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Yayın Assortment optimization with log-linear demand: application at a Turkish grocery store(Elsevier Ltd, 2019-09) Hekimoğlu, Mustafa; Sevim, İsmail; Aksezer, Sezgin Çağlar; Durmuş, İpekIn retail sector, product variety increases faster than shelf spaces of retail stores where goods are presented to consumers. Hence, assortment planning is an important task for sustained financial success of a retailer in a competitive business environment. In this study, we consider the assortment planning problem of a retailer in Turkey. Using empirical point-of-sale data, a demand model is developed and utilized in the optimization model. Due to nonlinear nature of the model and integrality constraint, we find that it is difficult to obtain a solution even for moderately large product sets. We propose a greedy heuristic approach that generates better results than the mixed integer nonlinear programming in a reasonably shorter period of time for medium and large problem sizes. We also proved that our method has a worst-case time complexity of O(n 2 )while other two well-known heuristics’ complexities are O(n 3 )and O(n 4 ). Also numerical experiments reveal that our method has a better performance than the worst-case as it generates better results in a much shorter run-times compared to other methods.Yayın Deprem sonrası planlamaya yönelik lojistik ağ tasarımı: Ümraniye bölgesinde farklı deprem senaryoları için bir uygulama(Pamukkale Univ, 2019) Temur Tekin, Gül; Turgut, Yelda; Yılmaz, Abdurrahman; Arslan, Şafak; Camcı, AlperIn this study, a logistic network design for post-earthquake planning was developed. The purpose of this design is to develop a plan that enables earthquake survivors to access to basic humanitarian needs in the shortest time. In this direction, humanitarian aid and need scores were calculated for each neighborhood in a selected region, using the humanitarian aid and need scores emergency material distribution center locations were defined and distribution centers to which each neighborhood will be connected were identified. An integrated two-stage model for logistic network design optimization is proposed. In the first stage, using Analytical Hierarchy Process (AHP), humanitarian aid and need scores of the neighborhoods, each of which is a candidate for a potential distribution center, were determined. In the next stage, with the help of humanitarian aid and need score weighted and distance minimization aiming p-median model, the neighborhoods in which the distribution centers should be located as well as the distribution centers which other neighborhoods should be connected to, were identified. The proposed model was applied to the problem of establishing humanitarian aid distribution centers in the post-earthquake period for the Umraniye district of Istanbul province, and the model was reconstructed for different earthquake scenarios.Yayın Economic impacts of increased U.S. exports of natural gas: An energy system perspective(MDPI AG, 2016-05-25) Sarıca, Kemal; Tyner, Wallace E.With the recent shale gas boom, the U.S. is expected to have very large natural gas resources. In this respect, the key question is would it be better to rely completely on free market resource allocations which would lead to large exports of natural gas or to limit natural gas exports so that more could be used in the U.S.. After accounting for the cost of liquefying the natural gas and shipping it to foreign markets, the current price difference leaves room for considerable profit to producers from exports. In addition, there is a large domestic demand for natural gas from various sectors such as electricity generation, industrial applications, and the transportation sector etc. A hybrid modeling approach has been carried out using our version of the well-known MARket ALlocation (MARKAL)-Macro model to keep bottom-up model richness with macro effects to incorporate price and gross domestic product (GDP) feedbacks. One of the conclusion of this study is that permitting higher natural gas export levels leads to a small reduction in GDP (0.04%-0.17%). Higher exports also increases U.S. greenhouse gas (GHG) emissions and electricity prices (1.1%-7.2%). We also evaluate the impacts of natural gas exports in the presence of a Clean Energy Standard (CES) for electricity. In this case, the GDP impacts are similar, but the electricity and transport sector impacts are different.Yayın Evaluation of proactive maintenance policies on a stochastically dependent hidden multi-component system using DBNs(Elsevier Ltd, 2021-07) Özgür Ünlüakın, Demet; Türkali, BusenurIn complex systems with stochastically dependent components which are not observed directly, determining an effective maintenance policy is a difficult task. In this paper, a dynamic Bayesian network based maintenance decision framework is proposed to evaluate proactive maintenance policies for such systems. Two preventive and one predictive maintenance strategies from a cost perspective are designed for multi-component dependable systems which aim to reduce maintenance cost while increasing system reliability at the same time. Tabu procedure is employed to avoid repetitive similar actions. The performances of the policies are compared with a reactive maintenance strategy and also with each other using different strategy parameters on a real life system confronted in thermal power plants for six different scenarios. The scenarios are designed considering different structures of system dependability and reactive cost. The results show that the threshold based maintenance which is the predictive strategy gives the minimum cost and maintenance number in almost all scenarios.
