Browsing by Author "Kara, Bahar Yetiş"

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Open Access
CCİ Ankara için talep toplama sistemi tasarımı
(TMMOB Makina Mühendisleri Odası, 2010) Anasoy, Şeyma; Kara, Semiha; Koçer, Oğuz; Taşkın, Gülce Simge; Yılmaz, Eren; Kara, Bahar Yetiş; Taysı, Tansel; Kökten, Simge
CCİ Satış ile müşteriler arasında bir arayüz oluşturan ön satış elemanları sabah ve akşam ofise gelmektedir. Harcanan bu zaman müşteri ilişkilerinin azalmasına, dolayısıyla CCİ’nin satış sistemindeki verimliliğinin azalmasına sebep olmaktadır. Ayrıca araç yakıt ve ofis giderlerini arttırmakta, ön satış elemanlarının yaşam kalitesini olumsuz yönde etkilemektedir. Bu projenin amacı, CCİ Ankara için yeni bir “Talep Toplama Sistemi” oluşturmaktır. Sistem, ön satış elemanlarının günlük iş akış süreçlerini yenilemiş ve talep toplama ağını yeniden oluşturmuştur. Ayrıca ön satış elemanlarının motivasyonlarının artırılması amacıyla yıllık aktivite plan takvimi düzenlenmiştir. Firmaya önerilen sistem şu an uygulanmakta olup yapılan analizler sonucunda giderlerde ofiste ortalama %30, araçta %12, personel servislerinde ise %60 oranında azalma olduğu saptanmıştır. Ön satış elemanlarının sayısı 84’ten 76’ya düşürülmüş olup, müşteri başına harcanan zaman artırılmıştır.
Open Access
Cumulative vehicle routing problems
(In-Teh, 2008) Kara, İmdat; Kara, Bahar Yetiş; Yetiş, M. K.; Caric, T.; Gold, H.
Open Access
Exact solution approaches for the minimum total cost traveling salesman problem with multiple drones
(Elsevier, 2023-01-04) Özbaygın Tiniç, G.; Karasan, Oya Ekin; Kara, Bahar Yetiş; Campbell, J.F.; Özel. A.
Deployment of drones in delivery operations has been attracting growing interest from the commercial sector due to its prospective advantages for a range of distribution systems. Motivated by the widespread adoption of drones in last-mile delivery, we introduce the minimum cost traveling salesman problem with multiple drones, where a truck and multiple drones work in synchronization to deliver parcels to customers. In this problem, we aim to find an optimal delivery plan for the truck and drones operating in tandem with the objective of minimizing the total operational cost including the vehicles’ operating and waiting costs. Unlike most studies in the literature where the objective is to minimize completion time, which means one needs to know only the arrival time of the latest arriving vehicle (truck or drone) at each synchronization point, we need to keep track of all the individual waiting times of the truck and the drones to properly account for waiting costs, which makes it more challenging to handle the synchronization. We provide a flow based and two cut based mixed integer linear programming formulations strengthened with valid inequalities. For non-compact models, we devise a variety of branch-and-cut schemes to solve our problem to optimality. To compare our formulations/algorithms and to demonstrate their competitiveness, we conduct computational experiments on a range of instances. The results indicate the superiority of utilizing branch-and-cut methodology over a flow based formulation. We also use our model to conduct sensitivity analyses with several problem parameters and to explore the benefits of launch and retrieval at the same node, the tradeoff between the number of drones and the operational cost, and the special case with a minimize completion objective with one drone. We also document very low waiting times for drones in optimal solutions and show solutions from minimizing cost have much lower cost than those from minimizing makespan.
Open Access
Mobile healthcare services in rural areas: An application with periodic location routing problem
(Springer Science and Business Media Deutschland GmbH, 2022-03-15) Savaşer, Sinem Kınay; Kara, Bahar Yetiş
In this study, we focus on the delivery of mobile healthcare services in rural areas, where doctors visit remote villages which do not have a healthcare facility nearby. The aim is to increase the accessibility of healthcare services for such population centers. We aim to determine the village assignments of the doctors, their monthly visit schedules and base hospitals where they start and end their tours. We model this as a periodic location routing problem and use the policies of Ministry of Health of Turkey as a basis for our mathematical formulation. These policies include the essential components of mobile healthcare services, namely, continuity of care and determining evenly distributed periodic visits. We determine the visit schedules, i.e. routes, of doctors endogenously while satisfying these policies. We also develop a heuristic algorithm based on a cluster first-route second approach and solve larger instances more effectively. The computational experiments support that this solution methodology can effectively find optimal or near-optimal solutions and improve the computational times significantly.
Open Access
Modeling and analysis of issues in hub location problem
(1999) Kara, Bahar Yetiş
The hub location problem has been around for more than 10 years. The first mathematical model was formulated by O’Kelly (1986) which is a quadratic integer program. Since then, nearly all of the researchers in this area have concentrated on developing ’good’ linearizations. However, there are many aspects of the problem that need to be analyzed. In this dissertation, we investigate some of these issues. We first study the application areas of the hub location problem and clarify the underlying assumptions of the real world problems which lead to the customarily defined hub location problem. We identify a certain problem characteristic of cargo delivery systems, which is one of the major application areas of the hub location problem, which is not satisfactorily modeled by means of the customarily defined hub location models. We propose a new hub location model that captures the specific requirements that are particular to cargo delivery systems. Another issue that we concentrate on is the identification, modeling and analysis of the hub location problem under different performance measures, namely minimax and covering criteria. We propose new integer programming models for the hub location problem under minimax and covering objectives. Both of the new models are the result of a different way of approaching the problem and their computational performance is far more superior than the performance of the various linearizations of the basic models proposed for these problems in the literature.
Open Access
Modeling the shelter site location problem using chance constraints: a case study for Istanbul
(Elsevier B.V., 2018) Kınay, Ö. B.; Kara, Bahar Yetiş; Saldanha-da-Gama, F.; Correia, I.
In this work, we develop and test a new modeling framework for the shelter site location problem under demand uncertainty. In particular, we propose a maxmin probabilistic programming model that includes two types of probabilistic constraints: one concerning the utilization rate of the selected shelters and the other concerning the capacity of those shelters. By invoking the central limit theorem we are able to obtain an optimization model with a single set of non-linear constraints which, nonetheless, can be approximated using a family of piecewise linear functions. The latter, in turn, can be modeled mathematically using integer variables. Eventually, an approximate model is obtained, which is a mixed-integer linear programming model that can be tackled by an off-the-shelf solver. Using the proposed reformulation we are able to solve instances of the problem using data associated with the Kartal district in Istanbul, Turkey. We also consider a large-scale instance of the problem by making use of data for the whole Anatolian side of Istanbul. The results obtained are presented and discussed in the paper. They provide clear evidence that capturing uncertainty in the shelter site location problem by means of probabilistic constraints may lead to solutions that are much different from those obtained when a deterministic counterpart is considered. Furthermore, it is possible to observe that the probabilities embedded in the probabilistic constraints have a clear influence in the results, thus supporting the statement that a probabilistic programming modeling framework, if appropriately tuned by a decision maker, can make a full difference when it comes to find good solutions for the problem.
Open Access
No country for young refugees: barriers and opportunities for inclusive refugee education practices
(Sage Publications, Inc., 2024) Demir, Sebnem Manolya; Sahinyazan, Feyza G.; Kara, Bahar Yetiş; Buluc, Elfe
The recent refugee crises in Ukraine (2022) and Syria (2011) have created millions of refugees, 40% of whom are children. The education systems of countries hosting refugees struggle to integrate such large populations. In addition, language barriers and the stigma associated with refugees hamper inclusive and equitable education opportunities for these children. There is thus a risk of "lost generations" distanced from education, who may eventually depend on social security systems and monetary aid in the long term. This study considers the following research question: How can a host country improve the inclusion of refugee children in the education system without overburdening its infrastructure? First, we document the availability and accessibility challenges and opportunities that refugee children face during the Syrian refugee crisis. We then develop an inclusive planning strategy aligned with existing capacity and resources and formulate two adaptations of the maximum covering problem (MCP): cooperative capacitated MCP with heterogeneity constraints (CCMCP-HC) to improve the current schooling access in T & uuml;rkiye and Modular CCMCP-HC to guide early planning in the case of a future crisis. Our computational analyses illustrate that the proposed approach yields higher schooling rates and capacity utilization than existing approaches. Our results emphasize the importance of having a planning strategy in the initial phases of a crisis that considers future integration possibilities. This study analyzes T & uuml;rkiye's experience and lessons learned to provide a road map for other ongoing and future refugee crises.
Open Access
Post-disaster assessment routing problem
(Elsevier, 2018) Oruç, Buse Eylül; Kara, Bahar Yetiş
In this study, we propose a post-disaster assessment strategy as part of response operations in which effective and fast relief routing are of utmost importance. In particular, the road segments and the population points to perform assessment activities on are selected based on the value they add to the consecutive response operations. To this end, we develop a bi-objective mathematical model that provides damage information in the affected region by considering both the importance of population centers and road segments on the transportation network through using aerial and ground vehicles (drones and motorcycles). The first objective aims to maximize the total value added by the assessment of the road segments (arcs) whereas the second maximizes the total profit generated by assessing points of interests (nodes). Bi-objectivity of the problem is studied with the ϵ-constraint method. Since obtaining solutions as fast as possible is crucial in the post-disaster condition, heuristic methods are also proposed. To test the mathematical model and the heuristic methods, a data set belonging to Kartal district of Istanbul is used. Computational experiments demonstrate that the use of drones in post-disaster assessment contributes to the assessment of a larger area due to its angular point of view. Also, the proposed heuristic methods not only can find a high-quality approximation of the Pareto front but also mitigates the solution time difficulties of the mathematical model.
Open Access
The refugee camp management: a general framework and a unifying decision-making model
(Emerald Group Publishing, 2019) Karsu, Özlem; Kara, Bahar Yetiş; Selvi, B.
Purpose: Motivated by the increasing need to provide support to refugees, which remains as a pressing issue in the agenda of many countries, the purpose of this paper is to consider the refugee camp management problem. Although each of these countries may have different procedures shaped by their own culture, rules and regulations, the main structure of the problem can be modeled utilizing a general framework which will apply to different practices. Design/methodology/approach: In this study, the authors consider the issue with an operations research (OR) perspective and provide such a framework utilizing Turkish Red Crescent (TRC)’s field expertise in many regions of the world. In the proposed framework, the overall refugee camp management problem is first categorized in two main phases: the establishment phase, which consists of one-time decisions like infrastructure design and the administration phase, which focuses on routine decisions that are taken on a periodic basis like aid distribution. Findings: The authors then provide a unifying decision-making model for the establishment phase and detail the administrative phase via subcategories, linking the relevant problems to the OR literature. The proposed framework is general enough to be used by practitioners and to be utilized by the academicians to define new OR problems to the literature. Originality/value: TRC’s know-how is very broad and extensive. Integrating that know-how with OR perspective, the authors provide a general framework that could be of use to practitioners as well as academicians. The proposed framework will constitute an example for countries of asylum and national or international NGOs to manage the refugee camps efficiently. The authors also highlight main challenges and dynamics of the decision-making problems encountered in different parts of the proposed framework, which may constitute many different problems to the OR literature each of which can open new venues for future research.
Open Access
A two-stage stochastic programming approach for reliability constrained power system expansion planning
(Elsevier, 2018) Peker, Meltem; Kocaman, Ayşe Selin; Kara, Bahar Yetiş
Probabilistic realizations of outages and their effects on the operational costs are highly overlooked aspects in power system expansion planning. Since the effect of randomness in contingencies can be more prominent especially when transmission switching is considered, in this paper we introduce contingency-dependent transmission switching concept to ensure N-1 criterion. To include randomness of outages and the outputs (i.e. flow on the lines/generation amounts) during the outages, we represent each contingency by a single scenario. Status of transmission lines, generation amounts and power flow decisions are defined as recourse actions of our two-stage stochastic model, therefore, expected operational cost during the contingencies are taken into account in a more accurate manner. A solution methodology with a filtering technique is also proposed to overcome the computational burden. The model and the solution methodology are tested on the IEEE Reliability Test System and IEEE 118-bus power system and the results show that the solution method finds the solutions for these power systems in significantly shorter solution times. The solution method is also tested on a new data set for the 380-kV Turkish transmission network. Suggestions for possible extensions of the problem and the modifications of the solution approach to handle these extensions are also discussed.