Waterloo Research

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Browsing Waterloo Research by Author "Alumur, Sibel A."

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    An alternative approach to address uncertainty in hub location
    (Springer, 2023) Janschekowitz, Marc; Taherkhani, Gita; Alumur, Sibel A.; Nickel, Stefan
    In this paper, optimization and simulation techniques are integrated to address single and multiple allocation hub network design problems under uncertainty. Using a scenario-based iterative optimization-simulation approach four sources of uncertainty are considered: the demand to be transported within the network, the associated transportation costs as well as the fixed costs for both opening hub facilities and establishing the connections between them. Additionally, flow-dependent economies of scale on all network connections are incorporated in the simulation phase. A value of simulation measure is introduced to evaluate the performance of the methodology. The computational tests conducted on the well-known CAB data set with varying levels of uncertainty show that the approach can result in better solutions with up to 6.6% lower cost compared to its deterministic counterpart.
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    Robust-stochastic models for profit maximizing hub location problems
    (INFORMS, 2021) Taherkhani, Gita; Alumur, Sibel A.; Hosseini, Mojtaba
    This paper introduces robust-stochastic models for profit maximizing capacitated hub location problems in which two different types of uncertainty including stochastic demand and uncertain revenue are simultaneously incorporated into the problem. First, a two-stage stochastic program is presented where demand and revenue are jointly stochastic. Next, robust-stochastic models are developed to better model uncertainty in the revenue while keeping the demand stochastic. Two particular cases are studied based on the dependency between demand and revenue. In the first case, a robust-stochastic model with a min-max regret objective is developed assuming a finite set of scenarios that describe uncertainty associated with the revenue under a revenue-elastic demand setting. For the case when demand and revenue are independent, robust-stochastic models with a max-min criterion and a min-max regret objective are formulated considering both interval uncertainty and discrete scenarios, respectively. It is proved that the robust-stochastic version with max-min criterion can be viewed as a special case of the min-max regret stochastic model. Exact algorithms based on Benders decomposition coupled with sample average approximation scheme are proposed. Exploiting the repetitive nature of sample average approximation, generic acceleration methodologies are developed to enhance the performance of the algorithms enabling them to solve large-scale intractable instances. Extensive computational experiments are performed to consider the efficiency of the proposed algorithms and also to analyze the effects of uncertainty under different settings. The qualities of the solutions obtained from different modeling approaches are compared under various parameter settings. Computational results justify the need to solve robust-stochastic models to embed uncertainty in decision making to design resilient hub networks.
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