Makale Koleksiyonu
Bu koleksiyon için kalıcı URI
Güncel Gönderiler
Öğe Investigation of the network made with the complementary motor of the thalamus with the dti method in individuals with tinnitus(2024) Elpen Karyemez, Pınar; Yağımlı, Mustafa; Tozan, HakanObjectives: This study aimed to investigate the differences in the networks between the thalamus and the supplementary motor area in brain MRI examinations of individuals with subjective chronic tinnitus and healthy control groups. Methods: In the study, brain MRI imaging was performed on 47 patients suffering from bilateral tinnitus for two years or more and 42 healthy individuals. The tensor information obtained using the diffusion tensor imaging sequence data obtained from both groups was processed in the DSI Studio program. Brain connection maps were obtained from the data obtained. Quantitative information of the individual connecogram maps of each case was recorded and the groups were compared. Results: In the tinnitus group, there is an increase in the pathways between the thalamus and the supplementary motor area. In addition, it was determined that the pathways of the thalamus with the hippocampus, superior and middle occipital area and frontal area increased, and the pathways with the hippocampus and cerebellum decreased. Conclusion: Important functions of the supplementary motor area and thalamus; These are cognitive actions in which attention plays an important role, such as noticing the stimulus from the environment, planning and implementing the response action. It is thought that changes in the pathways between the thalamus and the supplementary motor area will especially affect attention-related functions.Öğe Non-destructive covalent surface alkylation of graphitic carbon nitride for enhanced photocatalytic dye degradation in water(Elsevier B.V., 2024) Arat, Refik; Fidan, Tuçe; Yüce, Meral; Kurt, Hasan; Bayazıt, Mustafa KemalGraphitic carbon nitride (g-CN) is a promising material for various applications due to its unique electronic, optical, and photocatalytic properties, tunable by surface modifications. Herein, a novel and straightforward approach to the covalent addition of low molecular weight polyethylene glycol (PEG550) to g-CNs surface following non-destructive chemistry benefiting from simultaneous activation of hydroxyl and free-amine surface groups by a weak base, potassium carbonate, is for the first time described. The resulting g-CN-PEG550 exhibits almost two-fold enhanced water solubility due to 1 PEG550 chain addition for every ? 128 g-CN atoms, detected by thermogravimetric analysis. Complementary X-ray photoelectron spectroscopy elemental analysis of the isolated g-CN-PEG550 displays an increased C?O chemical environment attributed to the covalent addition of carbon- and oxygen-rich PEG550 to the g-CN surface. The g-CN-PEG550 photocatalyst performs 2.5-fold better in degrading rhodamine B due to its enhanced light absorption, improved water-dispersibility, and the efficient separation of photogenerated electron-hole pairs compared to the as-prepared g-CN. The study underscores the potential use of covalently PEGylated oxygen-rich g-CNs in photocatalytic applications.Öğe Plasmonic titanium nitride nanohole arrays for refractometric sensing(American Chemical Society, 2023) Günaydın, Beyza Nur; Gülmez, Mert; Torabfam, Milad; Pehlivan, Zeki Semih; Tütüncüoğlu, Atacan; Kayalan, Cemre Irmak; Saatçioğlu, Erhan; Bayazıt, Mustafa Kemal; Yüce, Meral; Kurt, HasanGroup IVB metal nitrides have attracted great interest as alternative plasmonic materials. Among them, titanium nitride (TiN) stands out due to the ease of deposition and relative abundance of Ti compared to those of Zr and Hf metals. Even though they do not have Au or Ag-like plasmonic characteristics, they offer many advantages, from high mechanical stability to refractory behavior and complementary metal oxide semiconductor-compatible fabrication to tunable electrical/optical properties. In this study, we utilized reactive RF magnetron sputtering to deposit plasmonic TiN thin films. The flow rate and ratio of Ar/N2 and oxygen scavenging methods were optimized to improve the plasmonic performance of TiN thin films. The stoichiometry and structure of the TiN thin films were thoroughly investigated to assess the viability of the optimized operation procedures. To assess the plasmonic performance of TiN thin films, periodic nanohole arrays were perforated on TiN thin films by using electron beam lithography and reactive ion etching methods. The resulting TiN periodic nanohole array with varying periods was investigated by using a custom microspectroscopy setup for both reflection and transmission characteristics in various media to underline the efficacy of TiN for refractometric sensing.Öğe Integrated entropy-EDAS methods for the electrified car selection problem(Kocaeli University, 2022) Çaloğlu Büyükselçuk, Elif; Tozan, HakanIncreasing air pollution affects the environment and life negatively. For a sustainable environment and life, people, voluntary organizations, and governments need to work on the solution of this problem. The biggest sources of air pollution are transportation vehicles. For this reason, many countries in Europe have stated that they will use solely electrified cars to reduce air pollution in the future. Therefore, in this study, it is aimed to determine the best electrified car. The result obtained can support consumers that to intend to buy an electrified vehicle in the decision-making process. This problem is a typical multi-criteria decision making (MCDM) problem and some MCDM techniques are used to solve these problems. Here, the Entropy method was used to determine the weights of the selection criteria. Selection criteria was determined according to comprehensive literature survey and interviews with sales representatives. The EDAS (Evaluation based on Distance from Average Solution) method was used to rank the electrified car alternatives that sold in Turkey. As a result of the evaluation, the most important criteria was determined by the price of the vehicle, the net battery capacity, and the electric motor power. According to these criteria, the electrified car manufactured in China was chosen as the best.Öğe An integrated performance measurement framework for restaurant chains: A case study in Istanbul(TMMOB Chamber of Mechanical Engineers, 2022) Pınarbaşı, Ayşegül; Aydın, Umut; Karadayı, Melis Almula; Tozan, HakanCompanies that continue to operate in a competitive market strive the most efficient use of their resources in order to remain competitive. Nowadays, with increasing customer feedback, properly analyzing customer needs and requests and producing services in accordance with expectations have become increasingly important due to the large number of companies competing in the same market, and this is especially important to be at the forefront of competitors in the food services industry. There are risks and uncertainties owing to the continuously changing demand for food service enterprises, the difficulty to regulate interest and comparable charges, the competitive environment, and currency rate hikes. In light of all of these circumstances, restaurants require a versatile tool to effectively measure and analyze their performance. Therefore, this study combines Principal Component Analysis (PCA) and Categorical Data Envelopment Analysis (CAT-DEA) to analyze the performance of 15 dealers in Istanbul, divided into three categories: steakhouse, kebab, and meatball-doner. The results demonstrate that each category has just one efficient restaurant, for a total of three efficient restaurants out of fifteen. In addition to the suggested CAT-DEA-based framework, three research hypotheses are constructed and analyzed to investigate the link between restaurant performance and various environmental factors (or relevant indicators) in the food service industry.Öğe How do airlines survive? An integrated efficiency analysis on the survival of airlines(Elsevier Ltd, 2023) Kaya, Gizem; Aydın, Umut; Ülengin, Burç; Karadayı, Melis Almula; Ülengin, FüsunAirline managers must prepare effective strategies to cope with fluctuations and enhance the performance and competitiveness of their companies. Airline performance evaluation also requires a multidimensional perspective that focuses on both financial and nonfinancial indicators. In this study; a two-stage super-efficiency data envelopment analysis (DEA) was used to better understand the sources of inefficiency, and an enhanced performance evaluation from five different dimensions was conducted by integrating the sustainability balanced scorecard and two-stage DEA approaches. The results indicated that Aeromexico and Icelandair had the highest efficiency scores among the 35 airlines, and they were labeled as efficient with respect to the customer, finance, internal process, learning and growth, and environmental dimensions of the SBSC. Finally; a regression analysis based on the Simar–Wilson method shows that an increase in the number of wide-body aircraft and return on assets negatively affects efficiency, net carrier airlines are more efficient than low-cost airlines, and airlines that are not members of any alliance are more efficient.Öğe Online routing and searching on graphs with blocked edges(Springer, 2022) Shiri, Davood; Tozan, HakanWe study routing and searching decisions of a search-and-detection (SDT) team on a road network under online uncertainty setting. Given an undirected edge-weighted bounded graph, a static target is positioned at an unknown vertex among potential target vertices in the graph. A non-negative search cost is given on each of the potential target vertices. The target is detected once the SDT searches its corresponding vertex. There may be some non-recoverable online blockages in the graph, but the existence of blockages is unknown to the SDT initially. If a blockage exists in the graph, it is disclosed online once the SDT visits one of its end-vertices. The graph stays connected when the blockages are omitted from it. The SDT begins from a certain vertex and aims to identify a route without any blocked edges which detects the target with minimum total traveling and search cost. We analyze this problem from a competitive analysis perspective under two scenarios with and without blockages. For the scenario with blockages, we provide a tight lower bound on the competitive ratio of deterministic solutions, an optimal deterministic solution, a randomized solution with a bounded expected competitive ratio, together with lower and upper bounds on the expected competitive ratio of the optimal randomized solutions. For the scenario without blockages, we provide tight lower bounds as well as optimal deterministic and randomized solutions.Öğe Microwave-promoted continuous flow synthesis of thermoplastic polyurethane-silver nanocomposites and their antimicrobial performance(Royal Society of Chemistry, 2022) Saleem, Qandeel; Torabfam, Milad; Kurt, Hasan; Yüce, Meral; Bayazıt, Mustafa KemalThermoplastic polyurethane-silver nanocomposites (PU-Ag NCs) have considerable potential in many medical applications due to their superior mechanical and antimicrobial properties. Herein, a microwave-promoted flow system is successfully employed for continuous in situ manufacturing of PU NCs having spherical silver nanoparticles (AgNPs) without any reducing agent at similar to 40 degrees C in approximately 4 minutes. The main experimental parameters, including microwave power, metal salt concentration, polymer concentration, and flow rate, are optimised for the reproducible synthesis of AgNPs (similar to 5 nm) in the PU matrix, characterised by HRTEM-EDS and DLS analysis. XRD patterns indicate an increase in PU crystallinity with decreased particle size. Conventional heating flow synthesis at similar to 50 degrees C or microwave-batch synthesis (MWB) at similar to 44 and similar to 50 degrees C is ineffective in preparing AgNPs, and only large AgNPs (>100 nm) are synthesised at 70 degrees C in the MWB reactor. PU-Ag NC films bearing small AgNPs (similar to 5 nm) exhibit superior antibacterial activity (>97%) against Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus compared to large NPs (similar to 218 nm). The proposed method may manufacture other metal-polymer matrix composites.Öğe A microwave-powered continuous fluidic system for polymer nanocomposite manufacturing: A proof-of-concept study(Royal Society of Chemistry, 2022) Torabfam, Milad; Nejatpour, Mona; Fidan, Tuçe; Kurt, Hasan; Yüce, Meral; Bayazıt, Mustafa KemalContinuous manufacturing of pure nanocrystals with a narrow size distribution in a polymer matrix is very challenging, although it is highly crucial to get their full potential for advanced applications. A long-lasting nanocomposite (NC) manufacturing challenge is, for the first time, overcome by a microwave-powered fluidic system (MWFS). The effect of microwave power (MWP), flow rate, and the concentration of the reagents are systematically studied. The nylon-6 NC bearing evenly distributed silver nanoparticles (AgNPs) with a mean size of similar to 2.59 +/- 0.639 nm is manufactured continuously in similar to 2 min at similar to 50-55 degrees C using a green solvent, formic acid. The AgNP size becomes smaller when increasing the polymer concentration gradually. Small NPs with a narrow size distribution are produced at high MWP (40 W), but large ones with a broad size distribution at low MWP (10 W). The nylon-6 crystallinity is NP size-dependent, and the gamma-phase (pseudo-hexagonal crystal) is dominant in the presence of small NPs as against the large counterparts. Given the small-sized AgNPs in the MWF-manufactured NCs, the antibacterial activity tests with Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa show superior activity compared to that of the large AgNP-bearing (similar to 50 nm) NCs produced in a conventional heating fluidic system. The proposed MWFS can manufacture other added-value NCs continuously.Öğe Integrated methodology for evaluating the efficiency of airports: A case study in Turkey(Elsevier Ltd, 2022) Kaya, Gizem; Aydın, Umut; Karadayı, Melis Almula; Ülengin, Füsun; Ülengin, Burç; İçken, AyhanAirports play an important role in the economic development of countries. Efficiency analyses of airports are crucial for providing an important roadmap to airport authorities and for helping to benchmark their airports with respect to a role model. Accordingly, in this study, an integrated analysis comprising a super-efficient data envelopment analysis (DEA) and an eigenvector centrality analysis is conducted. Additionally, a Simar–Wilson bootstrapping approach was used to identify the important factors influencing this ef?ciency. As a case study, the efficiencies of Turkey's 39 international airports were analyzed, and strategic recommendations were suggested. According to the results of the super-efficient DEA analysis and eigenvector centrality analysis, Istanbul Atatürk (closed as of 2019), Adana, and Diyarbakır airports are the “shining stars.” These airports are operationally efficient and act as role models for other airports. Moreover, the hypothesis testing conducted based on the Simar–Wilson bootstrapping approach shows that the efficiency of airports varies by region. The efficiency of airports in the Mediterranean region, which is used as a reference, was found to be significantly higher than that of airports in Eastern Anatolia, Southeast Anatolia, Central Anatolia, and Marmara regions. There also exists a difference between the efficiencies of private and public terminal operations in the favor of public terminal operations. The efficiency of an airport is also positively influenced by the tourist population and by an increase in the gross domestic product per capita of the corresponding city.Öğe An MCDM-based game-theoretic approach for strategy selection in higher education(Elsevier Science Inc., 2022) Ekinci, Yeliz; Orbay, Benan Zeki; Karadayı, Melis AlmulaThis study proposes a framework for universities and governments to select strategies by considering the strategic interactions. The strategic choices of universities and governments can be determined by analyzing the related literature and discussing it with experts in higher education (HE). Because these experts form their evaluations depending on various criteria, the outputs of multi-criteria decision-making (MCDM) models are used to determine payoff values for players by considering all strategic combinations. After constructing the payoff matrix, the Nash equilibrium concept of game theory is used to determine optimal strategies for the universities and governments for simultaneously played games. Sequential versions of the games are also analyzed using backward induction. The results show that in all games constructed using criteria with different weights, either the government or the university, or both, preferred to motivate high-quality academic research. The proposed methodology can be used by the policymakers in the higher education area, both by the central planners (usually the government) and the universities.Öğe A novel fuzzy framework for technology selection of sustainable wastewater treatment plants based on TODIM methodology in developing urban areas(Nature Research, 2022) Eseoğlu, Güneş; Yapsaklı, Kozet; Tozan, Hakan; Vayvay, ÖzalpOptimal technology selection of wastewater treatment plants (WWTPs) necessitates the adoption of data-driven scientific approaches that satisfy the sustainability requirements of the urban ecosystem. Such approaches should be able to provide actionable insights to decision makers constrained by factors such as population growth, land scarcity, and loss of functionality of wastewater treatment plants. The framework in this study proposes a hybrid fuzzy multi-criteria decision making (MCDM) model consisting of the analytical hierarchy process (AHP) and the TODIM (an acronym in Portuguese of interactive and multi-criteria decision-making) by using alpha cut series which takes into account the risk aversion of decision makers (DMs) to overcome uncertainties of environmental conditions. The literature to date indicates that the study is the first to presents how a systematic decision-making process is approached by interpreting the interaction of criteria for the selection of wastewater treatment technology through the membership function of Prospect Theory. The proposed methodology reveals that the prominent reference criterion manipulates other sub-criteria according to the function of risk-aversion behavior. The fuzzy sets based on alpha cut series are employed to evaluate both the criteria weight and the rank of the alternatives in the decision-making process to obtain compromise solutions under uncertainty. The dominance degrees of the alternatives are achieved by fuzzy TODIM integrated with the fuzzy analytic hierarchy process (FAHP) which deals with the uncertainty of human judgements. According to the ranking results determined by the dominance degree of alternatives, anaerobic-anoxic-oxic (A2O) without pre-clarification was the most effective process in relation to the sludge disposal cost (C25) calculated as reference criteria. The ranking of four full-scale WWTPs in a metropolitan city of an EMEA country based on 24 sub-criteria listed under the four main criteria, namely the dimensions of sustainability, is used as a case study to verify the usefulness of the fuzzy approach. Motivated by the literature gap related to the failure to consider the psychological behavior of DMs in technology selection problem for wastewater treatment, it is discussed how the proposed hybrid MCDM model can be utilized by reflecting human risk perception in wastewater treatment technology selection for developing urban areas.Öğe A multi-criteria decision-making approach for greenovative supplier selection(University of Cincinnati, 2022) Çaloğlu Büyükselçuk, Elif; Tozan, Hakan; Vayvay, ÖzalpIn today's rapidly changing business environment, green and innovative (greenovative) activities have become indispensable elements of sustainable supply network management. Realization of this fact obliges firms to consider greenovative as well as traditional criteria in determining their supplier. This study provides a new greenovative systematic approach to supplier selection for small and medium-sized enterprises. Fuzzy multi-criteria decision-making (FMCDM)-based techniques were used to determine the most appropriate supplier with the proposed model. To show the usability of the model, an application was carried out on an automotive supply company. Analytic Hierarchy Process (AHP) and Fuzzy Analytic Hierarchy Process (FAHP) approaches were used to calculate the weights of the supplier selection criteria. After determining criteria weights, different multi-criteria decision-making (MCDM) techniques that are often encountered in the literature were used to identify the best supplier.Öğe Integrated supply chain scheduling models: A literature review(2019) Ceylan, Zeynep; Bulkan, Serol; Tozan, HakanResearch on integration of supply chain and scheduling is relatively recent, and number of studies on this topic is increasing. This study provides a comprehensive literature survey about Integrated Supply Chain Scheduling (ISCS) models to help identify deficiencies in this area. For this purpose, it is thought that this study will contribute in terms of guiding researchers working in this field. In this study, existing literature on ISCS problems are reviewed and summarized by introducing the new classification scheme. The studies were categorized by considering the features such as the number of customers (single or multiple), product lifespan (limited or unlimited), order sizes (equal or general), vehicle characteristics (limited/sufficient and homogeneous/heterogeneous), machine configurations and number of objective function (single or multi objective). In addition, properties of mathematical models applied for problems and solution approaches are also discussed.Öğe Field-dependent charge collection model for thin film organic photovoltaics(Bingöl Üniversitesi, 2020) Kurt, HasanIn this study, a unified charge collection model is developed using an optical cavity dependent charge carrier generation and non-uniform built-in electric field distribution within a bulk heterojunction photovoltaic device. The charge collection model relies on the experimental inputs related to the charge carrier dynamics such as mobilities of charge carriers, recombination lifetime, and junction width of charge carrier species. Optical cavity modes and field strength were calculated using the experimental variable angle ellipsometry analysis of individual components of the devices. In order to evaluate the model, ambient processed PCDTBT:PC71BM based conventional and inverted derive architectures were utilized to underline the effect of unintentional doping and distinct optical cavity modes. The simulated external quantum efficiency and shortcircuit current density profiles from the model were compared to the experimental results with differing active layers thicknesses and device architectures. The proposed charge collection model presented a high degree of correlation with the experimental results and underlined its validity for further application on other types of organic photovoltaic devices.Öğe Experimental measurement of a UAV propeller's thrust(University of Osijek, 2022) Kosa, Patrik; Kisev, Marian; Vacho, Lukas; Toth, Ladislav; Olejar, Martin; Harnicarova, Marta; Valicek, Jan; Tozan, HakanAt present, there are several types of propellers in the field of the use of Unmanned Aerial Vehicles (UAVs) with unknown parameters, where it is necessary to provide information about their thrust, current consumption and maximal rotational speed (RPM). Commonly used methods for measurement of a propeller's thrust are mostly based on the usage of a single purpose system, on short measurements without data storage or on inaccurate sensors. The goal of this article is to develop a universal experimental measuring system for more accurate measurement of propeller's parameters (thrust, current consumption, maximal RPM). For more accurate measurement, the battery voltage, temperature and humidity of the environment were also measured. To acquire, measure and store the data safely on a micro SD card, a processing circuit based on an ATmega2560 microcontroller was developed. This innovative approach allowed to analyse the behaviour of the propeller and to measure the dependencies of the RPM on pulse width, of the current on RPM and of the thrust on RPM at different input conditions. The measurements have shown that the dependencies can be approximated by cubic functions. The mathematical description allows predicting the behaviour of the propeller in unmeasurable conditions.Öğe Application of a novel hybrid f-sc risk analysis method in the paint industry(MDPI, 2021) Hacıbektaşoğlu, Süleyman Enes; Mertoğlu, Bülent; Tozan, HakanOccupational health and safety (OHS) activities aim to ensure the full mental and physical well-being of employees in the workplace. For this reason, it is essential to determine the precautions to be taken and the suitability of risk assessments. In this study, f-SC, a novel hybrid risk analysis method using Step-Wise Weight Assessment Ratio Analysis (SWARA) and Complex Proportional Assessment (COPRAS) multi-criteria decision making methods (MCDM) based on fuzzy logic, was developed to perform a classical Fine–Kinney risk analysis method. There are few studies in the literature about the Fine–Kinney method compared to other risk analysis methods such as FMEA and FTA. Therefore, this work aimed to integrate this classical method with the proposed method to obtain more accurate and sensitive results in risk analyses. First, the criteria used in determining the risk score were weighted with the help of 10 OHS experts. As the criteria used in the classical method are evaluated with equal importance, this situation can cause serious errors in the risk scores obtained with the relevant calculations and in the risk priorities based on these calculations. We aimed to minimize the occurrence of such errors by determining the weights of the criteria with the proposed method. f-SWARA was used for this process. The weights of probability, exposure, and severity criteria were obtained as 0.196, 0.285, and 0.518, respectively. Thus, it was determined that severity is an important and effective criterion for calculating the risk score. In the proposed method, after the criterion weights were determined, an analysis of the hazards was conducted with the f-COPRAS method instead of the classical Fine–Kinney method. Contrary to the numerical values used in the classical method, in this method, decision makers use linguistic terms that are more intuitive than numerical values. These linguistic terms were converted into numerical values using this method based on fuzzy logic, and a ranking of hazards was obtained. As a result of the analyses, it was seen that the case study, H7, which had a 0.557 Ni value, was the most dangerous scenario and that H11, which had a 1.000 Ni value, was the least dangerous. In addition, for the same data, analyses were conducted using the fuzzy Vise Kriterijumska Optimizacija I Kompromisno Resenje (VIKOR) method, which has been previously used in the literature, and a comparison was made with the f-SC method to demonstrate the validity of the study. The results of the f-VIKOR and classical Fine–Kinney methods were similar to the developed f-SC method. This research provides three contributions: (1) criteria must be weighted to determine risk scores, (2) using intuitive linguistic terms in scoring criteria made the risk analysis method more sensitive and appropriate, and (3) using MCDM methods instead of classical methods for the risk analyses in the OHS field removes uncertainties.Öğe A coordinated scheduling problem for the supply chain in a flexible job shop machine environment(Springer Heidelberg, 2021) Ceylan, Zeynep; Tozan, Hakan; Bulkan, SerolIn this study, a new coordinated scheduling problem is proposed for the multi-stage supply chain network. A multi-product and multi-period supply chain structure has been developed, including a factory, warehouses, and customers. Furthermore, the flexible job shop scheduling problem is integrated into the manufacturing part of the supply chain network to make the structure more comprehensive. In the proposed problem, each product includes a sequence of operations and is processed on a set of multi-functional machines at the factory to produce the final product. Final products are delivered to the warehouses to meet customers' demands. If the demands of customers are not fulfilled, the shortage in the form of backorder may occur at any period. The problem is expressed as a bi-objective mixed-integer linear programming (MILP) model. The first objective function is to minimize the total supply chain costs. On the other hand, the second objective function aims to minimize the makespan in all periods. A numerical example is presented to evaluate the performance of the proposed MILP model. Five multi-objective decision-making (MODM) methods, namely weighted sum, goal programming, goal attainment, LP metric, and max-min, are used to provide different alternative solutions to the decision-makers. The performance of the methods is evaluated according to both objective function values and CPU time criteria. In order to select the best solution technique, the displaced ideal solution method is applied. The results reveal that the weighted sum method is the best among all MODM methods.Öğe An online optimization approach for post-disaster relief distribution with online blocked edges(Elsevier Ltd, 2022) Akbari, Vahid; Shiri, DavoodDisasters can disrupt road networks by blocking some of the roads and consequently impeding accessibility to critical locations. In the immediate post-disaster response phase, while the blockage information is yet to be collected, relief distribution crews (RDCs) should dispatch from warehouses to supply critical locations with first aid items. The RDCs are not capable of unblocking damaged roads and should find a way to bypass them once such edges are observed in their routes. With the objective of minimizing total latency of the critical nodes, we study the problem that addresses the relief distribution operations with k non-recoverable online blocked edges. The online blocked edges are not known to the RDCs initially and the blockage of a blocked edge is revealed when one of the RDCs arrives at one of its end-nodes. Once one of the RDCs knows about a blocked edge, this information is communicated among the rest of the RDCs and they will all be informed about that blocked edge. We first investigate the worst-case performance of online algorithms against offline optimal solutions using competitive ratio. We then prove a lower bound on the competitive ratio of deterministic online algorithms. We also provide an upper bound on the competitive ratio of the optimal deterministic online algorithms by introducing a deterministic algorithm which achieves a bounded competitive ratio. We then develop three heuristic algorithms to solve this problem. One of our algorithms is based on solving an Integer Programming model to find the assignment of the nodes to the RDCs and then finding the routes dynamically. The other algorithms are not based on solving optimization models and hence are more efficient in terms of their computational time. We compare our proposed heuristic algorithms with the best known algorithms from the literature that are developed for a single RDC variation of the problem. Finally, we provide a through computational analysis of our algorithms on instances adopted from real-world road networks.Öğe An online optimization approach to post-disaster road restoration(Elsevier Ltd, 2021) Akbari, Vahid; Shiri, Davood; Salman, F. SibelNatural disasters impact transportation networks adversely and cause road sections to be damaged or blocked. The road network may even become disconnected, impeding accessibility between disaster-stricken areas and critical locations such as hospitals, relief aid depots and transportation hubs. In the immediate response phase, a set of blocked edges should be selected and restored to reconnect the transportation network. While locations of the disrupted roads can be identified using drone or satellite images, an accurate estimation of time to restore a road segment can be carried out only after expert observations on the field. In this article, we study a post-disaster road restoration problem modeled on an undirected edge-weighted graph with k blocked edges, where the unblocking time of a blocked edge is revealed online once the road restoration team visits an end-node of that blocked edge. The objective is to minimize the time at which the road network is reconnected. We first investigate the worst-case performance of online algorithms against offline optimal solutions by means of the competitive ratio. We prove that any online deterministic algorithm cannot achieve a competitive ratio better than 2k?1. We also provide an optimal online algorithm that is proven to achieve this lower bound. In addition, to achieve good performance on realistic instances, we implement an algorithm that solves a mixed integer programming model each time new information is revealed. Since model solution is prohibitively time-consuming, we also propose a novel polynomial time online algorithm. We compare these two algorithms with two other benchmark online algorithms on both Istanbul road network instances and several other city instances from the literature. Our experiments show that the proposed polynomial time online algorithm performs superior to the benchmark ones and obtains solutions close to the offline optimum on all the tested instances.
