SCHOLARLY PUBLICATION

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Now showing 1 - 20 of 222

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3D simulation of droplet impact on static and moving walls
(Begell House Inc., 2022) Yılmaz, Anıl; Kayansalçik, Gökhan; Ertunç, Özgür; Mechanical Engineering; ERTUNÇ, Özgür; Yılmaz, Anıl; Kayansalçik, Gökhan
In the present study, the contact angle model and the origin of the parasitic current, precisely, the relation of the parasitic current with grid distribution, have been studied to accurately predict droplet impact on static and moving walls in the volume of fluid (VOF) framework. The authors have quantitatively shown that the number of neighboring cells of the central cell influences the gradient calculations regarding the generation and spatial distribution of parasitic current. Accordingly, the polyhedral cell structure provides smoother interface gradient distribution than the Cartesian grid structure. After implementing a modified Kistler contact angle model in OpenFOAM and using the polyhedral grid for the simulations, we could accurately validate transient droplet shapes formed upon impact with those obtained from experiments. Droplet outcomes obtained, such as deposition, partial rebound, and split deposition on stationary and moving smooth surfaces, are consistent with experimental results.
Open Access
Accuracy limits of pair distribution function analysis in structural characterization of nanocrystalline powders by X-ray diffraction
(Turkish Chemical Society, 2022) Baloochiyan, Abolfazl; Batyrow, Merdan; Öztürk, Hande; Mechanical Engineering; KAYMAKSÜT, Hande Öztürk; Baloochiyan, Abolfazl; Batyrow, Merdan
We report the minimum errors of structural parameters, namely lattice parameter, crystallite size, and atomic displacement parameters, expected from Pair Distribution Function (PDF) analysis of nanocrystalline gold powders for the first time by a self-consistent computational methodology. Although PDF analysis has been increasingly used to characterize nanocrystalline powders by X-rays, the current literature includes no established error bounds to be expected from the resulting structural parameters. For accurate interpretation of X-ray diffraction data, these error bounds must be determined, and the obtained structural parameters must be cleared from them. Our novel methodology includes: 1) simulation of ideal powder diffraction experiments with the use of the Debye scattering equation, 2) pair distribution function analysis of the diffraction data with the Diffpy-CMI analysis software, and 3) determination of the errors from PDF analysis of the simulated diffraction data by comparing them with real-space analysis of spherical gold nanocrystals that are 30 nm size and smaller. Our results show that except for the lattice parameters and even with an ideal crystalline powder sample and ideal diffraction data, the extracted structural parameters from PDF analysis diverge from their true values for the studied nanopowder. These deviations are dependent on the average size of the nanocrystals and the energy of the X-rays selected for the diffraction experiments, where lower X-ray energies and small-sized nanocrystalline powders lead to greater errors.
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Acoustics and heat transfer characteristics of piezoelectric driven central orifice synthetic jet actuators
(Taylor & Francis, 2022-09-19) Ikhlaq, M.; Yasir, M.; Ghaffari, O.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet
Growth in computational capacity combined with a decrease in the size of digital devices has led to increasing demand for more active and efficient cooling of electronics. In this study, an experimental investigation into two different sizes of central orifice Synthetic Jet Actuators (SJAs) is conducted to evaluate their heat transfer as well as noise generation characteristics. Two SJAs (40 mm and 20 mm) are examined, covering a distinct span of frequencies ranging from low to medium (<5500 Hz) in regards to the effect of SJA size over performance. The SJAs’ disk deflection, structural frequency, and jet exit velocity were measured to fully characterize the jet performance. The maximum Nusselt number for the largest SJA was 3 times more than the smallest SJA, where the evaluation of stroke length suggests no effective synthetic jet formation for the smallest SJA. The noise from the SJAs was measured in an anechoic chamber using three microphones, Fast Fourier Transform (FFT) of the sound pressure levels provide contributions to different tones in the resulting noise. 1/3 Octave Constant Percentage Bandwidth (CPB) analysis was performed to identify the frequency bands making the largest contribution to the noise. The largest SJA showed the highest heat transfer at acceptable noise levels during the operation below resonance frequency.
Open Access
Active-grid turbulence effect on the topology and the flame location of a lean premixed combustion
(Society of Thermal Engineers of Serbia, 2018) Alhumairi, Mohammed Khudhair Abbas; Ertunç, Özgür; Mechanical Engineering; ERTUNÇ, Özgür; Alhumairi, Mohammed Khudhair Abbas
Lean premixed combustion under the influence of active-grid turbulence was computationally investigated, and the results were compared with experimental data. The experiments were carried out to generate a premixed flame at a thermal load of 9 kW from a single jet flow combustor. Turbulent combustion models, such as the coherent flame model (CFM) and turbulent flame speed closure (TFC) model were implemented for the simulations performed under different turbulent flow conditions, which were specified by the Reynolds number based on Taylor’s microscale (Reλ), the dissipation rate of turbulence (ε) and turbulent kinetic energy (k). This study shows that the applied turbulent combustion models differently predict the flame topology and location. However, similar to the experiments, simulations with both models revealed that the flame moves toward the inlet when turbulence becomes strong at the inlet, that is, when Reλ at the inlet increases. The results indicated that the flame topology and location in the coherent flame model were more sensitive to turbulence than those in the turbulent flame speed closure model. The flame location behavior on the jet flow combustor significantly changed with the increase of Reλ.
Open Access
Actor-critic reinforcement learning for bidding in bilateral negotiation
(TÜBİTAK, 2022) Arslan, Furkan; Aydoğan, Reyhan; Computer Science; AYDOĞAN, Reyhan; Arslan, Furkan
Designing an effective and intelligent bidding strategy is one of the most compelling research challenges in automated negotiation, where software agents negotiate with each other to find a mutual agreement when there is a conflict of interests. Instead of designing a hand-crafted decision-making module, this work proposes a novel bidding strategy adopting an actor-critic reinforcement learning approach, which learns what to offer in a bilateral negotiation. An entropy reinforcement learning framework called Soft Actor-Critic (SAC) is applied to the bidding problem, and a self-play approach is employed to train the model. Our model learns to produce the target utility of the coming offer based on previous offer exchanges and remaining time. Furthermore, an imitation learning approach called behavior cloning is adopted to speed up the learning process. Also, a novel reward function is introduced that does take not only the agent’s own utility but also the opponent’s utility at the end of the negotiation. The developed agent is empirically evaluated. Thus, a large number of negotiation sessions are run against a variety of opponents selected in different domains varying in size and opposition. The agent’s performance is compared with its opponents and the performance of the baseline agents negotiating with the same opponents. The empirical results show that our agent successfully negotiates against challenging opponents in different negotiation scenarios without requiring any former information about the opponent or domain in advance. Furthermore, it achieves better results than the baseline agents regarding the received utility at the end of the successful negotiations.
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Addressing endogeneity in the causal relationship between sustainability and financial performance
(Elsevier, 2019-04) Soytaş, Mehmet Ali; Denizel, M.; Uşar, Damla Durak; Economics; SOYTAŞ, Mehmet Ali; Uşar, Damla Durak
The existing empirical literature on the relationship between corporate sustainability performance and corporate financial performance casts doubt on the direction of this relationship although more studies point out a direction from sustainability to performance. Literature also presents a gap in addressing the mechanism(s) of the relationship that hinders the convergence of the empirical findings and only recently the question of causality is being addressed with modern econometric techniques. We argue that due to the potential endogeneity problem in the relationship, an empirical strategy without a theoretical base may result in inconclusive or misleading conclusions. We address the potential endogeneity problem in the relationship and identify the possible causes of this endogeneity as: (i) firm level heterogeneity in financial returns, (ii) the relationship between firm's productivity level and the marginal cost of sustainability initiatives, and (iii) measurement error. We implement Instrumental Variable (IV) technique to overcome these biases. Our results present empirical evidence to support the hypothesis that corporate sustainability is positively related (possibly causally) with corporate financial performance. We further find that sustainability initiatives are more costly for companies that are more productive; thus, they have less incentive to invest. Finally, measurement error in the sustainability metrics does not play a crucial role.
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Adolescents’ conflict resolution with their parents and best friends: links to life satisfaction
(Springer Nature, 2019-10) Gözkan, Ayfer Dost; Psychology; GÖZKAN, Ayfer Dost
Objective The present study aimed to compare the frequency with which adolescents' used different conflict resolution styles in their three close relationships (mother, father and best friend); to examine the associations among adolescents' conflict resolution styles in these relationships and the links between conflict resolution styles and life satisfaction. Methods The participants were adolescents (N = 854; aged 11-19 years), from six secondary schools in a metropolitan city in Turkey, who completed self-report questionnaires. Results In conflicts with their parents, adolescents used constructive styles (problem solving and compliance) more often than dysfunctional styles (withdrawal and conflict engagement). Problem solving was the most frequently used strategy in conflicts with a best friend, followed by conflict engagement, withdrawal, and compliance. Path analysis indicated a strong association between the strategies used to resolve conflicts with parents and with one's best friend, chi(2) = 6.45, df = 6, p = 0.38, CFI/TLI = 1/0.999, RMSEA = 0.007. Across all three relationships problem solving was positively linked to life satisfaction, whereas withdrawal and conflict engagement were negatively linked to life satisfaction, chi(2) = 25.234, df = 24, p = 0.39, CFI/TLI = 0.999/0.999, RMSEA = 0.006. Conclusion There were similarities and differences in the conflict resolution strategies used in the different relationships, which provide support for the social problem solving model as well as for the contextual view of conflict resolution. The findings also underline the importance of constructive conflict resolution to the wellbeing of adolescents and point to the importance of conflict resolution training.
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Adolescents’ well-being with respect to the patterns of disclosure to and secrecy from parents and the best friend: A person-centered examination
(Springer Nature, 2020-08) Elsharnouby, E.; Gözkan, Ayfer Dost; Psychology; GÖZKAN, Ayfer Dost
Adolescents' disclosure and secrecy behaviors have important implications for their well-being. Previous research examined adolescent disclosure and secrecy mostly in one close relationship (i.e., one's mother). What remains to be addressed is the patterns of disclosure and secrecy considering adolescents' other close relationships and the well-being differences between these patterns. Adopting a person-centered approach, the current study examined constellations of disclosure and secrecy in adolescents' relationships with their mother, father and best friends, and the extent to which these patterns differ with respect to life satisfaction, problem-solving confidence, and anxiety. The sample consisted of middle adolescents (N = 1097; M-age = 15.12; range = 14-16; 61.6% female) from nine public schools in Istanbul, Turkey. Analysis conducted for the whole sample showed that best friends were the most frequently disclosed confidants followed by mothers, and secrecy was similar for best friends and mothers. Adolescents were least likely to disclose to their fathers and keep secrets from them more frequently. Comparisons of the five classes identified with latent profile analysis, however, indicated that the class with the best well-being status (highest life satisfaction, problem-solving confidence, and lowest anxiety) consisted of adolescents who disclosed most often to their mothers, seconded by best friends, and kept secrets less often from their parents than their best friends. The class with the lowest well-being status consisted of adolescents who had lowest disclosure and highest secrecy in three close relationships. Overall, findings underscored the usefulness of person-centered analytic approach showing that while best friends were the primary confidents when examined with a variable-centered approach, adolescents with the highest well-being status reported to disclose most often to their mothers. This finding underlines the importance of sharing personal information and being less secretive especially with mothers in adolescence.
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Analysis of asynchronous cognitive radio system with imperfect sensing and bursty primary user traffic
(Springer Science+Business Media, 2016-03) Ercan, Ali Özer; Electrical & Electronics Engineering; ERCAN, Ali Özer
This paper presents a theoretical analysis of the spectrum utilization levels in a cognitive radio system. We assume that the traffic of the primary network is bursty and asynchronous with the secondary network, which performs imperfect spectrum sensing. Collisions of the primary and the secondary packets are assumed to result in increased packet error probabilities. We present primary and secondary utilization levels under optimized secondary transmission periods for varying primary traffic characteristics and secondary sensing performance levels. The results are also validated by extensive Monte Carlo simulations. We find that an asynchronous cognitive radio network with imperfect spectrum sensing is feasible when optimized transmission periods are used. The effects of primary traffic’s burst pattern and secondary sensing performance are discussed.
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Analysis of near-field radiation transfer within nano-gaps using FDTD method
(Elsevier, 2014-10) Didari, Azadeh; Mengüç, Mustafa Pınar; Mechanical Engineering; MENGÜÇ, Mustafa Pınar; Didari, Azadeh
Enhancement of near-field radiative emission via coupling of surface plasmons in nano-gaps formed between thin films is important for understanding and implementation of energy harvesting using nano-thermophotovoltaic cells. Design and construction of such cells need to be carried out along with detailed modeling studies, necessitating accurate calculation of near-field emission within thin films. The objective of this paper is to provide a methodology based on finite difference time domain analysis for the calculation of the near-field thermal radiation emission based on local density of electromagnetic states. Near-field thermal emission is investigated within the nano-gap formed between thin silicon carbide layers where both support surface phonon polaritons. Modeling of this problem with the FDTD method is not trivial particularly for establishing the Drude–Lorentz permittivity model and the selection of the right boundary conditions. We present an effective boundary condition, for calculation of Local Density Of electromagnetic States (LDOS) via Finite Difference Time Domain Method (FDTD) for applications to nano-scale geometries. We conclude that Convolutional Perfectly Matched Layer (CPML) is the optimum boundary condition that gives the most accurate results compared against the other methodologies for parallel plates separated by nano-gaps. This boundary condition allows more streamlined simulations to be carried out when working with sub-wavelength structures. The challenges and the possible solutions to overcome these difficulties are discussed in detail.
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Analysis of perlite and pumice based building insulation materials
(Elsevier, 2016-06) Çelik, S.; Family, Roxana; Mengüç, Mustafa Pınar; Mechanical Engineering; MENGÜÇ, Mustafa Pınar; Family, Roxana
Insulation panels composed of perlite, pumice, and cement were fabricated. Perlite and pumice were used to achieve high thermal resistance, and low density. Experimentation was conducted for determining R-values of perlite-pumice-cement (PPC) panels at different moisture content values. Dry and moist sample panels were tested with and without soil and moss at the top where the one with the soil and plant coupling represents a green roof. Reverse heat leak method was used in determination of the R-values of the samples. Verification of theory was achieved via experimentation with an acceptable error range. Thermal diffusivity values of the developed samples were measured using flash method.
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Application of sequence-dependent traveling salesman problem in printed circuit board assembly
(IEEE, 2013-06) Alkaya, A. F.; Duman, Ekrem; Industrial Engineering; DUMAN, Ekrem
Optimization issues regarding the automated assembly of printed circuit boards attracted the interest of researchers for several decades. This is because even small gains in assembly time result in very important benefits in mass production. In this paper, the focus is on a particular placement machine type that has a rotational turret and a stationary component magazine. So far, this type of machine received little attention among the researchers. In this paper, the feeder configuration, placement sequencing, and assembly time minimization problems are formulated explicitly and completely (without simplifying assumptions) using nonlinear integer programming. In addition, the placement sequencing problem is shown to be a recently introduced new generalization of the traveling salesman problem (the sequence-dependent traveling salesman). These formulations show the complexity of the problems and the need for effective heuristic designs for solving them. We propose three heuristics that improve previously suggested solution methods and give comparable results when compared to simulated annealing that is a widely accepted good performing metaheuristic on combinatorial optimization problems. The heuristics are experimentally shown to improve previous methods significantly in assembly time that implies a huge economic benefit. The heuristics proposed could also be applied to other placement machines with similar operation principles.
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Approximate markov-nash equilibria for discrete-time risk-sensitive mean-field games
(Informs, 2020-11) Saldı, Naci; Basar, T.; Raginsky, M.; Natural and Mathematical Sciences; SALDI, Naci
In this paper, we study a class of discrete-time mean-field games under the infinite-horizon risk-sensitive optimality criterion. Risk sensitivity is introduced for each agent (player) via an exponential utility function. In this game model, each agent is coupled with the rest of the population through the empirical distribution of the states, which affects both the agent's individual cost and its state dynamics. Under mild assumptions, we establish the existence of a mean-field equilibrium in the infinite-population limit as the number of agents (N) goes to infinity, and we then show that the policy obtained from the mean-field equilibrium constitutes an approximate Nash equilibrium when N is sufficiently large.
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Aspirations among young refugees in Turkey: social class, integration and onward migration in forced migration contexts
(Taylor and Francis, 2022) Üstübici, A.; Elçi, Ezgi; International Relations; ELÇİ, Ezgi
The prevailing sentiment is that refugees desire to go to developed countries rather than stay in their first host country. Based on a critical reading of the literature on onward migration, this article analyzes the formation of aspirations for (im)mobility among young Syrian refugees in Turkey, considering their initial access to resources and integration. Our quantitative analysis suggests that obtaining legal status, satisfaction with life, perceived cultural similarities between the Turkish and Syrian communities, and hence perceived inclusion are the foremost drivers of aspirations to stay in Turkey. The analysis also shows that rather than migrants’ social class, migration-specific capital, such as a passport and networks abroad, drives onward migration aspirations. The qualitative analysis further unpacks the relationship between economic, cultural, and social capital as well as the subjective experience of integration and aspirations to move on or stay. Analyzing different trajectories, we highlight the importance of ‘start-up capital’ at the onset of displacement in shaping opportunities for settlement and future aspirations. Our discussion underscores that resources and opportunities explain onward mobility aspirations in protracted displacement contexts in relation to daily experiences of inclusion and to considerations regarding social class and social mobility in the future.
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Assembly-based flood repair cost and time models for industrial buildings in Turkey
(Elsevier, 2023-10) Ölmez, Hasan Numan; Deniz, Derya; Civil Engineering; DENİZ, Derya; Ölmez, Hasan Numan
Flooding is one of the most frequent hazard events significantly affecting the industry in Turkey, leading to severe economic losses and business disruptions. To predict the direct flood losses and business downtimes on the industry, this study proposes probabilistic cost and time models of repairing direct physical flood damage to industrial buildings in Turkey. Using field notes and literature reviews, a typical industrial building was first disassembled into a list of building components vulnerable to flooding. With a focus on façade and interior building components, the flood damage potential of each component was assessed at different flood depths and velocities. Damage state relationships were established for the building components to connect their damage levels under flood actions to their individual repair work. Using the assembly-based approach, costs and times of repairing each flood-damaged component and their associated variabilities were assembled in a probabilistic approach to develop total flood repair cost and time models. The results show that typical repair costs and times for facilities subjected to significant flood depths and velocities might reach substantial levels, up to 28% of building replacement costs and 165 working days on average. The proposed models systematically incorporate existing important uncertainties, therefore, provide reliable estimates.
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Asymptotic performance of generalized transmit laser selection over lognormal turbulence channels
(IEEE, 2020-08) Elamassie, Mohammed; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Elamassie, Mohammed
In free space optical communication systems, transmit laser selection (TLS) has been proposed as an efficient fading-mitigation technique over turbulence channels. In TLS, the transmitter is equipped with a number of laser sources and the best laser source is selected for transmission. In practice, feedback error or outdated selection due to temporal changes in the channel may result in selecting another source rather than the best one. In this letter, we consider a FSOC system with generalized TLS (GTLS) where the nth best laser is selected among the available N lasers. Under the assumption of lognormal turbulence channels in addition to pointing errors, we derive a closed-form expression for asymptotic bit error rate. We use our derived closed-form expression to determine the diversity gains. We finally present simulation results to corroborate our analytical findings.
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Attack to quantum cryptosystems through RF fingerprints from photon detectors
(IEEE, 2022-03) Durak, Kadir; Jam, Naser Chamani; Karamzadeh, S.; Electrical & Electronics Engineering; DURAK, Kadir; Jam, Naser Chamani
The well-known RF penetration method is implemented on the quantum cryptosystems. In this study, RF radiations from the single photon detectors in a quantum key distribution system are intercepted. It was found that the data contents of a quantum transmission system can be detected from the vicinity of the receiving parties by exploiting the radiations of avalanche effect of single photon detectors. We showed that any Geiger-mode avalanche photodetector acts like a downconverter that converts the optical-wavelength photons to radio-wavelength photons. In our experiment, the fingerprints of the avalanche radiations from commercial single photon detectors were fed to a trained deep learning neural network, and the bit content of quantum transmission was cloned with >99% accuracy up to a distance of 2 meters from the detectors. We also speculate on the enhancement of the key cloning accuracy and range of the eavesdropping.
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Automated flow rate control of extrusion for 3D concrete printing incorporating rheological parameters
(Elsevier, 2024-04) Ahi, Oğulcan; Ertunç, Özgür; Bundur, Zeynep Başaran; Bebek, Özkan; Civil Engineering; Mechanical Engineering; ERTUNÇ, Özgür; BUNDUR, Zeynep Başaran; BEBEK, Özkan; Ahi, Oğulcan
The use of inline quality assessment technologies is of great importance in meeting the consistent extrusion requirements of 3D concrete printing (3DCP) applications. This paper presents a system to regulate extrusion speed and maintain the flow rate at a target value during 3DCP processes. The system is based on a new equation that combines printing parameters and the material's rheological properties in the printing process. The proposed control strategy is designed to effectively function with various cement-based mixtures. Validation tests demonstrate that the proposed system can maintain an instantaneous flow rate within a certain range and eventually achieve a constant flow rate. During operation, the flow rate is consistently maintained around the targeted value with an average error rate of 6.7 percent. The flow rate control mechanism shows promise as a reliable and efficient solution for achieving precise and constant flow rates, regardless of the cement mix design used.
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Autotuning runtime specialization for sparse matrix-vector multiplication
(ACM, 2016-04) Yılmaz, Buse; Aktemur, Tankut Barış; Garzaran, M. J.; Kamin, S.; Kıraç, Mustafa Furkan; Computer Science; AKTEMUR, Tankut Bariş; KIRAÇ, Mustafa Furkan; Yılmaz, Buse
Runtime specialization is used for optimizing programs based on partial information available only at runtime. In this paper we apply autotuning on runtime specialization of Sparse Matrix-Vector Multiplication to predict a best specialization method among several. In 91% to 96% of the predictions, either the best or the second-best method is chosen. Predictions achieve average speedups that are very close to the speedups achievable when only the best methods are used. By using an efficient code generator and a carefully designed set of matrix features, we show the runtime costs can be amortized to bring performance benefits for many real-world cases.
Open Access
B → T transition form factors in light-cone sum rules
(American Physical Society, 2019-11-11) Aliev, T. M.; Dağ, Hüseyin; Kokulu, A.; Ozpineci, A.; Natural and Mathematical Sciences; DAĞ, Hüseyin
We present a new calculation of the semileptonic tree-level and flavor-changing neutral current form factors describing B-meson transitions to tensor mesons T ¼ D2, K2, a2, f2 (JP ¼ 2þ). We employ the QCD light-cone sum rules approach with B-meson distribution amplitudes. We go beyond the leading-twist accuracy and provide analytically, for the first time, higher-twist corrections for the two-particle contributions up to twist-four terms. We observe that the impact of higher-twist terms to the sum rules is noticeable. We study the phenomenological implications of our results on the radiative B → K 2γ and semileptonic B → D 2lν¯l, B → K 2lþl− decays.

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