Organizational Unit: Department of Mechanical Engineering
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Master ThesisPublication Metadata only Optimization of tpms lattice structures via hybridization and grading of the lattice morphologiesÖzdemir, Mirhan; Şendur, Polat; Şendur, Polat; Yapıcı, Güney Güven; Şendur, G. K.; Department of Mechanical Engineering; Özdemir, MirhanOwing to its excellent mechanical properties, triply periodic minimum surfaces (TPMS) lattice structures have recently gained more interest in engineering applications. The superior properties of these structures make it easier to achieve engineering design goals such as strength and weight. Thanks to recent developments in additive manufacturing, the fabrication of the lattices are easier compared to the traditional methods. Therefore, their usage in the designs are more popular in recent application. However, technological advancements compel the designer to enhance the traditional TPMS design qualities. This thesis covers two approaches to enhance the design's mechanical performance by infilling the design domain with the optimal lattice design parameters. Initially, homogenization-based topology (HMTO) and free-size optimization-based graded lattice generation (FOGLG) methods are studied to obtain optimum lattice thickness distribution. The optimization methods are conducted for the modal characterization of a sandwiched structure. In the second study, a new hybrid optimization framework in which genetic algorithm (GA) and homogenization-based topology optimization are used to enhance the mechanical performance of the design. The method initially selects suitable lattice mythologies via GA and then grades them by topology optimization. In addition, the graded multi-morphology design is reconstructed by a novel blending algorithm in the study. The results of the studies clearly show that the proposed methods enable the designer to improve the mechanical performance of the designs. The proposed methods are also experimentally validated to assess their accuracy.Master ThesisPublication Metadata only Robust whole-body control for legged robotsOral, Dilay Yeşildağ; Uğurlu, Regaib Barkan; Uğurlu, Regaib Barkan; Ünal, Ramazan; Öniz, Y.; Department of Mechanical Engineering; Oral, Dilay YeşildağThis thesis aims to propose a robust whole-body locomotion controller for legged robots. To this end, it offers the centroidal momentum observer control algorithm, which could be a very useful tool for providing robust dynamic motion control in eliminating parameter uncertainty for legged locomotion. The control method based on centroidal momentum dynamics is essential for whole-body control. The method considers floating base dynamics when synthesizing controllers such that the base frame is not firmly connected to the ground; the base frame is freely floating. Therefore it can be applied to a wide range of mobile robotic systems. The method was tested using a simulated one-legged robot and whole-body humanoid models. As a result, we observe that the centroidal momentum observer control algorithm could be beneficial for whole-body robot robustness and stabilization.Master ThesisPublication Metadata only An experimental research on the effects of frost formation on different environmental conditions and surface configurationsÖksüz, Enes Abdülhakim; Arık, Mehmet; Arık, Mehmet; Şendur, Polat; Başol, Altuğ Melik; Yiğit, K. S.; Çelik, M.; Department of Mechanical Engineering; Öksüz, Enes AbdülhakimFrost formation on finned surfaces is observed on heat exchangers in cooling systems such as refrigerators and air conditioners. Accumulated frost on finned surfaces blocks the air flow in between fins and reduces the effective surface area and it creates an additional resistance for heat transfer. The frost accumulation on finned surfaces increases the energy consumption of a cooling system and needs to be mitigated by the design of the finned surfaces. In this experimental study, the effects of air velocity, relative air humidity, surface temperature, fin length, fin space, surface orientation and surface coating on the heat transfer performance of the finned surface have been investigated. The measurements were conducted in a horizontal wind tunnel open to atmosphere. Air at 18 °C was used over temperature controlled cold finned surfaces positioned inside the test section of the tunnel. The heat transfer rate through the finned surface was measured during the experiment using an in-house built Heat Flux Measurement System (HFMS). Experiments showed that frost accumulation on finned surfaces blocks the air penetration into fin gaps and results in a sharp drop in the heat transfer rate. It has been found that high relative humidity, high incoming air velocity, high fin length increases the heat transfer rate from the finned surface by reducing the negative impact of frosting on the heat transfer rate. Condensed water droplets occur on the surfaces since higher parameter values provides higher heat transfer rates. Condensed water droplets turn to ice with the beginning of the frost formation. High thermal conductivity of ice increases the effective thermal conductivity of frost and provides higher heat transfer rates. Narrow fin space is found to be disadvantageous under the frosting conditions. The small gap between fins is easily blocked by the frost and the effective surface area of the finned surface is heavily reduced. Fin gaps larger than 4 mm are required if the surface is to be used under frosting conditions. The effect of different fin shapes has also been studied. Knife edge shaped fins are found to be better in terms of heat transfer rate as compared to staggered and inline type fins. Regarding the orientation of the finned surface with respect to air flow, it was found that the parallel flow arrangement results in the highest heat flow rate compared to the impinging flow arrangement. This is due to the fact the finned surfaces under parallel flow arrangement is subjected to less frost accumulation. Finally, the effect of the superhydrophobic coating on the heat transfer performance of the finned surface was studied. The coated surfaces do not have a significant amount of water condensation in the vertical orientation and also in the parallel flow arrangement. On the other hand, over the uncoated surface considerable amount of water condensation was found. It could be that the water droplets could not adhere to the coated surface because of the superhydrophobic coating. The absence of the water droplets on the coated surface resulted in an earlier frost growth which resulted in an earlier blockage of the fins and an earlier drop in the heat transfer rate. Finally, the measurements have shown that hydrophobic coatings show a negative impact on the heat transfer rate of the finned surfaces under impinging flow conditions.PhD DissertationPublication Metadata only Mechanical performance of layered metallic composites processed by accumulative roll bonding(2019-08-20) Aljashaami, Dhyai Hassan Jawad; Yapıcı, Güney Güven; Yapıcı, Güney Güven; Başol, Altuğ; Bundur, Zeynep Başaran; Oral, A.; Tezel, Y. Ş.; Department of Mechanical Engineering; Aljashaami, Dhyai Hassan JawadMechanical Performance of Layered Metallic Composites Processed by Accumulative Roll Bonding.--Multi-layered metal composites have received considerable attention due to their advanced mechanical and physical properties. The current work is an experimental study to fabricate the ultra-fine grained combination of similar and dissimilar composites utilizing accumulative roll bonding (ARB) process as a severe plastic deformation (SPD) technique. The experimental work was organized in two parts. The first part describes the combination of Al2024 and Al6061 in similar and dissimilar aluminum composites, while the second part has different Al/IF steel composites including Al6061, Al2024 and interstitial free (IF) steel in various stacking sequences. Microhardness and uniaxial tensile tests were applied to analyse the surface and bulk mechanical properties of processed materials, respectively. This study not only investigates the monotonic mechanical behavior of multi-layered metal composites but also inspects the cyclic behavior of the prepared composites by employing the fatigue test. The high cycle fatigue (HCF) properties of layered metallic composites were investigated by cyclic testing under stress control with positive mean stress. For the first part, the processed structure after four passes ARB contained the various layer combinations of Al2024 and Al6061. Remarkable enhancement was observed in the hardness level of the samples with increasing number of ARB passes. Accordingly, improvement levels, up to 1.5 and 2 times, were recorded for Al2024 and Al6061 layers, respectively. The tensile strength of the composite with an interchanging layer architecture reached over 320MPa after two cycles, coinciding with more than two-fold of the as-received Al6061. The fatigue life was also improved, especially at the high stress amplitude. Microstructural observations revealed a significant grain refinement in further ARB processing along with the explanation of possible fracture mechanisms under tensile straining. Additionally, the mechanical properties of processed materials were evaluated using shear punch testing (SPT). The correlation between the results of tension experiments and shear strengths was calculated. Experimental results demonstrated that the shear strength enhanced by increasing the number of ARB passes. However, the shear elongation exhibited a notable reduction when the number of ARB passes increased. Inspection of the tensile and SPT results revealed that they follow a similar trend for both strength and ductility. Therefore, it can be asserted that the shear punch test represents a useful and complementary tool in the mechanical analysis of the ARBed samples. According to the SEM micrographs, in multi-passes ARB process, the interface of the previous pass bonds strongly during the next cycle, due to the improvement of the atomic diffusion and high pressure with further passes. The first ARB pass imposed a moderate strain and materials showed a ductile fracture with microvoids and dimples. With increasing cycles, the fracture mode remained as ductile with the existence of shear rupture and dimples. Nevertheless, these dimples were shallow and elongated, especially for the Al2024 layers as compared to those observed in Al6061. For the second part, necking and fracture of IF steel layers were detected in the macrostructural observation after three passes of ARB process. Furthermore, after five ARB passes, a multi-layer IF steel/Al composite with homogeneously distributed IF steel lumps in aluminum matrix was attained for all stackings of IF/Al6061. However, the low difference between the hardness of the Al2024 and IF steel prevents the occurrence of the same phenomena in Al2024/IF steel composites. Thus, the continuity of the layers after the third and fourth passes has remained for all stackings of IF/Al2024. Microstructure and mechanical characteristics of a fourth layer architecture were analyzed within a number of ARB passes. The results revealed that the monotonic and cyclic behavior of all dissimilar composites were significantly increased compared to the base aluminum alloys, while the composites with the outer aluminum layers exhibited the highest fatigue life, due to crack branching at the interface region when it propagated from the softer to the harder layer. Fatigue fracture surfaces and crack propagation paths of the samples were observed by scanning electron microscopy (SEM). Also, fracture morphology analysis demonstrated that despite the surface cracks on the outer layers, indeed the fatigue cracks of interface layers were caused by the fracture of samples. The ARB process was simulated utilizing finite element analysis. The effective stress and the distributions of equivalent strain along the thickness of ARBed sheets were determined. Results showed a significant agreement between the numerical simulations and the experimental findings. Finally, high cycle fatigue analysis was carried out and the results of the simulations were in decent agreement with the empirical data in terms of fatigue life. Also, as expected, the experimental fatigue life values for all conditions were lower than the simulations in relation with the existence of microcracks and scratches on the sample surface.PhD DissertationPublication Metadata only Turbulence and flame interaction for control of flame location in diffuser combustor(2018-08) Nazzal, Ibrahim Thamer Nazzal; Ertunç, Özgür; Ertunç, Özgür; Başol, Altuğ; Mengüç, Mustafa Pınar; ÖZdemir, B.; Koşar, A.; Department of Mechanical Engineering; Nazzal, Ibrahim Thamer NazzalAchieving an appropriate flame location is desirable in many combustion system applications especial in the design of the combustion system. Given that the characteristics of turbulent flows influence flame behavior and the design of combustion systems, flame location can be controlled within desirable levels without distorting the design of the combustor geometry by selecting suitable characteristics. A feedback control method utilizes the characteristics of turbulent flows to stabilize the flame location within the desirable level. The primary objective of this study is to introduce a strategy for flame location control using characteristics of turbulence flow. Turbulence intensity and length scale are among the main parameters of turbulent flow. Therefore, the secondary goal of this study is to investigate the influence of turbulence intensity and length scale on flame location. The investigation of the dependency of flame location on turbulence is based on selecting suitable combustor geometry. For this purpose, the axisymmetric diffuser form is used to reveal the response of the flame location of a turbulent premixed flame that has been exposed to various turbulence intensities and length scales. The diffuser is selected because the flow slows down along the direction. Thus, the flame is expected to propagate towards the inlet when the flame speed increases. In this manner, the effect of turbulence can be studied without changing the thermal power. In addition, the diffuser combustor is used to avoid blow-off and flame extinction because the flow slows along the combustor. Two types of diffuser combustors are selected for this study. The first combustor is a cylindrical diffuser, while the second one is a cylindrical diffuser with a conical insert. Numerical simulations are applied to the diffuser combustor for turbulent premixed propane flames by using a coherent flame model integrated to the Reynolds-averaged Navier–Stokes flow model with k-epsilon turbulence model. Firstly, the influence of turbulence on flame location in a diffuser-type combustor is studied under steady-state conditions. Results show that the flame location moves towards the inlet of the diffuser combustor with the increase in turbulence intensity for moderate- and high-turbulence length scales. The behavior of flame location is different in the low-turbulence length scale. The flame location initially decreases with the increase in turbulence intensity and subsequently stabilizes. Furthermore, the flame area density influences the flame location with the increase in turbulence intensity and turbulence length scale. Turbulence intensity and length scale simultaneously influence the flame area density, flame shape, and flame location. Secondly, the results of the unsteady simulations indicate that turbulence intensity, length scale, and flow separation exert a significant effect on the flame location of the premixed turbulent combustion. The flame front moves toward the diffuser inlet as a result of the increase in turbulence intensity and length scale. The flame location drops to the middle of the diffuser for the high turbulence intensity. However, the effect of turbulence intensity is more visible than that of turbulence length scale within the tested range. An increase in turbulent length scale at a constant turbulence intensity causes a decrease in flame location. It is observed that the combustion and inlet turbulence cause a flow separation mainly downstream of the flame front. Consequently, the secondary flow structures influence the flame topology and location. Therefore, the flame location and shape are influenced by the flow separation and the turbulence intensity and length scale. Thirdly, a conical insert is placed in the middle of a diffuser-type combustor to eliminate the flow separation. The influence of turbulence on flame location in two diffuser-type combustors (with and without conical insert) is studied and compared. Results indicate that the flame moves towards the inlet of the diffuser with the increase in turbulence intensity and length scale in the two diffuser-type combustors. At a high-turbulence length scale, the flame rapidly drops at the inlet of the diffuser with a conical insert with the increase in turbulence intensity, whereas the flame drops to an intermediate level when the diffuser is not used with a conical insert. Moreover, a similarity was observed in the trends of the flame location at low turbulence intensities in both cases. Results show that the Taylor-scale Reynolds number is the influential parameter of flame location and not turbulence intensity and length scale. An increase in the Taylor-scale Reynolds number leads the flame location to move towards the combustor inlet. The flame drops to the inlet of the combustor at a high-turbulence Taylor-scale Reynolds number. Flow separation is observed in the diffuser without a conical insert, and flow separation is eliminated by using the conical insert. Finally, the control of the flame location in the diffuser combustor is studied under various turbulent flow characteristics. A control strategy is suggested for this purpose. Control algorithm written as a Java macro is implemented to a commercial CFD software, namely STAR CCM+. This framework is utilized to perform all the simulations for the premixed turbulent flame under unsteady-state controlled conditions. The algorithm is built to adjust the turbulent kinetic energy and turbulent dissipation rate. Feedback control is introduced to stabilize the flame location at the desired level. Results indicate that control of the turbulent kinetic energy at the inlet control the flame location within the targeted level. In addition, it is observed the flame location moved to a low level for high turbulent kinetic energy whilst it moved to the high level for low turbulent kinetic energy.Master ThesisPublication Metadata only Durability testing and vibration characterization of an electric vehicle batteryDüzgün, Halil Zınar; Şendur, Polat; Şendur, Polat; Ünal, Ramazan; Çınar, A.; Department of Mechanical Engineering; Düzgün, Halil ZınarAlthough the electrical system is important in the development of electric vehicles, the durability of the battery against mechanical loads and the performance of the battery against shock reactions are just as important. The objective of this study is to investigate the structural life of the battery of the C-platform sports utility vehicle (SUV). For this purpose, a power spectral density (PSD) durability test profile is generated and compared to battery test standards such as ISO 6469:2019, AK-LH 5.21 and SAE J2380. Analytical Virtual Proving Ground (VPG), a multibody dynamics simulation model, is also developed and correlated to test data. The results show that fatigue damage spectrum (FDS) values for AK-LH is higher than the fatigue damage of the collected vehicle data, while the FDS results for ISO standard are lower compared to the vehicle data. The results also indicate that the loads in the longitudinal (x-direction) and lateral (y-direction) directions are different, and therefore loads with different amplitudes should be used for these directions. Finally, it is concluded that the VPG model can be used for determining the fatigue life when there is no test data, thanks to its high accuracy.Master ThesisPublication Metadata only Investigation of pair distribution function method on structural analysis of x-ray diffraction data from nanocrystalline powdersBaloochiyan, Abolfazl; Kaymaksüt, Hande Öztürk; Kaymaksüt, Hande Öztürk; Fındıkçı, İlknur Eruçar; Şenses, E.; Department of Mechanical Engineering; Baloochiyan, AbolfazlPair Distribution Function analysis is a local characterization method gaining momentum recently. Conventional X-ray diffraction and Rietveld methods have proven not to be accurate in the case of analysis of diffraction data from nanocrystalline powders. In this thesis, we evaluate the accuracy of this alternative technique, which is based on a refinement algorithm in real space, and because of being in real space, the properties obtained from this method are more intuitive. Structural parameters like particle size, neighbor atomic distances, etc. are obtained by directly calculating the distances and area of each peak or distance. Large surface to volume atom ratio is stated to be the reason that makes it hard for conventional X-ray diffractive methods to characterize nanocrystals. Because conventional methods have a fundamental assumption about the materials and that is large crystalline domains in the scattering material. Unfortunately, nanocrystalline powders do not satisfy this assumption. Surface atoms, because of lack of neighbor atoms around them, have a small coordination number, as a result, they do not fulfill crystallinity. However, Pair Distribution Function can obtain the structure of the materials, regardless of their crystallinity. It is claimed that amorphous materials can also be analyzed in this method. The probability of finding two atoms at a special distance from each other, is the basic point of view. This method is being used in different areas of study including drug delivery, materials characterization, fracture mechanics, condensed matter physics, etc. and recently it has been gaining momentum, especially where the conventional diffractive characterization methods cannot have an exact answer for structural properties. Although being a fairly developed algorithm, Pair Distribution Method still needs work to become perfect. For instance, the strain of the particles still cannot be calculated directly in this method, and Diffpy-Complex Modeling Infrastructure is intricate for the ones who are not familiar with object-oriented programming. In this research, we are looking at different approaches towards obtaining the necessary parameters from Pair Distribution Function fitting, goodness of the fits, and validity of the results. The systematic analysis of our work consists of five different nanocrystalline sizes and five different wavelengths, for both ideal nanocrystalline materials and energy minimized nanocrystals in 0 K temperature. The results of Pair Distribution Function show as the nanocrystalline size decreases the amount of crystallinity for the energy-minimized nanocrystals decrease. As the nanocrystals decrease in their size, the obtained values of lattice parameter decrease in comparison with the nominal lattice parameters. Atomic Displacement Parameters for the energy-minimized nanocrystals show a higher value for the smallest nanocrystalline sizes. Also, the predicted nanocrystalline size from this method gives slightly smaller size values which are attributed to the prediction of the peak attenuation. The lowest wavelength X-ray has the highest energy. For the highest energy wavelength, the Pair Distribution Function fit results of the particle size shows the size of the minimized particles is slightly less than the size of the ideal counterparts.Master ThesisPublication Metadata only Real-time robust control of a series elastic actuator subject to uncertainties(2018-12-24) Kansızoğlu, Ahmet Talha; Uğurlu, Regaip Barkan; Uğurlu, Regaip Barkan; Bebek, Özkan; Tümerdem, U.; Department of Mechanical Engineering; Kansızoğlu, Ahmet TalhaHumans are able to perform highly complicated tasks in their daily activities by interacting with the environment. However, for a robotic system, these tasks are quite challenging. To overcome these problems, robotic systems benefit from torque control approaches for enhanced environmental interaction capabilities. Furthermore, having a so-called ideal torque source at the joints may provide human-like movement functionalities for such systems. To that end, series elastic actuators (SEA) are highly preferred as a torque generator due to its numerous advantages such as low output impedance, high output torque bandwidth, and safety. A SEA consists of an electric motor, a reduction gear, and an elastic element. Having an elastic element between the motor and the output of the actuator makes the control problem of SEA highly complicated. Apart from modeling errors and non-linear disturbances, the environment is also unknown beforehand. An ideal actuator has to be robust against such uncertainties, and therefore, high fidelity control problem of SEAs is still an active research area. Hence, this thesis presents a comprehensive comparison study on various off-the-shelf advanced control methods. The study is supported via real-life experiments using the SEA unit that was designed in the Biomechatronics Laboratory of Ozyegin University.Master ThesisPublication Metadata only A comparative study on the severe plastic deformation of metals by constrained groove pressing(2018-06) Güler, Zeynel; Yapıcı, Güney Güven; Yapıcı, Güney Güven; Şendur, Polat; İpekoğlu, M.; Department of Mechanical Engineering; Güler, ZeynelThe main purpose of the present thesis is to reveal the microstructural refinement and mechanical behavior improvement of metallic materials, some of which are difficult to work, through severe plastic deformation. Constrained Groove Pressing is implemented in this study as a Severe Plastic Deformation method. Five different routes of Constrained Groove Pressing are planned to analyze the route effect on the materials. In this research, three different materials, namely AISI 304 stainless steel, Ti-15V-3Cr-3Sn-3Al and commercially pure zinc, are studied at room temperature. After microstructure and mechanical behavior of the samples are examined through optical microscopy, tension and erichsen tests and microhardness measurements, the underlying mechanisms are analyzed. CGP process of AISI 304 and Ti-15V-3Cr-3Sn-3Al results in a slight grain refinement while a comparatively high refinement has been observed for commercially pure zinc. Also, CGP routes have been significantly effective for only pure zinc samples in dictating grain sizes. After deformation, mechanical twinning activation has taken place for stainless steel and pure zinc samples. Twins recorded in AISI 304 with FCC lattice structure are thinner and sharper than that of commercially pure zinc with HCP lattice structure. Strain path is not considerably effective for AISI 304 samples. For pure zinc samples, twins are thinner for the routes A, C and D, compared to the other routes after the final pass. They are also rather thick and few in number after the first pass. Moreover, intensity of twins is relatively low for route E. It has been observed that CGP causes comparatively high strength increase in AISI 304 which has an FCC crystal structure, but lower increase in both Ti-15V-3Cr-3Sn-3Al and commercially pure zinc having BCC and HCP crystal structures, respectively. For AISI 304, the best strength improvement is nearly 41% occuring in TD for route C. For Ti alloy, the highest strength increase is about 16%, occurring in LD for routes D and E. For pure zinc, the best strength improvement is nearly 28% in TD belonging to route D. This study mainly aims to demonstrate the effect of several novel CGP routes on the metals having different crystal structures by analyzing the relationship between resulting microstructures and mechanical properties.Master ThesisPublication Metadata only Investigation of thermal comfort performance of radiant heating systems: comparisons of different heating surface configurations(2018-02) Acet, Ruşen Can; Mengüç, Mustafa Pınar; Mengüç, Mustafa Pınar; Başol, Altuğ; Atayılmaz, Ş. Ö.; Department of Mechanical Engineering; Acet, Ruşen CanThis thesis is an experimental-numerical study for the thermal comfort assessment of radiant heating system for different heating configuration such as from a wall, ceiling and combination of both that is installed in a test room with dimensions of 4m x 4m x 3m. Comfort evaluation was done by using the PMV (The Predicted Mean Vote) - PPD (Predicted Percentage of Dissatisfaction) index developed by Fanger [1]. In addition, for each heating scenario, human body exergy balance was calculated and the effect of exergy consumption rate on thermal comfort was evaluated. The data generated during the tests are used in numerical model for the validation of it. Numerical model is used to investigate the air temperature distribution, velocity fields for different cases. Three different heating configurations were evaluated in numerical model as same as experimental study. Wall heating, ceiling heating wall and ceiling heating scenarios were explored in terms of PMV thermal comfort index and human body exergy balance approach. All the numerical analysis studies were conducted using the Academic version of ANSYS 17.1, which is a commercial package program for numerical modelling. It contains special modules for different stages of the modelling process. After the three-dimensional room geometry was created in the Design Modeler module, the meshing module was subjected to decomposition using the finite volume method. Numerical solutions were made in Fluent, a widely used computational fluid dynamics module. The temperature and velocity fields were visually inspected using CFD-Post software as the final processor program. The natural convection was modelled using the Boussinesq approach, and the standard k-ε model which is a common numerical solution was picked to model turbulence. A Discrete Ordinates model with no scattering was used for radiative heat transfer. Numerical solution results were compared with different mesh numbers and mesh independence was observed. Radiant panels have been investigated to provide and maintain thermal comfort at different surface set temperatures. In the given set values, temperature distribution in the vertical and horizontal direction, mean radiant temperature and air velocity values in the room were examined. It has been observed that the exergy consumption values in the radiant heating system are close to the lowest values stated in the literature. Also, the temperature distribution in the room is considerably lower than all conventional systems. This demonstrates that radiant systems using low quality energy sources provide efficient, environmentally-friendly comfort solutions. It should be stated that it is a preliminary study for the location-based heating technologies and this method can be an innovative solution for heating / cooling industry. Therefore, it can be further evaluated in future research studies.