Endüstri Mühendisliği Bölümü Koleksiyonu

Permanent URI for this collectionhttps://gcris.khas.edu.tr/handle/20.500.12469/48

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Citation Count: 5
(3+3+2) warped-like product manifolds with Spin(7) holonomy
(Elsevier Science Bv, 2011) Bilge, Ayşe Hümeyra; Bilge, Ayşe Hümeyra
We consider a generalization of eight-dimensional multiply warped product manifolds as a special warped product by allowing the fiber metric to be non-block diagonal. We define this special warped product as a (3 + 3 + 2) warped-like manifold of the form M = F x B. where the base B is a two-dimensional Riemannian manifold and the fibre F is of the form F = F-1 x F-2 where the F-i(i = 1 2) are Riemannian 3-manifolds. We prove that the connection on M is completely determined by the requirement that the Bonan 4-form given in the work of Yasui and Ootsuka [Y. Yasui and T. Ootsuka Spin(7) holonomy manifold and superconnection Class. Quantum Gravity 18(2001)807-816] be closed. Assuming that the F-i are complete connected and simply connected it follows that they are isometric to S-3 with constant curvature k > 0 and the Yasui-Ootsuka solution is unique in the class of (3 + 3 + 2) warped-like product metrics admitting a specific Spin(7) structure. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.
Citation Count: 5
An analysis of price spikes and deviations in the deregulated Turkish power market
(Elsevier, 2019) Bilge, Ayşe Hümeyra; Yücekaya, Ahmet; Bilge, Ayşe Hümeyra
The successful operation of a real time market is related to the planning in the day ahead market. We analyze the day ahead and real time market data for the Turkish power market for the period 2012-2015 to classify price spikes and their causes. We also focus on the levels of deviation between the day ahead market values and the real time market values. We define price deviation and load deviation ratios to measure the level of deviation both in price and demand. The analysis for the load is based on load shedding and cycling values. We analyze the mean and standard deviation in market prices and we determine the price spike as a two sigma deviation from the mean value. It is shown that 60% of the price deviation ratios are in the range of ( +/- 20%), while 44% are in the range of ( +/- 10%) and 35% are in the range of (+/- 5%). We also show that 56.9% of the spikes are due to problems in the generation of natural gas based power plants which affect the day ahead and real time prices. A total of 29.2% of the spikes are due to power plant and system failures that affect only real time prices. The share of high temperature based spikes is 13.9% which is a result of air conditioner usage.
Citation Count: 87
An analytic network process-based approach to concept evaluation in a new product development environment
(Taylor & Francis, 2007) Ayağ, Zeki; Özdemir, Rifat Gürcan
Selecting the best product concept is one of the most critical tasks in a new product development (NPD) environment. Making decisions at this stage becomes very difficult due to imprecise and uncertain product requirements. So the evaluation process of determining the most satisfying conceptual design has been a very vital issue for companies to survive in fast-growing markets for a long time. Therefore most companies have used various methods to successfully carry out this difficult and time-consuming process. Of these methods an analytic hierarchy process (AHP) has been widely used in multiple-criteria decision-making problems (i.e. concept selection equipment evaluation). In this study however we use an analytic network process (ANP) a more general form of AHP due to the fact that AHP cannot accommodate the variety of interactions dependencies and feedback between higher and lower level elements. Briefly in this paper an ANP-based approach is presented to evaluate a set of conceptual design alternatives in order to reach to the best concept satisfying the needs and expectations of both customers and company. In addition a numerical example is presented to illustrate the proposed approach.
Citation Count: 1
An approach to evaluate CAM software alternatives
(TAYLOR & FRANCIS LTD, 2020) Ayağ, Zeki
The selection process for the best computer-aided manufacturing (CAM) software among a set of potential number of alternatives in market has been critical issue for most companies that aim to make their design/manufacturing-related activities automated. Because this selection process is very vital for companies because a wrong decision might put them into a difficult position in terms of economical, market share and time spent. Therefore, today's companies have used different multiple-criteria decision making (MCDM) methods for the evaluation study of CAM software alternatives to make this complex process easily applicable and not time-consuming. Among the MCDM methods in literature, the analytic hierarchy process (AHP) method has been widely used for various MCDM problems in both academic researches and industrial practices. However, in some cases, because of the vagueness and uncertainty on the judgments of decision maker(s), the crisp pair wise comparison in the conventional AHP seems to be insufficient and imprecise to capture the right judgments of decision maker(s). Therefore, a fuzzy logic method is integrated in the pair wise comparison of AHP to make up for this deficiency in the conventional AHP, called as fuzzy AHP. Moreover, the proposed approach is also realized on a case study.
Citation Count: 0
Babbling through social media: A cross-country study mapping out social networks using eWOM intentions
(Springer, 2023) Zülal, İşler; Kıygı-Çallı, Meltem; El Oraiby, Maryam
This research aims to determine the factors affecting the users’ electronic word-of-mouth (eWOM) seeking and sharing intentions and to reveal the interactions among and within clusters using social network analysis (SNA). This study includes three hierarchical sub-studies conducted in two countries, Turkey and Poland. First, we develop a segmentation for social networking site (SNS) users based on the frequency of sharing product-related information on SNSs. Second, we investigate the impact of several factors that affect eWOM seeking and sharing intentions using regression analysis. In the second sub-study, we also include the identified segments developed in the first sub-study as another factor that may have differentiated eWOM intentions. Third, to understand the degree of interaction among SNS users, we apply an SNA using the forecasted eWOM intentions scores from the second sub-study, which gives us hypothetical social networks. The results of SNA present strong interactions inter- and intra-clusters in both countries. Some key findings include the identification of three SNS user segments, including “Middlers,” that may be of particular interest to brands. We also find that in terms of eWOM intentions, users in Turkey are more active than in Poland. Although some predictors of eWOM seeking and sharing intentions differ between the two countries, users intend to be more active in eWOM seeking than in eWOM sharing. The comparative study provides valuable insights for decision-makers to engage different market segments via SNSs with various proposed features using suggested information contents for selected product categories.
Citation Count: 0
Canonical forms for families of anti-commuting diagonalizable linear operators
(Elsevier Science Inc, 2012) Bilge, Ayşe Hümeyra; Bilge, Ayşe Hümeyra
It is well known that a commuting family of diagonalizable linear operators on a finite dimensional vector space is simultaneously diagonalizable. In this paper we consider a family A = {A(a)} A(a) : V -> V a = 1... N of anti-commuting (complex) linear operators on a finite dimensional vector space. We prove that if the family is diagonalizable over the complex numbers then V has an A-invariant direct sum decomposition into subspaces V(alpha) such that the restriction of the family A to V(alpha) is a representation of a Clifford algebra. Thus unlike the families of commuting diagonalizable operators diagonalizable anti-commuting families cannot be simultaneously digonalized but on each subspace they can be put simultaneously to (non-unique) canonical forms. The construction of canonical forms for complex representations is straightforward while for the real representations it follows from the results of [A.H. Bilge S. Kocak S. Uguz Canonical bases for real representations of Clifford algebras Linear Algebra Appl. 419 (2006) 417-439]. (C) 2011 Elsevier Inc. All rights reserved.
Citation Count: 3
Detached eddy simulation of shallow mixing layer development between parallel streams
(Elsevier Science Bv, 2015) Kirkil, Gökhan
Results of a high resolution Detached Eddy Simulation (DES) are used to characterize the evolution of a shallow mixing layer developing between two parallel streams in a long open channel with a smooth flat bed and medium size dunes. The study discusses the vertical non-uniformity in the mixing layer structure and provides a quantitative characterization of the growth of the large-scale quasi two-dimensional (2D) coherent structures with the distance from the splitter plate. Results show that in streamwise sections situated between 75D (D is the channel depth) and 1501) from the splitter plate the width of the mixing layer close to the free surface is 20-30% more than the width in the near-bed region. This is mostly because of the tilting of the mixing layer interface on the low-speed side toward the low speed stream as the free surface is approached. Power spectra of the horizontal velocity components near the free surface show the presence of a -3 subrange corresponding to inverse energy cascade in two-dimensional turbulence at streamwise locations situated more than 10D from the splitter plate consistent with the presence of large-scale quasi 2D horizontal eddies and the transfer of energy (inverse energy cascade) from the smaller scales toward these eddies. Consistent with visualizations of the mass transport of a passive scalar within the mixing layer close to the free surface the estimated streamwise length of the quasi 2D mixing layer eddies is about 2.5-3.0 times larger than the local width of the mixing layer. The presence of large-scale roughness elements in the form of an array of two-dimensional dunes with a maximum height of 0.25D (D is the channel depth) induces a much more rapid and larger shift of the centerline of the mixing layer due to the increased influence of the bottom roughness. (C) 2014 International Association for Hydro-environment Engineering and Research Asia Pacific Division. Published by Elsevier B.V. All rights reserved.
Citation Count: 17
Evaluation of Influenza Intervention Strategies in Turkey with Fuzzy AHP-VIKOR
(Hindawi LTD, 2019) Samanlıoğlu, Funda
In this study, a fuzzy AHP-VIKOR method is presented to help decision makers (DMs), especially physicians, evaluate and rank intervention strategies for influenza. Selecting the best intervention strategy is a sophisticated multiple criteria decision-making (MCDM) problem with potentially competing criteria. Two fuzzy MCDM methods, fuzzy analytic hierarchy process (F-AHP) and fuzzy VIsekriterijumska optimizacija i KOmpromisno Resenje (F-VIKOR), are integrated to evaluate and rank influenza intervention strategies. In fuzzy AHP-VIKOR, F-AHP is used to determine the fuzzy criteria weights and F-VIKOR is implemented to rank the strategies with respect to the presented criteria. A case study is given where a professor of infectious diseases and clinical microbiology, an internal medicine physician, an ENT physician, a family physician, and a cardiologist in Turkey act as DMs in the process.
Citation Count: 3
An evolutionary approach for tuning parametric Esau and Williams heuristics
(Palgrave Macmillan Ltd., 2012) Battarra, Maria; Oncan, Temel; Altinel, I. Kuban; Golden, Bruce; Vigo, Daniele; Phillips, E.
Owing to its inherent difficulty many heuristic solution methods have been proposed for the capacitated minimum spanning tree problem. On the basis of recent developments it is clear that the best metaheuristic implementations outperform classical heuristics. Unfortunately they require long computing times and may not be very easy to implement which explains the popularity of the Esau and Williams heuristic in practice and the motivation behind its enhancements. Some of these enhancements involve parameters and their accuracy becomes nearly competitive with the best metaheuristics when they are tuned properly which is usually done using a grid search within given search intervals for the parameters. In this work we propose a genetic algorithm parameter setting procedure. Computational results show that the new method is even more accurate than an enumerative approach and much more efficient. Journal of the Operational Research Society (2012) 63 368-378. doi:10.1057/jors.2011.36 Published online 1 June 2011
Citation Count: 8
A heuristic approach for allocation of data to RFID tags: A data allocation knapsack problem (DAKP)
(Pergamon-Elsevier Science Ltd, 2012) Samanlıoğlu, Funda; Samanlıoğlu, Funda; Jiang, Xiaochun; Mota, Daniel; Stanfield, Paul
Durable products and their components are increasingly being equipped with one of several forms of automatic identification technology such as radio frequency identification (RFID). This technology enables data collection, storage, and transmission of product information throughout its life cycle. Ideally all available relevant information could be stored on RFID tags with new information being added to the tags as it becomes available. However, because of the finite memory capacity of RFID tags along with the magnitude of potential lifecycle data, users need to be more selective in data allocation. In this research, the data allocation problem is modeled as a variant of the nonlinear knapsack problem. The objective is to determine the number of items to place on the tag such that the value of the "unexplained" data left off the tag is minimized. A binary encoded genetic algorithm is proposed and an extensive computational study is performed to illustrate the effectiveness of this approach. Additionally, we discuss some properties of the optimal solution which can be effective in solving more difficult problem instances. (C) 2011 Elsevier Ltd. All rights reserved.
Citation Count: 28
Hourly electricity demand forecasting using Fourier analysis with feedback
(Elsevıer, 2020) Yükseltan, Ergün; Yücekaya, Ahmet; Bilge, Ayşe Humeyra
Whether it be long-term, like year-ahead, or short-term, such as hour-ahead or day-ahead, forecasting of electricity demand is crucial for the success of deregulated electricity markets. The stochastic nature of the demand for electricity, along with parameters such as temperature, humidity, and work habits, eventually causes deviations from expected demand. In this paper, we propose a feedback-based forecasting methodology in which the hourly prediction by a Fourier series expansion is updated by using the error at the current hour for the forecast at the next hour. The proposed methodology is applied to the Turkish power market for the period 2012-2017 and provides a powerful tool to forecasts the demand in hourly, daily and yearly horizons using only the past demand data. The hourly forecasting errors in the demand, in the Mean Absolute Percentage Error (MAPE) norm, are 0.87% in hour-ahead, 2.90% in day-ahead, and 3.54% in year-ahead horizons, respectively. An autoregressive (AR) model is also applied to the predictions by the Fourier series expansion to obtain slightly better results. As predictions are updated on an hourly basis using the already realized data for the current hour, the model can be considered as reliable and practical in circumstances needed to make bidding and dispatching decisions.
Citation Count: 1
'Level grading' a new graded algebra structure on differential polynomials: application to the classification of scalar evolution equations
(IOP Publishing Ltd, 2013) Bilge, Ayşe Hümeyra; Bilge, Ayşe Hümeyra
We define a new grading which we call the 'level grading' on the algebra of polynomials generated by the derivatives u(k+i) over the ring K-(k) of C-infinity functions of x t u u(1) ... u(k) where . This grading has the property that the total derivative and the integration by parts with respect to x are filtered algebra maps. In addition if u satisfies the evolution equation u(j) = F[u] where F is a polynomial of order m = k + p and of level p then the total derivative with respect to t D-t is also a filtered algebra map. Furthermore if the separant partial derivative F/partial derivative u(m) belongs to K-(k) then the canonical densities (i) are polynomials of level 2i + 1 and (i) is of level 2i + 1 + m. We define 'KdV-like' evolution equations as those equations for which all the odd canonical densities rho((i)) are non-trivial. We use the properties of level grading to obtain a preliminary classification of scalar evolution equations of orders m = 7 9 11 13 up to their dependence on x t u u(1) and u(2). These equations have the property that the canonical density rho((-1)) is (alpha u(3)(2) + beta u(3) + gamma)(1/2) where alpha beta and gamma are functions of x t u u(1) u(2). This form of rho((-1)) is shared by the essentially nonlinear class of third order equations and a new class of fifth order equations.
Citation Count: 8
Master problem approximations in Dantzig-Wolfe decomposition of variational inequality problems with applications to two energy market models
(Pergamon-Elsevier Science Ltd, 2013) Çelebi, Emre; Fuller, David J.
In this paper a modification to Dantzig-Wolfe (DW) decomposition algorithm for variational inequality (VI) problems is considered to alleviate the computational burden and to facilitate model management and maintenance. As proposals from DW subproblems are accumulated in the DW master problem the solution time and memory requirements are increasing for the master problem. Approximation of the DW master problem solution significantly reduces the computational effort required to find the equilibrium. The approximate DW algorithm is applied to a time of use pricing model with realistic network constraints for the Ontario electricity market and to a two-region energy model for Canada. In addition to empirical analysis theoretical results for the convergence of the approximate DW algorithm are presented. (C) 2013 Elsevier Ltd. All rights reserved.
Citation Count: 2
Maximal linear subspaces of strong self-dual 2-forms and the Bonan 4-form
(Elsevier Science Inc, 2011) Bilge, Ayşe Hümeyra; Dereli, Tekin; Koçak, Şahin
The notion of self-duality of 2-forms in 4-dimensions plays an eminent role in many areas of mathematics and physics, but although the 2-forms have a genuine meaning related to curvature and gauge-field-strength in higher dimensions also, their "self-duality" is something which is almost avoided above 4-dimensions. We show that self-duality of 2-forms is a very natural notion in higher (even) dimensions also and we prove the equivalence of some scattered and rarely used definitions in the literature. We demonstrate the usefulness of this higher self-duality by studying it in 8-dimensions and we derive a natural expression for the Bonan form in terms of self-dual 2-forms and we give an explicit expression of the local action of SO(8) on the Bonan form. (C) 2010 Elsevier Inc. All rights reserved.
Citation Count: 13
Minimization of rest mismatches in round robin tournaments
(Pergamon-Elsevier Science Ltd, 2018) Çavdaroğlu, Burak; Çavdaroğlu, Burak
In sports tournaments an occurrence of a difference in the rest periods of opponent teams in a game which we refer to as a rest mismatch will disadvantage the less rested team. Thus it is only fair to expect opposing teams to have rested equally before their game. In this work we introduce and study the Rest Mismatch Problem where the goal is to minimize the number of rest mismatches in a round robin tournament. Two integer linear formulations and a constraint programming formulation are provided and their computational performances are compared for several problem instances. Moreover a heuristic algorithm is developed which finds a single round robin schedule with zero mismatches when the number of teams in the tournament is a multiple of 8 and four mismatches when it is a multiple of 4 but not 8. (C) 2018 Elsevier Ltd. All rights reserved.
Citation Count: 0
On the classification of scalar evolution equations with non-constant separant
(IOP Publishing Ltd, 2017) Bilge, Ayşe Hümeyra; Mizrahi, Eti
The ` separant' of the evolution equation u(t) = F where F is some differentiable function of the derivatives of u up to order m is the partial derivative partial derivative F/partial derivative u(m) where um u(m) = partial derivative(m)u/partial derivative x(m). As an integrability test we use the formal symmetry method of Mikhailov-Shabat-Sokolov which is based on the existence of a recursion operator as a formal series. The solvability of its coefficients in the class of local functions gives a sequence of conservation laws called the 'conserved densities' rho((i)) i = -1 1 2 3 ... We apply this method to the classification of scalar evolution equations of orders 3 <= m <= 15 for which rho((-)) = [partial derivative F/partial derivative u(m)](-1/m) and rho((1)) are non-trivial i.e. they are not total derivatives and rho((-1)) is not linear in its highest order derivative. We obtain the 'top level' parts of these equations and their ` top dependencies' with respect to the 'level grading' that we defined in a previous paper as a grading on the algebra of polynomials generated by the derivatives u(b+i) over the ring of C-infinity functions of u u(1) .. u(b). In this setting b and i are called 'base' and 'level' respectively. We solve the conserved density conditions to show that if rho((-)) depends on u u(1) ... u(b) then these equations are level homogeneous polynomials in u(b+i) ... u(m) i >= 1. Furthermore we prove that if rho((3)) is nontrivial then rho((-)) = (alpha mu(2)(b) (3) is trivial then ub 1/3 where b similar to 5 and a .. and mu are functions of u. ub-1. We show that the equations that we obtain form commuting flows and we construct their recursion operators that are respectively of orders 2 and 6 for non-trivial and trivial (3) respectively. Omitting lower order dependencies we show that equations with non-trivial (3) and b = 3 are symmetries of the ` essentially non-linear third order equation'
Citation Count: 1
On the Time Shift Phenomena in Epidemic Models
(Frontiers Media Sa, 2020) Bilge, Ayşe Hümeyra; Demirci, Ali; Bilge, Ayşe Hümeyra; Ahmetolan, Semra
In the standard Susceptible-Infected-Removed (SIR) and Susceptible-Exposed-Infected-Removed (SEIR) models, the peak of infected individuals coincides with the inflection point of removed individuals. Nevertheless, a survey based on the data of the 2009 H1N1 epidemic in Istanbul, Turkey displayed a time shift between the hospital referrals and fatalities. An analysis of recent COVID-19 data and the records for Spanish flu (1918-1919) and SARS (2002-2004) epidemics confirm this observation. We use multistage SIR and SEIR models to provide an explanation for this time shift. Numerical solutions of these models present strong evidence that the delay between the peak of R' (t) and the peak of J(t) = Sigma I-i(i)(t) is approximately half of the infectious period of the epidemic disease. In addition, we use a quadratic approximation to show that the distance between successive peaks of I-i is 1/gamma(i) , where 1/gamma(i) is the infectious period of the ith infectious stage, and we present numerical calculations that confirm this approximation.
Citation Count: 7
On the uniqueness of epidemic models fitting a normalized curve of removed individuals
(Springer Heidelberg, 2015) Bilge, Ayşe Hümeyra; Samanlıoğlu, Funda; Ergönül, Önder
The susceptible-infected-removed (SIR) and the susceptible-exposed-infected-removed (SEIR) epidemic models with constant parameters are adequate for describing the time evolution of seasonal diseases for which available data usually consist of fatality reports. The problems associated with the determination of system parameters starts with the inference of the number of removed individuals from fatality data, because the infection to death period may depend on health care factors. Then, one encounters numerical sensitivity problems for the determination of the system parameters from a correct but noisy representative of the number of removed individuals. Finally as the available data is necessarily a normalized one, the models fitting this data may not be unique. We prove that the parameters of the (SEIR) model cannot be determined from the knowledge of a normalized curve of "Removed" individuals and we show that the proportion of removed individuals, , is invariant under the interchange of the incubation and infection periods and corresponding scalings of the contact rate. On the other hand we prove that the SIR model fitting a normalized curve of removed individuals is unique and we give an implicit relation for the system parameters in terms of the values of and , where is the steady state value of and and are the values of and its derivative at the inflection point of . We use these implicit relations to provide a robust method for the estimation of the system parameters and we apply this procedure to the fatality data for the H1N1 epidemic in the Czech Republic during 2009. We finally discuss the inference of the number of removed individuals from observational data, using a clinical survey conducted at major hospitals in Istanbul, Turkey, during 2009 H1N1 epidemic.
Citation Count: 6
An Overview of the 2009 A(H1N1) Pandemic in Europe: Efficiency of the Vaccination and Healthcare Strategies
(Hindawi Ltd, 2016) Samanlıoğlu, Funda; Bilge, Ayşe Hümeyra
2009 A(H1N1) data for 13 European countries obtained from the weekly influenza surveillance overview (WISO) reports of European Centre for Disease Prevention and Control (ECDC) in the form of weekly cumulative fatalities are analyzed. The variability of relative fatalities is explained by the health index of analyzed countries. Vaccination and healthcare practices as reported in the literature are used to explain the departures from this model. The timing of the vaccination with respect to the peak of the epidemic and its role in the efficiency of the vaccination is discussed. Simulations are used to show that on-time vaccination reduces considerably the final value of R(t) R-f but it has little effect on the shape of normalized curve R(t)/R-f.
Citation Count: 93
Resolved turbulence characteristics in large-eddy simulations nested within mesoscale simulations using the weather research and forecasting model
(2014) Kirkil, Gökhan; Kosovic, Branko; Kirkil, Gökhan
One-way concurrent nesting within the Weather Research and Forecasting Model (WRF) is examined for conducting large-eddy simulations (LES) nested within mesoscale simulations. Wind speed spectra and resolved turbulent stresses and turbulence kinetic energy from the nested LES are compared with data from nonnested simulations using periodic lateral boundary conditions. Six different subfilter-scale (SFS) stress models are evaluated using two different nesting strategies under geostrophically forced flow over both flat and hilly terrain. Neutral and weakly convective conditions are examined. For neutral flow over flat terrain turbulence appears on the nested LES domains only when using the two dynamic SFS stress models. The addition of small hills and valleys (wavelengths of 2.4 km and maximum slopes of ±10°) yields small improvements with all six models producing some turbulence on nested domains. Weak convection (surface heat fluxes of 10 Wm-2) further accelerates the development of turbulence on all nested domains. However considerable differences in key parameters are observed between the nested LES domains and their nonnested counterparts. Nesting of a finer LES within a coarser LES provides superior results to using only one nested LES domain. Adding temperature and velocity perturbations near the inlet planes of nested domains shows promise as an easy-to-implement method to accelerate turbulence generation and improve its accuracy on nested domains. © 2014 American Meteorological Society.

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