Middle East Technical University
  • Çankaya, Ankara, Turkey
Recent publications
Identification and modeling of joint dynamics is an important issue in the dynamic modeling of structures with joints. Moreover, structures connected by joints may show nonlinear behavior due to the existence of dry friction between the connected elements, especially at higher levels of vibration. Therefore, nonlinear identification of joints and parameterization of them is a necessary step in the dynamic analysis of structures. In this study, a new method is proposed for the identification of nonlinear joint forces as functions of relative responses in the frequency domain, which are used to model joint dynamics. In the proposed method, nonlinear joint forces are calculated by using both the measured nonlinear frequency responses of the system, and the calculated or measured linear frequency response functions (FRFs) of the substructures or calculated linear FRFs of the coupled structure at certain points by using estimated values for linear joint parameters. It is important to note that the proposed method does not require measurements at the connection DOFs, which is an advantage since it is generally not practical to make measurements at joints. To demonstrate the performance of the identification method proposed, as well as its sensitivity to measurement errors, two lumped parameter systems connected to each other by nonlinear elements are considered. In these case studies, simulated results representing experimental measurements are used. It is observed that the proposed method is robust to measurement noise, and nonlinear parameters of the connection can be identified accurately.
Risk assessment in projects requires the integration of various information on project characteristics as well as external and internal sources of uncertainty and is based on assumptions about future and project vulnerability. Complexity is a major source of uncertainty that decreases the predictability of project outputs. In this research, the aim was to develop a decision-support tool that can estimate the level of risk and required contingency in a project by assessment of complexity factors as well as contextual information such as contract conditions and mitigation strategies. A process model and a tool were developed using the data of 11 mega construction projects. The tool was tested on a real project, and promising results were obtained about its usability. The tool has the potential to support decision-making during bidding in construction projects with its visualization and prediction features. On the other hand, as a limited number of cases and experts were involved in this study, findings on its performance cannot be generalized. The identified complexity and risk factors, proposed process model, and visual representations may help the development of similar decision-support tools according to different company needs.
This paper presents a family of implicit-explicit (IMEX) time stepping scheme for the optimal control problem of the unsteady Navier-Stokes equations (NSE). The main feature of this kind of optimal control problem is that stabilization terms are proportional to discrete curvature of the solutions. First and second order optimality conditions are used for optimality of the curvature based stabilized Navier-Stokes equations. Complete stability and error analyses of state, adjoint and control variables are presented. Numerical experiments verify theoretical findings and illustrate the improvement of approximate solutions enhancing the efficiency of numerical scheme.
Passive radiative cooling can dissipate heat load directly through the high transparency atmospheric window (8–13 μm). The conventional strategy for radiative cooling has mainly focused on simultaneously blocking the total solar heating and enhancing the infrared emission, which makes the appearance either specular or diffusive white. Recently proposed colored radiative coolers can realize the colored radiative cooling but with limited color gamut. In this work, we have designed both transmissive and reflective colored radiative coolers that realize radiative cooling with ultra-wide color gamut. The optimal structures are identified with Bayesian optimization by only evaluating less than 1% of the candidate structures, which dramatically accelerates the design process. The optimized transmissive colored radiative cooler can produce standard additive colors and realize sub-ambient cooling effect at moderate environment conditions. The reflective colored radiative cooler is highly transparent in visible light range and can largely reduce the thermal load as well as enhance heat dissipation by radiative heat transfer.
In this study, an enhanced heat pipe performance for grooved heat pipes has been demonstrated through capillary boosting with the introduction of the bifurcation of grooves. Wider grooves regularly branch to narrower grooves such that the total cross-sectional liquid flow area remains approximately the same. Following the computational framework drawn by a recently developed heat pipe analysis toolbox (H-PAT), we develop a numerical model for the heat pipes with tree-like groove architecture. Then we utilize the model to design a flat-grooved heat pipe with one step groove bifurcation at the evaporator. To verify our numerical findings, two heat pipes with and without groove bifurcation are manufactured and experimented under the same conditions. Experimental results show that the numerical model can predict the thermal performance quite accurately. The results reveal that groove bifurcation can be a viable option for a better thermal performance than that of heat pipes with standard grooved heat pipes with straight grooves which leads to at least 25% higher maximum heat transport capacity. The effect of number of branching on the temperature flattening across the heat pipe is also demonstrated for different evaporator lengths.
A growing number of cities are deploying circular economy practices to contribute to the Sustainable Development Goals (SDGs). Makerspaces are community-driven initiatives enabling the collaboration between a city's diverse stakeholders in that objective. They can enhance the citizens' contribution to circularity by engaging them in circular ''making'' activities (e.g., reuse, repair, sharing) and by providing them with novel means, skills, and knowledge in these activities. However, cities face various challenges during the process of engaging citizens to create a vibrant circular city. Based on an in-depth analysis of experiences and challenges in seven European cities, which attempted to embrace the SDGs through circular making activities, this paper presents ten engagement-related challenges and five strategies to overcome them. Integrating these challenges and strategies in a strategy-challenge matrix offers several recommendations for cities striving to address SDGs through establishing and maintaining circular makerspaces.
An additive cyclic code of length n over F4 can be defined equivalently as an F2[x]/〈xn+1〉-submodule of F4[x]/〈xn+1〉. In this paper we study additive cyclic and complementary dual codes of odd length over F4 with respect to the trace Hermitian inner product and the trace Euclidean inner product. We characterize subfield subcodes and trace codes of these codes by their generators as binary cyclic codes.
Climate change is disproportionately affecting vulnerable communities, increasing existing risks and leading to further global inequalities. Drawing on the concept of urban adaptation justice, we evaluated the inclusion of vulnerable communities in the climate change adaptation planning process of İstanbul, a European coastal megacity with considerable vulnerability to climate change. For this, a policy content analysis structured around four criteria: (i) participation, (ii) capacity enhancement, (iii) governance, and (iv) justice integration into spatial planning, was carried out and supplemented by local expert consultations. Our findings indicate that while the objective of incorporating some aspects of justice in adaptation planning was recognized, there was a distinct lack of specific actions or evaluation tools. The expert consultations largely confirmed these findings, which were then connected to the socio-historical and political context of İstanbul and the wider Turkish region. Key conclusions include the failure of current adaptation policies to adequately consider vulnerabilities arising from a combination of urban marginalization interacting with neoliberal authoritarianism. We identify the need for understanding and integrating equitable climate change adaptation as a key dimension of urban decision-making for future policy-relevant research and practice.
Passivated Emitter and Rear Contact (PERC) and Passivated Emitter Rear Totally Diffused (PERT) solar cell designs are now a market reality. In the production of these cells, ion implantation is another method for creating emitters and back surface field (BSF) regions, since it can facilitate the process flow and precise doping control. Here, Technology Computer Aided Design (TCAD) is utilized to assess the effects of ion implantation fluences, annealing temperature, and duration, on the performance of bifacial PERC solar cells. To ensure realistic simulation results, the solar cell model is developed using experimentally demonstrated bifacial counterparts. The process simulation models are also calibrated to match experimentally obtained doping profiles. Simulation results show that by replacing emitter diffusion with phosphorus ion implantation, both the front/rear efficiencies can be improved by 0.07%. The rear boron implantation is also optimized to provide an effective BSF, improving the front/rear efficiencies by 0.21/0.15%, respectively. Moreover, co-annealing of emitter and BSF regions exhibits a 0.2% increase in efficiency than separately annealed PERT cells. Hence an overall improvement of 0.48/0.42% is achieved in the front/rear efficiencies when compared to the original diffused emitter PERC design. The fabrication process sequence is also suggested for these solar cells.
In this study, we introduce a measure for machine perception, inspired by the concept of Just Noticeable Difference (JND) of human perception. Based on this measure, we suggest an adversarial image generation algorithm, which iteratively distorts an image by an additive noise until the model detects the change in the image by outputting a false label. The noise added to the original image is defined as the gradient of the cost function of the model. A novel cost function is defined to explicitly minimize the amount of perturbation applied to the input image while enforcing the perceptual similarity between the adversarial and input images. For this purpose, the cost function is regularized by the well-known total variation and bounded range terms to meet the natural appearance of the adversarial image. We evaluate the adversarial images generated by our algorithm both qualitatively and quantitatively on CIFAR10, ImageNet, and MS COCO datasets. Our experiments on image classification and object detection tasks show that adversarial images generated by our JND method are both more successful in deceiving the recognition/detection models and less perturbed compared to the images generated by the state-of-the-art methods, namely, FGV, FSGM, and DeepFool methods.
This paper describes a modal analysis technique to approximate the vibrations of incompressible elastic solids using a stabilized finite element method to approximate the associated eigenvalue problem. It is explained why residual based formulations are not appropriate in this case, and a formulation involving only the pressure gradient is employed. The effect of the stabilization term compared to a Galerkin approach is detailed, both in the derivation of the approximate formulation and in the error estimate provided.
In this study, a new grade of Inconel 718 alloy (IN718) strengthened by nano-oxides has been designed and produced. The alloy composition of 0.3 wt% Y2O3 – IN718 has been determined using CALPHAD-based thermochemical modeling approach. Designed ODS-IN718 alloy is produced by Selective Laser Melting (SLM) method with various power and velocity values to determine the best process parameters for this system. The SLM parameters are optimized for the production of ODS-IN718 alloy. To dissolve the non-equilibrium phases, which are detrimental to the mechanical properties, and maximize the number densities of nano-oxide and other strengthening phases of γ'/γ″, various heat treatment processes are applied based on the thermochemical calculations. The samples which are solutionized at 1050 °C for 1 h followed by aging at 650 °C for 5 h exhibit the best microstructural and mechanical properties. Tensile tests demonstrate that the strength and ductility of the solutionized and aged samples are much improved compared to the standard solutionized and double aged samples due to the fine and uniform microstructure, especially at elevated temperatures. It is observed that experimental results are consistent with the thermochemical modeling calculations. This clearly shows the promise of thermochemical calculations incorporating various databases for designing new alloy systems with nano-oxide strengthening.
We address the route planning problem of an unmanned air vehicle (UAV) tasked with collecting information from a radar-monitored environment for a reconnaissance mission. The UAV takes off from a home base, visits a set of targets, and finishes its movement at a final base. Collectable information at a target depends on the time the target is visited by the UAV. There are multiple trajectory alternatives between the target pairs with different travel time and threat attributes. A route plan involves the selection of the targets to visit, the order of visit to the targets, and the trajectories to follow between the targets. Multiple routing objectives, information collection, mission duration and mission safety, are considered to present the trade-offs among the objectives to the route planner. The problem is classified as a multi-objective orienteering problem with time-dependent prizes and multiple connection options. A mixed integer programming model that can be used for small-sized problems is formulated. Larger problems are addressed with a hybrid algorithm involving heuristics and exact approaches. A case study based on a terrain in the State of Colorado is presented. Finally, some practical issues for the UAV route planning problem is discussed.
This study presents a novel approach to account for the soil nonlinearity of nonhomogeneous soil deposits in foundation deflection analyses in the context of a modified Vlasov foundation model. We present an extension of the previously proposed formulation by developing a new formulation employing an improved algorithm that takes the modulus degradation curves at varying strain levels into account in an iterative manner. This new model, which takes the nonlinear soil behavior into account, was first verified against a linear elastic soil model given in the literature to ensure that the new model algorithm can capture the original solution when the soil behavior is assumed to be linear elastic. Later, the experimental data reported in the literature for a specific type of dense and loose sands were used in the example analyses. Example problems were considered for different cases, which presented (i) how the model captures nonlinear behavior and (ii) the significant effect of the nonlinear soil behavior. The result of the new model was also compared with the finite element model results, assuming elastoplastic soil. The results obtained from both models match well, especially for the maximum deflection value, provided that laterally constrained sections underneath the foundation are used.
Optical characterization of PbWO4 single crystals grown by Czochralski method was achieved in virtue of spectroscopic ellipsometry experiments carried out in the energy region of 1.0–5.6 eV at room temperature. Tetragonal scheelite structure with lattice parameters of a = b = 5.4619 Å and c = 12.0490 Å was determined for the bulk crystal utilizing from XRD analysis. Analyses of the ellipsometry data presented the photon energy dependencies of complex dielectric function of the crystal. The real part of the dielectric function exhibited increasing behavior with energy in the below 4.1 eV and then decreased immediately. Zero frequency refractive index and dielectric constant were determined to be 2.02 and 4.08, respectively, using Wemple and DiDomenico oscillator model. High frequency dielectric constant was calculated as 4.30 by Spitzer-Fan model. Optical band gap of PbWO4 was found to be 3.24 eV from the dielectric relaxation time spectrum. Moreover, existence of two critical points with energies of 3.70 and 4.58 eV was revealed from the analyses of extinction coefficient and second derivative of the dielectric function. These levels were considered to be due to creation of cation exciton (Pb²⁺ 6s² − Pb²⁺ 6s6p) and transitions in the [WO4]²⁻ group.
Bi12SiO20 single crystals grown by Czochralski method were optically investigated in detail in the present paper. Transmission and reflection measurements were performed at room temperature in the 400–800 nm spectral range on the (111) plane. Linear and nonlinear optical characteristics of the Bi12SiO20 crystal were determined analyzing the transmission and reflection spectra. The spectral dependencies of absorption coefficient, skin depth, refractive index, optical and electrical conductivities, real and imaginary parts of dielectric function were plotted. Analyses of optical parameters presented the direct band gap and Urbach energies as 2.55 and 0.33 eV, respectively. Static refractive index and dielectric constant, oscillator strength, dispersion and single oscillator energies, nonlinear refractive index, first- and third-order nonlinear susceptibilities were revealed. Structural properties of the Bi12SiO20 crystal were investigated by x-ray diffraction and scanning electron microscopy measurements.
The mutually exclusive authoritarianisms (i.e., nationalist, secular, and elitist vs. religious-Islamist) in West Asia (Middle East) do not correspond to the political structures in Euro-American settings (i.e., one traditional/conservative authority). This distinction suggests that the relationships of RWA and SDO with political ideologies and generalized prejudice might differ from the predictions of the dual-process motivational model of ideology and generalized prejudice (DPM). To investigate this idea, we used the DPM’s structure with culture-sensitive hypotheses in the light of the socio-cultural work on ideologies and social dynamics in Turkey. In Study 1 (N = 259), belief in a competitive jungle positively predicted SDO, and belief in a dangerous world did not predict RWA. RWA negatively predicted left-wing and positively predicted right-wing ideology. SDO negatively predicted left-wing ideology but did not predict right-wing ideology. In Study 2 (N = 312), the relations between worldviews and attitudinal orientations replicated. Further, we developed a context-specific (i.e., Turkey) generalized prejudice measure to test the hypotheses associated with intergroup prejudice, which had a factor structure compatible with the DPM. SDO predicted prejudice toward derogated groups positively and dangerous groups negatively, and it was not relevant to the attitudes toward dissident groups in Turkey. RWA predicted prejudice toward dissident and dangerous groups positively, and derogated groups negatively. These two studies showed the importance of analyzing political ideology and intergroup prejudice from a cultural-ecological perspective by revealing the contextual characteristics constructing social relations.
Coal fly ash (CFA) and coal bottom ash (CBA) are coal combustion's primary solid waste products. This solid waste management is a substantial challenge for power plants and local authorities. In order to apply coal ash as a pavement base material, the relationship of the mixture's mechanical properties with structural design parameters for pavement design and analysis must be explored. No study has investigated the mechanical behavior of the 100% coal ash cement stabilized material (CACSM) as a pavement base layer. Therefore, this study aimed to investigate the structural layer coefficient of CACSM used in the American Association of State Highway and Transportation Officials (AASHTO) 1993 pavement design guide. The study consisted of laboratory material characterization and field evaluation. The mechanical property test revealed that CACSM has a self-healing ability; coal ash appeared to influence the secondary curing of the specimens significantly. Strength towards the ends of stage I and stage II showed the influence of high pozzolanic on the CACSM, which resulted in improved long-term performance. The layer coefficients of the locations in the field test ranged from 0.22 to 0.24. By contrast, for the granular base and cement-treated base, these values were 0.07 to 0.14 and 0.17-0.20, respectively. The study findings show that CACSM as a pavement base is a viable solution that can help develop transportation infrastructure with enhanced concrete durability and lower carbon footprint emissions.
To ensure road safety and reduce traffic accidents, it is essential to determine geographical locations where traffic accidents occur the most. Spatial clustering methods of hot spots are used very effectively to detect such risky areas and take precautions to minimize or even avoid fatal or injury accidents. This study aims to determine speed-related hot spots in the pilot Mersin Region, which includes seven cities in the central-southern part of Turkey. Two different hot spot clustering methods, the Nearest Neighbourhood Hierarchical Clustering Method (NNH) and Density-Based Spatial Clustering of Applications with Noise (DBSCAN) Method, were employed to analyse traffic accident data between 2014-2021, obtained from the General Directorate of Highways. CrimeStat III program, which is free software, was used to detect NNH clusters, while the DBSCAN clusters were obtained using the open-source GIS program QGIS, which was also used to visualize and evaluate the results comparatively. As a result of the analysis, it was determined which method gave more effective results in determining the traffic accident risk clusters. These clusters were analysed based on road geometries (intersection or corridors). In addition, by considering the areas where repeated accidents have occurred over the years, future predictions of traffic accidents have been estimated.
Institution pages aggregate content on ResearchGate related to an institution. The members listed on this page have self-identified as being affiliated with this institution. Publications listed on this page were identified by our algorithms as relating to this institution. This page was not created or approved by the institution. If you represent an institution and have questions about these pages or wish to report inaccurate content, you can contact us here.
19,017 members
Huseyin Hacihabiboglu
  • Graduate School of Informatics
Mustafa Kurban
  • Department of Physics
Osman Yavuz Ataman
  • Department of Chemistry
Yavuz Topkaya
  • Department of Metallurgical and Materials Engineering
Üniversiteler Mah., Eskişehir Yolu No:1, 06800, Çankaya, Ankara, Turkey
Head of institution
Mustafa Verşan Kök