are satisfied. Surface tension force is treated as a tangent force and discretized in a semi-implicit form. Two different approaches for the computation of unit normal vector have been implemented: the least squares biquadratic surface fitting (LSBSF) and the mean weighted by sine and edge length reciprocals (MWSELR). The combination of MWSELR method and discontinuous treatment of density and viscosity reduced the parasitic currents to the machine precision. The resulting large system of algebraic equations is solved in a fully coupled manner in order to improve the time step restrictions. As a preconditioner, an approximate matrix factorization similar to that of the projection method is employed and the parallel algebraic multigrid solver BoomerAMG provided by the HYPRE library, which is accessed through the PETSc library, has been utilized for the scaled discrete Laplacian of pressure and the diagonal blocks of mesh deformation equations. The present calculations verify that the mass of the bubble can be conserved at machine precision independent of spatial and temporal resolutions. The accuracy of the proposed method is initially validated on the static bubble problem, since the surface tension force is highly sensitive to the accurate evaluation of the unit normal vector and the inaccuracies significantly contribute to unphysical velocities, called parasitic currents. The calculations indicate that the parasitic currents can be reduced to machine precision for the MWSELR method. The MWSELR approach, as far as our knowledge goes, has not been used for the evaluation of normal vectors in multiphase flows. In the second benchmark case, the proposed approach is applied to the single bubble rising in a viscous quiescent liquid for both low and high density ratios. The calculations produce accurate predictions of the bubble shape, center of mass, rise velocity, etc. Furthermore, the mass of each species is conserved at machine precision and discontinuous pressure field is obtained in order to avoid errors due to the incompressibility restriction in the vicinity of liquid-liquid interfaces at large density and viscosity ratios. The third benchmark case is rising of a Taylor bubble in 2D and in 3D. Taylor bubbles are large bullet shaped bubbles whose cross-section almost fill the cross-sectional area of the channel. Therefore, this benchmark case is numerically harder than the previous cases. It is seen, 3D bubble rises faster due to the smaller blockage effect (i.e. cross section of the bubble/cross section of the tube) of the bubble in three dimension with respect to the 2D bubble. In addition, drag force of the bubble decreases due to the three-dimensional relieving effect. The results are compared with the results available in the literature and it is shown that the obtained bubble shape and velocity field in the vicinity of the Taylor bubble are similar to that of the literature. In the fourth test case, rise of a single bubble in a quiescent, viscoelastic fluid due to buoyancy is simulated in 2D and the viscoelastic fluid is modeled as Oldroyd-B. By changing the size of the bubble, domain, placing the bubble to the different locations and changing the fluid properties, many simulations are performed and the change in bubble shape, rise velocity, circularity and sphericity are inspected. It is seen that the existence of the wall highly effects the outcome. In addition, the cusp at the trailing edge of the bubble and negative wake behind the bubble are observed in some cases. Therefore, it is shown that a viscoelastic fluid model that exhibits shear thinning is not essential for negative wake to arise. This result contradicts with the some published papers in the literature but is also supported by the others. The final benchmark case is similar to the previous one but this time viscoelastic fluid is modeled as FENE-CR and the problem is in 3D. Besides, subsequent simulations are performed for Newtonian bubble and Newtonian continuum, Newtonian bubble and viscoelastic continuum, and viscoelastic bubble and Newtonian continuum. It is observed that the bubble has a slight cusp at the trailing edge for Newtonian bubble and viscoelastic continuum. On the other hand, the bubble has a dimple at the trailing edge for the viscoelastic bubble and Newtonian continuum. In addition, it is shown that the results are in a good agreement with the result available in the literature. Finally, the methods used to develop and test the present multiphase solver for both Newtonian and viscoelastic fluids are summarized. Advantages and the drawbacks of the present solver are addressed with possible future applications.
Coherent structures and energy transfer in decelerated turbulent boundary layers

(Graduate School, 2023-02-10) Güngür, Taygun Recep ; Güngör, Ayşe Gül ; Maciel, Yvan ; 511162103 ; Aeronautical and Astronautical Engineering

This thesis aims to expand our knowledge about turbulent boundary layers (TBLs) developing under adverse pressure gradients (APG). The main focus of this thesis is coherent structures and energy transfer mechanisms in APG TBLs with small and large velocity defects. For this, two novel non-equilibrium APG TBL direct numerical simulation databases are generated. The first database is a non-equilibrium APG TBL with $Re_\theta$ reaching 8000 and a shape factor spanning between approximately $1.4$ and $3.2$. It is the main database utilized throughout the thesis. The second database has identical domain and boundary conditions to the first one. The difference between them is that turbulence in the inner layer of the second database is artificially eliminated. This second database is generated to examine the effect of the inner layer on the outer layer turbulence. For comparison purposes, a channel flow case, two zero pressure gradient (ZPG) TBLs and two homogeneous shear turbulence (HST) databases from the literature are employed. The energy-carrying and –transferring structures are examined using the spectral distributions and two-point correlations. The analysis reveals that energy-carrying structures in small defect APG TBLs and canonical flows have similar spatial and spectral features. In the large defect case, turbulence in the inner layer, which is the dominant region in canonical flows and small defect APG TBLs, loses its importance and outer-layer turbulence becomes dominant. The inner peak in the $\langle u^2\rangle$ spectra does not exist in the large-defect case. Moreover, two-point correlations show that the spatial organization becomes different in the large-defect case as well. Regarding the energy-transferring structures, production, pressure-strain and dissipation structures behave in a similar fashion to the energy-carrying structures. The spectral distributions show that the canonical flows and small defect APG TBLs behave very similarly. The shape of the spectra is qualitatively similar in both cases. In the large defect case, the wall-normal distributions of production and pressure-strain become different since the outer layer becomes dominant. However, the shape of 2D spectra and the aspect ratio of structures are alike in all cases. The production and pressure-strain structures are analyzed in more detail using the relative size and wall-normal positions with respect to each other and energetic structures using spectral distributions. The results show that production and pressure-strain spectra have similar features in both the inner and outer layers regardless of the velocity defect, despite the differences in energetic structures. In the inner layer, the results suggest that the near-wall cycle or another mechanism with similar spectral features exists in large defect APG. As for the outer layer, an interesting result is that in large-defect APG TBLs it acts more like a free shear layer than in small-defect APG TBLs or canonical flows. Besides that, production and inter-component energy transfer mechanisms are similar in all cases regardless of velocity defect. No inflection point instability in the outer layer of the large-defect APG TBLs was detected. The effect of the near-wall region on the outer-layer layer structures is examined through Reynolds-shear-stress carrying structures' spatial features by detecting individual structures using spatio-temporal volumetric data. The results show that the outer layer is not significantly affected by the inner-layer turbulent activity. The structures' spatial features mostly depend on the mean shear. The aspect ratio of Reynolds-shear-stress carrying structures remains almost identical in the outer layer when the inner-layer turbulence is eliminated. Moreover, the aspect ratio follows a similar trend in both outer layers of APG TBLs and HSTs when the structures' size is normalized with the Corrsin length scale. The overall conclusion is that energy transfer mechanisms remain the same within one layer regardless of the velocity defect. The reason why the wall-normal distribution of energy and energy transfer dramatically changes in the large defect case is probably the change in the mean shear profile due to the increasing velocity defect.
Design and optimization of variable stiffness composite structures modeled using Bézier curve

(Graduate School, 2022-06-09) Coşkun, Onur ; Türkmen, Halit S ; 511162115 ; Aeronautics and Astronautics Engineering

The usage of advanced fiber-reinforced polymer (FRP) matrix composites has been dramatically increased since the first carbon fiber patented in the 1960's. Particularly, the aerospace companies' interest has been gradually grown in carbon fiber-reinforced polymer (CFRP) aircraft structures due to major performance improvements such as high strength and stiffness to weight ratios and reduced weight. The traditional design approaches and manufacturing methodologies of CFRP structures in various industries have been well established and applied for more than 50 years. They are mainly developed for straight fibers and the optimum design solutions have been achieved by the choice of constituent materials, different fiber orientation angles that are often limited to 0, ±45, and 90 degrees, laminate stacking sequence and total number of plies. However, increasing complexity of structure geometries have resulted in complex lay-ups & contours; therefore, advanced manufacturing methodologies such as Automated Fiber Placement (AFP) and Tailored Fiber Placement (TFP) are developed to improve productivity and process reliability. Following the introduction of advanced manufacturing methods CFRP structures with complex geometry, complex lay-ups & contours have been manufactured with improved productivity and process reliability. In addition to that, composite materials can be tailored more effectively to meet design requirements by changing the design approach from straight to curvilinear fibers. The composite structures designed with curvilinear fibers have spatially varying stiffness due to local fiber orientations in the ply, and accordingly they are named as variable stiffness (VS) structures. In this dissertation, the variable stiffness composite plates and circular cylindrical shells modeled using parametric Bézier curves as curvilinear fiber paths are designed and optimized. The design method with parametric Bézier curves covers a wide and complex design space from simple linear angle variation to constant curvature path to highly nonlinear angle variations. The designed VS composite structures are expressed with new lay-up definition conventions that use simple and intuitive variables such as segment/station angles and multipliers/curvatures. The optimum structural designs in the complex design space of plates and circular cylindrical shells are searched using a multi-step optimization with multi-objective such as buckling and stiffness, and a novel pre-trained multi-step/cycle surrogate-based optimization (PMSO) framework with single objective, i.e. buckling, respectively. First, VS composite plates and circular cylinders are designed with 'Direct Fiber Path Parameterization' (DFPP) that uses continuous curve functions for fiber orientation angles at each point or grid in the laminate. The cubic and quadratic Bézier curves are used as curvilinear fiber path. The fiber paths as Bézier curves are constructed with approximation and interpolation formulations. The approximation curve captures the defined angles at the start point and the end point, and the shape of the curve changes with the position of the control points intuitively. On the other end, interpolation curve follows the exact positions of control points at the expense of control of the fiber angle. Therefore, fiber angles are different from the defined sector angles. Three types of parametric curves are formulated, i.e., cubic Bézier interpolation curve and quadratic and cubic Bézier approximation curves. Cubic Bézier approximation curves are specially formulated to define constant curvature fiber paths. Considering the characteristics of Bézier curves, intuitive conventions to define lay-ups of laminated VS plates and shells are proposed. The position of course boundaries within each ply are calculated using the reference fiber path, and resulting courses are shifted along one direction to cover VS plate and cylindrical shell surfaces. The reference fiber paths are defined with design variables such as sector/station angles and multipliers/curvatures, which are used to calculate control points. Current proposal for lay-up definition allows one to move stations using multipliers within an interval, hence it is possible to find lower curvature fiber paths with the same sector angles. The minimum curvature value is a major characteristic of curvilinear fiber paths due to manufacturing constraints. Golden Section Search and Downhill Simplex methodologies are used depending on the design approach together with Bézier curve formulations. The Golden Section Search method, which is a technique for finding an extremum, (minimum or maximum) of a unimodal function, is applied to approximation curves, and Downhill Simplex method is applied to interpolation curves due to a multidimensional space with n multipliers. The curvature values are significantly minimized without changing the lay-up definitions; especially for quadratic Bézier approximation curves, the curvature distribution along characteristic length gets close to the constant curvature results. Three different geometries for VS plates (b/a ≈ 1.8) and two different geometries for VS circular cylinder Cylinder 1 (L/R ≈ 2.67) and Cylinder 2 (L/R = 2) are modeled. Considering the cylindrical coordinates, the courses laid on the cylinder are axially shifted to have circumferentially varying stiffness and strength; however, the effective width of the ply is modified to have continuous fiber paths around the circumference. To have averaged boundaries, which is called no gap condition, minimum effective course width is used as the reference shifting value. The lay-up process is completed on developed plane of the cylinder, and then translated into cylindrical coordinates. Second, finite element models of laminated VS plates and cylindrical shells are generated using Ansys Mechanical APDL codes. Four node Shell 181 quadrilateral elements with full integration are used to mesh the VS plate and the VS composite shells with Cylinder 1 geometry, and FE models of layered VS composite shells with Cylinder 2 geometry are generated using eight node Shell 281 elements with reduced integration. Both shell elements are based on the first-order shear-deformation theory (referred to as Mindlin-Reissner shell theory). These elements with six degrees of freedom at each node (translations in the nodal x, y, and z directions and rotations about the nodal x, y, and z axis) are usually used to analyze thin to moderately-thick shell structures. The mesh convergence studies of reference QI plate and VS circular shells and plates are performed, and reference element edge lengths are chosen considering accurate mapping of curvilinear fiber paths on finite element mesh, buckling results, and computational efficiency. The curvilinear fiber paths for each ply are then mapped to related element centroids by APDL functions. Next, the VS laminates and circular cylinders are optimized for maximum stiffness and/or buckling load using surrogate-based NSGA-II algorithm. The NSGA-II is an evolutionary algorithm and supports multi-objective optimizations. The design space development strategy is an important part of surrogate modeling to get optimal distribution of fewest number of points with maximum insight into the design. Thus, experimental designs are generated with Optimal Space Filling (OSF) algorithm according to specified intervals. Then, surrogate models are generated with Genetic Aggregation. The Genetic Aggregation selects the best solution from Full 2nd-Order Polynomials, Non-Parametric Regression, Kriging, and Moving Least Squares. The algorithm generates the population of all methods and then it applies single response surface or combination of response surfaces according to fitness functions. The assemblage of Genetic Aggregation surrogate model is constructed with weighted average of selected meta-models. The weight and the combination of meta-models depend on design of experiment method and the behavior of VS structures designed with the approximation and interpolation curves. Two-cycle approach is used to increase the accuracy of the surrogate models. The first cycle consists of the design space between 80° and -80°, and the second cycle searches for ±20 degree of the optimum angle calculated at the first cycle. A better lay-up for Size 1 – Case 3 compared to results in literature is found by using reduced the domain in the second cycle. The best buckling performance is found for Size 3 plate with Case 3 boundary conditions that has 103% increase in buckling load against 44% reduction in equivalent stiffness compared to reference quasi-isotropic laminate. It is clear that increase in plate size increases the buckling performance of VS plates. This is due to wider design space with relaxed curvature constraint that allows higher angle differences between edge and the middle of the plate, accordingly fiber angle at the plate edges can align closer to loading direction while the fiber angles far from edge converges to smaller angles. The quadratic Bézier approximation curve is found to be a good alternative of cubic Bézier approximation curve with constant curvature, as it has similar edge load distribution and buckling mode shapes. Additionally, the stations, which are fixed for cubic Bézier approximation curve with constant curvature, can be shifted for definition with quadratic approximation without changing the lay-up definition according to designer's need. Finally, a novel pre-trained design optimization framework is proposed to optimize buckling load of VS composite circular cylinders under pure bending with curvature and strength constraints. By using Bézier curves, designers have more effective control on the design domain to improve the buckling performance in accordance with requirements such as curvature and strength. The strength constrain is calculated by using Tsai-Wu failure criterion. The optimizations are conducted using PMSO framework that utilizes NSGA-II. The main benefit of this framework is to gather prior knowledge about the design space at the first step by conducting pre-training optimizations using laminated VS composite shells with single ply definition. This narrows down the design space significantly before conducting a full lay-up design optimization with large number of parameters at the second step. Moreover, multiple cycle approach at each step helps to reduce the complexity of the optimization together with increased surrogate model accuracy. The optimization is completed for four different laminate stack-ups that are made up of all VS plies and partial VS plies in combination with unidirectional fibers (±45°, 0° and 90°). The maximum increase in buckling load is found to be 31% for Laminate 1 and 41% for Laminate 4 compared to reference QI shells. This gives 14% and 16% higher buckling load than the literature studies, and the Laminate 4 results are achieved for two times more design variables using approximately same number of sampling points. The gain in buckling load is due to the redistribution of stresses on compression and tension side as a consequence of variable angle distribution within each ply. The fiber angles close to axial direction on the tension side increase the strength and stiffness of the structure, and angles close to circumferential axis on the compression side reduce the stiffness of buckling critical region to distribute the compressive loads onto wider region.
Development of a fault tolerant flight control system for a UAV

(Graduate School, 2022-08-12) Vural, Sıtkı Yenal ; Hacızade, Cengiz ; 511082105 ; Aeronautics and Astronautics Engineering

It is important for the unmanned aerial vehicles that are used for various purposes including military missions, surveillance, security, atmospheric data gathering etc. to be autonomous and control systems that can work flawless even when faults are present are needed for such systems. The methods for achieving fault tolerant control are under development and are still not used much in practical applications however developing a fault tolerant control system for all types of applications including aerial vehicle control systems seems to be an ultimate control aim. In this thesis, developing a fault tolerant control system for a UAV is aimed mainly for the given reasons. In the study, active and passive control methods and Kalman filter based fault detection and isolation techniques are used together to build a fault tolerant controller for an unmanned aerial vehicle. Also, a hybrid controller including both active and passive fault tolerant controllers is developed in order to benefit from their different characteristics in dealing with faults. Kalman filter based fault detection and isolation algorithm which can be used to detect and isolate the faults in sensors and actuators , to determine the source of the fault and to find the unbiased sensor measurements is developed in the study and its effectiveness is shown through simulations. To detect and isolate the faults occuring in sensors/actuators Kalman filter innovation sequence analysis is used. On the other hand, to determine the source of the fault, Doyle-Stein method based Kalman filter and to rectify the biased sensor measurements Kalman filter insensitive to measurement failures are employed in the study. Unmanned aerial vehicle model is used to simulate the fault cases and to show the successfulness of the built system. One of the methods used in the thesis to build a fault tolerant controller is the active fault tolerant control method. In this method, fault detection and isolation technique is used to determine the faults occuring in the system and to find the severity of the fault and this info is used to reconfigure the controller. The actuators would not work effectively if hydraulic pressure decrease, partial blockage of a control valve, voltage reduction in electrical servosystems etc. occur in the system. In these cases, the effectiveness of the actuators decrease. The change in the mentioned actuator effectiveness can be represented in the system as the control effectiveness factor related with the actuator. In the study, two-stage Kalman filter is used to estimate the changes occurring in actuator control effectiveness factors which corresponds to the faults occurring in actuators. With the help of two-stage Kalman filter in which a second bias-estimation filter is used, the bias in the system can be estimated and the best state estimates can still be found. This type of filter,different than the augmented state filter in which all parameters are estimated in one stage, has the advantage of reducing calculation burden and thus giving results in small time period. In short, two-stage Kalman filter consists of a bias-free state estimator that estimates the states, a bias estimator to estimate the bias, the residual vector and the covariance matrix calculation equations and coupling equations that are used to relate the filters and update the bias free state estimator. In the simulations, the faults in actuators are modelled as changes in control distribution matrix B and these changes are tried to be estimated using two-stage Kalman filter and the reconfiguration of the controller is done using the determined new B matrix. In this method, one needs to determine when the fault occurs in the system and to decide when to reconfigure the controller. To that end, to determine the fault occuring in the system, weighted sum-squared bias estimate – WSSBE- fault detection algortihm is used. This algorithm uses the statistical variables that are based on bias- control effectiveness factor- estimates. The ratio of the square of the bias estimate to its covariance matrix is summed in a predetermined window length which corresponds to an iteration period. The resultant value should be between determined theoretical values if there is no fault in the system. In the decision-gain update algorithm, convergence of the control effectiveness factors is important and mean value of the estimates can be used for this purpose. In the study, using the mentioned methods, control of heading and altitude in cases where actuator faults are present in aileron and elevators are realized. It is shown through simulations that the unmanned aerial vehicle can be effectively controlled using the active fault tolerant controller despite the decrease in actuator control effectiveness factors which corresponds to effectivity loss in actuator controls. Another method used in the thesis to design a fault tolerant controller is passive fault tolerant control method. In this method, the pre-designed controller is relied upon in dealing with the faults occuring in the system. Thus, fault detection and isolation and controller reconfiguration are not needed in this scheme. Dynamic inversion technique is used together with robust integral of the signum of the error- RISE- method to design an asymptotic tracking passive fault tolerant controller that has the capability to cope with faults. The faults occuring in actuators are modelled as parametric uncertainties in control distribution matrix B. To build an asymptotic model following passive controller, control inputs that decrease the difference between model and system should be found. For this purpose, Lyapunov type functions are used and controller constants are determined as done in similar studies. In simulations, in longitudinal model, forward velocity, pitch rate and in lateral model, yaw rate and roll rate are the main states that are controlled. Using asymptotic tracking controller system that helps in maintaining control of mentioned states, an outer loop is also built which aranges heading and altitude changes by tuning reference model inputs using fedback state values from the main system. Simulations done using both longitudinal and lateral models show that the designed passive controller is effective in controlling the unmanned aerial vehicle at times when faults are present in actuators. Hybrid control method is also used in the study to build a fault tolerant controller. This method uses active and passive controllers at different times to achieve fault tolerant control. This way, at times when the fault detection and isolation algortihm based on two-stage Kalman filter determines the fault but still needs time to find the severity of the fault, passive fault tolerant controller can be used and the system can be kept under control continously. Reconfiguration can later be done after the fault severity is determined. As passive and active controllers are shown to be effective in controlling the unmanned aerial vehicle, hybrid control can be used for controlling faulty plants continously. In simulations done using lateral model of the unmanned aerial vehicle,it is shown that the hybrid controller is successful in keeping the vehicle under control and tracking the heading inputs at times when actuator faults are present. To achieve this result, active and passive controllers are used at different times after fault occurrence. In conclusion, a fault tolerant controller is designed for the unmanned aerial vehicle and it is shown that it can be effectively used when actuator faults – actuator control effectiveness loss cases corresponding to the problems in actuators- and/or sensor faults are present in the study.
Development of single-frame methods aided kalman-type filtering algorithms for attitude estimation of nano-satellites

(Graduate School, 2021-08-20) Çilden Güler, Demet ; Hacızade, Cengiz ; Kaymaz, Zerefşan ; 511162104 ; Aeronautics and Astronautics Engineering ; Uçak ve Uzay Mühendisliği

There is a growing demand for the development of highly accurate attitude estimation algorithms even for small satellite e.g. nanosatellites with attitude sensors that are typically cheap, simple, and light because, in order to control the orientation of a satellite or its instrument, it is important to estimate the attitude accurately. Here, the estimation is especially important in nanosatellites, whose sensors are usually low-cost and have higher noise levels than high-end sensors. The algorithms should also be able to run on systems with very restricted computer power. One of the aims of the thesis is to develop attitude estimation filters that improve the estimation accuracy while not increasing the computational burden too much. For this purpose, Kalman filter extensions are examined for attitude estimation with a 3-axis magnetometer and sun sensor measurements. In the first part of this research, the performance of the developed extensions for the state of art attitude estimation filters is evaluated by taking into consideration both accuracy and computational complexity. Here, single-frame method-aided attitude estimation algorithms are introduced. As the single-frame method, singular value decomposition (SVD) is used that aided extended Kalman filter (EKF) and unscented Kalman filter (UKF) for nanosatellite's attitude estimation. The development of the system model of the filter, and the measurement models of the sun sensors and the magnetometers, which are used to generate vector observations is presented. Vector observations are used in SVD for satellite attitude determination purposes. In the presented method, filtering stage inputs are coming from SVD as the linear measurements of attitude and their error covariance relations. In this step, UD is also introduced for EKF that factorizes the attitude angles error covariance with forming the measurements in order to obtain the appropriate inputs for the filtering stage. The necessity of the sub-step, called UD factorization on the measurement covariance is discussed. The accuracy of the estimation results of the SVD-aided EKF with and without UD factorization is compared for the estimation performance. Then, a case including an eclipse period is considered and possible switching rules are discussed especially for the eclipse period, when the sun sensor measurements are not available. There are also other attitude estimation algorithms that have strengths in coping well with nonlinear problems or working well with heavy-tailed noise. Therefore, different types of filters are also tested to see what kind of filter provides the largest improvements in the estimation accuracy. Kalman-type filter extensions correspond to different ways of approximating the models. In that sense, a filter takes the non-Gaussianity into account and updates the measurement noise covariance whereas another one minimizes the nonlinearity. Various other algorithms can be used for adapting the Kalman filter by scaling or updating the covariance of the filter. The filtering extensions are developed so that each of them is designed to mitigate different types of error sources for the Kalman filter that is used as the baseline. The distribution of the magnetometer noises for a better model is also investigated using sensor flight data. The filters are tested for the measurement noise with the best fitting distribution. The responses of the filters are performed under different operation modes such as nominal mode, recovery from incorrect initial state, short and long-term sensor faults. Another aspect of the thesis is to investigate two major environmental disturbances on the spacecraft close enough to a planet: the external magnetic field and the planet's albedo. As magnetometers and sun sensors are widely used attitude sensors, external magnetic field and albedo models have an important role in the accuracy of the attitude estimation. The magnetometers implemented on a spacecraft measure the internal geomagnetic field sources caused by the planet's dynamo and crust as well as the external sources such as solar wind and interplanetary magnetic field. However, the models that include only the internal field are frequently used, which might remain incapable when geomagnetic activities occur causing an error in the magnetic field model in comparison with the sensor measurements. Here, the external field variations caused by the solar wind, magnetic storms, and magnetospheric substorms are generally treated as bias on the measurements and removed from the measurements by estimating them in the augmented states. The measurement, in this case, diverges from the real case after the elimination. Another approach can be proposed to consider the external field in the model and not treat it as an error source. In this way, the model can represent the magnetic field closer to reality. If a magnetic field model used for the spacecraft attitude control does not consider the external fields, it can misevaluate that there is more noise on the sensor, while the variations are caused by a physical phenomenon (e.g. a magnetospheric substorm event), and not the sensor itself. Different geomagnetic field models are compared to study the errors resulting from the representation of magnetic fields that affect the satellite attitude determination system. For this purpose, we used magnetometer data from low Earth-orbiting spacecraft and the geomagnetic models, IGRF and T89 to study the differences between the magnetic field components, strength, and the angle between the predicted and observed vector magnetic fields. The comparisons are made during geomagnetically active and quiet days to see the effects of the geomagnetic storms and sub-storms on the predicted and observed magnetic fields and angles. The angles, in turn, are used to estimate the spacecraft attitude, and hence, the differences between model and observations as well as between two models become important to determine and reduce the errors associated with the models under different space environment conditions. It is shown that the models differ from the observations even during the geomagnetically quiet times but the associated errors during the geomagnetically active times increase more. It is found that the T89 model gives closer predictions to the observations, especially during active times and the errors are smaller compared to the IGRF model. The magnitude of the error in the angle under both environmental conditions is found to be less than 1 degree. The effects of magnetic disturbances resulting from geospace storms on the satellite attitudes estimated by EKF are also examined. The increasing levels of geomagnetic activity affect geomagnetic field vectors predicted by IGRF and T89 models. Various sensor combinations including magnetometer, gyroscope, and sun sensor are evaluated for magnetically quiet and active times. Errors are calculated for estimated attitude angles and differences are discussed. This portion of the study emphasizes the importance of environmental factors on the satellite attitude determination systems. Since the sun sensors are frequently used in both planet-orbiting satellites and interplanetary spacecraft missions in the solar system, a spacecraft close enough to the sun and a planet is also considered. The spacecraft receives electromagnetic radiation of direct solar flux, reflected radiation namely albedo, and emitted radiation of that planet. The albedo is the fraction of sunlight incident and reflected light from the planet. Spacecraft can be exposed to albedo when it sees the sunlit part of the planet. The albedo values vary depending on the seasonal, geographical, diurnal changes as well as the cloud coverage. The sun sensor not only measures the light from the sun but also the albedo of the planet. So, a planet's albedo interference can cause anomalous sun sensor readings. This can be eliminated by filtering the sun sensors to be insensitive to albedo. However, in most of the nanosatellites, coarse sun sensors are used and they are sensitive to albedo. Besides, some critical components and spacecraft systems e.g. optical sensors, thermal and power subsystems have to take the light reflectance into account. This makes the albedo estimations a significant factor in their analysis as well. Therefore, in this research, the purpose is to estimate the planet's albedo using a simple model with less parameter dependency than any albedo models and to estimate the attitude by comprising the corrected sun sensor measurements. A three-axis attitude estimation scheme is presented using a set of Earth's albedo interfered coarse sun sensors (CSSs), which are inexpensive, small in size, and light in power consumption. For modeling the interference, a two-stage albedo estimation algorithm based on an autoregressive (AR) model is proposed. The algorithm does not require any data such as albedo coefficients, spacecraft position, sky condition, or ground coverage, other than albedo measurements. The results are compared with different albedo models based on the reference conditions. The models are obtained using either a data-driven or estimated approach. The proposed estimated albedo is fed to the CSS measurements for correction. The corrected CSS measurements are processed under various estimation techniques with different sensor configurations. The relative performance of the attitude estimation schemes when using different albedo models is examined. In summary, the effects of two main space environment disturbances on the satellite's attitude estimation are studied with a comprehensive analysis with different types of spacecraft trajectories under various environmental conditions. The performance analyses are expected to be of interest to the aerospace community as they can be reproducible for the applications of spacecraft systems or aerial vehicles.
Dynamic and aeroelastic analysis of advanced aircraft wings carrying external stores

(Lisansüstü Eğitim Enstitüsü, 2021) Aksongur Kaçar, Alev ; Kaya, Metin Orhan ; 709160 ; Uçak ve Uzay Mühendisliği

Bu çalışma gelişmiş uçak kanatlarında harici yük ve takip edici kuvvet altında kanadın dinamik ve aeroleastik davranışlarını incelemektedir. Harici yüklerin ağırlığı, pozisyonu, birbirine göre yerleşimi, kompozit katmanların yönelimi ile itki kuvveti etkileri incelenmiş ve hepsinin kanadın doğal frekansı ve kritik çırpınma hızına olan etkileri tespit edilmiştir.
Experimental and numerical investigation of flapping airfoils interacting in various arrangements

(Graduate School, 2021-12-10) Yılmaz, Saliha Banu ; Ünal, Mehmet Fevzi ; Şahin, Mehmet ; 521082102 ; Aeronautical and Astronautical Engineering

In the last decades, flapping wing aerodynamics has gained a great deal of interest. Inspired by insect flight, the utilization of multiple wings has become very popular in Micro Air Vehicle (MAV) and Micromechanical Flying Insect (MFI) design. Therefore, studies aiming to disclose the characteristics of flow around interacting flapping airfoils has received a particular attention. However, the majority of these studies were done using real, complex, three dimensional parameters and geometries without making any assessment on basic two dimensional vortex dynamics. The aim of this study is to identify the baseline flow field characteristics in order to better understand the flapping wing aerodynamics in nature and thus to provide a viewpoint for MAV and MFI design. The thesis contains numerical and experimental investigation of tandem (in line) and biplane (side by side) arrangements of NACA0012 airfoils undergoing harmonic pure plunging motion by means of vortex dynamics, thrust and propulsive efficiency. Additionally, the "deflected wake phenomenon" which is an interesting and a challenging benchmark problem for the validation of the numerical algorithms for moving boundary problems is investigated for a single airfoil due to its flow characteristics which accommodates strong transient effects at low Reynolds numbers. Throughout the study, effects of reduced frequency, non-dimensional plunge amplitude, Reynolds number and phase angle between airfoils are considered. The vorticity patterns are obtained both numerically and experimentally whereas force statistics and propulsive efficiencies are evaluated only in numerical simulations. In the experimental phase of the study, Particle Image Velocimetry (PIV), which is a non-intrusive optical measurement technique, is utilized. Experiments are conducted in the large scale water channel in the Trisonic Laboratory of Istanbul Technical University. The motion of the wings is provided by two servo motors and their gear systems. To obtain a two dimensional flow around the wings, they are placed in between two large endplates one of which is having a slot to permit the connection between the wings and the servo motors. The flow is seeded with silver coated hollow glass spheres of 10µ diameter and illuminated with a dual cavity Nd-Yag laser. To visualize a larger flow area, two 16-bit CCD cameras are used together either inline or side by side depending on the positions of the wings. Dantec Dynamics's Dynamic Studio software is used for synchronization, image acquisition, image stitching and cross correlation purposes. Synchronization between servo motors and data acquisition system is done via LabView software. In post process, an in-house Matlab code is used for masking of the airfoils. CleanVec and NFILVB software are utilized for vector range validation and for filtering. In order to gather mean velocity fields, NWENSAV software is used. From the experimental velocity vector fields, two dimensional vorticity fields are obtained in order to understand the flow field characteristics. The experimental results are also used as a benchmark for the numerical studies. In the numerical phase of the study, an arbitrary Lagrangian-Eulerian (ALE) formulation based on an unstructured side-centered finite volume method is utilized in order to solve the incompressible Navier-Stokes equations. The velocities are defined at the midpoint of each edge where the pressure is defined at element centroid. The present arrangement of the primitive variables leads to a stable numerical scheme and it does not require any ad-hoc modifications in order to enhance pressure-velocity coupling. The most appealing feature of this primitive variable arrangement is the availability of very efficient multigrid solvers. The mesh motion algorithm is based on an algebraic method using the minimum distance function from the airfoil surface due to its numerical efficiency, although in some cases where large mesh deformation occurs Radial Basis Function (RBF) algorithm is used. To satisfy Discrete Geometric Conservation Law (DGCL), the convective term in the momentum equation is modified in order to take account the grid velocity. The numerical grid is created via Gambit and Cubit softwares with quadrilateral elements. Grid and time independencies are achieved by means of force statistics and vorticity fields. To make direct comparison Finite Time Lyapunov Exponent (FTLE) fields are calculated for some cases. FTLE fields characterize fluid flow by measuring the amount of stretching between neighbouring particles and the Lagrangian Coherent Structures (LCS) are computed as the locally maximum regions of the FTLE field. On the other hand, using a second-order Runge-Kutta method particle tracking algorithm is developed based on the integration of the massless particle trajectories on moving unstructured quadrilateral elements. Validation of results is performed by comparing the numerical results with the experimental results and also comparing with the corresponding cases in the literature. Accordingly, the results were substantially compatible within itself and also compatible with the literature. Highly accurate numerical results are obtained in order to investigate the flow pattern around a NACA0012 airfoil, undergoing pure harmonic plunging motion corresponding to the deflected wake phenomenon which are confirmed by means of spatial and temporal convergence. Present study successfully reproduces the details of the flow field which is not produced in literature such as fine vortical structures in opposite direction of the deflected wake and the vorticity structures close to airfoil surface which is dominated by complex interactions of LE with the plunging airfoil. Moreover, highly persistent transient effects and the calculations require two orders of magnitude larger duration than the heave period to reach the time-periodic state which is prohibitively expensive for the numerical simulations. This persistent transient effect is not reported before in the literature. The three-dimensional simulation also confirms highly persistent transient effects. In addition, the three-dimensional simulation indicates that the flow field is highly three-dimensional close to the airfoil leading edge. The three-dimensional structure of the flow field is not noted in the literature for the parameters used herein. In case of tandem arrangement of airfoils, the experimental results agree well with the numerical solutions. Major flow structures are substantially compatible in both numerical and experimental results at Reynolds number of 2,000. For the considered parameters, during upstroke and downstroke co-rotating leading and trailing end vortices merge at the trailing end of the forewing and interact with the downstream airfoil in either constructive or destructive way in trust production. Thrust production of forewing is maximum when airfoil moves from topmost position to mid position for the considered reduced frequencies at all configurations. It is hard to specify thrust-drag generation characteristics of the hindwing since it depends on not only plunge motion parameters, but also on interactions with vortices from the forewing. For the considered phase angles of 0°, 90°, 180° and 270°, in addition to stationary hind wing case, the force statistics are strongly altered due to the airfoil-wake interactions. In case of biplane arrangement of airfoils at phase angle of 180°, experimental and numerical vorticity results are also quite comparable. Regarding the parameters investigated, as the reduced frequency increases, vorticity structures get larger at constant plunge amplitude. However, vorticity structures do not change much after a certain reduced frequency value. As the plunge amplitude increases, the magnitude of vortices increases without depending on reduced frequency. Increasing plunge amplitude results in increased amount of fluid moving in the direction of motion in a constant period of time, commensurate with strong suction between airfoils as they move apart from each other. As a consequence of this suction force, energetic vortex pairs are formed which helps in thrust augmentation. For thrust production, among the phase angles considered, i.e. 0°, 90°, 180° and 270°, in addition to stationary lower wing case, the most efficient is φ=180°. Effect of three dimensionality is not observed at this phase angle for the considered parameters. Additionally, no remarkable difference is observed in general flow structure when Reynolds number is increased from 2,000 to 10,000.
Investigations on the effects of conical bluff body geometry on nonpremixed methane flames

(Graduate Institute, 2021) Ata, Alper ; Özdemir, İlyas Bedii ; 675677 ; Department of Aeronautics and Astronautics Engineering

This thesis is composed of three experimental studies, of which the first two are already published, and the third is under peer review. The first study investigates the effects of a stabilizer and the annular co-flow air speed on turbulent nonpremixed methane flames stabilized downstream of a conical bluff body. Four bluff body variants were designed by changing the outer diameter of a conically shaped object. The co-flow velocity was varied from zero to 7.4 m/s, while the fuel velocity was kept constant at 15 m/s. Radial distributions of temperature and velocity were measured in detail in the recirculation zone at vertical locations of 0.5D, 1D, and 1.5D. Measurements also included the CO2, CO, NOx, and O2 emissions at points downstream of the recirculation region. Flames were visualized under 20 different conditions, revealing various modes of combustion. The results evidenced that not only the co-flow velocity but also the bluff body diameter play important roles in the structure of the recirculation zone and, hence, the flame behavior. The second study analyzes the flow, thermal, and emission characteristics of turbulent nonpremixed CH4 flames for three burner heads of different cone heights. The fuel velocity was kept constant at 15 m/s, while the coflow air speed was varied between 0 – 7.4 m/s. Detailed radial profiles of the velocity and temperature were obtained in the bluff body wake at three vertical locations of 0.5D, 1D, and 1.5D. Emissions of CO2, CO, NOx, and O2 were also measured at the tail end of every flame. Flames were digitally photographed to support the point measurements with the visual observations. Fifteen different stability points were examined, which were the results of three bluff body variants and five coflow velocities. The results show that a blue-colored ring flame is formed, especially at high coflow velocities. The results also illustrate that, depending on the mixing at the bluff-body wake, the flames exhibit two modes of combustion regimes, namely fuel jet- and coflow-dominated flames. In the jet-dominated regime, the flames become longer compared to the flames of the coflow-dominated regime. In the latter regime, emissions were largely reduced due to the dilution by the excess air, which also surpasses their production. The final study examines the thermal characteristics of turbulent nonpremixed methane flames stabilized by four burner heads with the same exit diameter but different heights. The fuel flow rate was kept constant with an exit velocity of 15 m/s, while the co-flow air speed was increased from 0 to 7.6 m/s. The radial profiles of the temperature and flame visualizations were obtained to investigate the stability limits. The results evidenced that the air co-flow and the cone angle have essential roles in the stabilization of the flame: Increase in the cone angle and/or the co-flow speed deteriorated the stability of the flame, which eventually tended to blow-off. As the cone angle was reduced, the flame was attached to the bluff body. However, when the cone angle is very small, it has no effect on stability. The mixing and entrainment processes were described by the statistical moments of the temperature fluctuations. It appears that the rise in temperature coincides with the intensified mixing, and it becomes constant in the entrainment region.
Numerical and experimental study of fluid structure interaction in a reciprocating piston compressor

(Graduate School, 2022-01-14) Coşkun, Umut Can ; Acar, Hayri ; Güneş, Hasan ; 511132113 ; Aeronautics and Astronautics Engineering

Consisting of household refrigerators, cold storages, cold chain logistics, industrial freezers, air conditioners, cryogenics and heat pumps, refrigeration industry are a vital part of many sectors such as food, health care, air conditioning, sports, leisure, production of plastics and chemicals along with electronic data processing centers and scientific research facilities, which can not operate without refrigeration. There are roughly 5 billion in operation refrigeration systems which consumes 20% of the electricity used worldwide, responsible of 7.8% of GHG emission of the world, 500 billion USD cost of annual equipment sale, 15 million of employed people. Around 37% of global warming impact caused by refrigeration is direct emission of fluorinated refrigerants (CFCs, HCFCs and HFCs), 63% is due to indirect emission caused by electricity generation required for refrigeration. Both economic goals of making refrigeration units cheaper, more durable, and environment concerns of making these units more efficient and less hazardous for the world, require meticulous research and study on these refrigeration units. Approximately 40% of refrigeration units consist of domestic refrigeration systems alone where mostly hermetic, reciprocating type compressors are used. Design and improvement of such compressors is a multidisciplinary subject and requires deep understanding of heat and momentum transfer between refrigerant and solid component of compressor which can only be done through scientific investigation, using experimental and numerical techniques. In this thesis study, concerning the advantages of numerical studies, a multi-physics numerical model of flow through the gas line of a household, hermetically sealed, reciprocating piston compressor and the fluid structure interaction around the valve reeds including the contact between deformable parts was developed. Concerning the complexity of the model, the problem divided into several steps and at each step, numerical results are validated with experiments. In the first chapter of this thesis, the motivation behind the thesis study is discussed along with a theoretical background about refrigeration, compressors, fluid-structure interaction and a comprehensive literature survey are summarized to express the position of the thesis study among academic literature and it's novelty. In the second chapter, experimental studies conducted throughout the thesis are presented. Experimental studies divided into two sections. In the first section, the valve reed dynamics are investigated experimentally outside the compressor in multiple test conditions. A test rig is built for this reason, and the displacement of valve reed under constant point load, free oscillation and the impact of valve reed to valve plate from a pre-deformed form are measured, in order to validate the numerical work. In the second section, the compressor specifications such as cooling capacity, compression work, average refrigerant mass flow rate, along with surface temperature and instantaneous pressure variation from several locations inside the compressor are measured inside a calorimeter setup, to provide boundary conditions and validation for numerical analyses. Numerical work of the thesis study is explained in the third chapter. Modelling the whole compressor gas line between compressor inlet and outlet, including the strong coupled interaction between the refrigerant and deformable solid parts such as valve reeds is too complex of an attempt to do in a single step. Therefore, the numerical problem divided into seven smaller numerical problems and investigated consecutively. At each consecutive steps, problems are isolated, identified, solved and results are validated. The similarity of each step to the final model is increased along with it's complexity as a natural consequence at each consecutive steps. The numerical studies also briefly cover the advantages and disadvantages of using an open source or a commercial multi-physics solver, where OpenFOAM and Ansys Workbench software are utilized for this purpose, respectively. After the simplified steps of the numerical model are completed, the whole gas line of a compressor produced by Arçelik is modelled. The numerical results compared against experimentally obtained data and a good agreement is achieved between them. The developed method is further used for parametric investigation on compressor design to show the capabilities and the benefits of the numerical model. Finally, results of whole thesis study, the experience gained throughout the thesis work and the planned future work are discussed in the final chapter.
Numerical simulation of aircraft icing with an adaptive thermodynamic model considering ice accretion

(Institute of Science and Technology, 2022) Siyahi, Hadi ; Baytaş, A. Cihat ; 754795 ; Department of Aeronautics and Astronautics Engineering

The icing phenomenon is one of the most undesirable events in aircraft. We may see this phenomenon from different points of view. The safety of flight is undoubtedly the biggest concern of designers, nowadays. The icing causes the malfunctioning or even failure of the pressure and speed measurement devices, and consequently make difficulties for controllability of the flight. Icing in rudder, ailerons, and elevators can also make control of aircraft even impossible. During landing, the icing on the pilot window along with possible failures in the landing gears may cause major catastrophes. Besides, detachment of ice particles can cause serious mechanical damage to the aircraft when they collide with the body or sometimes with internal parts such as compressor blades. The other point of view is the degradation of the performance of aircraft, and consequently the increase of fuel consumption because of icing. Icing affects the aerodynamics of an airplane in an undesirable way and puts the aircraft in a situation that is far from what the aircraft is designed for. Therefore, it is necessary to study aircraft icing to provide a safer and more efficient flight. Since the icing in aircraft is of great importance, a precision analysis of this phenomenon should be performed. Tests in the wind tunnel and during the flight are very expensive. On contrary, the numerical-computational simulations can be cost-effective for studying aircraft icing. In the present study, the numerical-computational simulation of aircraft icing has been performed by writing a computer-code via FORTRAN. The computational simulation of aircraft icing is a modular procedure consisting of the grid generation, air solver, droplet solver and ice accretion modules. First, the computational domain is generated via elliptic grid generation. The differential methods based on the solution of the elliptic equations are commonly used for generating of the mesh for a geometry with arbitrary boundaries. Elliptic equations are also utilized for the unstructured grids. The most popular elliptic equation is the Poisson equation, which gives the wonderful possibility to satisfy smoothness, fine spacing, and orthogonality on the body surface by means of the controlling terms. Then, the velocity and pressure distributions of airflow around the wing have been found, and the convective heat transfer coefficient on the body will be calculated. The inviscid flow model has been selected in our simulation because it needs less effort and time in comparison with the Navier-Stokes codes. The two-dimensional, steady-state, inviscid, incompressible, irrotational flow (potential flow) model has been applied for solving airflow.
Optimization based-control of cooperative and noncooperative multi aircraft systems

( 2020) Başpınar, Barış ; Koyuncu, Emre ; 625456 ; Uçak ve Uzay Mühendisliği

In this thesis, we mainly focus on developing methods that ensure autonomous control of cooperative and noncooperative multi-aircraft systems. Particularly, we focus on aerial combat, air traffic control problem, and control of multiple UAVs. We propose two different optimization-based approaches and their implementations with civil and military applications. In the first method, we benefit from hybrid system theory to present the input space of decision process. Then, using a problem specific evaluation strategy, we formulate an optimization problem in the form of integer/linear programming to generate optimal strategy. As a second approach, we design a method that generates control inputs as continuous real valued functions instead of predefined maneuvers. In this case, we benefit from differential flatness theory and flatness-based control. We construct optimization problems in the form of mixed-integer linear programming (MILP) and non-convex optimization problem. In both methods, we also benefit from game theory when there are competitive decision makers. We give the details of the approaches for both civil and military applications. We present the details of the hybrid maneuver-based method for air-to-air combat. We use the performance parameters of F-16 to model the aircraft for military applications. Using hybrid system theory, we describe the basic and advanced fighter maneuvers. These maneuvers present the input space of the aerial combat. We define a set of metrics to present the air superiority. Then, the optimal strategy generation procedure is formulated as a linear program. Afterwards, we use the similar maneuver-based optimization approach to model the decision process of the air traffic control operator. We mainly focus on providing a scalable and fully automated ATC system and redetermining the airspace capacity via the developed ATC system. Firstly, we present an aircraft model for civil aviation applications and describe guidance algorithms for trajectory tracking. These model and algorithms are used to simulate and predict the motion of the aircraft. Then, ATCo's interventions are modelled as a set of maneuvers. We propose a mapping process to improve the performance of separation assurance and formulate an integer linear programming (ILP) that benefits from the mapping process to ensure the safety in the airspace. Thereafter, we propose a method to redetermine the airspace capacity. We create a stochastic traffic environment to simulate traffics at different complexities and define breaking point of an airspace with regards to different metrics. The approach is validated on real air traffic data for en-route airspace, and it is shown that the designed ATC system can manage traffic much denser than current traffic. As a second approach, we develop a method that generates control inputs as continuous real valued functions instead of predefined maneuvers. It is also an optimization-based approach. Firstly, we focus on control of multi-aircraft systems. We utilize the STL specifications to encode the missions of the multiple aircraft. We benefit from differential flatness theory to construct a mixed-integer linear programming (MILP) that generates optimal trajectories for satisfying the STL specifications and performance constraints. We utilize air traffic control tasks to illustrate our approach. We present a realistic nonlinear aircraft model as a partially differentially flat system and apply the proposed method on managing approach control and solving the arrival sequencing problem. We also simulate a case study with a quadrotor fleet to show that the method can be used with different multi-agent systems. Afterwards, we use the similar flatness-based optimization approach to solve the aerial combat problem. In this case, we benefit from differential flatness, curve parametrization, game theory and receding horizon control. We present the flat description of aircraft dynamics for military applications. We parametrize the aircraft trajectories in terms of flat outputs. By the help of game theory, the aerial combat is modeled as an optimization problem with regards to the parametrized trajectories. This method allows the presentation of the problem in a lower dimensional space with all given and dynamical constraints. Therefore, it speeds up the strategy generation process. The optimization problem is solved with a moving time horizon scheme to generate optimal combat strategies. We demonstrate the method with the aerial combats between two UAVs. We show the success of the method through two different scenarios.
Performance enhancing additives for hybrid rockets

(Graduate School, 2023-02-22) Karakaş, Hakkı ; Özkol, İbrahim ; Karabeyoğlu, Arif ; 511142111 ; Aeronautics and Astronautics Engineering

A comprehensive assessment of fuel additives for a paraffin-based hybrid rocket fuel and hybrid rocket test firings are presented in this thesis. The reason for the selection of paraffin wax fuel binder is discussed as well as the expected performance gain by the addition of energetic materials to the fuel. Al, Mg, LiAlH4 and NBH6 are selected by assessing the thermochemical calculation results and material availability. An experimental study with liquid nitrous oxide oxidizer is concluded which showed Al and LiAlH4 are promising materials for future studies. They increase the c* which in turn increase delivered Isp and decrease the nozzle erosion rate. Also, ammonium borane, which is a promising material because of the rich hydrogen content, is studied, but because of the problems in its procurement detailed tests are postponed in a future study. If the availability and cost problems are solved, ammonium borane is the best choice for theoretical Isp performance. However, it needs to be tested in real operating conditions to better understand its characteristics. First chapter of the thesis shows that there are improvements of the hybrid rocket regression rate, Isp and combustion efficiency with the energetic material addition. However, the most noteworthy improvement is the nozzle erosion rate reduction. Therefore, it is decided to study this characteristic in more detail. Due to its relatively low cost and wide availability, carbon graphite is one of the most widely used ablative nozzle material in hybrid rocket propulsion. The erosion characteristics of this material has paramount importance, since it directly influences the Isp performance. This is especially the case for upper stage or in-space rocket motors operating with very long burn times. In this study the effect of aluminum added fuel on the graphite nozzle erosion is studied. In the experimental studies, a high regression rate paraffin-based fuel is loaded with micron size aluminum powder for nozzle erosion reduction. In our approach, aluminum is added at high concentrations as a fuel ring in front of the main paraffin-based fuel which contains no aluminum. Based on the motor tests conducted with gaseous oxygen as an oxidizer, it is shown that aluminum addition decreased the nozzle erosion rate up to 45% and increased the nozzle erosion onset time by 1 to 3 seconds. The new method of introducing an energetic powder in a fuel ring positioned at the fore end of the motor offers an easy and scalable way of reducing the nozzle erosion and improving the Isp performance of the rocket motor. As pointed out using Al as additive for hybrid rocket motors, substantially reduce the nozzle erosion rate which increase the Isp performance. It is widely available, cost effective and easy to use material. The novel addition of the Al material as a high concentrated ring to the hybrid rocket fuel, makes this method highly scalable for larger rocket motors. In future, ammonium borane additive could be studied, but its cost and availabilty is a problem to be solved.
Safe motion planning and learning for unmanned aerial systems

(Graduate School, 2022-05-06) Perk, Barış Eren ; İnalhan, Gökhan ; 511142104 ; Aeronautics and Astronautics Engineering

To control unmanned aerial systems, we rarely have a perfect system model. Safe and aggressive planning is also challenging for nonlinear and under-actuated systems. Expert pilots, however, demonstrate maneuvers that are deemed at the edge of plane envelope. Inspired by biological systems, in this paper, we introduce a framework that leverages methods in the field of control theory and reinforcement learning to generate feasible, possibly aggressive, trajectories. For the control policies, Dynamic Movement Primitives (DMPs) imitate pilot-induced primitives, and DMPs are combined in parallel to generate trajectories to reach original or different goal points. The stability properties of DMPs and their overall systems are analyzed using contraction theory. For reinforcement learning, Policy Improvement with Path Integrals (PI2) was used for the maneuvers. The results in this paper show that PI2 updated policies are a feasible and parallel combination of different updated primitives transfer the learning in the contraction regions. Our proposed methodology can be used to imitate, reshape, and improve feasible, possibly aggressive, maneuvers. In addition, we can exploit trajectories generated by optimization methods, such as Model Predictive Control (MPC), and a library of maneuvers can be instantly generated. For application, 3-DOF (degrees of freedom) Helicopter and 2D-UAV (unmanned aerial vehicle) models are utilized to demonstrate the main results.
Teknoloji geliştirme bölgelerinin hizmet kalitesinin ölçümü: Türkiye genelinde bir uygulama

(Fen Bilimleri Enstitüsü, 2020) Özyurt, Mehmet Akif ; Özkol, İbrahim ; 656880 ; Uçak ve Uzay Mühendisliği Ana Bilim Dalı

Bilgi Üretimine ve bunun bir çıktısı olan teknolojik üretime dayalı ürünler bugün çağımıza damgasını vurmuş ve yaşadığımız zaman dilimi, bir çok düşünür tarafından "Bilgi Çağı" olarak adlandırılmıştır. Bu çağda yüksek teknoloji üretiminin merkezinde olan ülkelerin gücü, toprak ya da sermaye büyüklüğünden değil, kaliteli eğitilmiş insan gücünün büyüklüğünden ve bu gücün yüksek teknoloji içeren üretimlere aktarılmasından kaynaklanmaktadır. Eğitim seviyesi yüksek insanlara sahip ülkelerin, üretim kalite ve seviyeleri de yüksektir. Yaşadığımız yüzyılda ülkelerin bilimsel ve teknolojik gelişim hızı çok artmıştır. Bugüne kadar ortaya çıkan bu gelişmelerin çoğu, son 30 yıl içerisinde meydana gelmiş olup, bu hız her geçen gün katlanarak artmaktadır. Dolayısı ile, gelecek kısa vadeli zaman diliminde de, bilimsel ve teknolojik açıdan, şu an yaşadığımızdan çok daha ileride bir dünyanın ortaya çıkacağını öngörmek yanlış olmaz. Yüksek teknoloji üretimi günümüzde, rekabet üstünlüğü yarışının da en belirleyici unsuru haline gelmiştir. Bu nedenle, rekabet gücünün artırılması, sadece maliyetleri düşürmeye değil, tüketici tercih ve taleplerine hızlı bir şekilde yanıt vermenin ötesinde, sürekli gelişime, yenilik ve icatta bulunmaya bağlı bir duruma gelmiştir. Teknolojik bulguları, pazarlama şansı olan bir ürün ya da hizmete, yeni bir üretim veya dağıtım yöntemine, ya da yeni bir hizmet mekanizmasına dönüştürmede, yani teknolojik yenilik üretiminde (inovasyonda) başarılı olanlar artık, dünya pazarlarına egemen olmaktadırlar. Bu tür Ar-Ge'ye dayalı teknolojik gelişmelerin ve yeniliklerin ortaya çıkartıldığı, kaliteli eğitilmiş insan gücünün istihdam edildiği, yüksek katma değerli ürünleri üreten şirketleri ve kurumları bünyesinde barındıran bölgelere, "Teknopark" ya da ülkemizde ilgili yasanın verdiği ad ile "Teknoloji Geliştirme Bölgesi" (TGB) adı verilmektedir. Kavramsal olarak, teknoparklar, Ar-Ge yapıcılar ile, üniversiteler ve sanayi (firmaları) arasında bilim ve teknoloji akışını sağlamaya ve yaymaya yardımcı olan araçlardır. Ayrıca teknoparklar, kuluçka mekanizmalarının oluşturduğu sinerji ile, bilim ve teknoloji tabanlı firmaların gelişimini kolaylaştırmaktadırlar. Bu alanlarda, yüksek teknoloji ve destek araçları kullanılarak, firmalar yenilikçi olmaya teşvik edilmekte, bu yolla katma değeri yüksek ürünler ortaya çıkartılmaktadır. Uluslararası Bilim Parkı Birliği tarafından ise teknoparklar, temel amaçları yenilikçilik kültürünü ve işletmelerinin ya da bilgi merkezli kurumların rekabet gücünü artırmayı destekleyerek, toplumun refah seviyesini yükseltmek olan, alanında profesyonel ekipler tarafından yönetilen yapılar şeklinde tanımlanmaktadır. Bu hedeflere ulaşmak için teknoparklar, üniversiteler, Ar-Ge yapıcıları ve firmalar arasındaki bilgi ve teknoloji akışını sağlar, yönetir, kuluçka ve spin-off mekanizmaları ile yenilikçilik eksenli şirketlerin oluşmasını ve gelişmesini kolaylaştırır, kaliteli yapılar üreterek, diğer katma değer sunan şirket ve hizmetlerin de ortaya çıkmasına altyapı hazırlarlar. Bu tanımlamalar doğrultusunda, diğer adı ile TGB'lerin aslında bilim ve teknoloji kümelenmesi oldukları da söylenebilir. Çünkü genel anlamda teknoparklar, yenilikçi fikirlerle bir araya gelen, ileri teknoloji üreten veya kullanan ve aynı zamanda bu teknolojiyi pazarlayan, Ar-Ge merkezinden ya da üniversiteden faydalanan işletmelerin oluşturduğu bir küme olarak da tabir edilmektedirler. Teknoparklara yönelik yapılan bu tanımlamaların farklılığı büyüklüklerinden ve işkolu faaliyetlerindeki farklılıklardan kaynaklanmaktadır. Yüksek teknoloji üreticilerinin konumlanma merkezi olan teknoparklar, istihdam imkanlarının artırılmasında, gerekli bilgi birikimi sağlanarak sanayinin geliştirilmesinde, üniversiteler ile birlikte eğitim olanaklarının artırılması için firmalara destek verilmesinde ve KOBİ'lerin sayısının artırılmasının yanı sıra bunların desteklenmesinde de etkili bir araç olarak kullanılmaktadırlar. Bu açıdan teknoparkların en temel amaçlarından bir tanesi üniversite, sanayi ve devlet arasında iş birliği sağlamak ve buna bağlı olarak bilgi ve teknoloji ağırlıklı mekânların kurulması ile bölgesel, ulusal ve uluslararası rekabetçilik seviyesinin artırılarak, ülke kalkınmasına katkı sağlamaktır. Teknoparklar, ülkelerin istihdam yapısını olumlu yönde değiştiren ve işsizlik oranının düşmesinde önemli bir etken olan, yeni ve yüksek teknoloji altyapısına sahip alanlardır. Bunun örneklerini teknopark tecrübeleri eskiye dayanan gelişmiş ve sanayileşmiş ülkelerde görmek mümkündür. Bu değişim ve gelişmenin de etkisi ile istihdamın sektörel dağılım anlamında da farklılaştığı görülmektedir. Bilindiği gibi geçmişte gelişmişliğin bir ölçütü, işgücü dağılımının tarım ve sanayi sektörlerindeki durumu olarak görülmekteydi. Şimdilerde ise gelişmişliğin ölçütü olarak, teknoloji sektöründeki istihdam oranı bir ölçüt olarak görülmektedir. Örneğin gelişmiş bir ülke durumunda olan Almanya'da, tarım ve geleneksel sanayilerindeki yüksek istihdam oranı günümüzde ciddi bir azalış göstererek istihdam, yüksek teknolojik ürün üreten sektörlere doğru kaymıştır. Teknoparklarda, Üniversite - Sanayi - Devlet üçgeninde yer alan bütün aktörlerin karlı çıkması hedeflenerek, Ar-Ge için yatırım yapacak yeterli gücü olmayan firmaların da desteklenmesi ve üniversitelerde üretilen bilginin ticarileştirilerek bu firmalara aktarılması düşüncesi de gerçekleştirilmeye çalışılmaktadır. Buna bağlı olarak oluşturulan teknopark ara yüzünün, üniversite, sanayi, bölge ve ülke ekonomik yapısına önemli katkılar sağlaması beklenmektedir. Nitekim teknoparklardan sanayiye akan bu bilgi, sanayi üretiminin modern ölçülerde yapılmasında ve üretim tabanının bilgi ve teknoloji kaynaklı olmasında etkili bir rol oynamaktadır. Bir diğer deyiş ile teknoparklar vasıtası ile, sanayinin üniversitede üretilen bilgiye ulaşması ve üniversitede üretilen bu bilginin de sanayi tarafından uygulama alanı bulması hedeflenmektedir. Bu çalışmada, Türkiye'de faaliyet gösteren teknoparkların sunmuş olduğu hizmet kalitesi ile bu hizmetlerden istifade eden oyuncuların algıladığı hizmet kalitesi arasındaki farkı ortaya çıkarmak, Servqual ölçeğinden yararlanılarak müşterilerin (Ar-Ge yapıcılarının) memnuniyet düzeylerini belirlemek amaçlanmıştır. Çalışmada ayrıca teknoparkların faaliyette bulundukları süre ile müşterilerin teknoparklara ilişkin hizmet kalite algıları arasında bir ilişki olup olmadığı araştırılmıştır. Teknoparklar arasında geçiş yapan firmalarda, teknopark değiştirme kararı verirken hizmet kalitesinin etkisinin de belirlenmesi hedeflenmiştir. Çalışmada son olarak Vikor yöntemi kullanılarak Türkiye'de faaliyet gösteren teknoparklar, hizmet kalitesi açısından sıralanmıştır. Araştırmada Servqual ölçeğinde yer alan hizmet ölçüm faktörleri yer almıştır. Parasuraman ve ark. (1988) tarafından geliştirilen ve hizmet kalitesini belirlemek için ortaya koydukları Servqual ölçme aracı, bugüne kadar spor tesislerinden, otel hizmetlerine kadar tüm hizmet işletmelerinde sıklıkla kullanılmıştır. Bu ölçek hem yurtiçi hem de yurtdışında birer hizmet işletmesi olarak ele alınan teknoparkların hizmet kalitesini ölçmek için, ilk defa bu çalışmada kullanılmıştır. Bu nedenle öncelikle ölçeğin teknoparklara adaptasyonu yapılmış ve bu adaptasyonun güvenilirlik ve geçerlilik çalışması gerçekleştirilerek, analizlere geçilmiştir. Araştırmada ölçeğinde, Servqual Hizmet Kalitesi Ölçeğinde yer alan, "Fiziksel Özellikler", "Güvenilirlik", "Heveslilik", "Yeterlilik" ve "Empati (Duyarlılık)" faktörleri kullanılmıştır. Fiziki özellikler faktörü, binalarda kullanılmış olan cihazların, iletişim malzemelerinin ve çalışanların fiziki görünümünü kapsamaktadır. Güvenilirlik faktörü, teknoparkların verdikleri hizmetinin zamanında ve doğru olarak yerine getirmesi ile ilgili durumunu tespit etmek için kullanılmaktadır. Heveslilik faktörü, teknoparkların müşterilerine yardım etme, hızlı hizmet verme istekliliği ve işin zamanında bitirme yeteneğini ölçmektedir. Yeterlilik faktörü, teknoparklarda çalışan servis personellerinin gerekli ve yeterli bilgiye sahip olup olmadığını ölçmek için kullanılmıştır. Empati (Duyarlılık) faktörü ise müşteri ile direkt ilişki içinde olan çalışanların, saygı, nezaket ve samimiyet düzeylerini belirlemeyi amaçlamaktadır. Çalışmada teknoparkların hizmet kalitesi seviyelerinin ölçümünün sağlanması, ileride yapılabilecek bilimsel araştırmalar için de öncü bir rol oynayacaktır. Hem yurtiçinde hem de yurtdışında buna benzer bir çalışma olmaması nedeni ile sonuçlarının, teknopark yönetici şirketleri için de büyük önem arz edeceği düşünülmektedir.

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Başlık ile LEE- Uçak ve Uzay Mühendisliği-Doktora'a göz atma

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