WOS - Web of Science
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Publication Implementation of DBSCAN Method in Star Trackers to Improve Image Segmentation in Heavy Noise Conditions(2023) Nevsan ŞENGİL; Şengil, Nevsan; Türk Hava Kurumu Üniversitesi, Uzay Mühendisliği Bölümü, Ankara, TürkiyeStar trackers are currently the most accurate sensors for determining the attitude of a spacecraft. These sensors comprise not only highly capable optical detectors and processor units but also complicated software solvers. One of the main solvers employed in star trackers is image segmentation. In this study, the aim is to develop a hybrid image segmentation method which is a combination of both global thresholding and density-based spatial clustering of applications with noise (DBSCAN) method to increase detection probability of the stars in heavy noise. Secondly, a sorting algorithm is added to list the detected stars in terms of their brightness to increase the efficiency of the star tracking algorithm. Then, this new approach and two different conventional segmentation methods are applied to the Orion star constellation image polluted with Gaussian, salt and pepper, and uneven background noises. The resulting images of these segmentation methods are compared in terms of denoising capabilities. Although computationally more expensive, the proposed DBSCAN-based hybrid method displays a background pixel recovery performance of 99.5%, compared to Otsu global thresholding and adaptive thresholding methods’ 73.5% and 79.9% recovery values, respectively. Additionally, it has been demonstrated that the sorting algorithm successfully listed the detected stars in accordance with their brightness.Publication A 120 MHz fast automatic tuning CMOS active-RC lowpass filter(ELSEVIER SCI LTD, 2023) Yesil, Ercem; Zencir, Ertan; Zencir, Ertan; Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK); Turk Hava Kurumu University; Turkish Aeronautical AssociationIn this paper, a fast automatic cutoff frequency calibrating, CMOS lowpass filter with linear-in-dB tunable gain and constant in-band group delay is presented. The automatic frequency tuning (AFT) circuit can successfully tune the cutoff frequency to 120 MHz with less than 4.7% error. In-band group delay variation is handled using a capacitor process variation detection circuit (CPVD), and the overall tuning time is less than 700 ns. The design is done using STM PD-SOI 130 nm CMOS process, and the supply voltage is 1.2 V with a total power consumption of 21.7 mW. The overall filter provides a programmable linear-in-dB gain of 0.5 dB to 17.6 dB range with a cutoff tuning range of 80 to 180 MHz. The in-band group delay variation is below 500 ps with an adjacent channel ratio (ACR) of 85 dB. The spurious free dynamic range (SFDR) is over 66.6 dBc. The active area of the filter with tuning is 0.171 mm2.Publication Smart prospects for solar-based cooling and heating systems in the Middle East and Turkey(IEEE, 2022) Baba, Abdullatif; Al Shehabi, Shadi; Bonny, M. Talal; Baba, Abdullatif; Turkish Aeronautical Association; Turk Hava Kurumu University; University of SharjahSolar-based systems represent the focal interest field for most of the newest studies of renewable energy. Solar heating and cooling systems convert thermal energy coming from the sun into electricity or heat to provide residential, commercial, and industrial areas with hot water, or to retain a specific required temperature. These technologies replace the classical fossil energy sources that are characterized by their damaging environmental impacts as well as their expensive costs. In this paper, we look at the ambitious solar projects that are already established or will be constructed in different countries of the ME and Turkey. Then, we discuss in brief most of the available technologies that rely on solar-based approaches for heating and cooling purposes. To overcome some challenges that may affect the performance of solar plants like cloudy or night environments, smart management techniques for interconnected units, or the cross-border power units of neighbored countries, are presented here and clarified by a case study. Finally, we conclude with a summary describing the most important features of our paper.Publication On Modified Mellin-Gauss-Weierstrass Convolution Operators(SPRINGER BASEL AG, 2022) Aral, A.; Erbay, H.; Yilmaz, B.; Kirikkale University; Turk Hava Kurumu University; Turkish Aeronautical AssociationMellin transform has various applications to real-life problems in function approximation, signal processing, and image recognition, thus, it has been the main ingredient of many studies in diverse fields. This study is devoted to Mellin operators and their variants to improve approximation accuracy and approximate ratio. Two Mellin type operators are reconstructed by using two sequences of functions to enable lower pointwise approximation error as well as higher pointwise convergence rate. Keeping the idea of Mellin convolution, these classes aim to be associated with functions defined on the semi-real axis, and the affine and quadratic functions pairs are fixed points. It has been shown, both theoretically and numerically, that operators can be used to approximate functions pointwise. Indeed the approximation accuracy can be adjusted by tuning the parameters. Moreover, weighted approximation, as well as Voronovskaya type results, are studied throughout the paper. The advantages of each operator over the other in terms of both approximation errors and convergence rates are presented.Publication What's going on in my mind? The effects of cognitive differences on buying impulsiveness, cognitive dissonance, and price consciousness(WILEY, 2022) Ozyoruk, Huseyin Erbil; Turkish Aeronautical Association; Turk Hava Kurumu UniversityBuying impulsiveness, tendency for cognitive dissonance, and price consciousness may differ among consumers, primarily because of differences in individuals' time orientation, thinking style, risk perception, and cognitive flexibility. This exploratory hypothesis-testing study aims to examine whether the cognitive differences in future orientation, holistic thinking, risk perception, and cognitive flexibility of consumers affect buying impulsiveness, tendency for cognitive dissonance, and price consciousness, through hypothesis testing and structural equation modelling. Data were collected face-to-face from 414 participants across the two largest cities of Turkey (Istanbul and Ankara) using the convenience sampling method. The findings demonstrate that future orientation, holistic thinking, and risk perception negatively affect buying impulsiveness. The tendency for post-purchase cognitive dissonance is negatively affected by future orientation and holistic thinking and is positively affected by risk perception. Price consciousness, however, is affected positively by future orientation and negatively by risk perception. In addition, cognitive flexibility, which is also shaped by future orientation and holistic thinking, increases price consciousness and plays a mediating role in the relationship between future orientation and price consciousness.Publication Experimental and analytical investigation of the tensile behavior of 3D-printed composites based on micro-CT analysis(SAGE PUBLICATIONS LTD, 2023) Sik, Alp; Tanabi, Hamed; Cubukcu, H. Evren; Sabuncuoglu, Baris; Hacettepe University; Hacettepe University; Turk Hava Kurumu University; Turkish Aeronautical Association; Hacettepe University; Hacettepe UniversityMechanical properties of short fiber reinforced composites manufactured with fused deposition modeling is affected by variables such as printing properties and internal structure of the composite. In this study, the relation between the tensile properties of 3D-printed short fiber reinforced composites, printing orientation and internal structure is inspected. Samples of different printing orientations are manufactured. These samples are inspected under X-ray micro computed tomography (micro-CT) and their fiber orientation distributions and fiber properties are determined. Then, these internal structure properties are applied to analytical material models to estimate the tensile properties of composites. Estimated material properties are compared with tensile test results. It turns out that analytical models that consider internal structure properties provide more accurate estimations than other analytical models that does not take these into account.Publication A low-power 65-nm CMOS mixer linearized with IM2 injection for V2X applications(SPRINGER, 2022) Ozkan, Bahadir; Zencir, Ertan; Zencir, Ertan; Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK); Turkish Aeronautical Association; Turk Hava Kurumu UniversityIn this paper, high linearity, low power down-conversion mixer is presented with a 65-nm CMOS process for vehicle-to-everything (V2X) applications. 5G NR V2X standard has a carrier frequency of 5.9 GHz with 10 and 20 MHz narrow bandwidth options. The mixer design uses a double-balanced topology with a second-order intermodulation injection linearization technique to improve the linearity performance. The charge injection method is also used to decrease the noise figure of the circuit. The designed circuit shows a single sideband integrated noise figure of 16.5 dB with a total conversion gain of 2 dB. The third-order input intercept point is obtained as 19.86 dBm. The design consumes a total current of 6 mA from a 1.2-V supply voltage. To the best of the authors' knowledge, this technique is the first applied to mixer design that has been designed for 5G NR based C-V2X applications in the literature.Publication Gain and Bandwidth Programmable Fourth-Order Multiple Feedback Butterworth Low-Pass Filter for C-V2X Applications(WORLD SCIENTIFIC PUBL CO PTE LTD, 2022) Barin, Furkan; Zencir, Ertan; Zencir, Ertan; Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK); Turk Hava Kurumu University; Turkish Aeronautical AssociationA gain and bandwidth tunable active-RC multiple-feedback (MFB) fourth-order low-pass filter is presented, which exhibits four different bandwidths of 10, 20, 30 and 40 MHz and four different gain settings of 0, 4, 8 and 12 dB to meet the requirements of the cellular vehicle-to-everything (C-V2X) standards. The filter uses the cascade of two biquad MFB cells. Gain and bandwidth programmability is achieved by using programmable capacitor and resistor arrays. A logic block is implemented in the filter to adjust the gain transfer function for every tuning option. Also, two-stage miller op-amp topology is chosen to implement biquad MFB cells for minimum complexity and maximum efficiency in low voltage operation. The filter is designed in 65-nm CMOS technology and occupies a 0.181 mm(2) area and it totally consumes 13.41 mW from the 1.2 V supply voltage. To the best of the author's knowledge, this work is the first CMOS baseband filter design that includes both gain and bandwidth programmability implemented for C-V2X applications.Publication On the numerical solutions of some identification problems for one- and multidimensional parabolic equations backward in time(ELSEVIER, 2022) Sazaklioglu, Ali Ugur; Sazaklıoğlu, Ali Uğur; Turk Hava Kurumu University; Turkish Aeronautical AssociationIn this paper, an abstract (differential) inverse problem backward in time, that is a class of some identification problems for simultaneous determination of the source and initial condition, is considered. A finite difference scheme is constructed for the numerical solution of this abstract problem. Some stability and almost coercive stability estimates for the constructed difference scheme are established by the tools of operator theory. Furthermore, the proposed abstract difference scheme and the acquired results for that are extended for several applications involving identification problems for one-and multidimensional parabolic equations. Finally, sundry illustrative numerical results and visualizations are carried out and discussed.(c) 2022 IMACS. Published by Elsevier B.V. All rights reserved.Publication Numerical and experimental studies of thermal performance enhancement for parabolic trough solar collector using none-circulated CuO/synthetic oil nanofluid(EMERALD GROUP PUBLISHING LTD, 2023) Elmnefi, Mohamed; Al-Khazraji, Waqas; Turk Hava Kurumu University; Turkish Aeronautical AssociationPurposeOne of the existing and commonly used solar energy harvesting devices is the parabolic trough solar collector (PTSC). Because of their ability to operate in low and medium temperatures, parabolic trough concentrators are widely used in power generation plants and industrial process heating applications. Therefore, the investigation of how different operating conditions affect these devices' overall efficiency has received a great deal of attention in the recent decade. This study aims to enhance the thermal performance of the PTSC and reduce the system cost. Design/methodology/approachIn the novel configuration, a noncirculated nanofluid absorbs solar radiation through a glass wall. The base fluid was synthetic oil (5W30), and the nanoparticles used were copper oxide. The heat captured is immediately absorbed by the water circulating inside the copper tube immersed in the nanofluid. ANSYS FLUENT 15.0 was used for carrying out computational fluid dynamics simulations for two models of single and triple copper tubes. The experimental results obtained from a test rig constructed for this purpose were compared with the numerical outcomes of the single copper tube model. FindingsThe findings of the simulation demonstrated that performance was superior for the single copper tube model over the triple copper tube model. The numerical findings of the single copper tube model were compared with the experimental results. The numerical and experimental results differed from 3.17% to 5.6%. Investigations were carried out to study the effects of varying the volumetric flow rate of (20, 40, 60 and 80 L/h) and water inlet temperatures of (300, 315 and 330 K) on the effectiveness and performance of the newly developed model. Additionally, two nanofluid volume fractions of 0.05% and 0.075% were used for investigating their effect on the performance of the novel configuration. According to the findings, the highest thermal efficiency of 55.31% was recorded at 0.075% concentration and 80 L/h volume flow rate. Originality/valueIn this study, a novel direct absorption solar collector configuration using a noncirculated nanofluid was designed to enhance the thermal efficiency of PTSC. This new approach makes it possible to boost the thermal performance of the PTSC and lower the system's cost.