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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/3
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Publication Metadata only Pressurized infusion: a new and improved liquid composite molding process(ASME, 2019) Yalçınkaya, M. Akif; Güloğlu, Görkem E.; Pishvar, Maya; Amirkhosravi, Mehrad; Altan, M. Cengiz; Department of Mechanical Engineering; Sözer, Murat; Faculty Member; Department of Mechanical Engineering; College of Engineering; 110357Vacuum-assisted resin transfer molding (VARTM) has several inherent shortcomings such as long mold filling times, low fiber volume fraction, and high void content in fabricated laminates. These problems in VARTM mainly arise from the limited compaction of the laminate and low resin pressure. Pressurized infusion (PI) molding introduced in this paper overcomes these disadvantages by (i) applying high compaction pressure on the laminate by an external pressure chamber placed on the mold and (ii) increasing the resin pressure by pressurizing the inlet resin reservoir. The effectiveness of PI molding was verified by fabricating composite laminates at various levels of chamber and inlet pressures and investigating the effect of these parameters on the fill time, fiber volume fraction, and void content. Furthermore, spatial distribution of voids was characterized by employing a unique method, which uses a flatbed scanner to capture the high-resolution planar scan of the fabricated laminates. The results revealed that PI molding reduced fill time by 45%, increased fiber volume fraction by 16%, reduced void content by 98%, improved short beam shear (SBS) strength by 14%, and yielded uniform spatial distribution of voids compared to those obtained by conventional VARTM.Publication Metadata only Morphological evolution of sol-electrophoretic deposited ZnO nanostructures on anodic TiO2 nanotubes for back-side illuminated dye-sensitised solar cells(Elsevier Ltd, 2023) Babaie-Aghdam, Samaneh; Nasirpouri, Farzad; Peighambardoust, Naeimeh Sadat; Researcher; Koç University AKKİM Boron-Based Materials & High-technology Chemicals Research & Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM)Efficient photoanodes are essential for dye-sensitized solar cells (DSSCs). We report on sol-electrophoretic deposition of ZnO/TiO2 heterojunction structures with enhanced photocurrent-photovoltage performance for back-side illumination DSSCs. An anodic oxidation method was used for the preparation of TiO2 nanotube arrays (TNAs). ZnO nanostructures with different morphologies were successfully deposited on TNAs via sol-electrophoretic process. The results revealed that TNAs decorated with ZnO favors the photovoltaic properties reaching the best photoanode performance in a pH and temperature of 8.5 and 30°C, respectively, under an electrophoretic deposition (EPD) potential of 10 V. This photoanode generates short circuit current (Jsc) and open circuit voltage (Voc) of 3.28 mA/cm2 and 0.79 V, respectively. Consequently, the power conversion efficiency (PCE) acquired 1.525% with a fill factor of 72% for the best ZnO/TiO2 heterojunction enhances the DSSC performance by 107% with respect to similar TNAs due to the surface passivation of electronic states eliminating the recombination rate. © 2022 Elsevier LtdPublication Metadata only Time-resolved OCT-mu PIV: a new microscopic PIV technique for noninvasive depth-resolved pulsatile flow profile acquisition (vol 54, 1426, 2013)(Springer, 2013) Chen, Chia-Yuan; Menon, Prahlad G.; Kowalski, William; Department of Mechanical Engineering; Pekkan, Kerem; Faculty Member; Department of Mechanical Engineering; College of Engineering; 161845N/APublication Metadata only A novel unibody axial flow pump for the lubrication of inverter type hermetic reciprocating compressors(Elsevier, 2022) N/A; N/A; N/A; Shahzad, Aamir; Pashak, Pouya; Lazoğlu, İsmail; PhD Student; PhD Student; Faculty Member; Manufacturing and Automation Research Center (MARC); N/A; N/A; N/A; N/A; N/A; 179391Lubrication at low speeds is a general problem in the hermetic reciprocating compressors for household refrigerators. The aim of this article is to present a new method using a novel 3D printed unibody axial flow pump for the lubrication oil supply system of an inverter type hermetic reciprocating compressor during a low speed (< 2000 rpm) operation. The system is based on a 3D printed unibody bladeless impeller axial flow pump attached to the bottom end of the vertical rotating crankshaft partially immersed in the oil sump inside hermetic reciprocating compressor sealed casing. A Computational Fluid Dynamics (CFD) simulation besides the experiment is used to simulate the flow inside the pump to calculate the mass flow rate of the lubrication oil and to optimize the helix angle of the pump. Volume of Fluid (VoF) and Multi Reference Frame (MRF) methods are used for modeling the two-phase flow (air and lubrication oil) and rotary domain respectively. The mass flow rate and climbing time of the lubrication oil in the unibody axial pump are analyzed. The results show that the designed pump is capable to supply the lubrication oil at a low speed i.e., 1400 rpm. Moreover, results indicate that the mass flow rate of the lubrication oil increases as the viscosity decreases. The average climbing time is observed to be 1 s at 1400 rpm and 0.4 s at 2000 rpm.Publication Metadata only Interval algebra to deal with pattern loading and structural uncertainties - Closure(Asce-Amer Soc Civil Engineers, 1997) Çakmak, Ahmet Ş.; Nielsen, Søren R.K.; Department of Mathematics; Köylüoğlu, Hasan Uğur; Teaching Faculty; Department of Mathematics; College of Sciences; N/AN/APublication Metadata only Tribological behavior of tialn, altin, and alcrn coatings at boundary lubricating condition(Springer/Plenum Publishers, 2018) Ozkan, Dogus; Erarslan, Yaman; Sulukan, Egemen; Kara, Levent; Yilmaz, M. Alper; Yagci, M. Baris; N/A; Yağcı, Mustafa Barış; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/AIn this study, similar to 3.5 mu m thick multilayer titanium alumina nitride (TiAlN), alumina titanium nitride (AlTiN), and alumina chromium nitride (AlCrN) coatings were deposited on the H13 steel surface by cathodic arc physical vapor deposition(CAPVD) method. The tribological performance of the coatings was evaluated by a tribometer at boundary lubrication condition. Then, coating surfaces were observed by optical microscope, optical profilometer, and atomic force microscope to evaluate the morphological changes, wear volumes, and tribofilm thickness. Also, scanning electron microscopy (energy dispersive X-ray) and X-ray photoelectron spectrometry analyses were applied to coating surfaces for the tribochemical evolution of the tribofilm. Results showed that AlCrN coating performed the best tribological behavior at boundary lubricated condition, when compared to TiAlN and AlTiN coatings and it can be used as a wear resistant cam tappet coating in internal combustion engines.Publication Metadata only Prediction of properties of friction stir spot welded joints of AA7075-T651/Ti-6Al-4V alloy using machine learning algorithms(Springernature, 2022) Asmael, Mohammed; Nasir, Tauqir; Zeeshan, Qasim; Safaei, Babak; Kalaf, Omer; Hussain, Ghulam; N/A; Motallebzadeh, Amir; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/AIn the present study, experimental works on friction stir spot welding (FSSW) of dissimilar AA 7075-T651/ Ti-6Al-4V alloys under various process conditions to weld joints have been reviews and multiple machine learning algorithms have been applied to forecast tensile shear strength. The influences of welding parameters such as dwell period and revolving speed on the mechanical and microstructural characteristics of weld joints were examined. Microstructural analyses were conducted using optical and scanning electron microscopy (SEM-EDS). The maximum tensile shear strength of 3457.2 N was achieved at the revolving speed of 1000 rpm and dwell period of 10 s. Dwell period has significant impact on the tensile shear strength of weld joints. A sharp decline (74.70%) in tensile shear strength was observed at longer dwell periods and high revolving speeds. In addition, a considerable improvement of 53.38% was observed in tensile shear strength at low dwell periods and high revolving speeds. Most significant machine learning data-driven methods used in welding such as, artificial neural network (ANN), adaptive neuro-fuzzy inference system (ANFIS), support vector machine (SVM) and regression model were used to forecast the tensile shear strength of welded joints at selected welding parameters. The performance of each model was examined in training and validation stages and compared with experimental data. To evaluate the performance of the developed models, the two quantitative standard statistical measures of prediction error % and root mean squared error (RMSE) were applied. The performance of regression, ANN, ANFIS and SVM were compared and SVM regression model was found to perform better than ANN and ANFIS in forecasting the tensile shear strength of FSSW joints.Publication Metadata only Model validation and performance prediction in the design of micro systems(Sage Publications Ltd, 2008) N/A; Department of Mechanical Engineering; Anaç, Ozan; Başdoğan, İpek; Master Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 179940Micro Electro Mechanical Systems are among the new and emerging technologies of the future and have many applications in different disciplines. Predicting the performance of such systems early in the design process can significantly impact the design cost and also improve the quality of the design. This study presents the model validation techniques integrated with some design methodologies to predict the performance of the micro systems. A two-dimensional micro scanner mirror was chosen as the case study to demonstrate the developed methodologies. The model validation methodology includes the verification of the finite element model using an experimental modal analysis setup for measuring the out-of-plane vibrations of the micro devices. The setup includes a laser doppler vibrometer, a microscope, a camera, a laser positioning system, and a data acquisition system to acquire the data. An experimental procedure was developed to collect the vibration data and then modal analysis was performed to determine the modal frequencies, mode shapes, and modal damping coefficients. The finite element analysis and experimental results were compared to identify the inaccuracies in the modeling assumptions. A validated finite element model was used to obtain the state space representation of the micro scanner mirror to proceed further with additional design studies. The state space model was used for disturbance analysis that was performed using Lyapunov approach to obtain root mean square values of the mirror rotation angle under the effect of a disturbance torque. The disturbance analysis framework was combined with the sensitivity analysis to determine the critical design parameters for improving the system performance. In addition to the disturbance sensitivity analysis, modal sensitivities of the design parameters were also investigated. This analysis was performed by perturbing the design parameters and investigating the change in the modal frequencies.Publication Metadata only Local and modal damage indicators for rc frames subject to earthquakes(Asce-Amer Soc Civil Engineers, 1998) Nielsen, SRK; Abbott, J; Cahmak, AS; Department of Mathematics; Köylüoğlu, Hasan Uğur; Teaching Faculty; Department of Mathematics; College of Sciences; N/ALocal, modal, and overall damage indicators for reinforced concrete shear frames subject to seismic excitation are defined and studied. Each story of the shear frame is represented by a Clough and Johnston hysteretic oscillator with a degrading elastic fraction of the restoring force. The local maximum softening damage indicators are defined in a closed form based on the variation of the eigenfrequency of the local oscillators due to the local stiffness and strength deterioration. The modal maximum softening damage indicators are calculated from the variation of the eigenfrequencies of the structure during excitation. The linear and nonlinear parameters of the local oscillators are assumed to be known. Next, a statistical analysis is performed where a sample five-story shear frame is subject to sinusoidal and simulated earthquake excitations. The shear frame is subject to 30 independent simulations of the earthquake excitation, which is modeled as a stationary Gaussian stochastic process with the Kanai-Tajimi spectrum, multiplied by an envelope function. Equations of motion of the storys are solved by a Runge-Kutta fourth-order scheme, where the local softening value is recorded. The modal maximum softening indicators are calculated from the known instantaneous stiffness matrix, which is a function of the structural properties and local damage. Alternatively, a Fourier analysis is performed for consecutive time windows to measure the same evolution using the top story displacement. Finally, the relationship between local and modal damage indices are investigated statistically.Publication Metadata only Time-resolved OCT-mu PIV: a new microscopic PIV technique for noninvasive depth-resolved pulsatile flow profile acquisition(Springer, 2013) Chen, Chia-Yuan; Menon, Prahlad G.; Kowalski, William; Department of Mechanical Engineering; Pekkan, Kerem; Faculty Member; Department of Mechanical Engineering; College of Engineering; 161845In vivo acquisition of endothelial wall shear stress requires instantaneous depth-resolved whole-field pulsatile flow profile measurements in microcirculation. High-accuracy, quantitative and non-invasive velocimetry techniques are essential for emerging real-time mechanogenomic investigations. To address these research needs, a novel biological flow quantification technique, OCT-mu PIV, was developed utilizing high-speed optical coherence tomography (OCT) integrated with microscopic Particle Image Velocimetry (mu PIV). This technique offers the unique advantage of simultaneously acquiring blood flow profiles and vessel anatomy along arbitrarily oriented sagittal planes. The process is instantaneous and enables real-time 3D flow reconstruction without the need for computationally intensive image processing compared to state-of-the-art velocimetry techniques. To evaluate the line-scanning direction and speed, four sets of parametric synthetic OCT-mu PIV data were generated using an in-house code. Based on this investigation, an in vitro experiment was designed at the fastest scan speed while preserving the region of interest providing the depth-resolved velocity profiles spanning across the width of a micro-fabricated channel. High-agreement with the analytical flow profiles was achieved for different flow rates and seed particle types and sizes. Finally, by employing blood cells as non-invasive seeding particles, in vivo embryonic vascular velocity profiles in multiple vessels were measured in the early chick embryo. The pulsatile flow frequency and peak velocity measurements were also acquired with OCT-mu PIV, which agreed well with previous reported values. These results demonstrate the potential utility of this technique to conduct practical microfluidic and non-invasive in vivo studies for embryonic blood flows.