Browsing by Author "Erkip, Nesim K."

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    Collaborative decision making for air traffic management: A generic mathematical program for the rescheduling problem
    (Elsevier, 2019) Erkan, H.; Erkip, Nesim K.; Şafak, Özge
    We propose a model which can be utilized within a collaborative decision making (CDM) framework for rescheduling of arrivals and departures. The proposed mathematical program is expected to be utilized by major stakeholders, namely airlines and air navigation service providers. After providing the constraints, we list possible performance measures to be used as the objective function by a stakeholder. Performance measures include the ones that represent equity, as well as efficiency. We suggest guidelines to utilize the model for any stakeholder within CDM. Finally, a case study is prepared using publicly available data to demonstrate possible benefits.
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    Safety stock placement for serial systems under supply process uncertainty
    (Springer, 2020) Urlu, B.; Erkip, Nesim K.
    In this study, we address safety stock positioning when demand per period is a known constant but supply is uncertain. The supply is either available or not available, while the setting is that of a periodically reviewed, serial system following a base stock policy. Each stage is allowed to operate according to the guaranteed or stochastic service model. We use a Discrete Time Markov Chain model for expressing the expected on-hand inventories for each stage, along with other terms of interest, as a function of policy parameters determined by a given service level requirement for the end product. Exact models are constructed for single-stage and two-stage systems. As the number of states for a two-stage system grows exponentially, we propose an approximation for expressing the effect of the input stage using a single parameter. A generalization for the approximation is provided for a multi-stage problem. Computational evaluations of the approximation, as well as numerical comparisons of different cases, are presented.