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Publication Metadata only A bi-objective model for design and analysis of sustainable intermodal transportation systems: A case study of Turkey(Taylor & Francis Ltd, 2019) Reşat, Hamdi Giray; Department of Industrial Engineering; Türkay, Metin; Faculty Member; Department of Industrial Engineering; College of Engineering; 24956This paper presents a mixed-integer linear optimisation model to analyse the intermodal transportation systems in the Turkish transportation industry. The solution approach includes mathematical modelling, data analysis from real-life cases and solving the resulting mathematical programming problem to minimise total transportation cost and carbon dioxide emissions by using two different exact solution methods in order to find the optimal solutions. The novel approach of this paper generates Pareto solutions quickly and allows the decision makers to identify sustainable solutions by using a newly developed solution methodology for bi-objective mixed-integer linear problems in real-life cases.Publication Metadata only A discrete-continuous optimization approach for the design and operation of synchromodal transportation networks(Elsevier, 2019) Reşat, Hamdi Giray; Department of Industrial Engineering; Türkay, Metin; Faculty Member; Department of Industrial Engineering; College of Engineering; 24956This paper presents a multi-objective mixed-integer programming problem for integrating specific characteristics of synchromodal transportation. The problem includes different objective functions including total transportation cost, travel time and CO2 emissions while optimizing the proposed network structure. Traffic congestion, time-dependent vehicle speeds and vehicle filling ratios are considered and computational results for different illustrative cases are presented with real data from the Marmara Region of Turkey. The defined non-linear model is converted into linear form and solved by using a customized implementation of the e-constraint method. Then, the sensitivity analysis of proposed mathematical models with pre-processing constraints is summarized for decision makers.Publication Metadata only An inventory model where customer demand is dependent on a stochastic price process(Elsevier Science Bv, 2019) Canyakmaz, Caner; Department of Industrial Engineering; Department of Industrial Engineering; Özekici, Süleyman; Karaesmen, Fikri; Faculty Member; Faculty Member; Department of Industrial Engineering; College of Engineering; College of Engineering; 32631; 3579We investigate the optimal inventory operations of a firm selling an item whose price is driven by an exogenous stochastic price process which consequently impacts customer arrivals between ordering cycles. This case is typical for retailers that operate in different currencies, or trade products consisting of commodities or components whose prices are subject to market fluctuations. We assume that there is a stochastic input price process for the inventory item which determines purchase and selling prices according to a general selling price function. Customers arrive according to a doubly-stochastic Poisson process that is modulated by stochastic input prices. We analyze optimal ordering decisions for both backorder and lost-sale cases. We show that under certain conditions, a price-dependent base stock policy is optimal. Our analysis is then extended to a price-modulated compound Poisson demand case, and the case with fixed ordering cost where a price-dependent (s, S) policy is optimal. We present a numerical study on the sensitivity of optimal profit to various parameters of the operational setting and stochastic price process such as price volatility, customer sensitivity to price changes etc. We then make a comparison with a corresponding discrete-time benchmark model that ignores within-period price fluctuations and present the optimality gap when using the benchmark model as an approximation.Publication Metadata only Cyclic scheduling in synchronous production lines(Taylor & Francis, 1999) Kouvelis, P.; Department of Business Administration; Karabatı, Selçuk; Faculty Member; Department of Business Administration; College of Administrative Sciences and Economics; 38819In this paper we address the scheduling problem in unpaced synchronous mixed-model production lines operated under a cyclic scheduling policy. We first discuss operations of a production line with the synchronous transfer of parts. We then present an integer programming formulation of the problem. The problem, however, is NP-hard, and for its exact solution we propose an implicit enumeration scheme. We discuss a property of the scheduling problem which allows us to effectively solve large size instances of the problem. We also present an approximate solution procedure with very good average performance. Useful managerial insights are obtained as we search for ways to improve the performance of synchronous lines. The relaxation of one of our original assumptions in the scheduling problem formulation results in an easy problem whose solution generates the absolute best in throughput performance configuration of the production line. Implementation of this solution, however, requires increasing the number of buffers in the line. We suggest other performance improvement ways to better balance the tradeoff between throughput and average Work-In-Progress (WIP) inventory in the line.Publication Metadata only Decentralized inventory control in a two-stage capacitated supply chain(Taylor & Francis Inc, 2007) Jemai, Zied; Department of Industrial Engineering; Karaesmen, Fikri; Faculty Member; Department of Industrial Engineering; College of Engineering; 3579This paper investigates a two-stage supply chain consisting of a capacitated supplier and a retailer that faces a stationary random demand. Both the supplier and the retailer employ base stock policies for inventory replenishment. All unsatisfied demand is backlogged and the customer backorder cost is shared between the supplier and the retailer. We investigate the determination of decentralized inventory decisions when the two parties optimize their individual inventory-related costs in a noncooperative manner. We explicitly characterize the Nash equilibrium inventory strategies and identify the causes of inefficiency in the decentralized operation. We then study a set of simple linear contracts to see whether these inefficiencies can be overcome. Finally, we investigate Stackelberg games where one of the parties is assumed to be dominating.Publication Metadata only Dynamic ordering decisions with approximate learning of supply yield uncertainty(Elsevier, 2022) Gel, Esma S.; Department of Industrial Engineering; Salman, Fatma Sibel; Faculty Member; Department of Industrial Engineering; College of Engineering; 178838We consider the real-life problem of a coach bus manufacturer located in Turkey, facing the problem of setting ordering quantities for a part procured from an unreliable supplier, where the number of items delivered is binomially distributed with an unknown yield parameter, p. We use the well-defined finite-horizon planning context with deterministic demand per period, purchasing, holding, and shortage costs to investigate the effectiveness of a fill-rate based approximate learning scheme in comparison to an exact Bayesian learning scheme, where observations on the supplier's delivery performance are used to update the assumed distribution ofp. We formulate the exact optimal learning problem as a Bayes-adaptive Markov decision process and solve the corresponding finite horizon stochastic dynamic program to provide insights on the value of online learning in comparison to the unrealistic perfect information (PI) and no information (NT) benchmarks. We contrast the performance of the so-called Bayesian Updating (BU) policy to other practical approaches such as using an assumed/guessed value ofp and implementing a constant safety stock. Noting the significant value of learning, we finally study the effectiveness of an approximate learning formulation that does not enjoy the asymptotic consistency and convergence properties but involves much lower computational burden, and demonstrate its confounding performance, at times beating the BU policy with exact Bayesian updates.Publication Metadata only Dynamic policies of admission to a two-class system based on customer offers(Taylor & Francis, 2002) Burnetas, A.N.; Emmons, H.; Department of Industrial Engineering; Örmeci, Lerzan; Faculty Member; Department of Industrial Engineering; College of Engineering; 32863We consider the problem of dynamic admission control in a Markovian loss queueing system with two classes of jobs with different service rates and random revenues. We establish the existence of an optimal monotone policy. We also show that under certain conditions there exist preferred jobs from either class.Publication Metadata only Dynamic pricing of durable products with heterogeneous customers and demand interactions over time(Pergamon-Elsevier Science Ltd, 2013) N/A; Department of Industrial Engineering; Kaya, Onur; Faculty Member; Department of Industrial Engineering; College of Engineering; 28405In this study, we analyze a dynamic pricing problem in which the demand is interdependent over time and the customers are heterogeneous in their purchasing decisions. The customers are grouped into different classes depending on their purchase probabilities and the customer classes evolve over time depending on the demand realizations at every period, which are a function of the prices set by the company. To decide on the optimal prices at every period, we model this problem using a stochastic dynamic program (SDP) and we develop several approximation algorithms to solve this SDP since the size of the state space of the SDP makes the optimal solution almost impossible to find. We present the efficiencies of the heuristics and provide managerial insights through a computational study in which we compare the revenues obtained with each heuristic with an upper bound value that we find on the optimal revenues.Publication Metadata only Editorial: games and decisions in reliability and risk(Elsevier, 2018) Soyer, Refik; Department of Industrial Engineering; Özekici, Süleyman; Faculty Member; Department of Industrial Engineering; College of Engineering; 32631N/APublication Metadata only Five-axis additive manufacturing of freeform models through buildup of transition layers(Elsevier Sci Ltd, 2019) N/A; Department of Mechanical Engineering; Department of Mechanical Engineering; Isa, Mohammed A.; Lazoğlu, İsmail; Researcher; Faculty Member; Department of Mechanical Engineering; Manufacturing and Automation Research Center (MARC); College of Engineering; College of Engineering; N/A; 179391Acclaimed for enabling the fabrication of complex parts, additive manufacturing is confined to established processing and planning methods that contribute impediments to its industrial adoption. The requirement of support structures and poor quality of produced surfaces are some of these impediments. Extension of the manufacturing method to accommodate variable tool orientation can introduce new approaches in process planning that can resolve these obstacles. Therefore, a new 5-axis 3D printer is designed, built and programmed to facilitate implementation of novel 3D curve paths. Common layering methods in additive manufacturing are centered around the idea of intersection of a CAD model with parallel planes or offset surfaces without regards to the form of the part. The use of these inflexible layering patterns leads to staircase effect on the surfaces, inefficient toolpaths and low load-bearing capacity. This article suggests and develops new 5-axis path planning model that takes into account the surface profiles of the freeform part. Path and tool orientation conditions are reexamined to propose planning schemes that prevent staircase effects on shell and solid components. To accomplish this, the material is deposited on successive transition surfaces whose infra-layer thickness varies to allow changes in the form of the surfaces.
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