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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/3
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Publication Metadata only Ris-aided angular-based hybrid beamforming design in mmwave massive mimo systems(IEEE, 2022) Koc, Asil; Tho Le-Ngoc; Department of Electrical and Electronics Engineering; Yıldırım, İbrahim; Başar, Ertuğrul; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of EngineeringThis paper proposes a reconfigurable intelligent surface (RIS)-aided and angular-based hybrid beamforming (AB-HBF) technique for the millimeter wave (mmWave) massive multiple-input multiple-output (MIMO) systems. The proposed RIS-AB-HBF architecture consists of three stages: (i) RF beam-former, (ii) baseband (BB) precoder/combiner, and (iii) RIS phase shift design. First, in order to reduce the number of RF chains and the channel estimation overhead, RF beamformers are designed based on the 3D geometry-based mmWave channel model using slow time-varying angular parameters of the channel. Second, a BB precoder/combiner is designed by exploiting the reduced-size effective channel seen from the BB stages. Then, the phase shifts of the RIS are adjusted to maximize the achievable rate of the system via the nature-inspired particle swarm optimization (PSO) algorithm. Illustrative simulation results demonstrate that the use of RISs in the AB-HBF systems has the potential to provide more promising advantages in terms of reliability and flexibility in system design.Publication Metadata only Effect of finger moisture on tactile perception of electroadhesion(Institute of Electrical and Electronics Engineers, 2024) Lefevre, Philippe; Martinsen, Orjan Grottem; Department of Mechanical Engineering; Aliabbasi, Easa; Muzammil, Muhammad; Şirin, Ömer; Başdoğan, Çağatay; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of EngineeringWe investigate the effect of finger moisture on the tactile perception of electroadhesion with 10 participants. Participants with moist fingers exhibited markedly higher threshold levels. Our electrical impedance measurements show a substantial reduction in impedance magnitude when sweat is present at the finger-touchscreen interface, indicating increased conductivity. Supporting this, our mechanical friction measurements show that the relative increase in electrostatic force due to electroadhesion is lower for a moist finger.Publication Metadata only Numerical simulation of milling operations on flexible composite parts(MATERIALS RESEARCH FORUM LLC, 2024) Nutte, Matthias; Riviere-Lorphevre, Edouard; Dambly, Valentin; Arrazola, Pedro-Jose; Ducobu, Francois; Department of Mechanical Engineering; Lazoğlu, İsmail; Department of Mechanical Engineering; Manufacturing and Automation Research Center (MARC); College of EngineeringFiber-reinforced polymers (FRPs) are a widely used and growing material in industry, thanks to their excellent mechanical properties. Manufactured FRPs parts usually have thin walls. These parts also require finishing operations such as edge trimming. Problems like those encountered when machining thin metal parts are also encountered with FRPs: form error, chatter vibrations and poor surface finish. However, the study and numerical modelling of thin FRP parts are not well developed up to now. The aim of this paper is to demonstrate the feasibility of adapting a numerical model for metals to FRPs. The modelling of the shape error during the thinning of a CFRP (Carbon Fiber Reinforced Polymers) part is studied in this paper using a quasi-static analysis. Compared to metals, two adaptations are introduced here for the FRPs. First, the material properties are adapted from isotropic to orthotropic. Secondly, a mechanical model was applied to calculate cutting forces for FRPs. The results of the study show the feasibility of this adaptation and examination of form error in the case of FRPs.Publication Metadata only Numerical investigation of hybrid rocket nozzle heat transfer and test validation(IEEE, 2023) Öztürk, Şule; Özkol, İbrahim; Department of Mechanical Engineering; Karabeyoğlu, Mustafa Arif; Department of Mechanical Engineering; College of EngineeringRockets are the most convenient way to reach space. Rockets accelerate the molecules formed as a result of combustion in the rocket through a nozzle and throws it out, thus creating thrust. The thrust produced by energized and accelerated hot gas expelling through nozzle. Rockets are categorized by ways of putting energy to these particles. A rocket motor is an example of the energy transformation system. This energy transformation can be done by accelerating particle using an electric field, nuclear energy, or heating particles by chemical reactions. With high safety and reliability, hybrid rocket engines as an example of chemical rockets can be better alternative of bi-liquid propulsion systems in terms of development and production cost. The aim of this study is to investigate the nozzle behavior in a small-scale and low-cost hybrid rocket engine in oxygen rich environment under thermal motor is designed, developed, and tested. Theoretically the conjugate gradient method with adjoin problem for function estimation iterative technique is used to solve the Inverse Heat Conduction Problem (IHCP) to estimate heat flux and internal wall temperature of the nozzle. The convective heat transfer coefficient is calculated using Bartz equation. The results are compared with the test data.Publication Metadata only An inverse synthesis method for the determination of effective mechanical properties for additively manufactured aperiodic structures with finite thickness(IEEE, 2023) Acar, Eren; Şimşek, Uğur; Şendur, Güllü Kızıltaş; Department of Mechanical Engineering; Gökçay, Barış; Department of Mechanical Engineering; College of EngineeringThe production of thin-walled structures via Additive Manufacturing is common for thermo-mechanical applications such as heat exchanger cores. However, accurate and efficient models predicting the effective material response of complex geometries with periodic inclusions where the finite size effect or aperiodicity is considered are limited. To address this challenge, here an inverse synthesis approach to calculate the effective material properties of periodic structures with finite thickness or aperiodicity is studied based on the Hill-Mandel equivalence principle under specific loading conditions using ANSYS optimization solver. The method is demonstrated on periodic microstructures with finite thickness/aperiodicity and square unit cells with circular inclusions. Results show that boundary conditions need to be revised to capture the finite size effect, but the framework has the potential of incorporating metrics and loadings for a variety of geometries with aperiodicity.Publication Metadata only Flexible-rate learned hierarchical bi-directional video compression with motion refinement and frame-level bit allocation(IEEE, 2022) Department of Electrical and Electronics Engineering; Çetin, Eren; Yılmaz, Mustafa Akın; Tekalp, Ahmet Murat; Department of Electrical and Electronics Engineering; Koç Üniversitesi İş Bankası Yapay Zeka Uygulama ve Araştırma Merkezi (KUIS AI)/ Koç University İş Bank Artificial Intelligence Center (KUIS AI); College of Engineering; Graduate School of Sciences and EngineeringThis paper presents improvements and novel additions to our recent work on end-to-end optimized hierarchical bidirectional video compression [1] to further advance the state-of-the-art in learned video compression. As an improvement, we combine motion estimation and prediction modules and compress refined residual motion vectors for improved rate-distortion performance. As novel addition, we adapted the gain unit proposed for image compression to flexible-rate video compression in two ways: first, the gain unit enables a single encoder model to operate at multiple rate-distortion operating points; second, we exploit the gain unit to control bit allocation among intra-coded vs. bi-directionally coded frames by fine tuning corresponding models for truly flexible-rate learned video coding. Experimental results demonstrate that we obtain state-of-the-art rate-distortion performance exceeding those of all prior art in learned video coding.Publication Metadata only Predicting path loss distributions of a wireless communication system for multiple base station altitudes from satellite images(IEEE, 2022) Güntürk, Bahadır K.; Ateş, Hasan F.; Baykaş, Tunçer; Department of Electrical and Electronics Engineering; Shoer, İbrahim; Department of Electrical and Electronics Engineering; College of EngineeringIt is expected that unmanned aerial vehicles (UAVs) will play a vital role in future communication systems. Optimum positioning of UAVs, serving as base stations, can be done through extensive field measurements or ray tracing simulations when the 3D model of the region of interest is available. In this paper, we present an alternative approach to optimize UAV base station altitude for a region. The approach is based on deep learning;specifically, a 2D satellite image of the target region is input to a deep neural network to predict path loss distributions for different UAV altitudes. The neural network is designed and trained to produce multiple path loss distributions in a single inference;thus, it is not necessary to train a separate network for each altitude.Publication Metadata only Multicamera audio-visual analysis of dance figures(IEEE, 2007) N/A; N/A; Department of Computer Engineering; Department of Computer Engineering; Department of Electrical and Electronics Engineering; Ofli, Ferda; Erzin, Engin; Yemez, Yücel; Tekalp, Ahmet Murat; PhD Student; Faculty Member; Faculty Member; Faculty Member; Department of Computer Engineering; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; College of Engineering; College of Engineering; N/A; 34503; 107907; 26207We present an automated system for multicamera motion capture and audio-visual analysis of dance figures. the multiview video of a dancing actor is acquired using 8 synchronized cameras. the motion capture technique is based on 3D tracking of the markers attached to the person's body in the scene, using stereo color information without need for an explicit 3D model. the resulting set of 3D points is then used to extract the body motion features as 3D displacement vectors whereas MFC coefficients serve as the audio features. in the first stage of multimodal analysis, we perform Hidden Markov Model (HMM) based unsupervised temporal segmentation of the audio and body motion features, separately, to determine the recurrent elementary audio and body motion patterns. then in the second stage, we investigate the correlation of body motion patterns with audio patterns, that can be used for estimation and synthesis of realistic audio-driven body animation.Publication Metadata only Application QoS fairness in wireless video scheduling(Institute of Electrical and Electronics Engineers (IEEE), 2006) N/A; N/A; Department of Electrical and Electronics Engineering; Department of Electrical and Electronics Engineering; Department of Electrical and Electronics Engineering; Özçelebi, Tanır; Tekalp, Ahmet Murat; Civanlar, Mehmet Reha; Sunay, Mehmet Oğuz; PhD Student; Faculty Member; Faculty Member; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; College of Engineering; College of Engineering; N/A; 26207; 16372; N/AThe video pre-roll delay for filling up the client buffer can not be too long for user utility and buffer limitations in wireless point-to-multipoint streaming systems. Cross-layer design that deals with both physical and application layer aspects jointly is necessary for this purpose. We present a cross-layer optimized multiuser video adaptation and user scheduling framework for wireless video communication, where Quality-of-Service (QoS) fairness among users is provided with maximum video quality and video throughput. Both protocol layers are jointly optimized using a single Multi-Objective Optimization (MOO) framework that aims to schedule the user with the least remaining playback time and the highest video throughput (delivered video seconds per transmission slot) with maximum video quality. Experiments carried out in the IS-856 (1×EV-DO) standard and ITU pedestrian and vehicular environments demonstrate the improvements over the state-of-the-art schedulers in terms of video QoS fairness, video quality and throughput. / İstemci arabelleğini doldurmak için videodan önce gösterilen reklam gecikmesi, kablosuz noktadan çok noktaya akış sistemlerinde kullanıcı yardımcı programı ve arabellek sınırlamaları için çok uzun olamaz. Bu amaç için hem fiziksel hem de uygulama katmanı özelliklerini birlikte ele alan çapraz katman tasarımı gereklidir. Kablosuz video iletişimi için, kullanıcılar arasında Hizmet Kalitesi (QoS) adaletinin maksimum video kalitesi ve video çıkışı ile sağlandığı, katmanlar arası optimize edilmiş çok kullanıcılı bir video uyarlaması ve kullanıcı planlama çerçevesi sunuyoruz. Her iki protokol katmanı, kullanıcıyı maksimum video kalitesiyle en az kalan oynatma süresi ve en yüksek video verimi (iletim yuvası başına iletilen video saniyesi) ile programlamayı amaçlayan tek bir Çok Amaçlı Optimizasyon (MOO) çerçevesi kullanılarak ortaklaşa optimize edilmiştir. IS-856 (lxEV-DO) standardında ve ITU yaya ve araç ortamlarında gerçekleştirilen deneyler, video QoS adaleti, video kalitesi ve verim açısından en son teknoloji zamanlayıcılara göre iyileştirmeler göstermektedir.Publication Metadata only Multiscale coupling based on quasicontinuum method in nanowires at finite temperatures(IEEE, 2015) Sonne, Mads Rostgaard; Hattel, Jesper Henri; N/A; Department of Mechanical Engineering; Esfahani, Mohammad Nasr; Alaca, Burhanettin Erdem; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 115108Nanoelectromechanical systems have been developed for ultra-high frequency oscillators because of their small size and excellent material properties. Using flexural modes and electrothermal features in nanowires for frequency tuning necessitates a sound modeling approach. The quasicontinuum method was developed to link atomistic models with the continuum finite element method in order to study the material behavior across multiple length scales. These significant efforts to develop a continuum theory based on atomistic models have so far been limited to zero temperature. The purpose of this work is to develop the theoretical framework needed to study the mechanical response in nanoscale components such as nanowires at finite temperatures. This is achieved up to a temperature of 1000 K by integrating Engineering Molecular Mechanics and the Cauchy-Born hypothesis. The proposed method is verified with Molecular Dynamics and Molecular Mechanics simulations reported in literature. Bending properties of nanowires at finite temperatures were studied with the proposed method. Thermomechanical properties were investigated by including surface effects.