LEE- Uçak ve Uzay Mühendisliği-Doktora

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  • Öge
    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.
  • Öge
    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.
  • Öge
    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.
  • Öge
    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.
  • Öge
    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.