Mechanical Engineering / Makina Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/4129
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Article Citation - WoS: 1Citation - Scopus: 2Emergence of Taperedducts in Vascular Designs With Laminar and Turbulent Flows(Begell House, 2014) Çetkin, ErdalHere we show that tapered ducts emerge in volumetrically bathed porous materials to decrease the resistance to the flow in laminar and turbulent flow regimes. The fluid enters the volume from one point and it is distributed to the entire volume. After bathing the volume, it is collected and leaves the volume from another point, i.e., two trees matched canopy to canopy. This paper shows that the flow architecture (i.e., design of the void spaces in a porous material) should be changed to obtain the minimum resistance to the flow as its size increases. Tapering the ducts decreases the order of the transition size, i.e., the size for changing from one construct to another to obtain the minimum pressure drop. The decrease in the pressure drop is 16% and 38% with the tapered ducts when the flow is laminar and turbulent, respectively. In addition, the volume ratios and the shape of the tapered ducts are documented. There is no design existing in nature with diameters of constant size in order to distribute and/or collect heat, fluid, and/or stress such as bones, rivers, veins, and tree branches. The emergence of the tapered ducts in designed porous materials is natural.Article Citation - WoS: 12Citation - Scopus: 11Inverted Fins for Cooling of a Non-Uniformly Heated Domain(Yıldız Teknik Üniversitesi, 2015) Çetkin, ErdalThis paper shows that the peak temperature of a non-uniformly heated region can be decreased by embedding high-conductivity tree-shaped inserts which is in contact with a heat sink from its stem. The volume fraction of the high-conductivity material is fixed, and so is the volume of the solid region. The length scale of the solid domain is L. Inside there is a cube-shaped region with length scale of 0.1L and heat production 100 times greater than the rest of the domain. The location of this hot spot was varied to uncover how its location affects the peak temperature and the design of inverted fins, i.e. highconductivity tree-shaped inserts. The volume fraction of the high-conductivity tree was varied for number of bifurcation levels of 0, 1 and 2. This showed that increasing the number of the bifurcation levels decreases the peak temperature when the volume fraction decreases. The optimal diameter ratios and optimal bifurcation angles at the each junction level are also documented. Y-shaped trees promise smaller peak temperatures than T-shaped trees. The location of the vascular tree in the z direction also affects the peak temperature when the heat generation is non-uniform. In addition, the peak temperature is minimum when z = 0.65L even though the hot spot is located on z = 0.75L.Article The Effect of Time Delay of Fluid Flow in a Vascularized Plate(MIM Research Group, 2018) Yenigün, Onur; Coşkun, Turgay; Çetkin, ErdalIn this study, we show the effect of time delay of coolant fluid flow into a vascularized plate on the peak temperature. Coolant flows along vascular channels which were embedded in a rectangular plate. Two kinds of vascular channel designs were investigated experimentally: parallel and tree-shaped. In the study, the peak temperatures were monitored and the coolant was pumped when the peak temperature reaches to 50°C, 70°C and 90°C. The performance comparison of two distinct designs is based on two criteria: the time required for the steady state condition after the coolant is pumped and the peak temperature after the steady state condition is conformed. The results show that the time required to reach steady-state condition increases as the time delay increases. The parallel and tree-shaped designs show similar performance (time required to reach steady state) with slightly improved performance in the tree-shaped design as the preset temperature for time delay increases. For instance, 4% decrease in the time required to reach steady-state with the tree-shaped design relative to the parallel design was achieved when the preset temperature for time delay is 90°C.Article Optimum Design of Anti-Buckling Behavior of Graphite/Epoxy Laminated Composites by Differential Evolution and Simulated Annealing Method(MIM Research Group, 2019) Akçaır, Mehmet; Savran, Melih; Aydın, Levent; Ayakdaş, Ozan; Öztürk, Savaş; Küçükdoğan, NilayStacking sequence design and optimization of 64 layered symmetric-balance graphite/epoxy laminated composite have been performed. The optimization problems aim to find the optimum stacking sequence maximizing the critical buckling load by single objective optimization approach. Differential Evolution (DE) and Simulated Annealing (SA) optimization algorithms are proposed to solve the problems. The effect of the aspect ratios (a/b) and in-plane biaxial compressive loading ratios (Nx/Ny) on critical buckling load are investigated. In order to see the effect of discrete increments of fiber orientation angle on critical buckling load, 1°, 5°, 15°, 30° and 45° fiber angle increments are also considered. The results show that (i) the proposed algorithms DE and SA exhibit comparable performance in terms of critical buckling load when compared Genetic algorithm (GA) and Generalized pattern search algorithm (GPSA), (ii) DE and SA find distinct stacking sequence configurations in terms of buckling load for the same laminated structure design problems.Article Citation - Scopus: 2Effects of Earthquake Motion on Mechanism Operation: an Experimental Approach(Levrotto and Bella, 2015) Selvi, Özgün; Ceccarelli, M.; Aytar, Erman BarışThis paper presents an experimental characterization of the effects of earthquakes on the operation of mechanical systems with the help of CaPaMan (Cassino Parallel Manipulator), which is a 3 DOF robot that can fairly well simulate 3D earthquake motion. The sensitivity of operation characteristics of machinery to earthquake disturbance is identified and characterized through experimental tests. Experimental tests have been carried out by using a slider-crank linkage, a small car model, and LARM Hand as test-bed mechanisms that have been sensored with proper acceleration or force sensors. Results are reported and discussed to describe the effects of earthquake motion on the characteristics of mechanism operation as a service application of the robotic CaPaMan system.Article Optimal Design of the Type Iii Hydrogen Storage Tank for Different Carbon/Epoxy Materials by Modified Differential Evolution Method(MIM Research Group, 2019) Ayakdaş, Ozan; Aydın, Levent; Savran, Melih; Küçükdoğan, Nilay; Öztürk, SavaşIn this study, the main objective is to minimize the failure index of a cylindrical laminated composite hydrogen storage tank under internal pressure. The first step is to obtain the distribution of stress components based on Classical Laminated Plate Theory (CLPT). The second is to evaluate the burst pressure of the tank according to three different first ply failure criteria and then to compare the results with the experimental and numerical ones from literature. In the final part of the study, the best possible combination of winding angles, stacking sequences and thicknesses of laminates satisfying minimum possible stress concentration will be obtained for different Carbon/Epoxy materials by Differential Evolution Method. The stress components and, the burst pressures reached according to Hashin-Rotem, Maximum Stress, and Tsai-Wu first-ply failure criteria, have been complied with experimental and numerical results in the literature for Type III pressure vessels. Manufacturable Type-III tank designs have been proposed satisfying the 35 MPa burst pressure for different Carbon/Epoxy materials.Article Citation - WoS: 8Citation - Scopus: 8Pore Connectivity Effects on the Internal Surface Electric Charge of Mesoporous Silica(Springer, 2019) Şen, Tümcan; Barışık, MuratNano-scale confinements within mesoporous systems develop overlapping electric double layers (EDL) such that the existing theoretical models cannot predict the electric potential distributions and resulting surface charges. In addition, ionic conditions undergo local variation through connections between the pore voids and pore throats. For the first time in literature, we studied the charging behavior of mesoporous silica in terms of the pore to throat size ratio (R-pt) to characterize the pore connectivity effects, in addition to porosity (epsilon) and pore size (H). Both local and average surface charge densities inside mesoporous silica were examined by varying these parameters systematically. Results showed that the magnitude of surface charge density decreased with increasing EDL overlap and decreasing connectivity effects. We formulized this behavior and developed an extended model to predict mesoporous silica's internal charge as a function of porosity, pore size, and pore to throat size ratio.Article Citation - WoS: 14Citation - Scopus: 15Electrochemical Performance of La0.6sr0.4co0.2fe0.8o3-Ce0.9gd0.1o2 Composite Sofc Cathodes Fabricated by Electrocatalyst And/Or Electrocatalyst-Ionic Conductor Infiltration(Springer, 2019) Sındıraç, Can; Büyükaksoy, Aligül; Akkurt, SedatInfiltration of electrocatalyst precursor solutions into previously sintered porous ionic conductor scaffolds has been used recently as an alternative method to the conventional co-sintering route to fabricate electrocatalyst-ionic conductor composites for solid oxide fuel cell (SOFC) cathode applications. However, the aqueous nitrate solutions generally used to perform the infiltration process results in electrocatalyst precipitates that are disconnected from each other, yielding poor electrode performance. In this work, polymeric electrocatalyst (La0.6Sr0.4Co0.2Fe0.8O3-LSCF) precursors that produce interconnected thin films upon heat treatment were used to infiltrate porous ionic conductor Ce0.9Gd0.1O2-delta (GDC) scaffolds to overcome these issues. In addition, for the first time in the literature, a mixture of LSCF and GDC polymeric precursors, which would yield LSCF-GDC nanocomposite coatings on the grains of the porous GDC scaffold were used as the infiltration solution. Thus, further enhancement of the electrocatalyst/ionic conductor interfacial area and achievement of improved electrode performance was aimed. As a result of the optimization studies, the lowest measured area specific polarization resistance (ASR(cathode)) values of 0.47 and 0.73 omega.cm(2) were obtained for polymeric LSCF+GDC and LSCF precursor infiltrations respectively at 700 degrees C, in air. In addition, LSCF+GDC infiltration yielded electrodes with much improved long-term stability in comparison to those obtained by LSCF infiltration. [GRAPHICS] .Article Citation - WoS: 19Citation - Scopus: 20A Critical Review on Classification and Terminology of Scissor Structures(Int. Association for Shell and Spatial Structures, 2019) Maden, Feray; Akgün, Yenal; Kiper, Gökhan; Gür, Şebnem; Yar, Müjde; Korkmaz, KorayWhen the existing literature on the research of scissor structures is thoroughly investigated, it is seen that different researchers use different terminologies and classifications especially for the definition of the primary units and the motion type. Some of the studies define the whole geometry based on the geometric properties of the primary scissor units and the unit lines while some other studies define it according to the loops. All these studies use different names for similar elements. This article aims to review the literature on the classification and terminology of scissor structures and represent the state of art on the studies. Tables are represented showing all approaches in the literature. In addition, the article criticizes the missing points of each terminology and definition, and proposes some new terminology. In order to arrive at this aim, different definitions of the primary scissor units and motion types used in key studies in the literature are investigated thoroughly. With several examples, it is demonstrated that naming the scissor units according to the resulting motion type might be misleading and it is better to specify the motion type for the whole structure. A classification for transformation of planar curves is presented.Article Citation - WoS: 12Citation - Scopus: 13Assessment of Thermal Comfort Preferences in Mediterranean Climate a University Office Building Case(Vinca Inst Nuclear Sci, 2018) Turhan, Cihan; Gökçen Akkurt, GüldenThis study aims at evaluating the perceived thermal sensation of occupants with respect to thermal comfort standards, ASHRAE 55 and ISO 7730, for office buildings located in Mediterranean climate. A small office building in Izmir Institute of Technology Campus Area, Izmir, Turkey, was chosen as a case building and equipped with measurement devices to assess thermal comfort of occupants with respect to predicted mean vote and actual mean vote. Both objective and subjective measurements were conducted. The former included indoor and outdoor air temperature, mean radiant temperature, relative humidity and air velocity that were used for evaluating the thermal comfort of occupants. Oxygen concentration which can play an additional role in thermal comfort/discomfort, health and productivity of the office occupants, was also measured. Furthermore, occupants were subjected to a survey via a mobile application to obtain subjective measurements to calculate actual mean vote values. Based on objective and subjective measurements, the relationships among the parameters were derived by using simple regression analysis technique while a new combined mean vote correlation was also derived but this time by using multiple linear regression model. Neutral and comfort temperatures were obtained using indoor air temperature and actual mean vote values which were calculated from subjective measurements. The results showed that neutral temperature in the university office building was 20.9 degrees C whilst the comfort temperature range was between 19.4 and 22.4 degrees C for the heating season. By applying new comfort temperatures, energy consumption of the case building located in Mediterranean climate, can be reduced.