Browsing by Subject "Operations research"
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Item Open Access Age-based vs. stock level control policies for a perishable inventory system(2001) Tekin, E.; Gürler Ü.; Berk, E.In this study, we investigate the impact of modified lotsize-reorder control policy for perishables which bases replenishment decisions on both the inventory level and the remaining lifetimes of items in stock. We derive the expressions for the key operating characteristics of a lost sales perishable inventory model, operating under the proposed age-based policy, and examine the sensitivity of the optimal policy parameters with respect to various system parameters. We compare the performance of the suggested policy to that of the classical (Q,r) type policy through a numerical study over a wide range of system parameters. Our findings indicate that the age-based policy is superior to the stock level policy for slow moving perishable inventory systems with high service levels.Item Open Access An algorithm based on facial decomposition for finding the efficient set in multiple objective linear programming(Elsevier, 1996) Sayın, S.We propose a method for finding the efficient set of a multiple objective linear program based on the well-known facial decomposition of the efficient set. The method incorporates a simple linear programming test that identifies efficient faces while employing a top-down search strategy which avoids enumeration of efficient extreme points and locates the maximally efficient faces of the feasible region. We suggest that discrete representations of the efficient faces could be obtained and presented to the Decision Maker. Results of computational experiments are reported.Item Open Access A branch and price approach for routing and refueling station location model(Elsevier, 2016) Yıldız, B.; Arslan, O.; Karaşan, O. E.The deviation flow refueling location problem is to locate p refueling stations in order to maximize the flow volume that can be refueled respecting the range limitations of the alternative fuel vehicles and the shortest path deviation tolerances of the drivers. We first provide an enhanced compact model based on a combination of existing models in the literature for this relatively new operations research problem. We then extend this problem and introduce the refueling station location problem which adds the routing aspect of the individual drivers. Our proposed branch and price algorithm relaxes the simple path assumption generally adopted in the existing studies and implicitly takes into account deviation tolerances without the pregeneration of the routes. Therefore, the decrease in solution times with respect to existing models is significant and our algorithm scales very efficiently to more realistic network dimensions.Item Open Access A characterization of the optimal set of linear programs based on the augmented lagrangian(Taylor & Francis, 1999) Pınar, M. Ç.It is proved that in a certain neighborhood of the optimal set of multipliers, the set of minimizers of the augmented lagrangian nmction generates a new characterization of the optimal solution set of the linear program.Item Open Access Componentwise bounds for nearly completely decomposable Markov chains using stochastic comparison and reordering(Elsevier, 2005) Pekergin, N.; Dayar T.; Alparslan, D. N.This paper presents an improved version of a componentwise bounding algorithm for the state probability vector of nearly completely decomposable Markov chains, and on an application it provides the first numerical results with the type of algorithm discussed. The given two-level algorithm uses aggregation and stochastic comparison with the strong stochastic (st) order. In order to improve accuracy, it employs reordering of states and a better componentwise probability bounding algorithm given st upper- and lower-bounding probability vectors. Results in sparse storage show that there are cases in which the given algorithm proves to be useful. © 2004 Elsevier B.V. All rights reserved.Item Open Access Data dependent worst case bound improving techniques in zero-one programming(Elsevier BV, 1991) Oğuz, OsmanA simple perturbation of data is suggested for use in conjunction with approximation algorithms for the purpose of improving the available bounds (upper and lower), and the worst case bounds. The technique does not require the approximation algorithm (heuristic) to provide a worst case bound to be applicable.Item Open Access Duality, area-considerations, and the Kalai–Smorodinsky solution(Elsevier, 2017) Karagözoğlu, E.; Rachmilevitch, S.We introduce a new solution concept for 2-person bargaining problems, which can be considered as the dual of the Equal-Area solution (EA) (see Anbarcı and Bigelow (1994)). Hence, we call it the Dual Equal-Area solution (DEA). We show that the point selected by the Kalai–Smorodinsky solution (see Kalai and Smorodinsky (1975)) lies in between those that are selected by EA and DEA. We formulate an axiom–area-based fairness–and offer three characterizations of the Kalai–Smorodinsky solution in which this axiom plays a central role. © 2016 Elsevier B.V.Item Open Access Dynamic lot sizing and tool management in automated manufacturing systems(Elsevier, 2002) Aktürk, M. S.; Önen, S.The overall aim of this study is to show that there is a critical interface between the lot sizing and tool management decisions, and these two problems cannot be viewed in isolation. We propose "ve alternative algorithms to solve lot sizing, tool allocation and machining conditions optimization problems simultaneously. The "rst algorithm is an exact algorithm which "nds the global optimum solution, and the others are heuristics equipped with a look-ahead mechanism to guarantee at least local optimality. The computational results indicate that the amount of improvement is statistically signi"cant for a set of randomly generated problems. The magnitude of cost savings is dependent on the system parameters.Item Open Access Effect of load, processing time and due date variation on the effectiveness of scheduling rules(Taylor & Francis, 2002) Lejmi, T.; Sabuncuoğlu, İ.In real manufacturing environments, variations in production factors (i.e. pro- cessing time, demand, due-dates) are inevitable facts. All these dynamic changes, together with random disturbances (e.g. machine breakdowns) can seriously a ect the system performance. In this paper we focus on load, processing time and due date variation and analyse their impacts on a scheduling system. Speci®cally, we investigate the impact of variation on dispatching policies in a job shop environ- ment via simulation. The statistical analysis of the results leads to two major conclusions: ®rst, the relative performance of rules is not threatened much by PV (processing time variation), LV (load variation) or DDV (due date variation) Ð a result that can be a consolation for practitioners in the ®eld. Secondly, the performance of the rules deteriorates, in particular at high levels of PV, LV and DDV Ð a result that can provide new insights into the problem and produces useful information for researchers in their continuous e ort to develop better dispatching rules.Item Open Access Efficiency of the Turkish stock exchange with respect to monetary variables: a cointegration analysis(Elsevier BV, 1996) Muradoglu, Y. G.; Metin, K.In this study, we test the semistrong form of the efficient market hypothesis in Turkey by using the recently developed techniques in time series econometrics, namely unit roots and cointegration. The long run relationship between stock prices and inflation is investigated by assuming the possible existence of a proxy effect. Conclusions are made as to the efficiency of the Turkish Stock Exchange and its possible implications for investors. To our knowledge, this is among the pioneering studies conducted in an emerging market that uses an updated econometric methodology to allow for an analysis of long run steady state properties together with short run dynamics.Item Open Access Endogenous effects of hubbing on flow intensities(Springer, 2016) Taner, M. R.; Kara, B. Y.Location of hub facilities and the allocation decisions in transport networks endogenously affect both the flow intensities and the transportation costs. Since the introduction of the hub location problem to the operations research literature in mid-1980s, many researchers investigated different ways of modelling the effects of hub facilities on the transportation costs. On the other hand, there has been very limited research on their effect on the flow intensities. This study proposes a new approach, inspired by the Bass diffusion model, to forecast the change in the demand patterns generated at different locations as a result of the placement of new hubs. This new model is used in the context of the uncapacitated single allocation p-hub median problem to investigate the effects of endogenous attraction, caused by the spatial interaction of present hubs, on future hub location decisions. Computational results indicate that the location and allocation decisions may be greatly affected when these forecasts are taken into account in the selection of future hub locations.Item Open Access An equipment replacement problem with dynamic production planning and capacity considerations(Taylor & Francis, 1996) Gill, A.; Adil, G. K.; Bector, C. R.In this paper, we propose a 0-1 integer programming formulation for an equipment replacement problem in which the demand of a product varies from period to period and different types of equipment with different production capacities are available. Since all the variables involved in the model are restricted to have binary values, the model is fairly efficient and hence can be used for large-sized problems. The use of the model is demonstrated with the help of some numerical examples.Item Open Access Evaluation of mobilization and deployment plan of an armored battalion(Military Operations Research Society, 2005) Müslüm, S.; Sabuncuoğlu, I.Being ready for war as soon as possible and with minimum casualties during the crises is the first mission of the troops. Mobilization and deployment plans arise as the most important tools for Army as they cover all the activities that troops must execute to respond against enemy immediately. In this paper, we study the performance of the mobilization and deployment plans of a Turkish armored battalion via simulation. The proposed simulation model is developed for military operation planners to analyse mobilization and deployment operation of troops early in decision process; perform bottleneck analysis and take necessary actions for the main problem areas. The proposed model can also reduce the risk of military operations before conducting them in actual war conditions. Moreover, it is used to identify the significant factors of enemy threat, detect the most hazardous region and the most hazardous factor for each region, and discover the system boundaries. The simulation model is developed using ARENA simulation system. The output of the model is analysed by the experimental design and ranking/selection procedures. A related bibliography is also provided in the paper.Item Open Access An evaluative study of operation grouping policies in an FMS(Kluwer Academic Publishers, 2003) Sabuncuoǧlu, İ.; Lahmar, M.The increased use of flexible manufacturing systems to provide customers with diversified products efficiently has created a significant set of operational challenges for managers. This technology poses a number of decision problems that need to be solved by researchers and practitioners. In the literature, there have been a number of attempts to solve design and operational problems. Special attention has been given to machine loading problems, which involve the assignment of job operations and allocation of tools and resources to optimize specific measures of productivity. Most existing studies focus on modeling the problem and developing heuristics in order to optimize certain performance metrics rather than on understanding the problem and the interaction between the different factors in the system. The objective of this paper is to study the machine loading problem. More specifically, we compare operation aggregation and disaggregation policies in a random flexible manufacturing system (FMS) and analyze its interaction with other factors such as routing flexibility, sequencing flexibility, machine load, buffer capacity, and alternative processing-time ratio. For this purpose, a simulation study is conducted and the results are analyzed by statistical methods. The analysis of results highlights the important factors and their levels that could yield near-optimal system performance.Item Open Access A general theory on spectral properties of state-homogeneous finite-state quasi-birth-death processes(Elsevier, 2001) Fadıloğlu, M. M.; Yeralan, S.In this paper a spectral theory pertaining to Quasi-Birth–Death Processes (QBDs) is presented. The QBD, which is a generalization of the birth–death process, is a powerful tool that can be utilized in modeling many stochastic phenomena. Our theory is based on the application of a matrix polynomial method to obtain the steady-state probabilities in state-homogeneous finite-state QBDs. The method is based on finding the eigenvalue–eigenvector pairs that solve a matrix polynomial equation. Since the computational effort in the solution procedure is independent of the cardinality of the counting set, it has an immediate advantage over other solution procedures. We present and prove different properties relating the quantities that arise in the solution procedure. By also compiling and formalizing the previously known properties, we present a formal unified theory on the spectral properties of QBDs, which furnishes a formal framework to embody much of the previous work. This framework carries the prospect of furthering our understanding of the behavior the modeled systems manifest.Item Open Access A goal programming approach to mixed-model assembly line balancing problem(Elsevier, 1997) Gokcen, H.; Erel, E.In this paper, a binary goal programming model for the mixed-model assembly line balancing (ALB) problem is developed. The model is based on the concepts developed by Patterson and Albracht [1] and the model of Deckro and Rangachari [2] developed for the single-model ALB problem. The proposed model provides a considerable amount of flexibility to the decision maker since several conflicting goals can be simultaneously considered.Item Open Access The green network design problem(Elsevier, 2019) Dükkancı, Okan; Bektaş, T.; Kara, Bahar Y.; Faulin, J.; Grassman, S. E.; Juan, A. A.; Hirsch, P.Logistics activities are at the heart of world trade, but they also have unintended consequences on the environment due to the use of land, energy, and other types of natural resources. The significant energy usage by the more traditional means of transportation results in emissions, one of the most prominent of all negative externalities, that in turn causes air pollution affecting human health. One way to reduce such externalities is the (re-)design of the overall network on which logistics activities take place, giving rise to green network design problems, where the minimization of emissions is an integral and explicit part of the objective. The aim of this chapter is to present an overview and a classification of green network design problems arising at different levels of decision making, from operational to strategic, and will present definitions, optimization models, and practical applications for some of the key problems in this category.Item Open Access Hyperbolic efficiency and return to the dollar(Elsevier BV, 2002) Färe, R.; Grosskopf, S.; Zaim, O.This paper, after establishing the relations between hyperbolic graph measure of technical efficiency and the radial measures of technical efficiency, shows that the dual, cost and revenue interpretation of the hyperbolic efficiency measure is related to Georgescu-Roegen's notion of "return to the dollar" [N. Georgescu-Roegen, in: Koopmans, T. (Ed.), Activity Analysis of Production and Allocation, Wiley, New York, 1951, pp. 98-115]. Once this relation is established, it leads to a derivation of an allocative efficiency index, which measures the price distortions using data on observed costs and revenues without requiring information on prices. © 2002 Elsevier Science B.V. All rights reserved.Item Open Access An integrated evaluation of facility location, capacity acquisition, and technology selection for designing global manufacturing strategies(Elsevier BV, 1992) Verter, Vedat; Dincer, Mehmet CemalEmergence of global markets enhanced the emergence of global firms which have plants in different countries and implement an integrated management style. Due to the intensive competition in global markets, manufacturing performance is conceived as an important strategic weapon. Facility location, capacity acquisition and technology selection decisions constitute means to implement manufacturing strategies. We review the literature in order to contribute to a better understanding of global manufacturing strategies. As a result we observe that an integrated analysis of the location, capacity and technology decisions is vital for the design of effective global manufacturing strategies.Item Open Access Job shop scheduling with beam search(Elsevier Science B.V., Amsterdam, Netherlands, 1999) Sabuncuoglu I.; Bayiz, M.Beam Search is a heuristic method for solving optimization problems. It is an adaptation of the branch and bound method in which only some nodes are evaluated in the search tree. At any level, only the promising nodes are kept for further branching and remaining nodes are pruned off permanently. In this paper, we develop a beam search based scheduling algorithm for the job shop problem. Both the makespan and mean tardiness are used as the performance measures. The proposed algorithm is also compared with other well known search methods and dispatching rules for a wide variety of problems. The results indicate that the beam search technique is a very competitive and promising tool which deserves further research in the scheduling literature.