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    Publication
    A microstructure-sensitive model for simulating the impact response of a high-manganese austenitic steel
    (Asme, 2016) N/A; N/A; Department of Mechanical Engineering; Mirzajanzadeh, Morad; Canadinç, Demircan; PhD Student; Faculty Member; Department of Mechanical Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Engineering; N/A; 23433
    Microstructurally informed macroscopic impact response of a high-manganese austenitic steel was modeled through incorporation of the viscoplastic self-consistent (VPSC) crystal plasticity model into the ANSYS LS-DYNA nonlinear explicit finite-element (FE) frame. Voce hardening flow rule, capable of modeling plastic anisotropy in microstructures, was utilized in the VPSC crystal plasticity model to predict the micromechanical response of the material, which was calibrated based on experimentally measured quasi-static uniaxial tensile deformation response and initially measured textures. Specifically, hiring calibrated Voce parameters in VPSC, a modified material response was predicted employing local velocity gradient tensors obtained from the initial FE analyses as a new boundary condition for loading state. The updated micromechanical response of the material was then integrated into the macroscale material model by calibrating the Johnson-Cook (JC) constitutive relationship and the corresponding damage parameters. Consequently, we demonstrate the role of geometrically necessary multi-axial stress state for proper modeling of the impact response of polycrystalline metals and validate the presented approach by experimentally and numerically analyzing the deformation response of the Hadfield steel (HS) under impact loading.
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    Design and analysis of a new magneto rheological damper for washing machine
    (Korean Soc Mechanical Engineers, 2018) N/A; Department of Mechanical Engineering; Ulasyar, Abasin; Lazoğlu, İsmail; Researcher; Faculty Member; Department of Mechanical Engineering; Manufacturing and Automation Research Center (MARC); N/A; College of Engineering; N/A; 179391
    In this article, a new magneto rheological (MR) sponge damper is proposed for suppression of vibrations in a washing machine. The article presents design optimization of geometric parameters of MR sponge damper (MRSD) using the finite element analysis (FEA) and first order derivative techniques for a washing machine. The article explains the hysteresis behavior and the relationship of damping force with input current for the proposed MRSD. Moreover, the characteristics of the MRSD such as energy dissipation and equivalent damping coefficient are investigated experimentally in terms of input current and excitation amplitude. The passive dampers installed in washing machine are ineffective in reducing unwanted vibrations at resonant frequencies due to real time unbalanced mass. For this purpose, a test setup is established in order to compare the performance of passive dampers with the proposed MRSDs in a washing machine. It is noticed that MRSDs reduce average vibrations of 75.61 % in a low frequency band, whereas in a high frequency band, the MRSDs lessen average vibrations of 30.57 % in a washing machine. In order to determine the performance of proposed design MRSD, a detailed comparison of the performance parameters, such as total damping force, passive force, maximum average vibrations after suppression by MR dampers, maximum current and power ratings is provided with the existing designs of MR damper for washing machine from the literature.
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    PublicationOpen Access
    Effect of remote masking on detection of electrovibration
    (Institute of Electrical and Electronics Engineers (IEEE), 2019) Güçlü, Burak; Vardar, Yasemin; Department of Mechanical Engineering; Başdoğan, Çağatay; Jamalzadeh, Milad; Faculty Member; PhD Student; Department of Mechanical Engineering; College of Engineering; Graduate School of Sciences and Engineering; 125489; N/A
    Masking has been used to study human perception of tactile stimuli, including those created on haptic touch screens. Earlier studies have investigated the effect of in-site masking on tactile perception of electrovibration. In this study, we investigated whether it is possible to change the detection threshold of electrovibration at fingertip of index finger via remote masking, i.e. by applying a (mechanical) vibrotactile stimulus on the proximal phalanx of the same finger. The masking stimuli were generated by a voice coil (Haptuator). For eight participants, we first measured the detection thresholds for electrovibration at the fingertip and for vibrotactile stimuli at the proximal phalanx. Then, the vibrations on the skin were measured at four different locations on the index finger of subjects to investigate how the mechanical masking stimulus propagated as the masking level was varied. Finally, electrovibration thresholds were measured in the presence of vibrotactile masking stimuli. Our results show that vibrotactile masking stimuli generated sub-threshold vibrations around fingertip and, hence, probably did not mechanically interfere with the electrovibration stimulus. However, there was a clear psychophysical masking effect due to central neural processes. Electrovibration absolute threshold increased approximately 0.19 dB for each dB increase in the masking level.
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    PublicationOpen Access
    Haemodynamic recovery properties of the torsioned testicular Artery Lumen
    (Nature Publishing Group (NPG), 2017) Göktaş, Selda; Pişkin, Şenol; Çapraz, Can T.; Çakmak, Yusuf O.; N/A; Department of Mechanical Engineering; Yalçın, Özlem; Ermek, Erhan; Pekkan, Kerem; Other; Faculty Member; Department of Mechanical Engineering; School of Medicine; College of Engineering; 218440; 109991; 161845
    Testicular artery torsion (twisting) is one such severe vascular condition that leads spermatic cord injury. In this study, we investigate the recovery response of a torsioned ram testicular artery in an isolated organ-culture flow loop with clinically relevant twisting modes (90°, 180°, 270° and 360° angles). Quantitative optical coherence tomography technique was employed to track changes in the lumen diameter, wall thickness and the three-dimensional shape of the vessel in the physiological pressure range (10–50 mmHg). As a control, pressure-flow characteristics of the untwisted arteries were studied when subjected to augmented blood flow conditions with physiological flow rates up to 36 ml/min. Both twist and C-shaped buckling modes were observed. Acute increase in pressure levels opened the narrowed lumen of the twisted arteries noninvasively at all twist angles (at ∼22 mmHg and ∼35 mmHg for 360°-twisted vessels during static and dynamic flow experiments, respectively). The association between the twist-opening flow rate and the vessel diameter was greatly influenced by the initial twist angle. The biomechanical characteristics of the normal (untwisted) and torsioned testicular arteries supported the utilization of blood flow augmentation as an effective therapeutic approach to modulate the vessel lumen and recover organ reperfusion.
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    Impedimetric antimicrobial peptide biosensor for the detection of human immunodeficiency virus envelope protein GP120
    (Cell Press, 2024) Department of Mechanical Engineering; Uygun, Zihni Onur; Taşoğlu, Savaş; Department of Mechanical Engineering; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); KU Arçelik Research Center for Creative Industries (KUAR) / KU Arçelik Yaratıcı Endüstriler Uygulama ve Araştırma Merkezi (KUAR); College of Engineering
    This study presents the design and implementation of an antimicrobial peptide -based electrochemical impedance spectroscopy (EIS) based biosensor system. The biosensor consists of a gold coated carbon electrode with MXene and silver nanoparticles (AgNPs) for the label -free detection of the human immunodeficiency virus (HIV) envelope protein gp120. Scanning electron microscopy was used to confirm the presence and distribution of MXene and AgNPs on the biosensor surface. The employment of the antimicrobial peptide on the electrode surface minimized the denaturation of the biorecognition receptor to ensure reliable and stable performance. The biosensor exhibited a linear range of 10-4000 pg mL-1 for gp120 detection, demonstrating good repeatability in real samples. The limit of detection (LOD) and limit of quantification (LOQ) were also calculated as 0.05 pg mL-1 and 0.14 pg mL-1, respectively. This biosensing platform has promising applications in the detection of HIV in clinical and point -of -care settings./Yönetim kurulu üyelerinin azlini zorlaştıran düzenlemeler, bir taraftan farklı bağlamlarda şirket tüzel kişiliğinin, azınlık pay sahiplerinin ve yönetim kurulu üyelerinin menfaatlerinin korunmasına hizmet edebilmekte; ayrıca ülkemizdeki gibi blok pay sahipliği yapısının yaygın olduğu hukuk sistemlerinde, çoğunluk ve azınlık pay sahipleri arasında kontrol paylaşımını da mümkün kılmaktadır. Diğer taraftan ise, genel kurulun yönetim kurulu üyelerini görevden alma konusundaki mutlak ve devredilemez yetkisini sınırlandırmakta; böylelikle yönetim kurulu üyesi ve şirket arasındaki güvenin sarsıldığı durumlarda ilgili üyenin görevine son verilebilmesi imkânını daraltmaktadır. Öte yandan Türk Hukukunda, genel kurulun yönetim kurulu üyelerini azil yetkisini sınırlandıran düzenlemelerin ortaklık düzeninde geçerli olup olmadığı hususunda bir belirsizlik vardır. Bu çalışmada esas sözleşmedeki bir düzenlemeyle, genel kurulun yönetim kurulu üyelerini görevden alma yetkisinin sınırlandırılmasının mümkün olup olmadığı konusunda iki soruya cevap aranmıştır: Bunlardan biri, böyle bir kararın alınmasının belirli bir süreye yahut haklı sebeplerin veya bazı şartların varlığına bağlanıp bağlanamayacağı; diğeri ise, genel kurul toplantısı ve kararı açısından, özel yetersayılar aranıp aranmayacağıdır. İlk soruya öğretide genellikle olumsuz cevap verilmesine karşılık, ikinci soruya verilen cevap ise olumlu yöndedir. Çalışmada savunduğumuz görüş çerçevesinde, yönetim kurulu üyelerinin azli imkânının haklı sebep, koşul, süre veya yaptırım gibi düzenlemelerle sınırlandırılması mümkün olmamalıdır. Benzer şekilde yönetim kurulu üyelerinin görevden alınması hususunda toplantı ve/veya karar yetersayılarının da kanuna göre ağırlaştırılması imkân dahilinde olmamalıdır. Anılan türdeki düzenlemelerin esas sözleşmeye eklenmesi hâlinde ise, bunların bâtıl olacağının kabulü gerekir.
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    Investigation of 3-D multilayer approach in predicting the thermal behavior of 20 Ah Li-ion cells
    (Pergamon-Elsevier Science Ltd, 2019) N/A; N/A; Department of Chemical and Biological Engineering; Alipour, Mohammad; Esen, Ekin; Kızılel, Rıza; PhD Student; PhD Student; Researcher; Department of Chemical and Biological Engineering; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 114475
    Numerous research groups have adopted a 1D single-layer cell approach to model the thermal behavior of the Li-ion battery systems. However, as the size of a Li-ion cell increases, the 1D single-layer approach is not enough to determine the thermal behavior of the high capacity batteries. In this study, a multilayer approach is proposed to consider the effects of the number of layers on the thermal behavior of the cell. 3D electrochemical-thermal model with multilayer approach is designed and temperature predictions at various discharge rates are calculated. The results are validated at 30 degrees C for various discharge rates. Thermal behavior of the single-layer and multilayer cell approaches are compared with the experimental measurements. The results show that the error of estimates is halved if multilayer approach is applied. The proposed model is also used to study the effects of the number of layers on the temperature non-uniformity of the large sized Li-ion batteries. The results showed that multilayer cell approach represents the thermal behavior of the Li-ion cell more accurately. The study is promising for the development of an efficient thermal management system with a better prediction of the potential hot spots on the single cells and battery packs.
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    Left atrial ligation in the avian embryo as a model for altered hemodynamic loading during early vascular development
    (Journal of Visualized Experiments, 2023) Department of Mechanical Engineering; Sevgin, Börteçine; Çoban, Merve Nur; Karataş, Faruk; Pekkan, Kerem; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering
    Due to its four-chambered mature ventricular configuration, ease of culture, imaging access, and efficiency, the avian embryo is a preferred vertebrate animal model for studying cardiovascular development. Studies aiming to understand the normal development and congenital heart defect prognosis widely adopt this model. Microscopic surgical techniques are introduced to alter the normal mechanical loading patterns at a specific embryonic time point and track the downstream molecular and genetic cascade. The most common mechanical interventions are left vitelline vein ligation, conotruncal banding, and left atrial ligation (LAL), modulating the intramural vascular pressure and wall shear stress due to blood flow. LAL, particularly if performed in ovo, is the most challenging intervention, with very small sample yields due to the extremely fine sequential microsurgical operations. Despite its high risk, in ovo LAL is very valuable scientifically as it mimics hypoplastic left heart syndrome (HLHS) pathogenesis. HLHS is a clinically relevant, complex congenital heart disease observed in human newborns. A detailed protocol for in ovo LAL is documented in this paper. Briefly, fertilized avian embryos were incubated at 37.5 degrees C and 60% constant humidity typically until they reached Hamburger-Hamilton (HH) stages 20 to 21. The egg shells were cracked open, and the outer and inner membranes were removed. The embryo was gently rotated to expose the left atrial bulb of the common atrium. Pre-assembled micro-knots from 10-0 nylon sutures were gently positioned and tied around the left atrial bud. Finally, the embryo was returned to its original position, and LAL was completed. Normal and LALinstrumented ventricles demonstrated statistically significant differences in tissue compaction. An efficient LAL model generation pipeline would contribute to studies focusing on synchronized mechanical and genetic manipulation during the embryonic development of cardiovascular components. Likewise, this model will provide a perturbed cell source for tissue culture research and vascular biology.
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    Measurement and characteristic analysis of ris-assisted wireless communication channels in sub-6 ghz outdoor scenarios
    (IEEE, 2023) Lan, Jifeng; Sang, Jian; Zhou, Mingyong; Gao, Boning; Meng, Shengguo; Li, Xiao; Tang, Wankai; Jin, Shi; Cheng, Qiang; Cuit, Tie Jun; Department of Electrical and Electronics Engineering; Başar, Ertuğrul; Department of Electrical and Electronics Engineering; College of Engineering
    Reconfigurable intelligent surface (RIS)-empowered communication has recently drawn significant attention due to its superior capability in manipulating the wireless propagation environment. However, the channel modeling and measurement of RIS-assisted wireless communication systems in real environment has not been adequately studied. In this paper, we construct a channel measurement system using vector network analyzer (VNA) is used to investigate RIS-assisted wireless communication channel in outdoor scenarios at 2.6 GHz. New path loss (PL) models including angle domain information are proposed by refining the traditional close-in (CI) and floating-intercept (FI) models. In the proposed models, both influences of the distance from transmitter (TX) to RIS and the distance from receiver (RX) to RIS on the PL, are taken into account. In addition, the value of root mean square (RMS) delay spread of RIS-assisted wireless communication is found to be much smaller than that of the traditional non line-of-sight (NLOS) case, implying that RIS provides a virtual line-of-sight (LOS) link.
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    Measurement-based characterization of physical layer security for ris-assisted wireless systems
    (IEEE, 2023) Kesir, Samed; Wikelek, Ibrahim; Pusane, Ali Emre; Gorcin, Ali; Department of Electrical and Electronics Engineering; Kayraklık, Sefa; Başar, Ertuğrul; Department of Electrical and Electronics Engineering; CoreLab; Graduate School of Sciences and Engineering; College of Engineering
    There have been recently many studies demonstrating that the performance of wireless communication systems can be significantly improved by a reconfigurable intelligent surface (RIS), which is an attractive technology due to its low power requirement and low complexity. This paper presents a measurement-based characterization of RISs for providing physical layer security, where the transmitter (Alice), the intended user (Bob), and the eavesdropper (Eve) are deployed in an indoor environment. Each user is equipped with a software-defined radio connected to a horn antenna. The phase shifts of reflecting elements are software controlled to collaboratively determine the amount of received signal power at the locations of Bob and Eve in such a way that the secrecy capacity is aimed to be maximized. An iterative method is utilized to configure a Greenerwave RIS prototype consisting of 76 passive reflecting elements. Computer simulation and measurement results demonstrate that an RIS can be an effective tool to significantly increase the secrecy capacity between Bob and Eve.
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    PublicationOpen Access
    Multi-scale modeling of the impact response of a strain-rate sensitive high-manganese austenitic steel
    (Frontiers, 2014) Department of Mechanical Engineering; Canadinç, Demircan; Önal, Orkun; Özmenci, Cemre; Faculty Member; Department of Mechanical Engineering; College of Engineering; 23433; N/A; N/A
    A multi-scale modeling approach was applied to predict the impact response of a strain rate sensitive high-manganese austenitic steel. The roles of texture, geometry, and strain rate sensitivity were successfully taken into account all at once by coupling crystal plasticity and finite element (FE) analysis. Specifically, crystal plasticity was utilized to obtain the multi-axial flow rule at different strain rates based on the experimental deformation response under uniaxial tensile loading. The equivalent stress – equivalent strain response was then incorporated into the FE model for the sake of a more representative hardening rule under impact loading. The current results demonstrate that reliable predictions can be obtained by proper coupling of crystal plasticity and FE analysis even if the experimental flow rule of the material is acquired under uniaxial loading and at moderate strain rates that are significantly slower than those attained during impact loading. Furthermore, the current findings also demonstrate the need for an experiment-based multi-scale modeling approach for the sake of reliable predictions of the impact response.