Organizational Unit: Department of Mechanical Engineering
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Master ThesisPublication Metadata only Determining thermal comfort in built environment using computational fluid dynamics simulations(2017-06) Fidan, Güven; Mengüç, Mustafa Pınar; Mengüç, Mustafa Pınar; Ertunç, Özgür; Kapkın, Ş.; Department of Mechanical Engineering; Fidan, GüvenIn this thesis, three-dimensional time-dependent Thermal Comfort (TC) conditions in an occupied room is investigated and determined. The room is considered with furniture of varying materials (i.e. steel, wood, etc.) and under incident solar radiation. A Computational Fluid Dynamics (CFD) method, FloEFD software was used to enable designers to perform accurate and fast analysis. This computational tool allows detailed 3D visualizations and inclusion of geometrical details of the diffusers of the HVAC system. To determine the required TC conditions such as the I) air distribution, II) indoor temperature profiles, III) humidity, and IV) the mean radiant temperature profiles, the CFD method is used. This work is significantly different from the earlier works as it includes the spectral surface emissivity of the existing materials in the room and the spectral transmissivity of windows' glasses. In the analyses, both conduction and spectral radiation, which have led to calculate more accurate and detailed TC conditions, are considered. The mentioned analyses reach agreement with the experimental data obtained in the room. The visualization of four parameters is done for different scenarios by evaluation of the change of the window properties inside the room and the HVAC diffuser. Subsequently, these values were interpreted as PMV and PPD for thermal comfort with Fanger method, depending on human clothing and metabolic rate. The completion of the comparison scenarios during the sunshine time of 8 hours allowed considering the warm-up period of the materials in the room. Therefore, it has come to the conclusion that the long-wave radiation of heated materials can affect TC, not just direct radiation of sun. With the new diffuser scenario, the occupant is locally, positively influenced by the thermal comfort due to blowing the incoming air directly into the seating area. However, with this secondary diffuser, it has been found that the thermal comfort decreases more rapidly in the comfort zone during the 8 hours. The reason for this is that the primary diffuser in which the comparison is made delayed the heating of the material by blowing directly into the glass and the wall. The findings of this research can be used to evaluate TC in big glass façade cladding spaces.Master ThesisPublication Metadata only Application of interlayer friction stir spot welding on 304 stainless steel and aluminum alloyRaza, Muhammad Farhan; Yapıcı, Güney Güven; Yapıcı, Güney Güven; Başol, Altuğ Melik; İpekoğlu, M.; Department of Mechanical EngineeringThe Friction Stir Spot Welding (FSSW) is the modification of solid-state welding known as Friction Stir Welding (FSW). During the recent years, FSSW have become the most commonly used welding technique in the automotive, aerospace, and various manufacturing industries. The FSSW process is most suitable welding technique for joining such metals and alloys which cannot be welded using conventional fusion welding techniques. This welding technique can easily be employed to join dissimilar light weight alloys to reduce the weight of the components to manufacture energy efficient vehicles. Several researchers had applied this technique on various similar and dissimilar metal alloys to elaborate the dominance of this technique over other welding techniques and also to investigate the effects of processing parameters on the joint quality. Despite the superiority of this technique, keyhole that appears on the weld spot as a result of pin of the tool is a serious issue and several researchers focused on resolving this problem which results in the development of a novel technique called Interlayer Friction Stir Spot welding (IL-FSSW) which is developed in the Manufacturing Technologies Laboratory within MEMFIS at Ozyegin University. This study comprises of employment of IL-FSSW process in joining similar and dissimilar stainless steel and aluminium alloys. The weld joint fabricated by utilizing IL-FSSW technique has a magnificent surface appearance without any bulk deformation from the weld spot. The keyhole free weld joint has successfully been fabricated by utilizing this technique and this study elaborates the zero effect of the welding parameters on the surface appearance of the weld spot. The utilization of flat tip tool and intermediate layer helps to fabricate flat weld joint without keyhole in comparison of conventional FSSW techniques. The utilization of intermediate layer (IL) played an important role in improving the lap shear force (LSF) which is 1.5 times greater than the required limit of American Welding Society (AWS) standards and some of the welded samples showed 2 times higher LSF than the minimum required limit. To investigate the effect of operating parameters on the microstructures and mechanical behaviour of the weld joint Design of Experiments (DOE) based on Taguchi model L8 and L9 is developed for AISI304 stainless steel similar joint and Al6061/AISI304 dissimilar joint respectively, by using three parameters as variable by keeping other parameters constant and these three parameters as tool rotation speed, plunge depth and IL diameter. The analysis of variance (ANOVA) statistical model on the basis of lap shear force values elaborated that tool rotation speed has greatest influence on the joint strength. The mechanical behaviour of the weld joint was examined by utilizing tensile testing and Vicker's Hardness tests. Optical Microscopic analysis and micro-hardness tests performed on the nugget zone of AISI304 similar joint and Al6061/AISI304 dissimilar joint helped to discriminate the stir zone (SZ), thermomechanical affected zone (TMAZ), heat affected zone (HAZ) and base metal (BM) as micro-hardness values are different for each zone and OM differentiate these zones on the basis of average grain size (AGS). Scanning electron Microscopy (SEM) of the fracture surface for Al6061/AISI304 weld joint shows intergranular and transgranular fracture behaviour. The lap shear tensile tests showed that tool rotation speed and plunge depth have direct relation with lap shear force as with increasing tool rotation speed and plunge depth LSF is also increasing. The linear relation of plunge depth with LSF is up to certain limit because after that the thickness of spot weld starts to decrease which decrease the LSF as well. The OM analysis indicated the hook formation of intermediate layer with upper sheet and this hook formation plays important role in improving the strength of the weld joint.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 Development of a torque-controllable upper limb exoskeleton for industrial applicationsCoruk, Sinan; Uğurlu, Regaip Barkan; Uğurlu, Regaip Barkan; Bebek, Özkan; Barkana, D. E.; Department of Mechanical Engineering; Coruk, SinanChronic upper body pain and injuries are undesirable physical disorders that result from regular exposure to external forces that strain the musculoskeletal system. These symptoms reduce both the quality of life and employability of the person. Although these disorders are mostly seen in people working in heavy industry and factory workers, they can also be seen in anyone who works physically. The use of upper limb exoskeletons has been seen as a practical solution to prevent the aforementioned ailments. These systems have two basic approaches to prevent chronic upper body pain, depending on the nature of the physical work done. The first of these is to prevent the factory worker from being forced ergonomically while working; the second is to reduce external forces on the musculoskeletal system by providing strength support to the factory worker who lifts heavy loads. In this context, companies operating in many industrial areas, have provided different types of upper limb exoskeletons for their employees to use. Although these developed exoskeletons achieve their intended purpose, they have not been highly accepted by their users. While there are many different and individual reasons for this unacceptance, the most cited reasons are potential users' safety concerns and other types of difficulties in the expense of the system's promised conveniences, such as reduced mobility and increased time to get the work done. The main purpose of this thesis is to develop an upper limb exoskeleton with a robust actuator structure and innovative human-robot interface, which is intended to have high user acceptance, while supporting factory workers against the external forces they are exposed to in their work environment. During the thesis, a 4 degrees of freedom left arm exoskeleton was developed; an innovative sensor structure with a high weight/data input ratio has been developed for the aforementioned exoskeleton, which can read and process the limb force response from 16 different points in real time and thus perform intention estimation. The aforementioned exoskeleton is equipped with the developed sensor structure and driven by enhanced Series Elastic Actuators (SEA) for high-fidelity torque control.Master ThesisPublication Metadata only Design and implementation of a robust control scheme for a smart joint(2017-01) Jabeen, Saher; Bebek, Özkan; Bebek, Özkan; Yapıcı, Güney Güven; Uğurlu, Regaip Barkan; Samur, E.; İpekoğlu, Mehmet; Department of Mechanical Engineering; Jabeen, SaherThe invention of biomedical tools has made medical treatments more convenient; however, manual insertion of these tools requires years of practice and erroneous insertion into the body may cause ruptures and bleeding. Employing medical tools with smart joints can improve the medical procedures making them less traumatic. In this work, a shape memory alloy (SMA) actuator based joint, also known as smart joint, is controlled using a discrete-time integral sliding mode (DISM) control to guide the motion of a smart joint. Two Nitinol based SMA actuators are used in an antagonistic arrangement to provide bending motion. The controller is designed on the base of a simplified physical model of a single SMA actuator which eliminates the necessity of obtaining an accurate model. A disturbance observer (DOB) is integrated to the controller to compensate the model uncertainties and external disturbances to the system. The bandwidth of SMA actuator is relatively low. Due to the high sampling time of the hardware that is used, a discrete-time controller was designed. An experimental setup is designed to test the proposed controller with position feedback. In experimental results, DISM controller with DOB is shown to be robust against system model uncertainties and external disturbances. Different frequency responses are compared and it is shown that the response of 0.04 Hz can be achieved with RMS tracking error of 0.0112 radians. Multiple joints connected with rigid links are successfully tracked using Electromagnetic Tracking system as the position sensor.PhD DissertationPublication Metadata only Experimental and numerical investigation of rubber whether strip extrusion(2019-01-03) Talib, Nayyef Ahmed; Ertunç, Özgür; Ertunç, Özgür; Bundur, Zeynep Başaran; Başol, Altuğ Melik; Kırkköprü, K.; Uluer, Onuralp; Department of Mechanical Engineering; Talib, Nayyef AhmedExtrusion is the main method used to produce rubber weather strips in automotive industries. The product quality is dependent on many factors, which include the design parameters of the die, the processing variables, and the rheological behaviour of the used material. On the other hand, the flow of the rubber compound is complex due to both the shear thinning behaviour and the high viscoelastic character of this material. Therefore, the inclusion of many factors, which have an influence on product quality during the extrusion process, cause the relationship between the die design and the flow field to be not-intuitive. The current conventional die design employs trial-error process during which the manufactured die is reworked in several trials to guarantee the required product quality. Therefore, converting the running-in experiment into a virtual one by adopting numerical modelling is a powerful method leading to a reduction in waste material and time. In addition, it can also be used for the in-depth analysis of the rheological material variables in combination with the die design parameters to evaluate their effect on product quality. In extrusion process of high reactive material such as rubber, it is important to find less reactive conditions. This because of the time course of heating is very important factor in controlling the peroxide’ degradation or scorching inside the die. Therefore, estimation of the residence time and temperature of the material for a specific die design play an important role for products quality control. In the first part of the investigations, a special extrusion die instrumented with a special sensor was designed in an industrial scale size. Extrusion experiments were conducted in an extrusion line and flow rate, temperature and pressures were measured for extruder speeds. The rheological properties of the filled rubber compound were characterized using a capillary rheometer (Rosand) at different temperatures to evaluate the required material parameters for the numerical simulation. The curing characteristics were investigated using a rubber process analyser (RPA-2000) to construct a curing curve at different temperatures. A three-dimensional model was established for modelling the non-isothermal viscous flow of the ethylene propylene diene monomer (EPDM) rubber melts. A purely inelastic model was assumed through a power law model and a mixed finite element method to solve the complex flow in the extrusion die domain. The pressure-stabilized Petrov–Galerkin (PSPG) method and streamline upwind/Petrov–Galerkin numerical scheme were employed to solve the flow equations and increase numerical stability. The results confirmed that for the EPDM rubber compound, the screw speed exerted a remarkable effect on the temperature rise and pressure drop in the extrusion die. The impact of the viscous dissipation on the thermal behaviour and pressure drop prediction was also discussed. The obtained scorch time was compared with the estimated residence time in the flow domain to elucidate the influence of the extruder speed on the curing characteristic. The results suggested that there is neither premature vulcanisation nor the start of the scorching inside the flow domain within the studied extruder speed range. The velocity uniformity index and streamline were evaluated at the die exit and the entire flow domain, respectively. The analyses of the results obtained confirmed the ability of the proposed die design to produce a defect free product without the risk of the circulation or appearance of a large distortion. The validity of the model prediction was verified by the comparison between the simulation and the experimental results. The second part of the investigations was devoted to studying the swelling phenomenon which occurred during the extrusion of the rubber. Extrudate swell is an important phenomenon occurring when high viscoelastic materials, such as rubber and rubber compounds, are extruded. In this work, the effects of the relaxation time and the relaxation mode on the swell predictions using a nonlinear differential viscoelastic model, that is, the Giesekus model, were studied systematically for the Styrene-Butadiene rubber (SBR) extrusion in the capillary die. The corresponding 3D, steady-state finite element simulation for the predictions of the swelling was presented and compared with the experimental data for the validation. The velocity distribution, pressure drop and circulation flow in the die were analysed and discussed through the simulation. The results of the swell prediction revealed that the three-relaxation mode of the Giesekus model with a wide range of relaxation time reproduced the experimental data. In addition, the number of relaxation mode and range of relaxation time had a remarkable effect on the circulation flow at the die corner and some effect on the other field variables. After the validation of the proposed model, the same model was implemented on the capillary extrusion of the EPDM rubber, which is mainly used in the weather strip in automotive industries. The swelling of the EPDM rubber was compared with the swelling of the SBR. The results obtained showed that the swell ratio of the EPDM was less than the swell ratio of the SBR for all the studied parameters. This finding approved the ability of the invoke purely inelastic model in modelling this kind of material when a viscous effect is predominant. The influence of the die design parameter such as the die length and slippage at die wall on the swell was also discussed and analysed. Finally, viscoelastic simulation based Giesekus viscoelastic model was applied for modelling of extrudate swelling in an industrial scale extrusion die. The predicted extrudate profile was compared with basic profile at die exit. The results obtained show very slight deviation in extrudate profile from basic geometry at die exit which validate the proposed die design to produce the precise extrudate dimension without necessary to make die correction. The three dimensional stress field at die exit were discussed and analysed to evaluate their effect on the evolution of some defects such as melt fracture or surface defects.Master ThesisPublication Metadata only A numerical investigation of thermal comfort with different air diffusers at enclosed office spaceEraslan, Tolga Arda; Mengüç, Mustafa Pınar; Mengüç, Mustafa Pınar; Ertunç, Özgür; Ertürk, H.; Department of Mechanical EngineeringPeople spend most of their time in indoor environments, which highlights the significance of heating, ventilation, and air conditioning (HVAC) systems in these spaces. While ensuring the comfort of occupants in indoor spaces, it is also essential to maintain the efficient operation of these systems. Studies related to this topic are ongoing in the present day. HVAC should not only be seen as individual devices within it, but rather as a whole system. By doing so, the efficiency of each component can be enhanced. HVAC systems consist of various components, including heating and cooling units, ventilation units, dampers, air terminals, louvers, ducts, and pipes, among others. One of the most crucial elements that interact with the indoor environment is the air terminal. In this thesis, the interaction of air terminals with occupants in indoor spaces was observed and visualized using Computational Fluid Dynamics (CFD). Three types of diffusers were used in the study: square diffuser, four-way swirl diffuser, and a hexagonal diffuser designed by Cem Keskin. The interaction of these diffusers with the air inside the room, their airflow characteristics, and their impact on thermal comfort were investigated. The Fanger method's six variables were discussed in this context. Based on the findings, it was observed that the airflow characteristics of the hexagonal diffuser varied based on the openings, while the four-way swirl diffuser was found unsuitable for providing thermal comfort in an office environment. Additionally, when aiming for even air distribution, there is no energy difference among the diffusers. However, if only a specific area require conditioning, it's noticeable that the hexagonal diffuser consumes less energy compared to the other types of diffusers.Master ThesisPublication Metadata only Manufacturing and mechanical behavior of lightweight composites by accumulative roll bonding(2021-08-18) Tahir, Furqan; Yapıcı, Güney Güven; Yapıcı, Güney Güven; Başol, Altuğ Melik; İpekoğlu, M.; Department of Mechanical Engineering; Tahir, FurqanSevere plastic deformation (SPD) methods have received significant attention in fabricating a combination of similar and dissimilar metal composites. Accumulative roll bonding (ARB) is one of the SPD techniques employed to produce high strength, fine-grained, and multi-layered metal matrix composites (MMCs). The current work is characterized into two parts. The major part deals with the fabrication of different combinations between titanium and interstitial free (IF) steel interlayered with aluminum alloy (Al2024). While the second part studies the effect of post-ARB heat treatment on Al2024-IF steel composite. Microstructural evaluation and mechanical characteristics of processed composites were examined by optical microscope (OM), scanning electron microscope (SEM), and mechanical tests. Surface and bulk properties were analyzed through microhardness, uniaxial tensile, and shear punch test (SPT). Finally, experiment results were validated using numerical analysis. For the first case of titanium-IF steel composites, the optical micrographs indicated the reduction in layer thickness and grain size after each rolling cycle. Necking was initiated in some layers of the processed composite after the third (final) ARB cycle, suggesting the interlocking of layers and consequently stronger bonding. SEM figures from fracture surfaces revealed the delamination of layers from interfaces in early cycles, that lessened in the final cycle due to high strain exerted after each rolling cycle. Significant improvement was noticed in hardness levels as the number of ARB cycles increased. Enhancement levels for titanium and IF steel layers were up to 1.4 and 2.6 times, respectively after the ARB process. The tensile strength of the titanium-IF steel composite rolled at 400°C exceeded over 670MPa after three cycles. Typical to SPD processes, the ductility was reduced to less than 5% in all cases after three ARB cycles. Enhancement levels in tensile strength of each layer of the final composite compared to the initial metals were similar to hardness. SPT results showed the improvement in shear strength of the sample with the increasing number of cycles, a maximum shear strength of around 440MPa was achieved after three cycles. For the second part of aluminum-IF steel composite, microstructure observation revealed that long aging imparted partial coarsening of the average grain size in a three-cycle ARBed sample. Furthermore, the ductility was recovered after aging without losing tensile strength due to the occurrence of precipitation hardening along with recovery in individual layers. A maximum ultimate tensile strength (UTS) and ductility of around 420MPa and 18%, respectively, was obtained after aging at 150°C for 24h. SPT results showed a similar response as the tensile test, and maximum shear strength was reached up to 270MPa at the peak-aged condition. The ARB process, in the case of titanium-IF steel composite, was modeled by employing finite element analysis through ANSYS workbench. The effective plastic strain and effective stress (von-Mises) were calculated through the thickness of ARBed samples. A close agreement between experimental findings and numerical simulations was observed.Master ThesisPublication Metadata only Effect of garments on thermophysiological comfort(2019-11-24) Tatar, Ebru; Mengüç, Mustafa Pınar; Mengüç, Mustafa Pınar; Başol, Altuğ Melik; Kayakutlu, G.; Department of Mechanical Engineering; Tatar, EbruThis study investigates the effect of garment on human thermophysiological comfort. In this thesis simplified but effective model of heat transfer from human skin to the environment through clothing is proposed. A comprehensive and detailed literature search is provided. Thermal comfort models in literature are examined and based on the Fanger’s comfort model, the simplified thermal comfort model is developed. The objective is improving an algorithm which is easy to calculate numerically yet gives best possible results by matching well with the experimental data in literature. The objective is developing a thermal comfort algorithm that can be used in IOT applications such as smart phones and smart buildings, so that a user can control his/her thermal comfort state by changing his/her clothing according to outputs of the algorithm. Therefore, the algorithm has to give fast but good results that do not require any use of measurement instruments. In buildings, the use of PMV control showed 7.3 % less energy consumption than the dry-bulb air temperature control and showed 28.8 % less energy consumption for the annual cooling electricity consumption (Hong, 2018). Therefore, this study can be used for such building control applications. Fanger’s Predicted Mean Vote model is adopted to scale comfort values for this application. Different case studies that are focusing on clothing thermal resistance, evaporative resistance and PMV, are investigated. Codes are simulated in MATLAB. The main output of the algorithm in this study is suggestions on how a person should change his/her clothing to feel comfortable at any time in any environment. This algorithm is used for different strategies.