Scholarly Publications - Industrial Engineering
Permanent URI for this collectionhttps://hdl.handle.net/11693/115612
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Item Open Access Productivity enhancement in top-down VPP via concurrent grayscaling and platform speed profile optimization for symmetrical parts having micro scale features(Springer, 2024-06-14) Güven, Ege; Karpat, Yiğit; Çakmakcı, MelihVat Photopolymerization (VPP), a widely adopted additive manufacturing technique, has revolutionized the domain of 3D printing by enabling the precise fabrication of complex structures, including intricate details. However, challenges remain in achieving optimal print quality while improving speed. Conventionally, grayscaling has been used to improve part accuracy in continuous VPP systems as the build platform speed remains constant. Considering a detailed photocurable resin solidification model, together with grayscaling, this study aims to improve productivity by optimizing platform speed profile while maintaining the build quality. While the optimization formulation presented here can be applied to any part, the computational limitations due to the employment of a voxel-based approach and the nonlinear nature of the resulting optimization problem are simplified by adopting a novel discretization methodology utilizing the symmetric properties of the target 3D part. By employing ring elements instead of voxels for cylindrical symmetrical parts, the computational load of the optimization algorithm is dramatically reduced. Experimental results show the proposed concurrent optimization reduces print time by 56% while maintaining superior print surface quality on an hourglass-shaped test part having micro scale features.Item Open Access No country for young refugees: barriers and opportunities for inclusive refugee education practices(Sage Publications, Inc., 2024) Kara, Bahar Yetiş; Demir, Sebnem Manolya); Sahinyazan, Feyza G.; Buluc, ElfeThe 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.Item Embargo Mathematical programming models for multistage rural electrification planning: Off-grid, grid and mini-grid options(Pergamon-Elsevier Science Ltd., 2025-02-01) Yazıcı, Gözde; Karaşan, Oya Ekin; Kocaman, Ayşe Selin; Stoner, RobApproximately 9% of the global population lacks access to electricity. The majority of this population resides in rural areas, highlighting the critical importance of rural electrification efforts. In this study, we introduce novel mathematical programming models aimed at addressing the technology choice and network design challenges in rural electrification. These models determine the optimal electrification technology among off- grid, grid and mini-grid options for each demand point while designing the cost effective grid and mini-grid networks. Furthermore, we present multistage versions of these mathematical models, demonstrating the cost advantage of multistage modeling. These formulations can serve as a comprehensive framework that incorporates investment requirements for system roll-out. Through numerical experiments utilizing both real- life and synthetic instances, we offer new insights into electrification in diverse environments. Our research is expected to contribute to the socio-economic development of developing countries and aid in achieving the targets outlined in Sustainable Development Goal 7.Item Open Access MAD risk parity portfolios(Springer New York LLC, 2024-01-16) Ararat, Çağın; Cesarone, F.; Pınar, Mustafa Çelebi; Ricci, J. M.In this paper, we investigate the features and the performance of the risk parity (RP) portfoliosusing the mean absolute deviation (MAD) as a risk measure. The RP model is a recent strategyfor asset allocation that aims at equally sharing the global portfolio risk among all the assetsof an investment universe. We discuss here some existing and new results about the propertiesof MAD that are useful for the RP approach. We propose several formulations for finding MAD-RP portfolios computationally, and compare them in terms of accuracy and efficiency. Furthermore, we provide extensive empirical analysis based on three real-world datasets, showing that the performances of the RP approaches generally tend to place both in termsof risk and profitability between those obtained from the minimum risk and the Equally Weighted strategies.Item Open Access Unraveling the complex interplay between elastic recovery, contact pressure, and friction on the flank face of the micro tools via plunging-type testing(Elsevier Inc., 2024-07-06) Karpat, Yiğit; Güven, CanA good understanding of the interplay between the cutting tool edge radius, elastic recovery, friction, and contact pressure is essential for better modeling of ploughing forces during micro-scale cutting. This study conducts plunging tests on an ultra-precision CNC with engineered tungsten carbide cutting tools on commercially pure titanium alloy. The cutting tool edge radius is prepared to be around 3.5-4 mu m, which resembles those cutting tools used in micro scale machining. During plunging tests, the micro cutting tool is given a sinusoidal movement with an amplitude close to edge radius of the tool as the work material is rotated at a constant speed. The residual depth profiles of the webs corresponding to the commanded depths were investigated in detail to identify elastic recovery rate. The cutting and thrust force measurements during plunging experiments together with identified elastic recovery rate was employed in an analytical model of micro scale machining to obtain the variations of contact pressure and coefficient of friction as a function of commanded depth. Due to the scale of the experiments that were performed, the effects of surface topography of the cutting tool and possible alignment errors are also considered in the analytical model. A linear relationship between the contact pressure and elastic recovery has been identified during ploughing-dominated machining conditions for the work material and the cutting tool pair considered in this study. The proposed experimental technique is shown to be promising in terms of modeling ploughing forces during micro-scale cutting.Item Embargo Facility location decisions for drone delivery with riding: a literature review(Elsevier Ltd, 2024-04-18) Dükkancı, O.; Campbell, J.F.; Yetiş Kara, BaharThis study presents a comprehensive literature survey on facility location problems for drone (uncrewed vehicle) delivery in situations where drones can ride in or on other vehicles. This includes facilities visited by only one type of vehicle, as well as facilities visited by both drones and other vehicles. Unlike traditional facility location problems for delivery systems with one vehicle type, hybrid vehicle-drone delivery systems usually require determining locations where the two vehicle types meet and separate. The main goals of this paper are to review the large volume of drone delivery literature with riding from a facility location perspective to provide a connection between the studies from different research areas that cover similar problems, and to highlight future research directions in this area. We first review the functions of drones, including aerial and ground drones, and the different types of facilities used for hybrid vehicle-drone delivery systems. The literature is categorized based on the presence of resupply operations, the locations of drone launch and retrieval points, the types of drones (aerial or ground) and the location space (discrete or continuous). Each category is analyzed in terms of the modeling approach, decision(s), objective function(s), constraints and additional features. The paper concludes with promising future research directions.Item Embargo Finding robustly fair solutions in resource allocation(Elsevier Ltd, 2025-02) Karsu, Ozlem; Elver, Izzet Egemen; Kinik, Tuna ArdaIn this study, we consider resource allocation settings where the decisions affect multiple beneficiaries and the decision maker aims to ensure that the effect is distributed to the beneficiaries in an equitable manner. We specifically consider stochastic environments where there is uncertainty in the system and propose a robust programming approach that aims at maximizing system efficiency while guaranteeing an equitable benefit allocation even under the worst scenario. Acknowledging the fact that the robust solution may lead to high efficiency loss and may be over-conservative, we adopt a parametric approach that allows controlling the level of conservatism and present the decision maker alternative solutions that reveal the trade-off between efficiency and the degree of conservatism when incorporating fairness. We obtain tractable formulations, leveraging the results we provide on the properties of highly unfair allocations. We demonstrate the usability of our approach on project selection and shelter allocation applications.Item Open Access Sparsity penalized mean–variance portfolio selection: analysis and computation(SPRINGER HEIDELBERG, 2024-11-25) Akkaya, Deniz; Pınar, Mustafa Çelebi; Şen, BuseWe consider the problem of mean–variance portfolio selection regularized with an -penalty term to control the sparsity of the portfolio. We analyze the structure of local and global minimizers and use our results in the design of a Branch-and-Bound algorithm coupled with an advanced start heuristic. Extensive computational results with real data as well as comparisons with an off-the-shelf and state-of-the-art (MIQP) solver are reported.Item Open Access On the monotonicity of the Hilbert functions for 4-generatedpseudo-symmetric monomial curves(Taylor & Francis Inc., 2024-07-14) Şahin, NilIn this article we solve the following problem: “The Hilbert function of thelocal ring of a 4-generated pseudo-symmetric numerical semigroup is alwaysnon-decreasing.” We give a complete characterization of the standard baseswhen the tangent cone is not Cohen-Macaulay by showing that the number ofelements in the standard basis depends on some parameters we define. Sincethe tangent cone is not Cohen-Macaulay, non-decreasingness of the Hilbertfunction was not guaranteed, thus we proved the non-decreasingness fromour explicit Hilbert function computation.Item Open Access A robust Logistics-Electric framework for optimal power management of electrified ports under uncertain vessel arrival time(Elsevier Ltd, 2024-01-18) Sarantakos, I.; Nikkhah, S.; Peker, Meltem; Bowkett, A.; Sayfutdinov, T.; Alahyari, A.; Patsios, C.; Mangan, J.; Allahham, A.; Bougioukou, E.; Murphy, A.; Pazouki, K.Maritime transport is responsible for producing a considerable amount of environmental pollution due to the reliance of ports and ships on the carbon-based energy sources. With the increasing trend towards port elec trification to reduce carbon emissions, the operation of ports will be increasingly relying on the electricity network. This interconnection creates multiple challenges due to the complexity of power flow in the port network, uncertainty of vessel arrival time and fluctuation of power generation of renewable energy sources. These uncertainties can lead to an overload in electricity networks and delays in cargo-handling activities, resulting in increased vessel handling times and environmental emissions. This paper presents a joint logisticselectric framework for optimal operation and power management of electrified ports, considering multiple un certainties in the arrival time of vessels, network demand, and renewable power generation. An optimal power flow method is developed for a real-life port, with consideration for multiple port logistic assets such as cargo handling equipment, reefers, and renewable energy sources. The proposed model ensures feasible port operation for all uncertainty realisations defined by robust optimisation, while minimising operational costs. Simulation results demonstrate that the probability of a network constraint violation can be as high as 70% for an electrified major UK port if the uncertainty in the port operation is neglected, presenting an unacceptable risk of disruption to port activities. Furthermore, such uncertainty can cause 150% increase in emissions if the ships use their auxiliary engine instead of using shore power. The numerical study shows that such challenges can be handled by a 0.3% increase in the robustness in face of uncertainty, while the cost increase in the worst case does not exceed 4.7%. This shows the effectiveness of the proposed method enhancing robustness against uncertainty at the minimum cost.Item Open Access Computing the recession cone of a convex upper image via convex projection(Springer New York LLC, 2024-03-01) Kovácová, G.; Ulus, FirdevsIt is possible to solve unbounded convex vector optimization problems (CVOPs) in two phases: (1) computing or approximating the recession cone of the upper image and (2) solving the equivalent bounded CVOP where the ordering cone is extended based on the first phase. In this paper, we consider unbounded CVOPs and propose an alternative solution methodology to compute or approximate the recession cone of the upper image. In particular, we relate the dual of the recession cone with the Lagrange dual of weighted sum scalarization problems whenever the dual problem can be written explicitly. Computing this set requires solving a convex (or polyhedral) projection problem. We show that this methodology can be applied to semidefinite, quadratic, and linear vector optimization problems and provide some numerical examples.Item Open Access Heat partition evaluation during dry drilling of thick CFRP laminates with polycrystalline diamond drills(ELSEVIER, 2024-10) Shariar, Fahim; Karagüzel, Umut; Karpat, YiğitSince various material properties of carbon fiber-reinforced polymer (CFRP) are temperature dependent, dry drilling of CFRP is a delicate process. Thermal damage can be caused by a rise in temperature during drilling due to a large portion of heat being transferred into the material. Heat partition is used to quantify this, which represents the percentage of total heat being dissipated into the constituent objects during a machining operation. Drill margin and contact conditions at the tool-workpiece interface significantly affect the drilling of CFRP material. Drilling experiments were performed to measure thrust force, torque, and temperatures for five different sets of feed rates and rotational speeds. This study proposes a method for calculating heat partition values during CFRP drilling by developing a finite element-based thermal model. The FE model employs a Gaussian distributed ring-type heat flux that is a function of the equivalent contact length at the interface between the drill and the material surface and the geometry of the workpiece which operates as a moving heat source, emulating the progress of the drill through the CFRP laminate. The tool implements heat fluxes that use characteristic time-point-based step functions to represent the temperature on the drill as it advances through the workpiece during machining. The temperature profiles obtained from the FE analysis and the experiments for the workpiece and tool were subsequently matched iteratively to determine the corresponding heat partition valueItem Open Access Technical note—optimal procurement in remanufacturing systems with uncertain used-item condition(INFORMS Inst.for Operations Res.and the Management Sciences, 2023-05-08) Nadar, Emre; Akan, Mustafa; Debo, Laurens; Scheller-Wolf, AlanWe consider a single-product remanufacture-to-order system with multiple uncertain quality levels for used items, random procurement lead times, and lost sales. The quality level of a used item is revealed only after it is acquired and inspected; the remanufacturing cost is lower for a higher-quality item. We model this system as a Markov decision process and seek an optimal policy that specifies when a used item should be procured, whether an arriving demand for the remanufactured product should be satisfied, and which available item should be remanufactured to meet this demand. We characterize the optimal procurement policy as following a new type of strategy: state-dependent noncongestive acquisition. This strategy makes decisions, taking into account the system congestion level measured as the number of available items and their quality levels. We also show that it is always optimal to meet the demand with the highest-quality item among the available ones. We conclude with extensions of our model to limited cases when the used-item condition is known a priori (for two quality levels) and remanufacture-to-stock systems in which the standard push strategy is optimal in the remanufacturing stage. © 2023 INFORMS.Item Open Access Discovering sustainability practices in research and innovation sites(ARROW@TU, 2023-10-10) Downey, Robin AnnDiscovering sustainability pr ering sustainability practices in resear actices in research and inno ch and innovation This practice paper is a descriptive account of an experience with a sustainable development learning project for engineering students in a Science, Technology and Society (STS) course at Bilkent University. The students participated in the STS Sustainability Awards competition for two semesters in one academic year, an event that was inspired by Bilkent University’s 2021–2022 Sustainability Year. As part of the project, the students found a company or laboratory, consulted them on their innovation practices and asked questions that were grounded in Responsible Research and Innovation (RRI) approaches. RRI can provide an opening for students to explore how various values, including sustainability and privacy, are considered in innovation practices. The values by design approach can help engineering students to see that innovators consider both instrumental and qualitative values during the innovation process. Although the project has been used in other years, the sustainability awards motivated students to explore how innovators respond to concerns around a range of sustainability issues. The award recipients produced projects on smart homes, nanotechnology-based solar panels, clean meat, industry 4.0, geothermal energy, air cars and magnetic resonance imaging technology, and gave presentations in events hosted by the Faculty of Engineering administrators. Although future research in this area is needed, applied learning experiences, such as the one that is described in this paper, could have the potential to help bridge the disciplinary divide between STS and engineering.Item Open Access Student perspectives on sustainability in engineering education: multiple case study of european bachelor's programs in industrial engineering and management(ARROW@TU, 2023-10-10) Trigueiros, Francisca; Kaipainen, Jenni; Silva, Frederico; Geising, Niklas; Tosun, Erdem AtaThe global sustainability crisis is calling for engineers to take action. To enable and empower engineers to address this crisis, there must be a change in engineering education. Given the industry's key role in not only causing but also solving this sustainability crisis, it is especially crucial to improve how sustainability is addressed in industrial engineering and management (IEM) education. This paper examines (1) to which extent European IEM degrees are covering sustainability; (2) European IEM students’ motivations to learn and work with sustainability topics; and (3) their perceptions of their degree’s contribution to their knowledge and motivation regarding sustainability; and (4) which sustainability-related changes they would like to see in their degrees. Three IEM curricula covering different regions of Europe—Portugal, Germany, and Turkey—were analysed. The mixed-method analysis included a quantitative evaluation of the extent to which each course meets specific theory-based learning objectives pertinent to sustainability in engineering education. The analysis was complemented by students’ perspectives, which were gathered through group discussions and interviews. The results reveal how sustainability is addressed in IEM education in different European regions, its impact on students’ knowledge and motivation for sustainability issues, and how sustainability in engineering education should be developed based on students' perceptions. These findings contribute to the research on sustainability in engineering education and support university teachers in revising engineering study programs to provide adequate sustainability understanding and skills to students.Item Embargo Word of mouth on action: analysis of optimal shipment policy when customers are resentful(Elsevier Ltd, 2023-03-04) Çavdar, Bahar; Erkip, Nesim KohenWord-of-Mouth (WoM) communication via online reviews plays a vital role in customers’ purchasing decisions. As such, retailers must consider the impact of WoM to manage customer perceptions and future demand. This paper considers an online shopping system with premium and regular customers. Building on the behavioral and operations management literature, we model customer preferences based on the perceived service quality indicated by WoM and integrate this into the retailer's operational problem to determine a shipment policy regarding the timing of consolidated shipments and the treatment of regular demand. First, we study the e-tailer's problem when they have no knowledge of WoM and only react to the changes in demand. We analyze the long-term behavior of customer demand and show that potential market size and customer sensitivity are the key parameters determining this behavior. Then, we build a model to integrate the knowledge of WoM into operational decision-making and partially characterize the optimal solution. We show that (i) underpromise-and-overdeliver can be a hurtful strategy since it creates a false sense of fast delivery for the regular service, (ii) relaxations in operational constraints may hurt profitability due to the associated difficulties of managing perceptions, and (iii) seeking a stationary policy can lead to suboptimal solutions; therefore, cyclic policies should also be considered when appropriate. © 2023Item Open Access Outer approximation algorithms for convex vector optimization problems(Taylor and Francis Ltd., 2023-02-09) Keskin, İrem Nur; Ulus, FirdevsIn this study, we present a general framework of outer approximation algorithms to solve convex vector optimization problems, in which the Pascoletti-Serafini (PS) scalarization is solved iteratively. This scalarization finds the minimum ‘distance’ from a reference point, which is usually taken as a vertex of the current outer approximation, to the upper image through a given direction. We propose efficient methods to select the parameters (the reference point and direction vector) of the PS scalarization and analyse the effects of these on the overall performance of the algorithm. Different from the existing vertex selection rules from the literature, the proposed methods do not require solving additional single-objective optimization problems. Using some test problems, we conduct an extensive computational study where three different measures are set as the stopping criteria: the approximation error, the runtime, and the cardinality of the solution set. We observe that the proposed variants have satisfactory results, especially in terms of runtime compared to the existing variants from the literature. © 2023 Informa UK Limited, trading as Taylor & Francis Group.Item Open Access Computation of systemic risk measures: a mixed-integer programming approach(INFORMS Inst.for Operations Res.and the Management Sciences, 2023-09-22) Ararat, Çaǧın; Meimanjan, N.Systemic risk is concerned with the instability of a financial system whose members are interdependent in the sense that the failure of a few institutions may trigger a chain of defaults throughout the system. Recently, several systemic risk measures have been proposed in the literature that are used to determine capital requirements for the members subject to joint risk considerations. We address the problem of computing systemic risk measures for systems with sophisticated clearing mechanisms. In particular, we consider an extension of the Rogers-Veraart network model where the operating cash flows are unrestricted in sign. We propose a mixed-integer programming problem that can be used to compute clearing vectors in this model. Because of the binary variables in this problem, the corresponding (set-valued) systemic risk measure fails to have convex values in general. We associate nonconvex vector optimization problems with the systemic risk measure and provide theoretical results related to the weighted-sum and Pascoletti-Serafini scalarizations of this problem. Finally, we test the proposed formulations on computational examples and perform sensitivity analyses with respect to some model-specific and structural parameters. Copyright: © 2023 INFORMS.Item Open Access A stochastic programming approach to surgery scheduling under parallel processing principle(Elsevier Ltd, 2023-11-06) Çelik, Batuhan; Gül, Serhat; Çelik, MelihParallel processing is a principle which enables simultaneous implementation of anesthesia induction and operating room (OR) turnover with the aim of improving OR utilization. In this article, we study the problem of scheduling surgeries for multiple ORs and induction rooms (IR) that function based on the parallel processing principle under uncertainty. We propose a two-stage stochastic mixed-integer programming model considering the uncertainty in induction, surgery and turnover durations. We sequence patients and set appointment times for surgeries in the first stage and assign patients to IRs at the second stage of the model. We show that an optimal myopic policy can be used for IR assignment decisions due to the special structure of the model. We minimize the expected total cost of patient waiting time, OR idle time and IR idle time in the objective function. We enhance the model formulation using bounds on variables and symmetry-breaking constraints. We implement a novel progressive hedging algorithm by proposing a penalty update method and a variable fixing mechanism. Based on real data of a large academic hospital, we compare our solution approach with several scheduling heuristics from the literature. We assess the additional benefits and costs associated with the implementation of parallel processing using near-optimal schedules. We examine how the benefits are inflated by increasing the number of IRs. Finally, we estimate the value of stochastic solution to underline the importance of considering uncertainty in durations. © 2022 Elsevier LtdItem Open Access Random sets and choquet-type representations(American Institute of Mathematical Sciences, 2023-03) Ararat, Çağın; Çetin, U.As appropriate generalizations of convex combinations with un-countably many terms, we introduce the so-called Choquet combinations, Cho-quet decomposable combinations and Choquet convex decomposable combinations, as well as their corresponding hull operators acting on the power sets of Lebesgue-Bochner spaces. We show that Choquet hull coincides with convex hull in the finite-dimensional setting, yet Choquet hull tends to be larger in infinite dimensions. We also provide a quantitative characterization of Cho-quet hull, without any topological or algebraic assumptions on the underlying set. Furthermore, we show that the Choquet decomposable hull of a set coincides with its strongly closed decomposable hull and the Choquet convex decomposable hull of a set coincides with the Choquet decomposable hull of its convex hull. It turns out that the measurable selections of a closed-valued multifunction form a Choquet decomposable set and those of a closed convex-valued multifunction form a Choquet convex decomposable set. Finally, we investigate the operator-type features of Choquet decomposable and Choquet convex decomposable hull operators when applied in succession. © 2023, American Institute of Mathematical Sciences. All rights reserved.